Penobscot Experimental Forest:

60 Years of Research and 

Demonstration in Maine, 1950-2010

Forest

Service

United States Department of Agriculture

General Technical

Report NRS-P-123

Northern

Research Station January 2014
Visit our homepage at: http://www.nrs.fs.fed.us/

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January 2014 Fax: 740-368-0152

Abstract

The Penobscot Experimental Forest (PEF) in Maine has been the site of U.S. Department 

of Agriculture, Forest Service, Northern Research Station (previously Northeastern Forest 

Experiment Station) research on northern conifer silviculture and ecology since 1950. Purchased 

by forest industry and leased to the Forest Service for long-term experimentation, the PEF 

was donated to the University of Maine Foundation in 1994. Since that time, the University and 

the Forest Service have worked in collaboration to advance the PEF as a site for research, 

demonstration, and education. This publication reports the history of the PEF during its first 

60 years (1950 to 2010) and presents highlights of research accomplishments in silviculture, 

ecology, ecophysiology, nutrient cycling, botany, and other areas. Issues of data management 

and forest management planning are addressed. Also included is a bibliography of publications 

originating from research on the PEF, as well as recollections of a research forester stationed 

there for 30 years. 

More than half a century of work on the PEF has served as an important source of information 

for practitioners and policy makers in the Acadian Forest region of the northeastern United States 

and adjacent Canada, and informed the practice of silviculture nationally and internationally. 

Long-term consistency in treatment application and measurement; stand-level replication; and 

accessible, digital data, metadata, and records archives have facilitated hundreds of studies 

and made the PEF an invaluable and highly influential research site.

Cover Photos (all by U.S. Forest Service)

Top left: Sign at the entrance to the Penobscot Experimental Forest, 2001. 

Top right: U.S. Forest Service staff constructing a footbridge on the Penobscot Experimental 

Forest, 1950.

Center left: Logger working on the Penobscot Experimental Forest, 1952.

Center right: U.S. Forest Service technician T. Skratt taking measurements for a tree volume 

study on the Penobscot Experimental Forest, 1994. 

Bottom left: U.S. Forest Service research forester L. Kenefic leading a tour on the Penobscot 

Experimental Forest, 2011. 

Bottom right: Logging operation on the Penobscot Experimental Forest, 1950s.

The use of trade, firm, or corporation names in this publication is for the information and convenience of 

the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of 

Agriculture or the Forest Service of any product or service to the exclusion of others that may be suitable.
DEDICATION

Sustaining long-term research at experimental forests depends on the foresight, commitment, 

and dedication of many people over decades. For more than 60 years, research on forest ecology 

and silviculture has been conducted on the Penobscot Experimental Forest. The men and women 

who do this work day in, day out, deserve much of the credit for the longevity and success of 

this endeavor. Among those who have contributed or are continuing to contribute to this ongoing 

research are scientists, professionals, and technicians with the U.S. Department of Agriculture, 

Forest Service and the University of Maine, who work in partnership to maintain the studies and 

the site. Over the years, hundreds of undergraduate and graduate students from the University of 

Maine and elsewhere have worked side by side with others from the University and the Forest 

Service to carry out the research described in these pages.

We thank all those who have been involved with the Penobscot Experimental Forest since 1950 

collecting, managing, and analyzing data; implementing technology transfer; maintaining the 

forest and infrastructure; and providing administrative support. The culmination of these efforts 

is a remarkable body of knowledge that has advanced our understanding of forest science and 

improved our management of forest ecosystems.

This volume is dedicated to all who have given their time and talent to the Penobscot 

Experimental Forest. Whether they spend a season—or an entire career—at the forest, their 

legacy is a world-class research establishment. 

John C. Brissette and Laura S. Kenefic

U.S. Forest Service research forester R. Frank leading a tour on the Penobscot Experimental Forest, 1990s. 

Photo by U.S. Forest Service.
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Foreword

In terms of forest life, 60 years is not especially long. In terms of research, 60 years is a 

remarkable achievement. The Penobscot Experimental Forest is remarkable both for the duration 

of research that has been conducted here and for the collaboration behind that work, first with the 

forest industry and later with the University of Maine.

In this report, Northern Research Station scientists and collaborating scientists from a variety 

of institutions describe their work and how the Penobscot Experimental Forest furthered their 

understanding of a wide range of topics and changed how northern conifer forests are managed. 

Indeed, early research projects are still furthering our knowledge of forests as data collection 

and silvicultural treatments continue with modifications that allow us to use them to address 

contemporary issues in forest management. 

Sixty years of data is a treasure. Experiments such as those conducted at the Penobscot 

Experimental Forest generate data and knowledge that have become more important with time 

because the level of confidence in these results improves with each passing decade. 

Sharing data is as important to science as collecting data, and here too the Penobscot 

Experimental Forest is impressive. Today, the Penobscot database includes more than 1 million 

observations. These long-term data are publicly available through the Forest Service’s Research 

Data Archive, which puts the Penobscot Experimental Forest on the forefront of forestry research 

and science delivery.

This report will introduce you to the Penobscot Experimental Forest and the scientists who have 

used this landscape to experiment and discover. In a broader sense, it tells a story about Forest 

Service science and the Northern Research Station. I am deeply honored to be part of both, and 

very proud of the people and accomplishments about which you will be reading.

Michael T. Rains

Director, Northern Research Station

View of the Penobscot Experimental Forest from a canopy crane, 1998. Photo by U.S. Forest Service.
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Contents

History of the Penobscot Experimental Forest, 1950-2010 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

	 Laura S. Kenefic and John C. Brissette

Penobscot Experimental Forest: Resources, Administration, and Mission . . . . . . . . . . . . . . . 21

	 Alan J. Kimball

Centerpiece of Research on the Penobscot Experimental Forest: 

 the U.S. Forest Service Long-term Silvicultural Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

	 John C. Brissette and Laura S. Kenefic

Sixty Years of Research, 60 Years of Data: Long-term U.S. Forest Service 

 Data Management on the Penobscot Experimental Forest . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

	 Matthew B. Russell, Spencer R. Meyer, John C. Brissette, and Laura S. Kenefic

The Acadian Forest Ecosystem Research Program: 

 an Example of Natural Disturbance-based Silviculture in the Northeast . . . . . . . . . . . . . . . . 71

	 Mike R. Saunders, Robert S. Seymour, and Robert G. Wagner

The Cooperative Forestry Research Unit Commercial Thinning 

 Research Network: 9-Year Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

	 Robert S. Seymour, Spencer R. Meyer, and Robert G. Wagner

Silvicultural Options for Early-successional Stands in Maine: 

 6-Year Results of the Silvicultural Intensity and Species Composition Experiment . . . . . . . 91

	 Andrew S. Nelson, Robert G. Wagner, and Mike R. Saunders

Vascular Flora of the Penobscot Experimental Forest, 

 with Provisional Lists of Lichens and Bryophytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

	 Alison C. Dibble

Understanding Patterns and Regulatory Pathways in Conifer Ontogeny: 

 the Role of the Penobscot Experimental Forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

	 Michael E. Day, Michael S. Greenwood, Katherine Spencer, and Stephanie L. Adams

Studying the Role of Wood-decay Fungi in Calcium Cycling 

 on the Penobscot Experimental Forest: a Progress Report . . . . . . . . . . . . . . . . . . . . . . . . . 147

	 Walter C. Shortle, Jody Jellison, Kevin T. Smith, and Jonathan S. Schilling

Bob Frank’s Recollections Made on the Occasion of 

 the Penobscot Experimental Forest’s 60th Anniversary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

	 Robert M. Frank, Jr., and Laura S. Kenefic

Publications of the Penobscot Experimental Forest, 1950-2010 . . . . . . . . . . . . . . . . . . . . . . . 165

	 Laura S. Kenefic and John C. Brissette

About the Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
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60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 1

History of the Penobscot Experimental Forest, 1950-2010

Laura S. Kenefic and John C. Brissette

Abstract.—Though the U.S. Department of Agriculture, Forest Service has been 

studying the forests of the northeastern United States since the late 1800s, long-term 

studies were not common until experimental forests were introduced in the 20th 

century. These forests were established for long-term experimentation, and research 

questions were defined by local forest management needs. The Penobscot Experimental 

Forest (PEF) in east-central Maine is an example of the success and evolution of the 

experimental forest model. The PEF was purchased by forest industry for research by the 

Forest Service, and later donated to the University of Maine Foundation. Throughout its 

history, the PEF has been defined by successful collaboration in research. Today, the PEF 

is known for world-class research on northern conifer silviculture and ecology, and work 

continues to evolve to address research questions beyond the scope envisioned by the 

original proponents of the site.

INTRODUCTION

The conifer-dominated forests of northern New 

England (Fig. 1) and adjacent Canada have long been 

critical to the region’s economy. The northeastern 

United States was a leader in softwood lumber and 

pulpwood production by the mid- to late-1800s 

Figure 1.—Location of the northern conifer (previously called 

spruce-fir) forest in the northeastern United States, courtesy 

of B. Tyler Wilson, U.S. Forest Service, Forest Inventory and 

Analysis.

(Whitney 1994, Wilson 2005), and the region’s heavily 

utilized northern conifer forest was largely cut over 

by the early 20th century (Irland 1999). Widespread 

cutting of progressively smaller trees caused forest 

degradation and led to concerns about resource 

sustainability (Judd 1997), yet demand for wood 

products continued to grow.

Though the U.S. Department of Agriculture (USDA), 

Forest Service had been conducting observational 

studies in the northern conifer (previously called 

spruce-fir) forest type since the 1890s (e.g., 

Graves 1899, Hosmer 1902, Murphy 1917, Zon 

1914), manipulative research did not begin until 

experimental forests (EFs) were established in 

the 1920s. Establishment of EFs in the Northeast 

occurred shortly after the Forest Service formalized 

its research program in that region with the creation of 

the Northeastern Forest Experiment Station (now the 

Northern Research Station) in 1923 (Kenefic et al., in 

press). At that time, the northeastern pulp and paper 

industry manufactured more than half the nation’s 

wood pulp and contributed substantially to the region’s 

social and economic welfare (Meyer 1929, Westveld 

1938). 

Penobscot EF

Gale River EF

Paul Smith EF

Finch-Pruyn EF
2 GTR-NRS-P-123 Penobscot Experimental Forest: 

The Forest Service’s silvicultural experimentation in 

northern conifers began in 1926 at the Gale River EF 

(44°51′ N, 68°37′ W) in the White Mountains of New 

Hampshire. Research there was conducted under the 

direction of Marinus Westveld, the “Father of SpruceFir Silviculture.” Additional experiments in northern 

conifer silviculture were initiated in 1934 and 1945, 

respectively, at the Finch-Pruyn (44°00′ N, 74°13′ W)

and Paul Smith (44°26′ N, 74°14′ W) EFs in the 

Adirondacks of New York. Studies demonstrated 

the importance of establishing advance softwood 

regeneration prior to removing the overstory (Westveld 

1930, 1931, 1938), using mechanical and chemical 

treatments to release overtopped softwoods (Curry 

and Rushmore 1955, Westveld 1933), and retaining 

sawtimber in managed stands (Recknagel et al. 1933). 

Despite these accomplishments, all three EFs were 

closed by the middle of the 20th century. The Gale 

River EF was destroyed in the New England Hurricane 

of 1938 (U.S. Forest Service 1939), and changes in 

research priorities and staffing led to closure of the 

Finch Pruyn and Paul Smith EFs (Berven et al. 2013). 

Industrial use of the northern conifer forest continued 

to be heavy, particularly in Maine, where large 

acreages were owned by forest industry (Whitney 

1994). Without Forest Service research, forest product 

companies would have had little scientific basis for 

their management (Kenefic et al., in press). Prior to 

the McIntire-Stennis Act of 1962, the capacity of 

university faculty to conduct forestry research was 

limited (Thompson 2004). As a consequence, the 

Forest Service was the sole source of information 

about many forest management topics, especially in 

the northern conifer forest. 

A NEW EXPERIMENTAL FOREST

Louis Freedman, woods manager and superintendent 

of the Penobscot Chemical Fibre Company, suggested 

that forest industry purchase land for a new Forest 

Service experimental forest. The search for a suitable 

area in Maine began in earnest in the late 1940s. A 

number of criteria were specified for the property: 

2,500 to 4,000 acres of land, all-weather road access, 

and location within 30 to 35 miles of a town. Though 

more than 20 areas were considered, one was deemed 

“superior in every respect.”1

 The selected tract 

consisted of 3,800 acres owned by the Eastern Land 

Company in the towns of Bradley and Eddington on 

the east side of the Penobscot River (Fig. 2). 

Repeated partial cutting had occurred on this parcel, 

but the forest contained large acreages of operable 

red spruce (Picea rubens Sarg.), balsam fir (Abies 

balsamea [L.] Mill.), and eastern hemlock (Tsuga 

canadensis [L.] Carr.), as well as hardwood and 

mixedwood stands dominated by red maple (Acer 

rubrum L.) and birch and aspen (Betula and Populus

spp.). Two maps and unpublished reports2

 describe 

the forest land prior to the establishment of the 

experimental forest. A survey of the property in 1929 

reports:

The greater part of this area is second growth 

caused by an old burn and now has a growth of 

spruce, fir and poplar 5-8” dbh.3

 The balance of 

the area is old growth4

 having a stand of spruce, 

fir and hemlock, some pine and cedar. This area 

has all been cut off in [the] past five years for a 

mark of rather small saw logs, some as recent as 

last year, and there is now left standing spruce, 

fir and hemlock 5-8” [dbh] with a few trees of 

larger size. All of this land has a good growth of 

soft woods, spruce, fir and hemlock also poplar 

seedlings and saplings. A very thrifty stand.

An unpublished, undated report, “Statement Regarding 

a Proposed Experimental Forest, Bradley and 

Eddington Townships, Maine,” further describes the 

land. Reference to a cruise by the Sewall Company 

in 1947 suggests that the report was written after 

that date, but before the experimental forest was 

established in 1950. The site is described as follows:

1

 Unpublished reports are on file with the U.S. Forest 

Service, Northern Research Station in Bradley, ME.

2

 Unpublished reports are on file with the U.S. Forest 

Service, Northern Research Station in Bradley, ME.

3

 Diameter at breast height.

4

 The term “old growth” refers to older trees, not old-growth 

forest.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 3

Practically all the area is well suited for growing 

spruce and fir. Heavy cutting and fires in the 

past have reduced the softwood types to their 

present distribution but the presence of spruce 

and fir reproduction throughout is evidence that 

these species are on their way back. The growth 

rates of spruce and hemlock are very good. 

Indications are that balsam fir makes very rapid 

growth for a short period—perhaps 40 years—

then gradually goes into a decline.

The timber is all fairly young second growth, i.e. 

no overmature stands, in good condition, except 

for the fir which is showing signs of decay. One 

feature of the softwood stands which makes the 

tract particularly adaptable for research is the 

all-aged nature of the forest. This will permit 

immediate harvest cuttings on an experimental 

basis without the necessity for waiting until 

satisfactory stand conditions develop. A wide 

range of operating conditions is represented, 

from barely operable stands up to some carrying 

10 or 12 cords per acre. Similarly, the nonoperable portions range from young softwood 

stands which will be ready to cut in eight or ten 

years, to sprout hardwood stands where cutting 

will not be possible for 25 years or more.

The Penobscot Experimental Forest

The Eastern Land Company tract was selected for a 

new experimental forest and purchased in common 

and undivided ownership by nine industrial and 

land-holding companies: Great Northern Paper, 

Hollingsworth & Whitney, Oxford Paper, Eastern 

Corporation, S.D. Warren, Penobscot Chemical Fibre, 

International Paper, St. Regis Paper, and Dead River. 

Figure 2.—Location of the PEF; map by Dale Gormanson, U.S. Forest Service, Forest Inventory and Analysis.

Government

Road
4 GTR-NRS-P-123 Penobscot Experimental Forest: 

The land was leased to the Forest Service, which 

established the Penobscot EF (PEF, 44°53′ N, 

68°39′ W) (Fig. 3). An American Forest Products 

Industries, Inc., press release at the time stated that 

this was “the first instance in the annals of American 

forestry in which a group of wood-using industries 

have united to purchase a large tract of timberland 

solely for lease to the federal government for 

experimental work” (Fig. 4).

Figure 3.—Sign on the PEF listing names of the landowners, circa 1950s. Photo by U.S. Forest Service.

Figure 4.—Photograph taken on September 26, 1952 of representatives of the nine companies that purchased the land for 

the PEF, with U.S. Forest Service staff and cooperators. In 2010, Forest Service, industry, and university retirees identified 

the following: Robert I. Ashman (University of Maine, 2nd from left), Louis J. Freedman (Penobscot Chemical Fibre Co., 3rd 

from left), Dwight B. Demeritt (Dead River Co., 6th from left), and Ed Giddings (Penobscot Chemical Fibre Co., 11th from left). 

Also: Robert True (S.D. Warren, 2nd from right), Gregory Baker (University of Maine, 7th from right), Art Randall (University of 

Maine, 9th from right), Paul Patterson (Great Northern Paper Co., 11th from right), and Henry Plummer (University of Maine, 

13th from right). Photo by U.S. Forest Service.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 5

The PEF is located south of the large industrial 

ownerships of northern Maine, on the southern edge 

of the Acadian Forest (Braun 1950, Rowe 1972). It 

has a much larger component of eastern hemlock 

than forests to the north. To increase the relevance 

of the study to the industrial landowners, the longterm Forest Service experiment was established in 

the portions of the PEF with the most spruce and fir; 

these sites were also more poorly drained and less 

productive than the portions of the forest supporting 

northern hardwoods (Fig. 5). 

When the long-term experiment began, species 

composition across the study area was (in terms of 

basal area) 30 percent hemlock; 20 percent fir; 

16 percent spruce; 12 percent northern white-cedar 

(Thuja occidentalis L.); 9 percent red maple; and 4 

percent each eastern white pine (Pinus strobus L.), 

paper birch (B. papyrifera Marsh.), and “other.” 

Diameter distributions (graphs of number of trees 

per acre by diameter class) were reverse-J shaped, 

with few if any trees per acre in the large sawtimber 

classes. These distributions reflect the presence of 

scattered older residuals from past cutting in otherwise 

aggrading stands composed of released advance 

and new regeneration. The forest age structure was 

irregularly uneven-aged.

Forest Service Research on the PEF

Station employees held different opinions about the 

direction that Forest Service research should take in 

Maine. Westveld was nearing retirement, but continued 

observational studies of silvics and fundamental 

silviculture. New silviculturist Thomas F. McLintock 

Figure 5.—Location of the U.S. Forest Service management units and soil drainage on the PEF, courtesy of Alan Kimball, 

University of Maine.
6 GTR-NRS-P-123 Penobscot Experimental Forest: 

(Fig. 6) was focused on specific management 

problems, such as determining cutting cycle length 

and marking guidelines from growth and yield 

assessments. Though industrial support for forestry 

research was growing, the approach the Station would 

take in the future was not clearly defined. Ultimately, 

McLintock’s emphasis on growth and yield was 

adopted and served as the basis for experiments at the 

PEF; this decision was a turning point for the Forest 

Service’s research program in Maine.

Figure 6.—U.S. Forest Service, Northeastern Forest Experiment Station (now Northern Research Station) scientists assigned 

to the PEF, 1950-present. Dates research scientists were assigned to the PEF and served as Research Center or Project 

Leaders are shown, as determined from memos in the PEF archives, Forest Service Organizational Directories, published 

biographies, and personal communication with retirees. There may be slight errors (i.e., ± 1 to 2 years), particularly regarding 

earlier staff. Photos by U.S. Forest Service.

Thomas F. 

McLintock

1945-1956

Research 

Center 

Leader 

(1945-1956)

Expertise: 

silviculture

Arthur C. 

Hart, Sr.

1948-1969

Project 

Leader 

(1958-1969)

Expertise: 

silviculture

Grant Davis

1955-1958

Expertise: 

silviculture

Franklin R. 

Longwood 

1956-1961 

Research 

Center 

Leader 

(1956-1961)

Expertise: 

silviculture

John C. 

Bjorkbom 

1961-1970

Expertise: 

silviculture

Francis M. 

Rushmore 

1962-1972

Expertise: 

silviculture

Carter B. 

Gibbs 

1971-1972

Project 

Leader 

(1971-1972)

Expertise: 

silviculture

Robert M. 

Frank, Jr. 

1963-1996

Expertise: 

silviculture

Lawrence O. 

Safford 

1962-1970 

Expertise: 

soils
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 7

Figure 6 (continued).—U.S. Forest Service, Northeastern Forest Experiment Station (now Northern Research Station) 

scientists assigned to the PEF, 1950-present. Photos by U.S. Forest Service.

Dale S. 

Solomon 

1971-1988

Expertise: 

growth 

and yield 

modeling

Barton M. 

Blum 

1971-1991

Project 

Leader 

(1972-1991)

Expertise: 

silviculture 

Laura S. 

Kenefic 

1994-present

Expertise: 

silviculture

Miroslaw M. 

Czapowskyj 

1973-1986

Expertise: 

soils

John C. 

Brissette 

1992-present

Project 

Leader 

(1992-

present)

Expertise: 

silviculture

Daniel T. 

Jennings 

1976-1988

Expertise: 

entomology

Hewlette S. 

Crawford, Jr.

1975-1991

Expertise: 

wildlife 

biology

The core experiment proposed for the PEF was a 

“Compartment Study” of forest management options 

with different silvicultural treatments applied to 

replicated stand-level management units (MUs) (see 

Brissette and Kenefic, this volume). Described as 

“the heart of the research program,” this experiment 

would document tree growth, mortality, logging 

costs, and change in forest condition through frequent 

inventories. This large-scale (called “pilot plant” at 

the time) approach to experimentation was advocated 

within the Forest Service, and allowed assessment of 

forest management alternatives at an operational scale. 
8 GTR-NRS-P-123 Penobscot Experimental Forest: 

In addition, though earlier studies were unreplicated, 

advances in statistics (i.e., Fisher 1925a,b) led to at 

least minimal replication (n=2) in studies established 

on many mid-century EFs. On the PEF, researchers 

also set aside a small portion of the forest (less than 

50 acres) as an undisturbed area “closed to all 

operations and experiments, except those of an 

observational nature.” Though not intended as such, 

this area would later serve as an important reference 

for the long-term study, against which the outcomes 

of management could be evaluated.

In 1946, a national assessment of forest resources 

divided management practices into five categories, 

based on what was being done relative to what was 

deemed the most appropriate practice for the local 

forest type (Harper and Rettie 1946). These categories 

were: “high-order,” “good,” “fair,” “poor,” and 

“destructive.” The Northeastern Forest Experiment 

Station further promoted this categorization as a 

basis for research (U.S. Forest Service 1948). Many 

experimental forests in the Station established “cutting 

practice level” (CPL) studies which compared 

categories (usually the first four) of management, and 

served as demonstrations. 

A 1952 article in “Pulp and Paper Magazine” 

(Anonymous 1952) described the early stages of 

the research on the PEF. Work was initiated by 

McLintock with the objective of determining what 

types of forest management were economically 

practical for the spruce-fir-hardwood region of Maine. 

The first step was to establish a CPL (also called 

management intensity demonstration, MID) area on 

the forest; this 40-acre area was divided into 10-acre 

MUs representing high-order, good, fair, and poor 

management, as described above. These treatments, 

which have since been redefined as selection cutting 

on 5- and 15-year cycles, fixed diameter-limit cutting, 

and commercial clearcutting, mirrored those on many 

other EFs throughout the Station. They were initially 

intended to serve as demonstration areas, but have 

since allowed comparison of similar treatments across 

forest types, thus expanding the scope of the local 

studies to a regional scale (Kenefic and Schuler 2008). 

The PEF Compartment Study was also installed 

shortly after the PEF was established, with initial 

treatments applied between 1952 and 1957 to MUs 

averaging 25 acres in size. Though Westveld never 

worked on the PEF, the range of treatments there was 

greatly influenced by his earlier work in partial cutting 

(e.g., Belotelkin et al. 1942; Recknagel et al. 1933; 

Westveld 1938, 1953). The first draft of the study 

plan included variants of selection cutting, as well as 

common exploitative practices: diameter-limit cutting 

and commercial clearcutting (unregulated harvesting). 

This emphasis was consistent with broader trends in 

forestry research between 1925 and 1960, a period that 

has been called the “Selective Cutting Era” due to the 

national focus on selection and other forms of partial 

cutting (Seymour et al. 2006, Smith 1972). Though not 

initially included, variants of even-aged silvicultural 

systems (including shelterwood) were added to the 

study plan at the urging of cooperator David M. Smith, 

who was then a young faculty member at Yale. 

Smith’s suggestion proved to be an inspired one. In 

the 1960s, forestry nationwide shifted into what is 

now known as the “Production Forestry Era,” during 

which the management paradigm was one of evenaged, high-yield, low-cost wood production (Seymour 

et al. 2006). Studies of planting, fertilization, thinning, 

strip clearcutting, and whole-tree harvesting were 

initiated by Forest Service scientists on the PEF, in 

direct response to industrial needs (Table 1, Fig. 7). 

Arthur C. Hart, Sr., a silviculturist who had previously 

worked with Westveld at the Gale River EF, took over 

leadership of the PEF study from McLintock; Hart 

and new scientist Robert M. Frank, Jr., expanded the 

research to include regeneration and recruitment. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 9

Year initiated Studya PI(s) Study number Status Data available?b

1950 Cutting Practice Level Study 

(Management Intensity 

Demonstration)

McLintock NE-1101-3 Active Digital, online

1951 Small Woodland Management 

in the Spruce-Fir Region

McLintock, Hart — Closed No

1951 Costs and Returns from Pruning 

Red Spruce Trees

McLintock NE-1101-5 Closed No

1952 Compartment Management 

Study (Spruce-Fir Silviculture)

McLintock NE-1101-7 Active Digital, local and 

online

1953 Effect of Seedbed Preparation 

on Spruce and Hemlock 

Reproduction

 — — Closed No

1954 Compartment Inventory 

Sampling Study

 — — Closed Unknown

1954 Volume Table Study McLintock — Closed Unknown

1954 Balsam Fir Mortality Study — — Closed Unknown

1954 Physical Properties 

of Forest Soils

McLintock — Closed No

1955 White Pine Provenance Study Schreiner, Wright — Inactive Unknown

1955 Thinning Balsam Fir Thickets 

with Soil Sterilants

Hart — Closed No

1958 Seedbed Preparation and 

Regeneration of Paper Birch

Bjorkbom — Closed No

1958 Hybrid Spruce Plantations Hart NE-1101-10 Inactive Unknown

1958 Balsam Woolly Aphid 

Occurrence on the PEF 

Compartments

Hart NE-1101-13 Closed Unknown

1959 Influence of Mice and Birds 

on Spruce-Fir Reproduction

Hart — Closed No

1960 Rate of Growth of Heart Rot 

Fungi in Living Trees

Brandt, Shigo — Closed Unknown

Table 1.—Partial list of formal studies by U.S. Forest Service scientists on the PEF, 1950-present.

(Table 1 continued on next page)
10 GTR-NRS-P-123 Penobscot Experimental Forest: 

Year initiated Studya PI(s) Study number Status Data available?b

1961 Growth Response of 

Released White Spruce

Hart — Closed Unknown

1962 Revision: Compartment 

Management Study 

(Spruce-Fir Silviculture)

Hart NE-1101-7 Active Digital, local and 

online

1964 Amendment: Compartment 

Management Study 

(Regeneration)

Frank NE-1101-7 Active Digital, online

1964 Sapsucker Behavior and 

Feeding Habits

Rushmore — Closed No

1964 Strip Clearcutting and 

Slash Disposal Methods

Frank NE-1101-23 Inactive No

1964 Production and Germination 

of Paper Birch Seeds

Bjorkbom — Closed No

1966 Viability of Seeds in the Forest 

Floor After Clearcutting

Frank, Safford — Closed No

1967 Nutrient Content of Red Spruce 

Foliage on Different Soil Series

Safford — Closed No

1971 Effect of Fertilizer on Spruce 

Trees in a Thinned White 

Spruce-Balsam Fir Stands

Frank NE-1101-41 Closed No

1972 Height Growth Relationships 

Among Red Spruce, White 

Spruce, and Balsam Fir

Blum NE-1101-43 Closed Unknown

1973 Growth Response of Red 

Spruce, White Spruce, and Fir 

Along Edges of Strips

Frank NE-1101-45 Closed No

1973 Effects of Strip Harvesting and 

Slash Disposal Methods on Soils

Czapowskyj, Frank NE-1101-46 Inactive No

1975 Revision: Compartment 

Management Study 

(Spruce-Fir Silviculture)

Frank NE-1101-7 Active Digital, online

1975 Foliar Nutrient Concentrations of 

Young Balsam Fir Related to Soil 

and Slash Disposal Methods

Czapowskyj NE1101-49 Closed No

Table 1 (continued).—Partial list of formal studies by U.S. Forest Service scientists on the PEF, 1950-present.

(Table 1 continued on next page)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 11

Year initiated Studya PI(s) Study number Status Data available?b

1975 Influence of Residual 

Basal Area Density on 

Growth of Spruce-Fir Stands

Solomon NE-1101-56 Closed No

1975 Cultural Treatments Designed 

to Reduce Spruce Sawlog 

Rotation Age

Frank NE-1101-58 Active Digital, online

1976 Seasonal Foods Selected by 

Tractable Deer in Spruce-FirMixedwood Stands

Crawford NE-1151-61 Closed No

1977 Helicopter Propwash for 

Removal of Spruce Budworm

Jennings — Closed No

1977 Measuring Plant Growth 

with Radio Link Attenuation

Crawford NE-1151-73 Closed No

1978 Attraction of Male Spruce 

Budworm to Pheromone Traps

Jennings — Closed No

1978 Aspen and Red Maple Sprouting 

After Cutting

Blum NE-1151-77 Closed No

1978 Even-Aged and Shelterwood 

Regeneration of Residual Strips 

in Spruce-Fir Strip Harvests

Frank NE-1151-83 Inactive No

1979 Survival and Development 

of Advance Regeneration 

After Shelterwood Harvest

Blum NE-1151-87 Inactive Unknown

1979 Spruce Budworm Monitoring Blum NE-1151-89 Closed Unknown

1981 Variation in Bud Flushing Among 

White Spruce Provenances

Blum NE-1151-92 Closed No

1981 Early-Larval Dispersal 

of the Spruce Budworm

Jennings NE-1151-94 Closed No

1984 Comparison of Whole-Tree and 

Conventional Logging Damage 

to Spruce and Fir Regeneration

Frank NE-1151-100 Inactive Unknown

1994 Irregular Shelterwood 

(New Forestry)

Frank — Active Digital, local

1995 Age Structure of the Selection 

Compartments

Kenefic, Seymour — Inactive Digital, local

Table 1 (continued).—Partial list of formal studies by U.S. Forest Service scientists on the PEF, 1950-present.

(Table 1 continued on next page)
12 GTR-NRS-P-123 Penobscot Experimental Forest: 

Year initiated Studya PI(s) Study number Status Data available?b

1995 Leaf Area – Growth Efficiency 

Relationships in Multi-Cohort 

Stands

Kenefic — Inactive Digital, local

1995 Role of Fungi in 

Biotransformation and Nutrient 

Cycling in the Forest Ecosystem

Shortle, Jellison, 

Smith

NE-4505-95-2 Active No

2001 Quantifying Carbon in Northern 

Forests (Emphasis on Soils)

Hoover FS-NE-4152-177 Inactive Digital, local

2001 Timber Marking Costs in 

Northern Conifer Stands

Sendak — Inactive Digital, local

2005 Substrate Availability and 

Regeneration Microsites 

of Tolerant Conifers

Kenefic, Weaver — Closed Digital, local

2006 Relationships Between 

Understory Vegetation 

and Soil, Site, and Silviculture

Kenefic, Bryce — Inactive Digital, local

2008 Revision: Compartment 

Management Study 

(Northern Conifer Silviculture)

Brissette, Kenefic NRS-07-08-01 Active Digital, online

2008 Rehabilitation of Cutover 

Mixedwood Stands

Kenefic NRS-07-08-01 

Appendix

Active Digital, local

2009 Effects of Silvicultural Treatment 

on the Dynamics of Eastern 

White Pine in Mixed Stands

Brissette, Seymour — Inactive Digital, local

2010 Seedling Herbivory Kenefic, Weiskittel, 

Berven

 — Inactive Digital, local

2010 How Well Do the Permanent 

Sample Plots Represent Stand 

Conditions?

Brissette, 

Weiskittel, Kenefic

 — Active Digital, local

Table 1 (continued).—Partial list of formal studies by U.S. Forest Service scientists on the PEF, 1950-present.

— Indicates unknown (PI or study number) or unassigned (study number).

a This list includes studies conducted wholly or partly on the PEF and for which U.S. Forest Service scientists served as principal investigators. 

Active studies are ongoing with regularly scheduled treatments and inventories. Inactive studies are not being treated and/or inventoried at this 

time, but boundaries and/or plots are maintained for future remeasurement. Closed studies cannot be relocated or remeasured. This list is not 

complete; it includes only those studies for which documentation is on file.

b Data may be available for download (online) or in a locally stored digital format with limited accessibility (e.g., text, spreadsheet, or pdf). 

Availability of data from inactive or closed studies may be unknown due to the volume of records held by the Forest Service at the PEF; such 

data may be in paper or scanned (pdf) format or lost. Data that are not available are confirmed to be lost, with the exception of NE-4505-95-2, 

for which data are being processed at this time. Summaries of the data or results of analyses from most of the studies listed here have been 

published (see Kenefic and Brissette, “Publications,” this volume).
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 13

a. 1968 a. present

b. 1976 b. present

c. 1984 c. present

Figure 7.—Examples of production forestry research on the PEF: (a) eastern white pine provenance plantation (1968 and 

present), (b) precommercial thinning (1976 and present, after commercial thinning), and (c) biomass operation with whole tree 

harvesting (1984 and present). Photos by U.S. Forest Service.
14 GTR-NRS-P-123 Penobscot Experimental Forest: 

It was toward the end of this period, in the late 1970s 

and early 1980s, that a severe spruce budworm 

(Choristoneura fumiferana [Clemens]) infestation 

spread through southeastern Canada and northern 

New England. During the budworm years, Forest 

Service staffing in Maine and related research activity 

on the PEF increased. Though less severe than in 

forests farther north, the spruce budworm infestation 

on the PEF generated an abundance of literature 

related to budworm impacts and control (e.g., Blum 

1985; Collins and Jennings 1987; Houseweart et al. 

1980, 1982; Jennings and Crawford 1983; Jennings 

and Houseweart 1983, 1986, 1989; Jennings et al. 

1984; Kendall et al. 1982). In addition, heavy cutting 

throughout the region before and during the budworm 

era created large areas of naturally regenerated, 

softwood-dominated stands. Industrial landowners had 

questions about the best ways to manage these densely 

stocked stands, and whether early stand treatments 

were warranted. Some of the earliest studies on 

precommercial thinning in the region were conducted 

on the PEF, and would ultimately provide important 

information about the effectiveness of various 

operational methodologies and spacings (Brissette 

et al. 1999; Weiskittel et al. 2009, 2011). 

Over the years, numerous studies have been conducted 

on the PEF, and close to 300 technical and scientific 

publications written (see Kenefic and Brissette, this 

volume). In addition to an abundance of research on 

forest ecology and silviculture, studies done by or 

in cooperation with Forest Service scientists have 

covered a range of topics, such as measurement 

techniques (Brissette et al. 2003, Kidd 1952, 

Lindemuth 2007), tree growth (Blum and Solomon 

1980, Solomon and Frank 1983, Solomon and Seegrist 

1983), leaf area relationships (DeRose and Seymour 

2003, 2010; Gilmore and Seymour 1996; Kenefic and 

Seymour 1999; Maguire et al. 1998), root structure 

(Tian 2002), soils and site quality (Czapowskyj 

et al. 1977, McLintock 1959), wood properties and 

decay (Garber et al. 2005, Smith et al. 2007), genetics 

(Hawley et al. 2005), understory plants (Dibble et al. 

1999, Olson et al. 2011, Safford et al. 1969), songbirds 

(Horton and Holberton 2009, Johnston and Holberton 

2009), insects (Collins and Jennings 1987, Su and 

Woods 2001), spiders (Jennings and Houseweart 

1989, Jennings and Sferra 2002), and wildlife (Abbott 

and Hart 1961, Crawford 1982, Crawford and Frank 

1988, Grisez 1954). There has always been great 

potential for additional research, using the conditions 

represented within the long-term study to answer 

questions about ecology and management. In addition, 

many of the Forest Service’s data from the long-term 

silvicultural studies on the PEF are available online, 

facilitating collaborative research (Brissette et al. 

2012a, 2012b).

The University of Maine and the PEF

In the 1990s, forest product companies in the 

Northeast underwent profound changes involving 

consolidation, downsizing, and turnover in mill and 

forest ownership. Early mills had acquired large 

amounts of land and held it against wood shortages, 

but returns on investment were low. As long as 

timberland was cheap, the mills retained their forest 

property, but with land values rising and demand for 

pulpwood declining, many began to sell their property 

in the 1990s (Acheson 2000, Hagan et al. 2005). 

Frequent turnover of ownership within the forest 

industry, desire to increase university cooperation, 

and concerns about the Forest Service’s long-term 

commitment to the PEF (Frank and Kenefic, this 

volume) motivated the industrial owners to donate 

the property to the University of Maine Foundation 

in 1994. 

As a result of mergers and acquisitions of the original 

companies, the forest owners at the time of the 

donation were Boise Cascade, Champion International, 

Great Northern Paper, J.M. Huber, International 

Paper, J.D. Irving, James River Timber, Prentiss and 

Carlisle, Scott Paper, Seven Islands, and J.W. Sewall 

(Fig. 8). The Forest Service has continued its research 

under a memorandum of understanding since that 

time. In addition, University of Maine faculty and 

graduate students have expanded the research on the 

forest to include an additional 300 acres of forest 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 15

management experiments (see Nelson and Wagner, 

this volume; Saunders et al., this volume; Seymour et 

al., this volume), as well as 1,000 acres of wetlands 

and reserves and 1,500 acres of “working forest” 

managed by the University Forests Office for income 

generation, education, and research (see Kimball, 

this volume). The PEF is also open to many types of 

recreation. 

CONCLUSIONs

One of the articles published at the time of the PEF’s 

establishment concluded that “[t]here is every reason 

to think it [the PEF] should return rewards... It is a 

project established on a large enough scale, and to 

extend over a long enough period of time, to permit 

true scientific investigation” (Anonymous 1952). This 

Figure 8.—Sign on the PEF listing names of the landowners, circa 1990s. Photo by U.S. Forest Service.

prediction has proven to be true; the cutting practice 

level (MID) and large-scale Compartment Study have 

been continued by the Forest Service until the present 

day, making the PEF one of the oldest replicated, 

continuously operated and inventoried forest research 

sites in North America. The PEF studies provide 

invaluable information on the long-term consequences 

of various forest management alternatives. We give 

dozens of tours each year to landowners, researchers, 

forestry students, and land managers. Though visitors 

are usually from the Northeast and eastern Canada, 

we have had guests from throughout North America 

and as far away as Australia and Siberia. Perhaps 

most important, we continue to maintain and expand 

the research envisioned by our predecessors and in so 

doing, pay tribute to those Maine forestry pioneers.
16 GTR-NRS-P-123 Penobscot Experimental Forest: 

ACKNOWLEDGMENTS

We thank Robert M. Frank, Jr., and Lawrence O. 

Safford (USDA Forest Service, retired) for their 

assistance with the historical aspects of this paper. 

Robert Seymour and Justin Waskiewicz (University of 

Maine, School of Forest Resources) provided helpful 

reviews of an earlier version of this manuscript. 

Amy Doole of the National Forest Service Library 

provided invaluable assistance with research.

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along a vertical gradient to compare insect fauna 

in managed forests. Environmental Entomology. 

30(2): 400-408.

Thompson, D.H. 2004. Senator John C. Stennis: 

champion of forestry. Forest History Today. 

(Spring/Fall): 27-34.

Tian, S. 2002. Effect of precommercial thinning 

on root development and root and butt decay 

incidence of red spruce and balsam fir. Orono, 

ME: University of Maine. 265 p. Ph.D. dissertation.

U.S. Forest Service. 1939. Annual report for 1938.

New Haven, CT: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 34 p.

U.S. Forest Service. 1948. Annual report for 1948.

Upper Darby, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 98 p.

Weiskittel, A.R.; Kenefic, L.S.; Li, R.; Brissette, J.C. 

2011. Long-term influence of precommercial 

thinning treatments on stand-level attributes 

in a northern conifer stand of central Maine.

Northern Journal of Applied Forestry. 28(2): 92-96.
20 GTR-NRS-P-123 Penobscot Experimental Forest: 

Weiskittel, A.; Kenefic, L.S.; Seymour, R.S.; Phillips, 

L.M. 2009. Long-term effects of precommercial 

thinning on stem form, volume, and branch 

characteristics of red spruce and balsam fir crop 

trees. Silva Fennica. 43(3): 397-409.

Westveld, M. 1930. Suggestions of management 

of spruce stands in the Northeast. Cir. 134. 

Washington, DC: U.S. Department of Agriculture. 

24 p. 

Westveld, M. 1931. Reproduction on the pulpwood 

lands in the Northeast. Tech. Bull. 223. 

Washington, DC: U.S. Department of Agriculture, 

Forest Service, Washington Office. 52 p.

Westveld, M. 1933. Experimental cutting area in the 

Adirondacks. Journal of Forestry. 31(5): 599.

Westveld, M. 1938. Silvicultural treatment of spruce 

stands in northeastern United States. Journal of 

Forestry. 36(10): 944-950.

Westveld, M. 1953. Ecology and silviculture of the 

spruce-fir forests of eastern North America.

Journal of Forestry. 51(6): 422-430.

Whitney, G.G. 1994. From coastal wilderness to 

fruited plain. New York: Cambridge University 

Press. 451 p.

Wilson, J. 2005. Nineteenth century lumber surveys 

for Bangor, Maine: implications for preEuropean settlement forest characteristics in 

northern and eastern Maine. Journal of Forestry. 

103(5): 218-223.

Zon, R. 1914. Balsam fir. Bull. 55. Washington, DC: 

U.S. Department of Agriculture, Forest Service. 

68 p.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 21

Penobscot Experimental Forest: 

Resources, Administration, and Mission

Alan J. Kimball

Abstract.—The Penobscot Experimental Forest (PEF) was established more than 

60 years ago as a result of private forest landowners’ interest in supporting forest research 

in Maine. In 1950, nine pulp and paper and land-holding companies pooled resources and 

purchased almost 4,000 acres of land in east-central Maine. The property was named the 

Penobscot Experimental Forest and was leased to the U.S. Department of Agriculture, 

Forest Service for 99 years to provide a location for long-term forest management 

research in a mixed northern conifer forest.

The PEF is large enough to encompass a variety of stand, site, and soil conditions. 

From the very beginning, its proximity to the University of Maine has fostered a close 

working relationship among U.S. Forest Service and university researchers. In 1994, 

the landowners donated the PEF to the University of Maine Foundation. The deed 

and subsequent memorandum of understanding outlined the mission of the PEF, its 

management, and its governance as a joint venture by the U.S. Forest Service, the 

University of Maine, and the University of Maine Foundation. Today the PEF supports a 

broadened research mission; provides scholarships for students at the University’s School 

of Forest Resources; and exemplifies the benefits of a lasting, shared commitment to 

long-term silvicultural and ecosystem research, education, and outreach. 

PROPERTY HISTORY

The 3,855-acre Penobscot Experimental Forest (PEF) 

is centrally located in Maine, east of the Penobscot 

River in the towns of Bradley and Eddington 

(Fig. 1). Since 1950, the PEF has been the site of 

internationally recognized long-term forest ecosystem 

and silvicultural research on Northern Forest types. 

The University of Maine Foundation has owned the 

PEF since 1994. Today the PEF has a broadened 

research mission and provides scholarship support 

for students at the University’s School of Forest 

Resources.

The PEF arose from private forest landowners’ need 

for and willingness to support forest research in 

Maine. In the mid-1940s, nine pulp and paper and 

land-holding companies discussed establishing a longterm research area for the spruce-fir (Picea-Abies) 

forest type so important to Maine. 

The history of the PEF before 1950 is not well 

documented. It is known that by 1859 there were 

14 single-board mills, 3 mills with gangs of saws, 

4 clapboard mills, 4 lath machines, and 3 shingle mills 

in Bradley village alone (Town of Bradley 2012). Only 

a small portion of the PEF was cleared for agriculture 

or grazing, though most of the area was cut lightly 

in the 20 to 40 years before 1950 for pine (Pinus), 

hemlock (Tsuga), and spruce sawlogs. Earlier cutting 

may have been heavier. The presence of charcoal and 

old burned stumps in some areas indicates that fires 

occurred following the cutting of pine stands. In 1950, 

stands on the PEF were 60 to 100 years old with a few 

older trees scattered throughout the area (U.S. Forest 

Service 2006). 

In 1950, the nine companies pooled resources and 

purchased close to 4,000 acres of land in east-central 

Maine, 10 miles north of the city of Bangor and 
22 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 1.—Location of the Penobscot Experimental Forest.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 23

15 miles from the University of Maine campus. The 

property was named the Penobscot Experimental 

Forest and was leased to the U.S. Department of 

Agriculture (USDA), Forest Service, Northeastern 

Forest Experiment Station (now the Northern Research 

Station) for 99 years to provide a location for longterm forest management research in a mixed northern 

conifer forest.

In 1984, the then-12 private firms that owned the PEF 

in common, undivided ownership each deeded their 

respective interest in the 225 acres surrounding the 

historic Leonard’s Mills dam to the Maine Forest and 

Logging Museum, Inc. This transaction reduced the 

PEF to its present size. All 12 deeds reserved rights of 

way over and across Government Road and also across 

what is now known as the IP Road. 

In 1994, the landowners donated the PEF to the 

University of Maine Foundation. The deed and 

subsequent memorandum of understanding (MOU) 

between the Northeastern Forest Experiment Station 

and the University of Maine contained specific 

language outlining the mission of the PEF and its 

governance. According to the MOU, “The primary 

mission of the PEF is to provide a location where 

long-term research, developmental activities, and 

demonstrations on forested ecosystems may be carried 

out... . Closely related to the primary mission is the 

role that the PEF plays in the education of students, 

professionals, and the general public.”

Under the MOU between the University of Maine 

and the Northeastern Forest Experiment Station, 

the U.S. Forest Service will maintain control of its 

1,257 acres of long-term research sites for the balance 

of the original 99-year lease; any new research on the 

PEF is subject to approval by the Research Operations 

Team, a committee comprising University and 

U.S. Forest Service scientists. 

AREA DESCRIPTION

The PEF is located on the east side of the Penobscot 

River northeast of Bangor, Maine and presently 

contains 3,855 acres1

 in the towns of Bradley and 

Eddington. The PEF is accessible by the Chemo Pond 

Road from Maine Route 9 when traveling east from 

Brewer, or from Maine Route 178 to the main entrance 

in Bradley. From the main entrance the Government 

Road, a gravel road, traverses the PEF for 5 miles 

to the Chemo Pond Road; a gravel road continues 

southerly across the remaining PEF property for 

another 1.5 miles. Along the Government Road there 

are multiple side roads that provide gated access to 

various parts of the PEF. 

The PEF is in the heart of the region known as the 

lower Penobscot Valley. The cities of Bangor and 

Brewer are less than 15 miles from the PEF (Fig. 1) 

and represent one of the largest and fastest growing 

population centers in Maine. The towns around this 

urban center report increased residential home and 

second home construction. A recent report by the U.S. 

Forest Service suggests that this portion of Maine “is 

among the 15 watersheds in the Nation projected to 

experience the greatest increases in housing density 

on private forests by 2030” (Stein et al. 2005: 7). If 

expectations about population and home construction 

prove true, the forest land surrounding the PEF will 

be subjected to substantial development pressure. 

Consideration of these trends will have to be part 

of the management and research planning process. 

Increased growth holds a variety of implications for 

recreational use, wildlife habitat, invasive species, and 

the economics of the timber industry in the immediate 

area and entire region.

1

 Most acreages quoted in this document are based 

on geographic information systems analysis using a 

combination of photo interpretation and global positioning 

system technology. These are not survey acres and this 

document does not intend to represent them as such. 
24 GTR-NRS-P-123 Penobscot Experimental Forest: 

Forest Soils

The soil survey published by the USDA Natural 

Resources Conservation Service for Penobscot 

County (Soil Survey Staff 1963) includes a general 

soil map that shows the vicinity of the PEF as being 

covered by three distinct soil associations. These three 

associations are arranged from northwest to southeast 

and from the shores of the Penobscot River to the 

shores of Chemo Pond. The association that lies along 

the Penobscot River is the Suffield-Buxton-Biddeford 

association, described as silty, well-drained to very 

poorly drained soils on rolling and depressional 

topography that developed in the fine-textured 

lacustrine or marine materials deposited when the 

glaciers sunk Maine’s coastline. Soils of marine origin 

are found along the Penobscot River to Passadumkeag, 

and up the Passadumkeag River nearly to Saponac 

Lake. Farther from the river, the Plaisted-ThorndikeHowland association occurs and is characterized by 

moderately well to well-drained, stony and ledgy, deep 

to shallow, granitic and slaty soils that developed in 

the glacial tills of the upland. The Monarda-BurnhamDixmont association surrounding Chemo Pond is 

composed of wet, dominantly very stony soils that 

also developed in the glacial tills of the uplands. This 

coarse overview is useful for placing the soils into a 

landscape context. Figure 2 depicts the soils of the 

PEF categorized by drainage class.

Water Resources

About 6.5 miles of the PEF property boundary fall 

along Blackman Stream and Chemo Pond (Fig. 2). The 

pond, the stream, their tributaries, and several hundred 

acres of associated wetlands (both forested and 

nonforested) contribute to the beauty, diversity, and 

habitat value of the PEF. The many wetlands adjacent 

to and flowing into Blackman Stream and Chemo 

Pond from the PEF afford locally and regionally 

important habitat for a host of game and nongame 

species. Wood ducks, otters, mink, turtles, and frogs 

are abundant in these wetlands. Moose, beavers, great 

blue herons, black ducks, loons, and eagles are often 

seen by those enjoying the winding, flat water as they 

paddle upstream on Blackman Stream from Leonard’s 

Mills to Chemo Pond. 

Chemo Pond is a large, predominantly shallow 

(maximum depth 24 feet), warm-water pond. Anglers 

enjoy fast action for white perch, yellow perch, chain 

pickerel, and small-mouthed bass in both winter and 

summer. Chemo Pond has slightly below-average 

water quality. The surrounding peatlands and wetlands 

contribute phosphorus responsible for the tea-like 

color of the water. The Pond has a number of seasonal 

and year-round homes that ring the shore on all but the 

PEF frontage. These homes are reached from Route 9 

easterly via the private Chemo Pond Road; northerly 

via Bruckoff, Scott Point, and Getchell Roads; or 

westerly via Yawaca Road. There is a commercial 

campground and boat launch, Dean’s Landing, located 

where the Chemo Pond Road meets the pond shore. 

The stream and pond are thus important recreational 

resources for people living along them as well as for 

visitors from the entire region. 

