Resource Bulletin

NRS-20

October 2007

Pennsylvania’s Forest 2004

United States

Department of Agriculture DEPA RTMENTOFAGRICULT URE

Forest

Service

Northern

Research Station
Abstract

In 2000, the Pennsylvania Department of Conservation’s Natural Resources, Bureau 

of Forestry and the Northern Research Station’s Forest Inventory and Analysis unit 

implemented a new annual system for inventorying and monitoring Pennsylvania’s forests. 

This report includes data from 2000 to 2004. Pennsylvania’s forest-land base is stable, 

covering 16.6 million acres or 58 percent of land area. More than 660,000 acres of forest land 

were lost from 1989 to 2004, mostly to residential or industrial development. However, there 

was a 617,500-acre gain in forest land, mostly from agricultural land. Fifty-four percent of 

forest land is owned by families and individuals. Forest types with red maple as a dominant 

species have increased, while stands with sugar maple as a dominant have decreased. The 

distribution of forest land by stand-size class has been shifting toward large stands that now 

account for 6 of 10 acres. The area of forest has increased in the poor and moderate stocking 

classes and decreased in the full and overstocked classes. Hemlock, sugar maple, and oaks 

are poised to be less dominant in the future. Increases in red maple are slowing while black 

birch continues to increase. Sawtimber volume totals 88.9 billion board feet, an average of 

about 5,000 board feet per acre. Increases in sawtimber inventory have slowed over time. 

Currently, only half of the forest land that should have advance regeneration is adequately 

stocked with high-canopy species, and only one-third has adequate regeneration for 

commercially desirable timber species. Grass/forb and rhizomous ferns dominate understory 

communities, accounting for nearly one-third of the total nontree vegetative cover sampled. 

Several exotic diseases and insects threaten the health of Pennsylvania’s forests. Exoticinvasive plants threaten native plant diversity and forest health; however, monitoring efforts 

are only beginning to quantify their distribution and abundance. Stressors such as drought, 

acidic deposition, and ground-level ozone pollution are adversely affecting the State’s forests. 

Continued monitoring is required to gain a more complete understanding of these impacts on 

this valuable resource.

The Authors

WILLIAM H. MCWILLIAMS, CAROL L. ALERICH, BRETT J. BUTLER, MICHAEL L. 

HOPPUS, ANDREW J. LISTER, TONYA W. LISTER, RANDALL S. MORIN, CHARLES H. 

PERRY, JAMES A. WESTFALL, ERIC H. WHARTON and CHRISTOPHER W. WOODALL are 

foresters with the Northern Research Station’s Forest Inventory and Analysis unit in Newtown 

Square, PA. SETH P. CASSELL is communications coordinator and forest resources planner 

with the Pennsylvania Department of Conservation and Natural Resources, Bureau of 

Forestry, Harrisburg, PA. STEVE HORSLEY is a plant pathologist with the Northern Research 

Station at Irvine, PA.

Acknowledgments

We thank the following members of the field staff for their hard work and dedication in 

collecting the sample data used in this report:

Thomas Albright, David Berger, Amy Calehuff, Chris Coolbaugh, Jessica Deans, Gene 

Dressely, Michael Effinger, Adrienne Evans, Sam Fisher, Jason Gould, Marlin Gruver, Jeff 

Haibach, John Higham, Scott Hopton, Robert Ilgenfritz, Katherine Johnson, Karen Kubly, 

Brian LaPoint, Dan Levy, Trilby Libhart, Amanda Lyon, Chris Mate, William Moir, Paul 

Neubauer, Mark Peet, Mitchell Pennabaker, Edith Raymond, Todd Renninger, Todd Roffe, Ty 

Ryen, Richard Starr, Leland Swoger, Kathryn Tillman, Roffe Todd, Ryan Tye, Kathryn Tillman, 

Bryan Tirrrell, Mike Whitehill, Thomas Willard, and Ron Yaworsky.

All photos by the U.S. Forest Service unless otherwise stated.
Pennsylvania’s Forest 2004

William H. McWilliams

Seth P. Cassell

Carol L. Alerich

Brett J. Butler

Michael L. Hoppus

Stephen B. Horsley

Andrew J. Lister

Tonya W. Lister

Randall S. Morin

Charles H. Perry

James A. Westfall

Eric H. Wharton

Christopher W. Woodall

Northern Research Station—Forest Inventory and Analysis


Contents

Highlights............................................................................................. 2

Introduction.......................................................................................... 4

Chapter 1: Forest Land-Use Dynamics............................................. 5

Chapter 2: Forest-Land Ownership................................................. 12

Chapter 3: Composition and Structure........................................... 17

Low-Use Wood Resource............................................................. 25

 Timber Products Output............................................................... 47

Chapter 4: Understory Conditions................................................... 49

Chapter 5: Forest Health................................................................... 53

Forest Carbon................................................................................ 70

Chapter 6: Sustainability Issues and Knowledge Gaps................ 72

Literature Cited.................................................................................. 76

Appendix: Annual Inventory Overview........................................... 80

Definition of Terms............................................................................ 82

Sugar maple fall foliage.


Highlights

Forests account for 16.6 million acres or 58 percent of Pennsylvania’s land 

area, a decrease of 100,000 acres since 1989. More than 660,000 acres of forest 

land were lost from 1989 to 2004, an average of about 44,000 acres per year. 

Twenty-eight thousand acres per year were converted to residential and industrial 

development. However, losses of forest land were offset by additions from 

agricultural land reversion.

Fifty-four percent of forest land is owned by families and individuals. Family 

and individual owners cite numerous reasons for holding forest land, with 

timber harvesting a relatively minor objective. Only 2 percent of these owners 

have a written management plan and only 8 percent have sought professional 

management advice. Another 2.8 million acres (17 percent) are owned by other 

private entities such as corporations. Public agencies control 4.8 million acres 

(29 percent).

There were no substantial 

changes in the distribution 

of forest land by major 

forest-type group, but 

there have been significant 

changes in composition and 

structure. Forest types with 

red maple as a dominant 

species have increased while 

stands with sugar maple as a 

dominant have decreased.

The distribution of forest 

land by stand-size class has 

been shifting toward large 

stands that now account for 

6 of 10 acres.

The area of forest has 

increased in the poor and 

moderate stocking classes and decreased in the full and overstocked classes. These 

shifts were most prominent on private forest land.

The number of smaller trees is decreasing and the number of larger trees is 

increasing, characteristic of a maturing forest. Relative differences between species 

indicate that hemlock, sugar maple, and the oaks are decreasing in importance. 

Increases in red maple are slowing while black birch continues to increase.

The current sawtimber inventory (expressed in board feet, International 1⁄4-inch 

rule) is the highest recorded since the inception of the Forest Inventory and 

•

•

•

•

•

•

•

Cut-over forest at turn of the century. Photo from the Lycoming County Historical Society.


Analysis program in 1955. Sawtimber volume totals 88.9 billion board feet, 

or more than 5,000 board feet per acre. Increases in sawtimber inventory have 

slowed over time.

Currently, only half of the forest land that should have advance regeneration is 

adequately stocked with high-canopy species and only one-third has adequate 

regeneration for commercially desirable timber species. Northern tier and 

southeastern counties have the lowest levels of advance regeneration.

Grass/forb and rhizomous ferns dominate understory communities, accounting 

for nearly one-third of the total nontree vegetative cover sampled.

Several exotic diseases and insects are threatening the health of Pennsylvania’s 

forests. Gypsy moth, hemlock woolly adelgid, and beech bark disease are 

among those currently active. Sudden oak death (SOD) and the Asian longhorned beetle could inflict severe damage should they become established in 

Pennsylvania. The discovery of emerald ash borer in the State is particularly 

troublesome.

Exotic-invasive plants pose a threat to native plant diversity and forest health. 

Abundant understory 

exotics include 

multiflora rose, 

Russian/autumn 

olive, garlic 

mustard, Japanese 

stiltgrass, and bush 

honeysuckles. Tree-ofheaven has expanded 

to the point where 

there is an average of 

one stem for every 

acre in the State.