Blackman Stream and Chemo Pond are currently 

priority waters in the forefront of a major collaborative 

effort to improve passage for sea-run fish to their 

original habitats in several major watersheds in 

New England. One of the goals of this effort is to 

reestablish the impressive spawning runs of alewives 

in Blackman Stream that have been impossible since 

the construction of the first timber crib dams at Veazie 

and Great Works in the 1830s (Watts 2003). Currently 

the Penobscot River Restoration Trust is working to 

enhance energy production at the dams in Stillwater, 

West Enfield, and Medway so that the Veazie and 

Great Works dams can be breached or removed to 

allow passage for Atlantic salmon, alewives, herring, 

sturgeon, and eels (Penobscot River Restoration Trust 

2013). In addition, they plan to establish a bypass for 

fish around the Howland dam. Based on the apparent 

success of those efforts, the U.S. Department of the 

Interior, Fish and Wildlife Service and the Atlantic 

Salmon Federation approached the Maine Forest 

and Logging Museum and built an aesthetically 

complementary fishway around the historic Leonard’s 

Mills dam in Bradley. Protecting the shorelines, 

water quality, wetlands, and associated viewsheds 

of Blackman Stream and Chemo Pond all need to be 

considered when planning management and research 

activities within the PEF.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 25

Figure 2.—Soils of the Penobscot Experimental Forest by drainage class.
26 GTR-NRS-P-123 Penobscot Experimental Forest: 

Special Management Zones 

and Conservation Areas

State Shoreland Zoning (SLZ) regulations govern 

management on a sizable portion of the PEF. Maine’s 

minimum mandatory shoreland zoning requirements 

currently call for SLZ partial-harvest or no-harvest 

buffer distances of 75 to 250 feet around water bodies, 

riparian corridors, and significant wetlands, depending 

on the size and character of the feature. These 

buffers directly influence the management options on 

more than 225 acres of the PEF. Additional special 

habitat areas occur on the PEF and receive specific 

protections. 

An eagle nest on the south side of Chemo Pond 

receives special protection at the State and Federal 

levels. Management of the forest around the nest 

is conducted under guidelines from the Maine 

Department of Inland Fish and Wildlife. Harvesting is 

prohibited within 330 feet of the nest and this buffer 

area overlaps with the shoreland zoning buffer as well. 

The PEF staff takes extra care to identify nest sites to 

ensure that management activities do not negatively 

affect them.

ADMINISTRATION 

AND MANAGEMENT

The administration of the PEF is governed by the 

1994 deed from Penobscot Experimental Forest, Inc. 

to the University of Maine Foundation and was further 

clarified in the MOU that was signed in 1995 between 

the Northeastern Forest Experiment Station and the 

University of Maine’s College of Natural Sciences, 

Forestry and Agriculture (NFA). The deed specifies the 

provision of an annual Penobscot Experimental Forest 

Scholarship, which covers in-state undergraduate 

tuition to the University of Maine for 1 year for a 

student majoring in forest resources. It also specifies 

that this scholarship be fully funded each year before 

any other expenditures are made for research or 

management on the PEF. The MOU was to remain 

in effect for 50 years (the balance of the U.S. Forest 

Service’s 99-year lease), with periodic reviews every 

5 years. 

In compliance with the 1995 MOU, a Research 

Operations Team (ROT) was formed consisting of 

one representative from NFA appointed by the dean, 

one representative from the Maine Agricultural and 

Forest Experiment Station appointed by the director, 

and two representatives from the U.S. Forest Service 

appointed by the director of the Northeastern Forest 

Experiment Station. Any new research on the PEF is 

subject to approval by the ROT. The ROT administers 

the research funds derived from harvests on the PEF 

and as funds permit, issues requests for proposals and 

awards those research funds on a competitive basis.

Chairing of the ROT alternates annually between 

University and U.S. Forest Service personnel. The 

ROT works with the forest manager of the University 

of Maine (who is also the forest manager for the 

University of Maine Foundation) to set policies for 

the PEF. The University’s forest manager and the 

chair of the PEF ROT are responsible for the day-today operations, management, and maintenance of the 

PEF. The University’s forest manager is responsible 

for non-research management activities on the PEF, 

e.g., property boundary line and road maintenance, 

oversight of harvesting activities, and interaction with 

recreational users and municipal and state authorities. 

In 2007, a new MOU was signed between the U.S. 

Forest Service, Northern Research Station and the 

University of Maine’s School of Forest Resources. 

This MOU made only minor revisions (mostly 

recognizing the new names for the respective agencies) 

and was in effect for 5 years. Work is underway on the 

next MOU, which will add the University of Maine 

Foundation as signatory and reflect the evolution of 

the administration of the PEF. 

Areas of Management 

and Research Responsibility 

The original MOU gave the U.S. Forest Service full 

administrative control over all PEF areas containing 

ongoing research or demonstrations installed prior 

to the date of the agreement (see Figure 3). The U.S. 

Forest Service’s long-term research area is located 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 27

Figure 3.—Research and management responsibility at the Penobscot Experimental Forest.
28 GTR-NRS-P-123 Penobscot Experimental Forest: 

approximately in the middle of the PEF and covers 

1,257 acres (Table 1). This area has been host to 

a number of cooperative studies with University 

researchers including work on forest growth and 

development, wildlife habitat, forest insects, and 

studies of commercial thinning (the last in cooperation 

with the School’s landowner-funded Cooperative 

Forestry Research Unit).

Two University-sponsored programs—the Acadian 

Forest Ecosystem Research Program (AFERP) and the 

Land-Use Effects on Amphibian Populations Program 

(LEAP)—were initiated as long-term research on the 

PEF separate from the U.S. Forest Service studies. 

These two programs occupied 302 acres across the 

PEF (Table 1). Part of the AFERP study area was 

later used for the University’s Silvicultural Intensity 

and Species Composition (SIComp) Study. As of 

October 2009 the two LEAP arrays on the PEF were 

discontinued and now AFERP constitutes the largest 

area of University research outside of the U.S. Forest 

Service’s long-term research area (Fig. 3).

Deducting PEF land used in the U.S. Forest Service 

studies, AFERP, and SIComp (Table 1 and Fig. 3) 

leaves the working forest portion, which is managed 

by the University Forests office in accordance with the 

2009 forest management plan (Morrill and Kimball 

2009). Faculty, staff, and students at the School of 

Forest Resources, as well as U.S. Forest Service 

personnel and practicing professionals from across 

From PEF 2009 

management plan

U.S. Forest 

Service 

research 

area

University 

of Maine 

research 

area

Nonforested 

wetland

Roads 

and 

landings

Nonwetland 

reserves

SLZ 

75-foot 

buffer

No 

current 

access

Forest 

land under 

University 

of Maine 

management

Grand 

total 

acres

Compartment A NA 26 70 5.86 0 27 87 244 460

Compartment B NA 105 0 15.53 0 4 0 495 620

Compartment C NA 148 111 11.47 29 69 0 521 889

Compartment D NA 23 426 1.94 180 NA NA 0 631

Total acres 1,257 302 607 34.8 209 100 87 1,260 3,857

Table 1.—Acres of Penobscot Experimental Forest features and research

New England, worked together to develop this plan. 

A current forest inventory and forest simulation 

software were used to create multiple management 

scenarios, the results of which were evaluated both 

spatially and temporally. Sustainable harvest estimates, 

derived from growth modeling with the Landscape 

Management System software, were integrated into a 

comprehensive harvest planning schedule. 

Compartments A, B, and C (1,260 acres; Table 1) 

are managed under this plan. Most of Compartment 

D, which borders Blackman Stream to the north and 

includes a large percentage of forested and unforested 

wetland along the riparian zone, is designated as a 

reserve area (629 acres).

The original MOU states, “All forestry operations will 

be planned and executed in such a way that research 

needs are given priority” and further defines the role of 

the PEF to include educating students, professionals, 

and the public. The management objectives for the 

working forest portion of the PEF are designed to:

1. Yield a sustainable supply of timber and 

associated income to satisfy scholarship, 

research, and management goals. In addition, 

management actions will achieve the reasonable 

regulation of acreage and volumes harvested 

over the long term. The forest will be managed 

for a diversity of structural conditions using a 

variety of silvicultural systems. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 29

2. Continue support of current research projects 

and provide opportunities for new projects in the 

future. As outlined in the MOU, obligations of 

annual contributions to research and scholarship 

funds will be fulfilled. Venues and support for 

field demonstrations and tours open to students, 

forestry professionals, and the public will be 

provided. 

3. Support biodiversity by providing a diversity 

of plant species, developmental stages, and 

structures across all management compartments. 

Diverse and unique habitat types, significant to a 

broad spectrum of plant and animal species, will 

be maintained and enhanced where appropriate. 

Unique habitats and imperiled species will be 

protected.

4. Maintain water quality and soil integrity 

through attention to appropriate silvicultural 

and operational principles to include meeting or 

exceeding all applicable state water quality Best 

Management Practices.

5. Protect historical and cultural resources.

6. Provide a spectrum of safe recreational 

experiences for a variety of users insofar as 

that is possible without compromising the first 

five objectives. The PEF is first and foremost a 

research forest; recreation is therefore a secondtier objective. Nonetheless, given the projections 

of rapid development in the lower Penobscot 

watershed, the recreational pressure on the area’s 

three largest public forests—the Bangor City 

Forest, the University’s Dwight B. Demeritt 

Forest, and the PEF—can only be expected to 

increase.

THINKING BACK 

AND LOOKING FORW ARD

The PEF’s last 60 years stand as an example of what 

can be accomplished through cooperation and a 

lasting, shared commitment to long-term silvicultural 

and ecosystem research. The U.S. Forest Service, 

University of Maine, and University of Maine 

Foundation are committed to continuing the PEF’s 

long-standing leadership in supporting a diversity of 

research in the Acadian Forest Region. As we celebrate 

the accomplishments of the first 60 years, it is with 

renewed resolve to ensure that the PEF remains a 

focal point of applied forest research, education, and 

outreach vital to Maine and the entire Acadian Forest 

Region for decades to come. 

ACKNOWLEDGMENTS

Reviews of an earlier version of this manuscript 

were provided by John Kabrick and Laura Kenefic of 

the U.S. Department of Agriculture, Forest Service, 

Northern Research Station.

LITERATURE CITED

Morrill, R.; Kimball, A. 2009. Penobscot 

Experimental Forest forest management plan.

Available at https://forest.umaine.edu/files/2010/08/

PEF_Mgt_Plan_2009_FinalVersion_mar2010.pdf. 

(Accessed December 2012).

Penobscot River Restoration Trust. 2013. Homepage. 

Available at http://www.penobscotriver.org/. 

(Accessed February 2013).

Soil Survey Staff. 1963. Soil Survey of Penobscot 

County, Maine. Washington, DC: U.S. Department 

of Agriculture, Soil Conservation Service 

in cooperation with the University of Maine 

Agricultural Experiment Station. 

Stein, S.M.; McRoberts, R.E.; Alig, R.J.; Nelson, 

M.D.; Theobald, D.M.; Eley, M.; Dechter, M.; 

Carr, M. 2005. Forests on the edge: housing 

development on America’s private forests. 

Gen. Tech. Rep. PNW-GTR-636. Portland, OR: 

U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station. 16 p.

Town of Bradley. 2012. History of Bradley. Available 

at http://www.townofbradley.net/history.html. 

(Accessed December 2012).
30 GTR-NRS-P-123 Penobscot Experimental Forest: 

U.S. Forest Service. 2006. Penobscot Experimental 

Forest. Available at http://www.fs.fed.us/ne/

durham/4155/penobsco.htm. (Accessed August 

2009).

Watts, D.H. 2003. A documentary history of the 

alewife (Alosa pseudoharengus) in its native 

habitat in Maine and New England. Available 

at www.friendsofmerrymeeting.org. (Accessed 

April 2012).
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 31

Centerpiece of Research 

on the Penobscot Experimental Forest: 

the U.S. Forest Service Long-Term Silvicultural Study

John C. Brissette and Laura S. Kenefic

Abstract.—Established between 1952 and 1957, the U.S. Department of Agriculture, 

Forest Service experiment comparing several silvicultural treatments is not only the 

centerpiece of research on the Penobscot Experimental Forest in Maine, it is also one 

of the longest-running, replicated studies of how management techniques influence 

forest dynamics in North America. Ten treatments representing even- and unevenaged silvicultural systems and exploitative cutting are replicated twice on operationalscale experimental units averaging 21 acres in size. Treatments are applied uniformly 

to experimental units in accordance with prescriptions designed to direct both stand 

structure and composition. In some treatments harvests are scheduled at intervals 

(e.g., 5, 10, or 20 years); in others, harvests are triggered by stand conditions. 

Each experimental unit, or compartment (most recently termed management unit), 

has an average of 18 permanent sample plots (PSPs) for measuring attributes of trees 

≥0.5 inches in diameter at breast height. Tree regeneration and other vegetation are 

measured on multiple subplots within each PSP. Measurements are taken before and after 

harvests and, in many treatments, at intervals between harvests. Over the past 60 years, 

this long-term experiment and associated short-term studies have generated fundamental 

knowledge about forest ecosystems and silvicultural guidelines for the northern conifer 

forest type, and, in a more general sense, have contributed to our understanding of 

mixed-species forest science and management.

INTRODUCTION

Between 1952 and 1957 the U.S. Department of 

Agriculture, Forest Service established a longterm silvicultural experiment on the Penobscot 

Experimental Forest (PEF) in Maine. It is currently 

titled Silvicultural Effects on Composition, Structure, 

and Growth of Northern Conifers in the Acadian 

Forest Region: Revision of the Compartment 

Management Study on the Penobscot Experimental 

Forest. This experiment was one of a series of similar 

studies on experimental forests across the United 

States. These experiments were called “compartment 

management studies” because they were designed 

around large, essentially operational-scale, 

experimental units (≈20-40 acres) (metric conversions 

are in Appendix I) known as compartments. Very 

few of those studies were continued as planned, but 

research has proceeded on the PEF with periodic 

harvests and regular re-measurement of treatment 

effects on tree and stand growth and other response 

variables.

A series of study plans has guided the long-term 

silvicultural experiment on the PEF. The most recent 

plan, by J.C. Brissette and L.S. Kenefic, was approved 

January 2008 and was an update and revision of one 

submitted by R.M. Frank, Jr. and approved in May 

1975. Frank’s study plan superseded the original plan 

of January 1953 by T.F. McLintock and subsequent 

revision by A.C. Hart in June 1962. Each of the 

revisions updated the long-term study to adjust to 

changing research priorities, build on what had been 

learned thus far, and ensure the relevance of the 

experiment for future scientists and managers. Results 
32 GTR-NRS-P-123 Penobscot Experimental Forest: 

from the first 40 years of this study were summarized 

by Sendak et al. (2003). This paper focuses on the 

experiment as it is being carried out under the current 

study plan. Details about changes that have occurred 

over the years in treatment structure and response 

variables can be found in metadata associated with the 

measured data (Brissette et al. 2012).

Much has changed in the 60 years since this study 

was first conceived. Social and political ramifications 

of forest management have brought debate about 

appropriate silviculture into the public arena. Logging 

systems have advanced from hand felling and horse 

skidding to cut-to-length processors and forwarders. 

However, many of the fundamental issues that 

prompted installation of the study remain the same. 

Spruce budworm (Choristoneura fumiferana) is still 

a threat and discussions continue about the role of 

silviculture in reducing impacts during outbreaks. 

Diameter-limit harvesting is still practiced and its 

long-term effects debated. For social, economic, and 

biologic reasons, natural regeneration remains the 

predominant method of establishing new trees and 

stands in the northeastern United States, but many 

questions about ensuring adequate regeneration of 

desired species are yet unanswered. Because of the 

silvics of the major species in the northern conifer 

forest of which the PEF is representative—red spruce 

(Picea rubens Sarg.), balsam fir (Abies balsamea [L.] 

Mill), eastern white pine (Pinus strobus L.), eastern 

hemlock (Tsuga canadensis [L.] Carr), paper birch 

(Betula papyrifera Marsh), and red maple (Acer 

rubrum L.)—both even- and uneven-aged silvicultural 

systems can be used and no one system has achieved 

universal acceptance. Questions remain about the 

entire array of silvicultural options available to natural 

resource managers.

The long-term study on the PEF has experimental 

design limitations that cannot be corrected, the most 

serious being only two replicates of the treatments (see 

Frank and Kenefic, this volume) and separation of the 

control from the rest of the experiment (Kenefic et al. 

2005b). However, the study is unique because of its 

longevity, integrity of the original treatment structure, 

timeliness of treatment application, and the quality of 

the long-term database (Brissette et al. 2006; Kenefic 

et al. 2006; Russell et al., this volume). We feel that 

these qualities more than make up for the shortcoming 

in experimental design.

The primary objective of the study is to quantify tree 

and stand response to silvicultural treatment. Response 

variables are regeneration; species composition; and 

tree and stand growth, productivity, and quality. These 

data provide information about the interaction of 

natural and human disturbances and their effects on 

stand dynamics. To meet this objective, the hypotheses 

address some of the important unanswered questions 

about managing mixed northern conifer stands in the 

region. For example: Do responses vary between...

... managed and unmanaged stands?

... stands managed with clear silvicultural 

objectives and stands exploited for current timber 

production with no concern for future composition, 

structure, or condition?

... stands managed for one or two cohorts and 

stands managed for multiple cohorts?

... stands that once regenerated are left to develop 

naturally and stands that receive tending treatments 

such as cleaning or thinning?

Because of the range of response variables measured, 

this experiment not only answers questions about 

whether treatments differ but also addresses how

treatments differ and what about them differs.

Defining hypotheses to test is an important part 

of study planning. But in a long-term experiment 

such as this one, the most enlightening outcomes 

cannot be planned for; that is, an important aspect 

of this experiment is studying the unpredicted 

and unexpected. Although the unexpected cannot 

be articulated in a hypothesis statement, it can be 

stated that this study addresses questions about the 

uncertainty inherent in any silvicultural treatment 

because of the long-term nature of stand development 

and the unpredictability of sporadic natural disturbance 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 33

events and the likely prolonged effects of climate 

change. In addition to understanding the various 

pathways of stand development initiated by particular 

silvicultural manipulations, managers need to know 

the likelihood of achieving their desired objectives 

along those pathways. Such knowledge is best attained 

through long-term monitoring, where understanding 

increases incrementally with every measurement cycle.

A secondary objective of this study is to provide a 

variety of forest structures at one location to be used as 

the framework for short-term experiments in ecology 

and silviculture (see Appendix II for some examples). 

The long-term experiment can best be described as 

empirical; the short-term studies are often processoriented and thus can address why treatments differ.

Ultimately, results from this long-term experiment and 

associated short-term studies generate fundamental 

knowledge about forest ecosystems and sciencebased management guidelines for northern conifers 

and associated species in the Acadian Forest Region 

of Atlantic Canada and adjacent Maine. In a broader 

sense, results from this study influence forest science 

and management of shade-tolerant conifers globally.

To fully understand the design and significance of 

the experiment, it is important to put it into context 

regarding its location, the range of silvicultural 

alternatives represented in the treatment structure, 

and the silvics of the species under study.

Acadian Forest

The Acadian Forest contains a mixture of northern 

conifers and hardwoods dominated by spruces (Picea

spp.) and balsam fir. Species composition is highly 

variable and influenced by both latitude and site, with 

a greater proportion of conifers on low-lying and more 

northerly areas. Halliday (1937) first described the 

Acadian Forest Region in a classification of Canada’s 

forests. The Acadian Forest spans the provinces of 

New Brunswick, Nova Scotia, and Prince Edward 

Island, and in the United States, Maine and higher 

elevations of the Appalachian Mountains. The adjacent 

and closely related Great Lakes-St. Lawrence Forest 

Region extends west through southern Quebec and 

Ontario (Rowe 1972). The Boreal Forest Region lies 

north of the Acadian and Great Lakes-St. Lawrence 

regions. Maine juts into eastern Canada, with New 

Brunswick to the east and north, and Quebec to 

the north and west. The Laurentian Mixed Forest 

Province, Warm Continental Division (McNab and 

Avers 1994) north of Portland, Maine, has been 

identified with the Acadian Forest (Braun 1950).

The PEF is located in the southern extent of the 

Acadian Forest Region, in the towns of Bradley and 

Eddington in east-central Maine (44°54' N, 68°38' W) 

(Fig. 1). The dominant conifers are shade-tolerant 

and regenerate well under canopy cover. Advance 

regeneration is prolific (Brissette 1996), and without 

it regenerated stands are converted to a hardwood 

composition (Hart 1963). Balsam fir and spruce 

species are the principal commercial softwoods. 

Though the amount and early growth rates of fir 

regeneration surpass those of spruce, fir longevity and 

maximum diameter are approximately half those of 

the spruce species. Fir is also the preferred host of the 

spruce budworm (see below). Furthermore, the ability 

of fir to extend its root system on better sites gives it 

Figure 1.—Location of the Penobscot Experimental Forest in 

the northern conifer forest region.
34 GTR-NRS-P-123 Penobscot Experimental Forest: 

an additional advantage over spruce, which has a more 

shallow rooting system (Blum et al. 1983, Tian and 

Ostrofsky 2007). Management of spruce-fir stands 

should utilize a short (<70-year) rotation, and/or favor 

spruce over fir during intermediate treatments (Hart 

1963, Westveld 1946).

Natural stand-replacing disturbances are rare in the 

Acadian Forest Region. Partial disturbances resulting 

from windthrow and isolated pockets of insects and 

disease are common. The spruce budworm, an insect 

with cyclic outbreaks that causes mortality and growth 

suppression in balsam fir and spruce species, has a 

significant impact on forest structure and composition 

(MacLean 1984). Budworm mortality is positively 

related to the proportion of fir and poor-vigor trees 

(Baskerville 1975a, McLintock and Westveld 1946), 

drainage and hybrid index (Osawa 1989), and tree 

age (MacLean 1980, 1984). The relationship between 

stand structure and budworm susceptibility is less 

certain, and both even-aged structures (Baskerville 

1975b) and uneven-aged structures (Crawford 1984, 

Crawford and Jennings 1989, Westveld 1946) have 

been recommended. When an outbreak is at full 

strength, however, it may not matter because many 

ecological and stand relationships noted with spruce 

budworm at other times simply disappear (Osawa 

1989).

Though the budworm promotes the release of advance 

regeneration and thus naturally rejuvenates mature 

spruce-fir stands (Baskerville 1960), outbreaks 

threaten short-term production capacity (MacLean 

1984). Protection through spraying, although effective 

with regard to maintaining production, may reduce the 

outbreak interval by maintaining higher populations of 

host species (Baskerville 1975b).

The Acadian Forest has a long history of use by human 

beings. Virgin, or unharvested, forest is restricted to 

a few remote areas likely atypical of the region as 

a whole. Repeated diameter-limit cutting began in 

the 1800s and has continued until the present day 

(Cary 1896; Kenefic and Nyland 2005; Seymour 

1992, 1995; Westveld 1928). Preferential harvesting 

of large trees and desired species has resulted in a 

forest that is currently only 9 percent large sawtimber 

with a softwood to hardwood ratio of 0.7:1 while the 

underlying forest habitat suggests that ratio should be 

1.6:1 (McWilliams et al. 2005). Harvesting in response 

to the spruce budworm outbreak of the 1970s and 

1980s contributed to these imbalances.

Silvicultural Systems

A review of silvicultural concepts and terminology 

will set the stage for understanding and interpreting 

the long-term experiment on the PEF. Silviculture is 

the art and science of controlling the establishment, 

growth, composition, health, and quality of forest 

stands to meet specific objectives on a sustainable 

basis. Silvicultural systems are planned series of 

treatments for tending, harvesting, and regenerating 

stands (Helms 1998).

Even-aged Silviculture

Even-aged silviculture is applied to create and 

maintain stands with a single age class of trees. The 

even-aged regeneration methods include clearcut, seed 

tree, and shelterwood, and differ in terms of the source 

of regeneration and amount of cover provided during 

stand initiation.

Clearcutting allows regeneration to be established 

from seed or sprouts after the overstory is removed. 

It is not effective for natural regeneration of shadetolerant species, which will likely be outcompeted 

by fast-growing shade intolerants in an open stand. 

Additionally, research on the PEF has shown that 

northern conifer seed in the forest floor remains viable 

for only 1 year and is thus not a reliable source of 

regeneration following clearcutting (Frank and Safford 

1970). The seed tree method, which leaves scattered 

residual trees for the sole purpose of providing seed 

for the new cohort, is also not effective for the shadetolerant conifers because the intolerant hardwoods 

outcompete them and the shallow-rooted residuals lack 

windfirmness (Frank and Bjorkbom 1973, Seymour 

1995). The seed tree method has been applied with 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 35

some success for eastern white pine (Wendel and 

Smith 1990), a companion species in many northern 

conifer stands, but does not provide overhead 

protection from the white pine weevil (Pissodes 

strobi).

The most effective even-aged regeneration method in 

northern conifers is shelterwood (Brissette and Swift 

2006, Seymour 1995). In this method, the overstory 

is removed in two or more stages over the course of 

several years, providing seed and shade for the new 

cohort. This method can be used to regenerate dense 

stands of shade-tolerant trees, though the choice of 

seed trees, length of the overstory removal period, and 

intensity of the harvests determine the degree of shade 

and thus species composition of the new stand.

Additionally, shelterwood may be used to create twoaged stands if reserves, or trees from the older cohort, 

are retained after the regeneration harvest for reasons 

not related to regeneration. This shelterwood method 

may be implemented to increase growth and value 

during the next rotation, enhance vertical structure, 

improve aesthetics, and provide large trees for snags 

or downed logs (Nyland 2002).

Thinning is an intermediate treatment applied to 

immature even-aged stands to reduce stand density 

in order to improve overall growth of the stand or of 

individual trees, or capture mortality. These treatments 

may be precommercial, done as an investment before 

the trees are merchantable, or commercial. The timing, 

intensity, and type of thinning all vary depending on 

management objectives. The types of commercial 

thinning commonly applied (dominant, crown, and 

low thinning [Smith et al. 1997]) vary in terms of the 

crown classes from which trees are cut. Thinning of 

dominants (previously “selection” thinning) is used to 

remove poor form or otherwise undesirable dominants 

and should be applied only once to avoid high-grading 

(the removal of the most commercially valuable trees, 

often leaving a residual stand composed of trees of 

poor condition or undesirable species composition 

[Helms 1998]). Crown thinning is used to release 

desired crop trees in codominant and dominant canopy 

positions. Low thinning, which is generally lighter and 

more frequent, is applied to capture mortality in the 

intermediate and overtopped crown classes. Research 

on stand response to various combinations of timing, 

intensity, and types of commercial thinning has only 

recently begun in the northern conifer type (Wagner et 

al. 2002), although research on the PEF has established 

the positive effects of precommercial thinning (PCT) 

on species composition, growth, and mortality 

(Brissette et al. 1999; Weiskittel et al. 2009, 2011).

Uneven-aged Silviculture

Uneven-aged silviculture is used to create and 

maintain stands with three or more age classes of 

trees and is accomplished via selection cutting. The 

selection system has traditionally been applied to 

create a specific diameter distribution that is believed 

necessary for balancing growth and removals, and is 

manifest in long-term consistency and sustainability of 

structure and production (Meyer 1952, O’Hara 1996). 

Structural goals are defined in numerous ways, though 

primarily using empirical structures from previous 

experiments (e.g., Arbogast 1957) or mathematical 

derivations (Meyer 1952, Nyland 2002, Smith et al. 

1997).

The mathematical structures, such as q, have the 

advantage of being easy to use, but their relevance to 

biological processes is debated (Davis 1966, Oliver 

and Larson 1996). The approach historically applied 

on the PEF is the BDq method (Fiedler 1995, Guldin 

1991, Marquis 1978), in which a target residual basal 

area (BA), maximum diameter (D), and q-factor are 

determined based on financial or biological maturity, 

residual stocking goals, and desired distribution of 

growing stock among saplings, poles, and sawtimber 

(Kenefic and Brissette 2001). Using multiple q-factors 

to define a single structure has been suggested (Hansen 

and Nyland 1987, Leak and Filip 1975). The higher 

the q, the more growing stock in the smaller size class, 

and vice versa. The higher the basal area goal, the 

more trees in each size class, without any change in 

the proportional distribution of trees.
36 GTR-NRS-P-123 Penobscot Experimental Forest: 

The often-cited advantages of uneven-aged silviculture 

include comparatively little soil disturbance, high 

vertical structural diversity, high canopy cover, 

and continuous production of high-value sawlogs 

(Nyland 2002, Smith et al. 1997, Troup 1928). The 

last objective is best met through the application 

of structural goals that allocate a high proportion 

of growing space to the sawtimber classes. This 

approach is supported by research on the PEF that 

found that upper canopy trees generally produce more 

stemwood per unit leaf area than those lower in the 

canopy (Seymour and Kenefic 2002). However, too 

much overstory will suppress the development of 

poletimber and may impede regeneration and growth 

of small trees. The amount of overstory that can be 

carried without suppressing smaller trees to the point 

of structural instability has yet to be determined 

for northern conifers, though species’ competitive 

advantage is clearly related to amount and quality 

of overstory light (Moores et al. 2007). Data from 

the PEF demonstrate that even trees released from 

suppression do not grow as well as those that have 

been free growing; that is, older trees in the unevenaged stands grow less stemwood per amount of foliage 

than younger trees do (Seymour and Kenefic 2002). 

Unfortunately, preliminary assessment of sapling 

ingrowth dynamics in the uneven-aged PEF stands 

revealed slow growth and high mortality, generating 

additional questions about long-term sustainability 

(Kenefic and Brissette 2005). 

Although it is critical not to have too many trees 

in the sawtimber classes of uneven-aged northern 

conifer stands, it is also important not to create 

imbalances in other portions of the stand structure. 

The recommended diameter distribution should be 

followed for two reasons: to provide sufficient trees 

in each size class to replace those from larger classes 

as they grow in size or are cut, and to moderate 

growth of smaller trees (Arbogast 1957, Solomon and 

Frank 1983). Though timely regeneration of desired 

species is necessary to sustain uneven-aged stands, 

quality and distribution of growing stock should not 

be overlooked. In particular, it is necessary to tend 

immature trees in order to accumulate high-quality 

growing stock (Hart 1963). Thus, a deficit in the midsize classes, for example, both endangers sustainability 

of production as the sawtimber-sized trees are 

removed, and results in poor control over growth in 

the sapling classes.

Short-term sacrifices in quality and growth may 

be necessary for attainment of structural goals, 

particularly during periods of conversion to an unevenaged condition or rehabilitation of unmanaged or 

mismanaged stands (Nyland 2002). This approach is 

due in part to the need to sustain old trees in order to 

maintain an uneven-size structure during conversion 

(Nyland 2003). It has been suggested that such losses 

could be minimized in extreme cases by reducing the 

residual stocking goal (i.e., BA), and correspondingly 

lengthening the cutting cycle (Nyland 1987, 2002). 

This type of action would be short-term only and has 

the disadvantages of a delayed next entry and some 

loss of control over mortality and quality due to the 

longer cutting cycle.

The regeneration method utilized in uneven-aged 

silviculture is the selection method. Selection cuttings 

are applied on a fixed cutting cycle to remove mature 

timber, tend the immature classes, and establish new 

regeneration (Nyland 1987). The distribution of 

removals is across all size classes and may be singletree or in groups. Furthermore, though age and size are 

assumed to be equivalent, and thus size structures are 

utilized instead of age structures, research on the PEF 

has demonstrated that this relationship is poor in multiaged stands of shade-tolerant species (Blum 1973, 

Kenefic and Seymour 1999b, Seymour and Kenefic 

1998). However, the extreme difficulty of determining 

tree age from phenological characteristics of a tree 

requires use of the traditional diameter distribution but 

justifies exploratory age analysis and adjustment of 

growth expectations and structural goals based on the 

results of such.

Within the confines of the allowable cut per size 

class as determined by the structural goal defined 

above, removals are distributed to improve growth, 

quality, and species composition (Frank and Blum 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 37

1978, Leak et al. 1969). In traditional application, it 

is important that desires to make short-term gains in 

these factors do not jeopardize longer-term attempts 

to create a balanced structure. In applying such a 

treatment, species composition goals and marking 

guides are important, and all trees for harvest should 

be marked under the supervision of an experienced 

selection marker. The use of designated skid trails 

and directional felling are desirable because of the 

potential for residual stand damage associated with 

repeated partial harvests (Baker and Bishop 1986).

Much remains unknown about the short- and longterm dynamics of growth in managed uneven-aged 

northern conifer stands. Many questions of interest to 

researchers and practitioners, such as whether there is 

a production advantage to utilizing uneven- instead of 

even-aged silviculture, cannot be answered until both 

systems have been applied in a single experiment for 

the equivalent of a full rotation (approximately 80-100 

years in northern conifers). The PEF and the Acadia 

Research Forest in New Brunswick are the only 

locations with long-term experiments in the selection 

system in the Acadian Forest, and among few such 

sites in the world.

Exploitative Cutting

Exploitative cutting occurs when trees are removed 

without regard for residual stand condition. This 

type of harvesting occurs when short-term volume 

and value removals are given priority over long-term 

sustainability of composition and structure (Kenefic 

and Nyland 2005, Nyland et al. 1993). The intensity 

of the harvest varies, and ranges from diameter-limit 

cutting, in which valuable trees above specific size 

thresholds are removed, to commercial clearcutting, 

in which all merchantable trees are removed from 

a stand without tending or attention to regeneration 

(thus, as described here, commercial clearcutting is 

different from clearcutting as a silvicultural treatment). 

Both are examples of high grading, removing the most 

valuable trees from the stand. Though commonly 

practiced, removal-driven harvesting is rarely 

experimentally applied. The PEF is the site of the 

oldest known replicated experiment in diameter-limit 

and commercial clearcutting of northern conifers, and 

research on the PEF has documented the degrading 

effects of these practices on residual stand condition 

(Kenefic and Nyland 2005, 2006; Kenefic et al. 

2005a).

It has been theorized that stands subjected to repeated 

diameter-limit cuts will develop a structural imbalance 

that will ultimately suppress the establishment of 

regeneration and prevent periodic harvests (Roach 

1974). Modeling work in northern hardwoods has 

suggested a number of negative impacts, including 

reduced stand value, structural imbalance, and species 

and quality degradation (Nyland 2005, Nyland et al. 

1993). However, along with the experiment on the 

Fernow Experimental Forest in West Virginia (Schuler 

et al. 2005) and studies installed in the Central 

Hardwood Region (Fajvan 2006), the studies on the 

PEF are among the few sources of information about 

the results of experimentally controlled exploitative 

cutting. Though results from the PEF demonstrate 

shifts in species composition, degraded stand value, 

loss of sawtimber production, and increases in the 

proportion of unmerchantable trees, it is not yet 

known whether the repeated partial entries can be 

sustained. Modeling suggests, however, that the PEF 

fixed diameter-limit cut stands will not sustain another 

harvest of equal volume for many years (Kenefic et al. 

2005a).

Researchers in the Central Hardwoods have suggested 

an alternative to fixed diameter-limit cutting called 

modified (flexible) diameter-limit cutting. This 

alternative is similar to guiding diameter-limit 

cutting, which was developed for loblolly-shortleaf 

pine in the southern United States (Guldin 1987, 

Reynolds et al. 1984), although the allowable cut in 

modified diameter-limit cutting may not be restricted 

to growth as it is in guiding diameter-limit cutting. 

Because removals are based on pre-determined size 

thresholds, modified diameter-limit cutting does 

not create or maintain a specific residual condition. 

However, it is regarded by some as a compromise 

that allows landowners to accumulate the benefits of 

selection cutting without the necessity of tending the 
38 GTR-NRS-P-123 Penobscot Experimental Forest: 

unmerchantable classes (Miller and Smith 1993). As 

applied on the PEF, this treatment differs from fixed 

diameter-limit cutting in that trees below the diameter 

limits may be harvested if they are expected to die, and 

trees above the diameter limits may be left for wind 

protection or seed production. Preliminary analysis of 

data from the PEF suggests that stands treated with 

modified diameter-limit cutting are more similar to 

selection stands than to fixed-diameter-limit cut stands, 

and that these differences become more apparent over 

time (Kenefic et al. 2004).

Stand Development

Stand development is the competitive process of tree 

initiation, growth, senescence, and death (Smith et al. 

1997). It is important for managers to be familiar with 

expected stand development patterns when they are 

applying silvicultural treatments and assessing stand 

response. These patterns, described by Oliver (1981) 

and Oliver and Larson (1996), provide an ecological 

basis for understanding and communicating stand 

growth. In even-aged stands resulting from standreplacing disturbances, stands move sequentially 

through four stages: stand initiation, stem exclusion, 

understory reinitiation, and (in unmanaged stands) 

old growth. When this terminology is used to describe 

stand development, even people unfamiliar with 

the forest type may understand the processes and 

structures in the stands. Definitions (from Oliver 1981 

and Oliver and Larson 1996) are as follows:

• Stand initiation: Begins when a disturbance 

removes the existing stand and makes growing 

space available for a new cohort, and continues 

as long as trees are establishing.

• Stem exclusion: Begins when sufficient leaf 

area develops to prevent new cohorts from 

establishing, and continues as long as new 

cohorts are excluded. At this stage the processes 

of differentiation into crown classes (dominant, 

codominant, intermediate, and overtopped) and 

self thinning occur, and intermediate treatments 

and/or regeneration cuttings are applied.

• Understory reinitiation: Begins when gaps in the 

canopy (from crown abrasion or tree mortality) 

allow new cohorts to establish. An old-growth 

stand will result, unless a disturbance, such 

as harvesting, occurs. This is the stage when 

regeneration cuttings are often applied.

• Old growth: Begins when all trees from the 

initial cohort have died, and normally is not 

reached in stands managed for commodity 

production.

In uneven-aged stands the stem exclusion and 

understory reinitiation stages will likely occur in 

different places within the same stand at the same 

time. Additionally, in both even- and uneven-aged 

mixed-species stands, stratification occurs due to 

differences among species in height growth patterns, 

shade tolerance, and longevity, resulting in increased 

structural complexity. 

With this background on the Acadian Forest, 

silviculture, and stand development to provide 

context, we now consider the details of the long-term 

silvicultural experiment on the PEF.

METHODS

Treatment Overview

 The PEF long-term silvicultural experiment involves 

10 treatments (Table 1), each replicated twice in a 

completely random experimental design (Fig. 2). The 

compartments (now called management units in the 

PEF study) average 21 acres in size and the experiment 

covers 418 acres of the approximately 3,900-acre 

PEF. Considering that most of the compartment 

management studies established in the 1950s on 

experimental forests were either abandoned or scaled 

back, the long-term experiment on the PEF stands out 

for having remained true to its original intent. Harvest 

activities and sample plot remeasurements have stayed 

close to schedule throughout the life of the experiment 

(Fig. 3). In the early 2000s, the measurement interval 

between harvests was increased from 5 years to 10 

to accommodate measurement of several additional 

response variables.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 39

Treatment Management

System Code Description Unit

Even-aged silviculture SW2 Uniform shelterwood, 2-stage overstory removal 21, 30

SW3 Uniform shelterwood, 3-stage overstory removal; 23b, 29b 

without precommercial thinning

SW3 PCT Uniform shelterwood, 3-stage overstory removal; 23a, 29a 

with precommercial thinning

Uneven-aged silviculture S05 Single tree and group selection, 5-year cutting cycle 9, 16

S10 Single tree and group selection, 10-year cutting cycle 12, 20

S20 Single tree and group selection, 20-year cutting cycle 17, 27

Exploitative cutting CC Commercial clearcutting 8, 22

FDL Fixed diameter-limit cutting 4, 15

MDL Modified diameter-limit cutting 24, 28

Reference REF Unmanaged reference 32a, 32b

Table 1.—Treatments and compartments to which they are applied on the Penobscot Experimental Forest

Treatment Descriptions

Prior to treatment initiation, the study area was 

dominated by a second-growth forest of irregular age 

and size structure (Fig. 4a,b). Though land-use history 

before 1950 is not well documented, descriptions on 

maps indicate that it was “mixed softwood second 

growth” with pole-size spruce and fir, hemlock up to 

sawtimber size, scattered hardwoods, and good spruce 

and fir regeneration in 1929, and “operable sprucefir-hemlock” in 19491

. These conditions most likely 

resulted from a long history of periodic partial cutting 

and subsequent natural stand development (Kenefic et 

al. 2006, Sendak et al. 2003). 

The first study plan (McLintock 1953) presented the 

silvicultural treatments as a range of management 

options from “poor” to “high-order” and specified 

tentative residual stand structural and compositional 

goals as a basis for experimentation. Subsequent 

revisions of this plan by Hart (1962) and Frank (1975) 

clarified the silvicultural terminology and specifics of 

the treatments. The status of the treatments and current 

prescriptions, per the most recent study plan revision 

(Brissette and Kenefic 2008), are outlined in the 

following descriptions.

Even-Aged Silvicultural Treatments

Shelterwood System, Two-Stage Overstory 

Removal (SW2): This treatment is replicated in 

management units 21 (27 acres) and 30 (18 acres) 

(Fig. 2). In both management units the final overstory 

removal was completed in 1967 (Fig. 3), leaving wellestablished advance regeneration and an average of 

77 trees per acre in the 5-inch and larger diameter 

at breast height (d.b.h.) classes. The stands have 

two-storied structures with the larger residuals in 

the upper stratum. The new cohort reached the stem 

exclusion stage of stand development by the 1990s. 

Although the new cohort would benefit from removing 

1

 Unpublished documents on file at the Penobscot 

Experimental Forest and available from the authors.
40 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 2.—Locations of all U.S. Forest Service management units on the Penobscot Experimental Forest, including those in 

the long-term silvicultural experiment. Map courtesy of Alan Kimball, University of Maine.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 41

Year

Treatment/Compartment

Figure 3.—Timeline of treatments and inventories in the long-term silvicultural experiment on the Penobscot Experimental 

Forest through 2011.

Figure 4a,b.—Forest composition and structure prior to initiation of the long-term silvicultural experiment on the Penobscot 

Experimental Forest in the 1950s. Photos by U.S. Forest Service.

a b
42 GTR-NRS-P-123 Penobscot Experimental Forest: 

the overtopping residuals, there has not previously 

been enough merchantable volume to support a 

commercial thinning. The next planned intervention 

in this treatment will be a thinning. The authors and 

cooperators are working on a thinning prescription that 

will be applied in the next year or so; overstory BA 

will be reduced by no more than 40 percent.

Shelterwood System, Three-Stage Overstory 

Removal (SW3 and SW3 pct): The final overstory 

removal in MU23 was in 1971, and in MU29 in 1974 

(Fig. 3). Unlike SW2, all residual trees >2.5 inches in 

d.b.h. were cut during or immediately after the final 

overstory removal. About 10 years after the overstories 

were removed, these management units were split into 

approximately equal areas. Half of each management 

unit received PCT and half was left to develop without 

PCT.

Shelterwood System, Three-Stage Overstory 

Removal without PCT (SW3): The replicates for this 

treatment are MU23b (12 acres) and MU29b (8 acres) 

(Fig. 2). Both stands are in the stem exclusion stage 

of development, and self thinning is occurring. 

A thinning will be applied in this treatment when 

there are sufficient merchantable-sized trees in the 

new cohort to support a commercial harvest.

The likely thinning prescription will focus on 

maximizing stand-level volume production (see 

Seymour 1999) while also releasing high-quality 

eastern white pine and spruce crop trees from 

competition. The thinning method used will be a 

combination of crown and low thinning to capture 

mortality and release crop trees. Crown class, species, 

live crown ratio, and stem form and quality will be 

used to identify trees for either removal or retention.

Shelterwood System, Three-Stage Overstory 

Removal with PCT (SW3 pct): This treatment is 

replicated in management units 23a (12 acres) and 

29a (9 acres) (Fig. 2). Manual PCT to a residual 

spacing of approximately 6 feet by 9 feet was applied 

in MU23a in 1983 and in MU29a in 1984 (Fig. 3). 

The PCT lengthened the period of stand initiation 

and allowed new seedlings to become established. 

It enhanced diameter growth on the residual trees 

enough that these stands were further subdivided and 

commercially thinned. Both were included in the 

University of Maine’s Commercial Thinning Research 

Network (Seymour et al., this volume). MU23a and 

MU29a were commercially thinned in 2001 and 2010, 

respectively (Fig. 3).

Uneven-Aged Silvicultural Treatments

Selection System, 5-Year Cutting Cycle (S05):

Replicates of this treatment are MU9 (27 acres) and 

MU16 (16 acres) (Fig. 2). The eleventh selection 

cutting was in 2009 in MU9, and in 2011 in MU16 

(Fig. 3). Stands are vertically and horizontally diverse, 

with areas in both stem exclusion and understory 

reinitiation. The stands are highly stratified, and trees 

within each stratum are differentiated into crown 

classes.

The 2008 study plan revised the BDq structural goal 

to reflect species-specific growth rates and longevities. 

The previous version of the study plan did not account 

for species differences and had only one target 

diameter distribution (q=1.96 on 1-inch d.b.h. classes) 

and maximum diameter (MaxD, 19 inches d.b.h.) 

for the treatment. When all species are combined, 

the q for this treatment now averages 1.6 (decreasing 

from 1.8 in the saplings to 1.4 in the large sawtimber) 

and stand-level MaxD (excluding eastern white pine 

emergents) is 22 inches d.b.h. Species composition 

goals were also modified to better reflect the species 

assemblage occupying the site (the target BA was 

lowered for spruce and increased for hemlock). Efforts 

are being made to sustain spruce and reduce structural 

bimodality (too few trees in poletimber classes and 

too many in sawtimber) through increased recruitment 

and reduction of sawtimber excesses. An excess 

of seedlings and saplings has reduced the need to 

establish regeneration, and PCT is conducted to release 

spruce saplings from within-stratum competition.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 43

Species composition goals, expressed as a proportion 

of BA ≥4.5 inches d.b.h. are as follows: 

• eastern hemlock, 30 percent

• spruce species, 40 percent

• hardwoods, 15 percent 

• balsam fir, eastern white pine, and northern 

white-cedar (Thuja occidentalis L.), 

5 percent each

Marking guidelines by order of priority are: 

• remove cull trees, except northern white-cedar 

unless it exceeds the stand-level composition 

goal and/or is negatively impacting the growth 

of a merchantable tree 

• remove high-risk trees (i.e., trees expected to die 

before the next entry)

• remove unacceptable growing stock (UGS; trees 

without potential for volume or value increase)

• remove trees from d.b.h. classes and species that 

are in excess relative to the goals

• release or thin potential crop trees in the sapling, 

pole, and small sawtimber classes

• remove trees beyond species MaxD

Trees are not cut from size classes that are deficient 

relative to the diameter distribution unless they fall 

into the cull, high-risk, or UGS classifications. Trees 

with active cavities are not cut, nor are trees that will 

damage a snag with active cavities when felled. One 

to two trees greater than MaxD may be retained per 

management unit, if of exceptional size and quality 

for their species.

Target residual BA is 105 ft2

/acre ≥4.5 inches d.b.h., 

and the difference between actual and target stand 

BA in the 4.5-inch d.b.h. and larger classes equals the 

allowable cut. If allowable cut is less than 5 ft2

/acre 

(i.e., 1 ft2

/acre × cutting cycle length in years), then 

harvest is delayed until the next scheduled entry.

For structural control, the following species groups 

and maximum diameters are recognized:

• eastern hemlock and spruce species, 

22 inches d.b.h.

• balsam fir, 10 inches d.b.h.

• northern white-cedar, 12 inches d.b.h.

• hardwoods, 18 inches d.b.h.

• eastern white pine, 24 inches d.b.h.

Selection System, 10-Year Cutting Cycle (S10): This 

treatment is replicated in management units 12 (31 

acres) and 20 (21 acres) (Fig. 2). The fifth selection 

cutting was applied in 1994 in MU12, and in 1998 in 

MU20 (Fig. 3). Stands are vertically and horizontally 

diverse, with areas in both stem exclusion and 

understory reinitiation. The stands are highly stratified, 

and trees within each stratum are differentiated into 

crown classes.

Like the 5-year selection, this treatment had a single 

q-factor (1.96) and MaxD (18 inches d.b.h.) prior to 

the 2008 study plan revision. When all species are 

combined, the q for this treatment now averages 1.6 

(decreasing from 1.8 in the saplings to 1.4 in the large 

sawtimber) and stand-level MaxD (excluding eastern 

white pine emergents) is 20 inches d.b.h. Species 

composition goals and marking guidelines are the 

same as for the 5-year selection, and PCT is conducted 

to release selected spruce saplings.

Target residual BA is 90 ft2

/acre ≥4.5 inches d.b.h., 

and the difference between actual and target stand 

BA in the 4.5-inch d.b.h. and larger classes equals the 

allowable cut. If allowable cut is less than 10 ft2

/acre 

(i.e., 1 ft2

/acre × cutting cycle), then harvest will be 

delayed until the next scheduled entry.

For structural control, the following species groups 

and maximum diameters are recognized:

• eastern hemlock and spruce species, 

20 inches d.b.h.

• balsam fir, 8 inches d.b.h.
44 GTR-NRS-P-123 Penobscot Experimental Forest: 

• northern white-cedar, 12 inches d.b.h.

• hardwoods, 16 inches d.b.h. 

• eastern white pine, 24 inches d.b.h.

Selection System, 20-Year Cutting Cycle (S20):

The replicate management units for this treatment 

are MU17 (26 acres) and MU27 (20 acres) (Fig. 2). 

The third selection treatment was applied in 1994 

in MU17, and in 1996 in MU27 (Fig. 3). Stands are 

vertically and horizontally diverse, with areas in both 

stem exclusion and understory reinitiation. The stands 

are highly stratified, and trees within each stratum are 

differentiated into crown classes.

Like S05 and S10, this treatment had a single q-factor 

(1.96) and MaxD (16 inches d.b.h.) prior to the 2008 

study plan revision. When all species are combined, 

the q for this treatment now averages 1.6 (decreasing 

from 1.8 in the saplings to 1.4 in the large sawtimber) 

and stand-level MaxD (excluding eastern white pine 

emergents) is 18 inches d.b.h. Species composition 

goals, use of PCT, and marking guidelines are 

the same as those for the 5- and 10-year selection 

treatments.

Target residual BA is 70 ft2

/acre ≥4.5 inches d.b.h., 

and the difference between actual and target stand 

BA in the 4.5-inch d.b.h. and larger classes equals the 

allowable cut. As in the other selection treatments, if 

allowable cut is less than 20 ft2

/acre (i.e., 1 ft2/acre ×

length of cutting cycle), then harvest will be delayed 

until the next scheduled entry.

For structural control, the following species groups 

and maximum diameters are recognized:

• eastern hemlock and spruce species, 

18 inches d.b.h.

• balsam fir, 6 inches d.b.h.

• northern white-cedar, 10 inches d.b.h.

• hardwoods, 14 inches d.b.h.

• eastern white pine, 22 inches d.b.h.

Exploitative Cutting

Commercial Clearcut (CC): Replicates of this 

treatment are management units 8 (43 acres) and 22 

(34 acres) (Fig. 2). These management units were 

initially cut in 1953 (MU8) and 1957 (MU22); the 

second harvests were in 1982 and 1988 (Fig. 3). 

All merchantable trees were removed; lower 

merchantability standards resulted in heavier cuts 

in the second entries. The stands are in the stand 

initiation and stem exclusion phases of development. 

Portions of the management units in this treatment 

are being used to study a range of stand rehabilitation 

techniques (Kenefic et al. 2010).

Fixed Diameter-Limit Cutting (FDL): This treatment 

is replicated in management units 4 (25 acres) and 15 

(26 acres) (Fig. 2). The third diameter-limit cut was 

applied in MU4 in 1992 and in MU15 in 2001 (Fig. 3). 

Though some areas are in stem exclusion, much of 

the stand area is in the stand initiation phase. These 

management units will be harvested again when stand 

volume reaches initial (pre-first cut) treatment volume 

(2,000 ft3

/acre). At that time all merchantable trees 

at and above the following species-specific diameter 

limits will be cut:

• eastern white pine, 10.5 inches

• spruce species and eastern hemlock, 9.5 inches

• paper birch and northern white-cedar, 7.5 inches

• all other species, 5.5 inches

Modified Diameter-Limit Cutting (MDL): The 

two replicates of this treatment are MU24 (26 acres) 

and MU28 (18 acres) (Fig. 2). The third modified 

diameter-limit cut was applied in MU24 in 1995 and 

in MU28 in 1996 (Fig. 3). Portions of the stands are in 

the stem exclusion and understory reinitiation stages 

of development.

Unlike the fixed diameter-limit treatment, where 

the harvest interval depends on stand dynamics, 

this treatment has a defined cutting cycle of 

20 years. Furthermore, the diameter-limit classes are 

flexible, not proscriptively rigid as they are in the 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 45

fixed diameter-limit treatment. Consequently, at the 

next harvest entry all merchantable trees above the 

following species-specific diameter-limit classes will 

be cut unless they are needed for a seed source or to 

provide wind protection for smaller trees:

• eastern white pine and spruce species, 

14.5 inches

• eastern hemlock, 12.5 inches

• paper birch, 9.5 inches

• northern white-cedar, 7.5 inches

• all other species, 6.5 inches

Trees below the diameter limits may be harvested if 

they are expected to die before the next entry.

Experimental Control

Unmanaged Reference (REF): The reference 

replicates, MU32a (13 acres) and MU32b (6 acres), 

were originally one management unit, which was split 

in 1993 to take into account the distinctly different 

stages of stand development and to balance the 

experimental design. The stages of stand development 

were distinct because of an unrecorded natural 

disturbance event about the time the study was 

established that affected the area differently. MU32a 

is in the stand initiation and stem exclusion phases 

of development while MU32b is in the latter stages 

of stem exclusion and will soon enter the understory 

reinitiation phase. Neither management unit has been 

harvested since the late 1800s; prior to that, selective 

partial cuts were made.

Response Variables

Response variables are measured on a series of PSPs 

established at the beginning of the study. Currently 

there are 295 PSPs or, on average, one plot for each 

1.4 acres of the experiment. These nested circular 

fixed-radius plots have a common center point. Plot 

size varies depending on the size of tree or variable 

measured. Within these plots are three permanent 

circular milacre plots for inventorying regeneration 

in the treated management units and four such plots 

in the reference. Response variables are measured 

before and after harvests. The current study plan 

calls for additional inventories at 10-year intervals 

between harvests. (S05 and S10 have no betweenharvest inventories because of their cutting cycles.) 

Previously, that interval was 5 years. (S10 did have 

between-harvest inventories then.) It was changed 

to accommodate measuring additional response 

variables without adding substantially to the inventory 

workload. The current response variables are:

Species: Regardless of size, trees are recorded to 

species. Woody shrubs such as willow (Salix spp.), 

alder (Alnus spp.), and hazel (Corylus spp.) are not 

measured, even though they sometimes reach tree 

stature.

Regeneration: For each milacre plot the substrate 

is recorded as: undisturbed forest floor, disturbed 

forest floor, mineral soil, down coarse woody 

material, logging slash, rock, or water. If more than 

one substrate is present, the percentage of each is 

estimated to the nearest 10 percent. For tree species 

the number of seedlings >6 inches tall is counted 

according to height class: 0.5 to <1.0 foot, 1.0 to 

<2.0 feet, 2.0 to <4.5 feet, and ≥4.5 feet with d.b.h. 

<0.5 inches.

Understory vegetation: The milacre plots are also 

used to estimate percentage of cover of non-tree 

vegetation. Each milacre plot is visualized as a 

cylinder rising through the canopy, and the relative 

abundance for various taxa is classified within the 

cylinder (Witham et al. 1993). Non-tree taxa are 

recorded as: woody shrubs, herbaceous vegetation, 

grasses and sedges, ferns and similar plants, and 

mosses and lichens.

Diameter at breast height: Diameter at breast 

height is measured at 4.5 feet above the ground to 

the nearest 0.1 inch using a diameter tape. Tree size 

determines which plot it is measured on, as follows:

D.b.h. (inches) Plot size (acres)

0.5 to <2.5 1/50

2.5 to <4.5 1/20

≥4.5 1/5
46 GTR-NRS-P-123 Penobscot Experimental Forest: 

Diameter at breast height (continued): Since 

the mid 1970s, trees for which d.b.h. is measured 

have been numbered individually and a horizontal 

line is painted on the side of the tree facing plot 

center. Thus individual trees are followed over 

time and d.b.h. is consistently remeasured at the 

same location on the stem. Under the current 

measurement regime, more than 40,000 trees are 

measured in a typical year. In September 2010, the 

one-millionth d.b.h. measurement of a numbered 

tree was taken (Fig. 5). 

Spatial Distribution: On a subsample of at least 

30 percent of the plots in each management unit, 

the location of each numbered tree ≥4.5 inches 

d.b.h. is determined in relation to plot center, to 

the nearest 0.1 foot and nearest 2° of azimuth. 

The same plots are remeasured in subsequent 

inventories to add ingrowth trees and follow 

mortality.

Tree Height and Crown Attributes: On the 

same subsample of plots used to establish spatial 

distribution, height and crown attributes are 

measured on the sampled (i.e., spatially located) 

trees, as follows:

Total height—Measured to the nearest 0.1 foot.

Height to base of live crown—Measured to 

the nearest 0.1 foot. In this study, the base of 

the live crown is the center of the lowest live 

Figure 5.—Project leader John Brissette takes the onemillionth measurement in the long-term silvicultural study on 

the Penobscot Experimental Forest in 2010, assisted by a 

student technician (center) and forester Rick Dionne. Photo 

by U.S. Forest Service.

branch where it intersects the bole of the tree. 

The lowest live branch is the lowest branch that 

appears to be contributing more than it receives 

from the rest of the crown.

Crown projection—Distance from the center of 

the bole of each measured tree to the edge of its 

crown is measured to the nearest 0.1 foot in the 

four cardinal directions.

Tree Condition: A condition code is assigned to 

each numbered tree at each inventory. The codes 

provide information about the tree’s size class and 

general health and quality. Condition codes include 

such information as whether a tree is alive or dead 

(and the cause of mortality), whether it is ingrowth 

(first time measured as a sapling or pole-size tree) 

or was previously measured, and whether it is 

merchantable or cull. After trees ≥4.5 inches d.b.h. 

die, they stay in the inventory and the condition 

code reflects whether they are standing or down 

snags, and their state of decay.

DESIGN AND ANALYSIS

The study is laid out in a completely randomized 

experimental design (i.e., 2 replications of the 

10 treatments). Management units are the experimental 

units. Response variables are measured on the PSPs. 

On average there are 15 PSPs per management unit.

The reference was not included in the original 

experimental layout. It was added in 1954, after the 

experimental treatments were assigned to management 

units but before all initial treatments were applied. 

It is not contiguous with the rest of the experiment. 

However, because it is the best reference area we have 

to compare with the treated management units, it is 

considered an experimental control in analyses.

Data collected in this study are entered into a relational 

database before the next field season; details can be 

found in Russell et al. (this volume). In addition, an 

archived online database is maintained and is readily 

available to researchers working on the study and 

cooperators interested in testing various hypotheses or 

building models of northern conifer stand dynamics 

(Brissette et al. 2012).
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 47

OUTCOMES AND 

FUTURE DIRECTION

Results from the long-term silvicultural experiment 

on the PEF have improved our understanding of 

forest ecology and influenced the way forests are 

managed both regionally and internationally. Unlike 

most earlier silvicultural studies, the PEF long-term 

experiment was replicated and included an array of 

silvicultural systems. Research was initially restricted 

to sapling-size and larger trees, but that deficiency was 

recognized early on and measures of regeneration were 

added in the mid-1960s. Researchers quantified the 

competitive advantage of balsam fir over red spruce 

due to fir’s larger and less palatable seed (Abbott and 

Hart 1961), more frequent seeding, deeper rooting, and 

faster growth (Hart 1963). It became clear that natural 

regeneration of northern conifer stands was prolific, 

but questions remained about how to achieve desirable 

species mixtures. The spruce species were found to 

be less abundant than fir and hemlock under a range 

of selection and other partial cutting intensities, and 

hardwood-to-softwood ratios were higher in treatments 

with comparatively heavier removals (Brissette 1996).

Results of this study have been the basis of 

silvicultural guidance to forest managers. “The 

Silvicultural Guide for Spruce-Fir in the Northeast” 

(Frank and Bjorkbom 1973) has been used extensively 

by industrial, private, and government foresters 

throughout the northeastern United States and Atlantic 

Canada. In addition, management recommendations 

specific to uneven-aged silviculture were developed 

from the PEF selection treatments (Frank and Blum 

1978). Findings after 20 years of treatment showed 

decreases in the amount of unmerchantable volume, 

increases in seedling density and proportions of 

spruce, and improved diameter distributions.

The uneven-aged (selection) system was emphasized 

during the initial planning of the PEF study due to 

the shade tolerance of the most important commercial 

species and the preponderance of Forest Service partial 

cutting research prior to World War II (Westveld 

1946). Variants of even-aged systems were included in 

the experiment at the urging of David M. Smith from 

Yale University, who was asked to review a draft of the 

study plan. He told McLintock that “management and 

harvesting of spruce-fir types in this country would 

become pretty badly hog-tied in detailed refinements 

if an honest effort were made to superimpose the true 

selection principle... .” 2

 A national paradigm shift to 

even-aged silviculture focusing on high-yield, lowcost wood production occurred around 1960 (Seymour 

et al. 2006), largely because uneven-aged silviculture 

was regarded by many foresters as unnecessarily 

complex, prone to high-grading, and ill-suited 

for maximizing wood production. Thus, Smith’s 

suggestion to include even-aged treatments on the PEF 

proved to be an inspiration as studies of fertilization, 

PCT, strip clearcutting, whole-tree harvesting, and 

planting were initiated on the PEF between the 1960s 

and 1980s in direct response to the nationwide shift 

in forestry thinking. Because of the treatment design, 

the long-term silvicultural experiment on the PEF 

has demonstrated that northern conifer stands can be 

managed effectively with both uneven- and even-aged 

silvicultural systems, giving managers a broad range 

of options. That is not the case in most forest types.

The emphasis on even-aged silviculture began to 

wane in many parts of North America by the 1990s, 

when the idea of New Forestry (“a kinder and gentler 

forestry that better accommodates ecological values”) 

(Franklin 1989: 38) started to influence how both 

researchers and managers approached silviculture. 

On the PEF, the descriptor “spruce-fir” gave way to 

the more inclusive (and more accurate) “northern 

conifers” and new response variables were added 

to the long-term study, including standing and 

downed snags; structural characteristics such as tree 

location, height, crown projection, and crown length; 

and ground cover. Treatment prescriptions started 

emphasizing wildlife trees and canopy emergents by 

excluding a significant portion of them from cutting. 

2

 Smith, D.M. 1952 (November). Letter to T.F. McLintock. 

On file at the Penobscot Experimental Forest and available 

from the authors.
48 GTR-NRS-P-123 Penobscot Experimental Forest: 

In 1994, the industrial owners of the PEF donated 

the property to the University of Maine Foundation 

with the hope that new research would be initiated 

by faculty and graduate students. In the donation 

document they stated their expectation that the 

PEF would “afford a setting for long-term research 

conducted cooperatively among U.S. Forest Service 

scientists, University researchers and professional 

forest managers in Maine; to enhance forestry 

education of students and the public; and to 

demonstrate how the timber needs of society are met 

from a working forest.” 3

 With greater involvement 

by University researchers, the number of short-term 

studies overlain on the Forest Service’s long-term 

experiment has increased. These studies usually have 

a basic rather than applied focus and cover a range of 

topics important to sustainable forest management, 

including: wood decay (Smith et al. 2007), leaf area 

and growth efficiency (Kenefic and Seymour 1999a, 

Maguire et al. 1998, Seymour and Kenefic 2002), 

leaf morphology and gas exchange (Day et al. 2001), 

carbon storage (Hoover 2005), herbivory (Larouche 

et al. 2010), bird and insect diversity and habitat 

suitability (Johnston and Holberton 2009, Su and 

Woods 2001), and genetic diversity (Hawley et al. 

2005).

Studies of dead standing trees have provided new 

insights into the dynamics of wildlife habitat. Snag 

longevity, for example, was found to be a function 

of species, size, stand density, and cause of death, 

and was greatest in unharvested stands and least in 

stands with short cutting cycles (Garber et al. 2005). 

Investigation of decayed down wood established 

the importance of this substrate for regeneration 

of spruce and hemlock (Weaver et al. 2009). The 

effect of silviculture on spatial arrangement of trees 

was also investigated. Regeneration events were 

found to increase aggregation and reduce species 

mingling, particularly when treatment shifted species 

composition toward hardwoods (Saunders and Wagner 

2008).

Although non-tree vegetation received limited 

attention on the PEF in the past, an inventory of 

understory vegetation on the PSPs in the long-term 

study was recently completed. Understory species 

richness and diversity generally declined with 

decreasing silvicultural intensity (determined by 

BA removed and time since cutting); differences 

in diversity and composition of understory plants 

were related to canopy composition and forest floor 

disturbance (Bryce 2009). Nonnative invasive plants 

were uncommon in the experimental stands but 

abundant in adjacent old-field stands (Olson et al. 

2011).

The long-term silvicultural experiment on the PEF 

provides a unique perspective on forest dynamics, a 

perspective that is increasingly more relevant with 

time. One of the advantages of long-term experiments 

is that scientists can document treatment responses 

that vary over time. For example, the diameter 

distributions of the PEF selection treatments were 

close to their goals in the 1970s and researchers 

predicted that the stands would remain “essentially 

balanced” (Frank and Blum 1978). However, analysis 

of data from later remeasurements revealed structural 

and compositional imbalances that were not apparent 

in earlier assessments (Kenefic and Brissette 2001, 

Seymour and Kenefic 1998). In addition, though 

increases in the proportion of spruce growing stock 

led Frank and Blum (1978) to conclude that efforts to 

favor those species were successful, we now know that 

this outcome was a function of accretion rather than 

recruitment (Kenefic et al. 2007). Spruce trees in the 

selection treatments are almost all more than a century 

old (Seymour and Kenefic 1998) and new saplings 

have been growing at a rate of less than 1 inch in 

diameter per decade.4

Similarly, growth rates of seedlings in the selection 

treatments have been slow; the shade-tolerant conifers 

can take as many as 35 years to reach 1.5 feet in height 

(Weaver 2007). Analysis of relationships between 

3

 Unpublished document on file at the Penobscot 

Experimental Forest and available from the authors.

4

 Unpublished data on file at the Penobscot Experimental 

Forest and available from the authors.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 49

overstory stocking and growth of understory trees 

in the selection treatments revealed that there was 

no level of canopy closure that favored spruce over 

its competitors (Moores et al. 2007). These findings 

tell a story much different from those of the 1970s, 

and raise concerns about long-term sustainability of 

structure and composition of the selection treatments. 

These concerns can be addressed only by continuing 

to implement planned treatments and measuring the 

results over the next few decades.

In general, understanding of how forests respond 

to disturbances increases with time, but we must 

acknowledge that the localized impacts of climate 

change are still largely speculative. Iverson and 

Prassad (2001) concluded from their models that 

spruce-fir forests will be extirpated from New England 

within the century. Dawson et al. (2011) contend that 

although such models help identify exposure to climate 

change, assessing consequences requires considering 

not only exposure but sensitivity and adaptive 

capacity as well. Sensitivity is the degree to which the 

persistence and fitness of a species or species group 

depends on a particular climate. Adaptive capacity 

refers to whether species or communities tolerate 

change, shift their habitats, migrate to new regions, 

or become extinct (Dawson et al. 2011). 

Little is known about the sensitivity and adaptive 

capacity of northern conifers, but long-term 

experiments like the one on the PEF offer the best 

empirical evidence for evaluating the effects of 

climate change on these qualities. Studying phenotypic 

plasticity, genetic diversity, ecophysiology, and silvical 

traits like seed dispersal and microhabitat preferences 

can tell us much about the sensitivity and adaptive 

capacity of northern conifers. Many of these traits can 

be measured, and are being measured, in the PEF longterm experiment. In fact, many of these traits have 

been measured over the past 60 years (see Kenefic and 

Brissette, this volume) but not in the context of climate 

change. Evaluating how silvicultural treatments 

influence sensitivity and adaptive capacity will be 

a high priority for the PEF long-term silvicultural 

experiment over the next several decades.

SUMMARY

The long-term silvicultural study on the PEF has 

spanned the careers of four generations of researchers 

and has influenced the education and practices of 

untold numbers of foresters and other natural resource 

professionals, as well as landowners, from across 

the region. Field tours of the experiment are always 

dynamic events with many questions and much 

discussion. Two of the most frequently asked questions 

are: “What is the most important thing learned so far?” 

and “Why is it important to continue the study?”

Our answer to the first question is rather 

straightforward: Healthy, productive forests are 

maintained through careful harvesting based on 

informed planning. Harvesting for immediate gain 

alone leaves behind a low-quality forest with few 

options for the future.

Both even- and uneven-aged methods influence the 

composition and structure of northern conifer stands 

and thereby provide valuable timber, high-quality 

habitat, aesthetically pleasing views, and a broad 

range of management options for the future. However, 

management focused on short-term financial returns 

alone leaves stands that have few high-quality trees 

and require decades of growth before they once again 

provide a range of management options. In short, 

silviculture matters.

The answer to the second question is more subjective 

but perhaps more important: Knowledge accumulated 

through continued research leads to better, more 

certain management decisions.

Researchers turn data into knowledge. Managers turn 

knowledge into action. Knowledge based on shortterm results is incomplete at best and often wrong. 

The value of knowledge increases as it accumulates in 

two important ways: greater precision for prescribing 

treatments and greater certainty that prescriptions 

will achieve desired results. The PEF study is now 

more than halfway through an even-aged rotation and 

the overstory of the uneven-aged treatments is still 
50 GTR-NRS-P-123 Penobscot Experimental Forest: 

composed mostly of trees that were there when the 

experiment began. Consequently, we must continue 

to evaluate stand development patterns following 

the various treatments in order to provide managers 

the level of precision and certainty needed to ensure 

success.

This experiment represents a tremendous investment 

in time, effort, and dollars. It is also logical and 

appropriate to ask whether it has been worth it. We 

believe that it has, and that it continues to be worthy 

of our time and talents. Results of this study are of 

interest to a wide audience. Studies of underlying 

ecological processes and qualities like sensitivity 

and adaptive capacity with regard to climate change 

advance science and are presented via scientific 

meetings and peer-reviewed journal articles. Applied 

results such as management guidelines improve how 

forests are managed and are presented at practitioneroriented meetings and in publications. Additionally, 

field tours of the experiment are a key component 

of the technology transfer program on the PEF. This 

experiment not only has influenced the practice 

of forestry in the northern conifer type, but more 

importantly, has helped advance understanding of tree 

and stand growth and the relationship between human 

and natural disturbance at a fundamental level, not 

specific to a forest type. We maintain that the value 

of this study will continue to increase as its results 

are used to address the always-evolving compelling 

questions of the day.

ACKNOWLEDGMENTS

Sustaining long-term field research over decades 

requires the effort and talent of many people, some of 

whom work on the study for entire careers and others 

of whom are involved for only a season. All leave 

their mark on the research. The principal investigators 

of the experiment are all named in the paper and we 

are grateful for their foresight and diligence. Their 

dedication allowed the study to pass into our hands 

and we are humbled by the knowledge that we will 

some day pass it on to others. We also thank all those, 

too many to name, who over the years contributed to 

the study, especially the field crews who collected the 

data and the data managers who made it usable for 

the researchers. We also thank Mary Beth Adams and 

Edwin Swift for their thoughtful reviews of an earlier 

draft of the manuscript.

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form and branch characteristics of red spruce 

and balsam fir crop trees in Maine, USA. Silva 

Fennica. 43(3): 397-409.

Wendel, G.W.; Smith, H.C. 1990. Pinus strobus L., 

eastern white pine. In: Burns, R.M.; Honkala, 

B.H., tech. coords. Silvics of North America: 

1. Conifers. Agric. Hndbk. 654. Washington, DC: 

U.S. Department of Agriculture, Forest Service: 

476-488.

Westveld, M. 1928. Observations on cutover 

pulpwood lands in the Northeast. Journal of 

Forestry. 26: 649-664.

Westveld, M. 1946. Forest management as a means 

of controlling the spruce budworm. Journal of 

Forestry. 44: 949-953.

Witham, J.W.; Moore, E.H.; Hunter, M.L., Jr.; 

Kimball, A.J.; White, A.S. 1993. A longterm study of an oak pine forest ecosystem: 

techniques manual for the Holt Research Forest.

Tech. Bull. 153, Orono, ME: Maine Agricultural 

Experiment Station. 164 p.
58 GTR-NRS-P-123 Penobscot Experimental Forest: 

Appendix I. 

Conversion of English to metric values for units used in this paper.

Multiply by to obtain

Inches (in.) 2.54 Centimeters (cm)

Feet (ft) 0.3048 Meters (m)

Acres (ac) 0.4047 Hectares (ha)

Trees per acre (TPA) 2.471 Trees per hectare (TPH)

Square feet per acre (ft2/ac) 0.2296 Square meters per hectare (m2/ha)

Cubic feet per acre (ft3/ac) 0.06997 Cubic meters per hectare (m3/ha)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 59

Name Topic

Date 

Completed Degree Advisor Institution

Part 1. Graduate Student Research

Daniel Gilmore Crown structure, stem 

form, and leaf area 

relationships for balsam fir

1995 Ph.D. Robert Seymour University of Maine

Geoffrey Wilson Modeling early 

regeneration processes in 

mixed-species forests

1997 M.S. Douglas Maguire University of Maine

Jeffrey Jaros-Su Insect biodiversity in 

managed forests

1999 Ph.D. Stephen Woods University of Maine

Laura Kenefic Leaf area, stemwood 

volume growth, and 

structure in mixed-species, 

multi-aged stands

2000 Ph.D. Robert Seymour University of Maine

Michael Day Factors influencing net 

primary production in red 

spruce

2000 Ph.D. Michael Greenwood University of Maine

Kerry Sokol Effects of long-term 

diameter-limit cutting on 

radial growth and genetic 

diversity

2001 M.S. Michael Greenwood University of Maine

Suzhong Tian Effects of precommercial 

thinning on root structure

2002 Ph.D. William Ostrofsky University of Maine

Leah Phillips Crop-tree growth and 

quality after precommercial 

thinning

2002 M.S. Robert Seymour University of Maine

Andrew Moores Understory growth 

dynamics and mensuration 

techniques in unevenaged, mixed-species 

stands

2003 M.S. Robert Seymour University of Maine

R. Justin DeRose Leaf area index - relative 

density relationships in 

even-aged balsam fir - red 

spruce stands

2004 M.S. Robert Seymour University of Maine

Spencer Meyer Leaf area as a growth 

predictor of balsam fir and 

red spruce

2004 M.S. Robert Seymour University of Maine

Margaret Ward Age-related trends in red 

spruce needle anatomy 

and the relationship to 

declining productivity

2005 M.S. Michael Greenwood University of Maine

Appendix II. 

Examples of recent short-term studies in the U.S. Forest Service Long-Term Silvicultural Experiment on the 

Penobscot Experimental Forest, 1994-2010.

Appendix II continued on next page
60 GTR-NRS-P-123 Penobscot Experimental Forest: 

Name Topic

Date 

Completed Degree Advisor Institution

Michael Saunders Dynamics of forest 

structure under different 

silvicultural regimes

2006 Ph.D. Robert Wagner University of Maine

Stephanie Adams Age-related decline in 

photosynthesis in red 

spruce

2006 M.S. Michael Day, 

Michael Greenwood

University of Maine

Brent Horton Reproductive behavior of 

the white-throated sparrow

2007 Ph.D. Rebecca Holberton University of Maine

Jamie Weaver Regeneration and 

substrate availability in 

partially cut stands

2007 M.S. Laura Kenefic University of Maine

Jason Johnston Effects of forest 

management and food 

availability on condition 

and breeding of hermit 

thrushes

2007 Ph.D. Rebecca Holberton University of Maine

Robert Lindemuth Sampling methods for 

estimating basal area 

and volume in partially 

harvested stands

2007 M.S. Thomas Brann University of Maine

Ashley Thomson Comparative 

phylogeography of North 

American birches

2009 M.S. Christopher Dick University of Michigan

Catherine Larouche Regeneration of northern 

white-cedar in partially cut 

mixedwood stands

2009 Ph.D. Jean-Claude Ruel, 

Laura Kenefic

Laval University

Elizabeth Bryce Influence of silviculture 

and site on native and 

nonnative forest understory 

plant distribution

2009 M.S. Laura Kenefic University of Maine

Katherine Spencer Red spruce photosynthesis 

and maturation

2009 M.S. Michael Day University of Maine

Matthew Olson Temporal and spatial 

patterns of tree 

regeneration

2009 Ph.D. Robert Wagner University of Maine

Christopher Zellers Growth and financial 

performance of eastern 

white pine reserve trees

2010 M.S. Robert Seymour University of Maine

Kate Zellers Patterns of eastern white 

pine regeneration as 

influenced by reserve trees

2010 M.S. Robert Seymour University of Maine

Appendix II continued on next page

Appendix II (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 61

Principal Investigator(s) Topic Date completed Institution

Part 2. Examples of Additional Short-Term Research

John Brissette Red spruce and hemlock stem volume 1997 U.S. Forest Service, Northern 

Research Station (NRS)

Robert Shepard Wood properties in thinned and 

unthinned stands

1997 University of Maine

Doug Maguire et al. Crown structure and growth efficiency 

of red spruce

1998 University of Maine

Laura Kenefic, 

Robert Seymour

Age-size relationships in managed 

uneven-aged stands

1998 U.S. Forest Service, NRS 

University of Maine

Alison Dibble et al. Understory vegetation and red spruce 

regeneration

1999 U.S. Forest Service, NRS 

University of Maine

Gary Hawley et al. Genetic implications of diameter-limit cutting 2000 University of Vermont 

U.S. Forest Service, NRS

Mark Ducey et al. Point relascope sampling 

of down woody material

2003 University of New Hampshire 

U.S. Forest Service, NRS

Shawn Garber et al. Snag longevity in managed stands 2005 Oregon State University 

University of Maine

Coeli Hoover Carbon sequestration in thinned stands 2006 U.S. Forest Service, NRS

Aaron Weiskittel et al. Effect of precommercial thinning on tree 

and stand characteristics

2009 University of Maine 

U.S. Forest Service, NRS

Bruce Cook et al. Ecosystem structure and dynamics ongoing NASA

John Bradford et al. Relationship of climate and silviculture 

to tree growth response

ongoing U.S. Forest Service, NRS

Laura Kenefic et al. Rehabilitation options 

for cutover mixedwood stands

ongoing U.S. Forest Service, NRS 

University of Maine

Sam Droege, 

Jim Guldin

Native pollinators ongoing U.S. Geological Survey 

U.S. Forest Service, SRS

Walter Shortle, 

Jody Jellison

Biology and biochemistry of wood decay ongoing U.S. Forest Service, NRS 

University of Maine

Appendix II (continued)
62 GTR-NRS-P-123 Penobscot Experimental Forest: 

Sixty Years of Research, 60 Years of Data: 

Long-term U.S. Forest Service Data Management 

on the Penobscot Experimental Forest

Matthew B. Russell, Spencer R. Meyer, John C. Brissette, and Laura S. Kenefic 

Abstract.—The U.S. Department of Agriculture, Forest Service silvicultural experiment 

on the Penobscot Experimental Forest (PEF) in Maine represents 60 years of research in 

the northern conifer and mixedwood forests of the Acadian Forest Region. The objective 

of this data management effort, which began in 2008, was to compile, organize, and 

archive research data collected in the U.S. Forest Service silvicultural experiment and 

several auxiliary studies. Due to the hierarchical nature of these data, a relational database 

management system (RDMS) was used (Microsoft Office Access). The resulting data 

management system affords new opportunities for novel research through data mining 

and increased collaboration among researchers; many of the data have since been 

published online (Brissette et al. 2012a, 2012b). Data management efforts such as these 

bridge data collection and data analysis, and play an important role in preserving the 

integrity of long-term studies. The RDMS used in this project is contemporary and widely 

used, but data storage systems will continue to evolve. It is important that U.S. Forest 

Service data management efforts continue and that new systems are adopted as needed.

INTRODUCTION

The U.S. Department of Agriculture, Forest Service 

(USFS) silvicultural experiment on the Penobscot 

Experimental Forest (PEF) in Maine has generated 

60 years of research in the northern conifer and 

mixedwood forests common to the Acadian Forest 

Region of Atlantic Canada and adjacent Maine (Braun 

1950, Rowe 1972). Research began in the 1950s 

when the USFS initiated a study consisting of an 

array of silvicultural treatments applied to replicated 

experimental units (Sendak et al. 2003). Since then, 

many auxiliary studies have been implemented on the 

PEF, several of which are conducted by University 

of Maine faculty and students. Many of these studies 

are short-term; others include several years of 

measurements. These auxiliary studies were built 

upon the foundation of the long-term silvicultural 

experiment. Consequently, data from these studies are 

related and allow for synthesis and comprehensive 

analyses to address a range of intriguing questions. 

Adequate management of data records is an essential 

component of any long-term research program (Burton 

2006), but is often overlooked due to limited resources 

and the short-term nature of most projects.

Methods and data management practices for the 

USFS’s PEF database have evolved tremendously 

over these 60 years. Punch cards were used during 

the 1970s, but were phased out in the early 1980s 

when data transferred to electronic formats (e.g., 

computer tapes). Data were maintained for a time 

on the University of Maine mainframe computer 

system using FORTRAN programs. In the 1990s, the 

USFS PEF data were converted to ASCII files. These 

methods were appropriate for their time, though they 

are now outdated and inefficient.

By the early 2000s, nearly 60 years of PEF data were 

stored in 3,605 ASCII data files in 255 folders, and 

contained 374 megabytes of information. As a result of 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 63

the numerous files and folders, data were not readily 

accessible to researchers. The USFS recognized that 

the size and complexity of the PEF database warranted 

organization in a relational database management 

system (RDMS).

Serving as a tool for understanding the dynamics of 

northern conifer forests, data management is crucial 

for maintaining the integrity and value of research 

conducted on the PEF. This report describes a project 

initiated in 2008 to archive research data collected 

in the USFS silvicultural experiment and auxiliary 

studies. Specific objectives were to (1) organize and 

compile existing data, (2) test the functionality of 

an RDMS for archiving these data and making them 

available to users, and (3) develop and document a 

process for new data to be appended to the database. 

Many of the data have subsequently been published 

and are available online through the USFS Research 

Data Archive (Brissette et al. 2012a, 2012b).

METHODS

The Relational Database 

Research institutions have increasingly relied on 

the RDMS model to archive experiment data in a 

hierarchical structure. Such systems can be customized 

to meet the needs and design requirements of the 

information being stored. The RDMS appeared to be 

an ideal tool for an experiment like that of the USFS 

on the PEF for several reasons. First, the RDMS 

allows various types of data to be related under a 

single framework. As an example, one data table may 

describe the silvicultural treatments, while another 

includes information about the experimental units 

to which each treatment is applied. In addition, each 

experimental unit contains a network of permanent 

sample plots, each of which has spatial data. With 

an RDMS, plots can be related to the experimental 

unit, and the experimental unit can be related to the 

silvicultural treatment (Fig. 1). Second, through 

Figure 1.—Schematic displaying how U.S. Forest Service research on the Penobscot Experimental Forest fits into the 

structure of a relational database management system.
64 GTR-NRS-P-123 Penobscot Experimental Forest: 

powerful querying capabilities, analysts can rapidly 

manipulate, summarize, or extract data of interest. By 

drawing from different tables, queries allow users to 

interpret data without changing the underlying data 

structure. Lastly, the RDMS has the ability to store 

large amounts of data, which reside as a single, easily 

replicated, and shared file on a personal computer or 

server. In contrast to typical spreadsheet or other “flat” 

data management systems, an RDMS reduces data 

storage capacity by extracting redundant information 

and making use of hierarchical relationships.

With large data sets such as the USFS’s PEF 

experiment, data would be difficult to manage 

collectively as individual text files or spreadsheets. In 

the past, if researchers were interested in analyzing 

long-term data collected from a specific experimental 

unit, dozens of files from various inventories would 

first need to be compiled. Additional files that 

explained changing data collection methods and 

other associated metadata would similarly need to be 

compiled in order to interpret the data. This process 

of assembling data represented a cumbersome and 

time-consuming process for the analyst, and increased 

the probability of making errors. In addition, RDMS 

software is configured to work well with external 

software packages such as those used for statistical 

analyses by including fully featured input and output. 

RDMS software and database connection tools are 

available to work well with other operating systems 

such as Linux/UNIX and Apple operating systems 

(e.g., see R Development Core Team 2010). 

Compiling and Archiving 

60 Years of Information

Data collected on the PEF as part of the longterm USFS silvicultural experiment through the 

2006 field season were used to develop the base 

structure and organization of the database. Data 

were aggregated into groups according to the type 

of study. Groups were organized according to the 

kinds of treatments applied in the experiment and 

the types of data collected. Data that were part of 

the long-term USFS experiment were classified in 

one group while auxiliary studies were grouped in 

another. Datasets were normalized when possible, 

meaning that data were arranged and restructured 

to meet the assumptions of conventional relational 

database design, thus reducing redundant storage of 

information. Management of data followed general 

guidelines established for ecological studies (Borer et 

al. 2009). Data were archived in a Microsoft Office 

Access database. Non-proprietary ASCII files of these 

data were also archived.

U.S. Forest Service 

Long-term Silvicultural Study

Data records for the long-term USFS experiment on 

the PEF were previously stored solely in ASCII files. 

One file existed for overstory tree data collected in 

each experimental unit (called a management unit, or 

MU) at each inventory. Tree species, diameter, and 

status were universally recorded in these files. 

Given that the same variables were collected in 

all inventories, these data were first grouped by 

management unit. For example, data from the 

22 inventories that had occurred in MU 9 were 

previously stored in 22 separate files with related 

information. These were consolidated into one unified 

table in the database. After the files for each MU were 

aggregated, data were collapsed even further into a 

single table that contained all tree data collected on all 

MUs at all inventories. Tree regeneration data were 

organized in a similar manner as the overstory tree 

data. 

Each MU contains an average of 15 permanent 

sample plots, totaling more than 600 plots in the 

USFS experiment on the PEF. These plots differed in 

terms of the inventory design used and the level of 

measurement detail (Fig. 2). Measurement protocols 

differ between “compartments” (replicated MUs in 

the long-term experiment), “units” (nonreplicated 

MUs used for other research), and the “management 

intensity demonstrations” (MUs managed for 

demonstration purposes). The measurement protocols 

for these areas evolved during the study (Table 1).

Other files, such as those containing spatial 

distribution and tree height and crown data for the 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 65

Figure 2.—U.S. Forest Service permanent sample plot 

schematic displaying 1/5th-, 1/20th-, 1/50th-, and 1/1000thacre nested plots and the types of data associated with 

each as archived in the USFS’s Penobscot Experimental 

Forest database (key information archived in these tables is 

described in Table 2).

Table 1.—Overview of general historical data collection methods for trees in compartments, units, and 

the management intensity demonstration areas (MIDs) in the U.S. Forest Service’s long-term silvicultural 

study on the Penobscot Experimental Forest (1950-2010)

Plot size (ac) Minimum diameter at breast height measured (in) 

Before 2000 2000 and after Before 2000 2000 and after

Compartments 1/5, 1/20 1/5, 1/20, 1/50 0.5 0.5

Units 1/5, 1/20 1/5 1-in class 5-in class

MIDs varied 1/5, 1/20, 1/50 0.5 or 1.0 0.5

Table 2.—Key data tables for the U.S. Forest Service’s long-term silvicultural experiment, as archived in 

the Penobscot Experimental Forest Microsoft Office Access database (local-use only)

Data table Key information

Management units MU ID, silvicultural treatment, acreage, status

Plots MU ID, plot ID, spatial coordinates, depth to water table

Trees MU ID, plot ID, tree ID, species, diameter at breast height, status 

Trees (subsample) MU ID, plot ID, tree ID, height, height to crown, crown width, spatial location

Regeneration plot ID, species, count of stems by height class

Understory plot ID, ground cover class percentages 

compartments resided in separate Microsoft Office 

Excel spreadsheets. Similar to the tree data, these 

data existed in separate files for each MU. Given that 

identical variables were measured across the MUs, 

files of these types could be merged. Altogether, 

several key data tables were archived, and serve as the 

basis of the USFS silvicultural experiment on the PEF 

(Table 2).

Much of the supplementary information for the 

silvicultural experiment was obtained from scanned 

historical documents. Descriptions of silvicultural 

treatments, information on plot sizes, and tree species 

codes are examples of information obtained this way. 

After data through the 2006 field season were 

compiled and archived in Access, field data collected 

in 2007 and subsequent years were used to test the 

functionality of the database in terms of checking and 

appending subsequent remeasurement data.

Auxiliary Studies

A wealth of information existed in the USFS archives 

concerning auxiliary studies, i.e., those expanding 

upon the foundation of the long-term experiment 

but not directly a part of it. For example, complete 

metadata and tree-level data from a precommercial 
66 GTR-NRS-P-123 Penobscot Experimental Forest: 

thinning study (Brissette et al. 1999) were archived 

and well documented, as were data from a study of 

tree age. Some studies had limited data or metadata 

(e.g., for a study of growth efficiency on Study 58 

plots and a logging technology study in MUs 2A and 

2B). Auxiliary data that were well documented were 

imported directly into the database. 

Additional data from other auxiliary studies were 

obtained from individual researchers. Examples 

included studies of tree leaf area (Kenefic and 

Seymour 1999, Maguire et al. 1998) and additional 

tree size measurement data sets (Saunders and Wagner 

2008) (Table 3). Many of these data sets were archived 

following the overall database design used for the 

long-term silvicultural experiment; data manipulation 

was minor and done only to ensure consistency across 

all data tables archived within the database. 

Table 3.—Data sets for completed and ongoing 

studies included in the U.S. Forest Service’s 

Penobscot Experimental Forest local-use 

Microsoft Office Access database at the time of the 

60th anniversary in 2010. Data from USFS and S58 

have since been published online (Brissette et al. 

2012a, 2012b).

ID Experiment name

USFS Silvicultural Experiment

REGEN Regeneration Study

BRYCE Understory Vegetation and Cover

CORE Tree Core Analysis

MOORE Light/Seedling Experiment (Spruce, Fir, Hemlock)

SAUND Tree Measurements 

WEAV Seedling/Downed Woody Debris Study

S58 Study 58

PHLPS Study 58 Stem Analysis Measurements

REHAB Rehabilitation Study

LEAP Land Use Effects on Amphibian Populationsa

KZELL White Pine Study

GAP Expanding Gap Silvicultural Studya

CZELL White Pine Study

CTRN Maine Commercial Thinning Research Networka

WPINE White Pine Quality under Varying Silviculture

LKFOL Leaf Area of Eastern Hemlock

DMFOL Growth Efficiency of Red Spruce

WEATH PEF Weather Data/Weather Station

DAMAG PEF Harvest Damage Survey

2020 Agenda 2020 Vegetation Competition Studya

a Study overview only

RESULTS

At the time of the PEF’s 60th anniversary in 2010, 

the USFS PEF data were archived and resided as a 

fully integrated Microsoft Office Access database of 

80 megabytes in size. An additional 120 megabytes of 

supplementary information was linked to this database. 

This information included references to external files, 

such as maps (including maps of management units, 

plots, and soils) and key publications of PEF research. 

This database is for local use by researchers on the 

PEF; Russell and Meyer (2009)1

 serves as a guide for 

researchers using the database and details procedures 

for documenting future data. Many of the data have 

since been published and are publicly available via 

the Web (Brissette et al. 2012a, 2012b). The local-use 

database laid the groundwork for a smooth, timely 

transition between the multitude of ASCII files and full 

online access. It is a valuable resource for researchers 

and staff on the PEF, and allows management of data 

prior to publication. Those seeking to obtain data from 

this database should follow the appropriate procedures 

for acquiring data through the Northern Research 

Station. 

Twenty tables were initially archived within the localuse database. These tables included data collected as 

part of the silvicultural experiment, as well as auxiliary 

datasets (Table 3). In 2009, the “trees” data table 

contained more than 900,000 records with information 

on the species, diameter, and status of trees measured 

on permanent sample plots since the early 1950s. 

Several reference tables were included in the database 

to aid in interpreting and analyzing data. Examples of 

these tables include comprehensive tables of species 

codes used for all the experiments, coefficients for 

estimating tree volume (Honer 1967), and a list of 

all inventory and harvest dates for the MUs.

1

 Russell, M.B.; Meyer, S.R. 2009. Penobscot Experimental 

Forest: a guide for data management and the Microsoft 

Access database. 61 p. Internal report available by request 

from M.B. Russell, University of Minnesota, College 

of Food, Agricultural and Natural Resource Sciences, 

Department of Forest Resources, 1530 Cleveland Ave. N., 

St. Paul, MN 55108. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 67

Relationships were identified to associate data 

types in one table to similar types of data in another 

(Fig. 3). This step had important implications for 

using the database for querying tables and interpreting 

data. These data relationships form the backbone of 

the RDMS and allow the powerful query language to 

summarize similar data.

Action queries proved to be an effective and efficient 

tool for the database in several ways. Data collected 

after 2006, for example, were appended to existing 

data tables through action querying to take advantage 

of the structure of the database. In addition, several 

queries were designed to summarize stand-level 

statistics by using the underlying trees data table. 

These statistics included total and species-specific 

number of trees, basal area, and volume per acre for 

each of the inventories in the silvicultural experiment, 

as well as diameter distributions.

DISCUSSION 

The RDMS proved to be an effective tool for 

archiving and managing 60 years of research data 

Figure 3.—Relationships window displaying associations of data tables within the U.S. Forest Service’s Penobscot 

Experimental Forest local-use database.

collected in the USFS long-term experiment on the 

PEF. The RDMS structure allows data in one table 

to be associated with data in another, and is an ideal 

instrument for archiving forest inventory information. 

In the example of the USFS silvicultural experiment 

on the PEF, the “management units” table contains a 

list of areas of land that are managed in the different 

USFS experiments on the PEF, while the “plots” table 

is a list of measurement plots used in each MU. The 

ability of the RDMS to associate data of different 

types in a hierarchical fashion makes it well-suited to 

managing long-term forest inventory data sets such as 

those of the PEF. 

Querying functions allow users to interpret and 

analyze data found in the underlying data tables. By 

using the relationships defined among the different 

data tables, queries can be built that pull data from 

different source tables; this process allows users 

to summarize data sets quickly and repeatedly. For 

example, the “trees” table in the local-use database for 

the long-term silvicultural experiment can be queried 

to compute stand-level basal area, volume, and tree 
68 GTR-NRS-P-123 Penobscot Experimental Forest: 

diameter distributions across MUs, years, treatments, 

or other parameters. Users can readily graph stand 

development patterns throughout the duration of the 

study. A researcher may wish to routinely analyze 

trends in basal area among the differing selection 

system stands (Fig. 4). Once the analyst designs a 

query, it is instantaneously updated as new data are 

included in the database. This feature greatly 

facilitates analyses that are conducted annually.

Storing data in non-proprietary formats is an effective 

data management practice (Borer et al. 2009) and 

was accomplished as part of these efforts. Whereas 

Microsoft Office Access is proprietary software 

that (1) is subject to continuous updates, (2) could 

potentially become unavailable in the future, and (3) 

could be replaced by other newer and improved types 

of software, ASCII, or text files, can always be read. 

Similarly, analysis scripts have been maintained that 

Figure 4.—Trends in basal area per acre (trees ≥0.5 in 

diameter at breast height) for management units treated 

with the selection system on 5- and 20-year cutting 

cycles, as archived in the U.S. Forest Service’s Penobscot 

Experimental Forest database.

allow a user to import USFS PEF data into statistical 

packages such as R (R Development Core Team 

2010), MATLAB® (MathWorks, Inc., Natick, MA), 

and SAS (SAS Institute, Inc., Cary, NC). To preserve 

the relationships between different data tables in the 

RDMS, structured query language (SQL) scripts have 

been maintained of essential queries for importing 

data into other database systems, such as MySQL or 

PostgreSQL. Although currently the data are primarily 

managed within the Access RDMS, connectivity with 

other operating systems is offered. The open-source R 

statistical package is recommended for users seeking 

to use these coded scripts because of its compatibility 

with multiple operating systems and well-developed 

database connectivity packages (R Development Core 

Team 2010). Online USFS PEF data are in an Oracle®

database; both raw data and summary statistics can be 

downloaded. 

The ability to append data has tremendous value to the 

USFS long-term experiment. Data that are cohesively 

managed with a consistent structure provide a data 

format that can be easily maintained. For new data 

types and data from auxiliary experiments, new data 

tables can be created and incorporated into the existing 

database structure.

Opportunities

As an artifact of today’s technological age, computer 

technologies change and data management software 

continually evolves. Employing contemporary 

software used by scientists and managers is central 

to the research integrity of the USFS’s PEF data 

sets. In future years, the design and structure of the 

database should be evaluated to determine whether or 

not it is effectively meeting users’ needs. Similarly, 

new avenues of research and data management have 

arisen for the USFS’s PEF database. First, there are 

opportunities for spatially explicit data summary 

and analysis. Spatial technologies and geographic 

information systems software are now widely used, 

and technologies for bridging observed tree data 

with spatial data sets are available. Second, the 

database adds value to the long-term experiment by 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 69

providing a unified data set that can be readily used 

by researchers for forest growth and yield modeling, 

threat assessment, and other associated areas. Finally, 

opportunities for collaborations with new researchers 

have been established through Web-based PEF datasharing sites (e.g., http://www.fs.usda.gov/rds/archive/

Product/RDS-2012-0008 and http://www.fs.usda.

gov/rds/archive/Product/RDS-2012-0009). Such sites 

showcase the USFS’s data from the PEF and increase 

the real and perceived value of the experiments. 

CONCLUSIONS

Sixty years of research data from USFS experiments 

on the PEF have been compiled, archived, and made 

available to researchers. The relational database model 

proved effective given the design of the long-term 

silvicultural experiment and associated auxiliary 

studies. New opportunities and collaborations continue 

to arise as a result of these data management efforts.

LITERATURE CITED

Borer, E.T.; Seabloom, E.W.; Jones, E.B.; Schildhauer, 

M. 2009. Some simple guidelines for effective 

data management. Bulletin of the Ecological 

Society of America. 90(2): 205-214.

Braun, E.L. 1950. Deciduous forests of eastern 

North America. New York: Hafner. 596 p.

Brissette, J.C.; Frank, R.M.; Stone, T.L.; Skratt, 

T. 1999. Precommercial thinning results in a 

northern conifer stand: 18-year results. The 

Forestry Chronicle. 75(6): 967-972.

Brissette, J.C.; Kenefic, L.S.; Russell, M.B. 2012a. 

Precommercial thinning x fertilization study 

data from the Penobscot Experimental Forest.

Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northern Research 

Station. Available at http://dx.doi.org/10.2737/RDS2012-0009. (Accessed June 17, 2013).

Brissette, J.C.; Kenefic, L.S.; Russell, M.B.; Puhlick, 

J.J. 2012b. Overstory tree and regeneration data 

from the “Silvicultural Effects on Composition, 

Structure, and Growth” study at Penobscot 

Experimental Forest. Newtown Square, PA: 

U.S. Department of Agriculture, Forest Service, 

Northern Research Station. Available at http://

dx.doi.org/10.2737/RDS-2012-0008. (Accessed 

June 17, 2013).

Burton, P.J. 2006. The need for long-term research 

installations and datasets. In: Irland, L.C.; Camp, 

A.E.; Brissette, J.C.; Donohew, Z.R., eds. Longterm silvicultural and ecological studies: results for 

science and management. GISF Res. Pap. 005. New 

Haven, CT: Yale University: 136-138.

Honer, T.G. 1967. Standard volume tables and 

merchantable conversion factors for the 

commercial tree species of central and eastern 

Canada. Ottawa, ON: Canadian Department 

of Forestry and Rural Development, Forest 

Management Research and Services Institute. 

Info. Rep. FMR-X-5.

Kenefic, L.S.; Seymour, R.S. 1999. Leaf area 

prediction models for Tsuga canadensis in 

Maine. Canadian Journal of Forest Research. 

29: 1574-1582.

Maguire, D.A.; Brissette, J.C.; Gu, L. 1998. Crown 

structure and growth efficiency of red spruce 

in uneven-aged, mixed-species stands in Maine.

Canadian Journal of Forest Research. 28: 

1233-1240.

R Development Core Team. 2010. R data import/

export. Vienna, Austria: R Foundation for 

Statistical Computing. Available at http://cran.

r-project.org/doc/manuals/R-data.html. (Accessed 

October 25, 2010).

Rowe, J.S. 1972. Forest regions of Canada. Publ. 

1300. Ottawa, ON: Canadian Forest Service, 

Department of the Environment. 172 p.
70 GTR-NRS-P-123 Penobscot Experimental Forest: 

Saunders, M.R.; Wagner, R.G. 2008. Height-diameter 

models with random coefficients and site 

variables for tree species of Central Maine.

Annals of Forest Science. 65(2): 1-10.

Sendak, P.E.; Brissette, J.C.; Frank, R.M. 2003. 

Silviculture affects composition, growth, and, 

yield in mixed northern conifers: 40-year results 

from the Penobscot Experimental Forest.

Canadian Journal of Forest Research. 33(11): 

2116-2128.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 71

The Acadian Forest Ecosystem Research Program: 

An Example of Natural Disturbance-Based Silviculture 

in the Northeast

Mike R. Saunders, Robert S. Seymour, and Robert G. Wagner

Abstract.—The Acadian Forest Ecosystem Research Program (AFERP) began in 1994 

as one of the nation’s first trials of natural disturbance-based silviculture. The study 

tests the ecological impacts of two versions of expanding-gap silvicultural systems 

that are designed to emulate the spatial extent and frequency of natural disturbances 

in northeastern North America. The AFERP is now well into its second decade of 

monitoring. Inventory systems for overstory trees, saplings, herbaceous plants, tree 

regeneration, coarse woody debris, forest floor depth, and canopy light interception are 

described in this paper.

INTRODUCTION

Over the past 20 years, there has been a strong 

shift away from production-oriented management 

on public lands in the United States and Canada 

towards silvicultural practices guided by principles of 

disturbance ecology to increase ecosystem resilience 

and conserve biodiversity (Long 2009, Seymour et al. 

2006). Inherently, these new practices are conceptually 

and practically much more complex, often requiring 

extensive dendrochronological reconstructions of 

stand and landscape age structures to effectively 

emulate natural disturbance patterns for forest types 

in each region (D’Amato and Orwig 2008, North and 

Keeton 2008). However, both voluntary and regulated 

implementation of these systems has proceeded 

even without extensive research on economic or 

ecological sustainability of the practices (Gamborg 

and Larsen 2003, Long 2009). Earlier experiments by 

the U.S. Department of Agriculture, Forest Service 

and Canadian Forest Service were limited in focus, 

treatment types, or experimental design, and did not 

fully inform policy makers of repercussions of the shift 

in emphasis (Seymour et al. 2006).

The resulting paucity of pertinent research spawned 

numerous innovative large-scale management 

experiments by the U.S. and Canadian Forest 

Services, universities, and other organizations starting 

in the early 1990s (Puettmann 2005, Seymour et 

al. 2006). In northeastern North America, there 

are two notable experiments: the Vermont Forest 

Ecosystem Management Demonstration Project 

(FEMDP) (Keeton 2006, North and Keeton 2008) 

and the Acadian Forest Ecosystem Research Program 

(AFERP) (Saunders and Wagner 2005, Seymour 

et al. 2006). These stand-level experiments use 

silvicultural systems that are designed to emulate wind 

disturbance, senescence, and other tree- to gap-level 

disturbances that historically affected 1-5 percent yr -1 

of forest lands throughout the region (D’Amato and 

Orwig 2008, Lorimer and White 2003, Runkle 1982, 

Seymour et al. 2002).