Monitoring lichen 

communities can 

help gauge the impact 

of air pollution and 

indicate broad trends 

in biodiversity. Lichen species richness is higher in central Pennsylvania. Speciesrichness scores generally are lower in areas where sulfate deposition is high.

Stressors such as drought, acidic deposition, and ground-level ozone pollution 

are adversely affecting the Commonwealth’s forests. Continued monitoring 

is required to gain a better understanding of these impacts on this valuable 

resource.

•

•

•

•

•

•

Unbroken forested landscape.
Introduction

Credible information on Pennsylvania’s forests is essential to understanding the condition 

of this important natural resource. A comprehensive set of variables that consistently 

tracks and describes the forest through time is needed to accurately inform policies, guide 

management decisions, examine trends, chart trajectories, and formulate critical research 

questions. The Northern Research Station’s Forest Inventory and Analysis (NRS-FIA) unit 

is uniquely positioned to provide pertinent data and information about Pennsylvania’s 

forests to help achieve these and other objectives.

NRS-FIA has been conducting forest inventories in Pennsylvania since the 1950s. 

Periodic reports on the status of and changes in forest conditions were completed for 1955 

(Ferguson 1955), 1965 (Ferguson 1968), 1978 (Considine and Powell 1980), and 1989 

(Alerich 1993).

In 2000, the NRS-FIA and the Pennsylvania Department of Conservation and Natural 

Resources, Bureau of Forestry implemented a new annual system for inventorying and 

monitoring Pennsylvania forests, completing full inventories every 5 years. The new 

system combines features of the periodic system with a new sample-plot grid that now 

incorporates measurements of forest health (see Appendix). Also new to the program is 

the Pennsylvania Regeneration Study, which collects additional data on forest regeneration 

and understory conditions. The 2004 results represent the first complete set of annual 

inventory measurements collected over the first 5 years (2000-04).

Susquehanna River Log Boom. Photo from the Lycoming County Historical Society.
Chapter 1: Forest Land-Use Dynamics

Central to understanding Pennsylvania’s forest 

resource is placing it in context within the State’s 

evolving landscape. The forest is not an isolated 

entity but a product of the landscape interactions 

that occur within and aside its borders. In 

this chapter, we explore the complex land-use 

dynamics that affect the basic character and 

distribution of Pennsylvania’s forests.

Forest-Land Base:

Distribution and Trends

Historical data suggest that forest once covered 

more than 90 percent (27.3 million acres) of 

Pennsylvania’s land area in the pre-European 

settlement era (1630s) (Fig. 1). 

By the early 1900s, industrial 

timber harvesting and 

agricultural land clearing had 

diminished the forest land base 

to only 32 percent (9.2 millions 

acres). Following this period of 

extensive reduction, forest land 

increased steadily as it reclaimed 

former sites.

The forest-land base has been 

relatively stable for the last half 

century and now is the dominant 

land class at 58 percent. Forest 

land is defined as land at least 

10-percent stocked with trees 

of any size, or that formerly 

had such tree cover and is 

not currently developed for a 

nonforest use. The minimum 

area for classification of forest 

land is 1 acre. The 16.6 

million acres of forest land reported for Pennsylvania’s 

2004 inventory represents a slight but not statistically 

significant decrease from the previous inventory’s 

estimate (16.7 million acres).

Patterns in forest-land cover can be seen from satellite 

imagery (Fig. 2) taken in 2000 (Warner 2002). Large, 

0

5000

10000

15000

20000

25000

30000

1630 1907 1938 1955 1965 1978 1989 2004

Thousand acres 

16000

16500

17000

1989 2004

Thousand acres

Figure 1.—Area of forest land in Pennsylvania, 1630, 1907, 1938, 1955, 

1965, 1978, 1989, and 2004.

contiguous patches of forest extend across the Allegheny 

Plateau in the north-central portion of the State. In 

central Pennsylvania, forest-land distribution follows 

the topographical contours of the ridges that divide 

agricultural valleys. Smaller, more fragmented blocks of 

forest land are noticeable in more urban and agricultural 

regions, especially across southern-tier counties.

Figure 2.—Forest land in Pennsylvania, 2000.
A complex mix of biotic and abiotic factors determines 

forest composition, structure, and function. Traditional 

geopolitical boundaries, e.g., county maps, are not 

always correlated with these natural phenomena or 

specific resource issues. Figure 3 shows ecopolitical 

regions developed by the Bureau of Forestry to address 

sustainability issues. These partition Pennsylvania’s 

diverse landscape into meaningful areas that address 

ecological differences, e.g., Plateau versus Ridge and 

Valley forests, as well as cultural and political concerns, 

e.g., issues germane to the urban southeastern region 

versus the rural north-central region. These regions 

provide context for other maps and analyses in this 

report.

There was little net change in total area of 

forest land within all ecopolitical regions 

between 1989 and 2004 (Fig. 4). Although 

not statistically significant, all of the southern 

regions posted smaller acreages of forest land 

in the 2004 inventory. The north-central 

region, which includes the Allegheny National 

Forest and large tracts of state-owned forest, 

predictably contains the largest amount of 

forest land (79 percent). The southeastern 

region is the least forested at 22 percent. This 

is not surprising given that the region includes 

the Philadelphia metropolitan area and is host 

to the greatest proportion of agricultural land 

uses (35 percent) of any region in Pennsylvania. In fact, 

there is more agricultural land than forest in most of the 

counties in this region.

County-level changes in forest land are shown in Figure 

5. Many counties in the north-central and northeastern 

regions indicate an overall gain in forest land. Losses 

in forest land at the county level are prevalent in more 

urbanized counties, particularly in the southeastern 

region and in some counties in the south-central 

region. Many counties that show a net loss of forest 

land are located near urban centers or major connecting 

highways. Eastern Pennsylvania is part of the band of 

Northwestern North Central Northeastern

Southwestern South Central Southeastern

Figure 3.—Ecopolitical regions of Pennsylvania.

0

1000

2000

3000

4000

5000

6000

Northwestern Southwestern North-central South-central Northeastern Southeastern

Ecopolitical Region

Thousand Acres

1989

2004

Figure 4.—Area of forest land by ecopolitical region, Pennsylvania, 

1989 and 2004.
7

urban development that follows Interstate 95 along the 

East Coast. These areas are characterized by large cities, 

e.g., Philadelphia, with little forest land. Surrounding 

areas often include development patterns that have led to 

small patches of highly fragmented forests.

Forest-Land Loss and Gain

Although no signifi cant net change has occurred in 

Pennsylvania’s total forest area, both losses and gains 

in forest continue at various scales. In such a dynamic, 

the total acreage of forest area may remain the same 

while shifts occur in the forest-land base. Therefore, 

characterizing this base as having “no net change” 

may not accurately represent actual changes in forest 

distribution, character, and composition.

Percent Change 

Figure 5.—Percent change in area of forest land, Pennsylvania, 1989 to 2004.

NRS-FIA data indicate that more than 663,000 acres 

of forest land were lost from 1989 to 2004, an average 

of about 44,000 acres per year. Nearly two-thirds of 

the forest land, or 28,000 acres per year, was diverted 

to residential and industrial development and likely is 

permanent.

An example of forest lost to residential development is 

shown in Figure 6. Remote sensing data make it possible 

to identify pockets of land conversion, for example, in 

the northeastern region. This pattern of forest loss is 

common throughout Pennsylvania. Additional factors 

contributing to the loss of forest land include agricultural 

expansion and other localized disturbances such as 

mining. Timber harvesting does not contribute to this 

Figure 6.—Forest-land conversion to residential development, Pennsylvania, 1985 (a) and 1997 (b).

a b
type of forest loss so long as the tract continues to meet 

the definition of forest land.

During the same period, there was a 617,500-acre gain in 

forest land. About 350,000 acres (58 percent) of the gain 

was from agriculture. In this situation, abandoned fields 

commonly revert to forest through natural succession. 

This trend has offset most of the observed permanent 

loss of forest land and has allowed for the stable acreage 

in forest. That the most common agricultural land 

conversion is to urban land uses might limit this land 

type as a source for gain in forest land in the future. 

Reclaimed mine land and rights-of-way were other 

significant sources of forest gains.

Urbanization and Fragmentation

Urbanization of the landscape significantly affects the 

amount of forest land and also its species composition, 

health, and overall sustainability. Urbanization of 

forest land is the process of increasing urban (versus 

agricultural) development, either replacing or coming 

into increasing proximity to forest land. The process of 

urbanization is illustrated by comparing county-level 

estimates of forest area loss between 1989 and 2004 

and change in housing and population density over 

roughly the same period (Figs. 7-8). Many areas with 

the greatest loss of forest land also experienced the 

highest urbanization rates, supporting the finding that 

most forest is being lost to residential and commercial 

development.

Table 1 and Figure 9 show forest-land distribution by 

population density. Also shown is the proportion of 

forests by ecopolitical region that falls in Census Bureau 

urban areas, a more restrictive definition that includes 

contiguous densely settled area but doesn’t include many 

suburban and exurban areas (U.S. Census Bureau 2003). 

Averaged across the State, 6 percent of the forest is in 

Census-classified urban areas. The southeastern region 

has the highest amount at 28 percent.

Table 1.—Forest land in Pennsylvania by region and population density class 

(source: U.S. Bureau of Census and National Land Cover Data project)

People/mi2 Forest land

(no.) Northwestern Southwestern North-central South-central Northeastern Southeastern

--------------------------------------------------------------Percent--------------------------------------------------------------

0 to 25 9 11 63 27 20 0

26 to 50 40 25 23 37 32 4

51 to 100 34 30 10 22 24 11

101 to 250 12 21 3 11 18 45

251 to 500 3 7 1 2 4 24

501 to 1000 1 3 0 1 1 9

1001+ 1 2 0 0 1 7

Note: Percentage of total forest land in urban forest: Northwestern (3); Southwestern (9); North-central (0); South-central (1); 

Northeastern (2); Southeastern (28).

Land-clearing equipment.
Figure 8.—Change in population density, Pennsylvania, 1990 to 2000.

Figure 7.—Increases in housing density, Pennsylvania, 1990 to 2000.

Figure 9.—Forest land by population density and urban status, 2000.

Housing Density

(houses/sq. km)

no increase

0 - 5

5 - 50

50 - 100

100 and greater increase

Population Density

(people/sq. km)

-100 and greater decrease

-100 to -50

-50 to -5

-5 to 5

5 to 50

50 to 100

100 and greater increase

People/sq. km

0

1-25

25-50

50-100

100-250

250-500

500-1,000

> 1,000

Nonforest

Forest

Urban area
10

The rate of growth in Census-classified urban areas in the 

United States over the next several decades was modeled 

by Nowak and Walton (2005). Counties on the East 

Coast, including those in Pennsylvania, are projected 

to have some of the highest rates of urbanization over 

the next 50 years. The study projected that U.S. urban 

land will increase from 3 percent in 2000 to 8 percent 

in 2050. This growth could significantly transform the 

Commonwealth’s forests and attitudes regarding those 

forests, particularly in the northeastern and southern 

regions.

Forest fragmentation is defined as the division of 

contiguous or adjoining forest land into smaller patches. 

Fragmentation can be caused by urbanization, or as 

in Lancaster, Lebanon, and York counties, agriculture 

is a primary cause of fragmentation of forest land. 

Fragmentation analysis, the study of spatial patterns 

of forest patch distribution and dynamics, reveals 

information about the Pennsylvania’s forests that is not 

apparent from simple statistical summaries. For example, 

examining and measuring the relative distribution and 

interface between forest land and developed land uses 

provide insight into landscape characteristics that may 

influence the character and ecological function of the 

forest and aids in assessing its susceptibility to a broad 

range of anthropogenic impacts, e.g., invasive species. 

Such analyses also provide insight into the range and 

magnitude of ecosystem services that may be demanded 

of these remaining forests. Examples include waterquality protection, carbon sequestration, pollution 

removal, wildlife habitat, and 

recreation.

Losses of forest land to 

development can fragment 

remaining forest lands into 

smaller patches that are farther 

apart, reducing the chance that 

remaining wildlife populations are 

sustainable. And as the amount 

of direct interface or “forest edge” 

increases, interior or core forest 

areas are lost, increasing the chances for invasive species 

to be introduced and reducing the amount of habitat 

available for interior forest species. The characteristic of 

this edge environment and its effect on the forest varies 

according to the type of adjacent land use.

NRS-FIA researchers conducted a forest fragmentation 

assessment of Pennsylvania based on the 2001 National 

Land Cover Data project (Yang 2003). Calculating 

patch size from this dataset provides an indication of the 

continuousness of the forest cover (Fig. 10). Evident here 

are the smallest patch sizes in the southeast, followed 

by western Pennsylvania. In general, patches less than 

100 acres in size depend on the amount and proximity 

of other forest patches for sustaining viable wildlife 

populations of interior species. However, this analysis 

does not account for additional interruptions by roads. 

In Pennsylvania, 72 percent of the forest is less than 

0.25-mile from a road; Figure 11 shows the proportion 

of forest that close to a road by county.

Core forest is defined as forest 

more than about 100 feet from 

a nonforest edge. Figure 12

shows the proportion of core 

forest by county. Forests in the 

north-central region contain the 

highest proportion of core forest 

and also the least amount of 

fragmentation. Not surprisingly, 

the region hosts the largest 

forest-patch sizes. The Allegheny 

National Forest and State Forests 

contain the largest patches in Pennsylvania. Forest land 

in the southeastern region is the most highly fragmented, 

with small patches and minimal core forest.

New construction in the woods.

Mountain biker in the woods.
11

Figure 12.—Proportion of forest that is core forest by county, Pennsylvania, 2000.

Figure 11—Proportion of forest land that is less than 0.25-mile from a road, by 

county, Pennsylvania, 2001.

Figure 10.—Average forest patch size, Pennsylvania, 2000.

Proportion

Proportion

Patch Size (Acres)
12

Who owns Pennsylvania’s forest? While there is no simple answer, ownership is an important 

aspect in characterizing the status of the forest. Land owners often are positioned at a critical 

juncture where forest management practices intersect with broader societal values and 

economic pressures and realities. Discerning ownership trends and patterns is imperative 

to understand how Pennsylvania’s many forest-land owners collectively translate the 

Commonwealth’s economic and social landscape into that of the forests they own.

Pennsylvania hosts a diverse mix of public and private-forest land owners, including federal, 

state, and local governments; and corporations, individuals, and other private groups (Figs. 

13-14). The following tabulation shows the acres of forest land and associated estimates of 

the number of private owners:

Acres Private owners

Owner category Number Percent Owners Percent

Thousands Thousands

Federal 611.1 4

State 3,813.5 23

Local 413.7 2

Total public 4,838.3 29

Corporate:

 Forest industry 234.0 1 <1 <1

 TIMO 246.9 2 <1 <1

 Misc. corporate 1,658.4 10 26.5 5

Noncorporate:

 Families and

 individuals

8,906.4 54 501.5 94

Noncorporate 698.1 4 4.7 1

Total private 11,743.8 71 533.0

Total 16,582.1

Chapter 2: Forest-Land Ownership

Allegheny National Forest, northwestern Pennsylvania
13

The Commonwealth’s state-owned forest is the fourth largest in the 

United States at 3.8 million acres, or roughly one-fourth of the total 

forest land. The primary state agencies are the Bureau of Forestry with 

2.1 million acres and the Game Commission with 1.5 million acres. 

The Allegheny National Forest comprises most of the federal forest 

land in Pennsylvania; federal forests in total represent about 4 percent 

of the Commonwealth’s forest land (611,100 acres). Local, county, and 

municipal owners control 2 percent (413,400 acres).