The AFERP is the older experiment of the two with 

initial harvests occurring during winter 1995-96. 

Rather than using size-based structural control (e.g., 

a diameter distribution), the AFERP uses area control; 

all harvests are located within expanding gaps that are 

based loosely on the German “Femelschlag” system 

(Troup 1928). These treatments lead to distinctly 

irregular age and size structures, and may serve as a 

technique in a wide array of forest types to convert 

single-cohort stands to multi-aged stands more capable 

of sustaining selection-based forestry. Specifically, the 
72 GTR-NRS-P-123 Penobscot Experimental Forest: 

AFERP has three primary goals (Saunders and Wagner 

2005): (1) develop alternative silvicultural techniques 

and systems based on regional disturbance ecology, 

(2) evaluate ecosystem-scale influences of forest 

management practices, and (3) enhance understanding 

about forest ecosystems in the Acadian ecoregion.

Saunders and Wagner (2005) reported on results from 

the first 10 years of the AFERP, giving a very brief 

overview of the experimental design and inventory 

systems of the study. Since that publication, the 

experimental gaps within the AFERP have been 

expanded. Planning for these harvests revealed that 

one of the treatments was conceptually flawed and 

would not test the expanding gap concept. Changes 

were made to the treatment design to rectify this 

oversight. Further, several modifications to inventory 

systems have been made to make each more efficient. 

This paper documents those changes in detail.

METHODS

Study Area

The AFERP study is located on nine 9.4- to 11.3-

ha sites within the Penobscot Experimental Forest 

(PEF) (see Kenefic and Brissette’s “History” [this 

volume] for a description of the PEF). Although 

much of the PEF was originally heavily dominated by 

conifers, these sites have had a significant hardwood 

component for at least the past 50 years1

 and are 

generally of higher quality. Tree species with average, 

pretreatment basal areas greater than 1 m2

 ha-1 include 

(in decreasing order of dominance): eastern hemlock 

(Tsuga canadensis [L.] Carr.), red maple (Acer rubrum

L.), eastern white pine (Pinus strobus L.), balsam 

fir (Abies balsamea [L.] Mill), northern whitecedar (Thuja occidentalis L.), paper birch (Betula 

papyrifera Marsh.), red spruce (Picea rubens Sarg.), 

and trembling aspen (Populus tremuloides Michx.). 

Sixteen other tree species are also present across the 

sites.

1

 Seymour, R.S. [N.d.] FES 536: Forest stand dynamics 

(class files). On file with R.S. Seymour, University of 

Maine, School of Forest Resources, 5755 Nutting Hall, 

Orono, ME 04469-5755.

Topography in the AFERP sites is generally flat to 

rolling with slopes less than 20 percent. Soils are a 

fine-scaled mix of Spodosols and Histosols. Drainage 

is quite variable, ranging from moderately well drained 

to poorly drained. Based on stand growth projections, 

site qualities generally exceed 21 m for balsam fir 

(50 year base age) (Arsenault 2011).

Prior to initial harvests, sites did not differ 

significantly in basal area, density, or stand volume, 

although there were some minor compositional 

differences among the stands (Arsenault et al. 2011). 

Stand volume, basal area, and tree density averaged 

283.6 m3

 ha-1, 37.6 m2 ha-1, and 2,404 trees ha-1, 

respectively (Arsenault et al. 2011).

Experimental Design

Treatments

Three treatments were implemented in three blocks 

using a randomized block design across the nine 

sites. Blocks are associated with initial harvest date, 

during the winter of either 1995-96, 1996-97, or 

1997-98 (Table 1). Treatments were an unharvested, 

unmanaged experimental control and two expandinggap silvicultural systems. Both use a common 10-year 

cutting cycle over a 100-year rotation.

The more intensive large-gap, or 20:10, treatment was 

described in detail by Saunders and Wagner (2005). 

Briefly, the 20:10 is an extended group shelterwood 

with reserves with 20 percent of the area harvested 

during each of the first five cutting cycles (i.e., a 

10-year regeneration period) (Fig. 1). Initial gaps 

are approximately 0.2 ha in size. Study areas are 

scheduled to “rest” during the last five cutting cycles, 

although thinning and other intermediate treatments 

may occur if deemed necessary. The goal of this 

system is a mid-succession stand with a significant 

component of species that are intermediate in shade 

tolerance (i.e., white pine, paper birch, yellow birch 

[Betula alleghaniensis Britt.]). 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 73

Table 1.—Schedule of completed AFERP measurements and harvests, 1995-2010, by year and block 

number. With a few exceptions as noted, inventories were completed by replicate block defined as 

follows: Block 1 = research areas 1-3, Block 2 = research areas 4-6, and Block 3 = research areas 7-9. 

Planned intervals between inventories for a block are also given.

Inventory

Overstory Coarse Seedlings & Canopy light Retention Harvest

Year & saplings woody debris herb. plants Forest floor interception trees* (winter)

1995 1,2 1,2§ 1,2 1 1,2 1

1996 2§ 2 1

1997 3 3 3 3 3 2 2

1998 1 1 1 1,3 3

1999 2 2 2

2000 1,3† 3 3 3 1

2001 2 2

2002 3 3

2003

2004

2005 1 1 1 1

2006 2 2 2 2 1

2007 3 3 3 3 2

2008 3

2009

2010 1,2‡

Planned 

 Interval (yr) 5 10 5 20 5 5

* Before 2005, retention trees were only measured in 20:10 replicates.

§ Inventory started in 1995, but only the control replicate was finished. Inventory on other replicates was completed in 1996.

† Inventory only included 20:10 replicate in block 3.

‡ Inventory only included 20:10 and control replicates in block 2.

Figure 1.—Conceptual gap expansion diagrams for the 

two harvest treatments in the AFERP, color coordinated by 

year of entry. Initial entries into the 20:10 treatment (above) 

are in light red and expansion proceeds outward to the 

multi-colored strips. Initial entries into the 10:20 treatment 

(below) are the light and dark red areas, in 1995 and 2005, 

respectively. Successive expansions of these initial gaps 

occur every 20 years, alternating by gap cohort (i.e., 1995 

gaps or 2005 gaps). In both treatments, gaps are coalesced 

to avoid narrow strips and patches that could be inoperable 

in a future cycle. Expansions release regeneration whenever 

possible and use natural or anthropogenic breaks in a stand 

(i.e., streams, wet areas, skid trails) to define their boundary. 

Skid trails are shown as black dashed lines.
74 GTR-NRS-P-123 Penobscot Experimental Forest: 

The small-gap, or 10:20, treatment was refined during 

the 2005-2007 harvests to better reflect the original 

intent of the project. The 10:20 is an “expanding” 

group selection with 10 percent of the area harvested 

each cycle (Fig. 1). Initial gaps average 0.1 ha in 

size, and are expanded on alternative cutting cycles 

(i.e., a 20-year regeneration period); therefore two 

“cohorts” of gaps exist in each study area. This system 

encourages regeneration of shade-tolerant species, 

namely eastern hemlock, red spruce, sugar maple 

(Acer saccharum Marsh.), and balsam fir, and should 

accelerate the study area to a late-successional status.

Gap retention targets

Both systems use a permanent retention target of 

10 percent of the experiment-wide, pretreatment basal 

area, or roughly 3.75 m2

 ha-1. During gap creation and 

expansion, there are two possible retention scenarios 

depending on the amount of regeneration present 

(Fig. 2). If the gap or expansion has a sufficient 

stocking of large (at or above breast height or 

1.37 m) and well-distributed seedlings and saplings—

indicative of the understory reinitiation stage of stand 

development (Oliver and Larson 1996)—the gap 

or expansion is treated with a one-stage removal of 

the overstory (Fig. 2, top row), thereby lowering the 

residual basal area to the permanent 10 percent target. 

If the gap or expansion has an insufficient amount of 

regeneration (i.e., “nonregenerated”), the overstory 

removal will proceed in two stages to provide 

seed source and adequate shade for regeneration 

development (Fig. 2, bottom row). Initially, basal 

area will be reduced to a target of 30 percent, or 

11.25 m2

 ha-1. In the following gap expansion, 10 or 

20 years for the 20:10 and 10:20 treatments, 

respectively, the basal area is then reduced to the 

final 10 percent target.

20:10 Treatment 10:20 Treatment

Regenerated?

Yes No

Figure 2.—Examples of regenerated gaps (top) and nonregenerated gaps (bottom) for both the 20:10 treatment (left) and 

10:20 treatment (right). The 20:10 pictures are taken 10 years post-harvest; the 10:20 pictures were taken preharvest. 

Photos courtesy of Mike Saunders, Purdue University.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 75

Retention targets are meant to act as a flexible guide. 

Retention in individual openings may vary among 

replicates, but on average will be approximately 

10 percent and 30 percent for regenerated and 

nonregenerated areas, respectively.

Special areas

Within several of the replicates, there are temporary 

and perennial streams, vernal pools, seeps, and other 

wetland features. These features are protected by a 

“special area” designation that extends 10 m away 

from the resource. Harvesting these areas is included 

within the 10 or 20 percent area control for a treated 

replicate, but retention guidelines are much higher, 

60 percent of pretreatment basal area or 22.50 m2

 ha-1. 

Further, for special areas protecting a stream, both 

sides of the stream cannot be harvested in the same 

entry. Mature canopy cover is thereby kept above the 

stream and the need for new stream crossings 

is minimized.

Inventory Systems

There are several types of inventories conducted 

across the AFERP research areas, including overstory 

and sapling plots, coarse woody debris transects, 

seedling and herbaceous plant quadrats, forest floor 

sampling, canopy light interception readings, retention 

tree measurements, and a census of harvested stems. 

Most inventories are spatially linked to a 50 m by 

50 m grid that overlays each research area (Fig. 3a). 

All inventories had preharvest measurements in 

1995-97, with most post-harvest measurements in 

1998-2002 (Table 1). Planned resampling cycles are 

from 5-20 years, depending on inventory (Table 1). 

Data from nearly all inventories are contained within a 

geo-referenced database maintained by the School of 

Forest Resources and at the University of Maine.

Overstory and saplings

The purpose of this inventory is to monitor the growth 

and structural development in each of the replicates. 

Overstory and sapling plots are installed on 20 

randomly selected grid points. Overstory trees are 

defined as woody vegetation ≥9.5 cm in diameter at 

breast height (d.b.h.) and measured on circular 

0.05-ha plots. Saplings are defined as woody 

vegetation between 1.5 and 9.5 cm d.b.h. and 

measured on circular, nested 0.01-ha subplots 

(Fig. 3c). All overstory trees and saplings are spatially 

located within these plots and permanently tagged. 

Species, d.b.h., tree condition (e.g., live, dead, 

cull, broken top), canopy stratum, and light field 

(sensu Bechtold 2003) are recorded for both trees 

and saplings. For saplings, origin (i.e., seedling on 

mineral soil or forest floor, seedling on nurse log, or 

stump sprout), and overhead character (i.e., beneath 

contiguous overstory or within canopy gap) are also 

recorded.

Coarse woody debris

The purpose of this inventory is to monitor the 

biomass, volume, and spatial distribution of coarse 

woody debris by type and decay class within each 

replicate. Because individual pieces are tracked, decay 

rates and transitions between types may be estimated 

as well. In 2005, this inventory replaced a more 

costly and less efficient area-based sampling design; 

see Fraver et al. (2002) for a description of the prior 

system.

Coarse woody debris is defined within the AFERP 

as all standing snags and down wood >4.5 cm in 

diameter. Both snags and down wood are surveyed 

on 50-m or 100-m transects randomly oriented upon 

nearly every grid point in each research area (Fig. 3a). 

Transects are centered on the grid point and allowed to 

intersect one another. Transects that would otherwise 

extend beyond research area boundaries are either 

terminated or reflected back into the research area. 

Between 2,398 and 3,516 m of transects are installed 

per research area.

Transects are considered nested because coarse 

woody debris is measured in different size classes 

at different points along the transect (Fig. 3b). 

For 100-m transects, both snags and down wood 

≥4.5-cm in diameter are sampled from 0-5 m, 

45-55 m, and 95-100 m along the transect. Everywhere 
76 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 3.—Overview of the inventory systems for the AFERP. (a) Spatial layout of overstory plots and deadwood transects 

across the network of gridpoints in the first replicate block. (b) Schematic drawing of a 100-m-long nested, deadwood transect 

showing zones where sampling includes material ≥9.5 cm (thick red lines) and ≥4.5 cm (thin red lines). Items sampled 

include all down woody items crossing the transect line that are ≥0.3 m in length (gray cylinders) and standing snags that 

are measured “in” using a 4-m BAF variable radius prism centered on the transect perpendicular to the snag (gray circles). 

(c) Schematic drawing to scale of an 0.05-ha overstory plot, with a nested 0.01-ha sapling plot, four 1-m2

 herbaceous and 

seedling quadrats (black diamonds), 12 forest floor sampling points (black circles), and six canopy light interception points 

(X’s).
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 77

else, snags and down wood must measure ≥9.5 cm in 

diameter. To be sampled, down wood must intersect 

the transect and have a diameter exceeding the size 

threshold. Snags are sampled when their d.b.h. exceeds 

the size threshold for that part of the transect, and 

they can be counted as “in” from a 4-m basal area 

factor (BAF) prism held above a point on the transect 

perpendicular to the snag location (i.e., at the shortest 

point between the snag and the transect). Down wood 

with a diameter greater than its length and snags 

shorter than breast height are not sampled.

For snags, species (if it can be determined), decay 

class (Cline et al. 1980), origin (i.e., windsnap, 

windthrow, logging), d.b.h., height, and spatial 

location are recorded. Attributes such as cavities and 

history (e.g., dead tree from overstory inventory) are 

also noted.

For down wood, species, decay class (Maser et al. 

1979), origin, and transect intersection location are 

recorded. To estimate volume using line transectbased estimators (van Wagner 1968), diameter at 

transect intersection, diameter at large end, diameter 

at small end, horizontal (i.e., straight-line) length, 

curved length, number of pieces, and hollowness are 

measured. Lengths and diameters are taken only on 

that portion of a piece with a diameter greater than the 

4.5- or 9.5-cm threshold. Finally, suspension class 

(i.e., proportion of length not contacting the mineral 

soil or duff), history, and other special features 

are noted.

Seedlings and herbaceous plants

The purpose of this inventory is to both monitor 

changes in the herbaceous plant community and 

quantify tree regeneration up to 1.5 cm in d.b.h. 

Measurements take place on four 1-m2

 quadrats 

systematically located within the 0.05-ha overstory 

plots (Fig. 3c, black diamonds). 

In each quadrat, ferns, grasses, sedges, rushes, forbs, 

shrubs, and tree seedlings are identified to species 

whenever possible. Mosses and lichens are identified 

to genus only. In cases in which identification to 

species in the field is not possible, collections from 

outside of the quadrat are made if numerous other 

individuals of the species are present. For all plants, 

percentage cover is estimated to the nearest 0.1 percent 

for 0.1-5 percent cover, the nearest 1 percent for 

5-10 percent cover, the nearest 5 percent for 

10-25 percent cover, and the nearest 10 percent for 

greater than 25 percent cover. Number of stems is 

recorded for ferns, forbs, shrubs, and tree seedlings.

Forest floor

The purpose of this inventory is to quantify changes 

in forest floor organic horizons by stratum depth and 

mass. Forest floor depth measurements are taken at 

eight locations within the 0.05-ha overstory plots 

(Fig. 3c, inner small black circles). In addition, 15 cm 

by 15 cm removal subplots are taken at four locations 

surrounding each overstory plot (Fig. 3c, outer small 

black circles); organic matter layer depth is measured 

to the nearest 0.1 cm on each side of the removal 

subplot. Collected material from each subplot is ovendried at 70 oC for 48-72 hours and weighed to the 

nearest 0.01 g to determine mass. All samples are then 

archived for future use.

Canopy light interception

The purpose of these measurements is to estimate 

changes in leaf area index and gap fraction in response 

to treatments. Measurements are taken twice per 

summer, once in July and once in August, with a 

LiCOR LAI-2000 plant canopy analyzer (LiCOR, 

Inc. 1992) at six precise locations within each 0.05-ha 

overstory plot (Fig. 3c, X’s). Readings are taken on 

completely overcast days or on completely cloudless 

days in either early morning or late afternoon hours 

when diffuse light conditions predominate. Canopy 

masks (180o) are used to block out the southern portion 

of the sky for all readings. After downloading, average 

leaf area index and gap fraction of each overstory 

plot are calculated using the central 43o cone (rings 

1-3) (Puettmann and Reich 1995), although all mean 

contact frequency (CT#_) and gap fraction (GAP_) 

values are saved for recalculation of leaf area or gap 

fraction for any masking scheme.
78 GTR-NRS-P-123 Penobscot Experimental Forest: 

Retention trees

The purpose of this inventory is to quantify the growth 

and fate of trees retained during harvest operations. 

This inventory is not associated with plots or a grid, 

focusing on individual trees instead. Further, this 

inventory is not fully implemented. Records exist since 

1995-97 for trees that meet the 10 percent long-term 

target in the 20:10 treatment (n=535). Trees are being 

added from the 10:20 treatment areas and for meeting 

the 30 percent target for nonregenerated gaps. In total, 

907 individuals are now being monitored.

Each monitored tree is tagged and spatially located. 

Species, d.b.h., tree condition, height, lowest live 

crown (as defined by lowest whorl), canopy stratum, 

and light field are recorded. Presence of cavities, 

foliose lichen, and other special structural features 

are noted.

Harvested trees

The purpose of these measurements is to quantify 

the volume and quality of harvested material. This 

is a recent addition to the monitoring protocols as 

these data were not collected in the initial harvests in 

1995-97. Before each harvest, the species and d.b.h. 

of marked stems are recorded within each gap. For 

trees greater than 20 cm in d.b.h., merchantable height 

to the nearest 1.22 m (15 cm minimum inside bark 

diameter) and number of clear faces in the 2.59-m 

(including trim) butt log are recorded to approximate 

the quality of sawtimber and veneer being removed.

Other inventories

Several other kinds of data have been collected since 

the experiment began, largely for short-term studies 

that documented various community responses to 

the initial gap harvests. Nearly all these data have 

been spatially referenced with Universal Transverse 

Mercator coordinates. Results and methodology of 

some of these studies have been published elsewhere 

(Miller et al. 2007, 2008; Thomas et al. 2009) although 

several exist only as unpublished theses. For example, 

Schofield (2003) collected herbaceous vegetation and 

tree regeneration data in 4-m2

 quadrats placed along 

north-south transects bisecting 45 harvest gaps, 

23 recent (i.e., <25 years) natural gaps, and 23 closedcanopy areas across the experiment. Data collected 

were similar to the aforementioned inventory of 

seedlings and herbaceous plants, but also included 

spatial location in the gap, overstory basal area, 

canopy gap fraction, and increment cores and crosssectional disks (i.e., tree cookies) from gap edge trees 

to determine gap age and origin. In a study of gap 

harvest impacts on the amphibian community, Strojny 

(2004) measured volume, decay class, and abundance 

of down wood ≥10 cm within 17 harvest gaps, 

19 natural gaps, and 6 closed-canopy areas using 10-m 

line transects. Lastly, Olson (2009) superimposed 4-m2

vegetation quadrats in a repeating pattern, cyclical 

sampling design (sensu Scheller and Mladenoff 2002) 

upon the coarse woody debris transects to investigate 

the spatial pattern of both herbaceous communities and 

tree regeneration in response to overstory basal area 

and canopy gap fraction.

Two other data sources are of note. For several of the 

research areas, stand histories have been reconstructed 

based on dendrochronological analysis of increment 

cores collected from trees in the overstory inventory.2

Cores have been archived for future use. Second, 

airborne light detection and ranging data have been 

collected for five of the nine sites at a first return point 

density of >5 on a 1 m by 1 m grid. Raw and postprocessed data are available from the lead author. 

DISCUSSION

Puettmann et al. (2009) suggested that forests must 

be managed as complex adaptive systems in which 

silviculturists must produce forests with heterogeneous 

structures, composition, and functions in order to be 

resilient to changing biotic and abiotic conditions. 

Silvicultural research, they argued, thus must be 

designed to investigate aspects of scale and scaling 

2

 Seymour, R.S. [N.d.] FES 536: Forest stand dynamics 

(class files). On file with R.S. Seymour, University of 

Maine, School of Forest Resources, 5755 Nutting Hall, 

Orono, ME 04469-5755.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 79

to quantify nonlinear thresholds, local feedbacks, and 

other emergent properties (Puettmann et al. 2009).

The AFERP is a model example of this approach 

to forestry management and research. The AFERP 

uses silvicultural systems that can create quite 

heterogeneous stand structures and can be adapted 

to local conditions. Although inventory systems 

were initially designed to experimentally compare 

treatments with analysis of variance techniques 

(Seymour et al. 2006), redesigns over the past 5 years 

were done to explicitly investigate spatial patterns 

over multiple scales (e.g., Olson 2009). Retention tree 

inventories, for example, can be used to investigate 

tree growth in response to neighborhood competition; 

kriging can be used on the deadwood data to estimate 

relative salamander abundance.

Although the basic monitoring remains focused 

on vegetation, the AFERP continues to invite 

researchers interested in other biotic communities. 

This experiment, along with its more traditional PEF 

counterpart, the U.S. Forest Service Compartment 

Study (Brissette and Kenefic, this volume), will 

continue to be invaluable to both managers and 

researchers for years to come.

LITERATURE CITED

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silviculture systems with spatially independent 

growth models. West Lafayette, IN: Purdue 

University, Department of Forestry and Natural 

Resources. 144 p. M.S. thesis. 

Arsenault, J.A.; Saunders, M.R.; Seymour, R.S.; 

Wagner, R.G. 2011. First decadal response to 

treatment in a disturbance-based silviculture 

experiment in Maine. Forest Ecology and 

Management. 262: 404-412.

Bechtold, W.A. 2003. Crown position and light 

exposure classification—an alternative to fieldassigned crown class. Northern Journal of Applied 

Forestry. 20: 154-160.

Cline, S.P.; Berg, A.B.; Wight, H.M. 1980. Snag 

characteristics and dynamics in Douglas-fir 

forests, western Oregon. Journal of Wildlife 

Management. 44: 773-786.

D’Amato, A.W.; Orwig, D.A. 2008. Stand and 

landscape-level disturbance dynamics in oldgrowth forests in western Massachusetts.

Ecological Monographs. 78: 507-522.

Fraver, S.; Wagner, R.G.; Day, M. 2002. Dynamics of 

coarse woody debris following gap harvesting 

in the Acadian forest of central Maine, U.S.A.

Canadian Journal of Forest Research. 32: 

2094-2105. 

Gamborg, C.; Larsen, J.B. 2003. ‘Back to nature’ - A 

sustainable future for forestry? Forest Ecology 

and Management. 179: 559-571.

Keeton, W.S. 2006. Managing for late-successional/

old-growth characteristics in northern 

hardwood-conifer forests. Forest Ecology and 

Management. 235: 129-142.

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regimes as a basis for forest management: 

a North American view. Forest Ecology and 

Management. 257: 1868-1873.

Lorimer, C.G.; White, A.S. 2003. Scale and frequency 

of natural disturbances in the northeastern 

US: implications for early successional forest 

habitats and regional age distributions. Forest 

Ecology and Management. 185: 41-64.

Maser, C.M.; Anderson, R.G.; Cormack, K., Jr.; 

Williams, J.T.; Martin, R.E. 1979. Dead and down 

woody material. In: Thomas, J.W., ed. Wildlife 

habitats in managed forests: the Blue Mountains 

of Oregon and Washington. Agric. Handbk. 553. 

Washington, DC: U.S. Department of Agriculture, 

Forest Service: 78-95.
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Miller, K.M.; Wagner, R.G.; Woods, S.A. 2007. 

Effect of gap harvesting on epiphytes and 

bark-dwelling arthropods in the Acadian forest 

of central Maine. Canadian Journal of Forest 

Research. 37: 2175-2187.

Miller, K.M.; Wagner, R.G.; Woods, S.A. 2008. 

Arboreal arthropod associations with epiphytes 

following gap harvesting in the Acadian forest of 

Maine. The Bryologist. 111(3): 424-434.

North, M.P.; Keeton, W.S. 2008. Emulating natural 

disturbance regimes: an emerging approach for 

sustainable forest management. In: Lafortezza, 

R.; Chen, J.; Sanesi, G.; Crow, Th.R., eds. Patterns 

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Springer, Inc.: 341-372.

Oliver, C.D.; Larson, B.C. 1996. Forest stand 

dynamics. Update edition. New York: John Wiley 

& Sons, Inc. 520 p.

Olson, M.G. 2009. Spatial and temporal patterns 

of tree regeneration in the Acadian Forest 

of central Maine. Orono, ME: University of 

Maine, School of Forest Resources. 120 p. 

Ph.D. dissertation.

Puettmann, K.J. 2005. Do innovative experiments 

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ecosystem values: innovative experiments for 

sustainable forestry. Gen. Tech. Rep. PNW-625. 

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Natural disturbance regimes in northeastern 

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60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 81

The Cooperative Forestry Research Unit 

Commercial Thinning Research Network: 9-Year Results

Robert S. Seymour, Spencer R. Meyer, and Robert G. Wagner

Abstract.—The Commercial Thinning Research Network (CTRN) was established 

throughout the spruce-fir forest of Maine beginning in 2000 with substantial funding 

from the University of Maine’s Cooperative Forestry Research Unit. One of six replicates 

of the precommercially thinned stand condition in the CTRN is located in compartment 

23A in the U.S. Department of Agriculture, Forest Service’s long-term silviculture 

experiment on the Penobscot Experimental Forest (PEF), in which the experimental 

treatments are the timing of the first thinning entry and relative-density reduction. This 

paper presents 9-year results of this study, including detailed stand trajectories for the 

plots in compartment 23A. Contrary to expectations, there was no difference in periodic 

annual volume increment among the early-thinning treatments, owing to a much greaterthan-expected growth response from the heavily thinned (50 percent reduction) plots. 

The study has provided valuable calibration data for the Forest Vegetation Simulator, and 

has informed landowner thinning decisions for a large area of northern Maine that was 

regenerated in the 1970s and 1980s during the spruce budworm salvage era. The study 

is actively maintained and future publications will document the effects of the 10-year 

delayed treatments.

INTRODUCTION

The Cooperative Forestry Research Unit 

The Cooperative Forestry Research Unit (CFRU) is 

a partnership between Maine forest landowners and 

managers and the University of Maine. The purpose 

of the CFRU is to help member organizations advance 

forest management practices in Maine through 

applied scientific research. Member organizations 

contribute annual dues to support research projects 

that are guided by an advisory committee. In 2013, the 

CFRU had 31 members (representing approximately 

8 million acres), including private industrial, private 

nonindustrial, and public forest landowners; wood 

processors; and other private contributors.

The Commercial Thinning Research 

Network

The Commercial Thinning Research Network (CTRN) 

is a statewide system of study sites created with 

substantial funding from the CFRU to study questions 

surrounding commercial thinning of Maine’s sprucefir resource. The CTRN was installed in 2000-2001 

and consists of two experiments, each replicated at 

six sites (Fig. 1). The research questions for the two 

experiments are:

1. For naturally regenerated spruce-fir stands that 

have never received precommercial thinning 

(PCT), what is the influence of (a) method of 

commercial thinning and (b) residual density 

on subsequent stand response? 

2. For naturally regenerated spruce-fir stands that 

have received PCT, what is the influence of 

(a) timing of first commercial thinning entry 

and (b) residual density on subsequent stand 

response?

The Penobscot Experimental Forest (PEF) study site 

hosts one replicate of the second experiment type, 

hereafter referred to as the “PCT Study.” 
82 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 1.—Location of the 12 original CFRU Commercial 

Thinning Research Network study sites in Maine.

Relative density (RD) reduction (code)

Timing (code) 33% (LT) 50% (HT)

Immediate (0) Evenly space residual stand and reduce RD by 

33% in 2001-2002

Evenly space residual stand and reduce RD by 

50% in 2001-2002

Wait 5 years (5) Evenly space residual stand and reduce RD by 

33% in 2006-2007

Evenly space residual stand and reduce RD by 

50% in 2006-2007

Wait 10 years (10) Evenly space residual stand and reduce RD by 

33% in 2011-2012

Evenly space residual stand and reduce RD by 

50% in 2011-2012

Never Untreated control (NT)

Table 1.—Treatment prescriptions used in the PCT-origin stands. (Codes used in the subsequent text and 

analysis begin with the relative density reduction followed by the timing. For example, the 33 percent 

relative density removal waiting 5 years is coded “LT5.”)

METHODS

Treatment Prescriptions

In the PCT-origin stands of the CTRN, treatments were 

designed to answer two important questions regarding 

the (a) timing and (b) intensity of first commercial 

thinning entry (Table 1).

The “immediate” treatment should be interpreted as 

implementing a commercial thinning as soon as the 

stand is commercially harvestable, typically when the 

average height of all trees is about 35 feet if it has been 

precommercially thinned.

Treatments were 33 percent and 50 percent removal 

based on the original relative density, calculated 

from the diagram of Wilson et al. (1999). Relative 

density is the ratio of the current density (trees per 

acre) to the maximum number of trees possible based 

on the current average tree volume. Trees to be cut 

were marked with paint for the harvester operators. 

Marking generally favored the largest, most vigorous 

crop trees, while attempting to maintain fairly uniform 

spacing of residuals and discriminating against smaller 

merchantable firs. One exception is that balsam fir 

(Abies balsamea) over 9 inches in diameter at breast 

height (d.b.h.) were generally removed, based on the 

increasing likelihood of heart rots and susceptibility to 

the balsam woolly adelgid (Adelges piceae).

Treatment plots are nominally 1.0 acres with a 

0.2-acre measurement plot in the center (Fig. 2). 

Forwarder trails are spaced 100 feet apart with only 

one forwarder trail running through the sample plot. 

In the first two entries, small, single-grip processors 

used “ghost trails” spaced between forwarder trails to 

conduct thinnings. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 83

Figure 2.—Treatment and sample plot design for CFRU 

Commercial Thinning Research Network study sites.

Measurement Protocol

All trees ≥4.6 inches in d.b.h. have been measured 

annually since the study’s inception on seven 0.20-acre 

(87.2 feet by 100 feet) sample plots per site that are 

centered within the 0.92-acre (200 feet by 200 feet) 

treatment plots. Measurements are status (live or dead, 

with apparent cause of mortality), d.b.h., total height, 

and crown height of every tree. All logs removed from 

the plots during thinning were measured for length, 

diameter, and species. 

Initial Stand Conditions

All stands were originally precommercially thinned 

during the 1980s to a spacing of about 8 feet by 8 feet 

(600-800 trees per acre). Stand age at the time of our 

first commercial thinning in 2001 ranged from 23 to 

40 years; site index is high, ranging from 60 to 80 

(height in feet at a breast-height [bh] age of 50). All 

sites are dominated by balsam fir, some more than 

others (Table 2). The remaining stocking is mostly red 

spruce (Picea rubens), with occasional white spruce 

(Picea glauca) and other conifers such as eastern white 

pine (Pinus strobus) and eastern hemlock 

(Tsuga canadensis).

PEF Study Site Description

The study stand, compartment 23A, is one of two 

replicates of the three-stage shelterwood treatment in 

the U.S. Department of Agriculture, Forest Service’s 

long-term experiment on the PEF (Fig. 3). It originated 

after a shelterwood removal cutting that released 

small-sapling-size regeneration in 1972 (see Brissette 

and Kenefic, this volume). After the removal cutting, 

all stems >2.5 inches in d.b.h. were felled, resulting in 

a very uniform even-aged condition. Precommercial 

thinning was done in 1981 to a 2 m by 3 m spacing 

(slightly lower density than 8 feet by 8 feet). The first 

commercial thinnings were done in the winter of 

2001-2002, the second treatments were completed 

Table 2.—Preharvest stand metrics for the PCT-origin sites, averages or totals of all trees with d.b.h. 

≥4.6 inches. (Site Index is the height of the tallest 40 trees per acre at a bh age of 50, from the equations 

in the appendix of Wilson et al. [1999]. QMD is the quadratic mean d.b.h.; volumes are from Honer’s 

[1967] equations; merchantable limits are 4.6 inches d.b.h. to a 3.0-inch top diameter inside bark; relative 

density is from the equations in Wilson et al. [1999]; balsam fir percentage is based on the relative basal 

areas.) Site abbreviations from Figure 1 are Alder Stream (AS), Lake Macwahoc (LM), Lazy Tom (LT), the 

Penobscot Experimental Forest (PEF), Ronco Cove (RC), and Weeks Brook (WB).

 Basal area Density Total Merchantable

Age at Site (ft2 per (trees per QMD Height stemwood stemwood Relative Balsam

Site bh (yr) index acre) acre) (inches) (ft) volume (ft3) volume (ft3) density fir

AS 28 74 109 539 6.1 33 1,926 1,454 0.29 82%

LM 40 60 125 656 6.0 39 2,427 1,774 0.36 100%

LT 28 70 122 778 5.4 34 2,154 1,406 0.34 73%

PEF 29 67 101 606 5.5 35 1,854 1,254 0.29 69%

RC 23 80 113 736 5.3 31 1,802 1,169 0.29 97%

WB 29 67 124 831 5.3 35 2,273 1,477 0.36 92%
84 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 3.—Treatment map for the CFRU Commercial Thinning Study installation in compartment 23A of the PEF.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 85

in the winter of 2006-2007, and the final (10-year) 

treatment was completed during the winter of 

2011-2012. 

Analysis

All data are contained in a master Microsoft Office 

Access database which is updated annually. Data 

included here include the preharvest measurements 

from 2001, along with the annual measurements 

from 2002 through 2011. Because trees are generally 

measured early in the growing season, this analysis 

thus includes nine growing seasons, broken into two 

periods corresponding to the timing of the 5-year 

treatments: 2002 through 2006, and 2007 through 

2010. Data from the 5- and 10-year treatments were 

treated as unthinned controls during the years prior 

to thinning. The 10-year treatments were treated after 

the last measurement used here, so these plots are 

effectively the same as the controls during this period. 

Accretion is the volume increment on trees that are 

merchantable at the beginning of the growth period. 

Ingrowth is the ending-period volume of trees that 

grew across the 4.6-inch d.b.h. threshold. All volumes 

are merchantable, inside-bark cubic feet, from Honer’s 

(1967) equations; merchantable limits include trees 

≥4.6 inches in d.b.h. to a 3.0-inch top diameter inside 

bark. Statistical analyses were done with Systat’s 

version 12 General Linear Model procedure (Systat 

Software Inc., San Jose, CA). 

RESULTS and DISCUSSION

Commercial Harvests

The first harvests (the “immediate” treatment, done in 

2001-2002) yielded significant volumes of commercial 

products, ranging in value from about $500 to nearly 

$1,200 per acre on a mill-delivered basis (Fig. 4).

Over all sites, volume removals averaged between 8 

and 10 cords per acre in the first entries in 2001. The 

year-5 entries removed 26 percent more volume than 

the first, undoubtedly owing to the higher stocking 

levels and taller trees removed with essentially the 

same prescription (Table 3).

Figure 4.—Mill-delivered value by product class from thinning each of the six sites at 33 percent and 50 percent relative 

density reductions from first thinning (2001).
86 GTR-NRS-P-123 Penobscot Experimental Forest: 

Table 3.—Merchantable harvests (cubic feet per 

acre, cords in parentheses) from the commercial 

thinning treatments averaged over all six sites, by 

timing and removal rate

Timing LT (33%) HT (50%)

Immediate 673 (7.9) 890 (10.5)

Year 5 934 (11.0) 1039 (12.2)

Nine-Year Stand Development 

and Growth Responses 

Over all sites, yields increased from 1,694 ft3 

(19.9 cords) per acre in 2002 to 2,934 ft3

 (34.5 cords) 

in 2011, a 73 percent increase (Fig. 5). Net periodic 

annual increment (PAI) averaged 128 ft3

 (1.5 cords) 

per acre during the first 5 years, increasing to 143 ft3

 

(1.7 cords) during the next 4 years. Over the 9-year 

period, yields of many plots more than doubled. These 

stands are exceptionally productive by any relevant 

Figure 5.—Nine-year stand development of all 42 plots in the CTRN PCT study, grouped by thinning treatment. Yields include 

standing volumes plus removals in prior thinnings, if any.

0

500

1000

1500

2000

2500

3000

3500

4000

20 25 30 35 40 45 50 55

Merchantable yield (

ft3 per acre per year)

Age at BH (yr)

HT0 HT5 LT0 LT5 NT

standard. The only exception is the LM site, where 

the 5-year treatments suffered post-thinning mortality 

losses, as evidenced by the level or declining yields 

in two plots in the oldest site (>age 40, Fig. 5).

The overall analyses of variance (Table 4) revealed 

significant effects of treatment on the positive growth 

components, although this effect is due entirely to the 

lower growth of the “heavy thinning, wait 5 years” 

(HT5) treatment (Fig. 6). When each growth period 

was analyzed separately, accretion was not different 

among treatments during the first 5 years, averaging 

116 ft3

 (1.4 cords) per acre per year. During this 

period, but not the second, there was a significant 

difference in ingrowth, with the unthinned controls 

exceeding the treated plots. Evidently, many small 

(<4.6 inches d.b.h.) trees were harvested in the early 

treatments, thus preempting their subsequent ingrowth.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 87

Factor Accretion Ingrowth Mortality Net PAI

Treatment p-value	 0.002	 0.001	 0.097	 0.003

LT0 153.5 b 8.6 a 15.6 a 162.1 a

HT0 133.4 b 6.1 a 5.9 a 139.5 ab

LT5 137.6 b 11.6 ab 22.3 a 149.3 a

HT5 99.7 a 12.7 ab 17.7 a 112.4 b

NT (control) 137.6 b 14.3 b 6.8 a 152.5 a

Growth period p-value	 <0.001	 <0.001	 0.959	 <0.001

2002-2006 116.0 a 15.0 a 13.5 a 131.0 a

2007-2010 148.9 b 6.4 b 13.7 a 155.3 b

Site p-value	 <0.001	 <0.001	 0.001	 <0.001

Ronco Cove (RC) 176.8 a 14.1 bc 6.8 a 190.9 a

Alder Stream (AS) 156.1 ab 8.3 b 5.0 a 164.4 b

Weeks Brook (WB) 136.6 bc 18.1c 11.0 ab 154.7 b

Lake Macwahoc (LM) 114.2 cd 1.6 a 23.6 b 115.8 c

Lazy Tom (LT) 107.5 d 8.9 b 26.3 b 116.4 c

PEF 103.4 d 13.3 bc 8.8 a 116.8 c

Table 4.—Analyses of variance of four growth components using treatment and growth period as main 

effects and site as a blocking factor. P-values in italics are the probability of no differences in each 

growth component (columns) resulting from each factor. Data are least-squares means (in ft3 per acre 

per year); means followed by the same letter do not differ at p = 0.10 using Tukey’s Honestly-SignificantDifference test.

-25

0

25

50

75

100

125

150

175

200

HT0 LT0 NT HT0 HT5 LT0 LT5 NT

Growth component (

ft3 per acre per year)

Treatment

Accretion Ingrowth Mortality

First Period (2002-2006)

Second Period (2007-2010)

Figure 6.—Growth components (merchantable ft3 per acre per year) by thinning treatment and growth period, averaged 

over the six PCT sites. The positive height of the bars (accretion plus ingrowth) is the gross growth; gross growth minus the 

negative bars (mortality) is net growth.
88 GTR-NRS-P-123 Penobscot Experimental Forest: 

Except with the HT5 treatment, growth increased 

significantly over the course of the study. Net PAI 

was 24 ft3

 greater during the second period than the 

first (Table 4). The treatment effect on mortality was 

marginally significant overall, yet no mean separations 

reached this significance threshold. Note that mortality 

was very low on the unthinned controls during both 

periods (Fig. 6), suggesting that these stands are not 

yet self-thinning.

Growth was quite different among the six sites 

(Table 4), reflecting differences in their site indices 

(Table 2). The extraordinary growth (>2 cords per acre 

per year) and high site index (80 feet) of the Ronco 

Cove site are especially noteworthy. The net PAIs of 

the top three sites (Table 4) exceed the highest PAIs 

reported in the Green River spacing study (Pitt and 

Lanteigne 2008), and the Ronco Cove site approaches 

the highest PAIs reported by Pelletier and Pitt (2008) 

for white spruce plantations with a high site index in 

northwestern New Brunswick.

When relative density at the midpoint of the growth 

period was added to the analysis of variance model 

(Table 4) as a covariate, treatment effects became 

highly significant (Fig. 7). This difference is due 

entirely to the relatively lower growth of the unthinned 

control plots at a given relative density. The different 

slopes fitted to each treatment separately suggest an 

interaction between treatment and relative density, 

but this interaction was significant only at p = 0.29. 

Clearly the thinned treatments are more growthefficient at a given density, likely owing to their 

improved light environment. The pattern illustrated in 

Figure 7 is the main reason that Saunders et al. (2008) 

needed to perform extensive treatment-related growth 

calibrations to the Forest Vegetation Simulator for 

analyzing various commercial thinning regimes.

Figure 7.—Net PAI over relative density at the midpoint of the growth period, by treatment. Lines represent separate 

least-squares regressions fitted to each treatment.

0

50

100

150

200

250

0 0.1 0.2 0.3 0.4 0.5 0.6

Net PAI (

ft3 per acre per year)

Relative density (at midpoint of period)

HT0 HT5 LT0 LT5 NT
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 89

Stand Development of the PEF Site 

in Compartment 23A

When the PEF plots are graphed on the Wilson et 

al. (1999) density management diagram (Fig. 8), we 

see that the 50 percent removal (H) treatments left 

residual stands at a density of 240-260 trees per acre, 

or a spacing of about 13 feet between crop trees. The 

33 percent (L) treatments left about 100 more trees 

per acre (340-355), a spacing of 11 feet. Plots 1 (the 

control) and 4 (unthinned until 2012) began the study 

at a relative density of about 0.3 and have grown to 

nearly 0.5 over the 9-year period. No evidence of 

self-thinning or density-dependent mortality is yet 

apparent.

Growth components by treatment (Fig. 9) are 

somewhat lower than the overall study averages. 

During the first growth period, the L0 treatment grew 

much more than its heavier counterpart (H0), but 

during the second period, three large (8 inches, 

12 inches, and 13 inches d.b.h.) balsam firs died 

standing on this plot. We suspect they were attacked 

by the balsam woolly adelgid, which has not been 

observed on the other five installations.

FUTURE WORK

Although these results are interesting, conclusive 

findings must await future measurements and include 

the outcome of the 10-year delayed treatments 

implemented during the winter of 2012-2013. We 

hope to maintain this study until stands reach rotation 

age. Mean annual increments are still much lower 

than periodic growth, suggesting the culmination is at 

least 10-20 years in the future. The CTRN was also 

expanded in 2009-2010 to include three more stands 

of PCT origin with lower site indices and more spruce 

stocking, and thus more representative of the entire 

spruce-fir region than the six fir-dominated stands 

with high site indices reported here.

Figure 8.—Nine-year stand development of the PEF site plotted on the density management diagram of Wilson et al. (1999). 

Treatment symbols are identical to previous figures.
90 GTR-NRS-P-123 Penobscot Experimental Forest: 

-50

-25

0

25

50

75

100

125

150

175

HT0 LT0 NT HT0 HT5 LT0 LT5 NT

Growth component (

ft3 per acre per year)

Treatment

Accretion Ingrowth Mortality

First Period (2002-2006)

Second Period (2007-2010)

Figure 9.—Growth components (merchantable ft3 per acre per year) by thinning treatment and growth period for the PEF site. 

The positive height of the bars (accretion plus ingrowth) is the gross growth; gross growth minus the negative bars (mortality) 

is net growth.

LITERATURE CITED

Honer, T.G. 1967. Standard volume tables 

and merchantable conversion factors for 

the commercial tree species of central and 

eastern Canada. Information Report FMR-X-5. 

Ottawa, ON: Department of Forestry and Rural 

Development. 21 + 133 p. 

Pelletier, G.; Pitt, D.G. 2008. Silvicultural responses 

of two spruce plantations to midrotation 

commercial thinning in New Brunswick.

Canadian Journal of Forest Research. 38: 851-867.

Pitt, D.G.; Lanteigne, L. 2008. Long-term outcome 

of precommercial thinning in northwestern New 

Brunswick: growth and yield of balsam fir and 

red spruce. Canadian Journal of Forest Research. 

38: 592-610.

Saunders, M.; Wagner, W.G.; Seymour, R.S. 2008. 

Thinning regimes for spruce-fir stands in the 

northeastern United States and eastern Canada.

Orono, ME: University of Maine, Cooperative 

Forestry Research Unit. 186 p.

Wilson, D.S.; Seymour, R.S.; Maguire, D.A. 1999. 

Density management diagram for northeastern 

red spruce and balsam fir forests. Northern 

Journal of Applied Forestry. 16: 48-56.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 91

SILVICULTURAL OPTIONS FOR EARLY-SUCCESSIONAL STANDS 

IN MAINE: 6-YEAR RESULTS OF THE SILVICULTURAL INTENSITY 

AND SPECIES COMPOSITION EXPERIMENT

Andrew S. Nelson, Robert G. Wagner, and Mike R. Saunders

Abstract.—The Silvicultural Intensity and Species Composition (SIComp) experiment 

was installed in 2003 on a recently clearcut mixedwood site within the Penobscot 

Experimental Forest in east-central Maine. This study was initiated because the response 

of early-successional stands to various intensities of silviculture was poorly understood 

in the region. The goal of SIComp is to shift stand development and composition in 

multiple directions through a factorial combination of three silvicultural intensities (low, 

medium, and high) and three objectives for species composition (hardwood, mixedwood, 

and conifer). To date, the experiment has documented the survival, growth, and yield 

of hybrid poplar and planted white spruce plantations, the effects of precommercial 

thinning in juvenile aspen stands, the response of early-successional stands to conifer 

release treatments, and alternative approaches for managing young mixedwood stands. 

Current investigations are focused on mechanistic responses to the heterogeneous 

growing conditions created by the varied treatments in the experiment. Energy (labor, 

petroleum, and pesticide) inputs required to establish and maintain the experiment also 

will be incorporated into models estimating net carbon balance and value of the various 

treatments over time. 

INTRODUCTION

Young, naturally regenerated stands dominated by 

shade-intolerant hardwoods mixed with conifer 

species compose approximately 13 percent of the 

forest land in Maine (McWilliams et al. 2005). The 

majority of these stands originated after the largescale, post-budworm clearcuts during the 1980s and 

subsequent partial harvesting that continues today. 

Although these stands are prevalent in the region, their 

response to silviculture has not been well studied, 

which limits our ability to implement appropriate 

silvicultural systems to promote desirable composition 

and increase stand productivity. Additionally, without 

a firm understanding of early-successional stand 

responses to silviculture, current growth and yield 

models cannot accurately project stand development; 

therefore, the contribution of these stands to increasing 

regional wood fiber demands is poorly understood. In 

response to these concerns, the Silvicultural Intensity 

and Species Composition (SIComp) experiment was 

established in 2003 on a recently clearcut mixedwood 

site on the Penobscot Experimental Forest (PEF) in 

east-central Maine. Experimental plots were put on 

different trajectories by manipulating tree species 

composition and applying various intensities of 

silviculture, with the overall goal of documenting the 

resulting dynamics of young Acadian Forest stands. 

Long-term silvicultural experiments in Maine are 

limited, but the various experiments on the PEF have 

served as a basis for understanding the response of 

northern conifer stands to traditional silviculture 

(U.S. Department of Agriculture, Forest Service 

Compartment Study), ecological silviculture (Acadian 

Forest Ecosystem Research Program Expanding Gap 

Experiment), and intermediate treatments such as 

precommercial and commercial thinning (Commercial 

Thinning Research Network). The SIComp experiment 

complements these other long-term studies on the 

PEF. Long-term silvicultural experiments in the region 
92 GTR-NRS-P-123 Penobscot Experimental Forest: 

provide a basis for comparing the effectiveness of 

various treatments, such as clearcutting, shelterwood, 

and selection systems, and intensities of management 

on residual stand structure and regeneration dynamics 

(Brissette 1996). Responses to these various treatments 

will typically vary depending on species composition 

(Sendak et al. 2003). For example, clearcut-harvested 

stands in northern and central Maine typically 

regenerate to aspen (Populus spp.) and birch (Betula

spp.), but future composition and structure may 

differ depending on the density of conifer advance 

regeneration. 

The intensity of silviculture applied to a stand is 

directly related to the desired financial investment. 

In early-successional mixedwood stands in Maine, 

unmanaged stands often remain dominated by shadeintolerant hardwood species and growth rates may be 

inhibited by high stem densities. With a comparatively 

modest financial investment, desirable species can 

be promoted and growth rates can be increased with 

treatments such as conifer release (Newton et al. 1992) 

or hardwood thinning (Rice et al. 2001). Additional 

silvicultural investment, such as enrichment planting, 

can increase stand productivity and accelerate 

composition to mid- to late-successional species 

(Greene et al. 2002, Paquette et al. 2006). Currently, 

there are no long-term experiments in the Acadian 

region documenting the response of early-successional 

stands to different intensities of silviculture. Without 

these long-term data, it is difficult to assess the 

potential benefits of different treatments in earlysuccessional stands.

The lack of information on the responses of earlysuccessional stands to silviculture was the impetus 

for the establishment of SIComp. The three major 

objectives of the study are to: (1) quantify the growth 

and yield response of early-successional stands in 

central Maine to varying intensities of silviculture 

under different compositional objectives, 

(2) document the mechanisms affecting the dynamics 

and productivity of these young forest stands, and 

(3) compare the energy requirements and financial 

returns associated with implementation and eventual 

harvest under differing silvicultural intensities and 

compositional objectives. These objectives are longterm and require interdisciplinary collaboration. 

Currently, productivity is being investigated by 

modeling allometry, resource availability, and 

ecophysiological response in relation to the various 

growing conditions created by the treatments. 

Ongoing efforts to document the energy inputs (labor, 

petroleum, and pesticides) of the different treatments 

will provide managers with the costs and financial 

returns associated with the silvicultural strategies 

being tested.

Shortly after the study was established, the wide 

range of micro-environmental conditions created by 

the experiment allowed Kanoti (2005) to determine 

how silvicultural intensity can influence the natural 

regeneration (germination and survival) of Acadian 

tree species. He hypothesized that silviculture could 

be used to mitigate the possible negative effects of 

climate change on soil temperature and moisture 

regimes in a way that can encourage the germination 

and establishment of Acadian Forest tree species. 

Results showed that moisture availability was the most 

important factor influencing germination success. 

Drought tolerance decreased in the following order: 

trembling aspen (Populus tremuloides Michx.) > red 

spruce (Picea rubens Sarg.) = hybrid larch (Larix x 

eurolepis Henry) > Norway maple (Acer platanoides

L.) = paper birch (B. papyrifera Marsh.) = white pine 

(Pinus strobus L.) > balsam fir (Abies balsamea L.) 

= red maple (Acer rubrum L.). Additionally, Kanoti 

(2005) found that predator protection and sowing 

date strongly influenced emergence success, with a 

fall sowing date being detrimental to exotic species. 

After seedlings emerged, greater densities of grass 

competition and overstory density reduced survival 

of many of the species.

The importance of understanding the response 

of early-successional stands in Maine to various 

silvicultural intensities is increasing, yet no longterm silvicultural studies have previously addressed 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 93

these issues. Additionally, given the young age of the 

SIComp experiment, details of the study have not 

been documented elsewhere. Therefore, the goals 

of this paper were to: (1) describe the experimental 

design of the SIComp experiment, and (2) present 

initial findings on the difference in composition and 

individual tree size among species and treatments 

6 years after the experiment was started. 

METHODS

Study Area

This study was installed within the PEF near the 

towns of Bradley and Eddington (44°49'N, 68°38'W) 

between 2003 and 2004. Natural forest composition 

on the PEF is dominated by shade-tolerant conifer 

species, including balsam fir, eastern hemlock (Tsuga 

canadensis L.), and red spruce, and shade-intolerant 

hardwood species, such as trembling aspen, bigtooth 

aspen (Populus grandidentata Michx.), red maple, 

and paper birch (Sendak et al. 2003). Soils at the 

forest are of Wisconsin glacial till origin and the soil 

classifications at the study site range from loamy, 

mixed, active, acid, frigid, shallow, Aeric Endoquepts 

to coarse-loamy, isotic, frigid, Aquic Haplothods. The 

climate of eastern Maine is cool and humid. February 

is the coldest month on average (–7.1 °C) and July is 

typically the warmest (20.0 °C). Mean precipitation 

is 1070 mm, and the average growing season lasts 

approximately 160 days (Sendak et al. 2003). 

In 1995, the 9.2-ha site of the experiment was clearcut 

with approximately 2.3 m2

 ha-1 of residual basal area. 

Following harvest, the site naturally regenerated to 

shade-intolerant hardwoods (trembling aspen, bigtooth 

aspen, red maple, and paper birch) and smaller balsam 

fir, red spruce, white pine (Pinus strobus L.), and 

white spruce (Picea glauca [Moench] Voss). Shortly 

after harvest, Norway (Picea abies [L.] Karst.), red, 

black (Picea mariana [Mill.]), and white spruce were 

planted to increase the density of desirable conifer 

species, but much of the planting failed due to hare 

clipping during the first winter.

Study Design

The SIComp experiment is based on a mix of 

treatments needed to achieve the desired level of 

silvicultural intensity and tree species composition 

(Table 1). A specific set of regeneration and vegetation 

management treatments was then designed to achieve 

each treatment objective (Fig. 1). The treatment 

objectives consisted of three levels of silvicultural 

intensity (low, medium, and high) crossed with three 

species compositional objectives (conifer, mixedwood, 

and hardwood), plus an untreated control. Prior 

to the installation of the treatments, a vegetation 

survey across the site revealed a patchy pattern of 

conifer composition that required blocking of the 

treatment plots by species composition. Therefore, 

the experimental design is a 3 x 3 +1 factorial, 

restricted-randomized complete block design, with 

four replicates of each treatment. Abbreviations for 

the array of treatments are: low conifer (LC), low 

mixedwood (LM), low hardwood (LH), medium 

conifer (MC), medium mixedwood (MM), medium 

hardwood (MH), high conifer (HC), high mixedwood 

(HM), high hardwood (HH), and untreated control (C).

Each of the ten treatments was replicated four times 

for a total of forty 30 m by 30 m treatment plots 

(Fig. 2). In each low- and medium-intensity treatment 

plot, manipulations were applied to individual croptrees within each 2 m by 2 m growing space in the 

treatment plot (i.e., an average of 225 spaces per 

plot). Depending on treatment, these growing spaces 

were assigned one of four crop-tree types: naturally 

regenerated hardwood, planted hybrid poplar (Populus

clones), naturally regenerated conifer, or planted white 

spruce (Fig. 3). Naturally regenerated hardwoods 

and conifers were selected from high-quality, large 

individuals in each growing space using the following 

orders of priority: hardwoods—bigtooth aspen > 

trembling aspen > paper birch > sugar maple (Acer 

saccharum Marsh.) > red maple; conifers—eastern 

white pine > balsam fir > spruce species.
94 GTR-NRS-P-123 Penobscot Experimental Forest: 

Silvicultural 

intensity

Control of species colonization 

(regeneration)

Control of relative 

species performance 

(vegetation management)

Control of spacing 

among desired trees (thinning)

Zero None None None

Low Selection among natural 

regeneration for desired tree 

species at desired spacing; 

“holes” without desired trees 

not filled

Minimum control of neighboring 

woody plants needed to maintain 

desired trees in main canopy 

(i.e., one-time release)

None

Medium Selection among natural 

regeneration for desired tree 

species; “holes” fill-planted with 

desired tree species where 

needed to achieve desired 

spacing

Periodic control of neighboring 

woody plants around all desired 

trees that overtop or threaten to 

overtop desired trees

After crown closure of desired 

species, periodic thinning of 

desired trees to maintain a 

0.3 spacing:height ratio

High Genetically improved stock 

of desired tree species planted 

at desired spacing

Complete removal of all non-crop 

plants (woody and herbaceous) 

just before planting and through 

early stand development until 

crown closure of desired species

Periodic thinning of desired trees 

to maintain a 0.3 spacing:height 

ratio throughout life of stand

Table 1.—Description of the different levels of silvicultural intensity applied to treatment plots in the 

SIComp experiment.

Silvicultural 

intensity Pure conifer Conifer/hardwood Pure hardwood

Zero Natural succession 

(no composition target)

Low Naturally regenerated balsam 

fir and red spruce depending 

on relative abundance; “holes” 

without desired conifers not 

managed

Naturally regenerated conifers 

(balsam fir and red spruce) and 

hardwoods (aspen, red maple, 

or sugar maple) at 67:33 mixture 

depending on relative abundance

Naturally regenerated aspen, red 

maple, or sugar maple depending 

on relative abundance; “holes” 

without desired hardwoods not 

managed

Medium Naturally regenerated balsam fir 

and red spruce depending 

on relative abundance; “holes” 

fill-planted with white spruce 

Naturally regenerated conifers 

(balsam fir and red spruce) and 

hardwoods (aspen, red maple, 

sugar maple) at 67:33 mixture; 

“holes” fill-planted with white 

spruce and hybrid poplar 

Naturally regenerated aspen, red 

maple, or sugar maple depending 

on relative abundance; “holes” 

fill-planted with hybrid poplar

High Planted white spruce White spruce and hybrid poplar 

planted in 67:33 mixture

Planted hybrid poplar
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 95

Figure 1.—Treatments applied to achieve the various silvicultural objectives of the SIComp experiment. 

Figure 2.—Layout of the SIComp 

30 m by 30 m treatment plots across the 

9.2-ha clearcut site on the PEF. Treatment 

abbreviations are: C – untreated control, 

LC – low conifer, LM – low mixedwood, 

LH – low hardwood, MC – medium conifer, 

MM – medium mixedwood, MH – medium 

hardwood, HC – high conifer, HM – high 

mixedwood, and HH – high hardwood.
96 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 3.—Growing space allocation map for the SIComp experiment at study inception. The figure depicts each 2 m by 2 m 

growing space within measurement plots (20 m by 20 m) for each of the four replicates of a treatment. Cells are color-coded 

to species.

For both naturally regenerated crop-tree types, all 

woody vegetation within a 1-m radius around croptrees was chemically or mechanically controlled in 

2004. Hardwood competitors around conifer trees 

were killed using a basal application of 20 percent 

Garlon 4® (triclopyr ester) (Dow AgroSciences, 

Indianapolis, IN) mixed with Bark Oil Blue® (UAP 

Distribution Inc., Greeley, CO) (Table 2). Aspen trees 

of root-sucker origin were common across the site, so 

hardwood competitors around hardwood crop-trees 

were removed with motorized brushsaws to avoid 

potential herbicide damage to crop-trees through 

shared root systems with controlled trees. All conifer 

competitors were killed using brushsaws. 

Growing spaces assigned to fill-planted hybrid 

poplar and white spruce in the medium- and highintensity treatments were treated with herbicides 

(triclopyr basal-bark applications and glyphosate foliar 

treatments) and manually cleared of all pre-existing 

woody and herbaceous vegetation. In 2003 the highintensity treatments underwent a follow-up broadcast 

application of 2.80 acid equivalent (a.e.) kg ha-1

glyphosate (Accord® Concentrate, Dow AgroSciences) 

to control herbs, and residual sprouting by shrub and 

tree regeneration. Planted growing spaces within the 

MC, MM, HC, and HM treatments were then planted 

in spring 2004 with containerized white spruce 

obtained from a J.D. Irving, Ltd., tree nursery in 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 97

Proportion of basal area (%) Standard

Species C LC LM LH MC MM MH HC HM HH error

Paper birch 22.5 1.8 3.7 8.5 7.8 9.7 13.3 - - - 5.6

Gray birch 4.4 3.4 2.9 17.8 1.4 13.3 6.5 - - - 8.8

Bigtooth aspen 19.8 13.8 12.7 10.0 2.6 4.8 16.2 - - - 6.3

Trembling aspen 13.2 2.1 1.9 9.5 1.1 2.3 8.6 - - - 4.0

Red maple 17.6 12.3 8.8 23.9 5.2 13.0 27.0 - - - 6.6

Other hardwood species 10.4 12.1 10.3 21.0 15.9 11.3 21.8 - - - 7.0

Balsam fir 8.8 48.6 53.1 8.5 44.1 30.1 1.9 - - - 7.7

Red spruce 1.2 3.1 3.2 0.1 0.3 2.5 - - - - 1.2

Natural white spruce 0.2 1.4 1.8 - 0.3 - 0.9 - - - 0.5

Eastern white pine 1.6 1.4 1.4 0.6 4.1 5.7 1.5 - - - 1.1

Planted white spruce - - - - 17.1 6.3 - 100.0 80.7 - 3.2

Hybrid poplar - - - - - 0.6 2.2 - 19.3 100.0 1.8

Table 2.—Species composition (calculated as a proportion of total basal area) 6 years after the start of 

the experiment by treatment and for all treatments combined shown as least squares means. Treatment 

abbreviations are: C – untreated control, LC – low conifer, LM – low mixedwood, LH – low hardwood, 

MC – medium conifer, MM – medium mixedwood, MH – medium hardwood, HC – high conifer, HM – high 

mixedwood. The between-treatment standard error is also shown.

New Brunswick, Canada. The white spruce seedlings 

were 2+0 half-sib individuals grown in MP67 multipots with a 65-cm3

 rooting volume and planted with 

a Pottiputki® (a device with a hollow tube and duckbilled end that levers open a hole in soil suitable for 

seedling planting; BCC, Landscrona, Sweden).

Planted growing spaces within the MH, MM, HH, 

and HM treatments were planted with hybrid poplar 

cuttings obtained from the Woody Biomass Program 

at the State University of New York’s College of 

Environmental Science and Forestry. Four hybrid 

poplar clones were planted with dibbles using a 

split-plot design with equal proportions of each 

clone. Knowing asymmetry would occur early with 

the fast-growing hybrid poplars in the HM treatment, 

we spatially grouped the hybrid poplar into 

4-10 individuals each to minimize early interactions 

with the spruce (Fig. 3). Three Populus deltoides x 

Populus nigra clones (D51, DN10, and DN70) and one 

Populus nigra x Populus maximowiczii clone (NM6) 

were selected because it was unclear which variety 

would perform best under Maine soil and climatic 

conditions. 

Stubborn natural vegetation in the high-intensity plots 

required various treatments to ensure the success 

of the plantations. In 2004, a subsequent 1.68 a.e. 

kg ha-1 glyphosate broadcast was applied, but natural 

vegetation persisted. Following planting, a broadcast 

pre-emergent herbicide mixture was applied in 

2005 using backpack sprayers with a waving-wand 

method. The prescription consisted of 1.40 active 

ingredient (a.i.) kg ha-1 of isoxabin (Gallery® 75, Dow 

AgroSciences), 3.36 a.i. kg ha-1 of orayzalin (Surflan 

A.S.TM, Dow AgroSciences), and 0.56 a.i. kg ha-1 of 

oxyfluorfen (Goal® 2XL, Dow AgroSciences). The 

initial treatment was not completely successful in 

controlling all vegetation, requiring a subsequent 

1.68 a.e. kg ha-1 glyphosate broadcast application. 

Annually thereafter, all competition in the highintensity plots has been controlled with applications 

of glyphosate (2 to 5% concentration in water), which 

will be applied until crop-trees in all plots reach crown 

closure. Seedlings were protected from herbicide 

exposure using plastic bags for each application 

until they reached sufficient size for a directed 

application. Vegetation control in the southern plots 

has been relatively successful, but the northern plots 
98 GTR-NRS-P-123 Penobscot Experimental Forest: 

still support high densities of sedge (Carex spp.). 

Replanting of both poplar and spruce occurred during 

spring 2005 because of high initial mortality likely due 

to the poorly-drained soils across the site. 

Measurements

Within each treatment plot is a nested 20 m by 20 m 

measurement plot where 100 crop-trees were initially 

selected (Fig. 4). Two types of measurements are made 

annually from late spring to early summer (May-June). 

The crop-tree inventory measures the height, height 

to the base of the live crown, diameter at breast height 

(d.b.h.), basal diameter, and survival of the 100 croptrees in each measurement plot. The composition plot 

inventory consists of five 16-m2

 circular plots which 

are used to document changes in species composition 

and stand structure. Each of four composition plots 

is located 1.14 m inward from a measurement plot 

corner; the fifth is in the middle of the measurement 

plot. All woody vegetation ≥1.37 m tall within the 

subplot is tallied annually by species and d.b.h. is 

measured. For trees and shrubs <1.37 m tall, species 

and number of stems are recorded. 

Crown diameter, stem form, and damage rating of 

the crop-trees are measured periodically. Crown 

widths are measured in the north-south and east-west 

directions. Stem form is evaluated on the butt log 

(bottom 3 m) of the trees and placed into one of four 

categories: no defect, correctable form (e.g., minor 

crook), questionable form (e.g., moderate crook or 

minor sweep), or cull (e.g., major crook, sweep, or 

presence of forks). In conjunction with stem form, 

damage is periodically measured. Damage to the 

foliage and main stem including crown dieback, cracks 

or damage to bole, and decay are examined during this 

assessment.

Figure 4.—Layout of 20 m by 20 m measurement plots within each 30 m by 30 m treatment plot. The five shaded areas are 

compositional plots and total 80 m2. One crop or “pseudo-crop” tree is designated within each cell of the 2 m by 2 m grid 

(light dotted lines). 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 99

Since the start of the experiment, all of the energy 

inputs required to install and maintain the experiment 

have been measured. The energy inputs are herbicide 

a.i. (kg ha-1), petroleum (L ha-1), and labor (h ha-1).

Analysis

In this paper, the composition plot inventory from 

the sixth post-treatment growing season (2009) was 

used to calculate species composition (calculated as 

proportion of basal area) and mean d.b.h. by species 

and treatment. These two variables were analyzed with 

analysis of variance by species, where the independent 

variable in the models was treatment. Significance was 

assessed at the α = 0.05 level, and all analyses were 

performed in R version 2.13.2 (R Development Core 

Team 2011). Additionally, the energy inputs (herbicide, 

petroleum, and labor) were summed over the 6 years 

of the experiment and expressed as a unitless energy 

index to document the energy requirements of the 

various treatments.

FINDINGS

Tree Composition and Performance 

6 Years Post-treatment

Annual measurements of the SIComp experiment have 

documented the responses of individual crop-trees 

as well as stand-level compositional and structural 

changes to the wide range of early silvicultural 

treatments. Species composition and mean tree d.b.h. 

by species from the sixth post-treatment season (2009) 

composition plot inventory are shown in Tables 2 

and 3. The treatments were successful in promoting 

the desired species composition (expressed as a 

proportion of total basal area by species) after 6 years. 

The untreated control (C) and naturally regenerated 

hardwood treatments (LH and MH) were dominated 

by hardwood species, primarily trembling and bigtooth 

aspen, paper birch, and red maple (Table 2). The 

naturally regenerated conifer treatments (LC and MC) 

were dominated by balsam fir, but hardwood species 

were also well represented, especially bigtooth aspen 

and red maple with 13.8 percent (± 6.3%) and 

12.3 percent (± 6.6%) in the LC treatment and 

2.6 percent (± 6.3%) and 5.2 percent (±6.6%) in the 

MC treatment, respectively. 

Balsam fir was the dominant species in the LM 

treatment, accounting for 53.1 percent (± 7.7%) of 

the composition, followed by bigtooth aspen (12.7% 

± 6.3%) and red maple (8.8% ± 6.6%). Similarly, 

balsam fir was the most prevalent species in the 

MM treatment (30.1% ± 6.3%), followed by gray 

birch (Betula populifolia; 13.3% ± 8.8%), red maple 

(13.0% ± 6.6%), and paper birch (9.7% ± 5.6%). The 

Table 3.—Least squares mean d.b.h. (cm) of the most common species 6 years after treatment. Treatment 

abbreviations are: C – untreated control, LC – low conifer, LM – low mixedwood, LH – low hardwood, 

MC – medium conifer, MM – medium mixedwood, MH – medium hardwood, HC – high conifer, HM – high 

mixedwood, and HH – high hardwood. The between-treatment standard error (cm) is also shown.

Mean tree d.b.h. (cm) Standard

Species C LC LM LH MC MM MH HC HM HH error

Paper birch 1.1 0.6 0.6 1.3 0.6 1.5 1.0 - - - 0.4

Gray birch 1.4 0.2 0.4 0.3 0.2 0.4 0.5 - - - 0.4

Bigtooth aspen 3.8 6.3 6.3 3.5 1.9 7.2 4.8 - - - 1.6

Trembling aspen 2.5 4.0 3.0 7.8 0.6 3.4 3.5 - - - 1.6

Red maple 2.6 1.5 1.5 2.5 0.9 1.4 1.6 - - - 0.6

Balsam fir 1.9 3.5 3.2 1.7 3.3 3.3 0.6 - - - 0.4

Red spruce 1.3 2.2 1.7 0.1 1.5 3.9 - - - - 1.1

Natural white spruce 0.2 1.4 3.3 - 0.4 - 0.3 - - - 0.9

Eastern white pine 2.7 1.3 2.5 0.5 2.2 4.8 0.3 - - - 1.0

Planted white spruce - - - - 0.8 1.0 - 1.2 1.1 - 0.2

Hybrid poplar - - - - 0.6 0.6 - 6.1 4.6 0.8
100 GTR-NRS-P-123 Penobscot Experimental Forest: 

high-intensity treatments (HC, HM, and HH) were all 

successful in maintaining species composition, but 

in the HM treatment the proportion of white spruce 

had shifted to 80.7 percent (± 3.2%) from the initial 

proportion of 68.0 percent, likely due to the low 

hybrid poplar survival. 

Across the treatments, bigtooth and trembling aspen 

had the largest d.b.h. among all species (Table 3). Both 

of these species had larger stem diameters in the LH, 

MH, LM, and MM treatments than the C treatment. 

The d.b.h. of paper birch, gray birch, and red maple in 

the low- and medium-intensity treatments was similar 

to the C treatment. Among conifer species, d.b.h. of 

balsam fir was slightly larger in the low and medium 

treatments compared to the C treatment, but red 

spruce diameter was similar across treatments. Hybrid 

poplar d.b.h. was greater in the high treatments than 

the medium treatments, suggesting that fill-planting 

these shade-intolerant clones with natural regeneration 

reduced their performance. In contrast, d.b.h of 

the planted white spruce did not differ between the 

silvicultural intensities. 

Energy Investment through 2009

The energy input index shown in Figure 5 represents 

the sum of labor, herbicide, and petroleum inputs for 

each of the 10 treatment of the SIComp experiment 

through the 2009 spot-application of Accord®

Concentrate in the high-intensity treatments. Among 

treatments, the high-intensity plots had the greatest 

consumption of energy, most of which was from 

initial labor to prepare the sites for planting and the 

subsequent annual control of competing vegetation. 

In the low- and medium-intensity treatments, energy 

investments have been greater in the conifer and 

mixedwood treatments than the hardwood treatments. 

The dominance of early-successional hardwood 

species following the 1995 clearcut required greater 

energy investment to shift species composition. 

Figure 5.—Energy input index (a unitless measure of energy requirements to initiate and maintain the experimental design) for 

each of the 10 treatments maintained in the SIComp study through the sixth year of the study. The index is the sum of labor, 

herbicide, and petroleum inputs. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 101

FUTURE DIRECTIONS

The SIComp experiment is still in its early stages 

of development. However, the study has already 

documented substantial differences in the response 

of young stands to various silvicultural treatments 

to achieve a range of objectives. It has provided 

opportunities to use the extreme range of environments 

created by the treatments to study germination 

success. Ongoing research will afford other valuable 

research opportunities, including gaining a better 

understanding of mechanisms driving forest 

productivity and carbon accumulation, and collecting 

data for improving regional growth and yield models. 

The novel experimental design and unique growing 

conditions created by the treatments provide a 

basis for future work on multiple levels. A potential 

future investigation on the tree level may focus on 

investigating whether sink or source ecophysiological 

mechanisms are responsible for the differences 

observed in productivity. Such an investigation could 

be linked to environmental conditions and resource 

availability to assess whether the responses are more 

closely related to species composition or site-related 

factors. Plans also call for incorporating the energy 

data into financial and carbon balance analyses. These 

analyses will allow us to determine the value of the 

various species compositions and identify treatments 

that maximize stand value and carbon sequestration.

ACKNOWLEDGMENTS

The installation and maintenance of the SIComp 

experiment has been supported by the U.S. Department 

of Agriculture, Forest Service, Agenda 2020 Project 

(grant number 03-CA-11242343-031); University 

of Maine Cooperative Forestry Research Unit; 

Northeastern States Research Cooperative (NSRC 

Theme 3) (grant numbers 10-DG-11242307-049, 

09-DG-11242307-055); and the Henry W. Saunders’ 

Chair, School of Forest Resources, University of 

Maine. We would like to thank Keith Kanoti, Rick 

Dionne, John Brissette, Matthew Olson, and the 

numerous technicians involved in the establishment 

and measurement of the experiment over the years.

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Available at www.R-project.org. (Accessed 

February 18, 2013).
102 GTR-NRS-P-123 Penobscot Experimental Forest: 

Rice, J.A.; MacDonald, G.B.; Weingartner, D.H. 2001. 

Precommercial thinning of trembling aspen in 

northern Ontario: Part 1 - Growth responses.

The Forestry Chronicle. 77(5): 893-901.

Sendak, P.E.; Brissette, J.C.; Frank, R.M. 2003. 

Silviculture affects composition, growth, and 

yield in mixed northern conifers: 40-year results 

from the Penobscot Experimental Forest.

Canadian Journal of Forest Research. 33(11): 

2116-2128.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 103

Vascular Flora of the Penobscot Experimental Forest, 

with Provisional Lists of Lichens and Bryophytes

Alison C. Dibble

Abstract.—A compilation of plant lists from all available sources since the 1950s 

represents the flora of the Penobscot Experimental Forest (PEF), Bradley, Maine. 

More than 300 taxa of vascular plants in 71 families and 186 genera are included. 

Approximately 85 percent of the taxa are native to Maine. Ten of 45 nonnative species 

are considered invasive. Infraspecific taxa have not necessarily been resolved, though 

14 subspecies are included as they represent the species in the region. Two rare plants, 

Carex oronensis and Clematis occidentalis, have been documented. Omitted taxa 

overlap known species (e.g., “Salix sp.” in which a single species is indicated), or are 

thought to be misidentifications. Sixty-two lichen and 49 bryophyte species are included 

provisionally. More species could be found in surveys for (1) ruderal plants in disturbed 

ground; (2) species found in the 1960s that are unknown today at the PEF; (3) expected, 

common species of spruce-fir that have not been documented; (4) graminoids, which seem 

underrepresented; and (5) species in riparian zones and wetlands. The plant checklist 

could be especially useful in documenting shifts in the flora that might be attributable 

to climate change. Nomenclature in a new flora of New England differs from the 

U.S. Department of Agriculture, Natural Resources Conservation Service database in 

significant ways; both sources should be considered in vegetation research in the PEF. 

INTRODUCTION

Plant lists have value for estimating species diversity, 

summarizing large data sets, pointing out rare species 

and invasive plants, and stimulating additional study of 

an area, among many other uses (Palmer et al. 1995). 

This report is the first comprehensive vascular plant 

list for the Penobscot Experimental Forest (PEF) in 

Bradley, Penobscot County, Maine. The PEF is a longterm research site of the Northern Research Station of 

the U.S. Department of Agriculture (USDA), Forest 

Service, and is owned by the University of Maine 

Foundation. Ongoing research is conducted jointly and 

separately by the Forest Service and the university. 

The plants have been studied since the 1950s (Kenefic 

et al. 2006), yet plant species mentioned in peerreviewed publications have not been compiled into a 

plant list for the 1,618-ha forest until now. Vegetation 

has been reported especially regarding changes 

in overstory composition and tree regeneration in 

response to silvicultural experiments (Brissette 1996, 

Kenefic et al. 2006). Earliest studies focused entirely 

on valuable timber species, and by the late 1960s, 

105 woody plant species were on a list (Safford et 

al. 1969). Recent studies not only have included 

silvicultural treatments but also have broadened the 

focus, emphasizing the herb layer (Dibble et al. 1999, 

Schofield 2003), epiphytic lichens (Miller et al. 2007, 

2008), and invasive plants (Bryce 2009). Observations 

and surveys apart from the system of Continuous 

Forest Inventory (CFI) plots (also called permanent 

sample plots or PSPs) have included some of the 

roadsides, successional forest, and former agricultural 

land.

Any flora can have significance for conservation 

planning in that emphasis tends to fall on species that 

are seldom collected. Once their rarity is recognized, 

attention might flow toward further understanding 

of habitat requirements for such species, and 

management activities can help assure their continued 

occurrence within an area. However, common and 
104 GTR-NRS-P-123 Penobscot Experimental Forest: 

abundant species could be consequential if they are 

affected by disease or insect attack, with profound 

consequences for ecosystem processes, functions, and 

biodiversity (Ellison et al. 2005). Examples are the 

attack of American chestnut (Castanea dentata) by 

blight (Cryphonectria parasitica [Murrill] Barr), and 

the decimation of eastern hemlock (Tsuga canadensis) 

by hemlock woolly adelgid (Adelges tsugae Annand). 

Checklist and atlas preparation have been developing 

in recent years and numerous new opportunities are 

now available. For example, Allard (2004, updated 

through 2011) continually updates an online statewide 

list of bryophytes of Vermont. This atlas includes 

global rankings and synonyms, with information at 

the level of the township, rather than state or county. 

Ability to update rapidly and to obtain feedback 

increases the utility of the atlas. Internet technology 

allows expanded opportunities for understanding 

species distributions, habitat requirements, and gaps 

in knowledge. Eventually, overlays with forest cover 

type, natural community classification, soils, bedrock, 

drainage, and land use could enable prioritization of 

habitat protection or at least recognition of conditions 

that are conducive to certain rare species. 

At the same time, a push toward standardization of 

floras (Palmer et al. 1995) should help assure that 

the best possible data are reported in a manner that 

allows comparison across regions, continents, or the 

world. Palmer and associates have developed the 

Floras of North America project (Palmer 2013) with 

explorations of ways in which floristic inventories 

can be used across regions. We do not know all the 

uses that future researchers will find for the plant lists 

we prepare today, but those who work on checklists 

and atlases are alert to how easily errors might be 

perpetuated. These errors may occur because of 

(1) misidentifications, (2) duplicate entries that result 

when a species is identified and its genus (typically 

with “sp.” for an undetermined species) is also 

included, or (3) failure to represent nomenclatural 

changes. Despite the many challenges, the preparation 

of a flora is worthwhile for its many uses, not the 

least of which are serving as a hypothesis to test, and 

assigning research priorities. 

The purpose of this report is to establish a baseline 

list of vascular plants for the PEF in the form of a 

checklist. Though lichens and bryophytes have not yet 

been comprehensively surveyed in the PEF, these two 

groups are included as provisional lists. A secondary 

objective is to set the checklist in the context of what 

is known about plant species diversity in Maine. In 

this paper the PEF checklist is related to an ongoing 

effort to standardize floras, and the discussion includes 

a projection of uses for the checklist under several 

scenarios. 

METHODS

All data reported here were collected at the PEF 

(44°49.8' to 44°52.1' N, 68°39.5' to 68°36.2' W) in 

Bradley, Penobscot County, Maine. Since 1994, the 

property has been owned by the University of Maine 

Foundation, Orono, Maine, with its flagship campus 

only 1.6 km away as the crow flies. About 500 ha of 

adjacent properties in the Dwight Demeritt Forest are 

owned by the University of Maine system but are not 

part of the PEF. 

The PEF is in the Penobscot River watershed with 

a primary stream, Blackman Stream, as the major 

drainage. This is a glaciated low-elevation (< 75 m) 

landscape with mostly flat topography, ranging from 

29-77 m, without significant bedrock outcrops and 

containing only a few large glacial erratics. The soils 

are diverse, with an average depth of organic matter at 

<16 cm, over about 50-100 cm of glacial till. Safford 

et al. (1969) summarized the B-horizon as having a 

soil texture that ranges from silt-loam to sandy-loam, 

and drainage characteristics that range from good 

to poor. A cool, humid climate prevails, with mean 

annual temperature of 6.7 ±0.3 °C (±SD, 1971-2000). 

About half the annual precipitation of 1,066 ±137 mm 

falls between May and October, with average annual 

snowfall of 289 ±78 cm (Larouche et al. 2010). The 

growing season is 183 ±15 days (Brissette 1996). 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 105

Dominant vegetation consists of mixed northern 

conifers, and has been described as representative of 

the Acadian Forest (Sendak et al. 2003), an ecotone 

between the conifer-dominated boreal forest and 

the hardwoods prevalent southward. The type is 

characterized especially by red spruce (Picea rubens), 

an economically valuable conifer with low genetic 

variability (Hawley and DeHayes 1994) that is 

common in parts of Maine, New Brunswick, and Nova 

Scotia, with smaller populations in New Hampshire, 

eastern New York, Vermont, high elevations of the 

Appalachians farther to the south, and Quebec, and 

an outlying population in Ontario. With it grow 

balsam fir (Abies balsamea), eastern hemlock (Tsuga 

canadensis), eastern white pine (Pinus strobus), 

and northern white-cedar (Thuja occidentalis). Also 

present but rarely dominant are white spruce (Picea 

glauca), black spruce (P. mariana), tamarack (Larix 

laricina), and red pine (Pinus resinosa). Hardwoods 

include especially red maple (Acer rubrum), paper 

birch (Betula papyrifera), gray birch (B. populifolia), 

quaking aspen (Populus tremuloides), and bigtooth 

aspen (P. grandidentata). Additional hardwoods are 

American beech (Fagus grandifolia), northern red oak 

(Quercus rubra), white ash (Fraxinus americana), and 

sugar maple (Acer saccharum). 

In the Acadian Forest, natural disturbances tend to be 

in the form of small gaps rather than stand-replacing 

events. Fire-return interval and catastrophic windthrow 

events are thought to occur on a cycle of no less than 

800 years, though human disturbance can alter this 

frequency (Seymour et al. 2002). Longevity of red 

spruce, eastern hemlock, and northern white-cedar 

contribute to a stable shade environment unless stands 

are influenced by timber harvest, insect outbreak, or 

similar canopy disturbances. 

Land use at the PEF has consisted of some timber 

harvest since the 1790s, especially near Blackman 

Stream, but not much clearing for agriculture except 

at the west end of the property. The PEF has been the 

site of continuous, ongoing silvicultural treatments 

and monitoring conducted by the U.S. Forest Service, 

Northern Research Station since the 1950s. Repeated 

harvests have been conducted in 10 replicated 

treatments that include even-age and uneven-age 

prescriptions with entries from 5-20 years (Kenefic 

et al. 2006, Safford et al. 1969, Sendak et al. 2003). 

Approximately 580 CFI plots are arranged within the 

treatment compartments on a more-or-less evenly 

distributed pattern that typically avoids the road 

system and wetter areas. Data have been collected 

especially on more productive sites and uplands, 

whereas the wetlands contain fewer plots and have not 

been thoroughly inventoried. 

Valuable knowledge about sustainable forest 

management has been derived from the data 

collected in these experiments, with focus on timber 

management, spruce budworm, coarse woody material, 

economics, biodiversity, growth and yield modeling, 

avian habitats, invasive plants in relation to soil 

properties and silvicultural treatment, and much more. 

Few stands at the PEF are unharvested old growth; 

at one time or another, most or all of the forest has 

been cut. Numerous stump sprout hardwoods and 

cut stumps are in evidence in most stands. In some 

areas, entry might have been more than 100 years 

ago. A reference compartment, which lacks any recent 

harvest, represents baseline conditions, and features 

a hiking trail enjoyed by visitors to the Maine Forest 

and Logging Museum at Leonard’s Mills Historic 

Settlement, which is adjacent to the PEF. A general 

overview of the PEF as managed by the Northern 

Research Station and additional details can be found 

on the U.S. Forest Service’s Web page for the PEF 

(U.S. Forest Service 2012). 

For this report, a list of vascular plant, lichen, and 

bryophyte taxa on the PEF was derived from any 

relevant PEF publications and from some additional 

collecting in 2011. Most plant specimens were 

identified to the nearest species, though for 14 taxa a 

particular subspecies is the one known for this region; 

thus the list includes subspecific taxa. This aspect of 

plant identification was inconsistent between studies, 

and in some cases a taxon is represented at the genus 
106 GTR-NRS-P-123 Penobscot Experimental Forest: 

level only. Nomenclature for vascular plants follows 

the USDA Natural Resources Conservation Service 

(NRCS) database, an atlas of all vascular plants 

and some bryophytes in the United States, which is 

online. The NRCS database is used by the U.S. Forest 

Service’s Forest Inventory and Analysis Program 

and has gained acceptance for many uses, though its 

practicality as a sole nomenclatural source for a flora 

is questionable. Resources such as Haines (2011) that 

have gained popular usage among Maine botanists 

make a nationwide treatment less relevant unless the 

NRCS database reflects recent name changes. 

The papers consulted include, in chronological 

order: Safford et al. 1969, Rinaldi 1970, Dibble et al. 

1999 (including unpublished data for PEF vascular 

plant species that had not occurred with sufficient 

frequency to be included in analyses for the study), 

Schofield 2003, Miller et al. 2007, Miller et al. 

2008, and Bryce 2009. Effort was made to consult 

every written document that contains a plant list, 

including unpublished masters theses that are not in 

peer-reviewed journals. For observations of ferns, 

graminoids, shrubs, subshrubs, vines, and forbs, 

only growing-season data were used. For trees, data 

collected during other times of year were also used. 

Because study objectives and sampling methods 

differed between studies, plant lists are not directly 

comparable. For example, in some studies percentage 

cover of every vascular plant species was included 

(Bryce 2009, Dibble et al. 1999); in another, 

percentage cover of grasses, sedges, and rushes was 

not to the species level (Schofield 2003). The list was 

evaluated for plausibility as some identifications could 

be incorrect. Voucher specimens for questionable 

entries were examined if they were available. 

Nomenclature for species and family names, and 

native status (i.e., plants thought to be native to Maine 

rather than introduced or adventives) follow the NRCS 

database. Each taxon was assigned a growth form, e.g., 

fern (or fern ally), herb, graminoid, shrub, subshrub, 

tree, and vine. No abundance metric was assigned. 

In addition to published reports, the checklist includes 

data from an informal list of lichens that were 

observed by James W. Hinds and Patricia Hinds during 

a field meeting of the Josselyn Botanical Society at 

the PEF in 1994. Nomenclature follows Hinds and 

Hinds (2007). Bryophyte species information came 

from several sources. Some bryophytes were included 

in plot data by Dibble et al. (1999) and Bryce (2009) 

but most of those were at the genus level. Miller et 

al. (2007, 2008) found certain epiphytic bryophytes 

and lichens to be important to invertebrate diversity. 

Otherwise bryophyte and lichen observations have 

been incidental in just a few studies at the PEF. Some 

common species to be expected in such a large coniferdominated area were not yet listed. To increase the 

utility of this paper, I made additional observations in 

2010-11 in three locations: (1) a mature red sprucedominated stand at the Field Demonstration Trail, 

(2) a riparian mature northern white-cedar swamp 

called Dismal Swamp, and (3) low-lying mixed 

conifer forest near the freshwater marsh at Blackman 

Stream. Bryophyte nomenclature follows Allen (2005) 

through the Timmiaceae, and Crosby et al. (1999) for 

additional moss genera. For liverworts, nomenclature 

follows Stotler and Crandall-Stotler (1977).

Additional records, not published, that were 

considered for the plant list include (1) Orono sedge, 

Carex oronensis, which I documented at four locations 

in the PEF; (2) slippery elm, Ulmus rubra, determined 

by field crew during data collection for the Forest 

Inventory and Analysis Program in the 1990s, but not 

vouchered or confirmed, and otherwise undocumented 

in Maine since 1935; and (3) purple clematis, Clematis 

occidentalis, which is State Special Concern (Maine 

Department of Agriculture, Conservation, and Forestry 

2010).

To understand whether species richness is high or low, 

species richness at the PEF (minus 34 questionable 

taxa) was compared to that for several other areas 

in Maine that are of a relevant size and occupied 

almost entirely by forest and wetlands. The other sites 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 107

were: Massabesic Experimental Forest in Alfred and 

Lyman, York County (Dibble et al. 2004); Great Pond 

Mountain Wildlands in Orland, Hancock County; and 

Coastal Mountains Land Trust properties at Bald and 

Ragged Mountains, Camden, Knox County. A very 

well documented land holding, Acadia National Park 

with headquarters in Bar Harbor, Maine, was used 

as a far outlier in this comparison because the flora 

has been recently updated (Mittelhauser et al. 2010), 

and because bryophytes and lichens are especially 

well documented there. All these other areas are not 

necessarily similar to the PEF in terms of elevation; 

topography; soils; proximity to major water bodies, 

including the Atlantic Ocean; or forest management. 

Plant lists for two of the sites1

 are contained in inhouse natural resource inventory reports prepared 

for land trusts, and are used in development of 

management plans.

As part of the effort to standardize checklists 

worldwide, this report was contrasted with Palmer 

et al. (1995) and with a list of desired components 

for floras, which is under development (M. Palmer, 

Oklahoma State University, pers. communication). 

The PEF checklist of vascular plants reported here is 

in compliance with Palmer’s working list of features 

so that it could be referred to as an example in the 

standardization of florae and to assure best utility 

in the future.

RESULTS

More than 300 vascular plant taxa in 71 families and 

186 genera were considered appropriate for the PEF 

checklist (Appendix I), of which 45 species (about 

15 percent) are not native to Maine. The list contains 

five genera for which “sp.” is given, meaning that 

a species was not determined but, in my opinion, 

is likely to be other than those listed. Ideally the 

list would be fully resolved to infraspecific taxa; it 

includes 14 subspecific taxa but for some species 

it was not possible to resolve further. Vouchers 

are available for many of these taxa, but not all; 

collections by Olson et al. (2011), which were 

examined for this report, are especially useful in 

documenting the flora. Most are deposited at the 

Hart Building on the PEF, and unusual species were 

deposited at the University of Maine Herbarium in 

Orono. Families that are especially well-represented 

are the Asteraceae, Rosaceae, Cyperaceae, and 

Caprifoliaceae (Appendix II, based on NRCS 

designations). Perennials consisting of forbs, 

graminoids, shrubs, and trees were the majority 

of growth forms, with fewer ferns and fern allies, 

subshrubs, and only a few vines. Two rare plants, 

Carex oronensis (Fig. 1) and Clematis occidentalis, 

have been documented. Ten of the 45 nonnative plants 

are considered invasive or potentially so according to 

an unpublished list kept by the Maine Natural Areas 

Program: Berberis thunbergii, Celastrus orbiculatus, 

Euonymus alata, Frangula alnifolia (Fig. 2), Lonicera 

morrowii, Lonicera xylosteum, Lythrum salicaria, Poa 

nemoralis, Rosa multiflora, and Rhamnus cathartica. 

Several other nonnative species appear to persist 

and spread at the PEF under closed canopies or in 

openings, and might be considered invasive where 

they outcompete native vegetation, e.g., Epipactis 

helleborine, Hylotelephium telephium, Solanum 

dulcamara, Valeriana officinalis, and Veronica 

officinalis. Omitted taxa and unresolved genera are 

shown in Appendix III. They either are unlikely in 

southern Penobscot County and are thought to be 

misidentifications (e.g., Krigia virginiana, Rosa 

1

 Dibble, A.C.; Rees, C.A. 2006. Great Pond Mountain 

Wildlands Natural Resource Inventory. Proprietary 

document held by the Great Pond Mountain Wildlands 

Trust. 

Dibble, A.C. 2005. Ecological inventory of Bald Mountain 

Preserve, Camden, ME. Addendum 2008, in cooperation 

with C.A. Rees. Proprietary document held by the Coastal 

Mountains Land Trust.

Dibble, A.C. 2007. Ecological inventory of Ragged 

Mountain Preserve, Camden, ME. Addendum 2008, in 

cooperation with C.A. Rees. Proprietary document held by 

the Coastal Mountains Land Trust.
108 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 2.—Frangula alnifolia, glossy buckthorn. This shadetolerant tall shrub is spreading in the PEF in part because 

birds eat the fruits in autumn and spread them ever deeper 

into the forest. (Photo courtesy of A.C. Dibble.)

Figure 1.—Carex oronensis, Orono sedge. It is known from 

several small populations in the PEF and at Leonard’s Mills. 

(Photo courtesy of A.C. Dibble.)

johannensis) or are believed to overlap known species 

(e.g., “Salix sp.”); voucher specimens could not be 

found to check these. 

Plant name changes make preparation of a checklist 

more complicated. Of the taxa in Appendix I, revisions 

in Haines (2011) have led to 34 changes in family 

designation, compared to the NRCS database. For 

32 taxa, species became recognized at the subspecific 

level because that subspecies is the only one known 

in Maine. For 16 taxa, genus has changed, and 

these are likely to present particular challenge as 

some are common and likely in many parts of the 

PEF, such as northern starflower, Trientalis borealis

(now Lysimachia borealis), and bunchberry, Cornus 

canadensis (now Chamaepericlymenum canadense). 

Five taxa had a change in specific epithet, and there 

were numerous changes in naming authority, though 

some are slight.

Most species on the list are common and widespread in 

Maine and elsewhere in northeastern North America. 

Some are shade-associated, and are not usually 

abundant in forest openings; examples are Goodyera 

repens (Fig. 3), Mitchella repens, Monotropa uniflora, 

Moneses uniflora, Oxalis montana, and Trillium 

undulatum (Fig. 4). Their presence in the silvicultural 

treatment at the PEF suggests their resilience to 

canopy disturbance.

Plants listed as rare in Maine are not frequent or 

abundant in the PEF, but I documented one, Carex 

oronensis (Orono sedge) (Dibble and Campbell 

2001), state threatened, at two widely separated sites 

on the forest (at Leonard’s Mills and on a roadside at 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 109

Figure 4.—Trillium undulatum, painted trillium. It can tolerate 

the low shade under balsam fir and other conifers, where few 

other vascular plants thrive. (Photo courtesy of A.C. Dibble.)

Figure 3.—Goodyera repens, lesser rattlesnake plantain. 

This native terrestrial orchid is shade adapted and with 

potential as an indicator of closed-canopy conifer stands. 

(Photo courtesy of A.C. Dibble.)

Compartment 10) in 1991. In 2011, I found another 

subpopulation along a woods road. Another listed rare 

plant is Clematis occidentalis (western virginsbower), 

State Special Concern, documented by Molly 

Schauffler near the beaver dam at Compartment 26.

Some plants are unusual in Maine, though not 

yet on a state rare plant list. An example is ditch 

stonecrop (Penthorum sedoides), which occurs in 

sandy oxbows along small rivers. Its presence at the 

PEF is noteworthy because habitat was not typical, 

perhaps reflecting a general lack of knowledge about 

this undercollected plant, and not necessarily a status 

as rare. There was no reference specimen at the 

University of Maine Herbarium until recently when 

the gap was noticed, an omission that might indicate 

the plant is infrequent and local.

Sixty-two lichens (macrolichens and crustose lichens) 

have been documented in the PEF (Appendix IV) from 

published lists; in-house lists; recent, brief surveys 

at three sites in the PEF; and other sources. None of 

the lichens is rare or highly unusual. Nine liverworts 

species and 40 mosses were found (Appendix V). 

Again, none is considered rare.

DISCUSSION

Vascular plant species richness is not particularly 

high for this size area of forested land in Maine, and 

is at least 13.5 percent lower than for the other areas 

compared in Appendix VI. Only 16.5 percent of the 

total number of 2,103 vascular plant taxa recorded 

in Maine (Campbell et al. 1995) are documented on 

the PEF. Characteristic of the shady understory in 

spruce-fir forest types, low species richness is due 

in part to thin, acid soils; acidifying needle litter; 

and “low shade,” in which the conifer lower canopy 

excludes direct sunlight except for brief exposure to 

sun flecks. The proportion of the light spectrum in 

red : far red light is important for seed germination of 

forest plants (Jankowska-Blaszczuk and Daws 2007). 

Because this proportion differs between coniferous and 

deciduous canopies, growing conditions might be poor 

under spruce-fir and hemlock for otherwise common 

understory herbs and shrubs.

The percentage of the PEF flora comprising nonnative 

plants is lower than the overall percentage in Maine, 

which has 634 naturalized vascular plant species. 

At the PEF, 42 naturalized species have been found, 
110 GTR-NRS-P-123 Penobscot Experimental Forest: 

representing 12.1 percent of the flora, whereas in the 

entire state, 30.1 percent are nonnative. The percentage 

at the PEF is not particularly low for small florae 

(Palmer, pers. communication). 

The changes in focus over time for observations at 

the PEF are reflected in the checklist. Earliest studies 

focused on the trees valuable for timber; then shrubs 

were included in the list of Safford et al. (1969). 

Rinaldi (1970) quantified trees, shrubs and herbs and 

the latter were in broad groups, not to species. In the 

early 1990s, I included percentage cover estimates 

for all vascular plant species and some bryophytes 

and lichens in a study of red spruce regeneration 

habitat that included plots in the PEF, but species 

with low frequency were dropped for analyses, and 

a complete list for the PEF was not published. The 

most comprehensive plant list for the PEF was Bryce 

(2009), who found 234 plant species on CFI plots. 

That total includes some entities identified to genus 

only, with possible overlap for entities identified 

to species. Abundance data are available on plots 

and as a frequency of measured plots. Common 

lichens and bryophytes were included in that study, 

but were a low priority with 13 genera and only 

5 identified to species. I added 43 moss species based 

on observations at two sites in autumn 2010 and spring 

2011. Schofield’s (2003) list was for the Acadian 

Forest Ecosystem Research Program section of the 

PEF and is not as comparable to the other lists, though 

it contains many similarities, especially for woody 

plants. 

There are numerous sources of error in the exercise 

of preparing plant checklists. Selection of taxa 

for inclusion in the list is somewhat arbitrary. The 

list reported here could be improved if a group of 

botanists familiar with the flora of southern Penobscot 

County and the entire state were to go through the 

list line by line and reach a consensus about what 

must be excluded, but in this report, only one botanist 

made those choices. Standard methods for treating 

questionable species have not yet been adopted. 

In the PEF checklist, excluded taxa had a variety of 

problems that led to their removal. If a genus was 

already represented in the list by one or more likely 

species, then it seemed that duplication would result 

by also listing the genus with no specific epithet 

(e.g., Amelanchier sp.). 

Misidentifications were apparent—Safford et 

al. (1969) identified a plant as wickopee, Dirca 

palustris (Fig. 5), but the voucher specimen housed 

at the PEF is Viburnum. In recent years, Olson 

spotted D. palustris on the forest at the PEF, and 

verified it through use of a photo; this species is 

included in Appendix I. A few of Schofield’s (2003) 

determinations were omitted due to extreme rarity in 

Maine, out of known range, inappropriate habitat, lack 

of confirmation because no voucher could be located, 

or a combination of these reasons. Examples are: 

Asplenium sp., Corylus americana, Cystopteris sp., 

Krigia virginiana, and Rosa johannensis. 

Another challenge is plant name changes, which 

can be confusing—some names change and then 

change back again to the original name. Even more 

problematic is that name changes in taxon concepts or 

taxon ranks—such as when a subspecies is elevated 

to full species status—can generate complications 

when subspecies and varieties are lumped together 

(Palmer, pers. communication). Most of these entities 

Figure 5.—Dirca palustris, wickopee. Known at the PEF 

from a single small plant, it flowers in early spring. (Photo 

courtesy of A.C. Dibble.)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 111

can be updated and cross-referenced with powerful 

and widely available Web tools, but the NRCS plant 

database lags behind important taxonomic treatments 

including Haines (2011). There is wide expectation 

that Haines (2011) will serve as the standard for field 

botanists in New England, and eventually the NRCS 

plant database could adopt plant names that are likely 

to be in common usage. Further, the NRCS plant 

database contains at least a few subtle errors that could 

influence a plant checklist project, e.g., an erroneous 

name for Carex foenea, which has become confused 

with C. siccata. The NRCS currently includes only 

a few lichens and bryophytes, so its full utility for 

those groups is not realized. Haines (2011) includes 

only vascular plants. Additional complications in the 

PEF checklist can arise through published errors; for 

example, Miller et al. (2008) referred inadvertently 

to three species prominent in their study of arboreal 

arthropod diversity as “bryophytes,” but those are 

epiphytic lichens. 

The PEF checklist matches Palmer et al. (1995) 

regarding the recommended standards in most ways. 

The list is presented by genus, and other components 

such as elevational range are included. The PEF 

list departs in that precision of location data is 

not to standard. If latitude and longitude could be 

obtained for every population of each taxon, then 

relative abundance could be derived. This level of 

information might be prohibitive, even in a wellstudied forest area. Bryce (2009) calculated species 

relative abundance based on frequency of each taxon 

in her study plots, providing a start toward finding 

associations between certain plant species and 

overstory conditions, soils, and other environmental 

variables. 

Ways in Which the PEF Checklist 

Can Be Improved

Researchers can approach this checklist in several 

ways to identify gaps in our knowledge, and in doing 

so, can expand and improve the checklist itself. Not 

in any particular order by priority, these approaches 

include: 

1. Further document the weedy plants of roadsides 

and log landings. Disturbed areas should be 

checked for invasive plants on a regular basis 

because of the threat such plants place upon the 

long-term silvicultural experiments if they are 

not controlled. 

2. Survey the recently harvested 1,200 ha that are 

adjacent to the PEF. The parcel was recently 

added to ownership by the University of Maine 

System. Additional plant species are likely for 

the checklist. 

3. Monitor known rare plant populations in the 

PEF periodically, perhaps every 5 years, in 

conformity with the New England Wild Flower 

Society and Maine Natural Areas Program 

reporting protocols. 

4. Seek and document common plants of spruce-fir 

forests in Maine that are not yet on the checklist, 

such as: Dulichium arundinaceum, Equisetum 

sylvaticum, Glyceria canadensis, Monotropa 

hypopithys, Osmunda regalis, Vaccinium vitisidaea var. minus, and Viola macloskeyi ssp. 

pallens. 

5. Seek and document common nonnative 

plants, including Rumex acetosella, Trifolium 

pratensis, Phalaris arundinacea, and Festuca 

filiformis. These species might be present but 

were not found on plots. Because plots tended 

to be on better-drained soils, weedy plants of 

disturbed ditches might be underrepresented. 

Or there could be worker bias in that graminoid 

identification requires training and experience, 

is time-consuming, and might not be pertinent to 

project goals in some forest studies.