Despite these vast acreages of public forests, private owners hold the 

majority of Pennsylvania’s forest land. An estimated 533,000 private 

owners own 71 percent of the forest (11.7 million acres). The broadest 

grouping of private owners is corporate and noncorporate. The 506,000 

non-corporate owners account for 82 percent of Pennsylvania’s private 

forest (9.6 million acres); families and individuals are the dominant 

group in this category and overall. Miscellaneous noncorporate owners 

including nongovernmental organizations, e.g., Boy Scouts of America, 

clubs e.g., hunting clubs, and associations hold 4 percent. Corporate 

owners account for the remaining 18 percent (2.1 million acres) of 

Pennsylvania’s private forests. Corporate owners include:

Companies that own primary wood processing 

facilities,

Forest-management firms that do not own 

primary processing facilities,

Timber investment management organizations 

(TIMO) that manage land on behalf of 

institutional investors, and

Companies for which forest management is 

not the primary objective (such as mining or 

manufacturing firms).

•

•

•

•

Figure 14.—Distribution of forest land by ownership, Pennsylvania, 2004.

4%

23%

2%

1%

2%

10%

54%

4%

Federal

State

Local

Forest industry

TIMO

Misc. Corporate

Families and Individuals

Non-corporate

Figure 13.—Forest ownership in Pennsylvania, 2004.

Grey Towers, Milford, Pennsylvania

Owner Group

Private forest

Public forest

Nonforest land

Water
14

Timber Investment Management

Organizations

TIMO is a relatively new type of owner that emerged in 

the 1980s. Due to attractive rates of return and as hedges 

against stock market trends, timberland has become 

an attractive asset to such investors. TIMO purchase, 

manage, and sell forest land on the behalf of their clients, 

primarily institutional investors, e.g., pension funds. 

Most of these lands are in pooled funds with fixed-time 

horizons. On average, they mature in 7 to 12 years, at 

which time the assets are liquidated and redistributed 

to investors. Billions of dollars have been invested by 

TIMO across the United States and the world. They 

now control millions of acres of U.S. forest land and 

continue to acquire more. The large-scale divestment of 

forest holdings by traditional companies, i.e., vertically 

integrated firms, has provided ample investment 

opportunities for TIMO.

Six major TIMO currently operate in Pennsylvania. 

They own 246,900 acres, or 2 percent of the forest, and 

are located primarily in the north-central portion of the 

State. As with most asset managers, TIMO are focused 

on maximizing profits for their investors. Their activities 

are strongly tied to timber and land markets and the 

performance of other asset classes, i.e., alternative rates 

of return. Many of these funds will be maturing over 

the next several years, at which time their impact on 

Pennsylvania’s forests will be better understood.

National Woodland Owner Survey

FIA conducts the National Woodland Owner Survey 

(NWOS) to better understand family forest owners 

and their motivations and intentions (Butler and others 

2005). Family and individual-owned forests, hereafter 

referred to as family forests, includes forested parcels that 

are at least 1-acre in size, 10-percent stocked, and owned 

by individuals, couples, estates, trusts, or other groups 

of unincorporated individuals (Butler and Leatherberry 

2004).

Sixty-four percent of the family forest owners in 

Pennsylvania hold fewer than 10 acres and account for 

10 percent of the family forest-land base (Fig. 15-16). 

Thirty-three percent own forests ranging in size from 

10 to 99 acres; these owners represent 53 percent of the 

family forest land. Three percent own forest tracts larger 

than 100 acres; they represent 37 percent of the family 

forest land base.

The reasons for owning their forests are as diverse as 

the families and individuals themselves. Amenity values 

are reported to be more important to families and 

individuals than financial objectives. Four of the top 

five ownership objectives relate to beauty/scenery, home 

ownership, privacy, and nature protection (Fig. 17). The 

fifth objective relates to the family legacy value of the 

land. This is not to suggest that families and individuals 

oppose commercial activities on their forest land. Twentythree percent of the family forest owners, who hold 50 

percent of the family forest land, have commercially 

harvested trees at least once since purchasing their land.

A written management plan is one metric for measuring 

forethought and science-based forest management. 

According to the NWOS, only 2 percent of family 

forest owners, who hold 7 percent of the family forest 

land, indicated that they have a written plan to guide 

management activities. Federal and state outreach 

programs have focused on providing technical assistance 

to private owners through cost-share programs; however, 

Posted private forest land
15

0

10

20

30

40

50

60

1-9 10-49 50-99 100-499 500-999 1000-4999 5000+

Size of Holding (acres)

Percent

Owners

Area

Figure 15.—Distribution of forest land by size of forest holding, Pennsylvania, 2004.

Figure 16.—Percentage of area and number of families and individuals by size 

of holding, Pennsylvania, 2004.

0 10 20 30 40 50 60 70

Timber Production

Land Investment

Other Recreation

Part of Farm

Part of Home

Hunting

Nature Protection

Family Legacy

Privacy

Aesthetics

Reason for Owning

Area (%)

Figure 17.—Percentage of forest land held by families and individuals by reason for 

owning forest land, Pennsylvania, 2004.

Forest Holding (acres)

1 - 49

50 - 999

1,000+
16

less than 2 percent of the owners, who hold 6 percent of 

the family forest land, have participated in a cost-share 

assistance program.

Most family forest land is owned by people who do not 

have major activities planned for their land (Fig. 18). 

Of those who intend to actively manage their land, 

harvesting firewood or sawlogs are the most commonly 

planned activities. Ten percent of family forest owners, 

who hold 18 percent of the family forest land, intend 

to sell or transfer their land in the next 5 years. These 

owners represent a significant portion of Pennsylvania’s 

forest.

Family forest owners tend to be older, white males nearly 

one-third of whom have earned a college degree. Fifteen 

percent of family forest land is held by someone who is 

75 years or older and 38 percent is held by individuals 

older than 65 years (Fig. 19), another indication that 

these lands will soon change hands.

0

10

20

30

40

50

60

70

Sell Heirs Timber Minimal Firewood None

Plans for Next 5 Years

Area (%) 

0

5

10

15

20

25

30

< 35 35-44 45-54 55-64 65-75 75+

Age of Current Owner (years)

Owners (%) .

Figure 18.—Percentage of forest land held by families and individuals by plans for 

next 5 years, Pennsylvania, 2004.

Figure 19.—Percent of families and individuals by age of current owner, 

Pennsylvania, 2004.

Ownership Trends

Through purchases and other acquisitions, public forest 

land in Pennsylvania has increased by about 11 percent 

over the last decade, primarily due to increases of stateowned land.

Although the number of private forest owners has 

increased over the last decade by about 4 percent, their 

total private forest area actually decreased by nearly 5 

percent. This indicates more owners with smaller parcels, 

a trend likely to continue as aging landowners divest their 

properties. Forest industry also has divested much of its 

land over the last decade, with much of the former forest 

industry land now controlled by TIMO. It is not known 

whether the new owners have the same attitudes, needs, 

and management objectives as the current owners. What 

is certain is that private owners, including corporate and 

noncorporate, will have a profound impact on the future 

of Pennsylvania’s forests.
17

Chapter 3: Composition and Structure

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

White/Red Pine

MixedOak

Elm/Ash/Red Maple

Northern Hardwoods

Aspen Birch

Forest-Type Group

Thousand Acres

1989

2004

Figure 20. —Area of forest land by forest-type group, 

Pennsylvania, 1989 and 2004.

The NRS-FIA program collects a wealth of data that 

address forest composition and structure. Traditional 

measures of composition include forest types, relative 

importance of tree species, and spatial distribution of 

individual species. Measures of structure include stand-size 

class, stocking class, number of trees, volume, biomass, 

and down woody material. This information provides 

valuable insights into the condition, composition, and 

structure of Pennsylvania’s evolving forest resources.

Forest-Type Groups

NRS-FIA categorizes forests using a classification of 

forest land based on the species that form a plurality of 

live-tree stocking. Individual forest types are aggregated 

into forest-type groups to allow broad comparisons. 

The traditional forest-type names and conventions have 

remained the same over time to allow for consistent trend 

analysis. Changes in the distribution of forest land by 

forest-type group depend on natural and anthropogenic 

impacts on the forest canopy, for example, succession, pests 

and diseases, harvesting, and shifts in the forest-land base.

Regional analysis and comparisons of forest-type group, 

stand-size class, and stocking class are not available for 

inventories prior to 1989 because digital data could not 

be recomputed to meet current standards. Some statelevel comparisons can be made for inventories before 

1989 as appropriate.

The distribution of forest land by the major forest-type 

group has remained relatively stable since 1989 (Fig. 20). 

There were no significant decreases at the 95-percent 

confidence level; however, analysis of the means implies 

a decrease in the mixed oak group. This finding would 

substantiate trends in oak discussed elsewhere in this 

report.

Turn-of-the-century logging. Photo from the Lycoming County Historical Society.
18

Maps that generalize the distribution of mixedoak and northern hardwoods forest-type groups 

(Fig. 21) were derived from NRS-FIA inventory 

data using geostatistical techniques that represent 

the most probable distribution for a given group. 

(Maps of less common groups are not feasible due 

to limited sample size.) Maps of forest-type groups 

should not be confused with the distribution 

maps for individual species. Both types of map 

are included to provide a more complete analysis 

of species occurrence. For example, red maple 

is common in multiple forest-type groups but is 

singled out in the species distribution map. The 

maps provide a graphical depiction for forest-type 

groups often scattered geographically. In some areas, 

distributions overlap.

The northern hardwoods forest-type group 

(sometimes referred to as maple/beech/birch), while 

found across most of Pennsylvania, is concentrated 

primarily along the Allegheny Plateau in the northcentral region. The mixed-oak forest is concentrated 

along the ridges and valleys of central Pennsylvania.

Although forest-type groups were stable, there were 

noticeable changes in two forest types. Figure 22

shows statistically significant gains in specific forest 

types where red maple is the principal dominant and 

losses in the sugar maple/beech/yellow birch type.

Stand-Size Class

To gain a general indication of the stage of 

stand development, standard NRS-FIA tree-size 

measurements are used to arrive at stand-size 

class. Sampled stands are assigned to one of three 

categories—small, medium, and large—based on 

the class that accounts for the most stocking of 

live trees per acre. Small stands have a plurality of 

trees less than 5 inches in diameter at breast height 

(d.b.h.). Medium stands are dominated by trees at 

least 5 inches d.b.h. but less than large size. Large 

stands are at least 9 inches in d.b.h. for softwoods 

and 11 inches for hardwoods.

Figure 21.—Distribution of northern hardwoods and mixed-oak 

forest land, Pennsylvania, 2004.

0

500

1000

1500

2000

2500

3000

3500

Red Maple Sugar maple/beech/yellow birch

Forest Type

Thousand Acres

1989

2004

Figure 22. —Area of forest land classified as red maple and sugar 

maple/beech/yellow birch forest types, Pennsylvania, 1989 and 2004.

Northern Hardwoods

Mixed Oak
19

Major changes have occurred in the distribution 

of forest land by stand-size class. Statistically 

significant changes have occurred in all classes 

since 1989, with small stands and medium 

stands decreasing and large stands increasing 

(Fig. 23). This trend is more pronounced when 

these results are compared with those from the 

1955 inventory (Fig. 24). Since that time, small 

stands have decreased by 44 percent and large 

stands have increased by 33 percent. The current 

stand breakdown is 11 percent small, 30 percent 

medium, and 59 percent large.

The distribution of stand sizes across Pennsylvania 

is shown in Figure 25. The prevalence of large 

stands is evident. Medium stands are scattered 

throughout the State. Small stands, or typically 

early successional forest, also are distributed widely, 

with concentrations along the western edge of the 

ridges and valleys near Clearfield County. The 

map depicts generalized distribution. Each pixel is 

modeled according to the most likely stand class 

based on surrounding pixels. Large areas depicted 

as a single size class often are mixed with other 

classes.

Few of the changes in stand size for the six 

ecopolitical regions were significant. The 

progression of medium stands to large stands 

is apparent in the north-central, south-central, 

and northeastern regions. The decrease in small 

stands is occurring mostly in the southeastern, 

southwestern, and south-central regions and the 

northeastern region.

Stocking

Stocking is a measure of the occupancy of land 

by trees in relationship to the growth potential of 

the site (see Definition of Terms). Four stocking 

classes generally are reported: poor (10 to 34 

percent), moderate (35 to 59 percent), full (60 to 

100 percent), and overstocked (101+ percent). The 

nonstocked class (0 to 9 percent) is ignored in this 

analysis because it represents a small area.

0

2000

4000

6000

8000

10000

12000

Small Meduim Large

Stand-Size Class

Thousand Acres

1989

2004

0

2000

4000

6000

8000

10000

12000

Small Meduim Large

Stand-size Class

Thousand Acres

1955

2004

-44%

-16%

+33%

Large

Medium

Small

Figure 23. —Area of forest land by stand-size class, 

Pennsylvania, 1989 and 2004.

Figure 25.—Distribution of forest land by stand-size class, 

Pennsylvania, 2004.

Figure 24. —Area of forest land by stand-size class, Pennsylvania, 

1955 and 2004.
20

The NRS-FIA stocking classes roughly correspond 

to traditional stocking guides for the Eastern 

United States. Using the terminology of 

Gingrich (1967), Leak (1981), and others, the 

stocking classes relate to the A, B, and C levels of 

stocking guides. The overstocked class represents 

conditions above the A level. The full stocking 

class represents the area between the A and B 

levels where stocking is considered adequate. 

The moderate stocking class represents the 

area between the B and C levels where B level 

stocking is expected within 10 years. The poor 

class represents conditions below the C level—a 

stand that is considered in need of restocking or 

regeneration.

Since 1989, forest-land area in the poor and 

moderate classes increased by 82 and 34 percent, 

respectively (Fig. 26). Reductions in stocking can 

result from a variety of events and disturbances, 

e.g., destructive weather, pests and diseases, and 

timber harvesting. Forest-land area in the full and 

overstocked classes decreased by 16 and 

33 percent, respectively. This represents a 

reduction of 1.8 million acres in the two 

classes. However, most of the reduction was 

in the full stocking class. Changes appear 

to have occurred mostly in the western 

half of the State based on a map of canopy 

loss, a surrogate for stocking (Fig. 27). The 

reduction in both the full and overstocked 

classes occurred primarily on private land 

(Fig. 28).

Stocking was examined for all forest land 

and trees, including reserved areas and 

cull trees. Another way to analyze stocking 

levels across Pennsylvania is to examine only 

commercial timberland and growing-stock 

trees (Definition of Terms). The results for 

this type of analysis are similar to those for 

all forest land and trees (Figs. 29-30). The 

reduction in fully stocked stands is somewhat 

more pronounced because only commercial trees 

are considered by this method of analysis.

Canopy Loss (%)

0

2000

4000

6000

8000

10000

12000

Poor Moderate Full Overstocked

Stocking Class

Thousand Acres .

1989

2004

Figure 26. —Area of forest land by live-tree stocking class, 

Pennsylvania, 1989 and 2004.

Figure 27.—Distribution of forest land by percent canopy loss, 

Pennsylvania, 1990 to 2000.
21

Figure 28. —Area of forest land by live-tree stocking class and ownership, Pennsylvania, 1989 and 2004.

Figure 29. —Area of timberland by growing-stock 

stocking class, Pennsylvania, 1989 and 2004.

 stocking class

PUBLIC

SE CI 95 2004

0.38 67.1 268.3

0.18 123.4 1432.1

0.08 235.0 2779.4

0.2 108.9 350.5

PRIVATE

SE CI 95 2004

 PUBLIC

0

1000

2000

3000

4000

5000

6000

7000

8000

Poor Moderate Full Overstocked

Stocking Class

Thousand Acres

1989

2004

0.07 243.7 4133

0.042 306.6 5806.7

0.14 131.2 638.6

PRIVATE

0

1000

2000

3000

4000

5000

6000

7000

8000

Poor Moderate Full Overstocked

Stocking Class

Thousand Acres

1989

2004

0

2000

4000

6000

8000

10000

Poor Full

Stocking Class

Thousand Acres

1989

2004

Figure 30.—Area of timberland by growing-stock stocking class and ownership, Pennsylvania, 1989 and 2004.