6. Many fern, graminoid, and other plant species 

can be resolved to species or subspecies only 

when their spore-bearing structures, flowers, or 

fruits are present. For the sake of best-quality 

data, and mindful of budget constraints, efforts 

should be made to verify questionable species 

wherever possible by returning to a plant 

population later in the season and pressing a 

voucher specimen. 
112 GTR-NRS-P-123 Penobscot Experimental Forest: 

7. Survey habitats that are underrepresented in 

Appendix I such as wetter areas that have not 

been actively managed for timber. Riparian 

zones, forested wetlands, swales, and boggy 

areas have not yet been investigated beyond 

walk-throughs between plots and a few casual 

visits by botanists and other researchers. 

Bryophytes are not well-inventoried in any of 

the habitats and should be sampled as part of a 

rigorous inventory (see Newmaster et al. 2005).

8. Survey taxonomic groups that are 

underrepresented in PEF research, including 

the lichens, bryophytes, and fungi. Of these the 

crustose lichens and liverworts need particular 

attention to make the list more representative of 

the flora and thus more useful. Crustose lichens 

were a major influence on the lichen checklist 

for Katahdin in Baxter State Park (Dibble et al. 

2009, Hinds et al. 2009). At Acadia National 

Park, Sullivan (1996) found that more than half 

of the lichen diversity was in crustose lichens 

(198 of 379 species, Appendix VI). Although 

the PEF has hosted mycological field meetings, 

no list of fungi could be found for this report. 

I suggest that a requirement for use of the PEF 

as a field trip site for any botanical organization 

should be the understanding that species lists 

will be presented to the University of Maine and 

U.S. Forest Service.

9. Give particular thought to relative abundance 

when designing studies. An abundance rank 

for each species would be possible for many 

species in Appendix I, especially trees using the 

PEF plot data, and for many understory plants 

using Bryce’s thesis data (2009), but the actual 

abundance on the forest might not be accurate 

based on purposes for which the sampling was 

designed. Data collected on plots do not always 

represent actual abundance in the area. Relative 

abundance is of sufficient importance to warrant 

a thoughtful approach in other studies.

10. Seek the “lost” species. A few species were 

reported by Safford et al. (1969) and have 

not been documented since, including Acer 

saccharinum, Andromeda glaucophylla, 

Arceuthobium pusillum, Aronia melanocarpa, 

and Cephalanthus occidentalis. For each of 

these, relocation of a population seems likely. 

At Acadia National Park, 200 of the total 862 

species known for the park have not been seen 

for more than 20 years, such as numerous orchid 

species. This apparent loss could reflect change 

in land use, overcollection, or other factors 

(Greene et al. 2005, Mittelhauser et al. 2010).

On the other hand, Safford et al. (1969) featured 

plants such as Frangula alnifolia and Rosa 

multiflora, which are widely recognized now 

as invasive. They did not mention whether 

they considered them invasive. They did not 

list Oriental bittersweet, Celastrus orbiculatus, 

which can now be found in numerous places on 

the PEF, suggesting that it is a recent arrival. 

Because Oriental bittersweet spreads rapidly due 

to bird dispersal of the fruits, this invasive vine 

should be given priority in management of the 

PEF. Oriental bittersweet, perhaps more than 

most of the other invasive plants present, could 

impact forest regeneration on study plots in the 

silvicultural treatments.

11. Prepare vegetation maps for the PEF to include 

recently described natural communities of 

Gawler and Cutko (2010). Although forest 

types as categorized by the Society of 

American Foresters (SAF) and other vegetation 

classification schemes have been assigned to 

some of the vegetation in the PEF, especially 

regarding the silvicultural treatments, there is 

not yet a complete map of vegetation at the 

PEF. Broad forest types might not be sufficient 

to understand habitat requirements of certain 

plants of interest. Types assigned by timber 

stocking conditions might be used as a surrogate 

for canopy closure, which could be helpful in 

study of the shade-associated understory plants 

such as Goodyera repens (Fig. 3) and Trillium 

undulatum (Fig. 4). Bryce (2009) measured 

canopy closure on a subset of her plots and 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 113

found that species that had been shown in other 

studies to frequent shady understory conditions 

did not always do so at the PEF, so other factors 

could be involved in their distribution.

Plant checklists for land trusts are sometimes 

prepared by habitat or community type, and 

such an approach at the PEF would require 

some careful investigation for many of the plant 

species, to establish their plant associations and 

see how the natural community descriptions 

depart from what is actually found on the 

property. Natural communities as described by 

Gawler and Cutko (2010) in coordination with 

NatureServe have not yet been applied to the 

vegetation at the PEF, but some stands could 

be considered for possible classification as the 

spruce-pine woodland (state rank S4), sprucenorthern hardwoods forest (S5), lower elevation 

spruce-fir forest (S5), hemlock forest (S4), early 

successional forest (S5), with small patches of 

black spruce woodland (S2) or black spruce 

bog (S4), red maple swamp (S4), and northern 

white-cedar swamp (S4). Such community 

designations might be at a finer scale than the 

SAF forest types, and a plant checklist could 

eventually be prepared to reflect those natural 

communities. A purpose for such an exercise 

would be to recognize plant species that occur 

in only one or a few such communities; then 

management of the overstory might differ from 

what is otherwise being done. It should be noted 

that the northern white-cedar swamp at Dismal 

Swamp has not had any obvious recent harvest 

and apparently is outside of the CFI plot system. 

Cedar regeneration has been studied recently by 

Larouche et al. (2010) using data from the PEF, 

but not from Dismal Swamp.

12. To improve data quality in general, all studies 

in the PEF should include voucher specimens, 

particularly for any woody species not yet in 

Appendix I, and for herbs, grasses, sedges, and 

rushes; and lichens, mosses, and liverworts. 

If a plant is present in sufficient abundance, 

two specimens should be collected, one for 

the University of Maine Herbarium, where 

specimens can be examined if any questions 

arise, or for further study, and the other 

specimen for retention at the Hart Building on 

the PEF for handy access by field crews. This 

procedure would increase the utility of the 

specimens, but might involve administrative 

prioritization because a curation of vouchers 

takes up space, requires some preparation, 

and needs some maintenance over time. A 

maintenance schedule and curation protocols 

should be implemented at the PEF because 

even though the number of specimens is small 

compared to the Herbarium’s collection, the 

voucher specimens are of untold importance 

for future studies, and are vulnerable to insect 

attack, mold, and other damage.

Future of the PEF Checklist

The usefulness of a plant checklist is only partly 

known. The PEF checklist might become incorporated 

into a larger study with many other checklists from 

other areas (see Palmer et al. 1995). There could 

be vast changes to the PEF that would make this 

checklist a vital record by which to compare to future 

conditions. For instance, climate change could bring 

about disruption to the canopy due to increased 

intensity and frequency of storms, and spread of 

nonnative insect pests (e.g., balsam woolly adelgid, 

Adelges piceae) as minimum temperatures in winter 

are elevated. With increased canopy opening—apart 

from harvest activities related to ongoing experiments 

at the PEF—climate change could be accompanied by 

the accelerated spread of invasive plants and native 

ruderal plant species. Some of these plants might 

interfere with regeneration of desirable tree species. 

Increased shrub and graminoid cover might alter fuel 

characteristics in the PEF (Dibble and Rees 2005); 

in turn, these changes in fuel could affect fire-return 

interval and intensity of burns (Dibble et al. 2008). 

Presence of invasive plants might also alter fuels; plant 

species of northeastern North America differ in their 

combustion properties and some invasive plants are 

more flammable than their native counterparts (Dibble 
114 GTR-NRS-P-123 Penobscot Experimental Forest: 

et al. 2007). By comparing the number of more 

flammable species in a checklist to those thought to be 

relatively unflammable, differences in the fuels might 

be assessed. 

Many changes are likely to be made to the PEF 

checklist in coming years. Like any snapshot of data, 

a presence-absence checklist is not a true reflection of 

the vegetation so much as a tool by which workers can 

know whether they are within the realm of possibility 

as they identify plants they have found on the forest. 

Toward that end, this checklist will be especially 

useful.

ACKNOWLEDGMENTS

Thank you to John Brissette and Laura Kenefic 

for inviting me to write this paper and for helpful 

comments on earlier versions. Elizabeth Bryce Olson 

provided the plant list resulting from her thesis and 

made available voucher specimens for verification. 

Her publications are shown below under Bryce and 

also under Olson. Arthur Haines identified some 

specimens for E. Olson’s thesis. L. Kenefic and J. 

Brissette, with assistance from John Misener, helped 

in assuring that no studies were missed. Jim Hinds 

verified several lichen specimens, and Marnie Reeve 

refined identification of Sphagnum species. Reviews 

of an earlier version by Michael Palmer and Dorothy 

Allard led to substantive improvements in the paper. 

The Josselyn Botanical Society, especially Jim 

and Patricia Hinds, were helpful with lichen lists. 

Rick Dionne made available maps and records that 

contributed to this paper. Thank you to Robert Frank 

and Robert Seymour for introducing me to the PEF 

in 1993.

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60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 117

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Alismataceae Sagittaria latifolia Willd. forb 1

Anacardiaceae Rhus typhina L. Rhus hirta (L.) Sudworth shrub 1

Anacardiaceae Toxicodendron radicans (L.) 

Kuntze

subshrub 1

Apiaceae Hydrocotyle americana L. forb 1

Apiaceae Sium suave Walter forb 1

Aquifoliaceae Ilex mucronata (L.) 

Powell, Savolainen & Andrews 

shrub 1

Aquifoliaceae Ilex verticillata (L.) A. Gray shrub 1

Araceae Arisaema triphyllum (L.) Schott forb 1

Araceae Calla palustris L. forb 1

Araliaceae Aralia hispida Vent. ( Apiaceae) subshrub 1

Araliaceae Aralia nudicaulis L. ( Apiaceae) subshrub 1

Araliaceae Aralia racemosa L. Aralia racemosa L. ssp. racemosa 

( Apiaceae)

shrub 1

Araliaceae Aralia spinosa L. ( Apiaceae) shrub 1

Asteraceae Achillea millefolium L.* Achillea millefolium L. ssp. lanulosa

(Nutt.) Piper

forb

Asteraceae Anaphalis margaritacea (L.) 

Benth. & Hook.*

forb

Asteraceae Doellingeria umbellata (Mill.) 

Nees

forb 1

Asteraceae Erechtites hieraciifolia (L.) 

Raf. ex DC. 

Erechtites hieraciifolius (L.) 

Raf. ex DC. var. hieraciifolius

forb 1

Asteraceae Eurybia macrophylla L. forb 1

Asteraceae Eurybia radula (Aiton) 

G.L. Nesom

forb 1

Asteraceae Euthamia graminifolia (L.) Nutt. forb 1

Asteraceae Hieracium aurantiacum L.* forb

Asteraceae Hieracium caespitosum

Dumort.*

forb

Asteraceae Hieracium lachenalii 

C. C. Gmel.*

forb

(Appendix I continued on next page)

Appendix I.

Checklist of the vascular plants of the Penobscot Experimental Forest, Bradley, Maine, at the level of species, 

with family name, growth form, status as nonnative ( = *) or nonnative invasive ( = **). Nomenclature follows 

that used in the NRCS database (NRCS 2013). Changes in family (“”), genus, species, or subspecies in Haines 

(2011) are shown.
118 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Asteraceae Hieracium pilosella L.* forb

Asteraceae Hieracium piloselloides Vill.* forb

Asteraceae Lactua canadensis L. forb 1

Asteraceae Lapsana communis L.* forb

Asteraceae Leontodon autumnalis L.* Scorzoneroides autumnalis (L.) Moench forb

Asteraceae Oclemena acuminata (Michx.) 

Greene

forb 1

Asteraceae Petasites frigidus (L.) Fr. Petasites frigidus (L.) Fries 

var. palmatus (Ait.) Cronq.

forb 1

Asteraceae Solidago altissima L. Solidago altissima L. ssp. altissima forb 1

Asteraceae Solidago canadensis L. forb 1

Asteraceae Solidago gigantea Ait. forb 1

Asteraceae Solidago hispida Mulh. ex Willd. Solidago hispida Mulh. ex Willd. 

var. hispida

forb 1

Asteraceae Solidago juncea Ait. forb 1

Asteraceae Solidago nemoralis Ait. Solidago nemoralis Ait. var. nemoralis forb 1

Asteraceae Solidago puberula Nutt. Solidago puberula Nutt. var. puberula forb 1

Asteraceae Solidago rugosa Mill. forb 1

Asteraceae Symphyotrichum ciliolatum

(Lindl.) A. Löve & D. Löve

forb 1

Asteraceae Symphyotrichum lateriflorum

(L.) A. Löve & D. Löve

forb 1

Asteraceae Symphyotrichum novi-belgii (L.) 

G.L. Nesom

forb 1

Asteraceae Symphyotrichum puniceum (L.) 

A. Löve & D. Löve

forb 1

Asteraceae Symphyotrichum racemosum

(Eliott) G.L. Nesom

forb 1

Asteraceae Taraxacum officinale 

F.H. Wigg.*

Taraxacum officinale 

G.H. Weber ex Wiggers*

forb

Balsaminaceae Impatiens capensis Meerb. forb 1

Berberidaceae Berberis thunbergii DC.** shrub

Betulaceae Alnus incana (L.) Moench ssp. 

rugosa (Du Roi) R.T. Clausen

shrub 1

Betulaceae Betula alleghaniensis Briton tree 1

Betulaceae Betula papyrifera Marsh. tree 1

Betulaceae Betula populifolia Marsh. tree 1

(Appendix I continued on next page)

Appendix I (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 119

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Betulaceae Corylus americana Walter shrub 1

Betulaceae Corylus cornuta Marsh. Corylus cornuta Marsh. ssp. cornuta shrub 1

Betulaceae Ostrya virginiana (Mill.) K. Koch tree 1

Brassicaceae Erysimum cheiranthoides L.* forb

Callitrichaceae Callitriche palustris L. ( Plantaginaceae) forb 1

Campanulaceae Lobelia inflata L. forb 1

Caprifoliaceae Diervilla lonicera Mill. shrub 1

Caprifoliaceae Linnaea borealis ssp. longiflora

(Torr.) Hulten

Linnaea borealis L. ssp. americana

(Forbes) Hultén ex Clausen

subshrub 1

Caprifoliaceae Lonicera × bella Zabel 

[morrowii × tatarica]**

shrub

Caprifoliaceae Lonicera canadensis Bartram 

ex Marsh.

shrub 1

Caprifoliaceae Lonicera morrowii A. Gray** shrub

Caprifoliaceae Lonicera villosa (Michx.) Schult. shrub 1

Caprifoliaceae Lonicera xylosteum L.** shrub

Caprifoliaceae Sambucus nigra L. 

ssp. canadensis (L.) R. Bolli 

( Adoxaceae) shrub 1

Caprifoliaceae Sambucus racemosa L. var. 

racemosa

Sambucus racemosa L. 

( Adoxaceae)

shrub 1

Caprifoliaceae Viburnum acerifolium L. ( Adoxaceae) shrub 1

Caprifoliaceae Viburnum dentatum L. ( Adoxaceae) shrub 1

Caprifoliaceae Viburnum lentago L. ( Adoxaceae) shrub 1

Caprifoliaceae Viburnum nudum var. 

cassinoides (L.) Torr. & A. Gray

( Adoxaceae) shrub 1

Caprifoliaceae Viburnum opulus var. opulus L.* Viburnum opulus ssp. opulus L.* 

( Adoxaceae)

shrub

Caryophyllaceae Moehringia lateriflora (L.) Fenzl forb 1

Celastraceae Celastrus orbiculatus Thunb.** vine

Celastraceae Euonymus alata (Thunb.) 

Siebold**

Euonymus alatus (Thunb.) Siebold** shrub

Convolvulaceae Calystegia sepium L. forb 1

Cornaceae Cornus alternifolia L. f. Swida alternifolia (L. f.) Small shrub 1

Cornaceae Cornus amomum P. Mill. 

ssp. amomum

Swida amomum (P. Mill.) Small shrub 1

Cornaceae Cornus canadensis L. Chamaepericlymenum canadense (L.) 

Aschers. & Graebn.

subshrub 1

(Appendix I continued on next page)

Appendix I (continued)
120 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Cornaceae Cornus rugosa Lam. Swida rugosa (Lam.) Rydb. shrub 1

Cornaceae Cornus sericea L. Swida sericea (L.) Holub shrub 1

Crassulaceae Hylotelephium telephium (L.) 

H. Ohba ssp. telephium*

forb

Crassulaceae Penthorum sedoides L. ( Penthoraceae) forb 1

Cucurbitaceae Echinocystis lobata (Michx.) 

Torr. & A. Gray 

vine 1

Cupressaceae Juniperus communis L. 

var.depressa Pursh

shrub 1

Cupressaceae Thuja occidentalis L. tree 1

Cyperaceae Carex arctata Boott ex Hook. gramin 1

Cyperaceae Carex bromoides Schkuhr ex 

Willd.

Carex bromoides Schkuhr ex Willd. 

ssp. bromoides

gramin 1

Cyperaceae Carex brunnescens (Pers.) Poir. gramin 1

Cyperaceae Carex communis L.H. Bailey Carex communis Bailey var. communis gramin 1

Cyperaceae Carex debilis Michx. gramin 1

Cyperaceae Carex deflexa Horem. Carex deflexa Hornem. var. deflexa gramin 1

Cyperaceae Carex deweyana Schwein. Carex deweyana Schwein. 

var. deweyana

gramin 1

Cyperaceae Carex disperma Dewey gramin 1

Cyperaceae Carex gracillima Schwein. gramin 1

Cyperaceae Carex gynandra Schwein. gramin 1

Cyperaceae Carex intumescens Rudge gramin 1

Cyperaceae Carex lacustris Willd. gramin 1

Cyperaceae Carex leptalea Wahlenb. Carex leptalea Wahlenb. ssp. leptalea gramin 1

Cyperaceae Carex leptonervia (Fernald) 

Fernald

gramin 1

Cyperaceae Carex lucorum Willd. ex Link Carex lucorum Willd. ex Link 

ssp. lucorum

gramin 1

Cyperaceae Carex lurida Wahlenb. gramin 1

Cyperaceae Carex normalis Mack. gramin 1

Cyperaceae Carex oronensis Fernald gramin 1

Cyperaceae Carex projecta Mackenzie gramin 1

Cyperaceae Carex scoparia Schkuhr 

ex Willd.

gramin 1

Cyperaceae Carex stipata Muhl. ex Willd. Carex stipata Muhl. ex Willd. var. stipata gramin 1

(Appendix I continued on next page)

Appendix I (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 121

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Cyperaceae Carex tenera Dewey gramin 1

Cyperaceae Carex tribuloides Wahlenb. Carex tribuloides Wahlenb. 

var. tribuloides

gramin 1

Cyperaceae Carex trisperma Dewey gramin 1

Cyperaceae Scirpus cyperinus (L.) Kunth gramin 1

Cyperaceae Scirpus hattorianus Makino gramin 1

Dennstaedtiaceae Dennstaedtia punctilobula

(Michx.) T. Moore

fern 1

Dennstaedtiaceae Pteridium aquilinum (L.) 

Kuhn var. latiusculum (Desv.) 

Underw. ex A. Heller 

Pteridium aquilinum (L.) Kuhn 

ssp. latiusculum (Desv.) Hultén

fern 1

Dryopteridaceae Athyrium filix-femina (L.) Roth. Athyrium angustum (Willd.) C. Presl. 

( Woodsiaceae)

fern 1

Dryopteridaceae Dryopteris campyloptera

Clarkson

Dryopteris campyloptera (Kunze) 

Clarkson

fern 1

Dryopteridaceae Dryopteris carthusiana (Vill.) 

H. P. Fuchs

fern 1

Dryopteridaceae Dryopteris clintoniana 

(D.C. Eaton) Dowell 

fern 1

Dryopteridaceae Dryopteris cristata (L.) A. Gray fern 1

Dryopteridaceae Dryopteris intermedia (Mulh. ex 

Willd.) Gray

fern 1

Dryopteridaceae Dryopteris marginalis (L.) 

A. Gray

fern 1

Dryopteridaceae Gymnocarpium dryopteris (L.) 

Newman

( Woodsiaceae) fern 1

Dryopteridaceae Onoclea sensibilis L. ( Onocleaceae) fern 1

Dryopteridaceae Polystichum acrostichoides

(Michx.) Schott

fern 1

Equisetaceae Equisetum arvense L. fern 1

Equisetaceae Equisetum pratense Ehrh. fern 1

Ericaceae Andromeda polifolia L. 

var. glaucophylla (Link) DC. 

shrub 1

Ericaceae Chamaedaphne calyculata (L.) 

Moench

shrub 1

Ericaceae Epigaea repens L. subshrub 1

Ericaceae Gaultheria hispidula (L.) 

Muhl. ex Bigelow

subshrub 1

Ericaceae Gaultheria procumbens L. subshrub 1

(Appendix I continued on next page)

Appendix I (continued)
122 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Ericaceae Gaylussacia baccata

(Wangenh.) L. Koch

Gaylussacia baccata (Wangenh.) 

K. Koch

shrub 1

Ericaceae Kalmia angustifolia L. Kalmia angustifolia L. ssp. angustifolia shrub 1

Ericaceae Ledum groenlandicum Oeder. Rhododendron groenlandicum (Oeder) 

Kron & Judd

shrub 1

Ericaceae Rhododendron canadense (L.) 

Torr.

shrub 1

Ericaceae Vaccinium angustifolium Ait. shrub 1

Ericaceae Vaccinium corymbosum L. shrub 1

Ericaceae Vaccinium macrocarpon Ait. shrub 1

Ericaceae Vaccinium myrtilloides Michx. shrub 1

Ericaceae Vaccinium oxycoccos L. shrub 1

Euphorbiaceae Euphorbia sp.* forb

Fabaceae Lotus corniculatus L.* forb

Fabaceae Trifolium hybridum L.* forb

Fabaceae Trifolium repens L.* forb

Fabaceae Vicia cracca L.* Vicia cracca L. ssp. cracca forb

Fabaceae Vicia tetrasperma (L.) Schreb.* forb

Fagaceae Fagus grandifolia Ehrh. tree 1

Fagaceae Quercus rubra L. tree 1

Geraniaceae Geranium sp. forb 1

Grossulariaceae Ribes hirtellum Michx. shrub 1

Grossulariaceae Ribes lacustre (Pers.) Poir. shrub 1

Hamamelidaceae Hamamelis virginiana L. shrub 1

Iridaceae Iris versicolor L. forb 1

Juncaceae Juncus effusus L. gramin 1

Juncaceae Juncus sp. gramin 1

Juncaceae Juncus tenuis Willd. gramin 1

Juncaceae Luzula acuminata Raf. gramin 1

Juncaceae Luzula multiflora (Ehrh.) Lej. gramin 1

Lamiaceae Galeopsis tetrahit L.* forb

Lamiaceae Lycopus americanus Muhl. ex 

W. Bartram

forb 1

Lamiaceae Lycopus uniflorus Michx. forb 1

Lamiaceae Prunella vulgaris L.* forb

(Appendix I continued on next page)

Appendix I (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 123

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Lamiaceae Scutellaria galericulata L. forb 1

Lamiaceae Scutellaria lateriflora L. forb 1

Liliaceae Clintonia borealis (Aiton) Raf. forb 1

Liliaceae Maianthemum canadense Desf. ( Ruscaceae) forb 1

Liliaceae Maianthemum racemosa (L.) 

Link

Maianthemum racemosum (L.) Link 

ssp. racemosum ( Ruscaceae)

forb 1

Liliaceae Medeola virginiana L. forb 1

Liliaceae Polygonatum pubescens

(Willd.) Pursh

( Ruscaceae) forb 1

Liliaceae Streptopus lanceolatus (Aiton) 

Reveal

forb 1

Liliaceae Trillium erectum L. ( Melanthiaceae) forb 1

Liliaceae Trillium undulatum Willd. ( Melanthiaceae) forb 1

Liliaceae Uvularia sessilifolia L. ( Colchicaceae) forb 1

Lycopodiaceae Lycopodium annotinum L. Spinulum annotinum (L.) A. Haines fern 1

Lycopodiaceae Lycopodium clavatum L. fern 1

Lycopodiaceae Lycopodium hickeyi 

W.H. Wagner, Beitel & Moran 

Dendrolycopodium hickeyi 

(W.H. Wagner, Beitel & Moran) A. Haines 

fern 1

Lycopodiaceae Lycopodium obscurum L. Dendrolycopodium obscurum (L.) 

A. Haines

fern 1

Lythraceae Lythrum salicaria L.** forb

Monotropaceae Monotropa uniflora L. ( Ericaceae) forb 1

Myricaceae Comptonia peregrina (L.) 

J. M. Coult.

shrub 1

Myricaceae Myrica gale L. shrub 1

Oleaceae Fraxinus americana L. tree 1

Oleaceae Fraxinus nigra Marsh. tree 1

Oleaceae Fraxinus pennsylvanica Marsh. tree 1

Onagraceae Chamerion angustifolium (L.) 

Holub ssp. angustifolium

Chamerion angustifolium (L.) Holub 

ssp. circumvagum (Mosq.) Kartesz

forb 1

Onagraceae Circaea alpina L. Circaea alpina L. ssp. alpina forb 1

Onagraceae Circaea lutetiana L. 

ssp. canadensis (L.) 

Aschers. & Magnus

Circaea canadensis (L.) Hill 

ssp. canadensis

forb 1

Onagraceae Epilobium ciliatum Raf. forb 1

Onagraceae Epilobium coloratum Biehler forb 1

Onagraceae Epilobium leptophyllum Raf. forb 1

(Appendix I continued on next page)

Appendix I (continued)
124 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Onagraceae Ludwigia palustris (L.) Elliott forb 1

Onagraceae Oenothera perennis L. forb 1

Orchidaceae Cypripedium acaule Aiton forb 1

Orchidaceae Epipactis helleborine (L.) 

Crantz*

forb

Orchidaceae Goodyera repens (L.) R. Br. Goodyera repens (L.) R. Br. 

in Ait. & Ait. f.

forb 1

Orchidaceae Goodyera tessellata Lodd forb 1

Osmundaceae Osmunda cinnamomea L. Osmundastrum cinnamomeum (L.) 

C. Presl

fern 1

Osmundaceae Osmunda claytoniana L. fern 1

Osmundaceae Osmunda sp. fern 1

Oxalidaceae Oxalis corniculata L. forb 1

Oxalidaceae Oxalis montana Raf. forb 1

Oxalidaceae Oxalis stricta L. forb 1

Pinaceae Abies balsamea (L.) Mill. tree 1

Pinaceae Larix laricina (Du Roi) K. Koch tree 1

Pinaceae Picea abies (L.) Karst* tree

Pinaceae Picea glauca (Moench) Voss tree 1

Pinaceae Picea mariana (Mill.) 

Britton, Sterns & Poggenb.

tree 1

Pinaceae Picea rubens Sarg. tree 1

Pinaceae Pinus resinosa Aiton tree 1

Pinaceae Pinus strobus L. tree 1

Pinaceae Tsuga canadensis (L.) Carriere tree 1

Poaceae Agrostis perennans (Walter) 

Tuck.

gramin 1

Poaceae Agrostis scabra Willd. gramin 1

Poaceae Anthoxanthum odoratum L.* gramin

Poaceae Brachyelytrum aristosum

(Michx.) Trel. 

Brachyelytrum aristosum (Michx.) Trel. 

in Branner & Coville

gramin 1

Poaceae Calamagrostis canadensis

(Michx.) P. Beauv.

gramin 1

Poaceae Cinna latifolia 

(Trevis ex Goepp.) Griseb.

gramin 1

Poaceae Danthonia compressa Austin Danthonia compressa Austin ex Peck gramin 1

(Appendix I continued on next page)

Appendix I (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 125

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Poaceae Danthonia spicata (L.) 

P. Beauv. ex Roem. & Schult.

gramin 1

Poaceae Dichanthelium acuminatum

(Sw.) Gould & C.A. Clark 

gramin 1

Poaceae Glyceria striata (Lam.) 

A. S. Hitchcock

gramin 1

Poaceae Muhlenbergia uniflora L. Muhlenbergia uniflora (Muhl.) Fern. gramin 1

Poaceae Oryzopsis asperifolia Michx. gramin 1

Poaceae Poa nemoralis L.** gramin

Polygalaceae Polygala paucifolia Willd. forb 1

Polygalaceae Polygala sanguinea L. forb 1

Polygonaceae Fallopia scandens (L.) Holub. forb 1

Polygonaceae Polygonum convolvulus L. 

var. convolvulus

Fallopia convolvulus (L.) A. Löve forb 1

Polygonaceae Polygonum sagittatum L. Persicaria sagittata (L.) H. Gross vine 1

Polygonaceae Polygonum sp. forb 1

Polygonaceae Rumex orbiculatus A. Gray Rumex britannica L. forb 1

Primulaceae Lysimachia quadrifolia L. Lysimachia quadrifolia Sims 

( Myrsinaceae)

forb 1

Primulaceae Lysimachia terrestris (L.) B.S.P. ( Myrsinaceae) forb 1

Primulaceae Trientalis borealis Raf. Lysimachia borealis (Raf.) 

U Manns & A. Anderb. ( Myrsinaceae)

forb 1

Pyrolaceae Moneses uniflora (L.) A. Gray ( Ericaceae) forb 1

Pyrolaceae Orthilia secunda (L.) House ( Ericaceae) forb 1

Pyrolaceae Pyrola americana Sweet ( Ericaceae) forb 1

Pyrolaceae Pyrola elliptica Nutt. ( Ericaceae) forb 1

Ranunculaceae Actaea rubra (Aiton) Willd. forb 1

Ranunculaceae Anemone quinquefolia L. Anemone quinquefolia L. 

var. quinquefolia

forb 1

Ranunculaceae Clematis occidentalis (Hornem.) 

DC.

Clematis occidentalis (Hornem.) DC. 

ssp. occidentalis

vine 1

Ranunculaceae Coptis trifolia (L.) Salisb. forb 1

Ranunculaceae Ranunculus abortivus L. forb 1

Ranunculaceae Ranunculus acris L.* forb

Ranunculaceae Ranunculus hispidus L. Ranunculus hispidus Michx. forb 1

Ranunculaceae Ranunculus recurvatus Poir. Ranunculus recurvatus Poir. 

var. recurvatus

forb 1

(Appendix I continued on next page)

Appendix I (continued)
126 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Ranunculaceae Thalictrum pubescens Pursh forb 1

Rhamnaceae Frangula alnus Mill.** shrub

Rhamnaceae Rhamnus alnifolia L. Her. shrub 1

Rhamnaceae Rhamnus cathartica L.** shrub

Rosaceae Amelanchier arborea (Michx. f.) 

Fernald

tree 1

Rosaceae Amelanchier bartramiana

(Tausch) M. Roemer

shrub 1

Rosaceae Amelanchier canadensis (L.) 

Medik.

shrub 1

Rosaceae Amelanchier laevis Wiegand shrub 1

Rosaceae Crataegus macrosperma Ashe tree 1

Rosaceae Dalibarda repens L. Rubus dalibarda L. forb 1

Rosaceae Fragaria vesca L.* forb

Rosaceae Fragaria virginiana Duchesne forb 1

Rosaceae Geum laciniatum Murray forb 1

Rosaceae Malus pumila Mill.* tree

Rosaceae Malus sylvestris (L.) Mill.* tree

Rosaceae Photinia melanocarpa (Michx.) 

K.R. Robertson & Phipps 

Aronia melanocarpa (Michx.) Ell. shrub 1

Rosaceae Potentilla norvegica L. forb 1

Rosaceae Potentilla simplex Michx. forb 1

Rosaceae Prunus pensylvanica L. f. Prunus pensylvanica L. f. 

var. pensylvanica

tree 1

Rosaceae Prunus serotina Ehrh. Prunus serotina Ehrh. var. serotina tree 1

Rosaceae Prunus virginiana L. Prunus virginiana L. var. virginiana tree 1

Rosaceae Rosa multiflora Thunb.** Rosa multiflora Thunb. ex Murr. shrub

Rosaceae Rosa palustris Marsh. shrub 1

Rosaceae Rosa virginiana Mill. shrub 1

Rosaceae Rubus alleghaniensis Porter shrub 1

Rosaceae Rubus cf. vermontanus Blanch. shrub 1

Rosaceae Rubus flagellaris Willd. shrub 1

Rosaceae Rubus hispidus L. subshrub 1

Rosaceae Rubus idaeus ssp. strigosus

(Michx.) Focke

forb 1

Rosaceae Rubus occidentalis L. shrub 1

(Appendix I continued on next page)

Appendix I (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 127

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Rosaceae Rubus pensilvanicus Poir. shrub 1

Rosaceae Rubus pubescens Raf. subshrub 1

Rosaceae Sorbus americana Marsh. tree 1

Rosaceae Spiraea alba Du Roi var. latifolia shrub 1

Rosaceae Spiraea tomentosa L. shrub 1

Rubiaceae Cephalanthus occidentalis L. shrub 1

Rubiaceae Galium asprellum Michx. forb 1

Rubiaceae Galium palustre L. forb 1

Rubiaceae Galium trifidum L. forb 1

Rubiaceae Galium triflorum Michx. forb 1

Rubiaceae Houstonia caerulea L. forb 1

Rubiaceae Mitchella repens L. subshrub 1

Salicaceae Populus balsamifera L. Populus balsamifera L. ssp. balsamifera tree 1

Salicaceae Populus grandidentata Michx. tree 1

Salicaceae Populus tremuloides Michx. tree 1

Salicaceae Salix bebbiana Sarg. shrub 1

Salicaceae Salix discolor Muhl. shrub 1

Salicaceae Salix eriocephala Michx. Salix eriocephala Michx. 

ssp. eriocephala var. eriocephala

shrub 1

Salicaceae Salix lucida Muhl. Salix lucida Muhl. ssp. lucida shrub 1

Salicaceae Salix pedicellaris Pursh shrub 1

Salicaceae Salix sericea Marsh. shrub 1

Sapindaceae Acer pensylvanicum L. tree 1

Sapindaceae Acer platanoides L.** tree

Sapindaceae Acer rubrum L. tree 1

Sapindaceae Acer saccharinum L. tree 1

Sapindaceae Acer saccharum Marsh. Acer saccharum Marsh. var. saccharum tree 1

Sapindaceae Acer spicatum Lam. tree 1

Saxifragaceae Mitella nuda L. forb 1

Saxifragaceae Tiarella cordifolia L. Tiarella cordifolia L. var. cordifolia forb 1

Scrophulariaceae Chelone glabra L. ( Plantaginaceae) forb 1

Scrophulariaceae Gratiola neglecta Torr. ( Plantaginaceae) forb 1

Scrophulariaceae Linaria vulgaris Mill.** ( Plantaginaceae) forb

Scrophulariaceae Melampyrum lineare Desr. ( Orobanchaceae) forb 1

(Appendix I continued on next page)

Appendix I (continued)
128 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family

NRCS species 

with naming authority

Haines (2011), new name 

and change in family if applicable

Growth 

form Native

Scrophulariaceae Veronica officinalis L.* ( Plantaginaceae) forb

Scrophulariaceae Veronica serpyllifolia L.* ( Plantaginaceae) forb

Solanaceae Solanum dulcamara L.** vine

Sparganiaceae Sparganium herb 1

Taxaceae Taxus canadensis Marsh. shrub 1

Thelypteridaceae Phegopteris connectilis (Michx.) 

Watt

fern 1

Thelypteridaceae Thelypteris noveboracensis (L.) 

Nieuwl.

Parathelypteris noveboracensis (L.) 

Ching

fern 1

Thelypteridaceae Thelypteris palustris Schott 

var. pubescens (Lawson) Fern.

Thelypteris palustris Schott 

var. pubescens (G. Lawson) Fern.

fern 1

Thelypteridaceae Thelypteris simulata

(Davenport) Nieuwl.

Parathelypteris simulata (Davenport) 

Holttum

fern 1

Thymelaeaceae Dirca palustris L. shrub 1

Tiliaceae Tilia americana L. ( Malvaceae) tree 1

Ulmaceae Ulmus americana L. tree 1

Valerianaceae Valeriana officinalis L.** ( Caprifoliaceae) forb

Violaceae Viola blanda Willd. forb 1

Violaceae Viola cucullata Ait. forb 1

Violaceae Viola pubescens Aiton forb 1

Violaceae Viola renifolia A. Gray forb 1

Viscaceae Arceuthobium pusillum Peck forb 1

Vitaceae Parthenocissus quinquefolia

(L.) Planch.

vine 1

Appendix I (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 129

Family fern graminoid herb shrub subshrub tree vine Total

Alismataceae 1 1

Anacardiaceae 1 1 2

Apiaceae 2 2

Aquifoliaceae 2 2

Araceae 2 2

Araliaceae 2 2 4

Asteraceae 31 31

Balsaminaceae 1 1

Berberidaceae 1 1

Betulaceae 3 4 7

Brassicaceae 1 1

Callitrichaceae 1 1

Campanulaceae 1 1

Caprifoliaceae 13 1 14

Caryophyllaceae 1 1

Celastraceae 1 1 2

Clusiaceae 1 1

Convolvulaceae 1 1

Cornaceae 4 1 5

Crassulaceae 2 2

Cucurbitaceae 1 1

Cupressaceae 1 1 2

Cyperaceae 26 26

Dennstaedtiaceae 2 2

Dryopteridaceae 10 10

Equisetaceae 2 2

Ericaceae 11 3 14

Euphorbiaceae 1 1

Fabaceae 5 5

Fagaceae 2 2

(Appendix II continued on next page)

Appendix II.

Number of species per plant family, and growth form of vascular plants of the Penobscot Experimental Forest, 

Bradley, Maine. This summary follows the NRCS database, not Haines (2011) (see Appendix I), and includes 

some of the unresolved genera. 

Growth form
130 GTR-NRS-P-123 Penobscot Experimental Forest: 

Family fern graminoid herb shrub subshrub tree vine Total

Geraniaceae 1 1

Grossulariaceae 2 2

Hamamelidaceae 1 1

Iridaceae 1 1

Juncaceae 5 5

Lamiaceae 6 6

Liliaceae 9 9

Lycopodiaceae 4 4

Lythraceae 1 1

Monotropaceae 1 1

Myricaceae 2 2

Oleaceae 3 3

Onagraceae 8 8

Orchidaceae 4 4

Osmundaceae 3 3

Oxalidaceae 3 3

Pinaceae 9 9

Poaceae 13 13

Polygalaceae 2 2

Polygonaceae 4 1 5

Primulaceae 3 3

Pyrolaceae 4 4

Ranunculaceae 8 1 9

Rhamnaceae 3 3

Rosaceae 6 15 2 8 31

Rubiaceae 5 1 1 7

Salicaceae 6 3 9

Sapindaceae 6 6

Saxifragaceae 2 2

Scrophulariaceae 6 6

Solanaceae 1 1

Sparganiaceae 1 1

Taxaceae 1 1

(Appendix II continued on next page)

Growth form

Appendix II (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 131

Family fern graminoid herb shrub subshrub tree vine Total

Thelypteridaceae 4 4

Thymeliaceae 1 1

Tiliaceae 1 1

Ulmaceae 1 1

Valerianaceae 1 1

Violaceae 4 4

Viscaceae 1 1

Vitaceae 1 1

Grand total 25 44 132 71 11 38 6 327

Growth form

Appendix II (continued)
132 GTR-NRS-P-123 Penobscot Experimental Forest: 

Appendix III.

(a) Some vascular plant taxa that have been proposed for inclusion by various researchers, but are omitted from 

the list. These taxa may lack appropriate habitat at the PEF or be out of known range. Unavailability of 

voucher specimens prevents their listing. 

Asplenium sp.

Cystopteris sp.

Krigia virginica

Lactuca sativa

Pyrola chlorantha

Rosa johannensis

(b) Unresolved genera, some of which probably duplicate species already listed in Appendix I. During field work, 

plant material might have lacked flowers or fruits and could not be resolved below genus level, yet the genus 

is represented already by known species or subspecies in Appendix I.

Agrostis sp.

Amelanchier sp.

Aster sp.

Betula sp.

Bidens sp.

Carex sp.

Circaea sp.

Cornus sp.

Crataegus sp.

Danthonia sp.

Dryopteris sp.

Epilobium sp.

Equisetum sp.

Fraxinus sp.

Galium sp.

Geum sp.

Hieracium sp.

Hypericum sp.

Ilex sp.

Lonicera sp.

Luzula sp.

Lycopodium sp.

Oxalis sp.

Picea sp.

Poa sp.

Polygala sp.

Populus sp.

Potentilla sp.

Prenanthes sp.

Prunus sp.

Pyrola sp.

Ranunculus sp.

Ribes sp.

Rosa sp.

Rubus sp.

Salix sp.

Silene sp.

Solidago sp.

Sorbus sp.

Sparganium sp.

Thelypteris sp.

Trifolium sp.

Trillium sp.

Vaccinium sp.

Viola sp.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 133

Appendix IV.

Provisional list of lichens of the Penobscot Experimental Forest, Bradley, Maine. Nomenclature follows Esslinger 

(2011).

Bryoria furcellata (Fr.) Brodo & D. Hawksw. 1

Bryoria nadvornikiana (Gyelnik) Brodo & D. Hawksw. 1

Caloplaca sp. 1

Candelariella sp. 1

Cetrelia olivetorum (Nyl.) W.L. Culb. & C.F. Culb. 1

Cladina sp. 1

Cladonia chlorophaea group 1

Cladonia coniocraea (Flörke) Sprengel 1

Cladonia fimbriata (L.) Fr. 1

Cladonia furcata (Hudson) Schrader 1

Cladonia squamosa Hoffm. 1

Cladonia sp. 1

Cladonia spp. (squamulose) 1

Collema subflaccidum Degel. 1

Evernia mesomorpha Nyl. 1

Flavoparmelia caperata (L.) Ach. 1

Hypogymnia physodes (L.) Nyl. 1

Lecanora sp. 1

Lepraria sp. 1

Leptogium corticola (Taylor) Tuck. 1

Leptogium cyanescens (Rabenh.) Körber 1

Leptogium saturninum (Dickson) Nyl. 1

Lobaria pulmonaria (L.) Hoffm. 1

Lobaria quercizans Michaux 1

Melanelia halei (Ahti) Essl. 1

Melanelia subaurifera (Nyl.) Essl. 1

Myelochroa galbina (Ach.) Elix & Hale 1

Nephroma parile (Ach.) Ach. 1

Parmelia squarrosa Hale 1

Parmelia sulcata Taylor 1

Parmeliopsis ambigua (Wulfen) Nyl. 1

Parmeliopsis hyperopta (Ach.) Arnold 1

Peltigera canina (L.) Willd. 1

Peltigera horizontalis (Hudson) Baumg. 1

Peltigera polydactylon (Necker) Hoffm. 1

Peltigera praetextata (Flörke ex Sommerf.) Zopf 1

Peltigera rufescens (Weiss) Humb. 1

Peltigera cf. membranacea (Ach.) Nyl. 1

Lichens Macrolichen Crustose lichen

(Appendix IV continued on next page)
134 GTR-NRS-P-123 Penobscot Experimental Forest: 

Peltigera aphthosa (L.) Willd. or leucophlebia (Nyl.) Gyelnik 1

Phaeophyscia pusilloides (Zahlbr.) Essl. 1

Phaeophyscia rubropulchra (Degel.) Essl. 1

Physcia millegrana Degel. 1

Physconia detersa (Nyl.) Poelt 1

Platismatia glauca (L.) W.L. Culb. & C.F. Culb. 1

Platismatia tuckermanii (Oakes) W.L. Culb. & C.F. Culb. 1

Punctelia rudecta (Ach.) Krog 1

Pyxine sorediata (Ach.) Mont. 1

Ramalina americana Hale 1

Ramalina dilacerata (Hoffm.) Hoffm. 1

Ramalina intermedia (Delise ex Nyl.) Nyl. 1

Tuckermannopsis ciliaris (Ach.) Gyelnik grp. 1

Usnea filipendula Stirton 1

Usnea lapponica Vainio 1

Usnea merrillii Motyka 1

Usnea mutabilis Stirt. 1

Usnea strigosa subsp. strigosa 1

Usnea subfloridana Stirton 1

Usnocetraria oakesiana (Tuck.) M.J. Lai & C.J. Wei 1

Verrucaria sp. 1

Vulpicida pinastri (Scop.) J.-E. Mattsson & M.J. Lai 1

Xanthoparmelia conspersa (Ehrh. ex Ach.) Hale 1

Xanthoparmelia tasmanica (Hooker f. & Taylor) Hale/angustiphylla (Gyelnik) Hale 1

Total 59 3

Lichens Macrolichen Crustose lichen

Appendix IV (continued)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 135

Appendix V.

Provisional list of bryophytes at the Penobscot Experimental Forest, including 9 liverworts and 40 mosses. 

Nomenclature for liverworts follows Crosby and Magill (2005, 2006) and Stotler and Crandall-Stotler (1977). 

Nomenclature for mosses follows Allen (2005), except for pleurocarpous mosses (Crosby et al. 1999).

Bryophytes

Liverworts

Bazzania trilobata (L.) S. Gray var. trilobata

Frullania bolanderi Austin

Frullania tamarisci (L.) Dum. subsp. asagrayana (Mont.) Hatt.

Nowellia curvifolia (Dicks.) Mitt.

Pellia epiphylla (L.) Corda

Ptilidium ciliare (L.) Hampe

Ptilidium pulcherrimum (G. Web.) Hampe

Radula complanata (L.) Dum.

Scapania nemorosa (L.) Dum.

Mosses

Anomodon attenuatus (Hedwig) Hübener

Atrichum oerstedianum (C. Müller) Mitten

Atrichum sp.

Brachythecium cf. laetum (Brid.) B.S.G.

Brachythecium erythrorhizon W.P. Schimper in B.S.G.

Bryhnia novae-angliae (Sullivant & Lesquereux) Grout

Climacium dendroides (Hedwig) Weber & D. Mohr

Dicranum spp.

Dicranum montanum Hedwig

Dicranum polysetum Swartz

Dicranum scoparium Hedwig

Drepanocladus aduncus (Hedwig) Warnstorf

Hedwigia ciliata (Hedwig) Palisot de Beauvois

Homalia trichomanoides (Hedwig) W.P. Schimper in B.S.G.

Hylocomium splendens (Hedwig) W.P. Schimper in B.S.G.

Hypnum imponens Hedwig

Isopterygiopsis muelleriana (W.P. Schimper) Iwatsuki

Leucobryum glaucum (Hedwig) Ångström in Fries

Leucodon andrewsianus (H. Crum & L.E. Anderson) W.D. Reese & L.E. Anderson

Mnium hornum Hedwig

Neckera pennata Hedwig

Othodicranum flagellare (Hedw.) Loeske

Pleurozium schreberi (Willdenow ex Bridel) Mitten

Polytrichum sp.

Polytrichum commune Hedwig

Polytrichum ohioense Ren. & Card.

(Appendix V continued on next page)
136 GTR-NRS-P-123 Penobscot Experimental Forest: 

Mosses (continued)

Ptilium crista-castrensis (Hedwig) De Notaris

Rhizomnium appalachianum

Rhytidiadelphus triquetrus (Hedwig) Warnstorf

Sphagnum affine Renauld & Cardot

Sphagnum capillifolium (Ehrhart) Hedwig

Sphagnum fimbriatum Wils. 

Sphagnum girgensohnii Russ.

Sphagnum palustre L.

Sphagnum squarrosum Crome

Sphagnum wulfianum Girg.

Tetraphis pellucida Milde.

Thuidium delicatulum (Hedw.) Schimp.

Ulota crispa (Hedw.) Brid.

Warnstorfia fluitans (Hedw.) Loeske

Appendix V (continued)

Bryophytes
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 137

Appendix VI.

Comparison of species richness in the checklist of taxa for Penobscot Experimental Forest, Bradley, Maine, with 

that of some other land bases in Maine. Numbers are approximate and do not reflect some recent additions and 

name changes.

Land base

Number of 

hectares

Number of 

vascular plant 

taxa

Number of 

nonnative taxa 

(percentage of 

total known)

Number of 

lichen species 

reported

Number of 

liverwort 

species 

reported

Number of 

moss species 

reported

Maine, entire state 9,164,673 2,103 634 (30%) (ca. 700) 147 (ca. 430)

Penobscot Experimental 

Forest, Bradley

1,618 344 45 (15%) 59 9 40

Massabesic Experimental 

Forest, Alfred and Lyman, 

York County

1,497 464 43 (9%) Not reported Not reported Not reported

Great Pond Mountain 

Wildlands, Orland, 

Hancock County

1,700 400+ 40 (10%) 12 5 14

Acadia National Park, Bar 

Harbor, Hancock County 

14,648 1,135 284 (25%) 379, of which 

198 are 

crustose

11+ 51+
138 GTR-NRS-P-123 Penobscot Experimental Forest: 

UNDERSTANDING PATTERNS 

AND REGULATORY PATHWAYS IN CONIFER ONTOGENY: 

THE ROLE OF THE PENOBSCOT EXPERIMENTAL FOREST

Michael E. Day, Michael S. Greenwood, Katherine Spencer, and Stephanie L. Adams

Abstract.—Multi-cohort stands maintained by the U.S. Department of Agriculture, 

Forest Service on the Penobscot Experimental Forest (PEF) have played a central role 

in research into the mechanisms that regulate ontogenetic trends in forest conifers as 

they control effects due to population, climatic, and edaphic factors. These long-term 

silvicultural studies have permitted direct comparisons of trends in morphology and 

physiology across life-stages, and experimental reciprocal grafting research among 

juvenile, mid-aged and old-growth individuals has proved an important approach to 

discriminating between trends due to tree size and those related to life-stage. The results 

of this research have provided valuable insight into the physiological mechanisms that 

underlie age-related trends in growth and development. Two decades of study on the PEF 

have identified a surprising paradox of high photosynthetic rates and declining stemwood 

production that characterizes the transition from mid-aged to old-growth life-stages in 

spruce. Ultimately, this research has led to a novel explanation for that paradox in the 

ecologically stable strategy hypothesis, which integrates environment, intrinsic regulation 

of development, and life-stage specific challenges.

INTRODUCTION

For the past two decades the Penobscot Experimental 

Forest (PEF) has been a key asset for the University 

of Maine’s research program in tree physiology 

and physiological ecology in the School of Forest 

Resources. A major focus of research by the school’s 

plant physiology and physiological ecology group 

is how external factors, such as climate, and internal 

factors, such as life-stage (ontogeny), control tree 

productivity and how they relate to tree form (anatomy 

and structure) and function (physiology). In turn, we 

seek to understand how the influence of these factors 

relates to species’ competitive abilities, range-limits, 

and response to changing climates. While working 

on the gas exchange physiology of red spruce (Picea 

rubens Sarg.) in the early 1990s, we were struck 

by substantial differences in morphological and 

physiological attributes between various tree age 

classes. The older age classes in multi-cohort stands 

differed from younger trees not only in overall form, 

but also in branching patterns, needle morphology, 

and physiological properties such as stomatal 

sensitivity to atmospheric humidity and, potentially, 

in photosynthetic rates. It had been well established 

that stemwood production declines in forest trees 

after they reach roughly the midpoint of their normal 

lifespan (Assmann 1970). However, a physiological 

explanation for this and other observed age- and sizerelated trends was lacking.

The PEF stands that the U.S. Department of 

Agriculture (USDA), Forest Service has maintained 

under long-term selection silvicultural systems 

provided an ideal field laboratory to experimentally 

study age- and size-related phenomena in search of a 

physiological explanation for these trends. Stands in 

the Forest Service compartments on the PEF, managed 

for more than 50 years with an emphasis on red 

spruce, contain individuals from a common population 

ranging from germinants to old-growth trees, over a 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 139

century and a half in age, all growing on similar soil 

types. Moreover, by using adjacent stands managed 

under selection and shelterwood systems, individuals 

of both free-to-grow and shaded crown status of all 

age classes could be selected to control for the effects 

of light environment on form and function.

The decline in stemwood growth efficiency 

(production per unit foliage) from mid-age to old age 

is of particular importance to forest management and 

silvicultural decisions. The paradigm of decreasing 

productivity with tree age forms the basis for 

silvicultural decisions on harvesting by defining 

expected returns on tree growth. This model is 

particularly important when considering the value 

of old trees in uneven-aged silviculture, carbon 

sequestration, and biomaterials production, and can 

result in a tension between short- and long-rotation 

silvicultural approaches.

EARLY STUDIES

Our initial assessments showed that red spruce on 

the PEF did indeed decrease stemwood production 

efficiency approximately 50 percent between the 

mid-aged time of maximum production (60 yr) and 

the oldest individuals (Fig. 1) (Day et al. 2001). This 

trend was subsequently confirmed by Seymour and 

Kenefic (2002). In needle morphology, older trees 

produce foliage that is more massive (lower specific 

leaf area, SLA) irrespective of sun-foliage or shadefoliage status (Fig. 2), demonstrating the interplay of 

heteroblasty and heterophylly in foliar development 

(Greenwood et al. 2009). In addition, old conifers 

deviate from the pyramidal crown form of younger 

individuals and develop a characteristic flat-topped 

form with low rates of stem and branch elongation and 

increased rates of lateral branch initiation (Fig. 3). 

Figure 1.—Foliar efficiency in wood production for reproductively mature red spruce in the PEF (after Day et al. 2001).
140 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 2.—Trends with tree age in specific leaf area for red spruce foliage from the PEF. Left, juvenile and old-growth 

foliage after grafting to common juvenile rootstock showing differences in overall needle morphology and robustness; right, 

independent trends associated with ontological heteroblasty and sun-shade heterophylly. (Means ± standard error.) (Data from 

Day et al. 2001). Photo courtesy of University of Maine.

Figure 3.—Typical age- and size-related changes in crown structure, branch elongation, and branches per stem length for red 

spruce in the PEF with a mid-aged (60 yr) tree on the left and old-growth (150 yr) on the right. Insets detail upper crown lateral 

branches. Photos courtesy of University of Maine.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 141

Studies in western North America during the 1990s 

suggested that observed trends in productivity 

and perhaps morphology were due to decreased 

photosynthetic rates and/or enhanced midday 

depression of photosynthesis in old trees because 

of the increased hydraulic stress associated with 

longer water transport pathways (Ryan and Yoder 

1997, Yoder et al. 1994). Others suggested restricted 

availability of nutrients, as more became incorporated 

in living biomass and coarse woody material (e.g., 

Binkley et al. 1995). Research on redwoods (Sequoia 

sempervirens) (Koch et al. 2004) and Douglas-fir 

(Pseudotsuga menziesii) (Woodruff et al. 2008) 

further suggested that height gradients in needle SLA 

are correlated with increasing water stress with tree 

height through limitations on cell number and/or 

expansion growth of developing tissues. Because 

most studies on the issue of age- and size-related 

declines in productivity, foliar morphology, and crown 

form were based on very tall (60-100+ m) trees in 

the Pacific Northwest, a major thrust of our research 

has been the applicability of “tall tree” hypotheses to 

the 20- to 30-m-high species typical of the Acadian 

Forest of northeastern North America. In some of 

the tallest trees, the biophysical mechanisms that 

transport water from soil to foliage may reach their 

upper bounds (Koch et al. 2004). The influences of 

gravity and resistance of long transport pathways on 

water potential (approximately -0.02 MPa m-1) are 

likely to play various roles such as limiting CO2

 uptake 

for photosynthesis, and limiting the turgor pressure 

within developing cells required for their expansion. 

But it also seems likely that biophysical restrictions 

on water transport and their effects on physiology and 

morphology are much diminished in species of shorter 

stature growing in mesic forests.

RESEARCH ON GRAFTING

Although age-related trends in productivity and 

outward morphology of forest trees and their organs 

are manifest, understanding the bases for these 

phenomena is complicated by the confounding effects 

of size, chronological age, and external environment 

on long-lived organisms that progress in biomass 

through many orders of magnitude during their 

lifespan. One approach to addressing these potentially 

confounding factors is through grafting experiments 

in which scions from donors of various ages or sizes 

are grafted onto rootstock of a common age. A more 

comprehensive approach is reciprocally grafting scions 

from different ages onto rootstock representing the 

ages of the scion donors (Day et al. 2002). Earlier 

experiments using the former approach, where scions 

from older age classes are grafted onto juvenile 

rootstock, suggested that specific foliar traits, such as 

needle width, and growth habits such as plagiotropy 

are maintained in the genetic “memory” of scions from 

older donors (Day et al. 2001, Rebbeck et al. 1992). 

In 2002 we initiated a reciprocal grafting study on 

the PEF between juvenile (< 10 yr, 1 m in height), 

mid-aged (60 yr, 10 m), and old (120+ yr, 20 m) red 

spruce, using multiple grafts in the crowns of eight 

trees in each of the two mature age-classes, and single 

grafts on juvenile rootstock. Three years into the 

study we made extensive in situ physiological and 

morphological measurements, and we completed a 

second series of measurements including destructive 

sampling when scions were harvested 7 years after 

grafting. Using this approach, we were able to separate 

the intrinsic and extrinsic mechanisms that regulate 

numerous morphological and physiological traits (Day 

and Greenwood 2011, Greenwood et al. 2010). In 

this regulatory scheme, intrinsic traits are those that 

are mainly influenced by mechanisms originating in 

shoot apical meristems (SAMs), the growth points 

for production of new cells that elongate shoots and 

branches. Intrinsic changes in SAM behavior in red 

spruce include age-related trends in rooting ability, 

decreased apical control of lateral buds, wider and 

more massive foliage, and increased reproductive 

competency. In contrast, extrinsic regulation results 

from influences outside of the SAM, and includes gas 

exchange behavior and resource allocation patterns. 

Other age- and size-related trends were explained 

by a complex interaction of intrinsic and extrinsic 

regulatory pathways (Fig. 4) (Day and Greenwood 

2011). The influence of intrinsic and extrinsic factors 
142 GTR-NRS-P-123 Penobscot Experimental Forest: 

in regulating branching patterns proved of particular 

significance in understanding age-related trends in 

crown form (Greenwood et al. 2010).

STUDIES ON RESOURCE 

SUPPLY LIMITATIONS

During the past decade our lab also tested the 

relevance of resource supply restrictions on the 

question of age- and size-related decreases in 

stemwood increment beyond mid-age in red spruce. 

Again, the PEF multicohort populations proved 

invaluable by providing trees that were free to grow 

on common soil types and with documented harvest 

histories extending over five decades. In addition, 

the PEF road and landing system allowed the use of 

a self-propelled hydraulic lift to reach upper crowns 

for nondestructive measurements. Our research has 

demonstrated that neither xylem conductivity, nutrient 

availability, photosynthetic capacity, nor diurnal 

trends in stomatal conductance and photosynthesis 

differ between mid-aged and old red spruce (Fig. 5) 

(Adams 2006, Day and Greenwood 2011, Greenwood 

Figure 4.—Reciprocal grafting results showing complex 

intrinsic-extrinsic (relative to meristem) regulation of foliar 

morphology. Grafting combinations (left) are compared to 

understock foliage on the right. Scions from old donors 

maintained greater width than scions from juvenile donors on 

juvenile rootstock from intrinsic regulation, but were narrower 

than would be expected without extrinsic influence. Scions 

from juvenile donors grafted onto old rootstock maintained 

a narrower width of juvenile foliage, but were wider than 

“normal” juvenile needles. Dissimilar letters indicate 

significant differences (α = 0.05; Tukey’s honestly 

significant difference).

et al. 2008). In addition, a preliminary study of nonstructural carbohydrates (NSC) in red spruce foliage 

suggested that net assimilation, the availability of 

photosynthetic products beyond the respiratory need 

of foliage, was not limiting in old trees (Fig. 6) (Day 

and Greenwood 2011). Subsequently, Spencer (2010) 

found no differences in soluble sugar concentrations 

between mid-aged and old trees across seasons and 

between two successive years. Similar trends in 

NSC concentrations have recently been reported 

for lodgepole pine (Pinus contorta Douglas ex 

Loudon) and redwoods in western North America and 

Australian eucalypts (Eucalyptus spp.) (Sala and Hoch 

2009, Sillet et al. 2010).

Taken as a whole, the lack of evidence for resource 

limitations to observed age- and size-related decreases 

in tree productivity suggests that these trends may 

be regulated by demand-side (allocation and growth) 

pathways (Day et al. 2002). Further, Sillet et al. (2010) 

have suggested that the conventional use of stemwood 

production as a measure of tree productivity may 

suffer from a conceptual flaw stemming from a view 

of trees as wood production systems, as age- and 

size-related shifts in resource allocation patterns are 

not recognized. The results of our reciprocal grafting 

study supported continued high potential productivity 

in old red spruce (Greenwood et al. 2010). Despite 

less extension growth and greater branching of all age 

classes of scions grafted into the tops of old trees, all 

scions showed the same growth in total biomass after 

7 years. Additionally, there were no differences in 

7-year biomass increment between scions grafted 

on old rootstock and those on mid-aged rootstock, 

suggesting that growth potential was not influenced 

by age or size of rootstock.

STUDIES EXPLORING THE 

ECOLOGICALLY SUSTAINABLE 

STRATEGIES HYPOTHESIS

To address the adaptive significance of these 

ontological differences in growth habit, we have 

recently advanced the ecologically sustainable 

strategies (ESS) hypothesis (Day and Greenwood 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 143

Figure 5.—Diurnal tracking of (A) stomatal conductance and 

(B) photosynthesis in the sun-adapted foliage of juvenile 

(10 yr, 1 m), mid-aged (60 yr, 10 m), and old (125+ yr, 

20+ m) red spruce trees in the PEF. Juvenile trees exhibited 

higher net rates. Mid-aged and old trees showed the same 

rates and pattern of midday decrease in gas exchange. 

(Means ± standard error, n=24.) This pattern corresponds 

to those of electron transport rates (Jmax) and carboxylation 

capacity (Vcmax) in the same trees (Greenwood et al. 2007). 

Figure 6.—Midday concentrations of the non-structural 

carbohydrates (A) sugar and (B) carbohydrates in red 

spruce foliage for juvenile (12 yr), mid-aged (60 yr), and old 

(120+ yr) red spruce in the PEF. The concentration of both 

soluble sugars and starch was greatest in the most rapidly 

growing age classes.

2011). This hypothesis is built on the evolutionarily 

sustainable strategy concept from game theory 

(Vincent et al. 1996), where an individual (or, in 

our case, a tree species) evolves a life-strategy that 

permits it to indefinitely occupy a niche regardless 

of competition from other species. In long-lived 

tree species such a life-strategy not only requires a 

degree of plasticity, but will vary through ontogeny 

in a pattern adapted to confront the highly variable 

challenges faced by the tree as it progresses through 

its life-stages. In its overarching concept, the ESS 

hypothesis predicts that trees growing in regions 

with high seasonal variability in climate such as the 

Acadian Region and in stochastic disturbance events, 

such as downburst cells and extra-tropical or tropical 

cyclones, will co-evolve a strategically uniform 

canopy. In these systems, emergent tree species would 

be evolutionarily penalized by increased risks from 

disturbance agents, a requirement for greater allocation 

of resources to stem diameter to counter dynamic 

loads, and increased likelihood of a shorter residence 

time in the upper canopy where reproductive effort has 

the greatest influence on population size. In contrast, 

maximum height growth resulting from positivefeedback “runaway” competition would be most likely 

in regions where relatively uniform climates and 

lower-intensity disturbances reduce the evolutionary 

costs of maximizing height growth and even provide 

an advantage to species that grow to their biophysical 

height limits.
144 GTR-NRS-P-123 Penobscot Experimental Forest: 

Our continuing research based on the PEF provides 

support for the ESS hypothesis. We have shown 

that allocation patterns in conifer germinants are 

largely under the control of intrinsic mechanisms 

(Greenwood et al. 2008, Zazzaro 2009). At this lifestage, individuals have very limited capacity to assess 

their environment beyond a few centimeters’ distance 

and any delay or resource/energy cost associated 

with sensing and responding to environmental cues 

decreases the individual’s fitness to establish its roots 

in a water source and move towards a positive carbon 

balance. Therefore, intrinsically regulated allocation 

patterns that have been evolutionarily established 

as effective in previous generations are favored. 

Once established in the understory, seedlings tend 

to develop highly branched, flattened crowns and 

allocate minimal resources to extension growth of 

the main stem. When the seedling/sapling detects the 

presence of an overstory gap from changes in incident 

light quality and quantity, it begins its extension 

growth. During this phase of increasing stemwood 

productivity, allocation to the stem, apical downregulation of lateral branching, and extension growth 

are maximized. Research on other conifer species 

suggests a role for a phytochrome-mediated response 

to altered red:far-red light wavelength composition 

resulting from gap formation or the presence of 

competitors in this ontogenetic stage (Hoddinott and 

Scott 1996). For PEF red spruce, Day and Greenwood 

(2011) provide evidence for a strong extrinsic control 

pathway in mid-aged individuals mediated by the 

external environment. Finally, once the tree reaches 

a position in the upper canopy, height growth is again 

decreased in favor of less apical control, increased 

branching, and more robust foliage, ultimately 

developing the spreading crown that characterizes 

the old life-stage. We believe this old-growth strategy 

minimizes risk and sustains long-term reproductive 

effort.

The ontogenetic pattern of growth allocation 

described above is supported by our long-term 

reciprocal grafting study. Scions from both old and 

juvenile donors showed the greatest tendency for 

branch production per centimeter of stem length and 

those from mid-aged donors showed the greatest 

tendency for elongation growth on rootstock of all 

ages (Greenwood et al. 2010). Our research to date 

suggests that the reversion to higher branchiness in old 

trees is under a complex mix of extrinsic and intrinsic 

regulatory mechanisms, but we have yet to identify 

potential pathways that alter this and other old-tree 

allocation patterns. 

Having established that the foliage of old trees is 

equal in photosynthetic assimilation to that of midaged individuals that show maximum accumulation 

of stemwood, our current research on the PEF red 

spruce population seeks to identify the old-tree sinks 

for the “missing” photosynthate. The more massive 

needles on old trees also have a 28 percent decrease in 

internal air space, resulting in a greater specific gravity 

(Greenwood et al. 2008), and may support thicker 

cuticles and increased lignin content, all of which 

may add to both resource cost and foliar longevity. 

Our preliminary data also suggest that the more 

conservative strategy in this ontogenetic stage includes 

substantially increased allocation of carbon to starch 

reserves in the stem. When reserves are standardized 

on a unit-foliage basis, old trees are holding more than 

4.5 times as much carbon in stem reserves as are midaged spruce (Fig. 7), greatly enhancing their resiliency 

to external stresses.

Figure 7.—Starch in stemwood reserves per unit foliage for 

mid-aged (65 yr) and old (120+ yr) red spruce age classes in 

the PEF. (Means ± standard errors, P < 0.001, n=16.)
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 145

CONCLUSIONS

The Forest Service long-term silvicultural experiment 

on the PEF has proven a critical resource in advancing 

our understanding of the complex and recalcitrant 

questions associated with age- and size-related trends 

in forest tree physiology and morphology. The answers 

to these questions will have substantial influence 

not only within the scientific understanding of tree 

ontogeny, but also practical application in defining 

paradigms for multicohort silviculture and the carbon 

economy of old trees. Our research group is continuing 

the study of age-related changes in temperate conifers 

with red spruce as our model species. Current projects 

include quantifying carbon dynamics and phenological 

cycles, variation in cell wall and foliar cuticular 

allocation, and the role of apical dominance in stagespecific crown attributes. The multicohort red spruce 

populations on the PEF continue to play a key role in 

our research.

ACKNOWLEDGMENTS

This material is based on work supported by the 

National Science Foundation under Grant No. IBN0110142. (Any opinions, findings, conclusions, or 

recommendations expressed here are those of the 

authors and do not necessarily reflect the views of 

the National Science Foundation.) Additionally, the 

authors’ research on age-related trends has been 

supported by the USDA Forest Service, Northern 

Research Station; Northeastern States Research 

Cooperative; and the Maine Agricultural and Forest 

Experiment Station. We are indebted to the USDA 

Forest Service for permitting research in its longterm silvicultural studies, and to the many graduate 

and undergraduate students who have made these 

studies possible. This is Maine Agricultural and Forest 

Experiment Station Publication Number 3314. 

LITERATURE CITED

Adams, S.L. 2006. A physiological examination of 

the age-related decline in photosynthesis in Picea 

rubens. Orono, ME: University of Maine. 75 p. 

M.S. thesis.

Assman, E. 1970. Principles of forest yield study.

Oxford, UK: Pergamon Press Ltd. 506 p.

Binkley, D.; Smith, F.W.; Son, Y. 1995. Nutrient 

supply and declines in leaf area and production 

in lodgepole pine. Canadian Journal of Forest 

Research. 25: 621-628.

Day, M.E.; Greenwood, M.S. 2011. Regulation of 

ontogeny in temperate conifers. In: Meinzer, F.C.; 

Lachenbruch, B.; Dawson, T.E., eds. Size- and agerelated changes in tree structure and function. New 

York: Springer: 91-120.

Day, M.E.; Greenwood, M.S.; White, A.S. 2001. 

Age-related changes in foliar morphology and 

physiology in red spruce and their influence on 

declining photosynthetic rates and productivity 

with tree age. Tree Physiology. 21: 1195-1204.

Day, M.E.; Greenwood, M.S.; Diaz-Sala, C. 2002. 

Age- and size-related trends in woody plant 

shoot development: regulatory pathways and 

evidence for genetic control. Tree Physiology. 

22: 507-513.

Greenwood, M.S.; Ward, M.H.; Day, M.E.; Adams, 

S.A.; Bond, B.J. 2008. Age-related trends in red 

spruce foliar plasticity in relation to declining 

productivity. Tree Physiology. 28: 255-232.

Greenwood, M.S.; Day, M.E.; Berlyn, G.P. 2009. 

Regulation of foliar plasticity in conifers: 

developmental and environmental factors.

Journal of Sustainable Forestry. 28: 48-62.

Greenwood, M.S.; Day, M.E.; Schatz, J. 2010. 

Separating the effects of tree size and meristem 

maturation on shoot development of grafted 

scions of red spruce (Picea rubens Sarg.). Tree 

Physiology. 30: 459-468.

Hoddinott, J.; Scott, R. 1996. The influence of light 

quality and carbon dioxide enrichment on the 

growth and physiology of seedlings of three 

conifer species. I. Growth responses. Canadian 

Journal of Botany. 74: 383-390.

Koch, G.E.; Sillet, S.C.; Jennings, G.M.; Davis, S.D. 

2004. The limits of tree height. Nature. 428: 

851-854.
146 GTR-NRS-P-123 Penobscot Experimental Forest: 

Rebbeck, J.; Jesen, K.F.; Greenwood, M.S. 1992. 

Ozone effects on grafted mature and juvenile red 

spruce: photosynthesis, stomatal conductance, 

and chlorophyll concentration. Canadian Journal 

of Forest Research. 23: 450-456.

Ryan, M.G.; Yoder, B.L. 1997. Hydraulic limits to 

tree height and tree growth. Bioscience. 47(4): 

235-242.

Sala, A.; Hoch, G. 2009. Height-related growth 

declines in ponderosa pine are not due to carbon 

limitation. Plant Cell and Environment. 32: 22-30.

Seymour, R.S.; Kenefic, L.S. 2002. Influence of age 

on growth efficiency of Tsuga canadensis and 

Picea rubens trees in mixed-species, multi-aged 

northern conifer stands. Canadian Journal of 

Forest Research. 32: 2032-2042.

Sillet, S.C.; Van Pelt, R.; Koch, G.W.; Ambrose, A.R.; 

Carroll, A.L.; Antoine, M.E.; Mifsud, B.M. 2010. 

Increasing wood production through old age in 

tall trees. Forest Ecology Management. 259: 

976-994.

Spencer, K.J. 2010. Addressing the age-related 

growth decline in red spruce productivity: 

integrating physiological and genomic 

approaches. Orono, ME: University of Maine. 

71 p. M.S. thesis.

Vincent, T.L.; Van, M.V.; Geh, B.S. 1996. Ecological 

stability, evolutionary stability and the EES 

maximum principle. Functional Ecology. 10: 

567-591.

Woodruff, D.R.; Meinzer, F.C.; Lachenbruch, B.; 

Johnson, D.M. 2008. Coordination of leaf 

structure and gas exchange along a height 

gradient in a tall conifer. Tree Physiology. 

29: 261-272.

Yoder, B.J.; Ryan, M.G.; Waring, R.H.; Schoettle, 

A.W.; Kaufmann, M.R. 1994. Evidence of reduced 

photosynthetic rates in old trees. Forest Science. 

40: 513-527.

Zazzaro, S. 2009. Plasticity in response to changing 

light environment for red spruce and balsam fir.

Orono, ME: University of Maine. 74 p. M.S. thesis.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 147

Studying the Role of Wood-decay Fungi in 

Calcium Cycling on the Penobscot Experimental Forest: 

a Progress Report

Walter C. Shortle, Jody Jellison, Kevin T. Smith, and Jonathan S. Schilling

Abstract.—Depletion of essential mineral nutrients from the rooting zone of trees in 

northern forests may reduce health and productivity. Long-term field investigations 

coupled with detailed laboratory studies enhance understanding of the biological 

processes and suggest means to address potential threats. One such investigation by the 

U.S. Department of Agriculture, Forest Service, in cooperation with the University of 

Maine, involves the role of wood-decay. fungi in cycling calcium (Ca) and other essential 

mineral nutrients. The investigation was established on the Penobscot Experimental 

Forest (Maine) in 1996 and 1997 and replicated in part on the Bartlett Experimental 

Forest (New Hampshire) in 1995. Some initial findings were: (1) A significant gain in 

Ca concentration in decaying wood occurred by 6 yr and that gain was sustained through 

10 yr; (2) a significant gain in wood potassium was observed at 2 yr, but the gain was not 

sustained; and (3) observed changes in magnesium concentration in decaying wood were 

variable. Plans to continue this unique and important long-term study are described in 

this report. 

INTRODUCTION

Calcium (Ca) is the fifth most abundant element 

in the Earth’s crust. Other than nitrogen (N), Ca is 

considered the most important essential mineral for 

managing plant diseases (Rahman and Punja 2007). It 

is also the fifth most abundant element in trees after 

hydrogen (H), carbon (C), oxygen (O), and N (Shortle 

et al. 2008). Calcium is a structural link for wood 

components and regulates acidity, signals changes in 

various biological functions, and is needed to form 

protective layers in wood and bark (McLaughlin and 

Wimmer 1999). Therefore, living trees require a steady 

supply of Ca for wood formation and protection.

The depletion of root-available Ca from northern 

forest soils can occur over time by the processes 

of podzolization (Ponomareva 1969) and can be 

accelerated by acid deposition, the input of nonbiological acidity resulting from regional emissions 

of sulfur (S) and N oxides (Shortle and Bondietti 

1992). Declines in stem growth and mortality due to 

Ca depletion followed by aluminum (Al) mobilization 

have been documented in spruce (Picea spp.) 

(Lawrence et al. 1995, 2005; Shortle and Smith 1988; 

Shortle et al. 1997) and maple (Acer spp.) (Johnson 

et al. 2008, Lawrence et al. 1999, Long et al. 2009, 

St.Clair et al. 2008, Zaccherio and Finzi 2007).

As trees die and woody parts are shed or broken, 

wood is added to the forest floor. Root-available Ca 

is replaced in depleted sites by the action of wooddecay fungi that both release the solar energy stored 

in cellulose and lignin, the two most abundant organic 

substances in nature, and enrich the decayed wood 

residue with Ca from external sources. Microcosm 

tests demonstrate movement of Ca into decaying 

wood of conifers (Connolly et al. 1999, Ostrofsky 

et al. 1997) and hardwoods (Clinton et al. 2009). 

Many of the fungi that decompose wood are large, 

long-lived organisms that produce extensive mycelial 

networks, including cords and rhizomorphs, which 

move essential elements for many meters through 

the forest floor in and out of decaying wood (Boddy 

and Watkinson 1995, Connolly and Jellison 1997, 
148 GTR-NRS-P-123 Penobscot Experimental Forest: 

Lindahl et al. 2001). Although commonly regarded as 

microorganisms, the dominant wood-decay fungi are 

anything but “micro-”.

The purposes of the studies established on the 

Penobscot Experimental Forest (PEF) in Maine in 

1996 and 1997 and on the Bartlett Experimental 

Forest (BEF) in New Hampshire in 1995 were (1) to 

determine changes in Ca and the other two essential 

base cations, magnesium (Mg) and potassium (K), as 

well as the acid-mobilized metals manganese (Mn), 

Al, and iron (Fe) in wood decaying in ground contact 

for at least 15 yr; (2) to determine changes in these 

elements in the organic and underlying mineral soil 

contiguous with the decaying wood; and (3) to archive 

wood samples in progressive stages of decay that 

preserve features indicative of biological processes 

of decay and the incorporation of residues into soil. 

Some key features being studied are differences in 

decay type (brown-rot, white-rot), variations in cation 

solubility and exchange properties, and modifications 

to cell wall polymers, especially lignin.

METHODS

The tree species selected for study were red spruce 

(P. rubens Sarg.) on the PEF and BEF, red maple 

(A. rubrum L.) at both locations, eastern hemlock 

(Tsuga canadensis [L.] Carr.) on the PEF, and paper 

birch (Betula papyrifera Marsh.) on the PEF. For 

each combination of location and species, 3 groups of 

10 trees in a dominant or co-dominant position and 

15- to 45-cm diameter at 1.3 m above ground were 

selected and tagged to identify the tree and treatment. 

Treatments were assigned at random. One group was 

used as an uncut reference for soil samples. Trees in 

a second group were cut and the felled trees left in 

place to decay in a gap in the canopy. Trees in the third 

group were cut and sections of the felled trees were 

hauled and placed under the forest canopy to decay 

while the gap was left with only stumps to decay (all 

tops and branches were removed). 

Time-zero reference disks, 5 cm thick, were cut from 

all felled stems at 3 m and 7 m above the stumps. 

Decaying wood was subsequently collected at 2-yr 

intervals from the intervening 4-m bolt. Small soil 

pits dug next to the bolt provided samples of the 

O-horizon and the underlying 10 cm of mineral soil. 

Soil pits in the reference group and the stump-only 

area were taken at approximately where the 4-m bolt 

would have been if trees were felled. The position of 

each decaying bolt was mapped for future reference. 

Decaying wood was collected for chemical analysis 

by removing and discarding a 10-cm length from the 

lower end of each bolt followed by the removal and 

retention of a 5-cm-thick sample disk. Small soil pits 

were used to sample soil contiguous with the decaying 

stem at 10 and 15 yr, taking care to avoid the location 

of the initial sampling pits.

For chemical analysis, wood disks were air-dried at 

room temperature and then oven-dried at 90 °C. 

Rectangular prism blocks were split from the 

sapwood of dried disks from a position 90° around 

the stem from the point of soil contact. The volume 

of each block was calculated from the mean of four 

measurements of each dimension (longitudinal, radial, 

and tangential) and weighed (± 1 mg). Density 

(g cm-3) was calculated as the mass to volume ratio. 

After density was measured, blocks were chiseled 

into small pieces and ground in a benchtop Wiley mill 

(Thomas Scientific, Swedesboro, NJ) to pass a 1-mm 

mesh. One-gram portions of milled wood powder were 

ashed for 6 h at 550 °C, cooled, dissolved in 3 mL 

of 6 M HCl, and brought to a volume of 50 mL with 

deionized water. Concentrations of Ca, Mg, K, Mn, 

Al, and Fe in ash solutions, analytical standards, and 

blanks were measured by inductively coupled plasma 

optical emission spectroscopy (ICP-OES) (Model 750, 

Thermo Jarrell Ash Corp., Franklin, MA).

Element concentrations determined by ICP-OES were 

converted from parts per million (ppm) to mmol kg-1

by dividing ppm by the atomic weight. Concentrations 

for comparison on a constant volume basis, mol m-3, 

were obtained as the product of the mass-based 

concentration, mmol kg-1, and the wood density, 

g cm-3. For each set of wood samples taken at 2-yr 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 149

intervals the mean and 95-percent confidence intervals 

of replicate samples were determined. Statistical 

analysis is presented in the initial publication of results 

through 6-8 yr (Smith et al. 2007).

Soil samples taken at 0 yr for each location and species 

and at 10 yr for spruce on the PEF, maple on the BEF 

and PEF, and birch on the PEF were analyzed using a 

suite of protocols approved by the U.S. Environmental 

Protection Agency (EPA) for forest soil analysis (loss 

on ignition, pH, acidity, cation exchange capacity, total 

Kjeldahl nitrogen, P, Na, Ca, K, Mg, Fe, Al, Zn) in the 

EPA-certified analytical laboratory at the University 

of Maine, Orono. Soil sampling was planned for 15 yr 

for each location and species. Decaying wood samples 

taken at 10, 12, and 15 yr are being analyzed by the 

same forest soil protocols applied to the O-horizon 

samples, in addition to the plant tissue protocol 

previously described, so that the nutrient status of the 

forest floor and wood decaying on the floor can be 

compared after the first decade of ground contact.

PRELIMINARY RESULTS

Preliminary results through the first 6-8 yr (Smith et 

al. 2007) indicated a significant accumulation of Ca in 

decaying wood in all tree species at both locations 

(30-90 percent increase after 6-8 yr). As the wood 

decayed and Ca was accumulated, Mg concentrations 

were sustained at approximately the initial 

concentration or had a small decrease of about 

20 percent in some cases. More-detailed results for 

changes taking place at 2-yr intervals in decaying 

wood for the first 12 yr will soon be available, along 

with comparisons of decaying wood and organic soil 

at 10 and 12 yr.

Sampling decaying wood and soil at 15 yr on the BEF 

has been completed and chemical analyses are being 

performed. Sampling decaying wood of spruce and 

maple on the PEF was completed at 15 yr in 2011, 

but soil sampling was delayed until 2012 due to 

standing water on the plots. Soil samples were taken 

at the maple plots on the PEF in 2012, but standing 

water again delayed soil sampling at spruce plots until 

2013. Sampling decaying wood and soil of hemlock 

and birch on the PEF has been suspended until work 

on spruce and maple has been completed. Subsets 

of archived decaying wood samples previously 

maintained at the University of Maine have been 

moved to other locations and are available to those 

interested in studying wood decay processes.

OUTLOOK

With our work on the Penobscot and Bartlett 

Experimental Forests, we have demonstrated an 

important dynamic of the biogeochemistry of 

northern forests, the translocation of essential Ca 

by wood-decay fungi. This work on the long-term 

effects of wood decay complements the existing 

understanding of the effects of forest management 

on Ca cycling. This process is driven by large, longlived fungi in these forests and is far more dynamic 

than decomposition of a carbon stock—it is a 

unique connection between the forest floor and the 

atmosphere. 

LITERATURE CITED

Boddy, L.; Watkinson, S.C. 1995. Wood 

decomposition, higher fungi, and their role in 

nutrient redistribution. Canadian Journal of 

Botany. 73(Suppl. 1): S1377-S1383.

Clinton, P.W.; Buchanan, P.K.; Wilke, J.P.; Small, 

S.J.; Kimberley, M.O. 2009. Decomposition 

of Nothofagus wood in vitro and nutrient 

mobilization by fungi. Canadian Journal of Forest 

Research. 39: 2193-2202.

Connolly, J.H.; Jellison, J. 1997. Two-way 

translocation of cations by the brown rot 

fungus, Gloeophyllum trabeum. International 

Biodeterioration and Biodegradation. 39: 181-188.

Connolly, J.H.; Shortle, W.C.; Jellison, J. 1999. 

Weathering of mineral matter by a nonmychorrhizal fungus. Canadian Journal of 

Botany. 77: 179-187.
150 GTR-NRS-P-123 Penobscot Experimental Forest: 

Johnson, A.H.; Moyer, A.; Bedison, J.E.; Richter, 

S.L.; Willig, S.A. 2008. Seven decades of calcium 

depletion in organic horizons of Adirondack 

forest soils. Soil Science Society of America 

Journal. 72: 1824-1830.

Lawrence, G.B.; David, M.B.; Shortle, W.C. 1995. A 

new mechanism for calcium loss in forest-floor 

soils. Nature. 378: 162-165.

Lawrence, G.B.; David, M.B.; Shortle, W.C.; Bailey, 

S.W.; Lovett, G.M. 1999. Mechanisms of basecation depletion by acid deposition in forest soils 

of the northeastern U.S. Gen. Tech. Rep. NE-261. 

Radnor, PA: U.S. Department of Agriculture, Forest 

Service: 75-97.

Lawrence, G.B.; Lapenis, A.G.; Berggren, D.; 

Aparin, B.F.; Smith, K.T.; Shortle, W.C.; Bailey, 

S.W.; Varlyguin, D.; Babakov, B. 2005. Climate 

dependency of tree growth suppressed by acid 

deposition effects on soils in northwest Russia.

Environmental Science and Technology. 39: 

2004-2010.

Lindahl, B.; Finlay, R.; Olsson, S. 2001. 

Simultaneous, bidirectional translocation of 

32P and 33P between wood blocks connected by 

mycelia cords of Hypholoma fasciculare. New 

Phytologist. 150: 189-194.

Long, R.P.; Horsley, S.B.; Hallet, R.A.; Bailey, 

S.B. 2009. Sugar maple growth in relation to 

nutrition and stress in the northeastern United 

States. Ecological Applications. 19: 1454-1466.

McLaughlin, S.B.; Wimmer, R. 1999. Tansley Review 

No. 104: calcium physiology and terrestrial 

ecosystem processes. New Phytology. 142: 

373-417.

Ostrofsky, A.; Jellison, J.; Smith, K.T.; Shortle, W.C. 

1997. Changes in cation concentrations in red 

spruce wood decayed by brown rot and white 

rot fungi. Canadian Journal of Forest Research. 

27: 567-571.

Ponomareva, V.V. 1969. Theory of podzolization.

Gourevitch, A., transl. Israel Program for Scientific 

Translation, TT 68-50442. Springfield, VA: 

National Technical Information Service. 309 p.

Rahman, M.; Punja, Z.K. 2007. Calcium and plant 

disease. In: Datnoff, L.E.; Elmer, W.H.; Huber, 

D.M., eds. Mineral nutrition and plant disease.

St. Paul, MN: APS Press: 79-93. 

Shortle, W.C.; Bondietti, E.A. 1992. Timing, 

magnitude, and impact of acidic deposition on 

sensitive forest sites. Water, Air, and Soil Pollution. 

61: 253-267. 

Shortle, W.C.; Murdoch, P.S.; Smith, K.T.; Minocha, 

R.; Lawrence, G.B. 2008. Monitoring recovery 

from calcium depletion and nitrogen saturation.

In: Murdoch, P.S.; Jenkins, J.; Birdsey, R.A., 

eds. The Delaware River Basin Collaborative 

Environmental Monitoring and Research Initiative: 

foundation. Gen. Tech. Rep. NRS-25. Newtown 

Square, PA: U.S. Department of Agriculture, Forest 

Service, Northern Research Station: 52-62.

Shortle, W.C.; Smith, K.T. 1988. Aluminum-induced 

calcium deficiency syndrome in declining red 

spruce. Science. 240: 1017-1018.

Shortle, W.C.; Smith, K.T.; Minocha, R.; Lawrence, 

G.B.; David, M.B. 1997. Acidic deposition, 

cation mobilization, and biochemical indicators 

of stress in healthy red spruce. Journal of 

Environmental Quality. 26: 871-876.

Smith, K.T.; Shortle, W.C.; Jellison, J.; Connolly, J.; 

Schilling, J. 2007. Concentrations of Ca and Mg 

in early stages of sapwood decay in red spruce, 

eastern hemlock, red maple, and paper birch.

Canadian Journal of Forest Research. 37: 

957-965. Available at http://nrs.fs.fed.us/pubs/2888. 

(Accessed 2 March 2011).

St.Clair, S.B.; Sharpe, W.E.; Lynch, J.P. 2008. Key 

interactions between nutrient limitations and 

climatic factors in temperate forests: a synthesis 

of the sugar maple literature. Canadian Journal of 

Forest Research. 38: 401-414.

Zaccherio, M.T.; Finzi, A.C. 2007. Atmospheric 

deposition may affect northern hardwood forest 

composition by altering soil nutrient supply.

Ecological Applications. 17: 1929-1941.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 151

Bob Frank’s Recollections Made on the Occasion of the

Penobscot Experimental Forest’s 60th Anniversary

Robert M. Frank, Jr., and Laura S. Kenefic

Abstract.—Robert M. (Bob) Frank, Jr., spent his career with the U.S. Forest Service 

and oversaw the long-term silvicultural research on the Penobscot Experimental Forest 

in Maine for nearly 30 years. His reflections here span more than four decades, from 

his first days with the Forest Service until his retirement in 1996. He touches upon the 

agency’s relations with members of the forest industry and the public, changes in the 

agency’s culture and funding over time, and his role in establishing and sustaining 

long-term studies that continue to this day.

INTRODUCTION

Robert M. (Bob) Frank, Jr., (Fig. 1) began his career 

with the U.S. Department of Agriculture, Forest 

Service as a permanent employee in 1957 on a Forest 

Survey (now Forest Inventory and Analysis) crew at 

Shin Pond, Maine. He later worked in the Anthracite 

Region of Pennsylvania before being transferred to the 

research office in Orono, Maine. Frank was a research 

forester (silviculturist) at the Penobscot Experimental 

Forest (PEF) from 1963 until his retirement in 1996. 

He had primary responsibility for the Forest Service’s 

long-term silvicultural experiments on the PEF from 

the late 1960s until the end of his career. 

Figure 1.—Research forester Robert M. (Bob) Frank, Jr., in his U.S. Forest Service office in Orono, Maine (1993). Photo by 

U.S. Forest Service.
152 GTR-NRS-P-123 Penobscot Experimental Forest: 

The following text represents the highlights of a 

2-hour conversation Frank had with Laura Kenefic on 

February 15, 2011. This conversation took the form of 

an interview in which Frank answered questions posed 

by Kenefic and John Brissette about his career. The 

text was transcribed by Matsuye Mairs, excerpted and 

edited by Kenefic, and reviewed and revised by Frank. 

Frank’s perspective is important not only because of 

his long tenure at the PEF, but because he is credited 

with sustaining the long-term research through years 

of waning organizational interest and investment. 

Frank is regarded by many of his peers as largely 

responsible for the existence of the more than 60-yearold Forest Service silvicultural study on the PEF today. 

He received the David M. Smith Award in Silviculture 

from the New England Society of American Foresters 

in 2012 in honor of his contributions to his profession.

EARLY INFLUENCES

I was always interested in wood, even in grammar 

school and high school. I lived in Newark, New Jersey. 

There were not many trees in Newark. My uncle had 

a camp in northern New Jersey and we would go there 

quite frequently. I just fell in love with being away 

from the Ironbound section of Newark. When I was 

10 years old, in 1942, I saw a movie called “The 

Forest Rangers,” starring Rod Cameron, Susan 

Hayward, and Fred MacMurray. It was about a forest 

fire and had some Hollywood romance. I said “Boy, 

it would be great to be a forest ranger!” I was only 

10 years old, and that was the start of it. Fortunately, 

the pieces came together and I am where I am today.

START OF CAREER 

WITH THE FOREST SERVICE

My first job with the U.S. Forest Service was in 1953 

in the state of Washington on the Gifford Pinchot 

National Forest, at the Randle Ranger District. Back 

in the 1950s and 1960s, it was highly recommended 

by the deans and directors of forestry schools that 

students spend one summer with the Forest Service. 

A lot of Forest Service jobs were available back then. 

In my training it was suggested that we get experience 

in different timber types. Coming from Penn State in 

central Pennsylvania, it was nice going out west and 

seeing the different ecosystems there. I was hired as 

a fire guard, but 1953 was not a season of fire threats. 

I spent most of my time hammering wooden shakes 

on warehouse building roofs. But I got to know some 

Forest Service personnel and we remained friends for 

a long time. I just felt comfortable perhaps pursuing a 

Forest Service job. 

When I finished my Master’s degree I moved to 

Boston to sell wholesale lumber. After a few months, 

I said to myself, “What am I doing in downtown 

Boston with over 6 years of education in forestry?” So 

I wrote to my advisor at Penn State. I told him I wasn’t 

happy, and asked if there was anything else he might 

suggest. He sent me a list of six possibilities around 

the country. That was in 1957 and there were a lot of 

jobs for foresters back then. I saw one possibility: a 

temporary job in northern Maine on Forest Inventory, 

called Forest Survey back then. So I applied for it and 

was granted that position. 

That was the start of what led to my permanent Forest 

Service career. I reported for work at Shin Pond, 

Maine—population 16—on June 17, 1957. Research 

folks had a good policy at that time; Station Director 

Ralph Marquis thought it wise for budding researchers 

to get their feet wet by spending some time on Forest 

Inventory. Many of us came from suburban or city 

environments and this got us out in the woods. I would 

still recommend that this be done today, but of course 

things have changed. I stayed with Inventory for 

almost 4 years. 

A vacancy occurred in Maine where a temporary office 

was established in what became the Orono Unit. I was 

ready to move to Maine when another chance came 

up. A pastor in Mount Carmel, Pennsylvania, was 

upset about strip mining in the anthracite coal fields 

of Pennsylvania. He had contacted a Congressional 

person, who contacted the Chief of the Forest Service, 

and—to make a long story short—the Station was 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 153

asked to conduct a survey of the lands disturbed by 

strip mining and coal processing plants throughout 

the entire anthracite area. This was before remote 

sensing, in the early 1960s. They asked me if I would 

be interested in doing this project, which would take 

a year or two. I asked, “Well, what about the offer to 

go to Maine?” The Director shook my hand and said, 

“It’s a deal. When you finish this job, we’ll ship you 

to Maine.” Without any written documentation or 

bureaucratic bumbling, the Director arranged that 

I should go to Maine. 

After I completed my assignment in Pennsylvania, I 

reported to duty in Maine on April Fools’ Day in 1963. 

As I remember, the temporary office was on the third 

floor of the library on campus at the University of 

Maine in Orono. The building which would contain 

the USDA office was being constructed at that time. 

I remember visiting there during February of that year 

to see the new building. I asked the foreman to let me 

walk the steel girders, and he showed me where my 

room was going to be. That was kind of neat. 

We only stayed a short time in the library and then 

the building was ready to be occupied. We moved to 

our new building sometime in the spring of 1963. We 

stayed in the USDA building until near the end of my 

career, when the university needed the space and we 

were asked to vacate. We moved to a new building off 

campus, on Godfrey Drive in Orono.

FIRST IMPRESSIONS OF THE PEF

The roads on the Experimental Forest were narrower 

than they are now, and we did not have the signage 

that we later developed. I had some experience with 

the timber type because I spent the better part of two 

growing seasons in northern Maine. One of the big 

differences was the amount of hemlock we had in our 

stands here on the Experimental Forest. Also, there 

was less spruce than you would see in the so-called 

spruce-fir part of Maine. We had more hardwoods in 

some of our sites. They were managed even at that 

point in time; they were eliminating red maple, as 

I recall. The plantation of pine was very young; it was 

planted in the late 1950s. Also, the pine trees at the 

museum1

 site are certainly a lot bigger now than they 

were 50 years ago.

FRANK’S ROLE AT THE PEF

I was told before I left Upper Darby [Pennsylvania] 

that the main reason I would be going to Maine was to 

address the problems of spruce-fir regeneration in the 

Compartment Study2

. The Station wanted to establish 

a system of inventory for regeneration. Regeneration 

was not being studied on the Experimental Forest. That 

was a top-down decision from Station headquarters, 

but you have to remember, the Station was much 

smaller then.

So that was my first job: to establish a measurement 

system for very small trees [Fig. 2]. Up to that 

point, very little was known about the effect of the 

various treatments on regeneration. We researched 

the measurement problem and had statisticians help 

us and suggest what we should do. The methodology 

of putting in the plots, we actually got that from 

reviewing the literature. We developed a system that 

I guess to some degree is still in operation. You really 

get to know the different areas on the Experimental 

Forest when you get on your hands and knees and 

spend many, many hours looking at regeneration.

I want to mention Orman Carroll [Fig. 3], who was our 

first technician. Orman was in the logging crew that 

helped establish our long-term study, which started in 

1952. But somewhere near the end—it took several 

years to install the study; I believe until 1957—they 

asked Orman if he would be willing to become an 

employee of the Forest Service. This happened before 

I was stationed in Maine. The Forest Service wanted 

him to take over from the logging crews, because 

they knew there were going to be periodic reentries. 

1

 Maine Forest and Logging Museum at Leonard’s Mills.

2

 The long-term, large-scale silvicultural experiment on 

the PEF, consisting of even- and uneven-aged silvicultural 

treatments and exploitative cuttings, replicated at the stand 

level.
154 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 2.—Bob Frank inspecting red spruce sapling growth 

on the Penobscot Experimental Forest (ca. 1965). Photo by 

U.S. Forest Service.

Orman accepted and stayed with us until 1979, when 

he retired. But he was a Cracker Jack technician. He 

was a hands-on person and could repair equipment. 

He knew his trees, he knew how to get around in 

the woods, and he was dedicated. Hours did not 

mean anything to him; rain did not mean anything 

to him. He was just the all-around technician that an 

experimental forest needs. And a great guy to work 

with, too.

Arthur Hart [Fig. 4] was Project Leader; he had taken 

over from Frank Longwood about that time. Arthur 

Hart was a marvelous person. He not only became 

my advisor, but he was my friend and, most of all, my 

mentor. Unfortunately, he became ill with cancer early 

in 1968 and passed away in 1969. That changed things 

on the Experimental Forest. I was still relatively new. 

Much of the responsibility fell on me to continue many 

PEF activities. The Compartment Study was a large 

study. There were 28 compartments at the time.

Figure 3.—Penobscot Experimental Forest technician Orman 

Carroll (1976). Photo by U.S. Forest Service.

Figure 4.—Arthur Hart measuring tree diameter on the 

Penobscot Experimental Forest (ca. 1960). Photo by U.S. 

Forest Service.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 155

THE PEF OPERATING COMMITTEE

The PEF Operating Committee was a small group of 

industry people—representatives of the landowners—

that kept tabs on what we were doing. They were there 

when we needed help from them. What was nice about 

that was we always had a little cash from the sale of 

timber from our experiments. This money was used for 

research, mainly to hire students for the summer when 

our Forest Service budgets wouldn’t allow for it. At 

that time there was a lot of comradeship amongst the 

forestry fraternity and that does not exist as strongly 

today. It was a time when we had the great companies 

that no longer exist. These were the folks that wanted 

the Forest Service to establish a research unit in 

Maine. It happened and we should be thankful that 

it is still progressing after 60 years.

MEETING THE NEEDS 

OF FOREST INDUSTRY

Of course the long-term experiment—the 

Compartment Study—did not occupy all my time. 

In the late 1960s forest industry was facing a labor 

shortage. It was difficult to get people to work in 

the woods. When I started at the PEF, some of the 

logging was done with horses. At the same time 

that we had horse logging, we would occasionally 

get operators who had “jitterbugs” or small cleat 

tractors. I remember in early 1969, one of the last 

operations that Mr. Hart was involved with, we had 

our first skidder come on site. He was apprehensive 

and said, “My gosh, this might be the end of some 

of our regeneration.” Well, that proved not to be the 

case, because we could detect very little difference 

between operations done with horses, jitterbugs, 

or small skidders. There was a need for some other 

means of getting trees from the stump to roadside, and 

mechanization came into play. 

Clearcutting was drawing the attention of some 

segments of society at that time, and generating 

controversy. Many clearcuts were commercial 

clearcuts, even on company lands—by no means 

true clearcuts. There was little market for low-grade 

hardwoods and the smaller softwoods, so they were 

left on site. I remember the PEF Operating Committee 

stating, “We need studies to show what will happen to 

advance regeneration when we use larger equipment 

and when we clearcut.” And that was the reason for the 

strip cutting study3

 here on the Experimental Forest—

perhaps one of the first times this harvesting method 

was tried in Maine [Fig. 5].

About that same time, work was being done in Fish 

River country in Aroostook County where the first 

mechanical harvester—a Beloit tree harvester—was 

brought onto land owned by Seven Islands Land 

Company, I believe Prentiss and Carlisle, and Great 

Northern Paper Company. This was a big machine 

with tracks. The plan was to harvest by clearing strips 

of different widths and different orientations. Many 

people were invited; there were probably 30 to 40 

forestry-oriented people watching this machine operate 

[Fig. 6]. You could hear comments like “This will be 

the end of the Maine forest” and “The Maine forest 

will not survive this machine.” The machine had a 

60-foot boom. It drove up to one tree at a time, 

3

 Compartment 33; see Bjorkbom and Frank (1968), 

Czapowskyj et al. (1977), and Frank and Safford (1970).

Figure 5.—Bob Frank explaining the experimental design 

for the strip cutting experiment (Compartment 33) on the 

Penobscot Experimental Forest (1969). Photo by U.S. 

Forest Service.
156 GTR-NRS-P-123 Penobscot Experimental Forest: 

Figure 6.—Foresters on a field trip to see the Beloit tree harvester in operation in northern Maine (1967). Photo by U.S. 

Forest Service.

delimbed it, topped it, and laid entire stems down in 

small bunches. Then a rubber-tired grapple skidder 

moved out bunches of these long stems to a landing.

I saw this as an opportunity to conduct research. It 

was one of the most miserable jobs I worked on. 

The resulting tangle of slash and other debris was 

almost overwhelming. The crew and I were putting 

in temporary regeneration plots on industry land to 

try and assess the effects on advance regeneration. 

I worked with a forester from Great Northern and 

we published some of the results.4

 We found that 

the machine itself did not cause problems because it 

rode on its own brush, but where the grapple skidder 

repeatedly traveled, that was the problem. I believe 

those cleared strips have since been logged again.

4

 Frank and Putnam (1972).

That was the beginning of mechanization and 

clearcutting, and of course the clearcutting debate 

continued. That forced us here on the Experimental 

Forest to look more at machines. What is unique about 

this property is that we would use logging systems that 

were in vogue at the time as much as possible. I think 

that has served us well. 

MANAGEMENT INTENSITY 

DEMONSTRATION AREAS

In the late 1960s, Arthur Hart was notified, I think by 

the Washington Office, that we were an experimental 

forest, not a demonstration forest. Therefore, we had 

to eliminate our demonstration areas. At that time 

we had a 40-acre block as a demonstration area. 

These MIDs, or management intensity demonstration 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 157

areas, existed on many Forest Service experimental 

forests5

. We decided that if we eliminated this 40-acre 

area, it would be difficult to take many of the groups 

we had then, like schoolchildren or high school or 

college students, on silvicultural tours. In a short 

period of time in the woods, this demonstration area 

gave many individuals their initial exposure to forest 

management procedures; perhaps their appetites for 

additional forestry knowledge would start on this area! 

We decided to modify and streamline our procedures 

in order to reduce the work required to maintain the 

demonstration. 

The MIDs were small in area for statistical analysis. 

We pretty much had to do 100-percent inventories. 

People back then didn’t have the finesse we have now 

in statistics. So we originally measured every tree, 

which was too much. But we could really measure 

those trees! We had technicians who would run 

through those trees, and we got results. 

We modified the treatments through time; I am happy 

to hear that they are being kept up. 

FOREST SERVICE CULTURE

I was first exposed to Forest Service culture in 

Washington State at the Ranger District in Randle. I 

liked it. I saw cohesiveness amongst the workers, from 

the District Rangers to the mule packers. It was great. 