PUBLIC

0

1000

2000

3000

4000

5000

6000

7000

8000

Poor Moderate Full Overstocked

Stocking Class

Thousand Acres

1989

2004

PRIVATE

0

1000

2000

3000

4000

5000

6000

7000

8000

Poor Moderate Full Overstocked

Stocking Class

Thousand Acres

1989

2004
22

Distribution of Tree Species

Maps showing the distribution of selected tree species 

were developed using percent basal area on sampled 

plots (Fig. 31). Twelve species provided the sample size 

necessary for this analysis. The resulting maps represent 

the probability of the level of select species dominance. 

Most of the species mentioned are found throughout 

the State.

Red maple

Black cherry

Northern red oak

Chestnut oak

Figure 31.—Estimated percentage of basal area for selected species, Pennsylvania, 2004.

Red maple is a useful species when interpreting 

distribution maps. Although it is most prevalent across 

the northern tier counties, with pockets in the western 

regions, red maple is common throughout Pennsylvania.

Allegheny hardwoods is a subgroup within the northern 

hardwoods forest-type group that occurs primarily on 

the Allegheny Plateau. NRS-FIA does not compute the 
23

area of Allegheny hardwoods because it is not part of the 

national forest-typing methodology. The dominance of 

black cherry, a major species within the type, serves as 

an indicator for Allegheny hardwoods as its dominance 

across the Allegheny Plateau is evident. Black cherry also 

is common in western Pennsylvania but is a relatively 

minor species in the eastern counties.

The distribution map for northern red oak shows clumps 

of this species through the central part of the State, 

particularly in the Ridge and Valley area. Red oak is 

relatively rare across the northern tier.

Chestnut oak also is concentrated in the ridges and 

valleys, occurring north of Blue Mountain and westward 

to the Allegheny Front. Sugar maple is clearly a northern 

Pennsylvania species. Concentrations are heaviest 

in glaciated soils that favor the development of this 

species. Eastern hemlock also is concentrated in the 

northern counties, with a strong presence west of the 

Sugar maple

Eastern hemlock

Black birch

White oak

Figure 31.—continued.
24

Allegheny Front and in the Pocono Mountains. Black 

birch is scattered throughout the Ridge and Valley area 

and in pockets across the northern counties. White oak 

is common across the State except along the northern 

tier. The heart of the Ridge and Valley area hosts 

concentrations of this species. White ash is dominant in a 

several areas in the southern half of the State, particularly 

southeastern Pennsylvania; it is widely scattered elsewhere. 

Beech’s prevalence across the northern counties is 

readily apparent. There are several clusters in western 

counties and a sizeable concentration in the Pocono 

Mountains. Yellow-poplar is clearly a southern tier 

species with heaviest concentrations in the southwestern 

and southeastern regions. The high concentration in 

the extreme southeastern portion of the State is notable 

because this area contains some of the Commonwealth’s 

oldest and largest forests. Yellow-poplar is the dominant 

species in many of these forests. White pine is highly 

concentrated in some locations, though occurrence is 

dispersed and this species is a minor component of the 

forest types in which it is found.

White ash

Beech

Yellow-poplar

White pine

Figure 31.—continued.
25

Low-Use Wood Resource

Interest in utilizing wood that has traditionally been in low demand has 

increased recently due to higher energy costs, improved utilization technology, 

and opportunities for improving rural economies. Pennsylvania’s forests are 

characterized by expansive tracts of relatively similar age that are developing 

into a highly valuable sawtimber resource. As the process of economic maturing 

continues, there also has been a tremendous buildup of wood material that is 

underused. This resource includes trees that are of poor form, quality, size, 

and value. Utilization of low-use wood (LUW) also improves forest health and 

sustainability. LUW removal is essentially an intermediate treatment that opens 

stands to light, thus enhancing the establishment and development of tree 

seedlings and understory flora, and improving wildlife habitat. Currently, the use 

of LUW is being recommended to policymakers as an opportunity that ranges 

from wood-chip utilization in energy generation, composite panels, paper, and 

cellulosic ethanol to associated secondary industries. This opportunity supports 

policies aimed at sustainable development for rural Pennsylvania.

LUW Estimation

The use of the term LUW was 

chosen over low grade or low 

value because many candidate 

trees have no grade or even 

value when the economics of 

availability, harvest, transport, 

and use are considered (Luppold 

and Baumgardner 2003). NRSFIA has developed estimates 

of LUW under the guidance of 

the Secretary of Agriculture’s 

Hardwood Blue Ribbon Task 

Force. A series of simple filters 

and constraints was developed 

and used to estimate the amount 

of LUW that may be available. 

For example, all rough and rotten 

trees, those with intermediate or 

overtopped crown positions, and 

stems with less than 10 percent crown ratio were included as LUW.

Actual availability of wood resources depends on multiple factors, e.g., 

operability, economic, legal, social, and other interacting factors (Luppold and 

McWilliams 2000; Luppold and Bumgardner 2003). The principal variables for 

reporting are ownership (public and private), biomass per acre in classes, and 

slope class. To approximate actual availability of LUW based on operability 

constraints, the analysis of “available LUW” excludes wood on sites with fewer 

than 30 tons of LUW per acre and on slopes greater than 40 percent.

Pile of wood chips.
26

LUW Resource

The total wood biomass on Pennsylvania’s timberland is 1,145.8 million green 

tons. Fifty-seven percent of the total biomass—657.8 million tons—is classified 

as LUW. Applying the LUW operability constraints yields an estimate of available 

LUW of 468.7 million tons, or 71 percent of the LUW in the State. Nearly threefourths of this material is on privately owned timberland (Fig. 53).

LUW is an abundant resource across Pennsylvania. Opportunities for supporting 

a variety of wood-use industries are apparent. Two important issues facing the 

State’s forestry community are how to manage the tremendous volume/value of 

existing stands while improving current advance regeneration and biodiversity 

for the Commonwealth’s future forest. As mentioned earlier, opening stands 

to improved light conditions promotes the development of tree seedlings 

and healthy understory flora (Marquis 1994). So long as den trees and other 

conditions are considered, wildlife should prosper from an increase in available 

food. Other benefits of LUW utilization include ecosystem health, economic 

growth, rural development, and forest sustainability.

Factors related to supply and demand and owner preferences are broad 

with respect to the assumptions in this analysis. Additional information can 

be obtained and tailored to more specific resource questions, such as citing 

industrial plant. Summaries of NRS-FIA data are available at http://fia.fs.fed.

us/tools-data/tools/.

488.0 468.7

189.1

27 73

Percent

Million tons

LUW Available

LUW Other

Other Wood

Private

Public Figure 53.—Low-use wood by 

component and percent of available 

low-use wood by broad ownership 

class, Pennsylvania, 2004.
27

Number of Trees

Forest stands are commonly examined by a “stand table” 

that charts the number of trees by diameter class. This 

information highlights changes in stand structure and 

indicates future trends. For example, a species may 

show gains in larger diameters but decreases in smaller 

diameters, suggesting lesser importance in the future 

forest.

Changes in stand structure occur due to natural and 

anthropogenic influences. As the forests of Pennsylvania 

mature, decreases in smaller diameter classes should be 

expected as natural thinning occurs and trees grow larger. 

Figure 32 aptly illustrates this trend as the 4- through 

8-inch classes posted decreases from 1989 to 2004. There 

was a slight increase in the 10-inch class and moderate 

increases in the 12- and 14-inch classes. Increases in the 

16-inch and larger classes exceeded 20 percent. Although 

not statistically significant, there was a slight increase in 

the number of 2-inch trees statewide. Mature stand of Allegheny hardwoods.

State

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent .

0

1000

2000

3000

4000

5000

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Figure 32.—Percent change and number of live trees on forest land by diameter 

class, by ecopolitical region, all species, Pennsylvania, 1989 to 2004.
28

Northwestern

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

200

400

600

800

1000

1200

1400

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Southwestern

-40

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

200

400

600

800

1000

1200

1400

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees Figure 32.—continued.
29

North Central

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

200

400

600

800

1000

1200

1400

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

South Central

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

200

400

600

800

1000

1200

1400

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Figure 32.—continued.
30

0

200

400

600

800

1000

1200

1400

2 4 6 8 10 12 14 16 18 20+

Diameter Class

Million Trees

Northeastern

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

10

20

30

40

50

60

70

80

90

100

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Southeastern

-40

-20

0

20

40

60

80

100

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

Figure 32.—continued.
31

Figure 33 summarizes changes 

among species for all size classes. 

Included are all species that 

contributed at least 1 percent of the 

total number of live trees in 2004. 

The results are influenced heavily 

by small trees because of the typical 

reverse J shape of the stand tables. 

This shape reflects typical conditions 

for most deciduous forests in the 

Mid-Atlantic region, i.e., there are 

numerous small stems and a smaller 

number of large trees.

Species with decreases in the total 

number of trees include red maple, 

sugar maple, hemlock, northern 

red oak, chestnut oak, hawthorn, 

sassafras, white oak, serviceberry, 

and hornbeam. The most significant 

increase was in the number of black 

birch trees.

Figure 34 shows how stand tables for selected species are changing by diameter class. For 

example, recent trends in red maple indicate that this species decreased in the 2- to 6-inch 

classes and increased in larger diameters. Black cherry showed increases in all diameter 

classes except the 2-inch class.

Oak species were combined to 

ensure statistical confidence and 

to allow for a concise evaluation of 

their status. Oaks in Pennsylvania 

continue to mature, though all of the 

smaller diameter classes are showing 

decreases. In the case of the 2-inch 

class, no statistically significant 

change was found, indicative of 

prospective regeneration. The stand 

table for sugar maple similarly 

revealed increases in larger diameters 

but decreases in all diameter classes 

from 2 through 10 inches. Black 

birch is a prolific species with a 

noted advantage in the regeneration 

component. This is demonstrated in 

recent stand table changes. 

Figure 33.—Percentage of live trees on forest land by species/species group, all sizes, 

Pennsylvania, 1989 to 2004.

Young stand of black birch.

0 5 10 15 20 25

Yellow-poplar

American hornbeam

Serviceberry

White oak

Sassafras

Hawthorn

White pine

Striped maple

Blackgum

Chestnut oak

Northern red oak

White ash

Eastern hemlock

Sugar maple

Beech

Black cherry

Black birch

Red maple

Percent

2004

1989
32

Figure 34.—Percent change and number of live trees on forest land by diameter class, 

selected species, Pennsylvania, 1989 to 2004.

Red maple

-40

-20

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

-20

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

Black cherry

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees
33

Figure 34.—continued.

-40

-20

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

P e rc e n t

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Oak

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Sugar maple

-40

-20

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent
34

Figure 34.—continued.

Black birch

-20

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

Yellow-poplar

-20

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees
35

Since 1989, this species has increased by more than 100 

percent in the 2-inch class and by nearly 50 percent in 

the 4-inch class. Yellow-poplar is maintaining its relative 

importance in all classes except for a slight decrease 

in the 2-inch class. The findings for white pine show 

little change for the midrange diameters but significant 

increases for the smaller and larger diameters. Increases 

in the 2- to 8-inch classes suggest that white pine is 

surviving within the regeneration component and 

maintaining its importance among medium trees.

Volume

The volume of sampled trees can be categorized by 

several measurements, the most common of which are 

biomass, live volume (cubic feet), and sawtimber volume 

(board feet expressed using the International 1⁄4-inch 

rule). Total biomass of live trees is a useful indicator of 

relative tree species dominance. Cubic-foot volume is 

the measurement of the merchantable portion of the tree 

stem and represents the amount of wood available for 

pulp, paper, and secondary roundwood products such as 

pallets. Sawtimber volume represents the most valuable 

wood available and is computed for sawtimber-size trees 

for only the saw log portion (Definition of Terms).

Biomass

The total above-ground biomass of live trees at least 1

inch in d.b.h. is 2 billion green tons. Ninety percent of 

Pennsylvania’s biomass is in hardwood species (Fig. 35). 

Ranking by species reveals the dominance of red maple, 

with 18 percent of the total biomass (Fig. 36). The 

top 10 species account for three-fourths of the State’s 

biomass. Figure 37 shows that, for all species, the main 

stem accounts for more than half of the total biomass, 

followed by stumps/roots, branches, cull trees, foliage, 

and saplings.

Live-Tree Volume

Pennsylvania’s total inventory volume comprises largeand medium-size trees (growing stock), as well as rough 

and rotten trees. Large trees make up nearly two-thirds 

of the inventory, followed by medium trees (30 percent), 

rough trees (4 percent), and rotten trees (1 percent) 

(Definition of Terms).

Figure 34.—continued.

0

100

200

300

400

500

600

700

800

2 4 6 8 10 12 14 16 18 20+

Diameter Class (inches)

Million Trees

White pine

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20

Diameter Class (inches)

Percent 
36

Softwoods

Hardwoods

0 5 10 15 20

Other

Hickory

Yellow -poplar

White/red pine

Beech

White ash

Other oak

White oak

Black birch

Eastern hemlock

Chestnut oak

Sugar maple

Northern red oak

Black cherry

Red maple

Species/Species Group .

Percent

Figure 37.—Distribution of tree biomass (green weight) on forest land by 

component, all species Pennsylvania, 2004.

Figure 35.—Distribution of tree biomass 

(green weight) on forest land by broad 

species group, Pennsylvania, 2004.

Figure 36.—Distribution of tree biomass (green weight) on forest land by species/

species group, Pennsylvania, 2004.

All Species

0 10 20 30 40 50 60

Saplings

Cull trees

Stumps/roots

Foliage

Branches

Main stem

Component

Percent
37

The total net volume of live trees has increased by 11

percent since 1989 and now totals 31.7 billion cubic feet. 

The current inventory is the highest recorded since NRSFIA initiated the inventory process. Volumes increased 

in all ecopolitical regions (Fig. 38). The northwestern, 

north-central, and south-central regions posted increases 

in excess of the State average.

The continued growth of Pennsylvania’s forest is evident 

from changes in volume by diameter class (Fig. 39). 

Volume decreased in smaller diameters and increased in 

the larger classes. The greatest gains were in the 16- and 

0

1000

2000

3000

4000

5000

6000

6 8 10 12 14 16 18 20 22+

Diameter Class (inches)

Million Cubic Feet .

1989

2004

18-inch classes. Volumes in the ecopolitical regions 

followed the same trend. The north-central region had 

the greatest increases in the larger classes.

Examining the top 10 species/species groups revealed 

that all increased in volume except sugar maple (Fig. 40). 

Some of the increases were not statistically significant 

at the 95-percent confidence level, but the means were 

higher. The decrease in sugar maple was not significant 

but the decrease likely is a reality given other indications 

of general decline across the northern-tier counties.

Figure 38.—Volume of live trees by ecopolitical region, Pennsylvania, 1989 and 2004.

Figure 39.—Net volume of live trees on forest land by diameter class, Pennsylvania, 1989 

and 2004.

0

2000

4000

6000

8000

10000

12000

14000

Northwestern Southwestern North-central South-central Northeastern Southeastern

Ecopolitical Region

Million Cubic Feet

1989

2004
38

Sawtimber Volume

Sawtimber volume is measured in board feet. In current 

markets, sawtimber is the most economically valuable 

wood product in Pennsylvania’s forests. As with live-tree 

volume, the current sawtimber inventory is the highest 

ever recorded in NRS-FIA inventories.

The accumulation of sawtimber volume is shown in 

Figure 41. Sawtimber volume has nearly tripled since 

1955 and the current inventory is 88.9 billion board feet. 