Being a student at the time, I was invited to loggers’ 

homes for dinner and so forth, and we got to know 

people quite well. When I came to Orono, because 

we were a small unit at the time, we were surrounded 

by non-Forest Service personnel, mainly industrial. 

You always stood out in the crowd. You were the only 

federal person, or maybe one other with you. But 

because of our exposure to various groups, we got to 

know these people not only professionally but socially. 

Back in the 1960s and 1970s, I remember being 

invited to my project leader’s home for dinner. I will 

never forget the first visit. God bless Min Hart, Arthur 

5

 Also called Cutting Practice Level plots.

Hart’s widow. She is still alive and lives next door 

to us; she is 93. The first time my wife Dorothy and 

I visited them was in 1964. I know Min wanted to 

meet my wife and that was one of the reasons we were 

invited. We had a delightful evening. When we left, 

she said—this was in the summertime—“Now, we 

go to bed early, so please don’t contact us after 8:30 

because we get up at 3 in the morning. We do all our 

chores and tend to our garden before we go to work 

at 8 o’clock.” And I respect that time to this day and 

never call her after 8:30. 

Even earlier, in the 1950s, we had Christmas parties in 

Upper Darby. Everyone at Station Headquarters would 

be invited to a restaurant and you got to talk to the 

Division Chiefs, Directors, etc. After I was transferred 

to the Orono Unit, something they did—that I believe 

is not done today—when someone from Headquarters 

or from another unit visited, you invited them over for 

dinner. We would be invited to one of the scientists’ 

homes. When Director Marquis, or Assistant Director 

Warren Doolittle, or others including the Station 

Editor or Station Statistician visited, we would always 

entertain in someone’s home. I remember once the 

Station Editor came to talk about the preparation of 

publications. My wife Dorothy and I invited him over 

for dinner. It was an awful, icy winter night; we had 

to walk about 400 feet down to the house. Everything 

went well, we had a nice dinner, it was a good visit, 

and I drove him back to his motel. We did a lot of 

that and even went on picnics in the summertime. We 

do less of that in the Forest Service now. Society has 

changed.

TECHNOLOGY TRANSFER

Throughout my career there was a pull and tug. I knew 

I had to produce manuscripts in order to stay in the 

good graces of the organization. On the other hand, 

I dealt with so many people in forestry who were 

potential users of our results who I knew would not 

spend much time reading publications. I tried to make 

the publications I did write as practical as I could. I 

wanted them to be guides for people to use in their 

work. 
158 GTR-NRS-P-123 Penobscot Experimental Forest: 

I did not solicit people for tours on the Experimental 

Forest. I did not suggest that when they go home, 

they tell others to come. They just came. And it 

increased year after year. Most of them were industrial 

foresters and government foresters. We also had 

many organizational groups including The World 

Forestry Center from Portland, Oregon [Fig. 7]. We 

had foreigners and I believe that continues to this 

day. We had many visitors from academia, from the 

local colleges and other places including Canadian 

provinces from Ontario to Newfoundland. It was 

always a treat when you could spend some time, 

have all your ducks in a row, have nice signs that 

impress people, have literature to hand them, and 

even publications. It was the entire package I tried 

to present. I was always willing to answer questions 

while they were here. I think it worked well with 

industrial people, with graduate students and so forth. 

And I might add, since I retired, the ownership has 

gone to the University [of Maine] and the use of the 

Experimental Forest—based on the base that was built 

here—has increased many, many fold. 

REPLICATION IN THE 

COMPARTMENT STUDY

If there was one mistake I made—though I was never 

the final decisionmaker—it had to do with replication 

in the Compartment Study. The study plan was revised 

in 1974. We decided to reduce the work load without 

impacting the overall results. Unfortunately, we only 

had two replicates of our treatments. At the time, 

we eliminated some compartments. Most of these 

were eliminated based on the soil-site conditions; we 

eliminated those that seemed to have variation from 

the norm. Those became what we called units. We kept 

them in the state of readiness for future research. Had 

we created another replicate from them, we might have 

bolstered the strength of the experiment. 

We did not have much additional suitable land 

within the Experimental Forest, but we might have 

found some areas where we could have started new 

compartments. That was about 20 years into the 

experiment; now we are 60 years in and we would 

Figure 7.—Visitors from the World Forestry Center in Portland, Oregon, view Bob Frank’s model of tree growth response by 

species and treatment following precommercial thinning in Study 58 on the Penobscot Experimental Forest (1987). Photo by 

U.S. Forest Service.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 159

have had 40 years more experience. But we didn’t do 

that. Nor did we get together with other experimental 

forests. Maybe these could be future goals.

MOTIVATION TO 

CONTINUE THE WORK

I was having fun. I really enjoyed my work. It was 

satisfactory and I truly felt we were producing results 

beneficial to many user groups. If I had to do it over 

again—only a fool would say they wouldn’t change 

anything—but as far as my forestry career was 

concerned, I would do it again. 

I do not know if this is common knowledge or not, 

but I was offered a position with a company in Maine, 

to start a research unit. It was Great Northern Paper 

Company. I had 3 weeks to make a decision. I was 

mid-career, and I was thinking, “I am established in 

my home, in my job, and in my church. My kids are in 

school. Do I want to change that?” I really wanted to 

work for industry. Now, when I think of that offer, I’m 

thankful I did not take it. 

I had passion to continue this work. I was always 

hoping it would continue beyond me, and it is in 

good hands now. One of the things that really made 

it happen was computer systems. How could we ever 

acquire information about problem insects, invasive 

species, you name it, if we did not have a good data 

set?

COMPUTERS

Managing the data was always a big job before we 

had computers. My initial and continuing fear with 

computers is that when the analysis is done manually, 

you are likely to only make little mistakes, but with 

computers you are likely to make big mistakes. To 

minimize mistakes, you need someone familiar with 

what is happening in the field to look at the data in 

order to detect grave mistakes. 

When computerization was in its infancy, we started 

numbering individual trees in the field [in the 

Compartment Study]; I thought that was an interesting 

improvement. In order to number trees, we had to 

create a system to minimize the effort and control 

errors on remeasured plots, because much of the 

field work was done by temporary people during the 

summer months. I went out into the field as much as I 

dared. We usually had good technician coverage, but 

mistakes were being made. We reviewed the research 

on how to number trees, and there were several ways 

of doing it. The student (I always call them students; 

they were temporary employees) we had at the time 

was Bruce Birr. It’s funny that we are talking about 

this now; just the other day I got a letter from John 

Brissette saying Bruce now works at Rhinelander 

[Wisconsin]. I haven’t seen him in 35 years. He was 

here in the early 1970s and was the one who went to 

the library to find ways to number trees. We came up 

with the “wagon wheel” approach. That worked well 

until we started to put numbers on trees down to the 

1-inch class. We struggled. But we kept the trees 

in order by dividing the circular plots into ten 36° 

pie-shaped segments. This procedure enhanced the 

accuracy of the measurements we were taking. I 

don’t know exactly how it’s done now.6

Our concern was: How many digits will fit on a small 

tree? If you add just one more digit, that multiplies 

how much numbering must be done. Do our plots have 

to be 1/20-acre for the smallest trees? Could we make 

these plots smaller and maintain statistical accuracy? 

I’m not sure how that is unraveling now.7

I thought when we started numbering trees and 

accumulating data and were able to measure the 

growth of individual trees, that somehow we could 

correlate this growth with soil type; I mean specific 

soil type, or the soil on which that one tree was 

growing! I remember talking to soil scientists about 

this over the years. I looked at available soils maps and 

knew these were of little help. You actually have to 

determine what soil each tree is growing in. It would 

be an interesting project to get the data necessary to 

6

 Comment: This system is still in use.

7

 Comment: The measurement plots for saplings are now 

1/50-acre instead of 1/20-acre.
160 GTR-NRS-P-123 Penobscot Experimental Forest: 

correlate individual tree growth with specific soil data. 

An additional enhancement in an analysis of this type 

would be to factor in the relation of the subject tree to 

adjacent trees. Are they free to grow on one side, two 

sides, three sides? You should be able to get a better 

handle on the association between the growth of trees 

and soil.

BUDGET CUTS OF THE 1980s

The budget cuts were large. Here’s how we survived. 

In a sense, it was a tense time. We all knew how 

budgets were shrinking. I believe a portion of one 

budget got cut 85 percent! I remember employing 

work-study students and volunteers to accomplish 

our field work. Once, we were able to enlist one or 

two elderly folks. I think it was a service group. That 

didn’t work out. We struggled. We also asked for and 

received some money from the companies from the 

PEF stumpage account.

But here’s another part of the budget story. It didn’t 

happen all at once; it took some time to unravel. Orono 

was on the list for possible closure as budgets were 

being reduced. Now, this is my interpretation. We 

heard from different sources, above the Project Leader 

level, perhaps above the Director level, that research 

done on the experimental forests was essentially for 

the National Forests. So what you guys are doing up 

there in Maine just didn’t fit the mold. 

These were hard times. Morale was down, people were 

transferred, and I was going to be transferred to New 

Hampshire. As I recall, the plan was that I would come 

back in the summertime and continue to work on the 

Experimental Forest. Of course you had to accept that 

possibility. I was not at the higher echelons of any 

decisionmaking, but a white paper was produced by 

the Director at the time, and it was suggested that it 

be distributed to our consumers. 

We could not lobby for increased funding. That was 

illegal; we couldn’t do that. So Project Leader Bart 

Blum and I put together lists of people we were going 

to visit. I took the northern part of the state and he 

took the southern part. We went to different people 

explaining, “You’ve been consumers of our work in 

the past, this is what we’ve done, and these are our 

publications.” We showed them the budgets, but we 

didn’t say anything like “Please, please, will you help 

us get more financing?” I do not know if that’s a blind 

elephant or not, but it might have appeared desperate 

at the time. 

Many, many letters were written to the Director 

supporting this unit. He later said, “I was impressed 

with the response and the support you had from all the 

difference agencies, companies, and organizations. 

I was really impressed with what you got from the 

Maine potato growers. What was that all about?” I 

said, “Potato fields are just a small percentage of the 

land they own. They own a lot of spruce-fir country.” 

That was the only way I knew how to answer. 

I think the letters had a significant influence; there 

was another influence also. I received a telephone call 

from a very influential person wanting to know if I 

would be willing to talk to Senator George Mitchell 

about our budget problems. He was scheduled to 

talk at Husson College [in Bangor, Maine] at a 

Businessman’s Breakfast. I attended those meetings. 

Many forestry people and business leaders in the 

community attended, also. Senator Mitchell was giving 

a talk. I was told that I had his time from when he left 

the lectern where he was giving his speech until he 

reached his automobile and I would walk with him. 

I was thinking, “What can I say to the Senator without 

violating what I’m not supposed to ask?” 

Here’s the interesting part. At this time there were 

two forces working against each other in Congress. 

George Mitchell was pushing the Clean Air Act and 

Robert Byrd did not like it. Senator Byrd was from 

West Virginia and he was concerned about the effect 

the Clean Air Act would have on the coal industry. So, 

somehow I had to weave this together. At that time 

I think we still had eight scientists in the Orono unit. 

I looked up how many scientists were in West Virginia. 

I thought, “Well, I can mention that.” 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 161

As I walked Senator Mitchell down to his automobile, 

I walked as slowly as I could. He was being very 

cordial and asked me questions about the work I was 

doing, who we were, and who we were doing the 

work for, and so forth. Trying to be as encompassing 

as I could, I said, “We have eight scientists here in 

Maine, they have X numbers here and there, and in 

West Virginia they have 52 scientists.” He stopped and 

looked at me. And he said, “I didn’t know that.” That 

was the only time he stopped. Shortly after that walk 

with Senator Mitchell, I believe the Director received 

a communication from him. And, as I was told—I’m 

getting this down through the levels—Senator Mitchell 

said, “If there is going to be research done for Maine, 

it is going to be done in Maine.” And that was the end 

of the unit moving from Orono, at least at that time. 

That’s the way I understood it.

TRANSFER OF THE PROPERTY 

TO THE UNIVERSITY OF MAINE 

FOUNDATION

What precipitated this was the perception of the PEF 

owners—and I got this right from the horse’s mouth—

that because of reduced budgets and conversations 

they had with other people that I was unaware of, 

that the Forest Service was probably going to give 

up and pull out [of the PEF]. I was in most of the 

meetings when the owners met. I remember one 

meeting where the decision had to be made: What 

were the owners going to do? I was at this meeting—I 

don’t recall anyone else from the Forest Service being 

there—a dean [from the University of Maine] actually 

asked me, “Bob, is it something the museum could 

undertake?” I said, “There is no way the museum 

could do it, though there may be a part it can play 

somehow.” He was thinking in terms of gearing up 

for it with an employee or two, or something like that. 

The university said, “There is no way we can handle 

the work load, the data collection, and so forth.” So it 

eventually wound up with the agreement we have now. 

The university owns it and we have a memorandum of 

understanding. I think that is working out fine. 

Would I have liked it to go any other way? I can’t say 

at this point in time. The majority of what might be 

considered pertinent work or studies with firm results 

are still being maintained. We now have the university 

overlaying their own set of studies on other parts of 

the Forest, plus an increase in graduate student work 

because of this. 

THOUGHTS ON CHANGES 

SINCE FRANK’S RETIREMENT

I don’t know all the work you are involved with, 

Laura. One of the potential opportunities that bothered 

me when I was working here is we did so little with 

northern white-cedar. I love cedar. I’ve hammered 

thousands of wooden shingles on my home, my 

barn, my garage, my camp, and outbuildings. I’ve 

even replaced, in one case, a set of shingles. I use a 

lot of cedar for posts, I use a lot of cedar lumber for 

paneling, for railings ... I just love cedar. It’s good 

to see an increase in cedar research. Much of the 

initiative with cedar came from you and others. It was 

a void, no question about it. So that’s good. 

I am happy that the research is being maintained. 

I think I was somewhat more fussy about how the 

Experimental Forest looked when I was here, but that’s 

a natural reaction when someone leaves. If a sign is 

falling over or you can’t read it, you should get it out 

of there. That sort of thing. Some of the roads could 

be improved. I realize that there are constraints. As 

far as the work is concerned, I think that with the staff 

you have and the means you have to do it, I feel it is 

being utilized and progress is being made. I know that 

you are involved in so many things and John is spread 

so thin. And you have one technician who keeps 

everything else going here. 

ANECDOTES ABOUT THE 

OLD FOREST SERVICE BUILDING 

ON THE PEF

When a new study had to be installed, I spent quite 

a bit of time on the PEF. I tried to pare it down so I 
162 GTR-NRS-P-123 Penobscot Experimental Forest: 

could get my work done behind the desk. We would 

bring our lunches out and meet in the old building 

[Fig. 8]. I call it the old building; I think it was built in 

1952. Naturally I was sad to see it go.8

On hot days we would all go into the building and it 

would be cool. We never had more than one permanent 

technician assigned to the Experimental Forest. We 

also had summer students hired as work-study or 

government employees. We always had a crew. There 

would usually be a minimum of four of us, maybe 

more. We even had loggers that would come and 

chitchat. We renovated that building and installed a 

bathroom, shower, an oven, and a wood stove. On 

one occasion we entertained a busload of budworm 

Figure 8.—The Forest Service building on the Penobscot Experimental Forest (1952). Photo by U.S. Forest Service.

8

 The building was torn down in 2010.

research people from Vermont for a 2-day visit. The 

night they were here, we cooked a turkey in the oven 

for them and served them a complete meal. It was an 

adequate building for many purposes. It’s all gone 

now.

One of the most humorous stories I remember from 

the Experimental Forest took place in the old building. 

Our technician Orman Carroll had three daughters. 

His wife would line up the lunches on the counter so 

when they went out the door they could pick a lunch. 

One day at lunchtime, we opened our lunches. Orman 

didn’t talk too much, but I heard him say “What in the 

world is this?!” He had, instead of lunch, a daughter’s 

gym suit! We all had a good laugh and shared our 

lunches with him. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 163

FRANK’S FOREST SERVICE 

LEGACY

I know how eastern spruce-fir research began. I never 

met the man who began the research in the 1920s—

Marinus Westveld—but I worked with someone who 

knew him and worked with him. That was Arthur Hart, 

who was Project Leader toward the end of his career. 

I currently live next door to a lady that met Marinus 

Westveld—it is Arthur’s wife. I have much respect 

for that early work. We look at it now with so much 

added knowledge that we think, “My goodness, that 

was pretty basic.” But when you build a pyramid you 

have to start at the base and eventually you put that top 

stone in. 

So if this experiment—the PEF—continues, and I 

might add that the Penobscot Experimental Forest is 

probably as well known now as any time in history, 

I would like to think that I was an important cog in the 

wheel that kept it going [Fig. 9]. 

I think I did the best I could. Fortunately, I think I 

was able to do it in both a satisfactory and meaningful 

manner. I had good people to be associated with, 

below me, above me, and equal to me. 

I certainly appreciate the opportunity to talk with you 

in this way. And it is still fun!

ACKNOWLEDGMENTS

We thank John Brissette for his assistance in crafting 

the interview questions and Matsuye Mairs for her 

careful work in transcribing the audio recording. Susan 

Stout and Peter Stine provided helpful reviews of the 

manuscript.

Figure 9.—Bob Frank at the entrance to the Penobscot Experimental Forest shortly before his retirement (1995). Photo by 

U.S. Forest Service.
164 GTR-NRS-P-123 Penobscot Experimental Forest: 

LITERATURE CITED

Bjorkbom, J.C.; Frank, R.M. 1968. Slash burning 

and whole-tree skidding fail to provide mineralsoil seedbeds for spruce-fir. The Northern Logger 

and Timber Processor. 16(7): 20, 45.

Czapowskyj, M.M.; Rourke, R.V.; Frank, R.M. 1977. 

Strip clearcutting did not degrade the site in 

a spruce-fir forest in central Maine. Res. Pap. 

NE-367. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Research Station. 8 p.

Frank, R.M.; Putnam, E.L. 1972. Seedling survival 

in spruce-fir after mechanical tree harvesting 

in strips. Res. Pap. NE-224. Upper Darby, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Research Station. 

16 p.

Frank, R.M.; Safford, L.O. 1970. Lack of viable seeds 

in the forest floor after clearcutting. Journal of 

Forestry. 68: 776-778.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 165

Publications of the Penobscot Experimental Forest, 

1950-2010

Laura S. Kenefic and John C. Brissette

Abbott, Herschel G.; Hart, Arthur C. 1961. Mice and 

voles prefer spruce seed. Station Paper 153. Upper 

Darby, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 

12 p.

Adams, Stephanie L. 2006. A physiological 

examination of the age-related decline in 

photosynthesis in Picea rubens. Orono, ME: 

University of Maine. 75 p. M.S. thesis.

Alkon, Philip U. 1961. Nutritional and acceptability 

values of hardwood slash as winter deer browse.

Journal of Wildlife Management. 25(1): 77-81.

Bailey, Michael E. 1977. Production and deer 

utilization of vegetation on small clearcuts in 

central Maine. Orono, ME: University of Maine. 

99 p. M.S. thesis.

Bjorkbom, John C. 1967. Seedbed preparation 

methods for paper birch. Research Paper NE-79. 

Upper Darby, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 15 p.

Bjorkbom, John C. 1971. Production and 

germination of paper birch seed and its dispersal 

into a forest opening. Research Paper NE-209. 

Upper Darby, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 14 p.

Bjorkbom, John C. 1972. Stand changes in the first 

10 years after seedbed preparation for paper 

birch. Research Paper NE-238. Upper Darby, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 10 p.

Bjorkbom, John C.; Frank, Robert M. 1968. Slash 

burning and whole-tree skidding fail to provide 

mineral-soil seedbeds for spruce-fir. The 

Northern Logger and Timber Processor. 16(7): 

20, 45.

Blomquist, Sean M. 2008. Relative fitness and 

behavioral compensation of amphibians in a 

managed forest. Orono, ME: University of Maine. 

185 p. Ph.D. dissertation.

This bibliography includes all journal articles, 

technical reports, proceedings, U.S. Forest Service 

publications, and theses published about or using data 

from the Penobscot Experimental Forest (PEF) during 

its first 60 years. Abstracts, Web pages, tour handouts, 

unpublished reports, and articles from newsletters, 

magazines, and newspapers are not included. This list 

was compiled using records maintained by the Forest 

Service, Internet resources, and the literature cited 

sections of known PEF publications, with input from 

current and former PEF scientists and cooperators. 

Each publication was obtained in digital format and 

the use of PEF data confirmed; if the study site was not 

named, expert opinion (e.g., type and general location 

of the research and knowledge of former studies on the 

experimental forest) was used to determine if the study 

was conducted on the PEF. Although we are confident 

that we have not made errors of inclusion, it is possible 

that we have made errors of omission due to the length 

of record, large number of cooperators and publication 

outlets, and abundance of gray literature originating 

from the PEF. Note that many of the PEF publications 

with Forest Service authors can be obtained from the 

online Treesearch database (http://www.treesearch.

fs.fed.us/).
166 GTR-NRS-P-123 Penobscot Experimental Forest: 

Blomquist, Sean M.; Hunter, Malcolm L., Jr. 2007. 

Externally attached radio-transmitters have 

limited effects on the antipredator behavior 

and vagility of Rana pipiens and Rana sylvatica.

Journal of Herpetology. 41 (3): 430-438. 

Blomquist, Sean M.; Hunter, Malcolm L., Jr. 2009. A 

multi-scale assessment of habitat selection and 

movement patterns by Northern leopard frogs 

(Lithobates [Rana] pipiens) in a managed forest.

Herpetological Conservation and Biology. 4: 

142-160. 

Blomquist, Sean M.; Hunter, Malcolm L., Jr. 2010. 

A multi-scale assessment of amphibian habitat 

selection: wood frog response to time of 

harvesting. Ecoscience. 17: 251-264.

Blomquist, Sean M.; Zydlewsk, Joseph D.; Hunter, 

Malcolm L., Jr. 2008. Efficacy of PIT tags for 

tracking the terrestrial anurans Rana pipiens

and Rana sylvatica. Herpetological Review. 39: 

174-179.

Blum, Barton M. 1973. Some observations on age 

relationships in spruce-fir regeneration. Research 

Note NE 169. Upper Darby, PA: U.S. Department 

of Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 4 p.

Blum, Barton M. 1975. Regeneration and unevenaged silviculture—the state of the art. In: 

Uneven-aged silviculture and management in 

the eastern United States; combined proceedings 

of two in-service workshops; 1975 July 15-17; 

Morgantown, WV; 1976 October 19-21; Redding, 

CA. Gen. Tech. Rep. WO-24. Washington, DC: 

U.S. Department of Agriculture, Forest Service: 

63-83.

Blum, Barton M. 1977. Animal damage to young 

spruce and fir in Maine. Research Note NE-231. 

Upper Darby, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 4 p.

Blum, Barton M. 1984a. Selection system of 

silviculture in spruce-fir. Forest Technique: 

Micro-Journal for Information Transfer. 84(8): 10. 

Blum, Barton M. 1984b. U.S. Forest Service research 

-- alive and well in Maine. National Woodlands. 

7(4): 8-10.

Blum, Barton M. 1985. Appropriate silviculture.

In: Spruce-fir management and spruce budworm: 

Society of American Foresters, Region VI, 

Technical Conference; 1984 April 24-26; 

Burlington, VT. Gen. Tech. Rep. NE-99. Broomall, 

PA: U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station: 185-191.

Blum, Barton M. 1987. Forest research, Orono, 

Maine. NE-INF-74-87. Broomall, PA: U.S. 

Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 7 p.

Blum, Barton M. 1988. Variation in the phenology of 

bud flushing in white and red spruce. Canadian 

Journal of Forest Research. 18: 315-319.

Blum, Barton M.; Solomon, Dale S. 1980. Growth 

trends in pruned red spruce trees. Research 

Note NE-294. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 6 p.

Bondietti, Ernest A.; Momoshima, Noriyuki; Shortle, 

Walter C.; Smith, Kevin T. 1990. A historical 

perspective on divalent cation trends in 

red spruce stemwood and the hypothetical 

relationship to acidic deposition. Canadian 

Journal of Forest Research. 20: 1850-1858.

Brissette, John C. 1996. Effects of intensity and 

frequency of harvesting on abundance, stocking 

and composition of natural regeneration in the 

Acadian Forest of eastern North America. Silva 

Fennica. 30(2-3): 301-314.

Brissette, John C.; Ducey, Mark J.; Gove, Jeffrey H. 

2003. A field test of point relascope sampling of 

down coarse woody material in managed stands 

in the Acadian Forest. Journal of the Torrey 

Botanical Society. 130(2): 79-88.

Brissette, John C.; Frank, Robert M., Jr.; 

Stone, Timothy L.; Skratt, Thomas A. 1999. 

Precommercial thinning in a northern conifer 

stand: 18-year results. The Forestry Chronicle. 

75(6): 967-972.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 167

Brissette, John C.; Kenefic, Laura S. 2000. Eastern 

hemlock response to even- and uneven-age 

management in the Acadian Forest: results from 

the Penobscot Experimental Forest long-term 

silvicultural study. In: McManus, K.A.; Shields, 

K.S.; Souto, D.R., eds. Proceedings: symposium on 

sustainable management of hemlock ecosystems in 

eastern North America; 1999 June 22-24; Durham, 

NH. Gen. Tech. Rep. NE-267. Newtown Square, 

PA: U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station: 23-28.

Brissette, John C.; Saunders, Michael R.; Kenefic, 

Laura S.; Sendak, Paul E. 2006. Long-term 

research on classical silvicultural approaches 

in the Acadian Forest: Penobscot Experimental 

Forest Part I. In: Proceedings, Society of 

American Foresters 2006 national convention; 2006 

October 25-29; Pittsburgh, PA. Bethesda, MD: 

Society of American Foresters. [CD-ROM]. 10 p.

Bryce, Elizabeth K. 2009. Influence of silvicultural 

treatment, site characteristics, and land 

use history on native and nonnative forest 

understory plant composition on the Penobscot 

Experimental Forest in Maine. Orono, ME: 

University of Maine. 123 p. M.S. thesis.

Bulger, John M.; Durgin, Jerry J.; Giffen, R. Alec; 

Tyron, Theodore C.; Griffen, Ralph H.; Weiland, 

George W.; Frank, Robert M. 1974. Forest practice 

goals and standards for the improvement, 

maintenance, and protection of the forest 

resources of Maine. The Maine Forest Review. 

7: 1-21.

Chalmers, Rebecca J. 2004. Wetland and nest 

scale habitat use by the four-toed salamander 

(Hemidactylium scutatum) in Maine, and a 

comparison of survey methods. Orono, ME: 

University of Maine. 109 p. M.S. thesis.

Chalmers, Rebecca J.; Loftin, Cynthia S. 2006. 

Wetland and microhabitat use by nesting 

four-toed salamanders in Maine. Journal of 

Herpetology. 40(4): 478-485.

Chapman, Gordon L.; Grove, Edwin R.; McLintock, 

Thomas F. 1951. Influence of silvicultural 

practice and species on the solid wood contents 

of peeled, stacked pulpwood. Technical Note 

No. 9. Orono, ME: University of Maine, Forestry 

Department. 2 p.

Collins, Judith A.; Jennings, Daniel T. 1987. 

Nesting height preferences of eumenid wasps 

(Hymenoptera: Eumenidae) that prey on spruce 

budworm (Lepidoptera: Tortricidae). Annals of 

the Entomological Society of America. 80: 

435-438.

Crawford, Hewlette S. 1982. Seasonal food selection 

and digestibility by tame white-tailed deer in 

central Maine. Journal of Wildlife Management. 

46(4): 974-982.

Crawford, Hewlette S.; Frank, Robert M. 1986. Is 

good forestry good wildlife management?--

Some examples. In: Is good forestry good wildlife 

management? Proceedings of a joint conference of: 

New England Chapter of the Society of American 

Foresters, Maine Chapter of the Wildlife Society, 

Atlantic International Chapter of the American 

Fisheries Society; 1985 January 6-8; Portland, ME. 

University of Maine Miscellaneous Publication 

689. Orono, ME: Maine Agricultural Experiment 

Station, University of Maine: 17-22.

Crawford, Hewlette S.; Frank, Robert M. 1987. 

Wildlife habitat responses to silvicultural 

practices in spruce-fir forests. In: McCabe, 

Richard E., ed. Transactions of the 52nd North 

American wildlife and natural resources 

conference; 1987 March 20-25; Quebec City, 

Quebec. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station: 92-100.

Crawford, Hewlette S.; Frank, Robert M. 1988. Rating 

spruce-fir silviculture for wildlife and forestry.

Broomall, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station, Information Management Group. 23 p.
168 GTR-NRS-P-123 Penobscot Experimental Forest: 

Crawford, Hewlette S.; Hankinson, Diane H. 1984. 

White-tailed deer vs. bovine inocula for in vitro 

digestibilities. Journal of Wildlife Management. 

48(2): 649-652.

Crawford, Hewlette S.; Stutzman, Warren L. 1983. 

Microwave attenuation as an indicator of 

sampling weight of herbaceous and woody plants 

in the field. Forest Science. 29(4): 726-734.

Cronan, Christopher S.; Lytle, David; Piampiano, 

Jeffrey. 1998. Influence of seasonal drought on 

water-table levels and hydrochemistry in two 

forested wetlands, Maine. Northeastern Naturalist. 

5(4): 313-322.

Czapowskyj, Miroslaw M. 1977. Status of 

fertilization and nutrition research in northern 

forest types. In: Proceedings of the symposium of 

intensive culture of northern forest types; 1976 July 

20-22; Orono, ME. Gen. Tech. Rep. NE-29. Upper 

Darby, Pa: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station: 

185-203.

Czapowskyj, Miroslaw M. 1979. Foliage nutrient 

concentration in balsam fir as affected by soil 

drainage and methods of slash disposal. Research 

Note NE-278. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 4 p.

Czapowskyj, Miroslaw M.; Rourke, Robert V.; Frank, 

Robert M. 1977. Strip clearcutting did not 

degrade the site in a spruce-fir forest in central 

Maine. Research Paper NE-367. Upper Darby, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Research Station. 

8 p.

Davis, Grant. 1958. Growth of white pine and red 

spruce trees after pruning. Research Note No. 83. 

Upper Darby, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment 

Station. 2 p.

Davis, Grant; Hart, Arthur C. 1961. Effect of seedbed 

preparation on natural reproduction of spruce 

and hemlock under dense shade. Station Paper 

No. 160. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 12 p.

Day, Michael. 2000a. Factors influencing net 

primary production in red spruce. Orono, ME: 

University of Maine. 172 p. Ph.D. dissertation.

Day, Michael E. 2000b. Influence of temperature 

and leaf-to-air vapor pressure deficit on net 

photosynthesis and stomatal conductance in red 

spruce (Picea rubens). Tree Physiology. 20: 57-63.

Day, Michael E.; Greenwood, Michael S.; White, 

Alan S. 2001. Age-related changes in foliar 

morphology and physiology in red spruce and 

their influence on declining photosynthetic rates 

and productivity with tree age. Tree Physiology. 

21: 1195-1204.

Dean, Thomas J.; Roberts, Scott D.; Gilmore, Daniel 

W.; Maguire, Douglas A.; Long, James N.; O’Hara, 

Kevin L.; Seymour, Robert S. 2002. An evaluation 

of the uniform stress hypothesis based on stem 

geometry in select North American conifers.

Trees. 16(8): 559-568.

deMaynadier, Philip G. 1996. Patterns of movement 

and habitat use by amphibians in Maine’s 

managed forests. Orono, ME: University of 

Maine. 207 p. Ph.D. dissertation. 

deMaynadier, Phillip G.; Hunter, Malcolm L., Jr. 1998. 

Effects of silvicultural edges on the distribution 

and abundance of amphibians in Maine.

Conservation Biology. 12: 340-352. 

deMaynadier, Phillip G.; Hunter, Malcolm L., Jr. 1999. 

Forest canopy closure and juvenile emigration 

by pool-breeding amphibians in Maine. Journal 

of Wildlife Management. 63: 441-450.

deMaynadier, Phillip G.; Hunter, Malcolm L., Jr. 

2000. Forest roads and amphibian movements in 

Maine. Natural Areas Journal. 20: 56-65.
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 169

DeRose, R. Justin. 2004. Leaf area index - relative 

density relationships in even-aged Abies 

balsamea - Picea rubens stands in Maine. Orono, 

ME: University of Maine. 83 p. M.S. thesis.

DeRose, R. Justin; Seymour, Robert S. 2003. 

Relationships between leaf area, structure and 

relative density in even-aged spruce-fir stands 

in Maine. In: Cooperative Forestry Research 

Unit: 2003 annual report. Miscellaneous Report 

2684. Orono, ME: Maine Agricultural and Forest 

Experiment Station: 28-30.

DeRose, R. Justin; Seymour, Robert S. 2009. The 

effect of site index on growth efficiency of upper 

crown class Picea rubens and Abies balsamea in 

Maine, USA. Canadian Journal of Forest Research. 

39: 1-8.

DeRose, R. Justin; Seymour, Robert S. 2010. Patterns 

of leaf area index during stand development 

in even-aged balsam fir - red spruce stands.

Canadian Journal of Forest Research. 40: 629-637.

Dibble, Alison C. 2004. Use of comparison areas 

rather than controls in a study of fuels in 

invaded forests of the Northeast and midAtlantic states. In: Proceedings, Society of 

American Foresters 2003 national convention; 

2003 October 25-29; Buffalo, NY. SAF Publication 

04-01. Bethesda, MD: Society of American 

Foresters: 319-324.

Dibble, Alison C.; Brissette, John C.; Hunter, 

Malcolm L., Jr. 1999. Putting community data 

to work: Some understory plants indicate red 

spruce regeneration habitat. Forest Ecology and 

Management. 114(2-3): 275-291.

Dibble, Alison C.; Rees, Catherine A.; Ducey, Mark 

J.; Patterson, William A., III. 2003. Fuelbed 

characteristics of invaded forest stands. In: 

2003 workshop proceedings: using fire to control 

invasive plants: What’s new, what works in the 

Northeast? 2003 Jan 24; Portsmouth, NH. Durham, 

NH: University of New Hampshire Cooperative 

Extension: 26-29.

DiMauro, Danielle. 1998. Reproduction of 

amphibians in natural and anthropogenic 

temporary pools in managed forests. Orono, ME: 

University of Maine. 77 p. M.S. thesis.

DiMauro, Danielle; Hunter, Malcolm L., Jr. 2002. 

Reproduction of amphibians in natural and 

anthropogenic temporary pools in managed 

forests. Forest Science. 48: 397-406.

Filauro, Anthony. 1973. Movement of applied 

nutrients through Plaisted and Howland soils 

under mixedwood forest cover types. Orono, ME: 

University of Maine. 111 p. M.S. thesis.

Frank, Robert M. 1973. The course of growth 

response in released white spruce -- 10-year 

results. Research Paper NE-258. Upper Darby, PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 6 p.

Frank, Robert M. 1974. Options for and probable 

benefits of timber stand improvement in Maine.

Maine Forest Review. 6: 2-4.

Frank, Robert M. 1977a. Indications of silvicultural 

potential from long-term experiments in sprucefir types. In: Proceedings of the symposium on 

intensive culture of northern forest types; 1976 July 

20-22; Orono, ME. Gen. Tech. Rep. NE-29. Upper 

Darby, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station: 

159-177.

Frank, Robert M. 1977b. Silvicultural options for 

obtaining high yields in spruce-fir. In: Filip, 

Stanley M.; Frank, Robert M.; Smith, David M., 

eds. High-yield forestry in northern hardwoods, 

spruce-fir and white pine. Concord, NH: Society of 

the Protection of New Hampshire Forests: 5-9, 16.

Frank, Robert M. 1983. Balsam fir (Pinaceae Abies 

balsamea) silvics, silviculture, and natural 

regeneration methods. In: Proceedings of the 

silvicultural guides workshop, Chippewa and 

Superior National Forests; 1983 February 14-18; 

Grand Rapids, MN. Duluth, MN: U.S. Forest 

Service, Superior National Forest: 114-136.
170 GTR-NRS-P-123 Penobscot Experimental Forest: 

Frank, Robert M. 1984. Shelterwood system 

of silviculture in spruce-fir forests. Forest 

Technique: Micro-Journal for Information Transfer. 

84(8): 9.

Frank, Robert M. 1986. Shelterwood -- an ideal 

silvicultural strategy for regenerating spruce-fir 

stands. In: Integrated pest management symposium 

for northern forests: the proceedings; 1986 March 

24-27; Madison, WI. Madison, WI: University of 

Wisconsin, Cooperative Extension Service: 

139-150.

Frank, Robert M. 1987. The Penobscot Experimental 

Forest -- a unique laboratory. Forest Technique: 

Micro-Journal for Information Transfer. 85(3): 20.

Frank, Robert M. 1990. Abies balsamea (L.) Mill. 

balsam fir. In: Silvics of North America. 1: 

Conifers. Agricultural Handbook 654. Washington, 

DC: U.S. Department of Agriculture, Forest 

Service: 26-35.

Frank, Robert M. 1992. Tree and wildlife diversity 

responses to silvicultural practices in northern 

conifers. In: Proceedings of the seminar on 

integrated resource management; 1992 April 

7-8; Hugh John Flemming Forestry Centre, N.B. 

Information Report M-X-183E. Fredericton, NB: 

Forestry Canada-Maritimes Region: 59-76.

Frank, Robert M.; Bjorkbom, John C. 1973. A 

silvicultural guide for spruce-fir in the northeast.

Gen. Tech. Rep. NE-6. Upper Darby, PA: U.S. 

Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 29 p.

Frank, Robert M.; Blum, Barton M. 1978. The 

selection system of silviculture in sprucefir stands -- procedures, early results, and 

comparisons with unmanaged stands. Research 

Paper NE-425. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 15 p.

Frank, Robert M.; OKeefe, Timothy G. 1983. A 

transfer system to reach a selected audience: 

woodlot owners and mill managers. Proceedings 

of the Technology Transfer Society International 

Symposium: 371-378.

Frank, Robert M.; Safford, Lawrence O. 1970. Lack of 

viable seeds in the forest floor after clearcutting.

Journal of Forestry. 68: 776-778.

Frank, Robert M.; Stone, Timothy L.; Crawford, 

Hewlette S. 1996. Tree/wildlife diversity 

responses to silvicultural practices in northern 

conifers. In: Forests sustaining people: proceedings 

of the 1995 Society of American Foresters 

convention; 1995 October 28-November 1; 

Portland, ME. Bethesda, MD: Society of American 

Foresters: 413-414.

Fraver, Shawn; Wagner, Robert G.; Day, Michael. 

2002. Dynamics of coarse woody debris 

following gap harvesting in the Acadian Forest 

of central Maine, U.S.A. Canadian Journal of 

Forest Research. 32: 2094-2105.

Garber, Sean M.; Brown, John P.; Wilson, Duncan S.; 

Maguire, Douglas A.; Heath, Linda S. 2005. Snag 

longevity under alternative silvicultural regimes 

in mixed species forests of central Maine.

Canadian Journal of Forest Research. 35: 787-796.

Garrett, Peter W.; Schreiner, Ernst J.; Kettlewood, 

Harry. 1973. Geographic variation of eastern 

white pine in the Northeast. Research Paper 

NE-274. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 14 p.

Georgitis, Kathryn M. 2001. Evaluating shade bias 

in insect trap catch and assessing the short- and 

long-term impacts of herbicide application 

in regenerating clearcuts on flowering plant 

communities. Orono, ME: University of Maine. 

81 p. M.S. thesis. 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 171

Gibbs, Carter B.; Carlaw, James B. 1973. The New 

England spruce-fir seed orchard program. In: 

Joint proceedings of the 10th Lake States forest 

tree improvement conference and the 7th Central 

States forest tree improvement conference; 1971 

September 22-24; Madison, WI. St. Paul, MN: U.S. 

Department of Agriculture, Forest Service, North 

Central Forest Experiment Station: 53-56.

Gilmore, Daniel W. 1995. Crown structure stem 

form development and foliage-sapwood area 

relationships for balsam fir in central Maine.

Orono, ME: University of Maine. 137 p. 

Ph.D. dissertation. 

Gilmore, Daniel W.; Seymour, Robert S. 1996. 

Alternative measures of stem growth efficiency 

applied to Abies balsamea from four canopy 

positions in central Maine, USA. Forest Ecology 

and Management. 84(1-3): 209-218.

Gilmore, Daniel W.; Seymour, Robert S. 1997. Crown 

architecture of Abies balsamea from four canopy 

positions. Tree Physiology. 17: 71-80.

Gilmore, Daniel W.; Seymour, Robert S. 2004. 

Foliage-sapwood area equations for balsam fir 

require local validation. Forest Science 50(4): 

566-570.

Gilmore, Daniel W.; Seymour, Robert S.; Halteman, 

William A.; Greenwood, Michael S. 1995. Canopy 

dynamics and the morphological development of 

Abies balsamea: effects of foliage age on specific 

leaf area and secondary vascular development.

Tree Physiology. 15(1): 47-55.

Gilmore, Daniel W.; Seymour, Robert S.; Maguire, 

Douglas A. 1996. Foliage sapwood area 

relationships for Abies balsamea in central 

Maine, USA. Canadian Journal of Forest Research. 

26(12): 2071-2079.

Greenwood, Michael S.; Day, Michael E.; Berlyn, 

Graeme P. 2009. Regulation of foliar plasticity 

in conifers: developmental and environmental 

factors. Journal of Sustainable Forestry. 28: 48-62. 

Greenwood, Michael S.; Day, Michael E.; Schatz, 

Jason. 2010. Separating the effects of tree size 

and meristem maturation on shoot development 

of grafted scions of red spruce (Picea rubens

Sarg.). Tree Physiology. 30: 459-468.

Greenwood, Michael S.; Livingston, William H.; 

Sokol, Kerry A. 1999. The legacy of diameterlimit cutting: a case study for red spruce. In: 

Cooperative Forestry Research Unit: 1999 annual 

report. Miscellaneous Report 417. Orono, ME: 

Maine Agricultural and Forest Experiment Station: 

33-35.

Greenwood, Michael S.; McConville, Daniel. 2002. 

Factors affecting the regeneration of red spruce 

and balsam fir. In: Cooperative Forestry Research 

Unit: 2002 annual report. Miscellaneous Report 

431. Orono, ME: Maine Agricultural and Forest 

Experiment Station: 37-40.

Greenwood, Michael S.; Ward, Margaret H.; Day, 

Michael E.; Adams, Stephanie L.; Bond, Barbara 

J. 2008. Age-related trends in red spruce foliar 

plasticity in relation to declining productivity.

Tree Physiology. 28: 225-232.

Grisez, Ted J. 1954a. 1954 hurricane damage of 

Penobscot Experimental Forest. Forest Research 

Note No. 39. Upper Darby, PA: U.S. Department 

of Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 2 p.

Grisez, Ted J. 1954b. Results of a mouse census in 

a spruce-fir-hemlock forest in Maine. Forest 

Research Note No. 28. Upper Darby, PA: U.S. 

Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 2 p.

Hale, Kathy N. 1977. The genetic improvement 

of northeastern spruce species. Orono, ME: 

University of Maine. 105 p. M.S. thesis.

Hart, Arthur C. 1952. A wrist guard for pulpwood 

handlers. The Northeastern Logger. 1(5): 37.
172 GTR-NRS-P-123 Penobscot Experimental Forest: 

Hart, Arthur C. 1953. Small-forest management 

in the spruce-fir region. Northeastern Research 

Note No. 21. Upper Darby, PA: U.S. Department 

of Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 4 p.

Hart, Arthur C. 1956a. Logging on the Penobscot 

Experimental Forest. The Northeastern Logger. 

5(6): 33, 90-91.

Hart, Arthur C. 1956b. Results of small woodlot 

cuttings on Penobscot Experimental Forest, 1953 

and 1954. Forest Research Note No. 61. Upper 

Darby, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 

2 p.

Hart, Arthur C. 1958. Penobscot woodlands yield 

annual cuts. Forest Research Note No. 84. Upper 

Darby, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 

2 p.

Hart, Arthur C. 1959. Silvical characteristics of 

balsam fir (Abies balsamea). Station Paper 

No. 122. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 21 p.

Hart, Arthur C. 1961. Thinning balsam fir thickets 

with soil sterilants. Station Paper No. 152. Upper 

Darby, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station. 

8 p.

Hart, Arthur C. 1963. Spruce-fir silviculture in 

northern New England. In: Proceedings, Society 

of American Foresters national convention; Boston, 

MA. Society of American Foresters: 107-110.

Hart, Arthur C. 1964. The Penobscot managementintensity demonstration plots. Research Paper 

NE-25. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 24 p.

Hart, Arthur C. 1966. Research programs of the 

Northeastern Forest Experiment Station.

American Pulpwood. January: 23-25.

Hart, Arthur C.; Abbott, Herschel G.; Ladd, Edward 

R. 1968. Do small mammals and birds affect 

reproduction of spruce and fir? Research Paper 

NE-110. Upper Darby, PA: U.S. Department of 

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60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 183

ACKNOWLEDGMENTS

Invaluable assistance was provided by University 

of Maine student employee John Misener and staff 

of the University of Maine Fogler Library and 

National Forest Service Library. U.S. Forest Service 

retirees Robert M. Frank, Jr., Lawrence Safford, 

and Timothy Stone helped us determine if some 

publications originated from the PEF. Mac Hunter of 

the University of Maine provided useful lists of theses 

and publications that originated from the Maine LandUse Effects on Amphibian Populations (LEAP) study. 

A draft of this bibliography was reviewed by Robert 

Frank and Robert Seymour (University of Maine).

Young, Harold E. 1955. Local pulpwood volume 

tables. Technical Bulletin 37. Orono, ME: Maine 

Agricultural and Forest Experiment Station. 5 p.

Zellers, Christopher E. 2010. The growth, yield, and 

financial performance of isolated eastern white 

pine (Pinus strobus L.) reserve trees. Orono, ME: 

University of Maine. 73 p. M.S. thesis.

Zellers, Christopher E.; Seymour, Robert S.; 

Weiskittel, Aaron; Benjamin, Jeffrey. 2009. The 

growth, yield and financial performance of 

isolated archetypal eastern white pine trees.

In: Meyer, Spencer R., ed. Cooperative Forestry 

Research Unit: 2008 annual report. Orono, ME: 

University of Maine: 29-31.

Zellers, Kate E. 2010. Patterns of regeneration 

of eastern white pine (Pinus strobus L.) as 

influenced by large isolated reserve trees and 

precommercial thinning. Orono, ME: University 

of Maine. 86 p. M.S. thesis.

Zellers, Kate E.; Seymour, Robert S.; Livingston, 

William H.; Wagner, Robert G. 2009. Patterns of 

regeneration of eastern white pine as influenced 

by large, isolated crop trees and precommercial 

thinning. In: Meyer, Spencer R., ed. Cooperative 

Forestry Research Unit: 2008 annual report. Orono, 

ME: University of Maine: 27-28.
184 GTR-NRS-P-123 Penobscot Experimental Forest: 
60 Years of Research and Demonstration in Maine, 1950-2010 GTR-NRS-P-123 185

ABOUT THE AUTHORS

Stephanie L. Adams was a graduate research 

assistant at the University of Maine, School of Forest 

Resources, Orono, Maine.

John C. Brissette is a project leader and research 

forester with the U.S. Forest Service, Northern 

Research Station, Durham, New Hampshire. Mailing 

address: U.S. Forest Service, Northern Research 

Station, 271 Mast Road, Durham, NH 03824; contact 

at 603-868-7632, or jbrissette@fs.fed.us.

Michael E. Day is an associate research professor at 

the University of Maine, School of Forest Resources, 

Orono, Maine.

Alison C. Dibble is a botanist and volunteer with 

the U.S. Forest Service, Northern Research Station, 

Bradley, Maine.

Robert M. Frank, Jr., is retired from the U.S. Forest 

Service, Northeastern Forest Experiment Station, 

Bradley, Maine. He was a research forester at the 

Penobscot Experimental Forest.

Michael S. Greenwood is retired from the University 

of Maine, School of Forest Resources, Orono, 

Maine. He was the Ruth Hutchins Professor of Tree 

Physiology and Forestry.

Jody Jellison is a professor at Virginia Tech, 

Department of Plant Pathology, Physiology, and Weed 

Science, and associate director of the Agricultural 

Experiment Station, Blacksburg, Virginia.

Laura S. Kenefic is a research forester with the 

U.S. Forest Service, Northern Research Station, 

Bradley, Maine and is the Station’s lead scientist at 

the Penobscot Experimental Forest. Mailing address: 

School of Forest Resources, 5755 Nutting Hall, 

University of Maine, Orono, ME 04469; contact at 

207-581-2794, or lkenefic@fs.fed.us.

Alan J. Kimball is an associate professor at the 

University of Maine, School of Forest Resources, and 

forest manager, University Forests Office, Orono, 

Maine.

Spencer R. Meyer is an associate scientist at 

the University of Maine, Center for Research on 

Sustainable Forests, Orono, Maine.

Andrew S. Nelson is an assistant professor at the 

University of Arkansas at Monticello, Monticello, 

Arkansas.

Matthew B. Russell is a postdoctoral research 

associate at the University of Minnesota, Department 

of Forest Resources, St. Paul, Minnesota.

Mike R. Saunders is an associate professor at Purdue 

University, Department of Forestry and Natural 

Resources, West Lafayette, Indiana.

Jonathan S. Schilling is an associate professor at the 

University of Minnesota, Department of Bioproducts 

and Biosystems Engineering, St. Paul, Minnesota.

Robert S. Seymour is the Curtis Hutchins Professor 

of Silviculture at the University of Maine, School of 

Forest Resources, Orono, Maine.

Walter C. Shortle is a senior research plant 

pathologist with the U.S. Forest Service, Northern 

Research Station, Durham, New Hampshire. 

Kevin T. Smith is a plant physiologist with the U.S. 

Forest Service, Northern Research Station, Durham, 

New Hampshire.

Katherine Spencer is a project supervisor with the 

Alberta Environmental and Sustainable Resource 

Development. 

Robert G. Wagner is the Henry W. Saunders 

Distinguished Professor in Forestry at the University 

of Maine, School of Forest Resources, and director 

of the Cooperative Forestry Research Unit, Orono, 

Maine. 
Printed on recycled paper

Kenefic, Laura S.; Brissette, John C., compilers. 2014. Penobscot Experimental 

Forest: 60 years of research and demonstration in Maine, 1950-2010. Gen. 

Tech. Rep. NRS-P-123. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. 185 p.

The Penobscot Experimental Forest (PEF) in Maine has been the site of U.S. 

Department of Agriculture, Forest Service, Northern Research Station (previously 

Northeastern Forest Experiment Station) research on northern conifer silviculture 

and ecology since 1950. Purchased by forest industry and leased to the Forest 

Service for long-term experimentation, the PEF was donated to the University 

of Maine Foundation in 1994. Since that time, the University and the Forest 

Service have worked in collaboration to advance the PEF as a site for research, 

demonstration, and education. This publication reports the history of the PEF 

during its first 60 years (1950 to 2010) and presents highlights of research 

accomplishments in silviculture, ecology, ecophysiology, nutrient cycling, botany, 

and other areas. Issues of data management and forest management planning are 

addressed. Also included is a bibliography of publications originating from research 

on the PEF, as well as recollections of a research forester stationed there for 

30 years. 

More than half a century of work on the PEF has served as an important source 

of information for practitioners and policy makers in the Acadian Forest region of 

the northeastern United States and adjacent Canada, and informed the practice 

of silviculture nationally and internationally. Long-term consistency in treatment 

application and measurement; stand-level replication; and accessible, digital data, 

metadata, and records archives have facilitated hundreds of studies and made the 

PEF an invaluable and highly influential research site.

Key words: silviculture, Acadian Forest, northern conifers, red spruce, 

 balsam fir, eastern hemlock 

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