On a per-acre basis, sawtimber volume exceeds 5,000 

board feet. Although sawtimber inventory is increasing, 

the rate is slowing. The current increase of 18 percent is 

much smaller than increases of 77 and 63 percent in the 

two prior inventories.

Sawtimber quality is gauged by examining volume 

distribution by tree grade (National Hardwood Lumber 

Association grades 1-3 and others). Tree grades are 

assigned to each sawtimber-size tree based on bottom log 

characteristics using both eastern hardwood grades and 

conifer grades. Trees that do not qualify for tree grades 

are placed into the “other” category. Volume estimates 

by grade were developed to approximate the output of 

standard lumber. Currently, grades 1 and 2 generally are 

preferred by sawmills.

Figure 40.—Net volume of live trees on forest land by species/species group, 

Pennsylvania, 1989 and 2004.

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

1955 1965 1978 1989 2004

Million Board Feet

+15%

+63%

+18%

+77%

Figure 41.—Volume of sawtimber by inventory date, Pennsylvania, 

1955, 1965, 1978, 1989, and 2004.

0 1000 2000 3000 4000 5000 6000 7000

Other oak

White ash

Eastern hemlock

White oak

Chestnut oak

Yellow-poplar

Sugar maple

Black cherry

Northern red oak

Red maple

Species/Species Group

Million Cubic Feet

2004

1989
39

The distribution of volume by grade reveals that more 

than half of the current volume is in tree grades 1 or 

2 (Fig. 42). This segment of the resource has been 

expanding, more than doubling since 1989, as more trees 

reach the minimum size for grading purposes.

Quality profiles show that at least half of the sawtimber 

volume of black cherry, northern red oak, white oak, 

white ash, and yellow-poplar is in tree grades 1 and 2

(Fig. 43). Public forests have a higher amount of 

sawtimber volume in the better grades than private land 

(Fig. 44).

All ecopolitical regions experienced significant increases 

in sawtimber volume (Fig. 45). The north-central and 

south-central regions had the largest percentage increases 

and accounted for 70 percent of the statewide increase in 

sawtimber inventory.

Statewide, all the major species/species groups had 

statistically significant increases or higher means (Fig. 46). 

Red maple had the largest increase in sawtimber volume 

and all oak species had significant increases. Larger oaks 

continue to expand in volume while the influx of younger 

oak trees is decreasing, as shown in the volume trends for 

live trees.

0

20000

40000

60000

80000

100000

1989 2004

Million Board Feet

Other

Grade 3

Grade 2

Grade 1

26

27

30

17

19 9

38

33

Figure 42.—Net volume of sawtimber and percent of total on timberland by tree 

grade, Pennsylvania, 1989 and 2004.

Figure 43.—Net volume of sawtimber and percentage of total on timberland by species/species group 

and tree grade, Pennsylvania, 2004.

34

52

8

34

27

19

16

45

37

13

28

24

29

29

32

30

26

27

24

25

26

17

49

26

32

40

39

18

25

37

12

7

15

11

8

10

19

9

13

24

0% 20% 40% 60% 80% 100%

Other oaks

Yellow-poplar

Black birch

White ash

White oak

Chestnut oak

Sugar maple

Norther red oak

Black cherry

Red maple

Percent

Grade1

Grade 2

Grade 3

Other
40

Distribution maps of sawtimber volume depict the 

location of inventory volume for major species/species 

groups (Fig. 47). When all species are considered, the 

areas with the largest sawtimber volume inventories are 

in the heavily forested northern-tier counties as well 

as the smaller forested parcels in southeastern corner. 

Some areas of the State contain more than 14,000 

board feet per acre.

Down Woody Material

Down woody material (DWM) is defined as 

dead material on the ground in various stages of 

decay. DWM is an important component of forest 

ecosystems, indicating critical attributes such as quality 

of wildlife habitats, structural diversity, fuel loading 

and fire behavior, carbon sequestration, and water 

storage and cycling (USDA For. Serv. 2002c).

Components measured by the DWM indicator include 

coarse woody debris (CWD); fine woody debris 

(FWD); duff; litter; herbs/shrubs; and fuelbed depth. 

DWM is measured at the transect line. CWD is dead 

wood 3 inches or larger in diameter (1,000-hour fuels); 

FWD is dead wood from 0.1 to 2.9 inches in diameter 

(1-, 10-, and 100-hour fuels). Litter is defined as the 

loose plant material on top of the forest floor where 

little decomposition has occurred. Duff is the layer just 

below the litter, consisting of decomposing leaves and 

other organic material.

Figure 45.—Volume of sawtimber by 

ecopolitical region, Pennsylvania, 1989 

and 2004.

Figure 44.—Distribution of hardwood sawtimber volume per 

acre by grade, size class, and ownership, Pennsylvania, 2004.

0.0

1000.0

2000.0

1 2 3 4 5

Hardwood grade (all diameters)

Board feet/acre (no.) ..

PUBLIC

PRIVATE

0

5000

10000

15000

20000

25000

30000

35000

40000

Northwestern Southwestern North-central South-central Northeastern Southeastern

Ecopolitical Region

Million Board Feet

1989

2004

0.0

1000.0

2000.0

1 2 3 4 5

Hardwood grade ( 15 inches d.b.h.)

Board feet/acre (no.) ..

PUBLIC

PRIVATE
41

Figure 46.—Net volume of sawtimber on forest land by species/species 

group, Pennsylvania, 1989 and 2004.

All species

Red maple

Figure 47.—Distribution of sawtimber volume, Pennsylvania, 2004.

0 2000 4000 6000 8000 10000 12000 14000 16000

Yellow-poplar

Black birch

White ash

White oak

Eastern hemlock

Chestnut oak

Sugar maple

Northern red oak

Black cherry

Red maple

Species/Species Group

Million Board Feet

2004

1989
42

Figure 47.—continued.

Black cherry

Northern red oak

Chestnut oak

Sugar maple
43

Hemlock

Black birch

White oak

White ash

Figure 47.—continued.
44

Beech

Figure 47.—continued.

Quantities of FWD and CWD in the mixed oak and 

northern hardwoods forest-type groups, measured in tons 

per acre, follow similar patterns. Table 2 lists fuel classes 

in an ascending manner (1-, 10-, 100-, and 1,000-hour), 

which is based on increasing diameter-class ranges (0.01

to 0.24, 0.25 to 0.9, 1.0 to 2-9, and 3.0+).

Fuel loadings for both forest-type groups also increase 

with each increase in fuel class (diameter-size class). The 

forest-type group designated as other displays the only 

variant in the this pattern. However, in the All Plots 

column, the original pattern persists.

Northern hardwoods had the largest amount of wood 

debris (FWD and CWD combined) with nearly 9 tons 

per acre. Mixed oak had 7.7 tons per acre, and other had 

7.2 tons per acre. The largest amount of litter (1.4 tons 

per acre) was in the mixed oak type, while the northern 

hardwood type averaged 1 ton per acre. The duff 

component of DWM contained the most tons per acre 

of all components. Northern hardwoods contained 8.4 

tons per acre, mixed oak 7.2, and other 4.3.

The shrub/herb component is an integrated 

measurement of percent cover and height representing 

Table 2.—Mean fuel loadings and associated standard errors for forest-type groups a, Pennsylvania, 2001-2004

Down woody Fuel Mixed oak Northern hardwoods Other All Plots

debris component loading Tons/acre SE Tons/acre SE Tons/acre SE Tons/acre SE

FWD 1-hour 0.24 0.03 0.38 0.05 0.22 0.06 0.30 0.03

FWD 10-hour 0.89 0.10 0.93 0.12 0.68 0.18 0.90 0.07

FWD 100-hour 3.10 0.81 2.82 0.35 3.86 2.44 3.03 0.44

CWD 1000-hour 3.42 0.83 4.83 0.86 2.45 1.42 4.00 0.56

Litter 1.40 0.17 1.02 0.14 0.33 0.14 1.18 0.11

Duff 7.19 1.00 8.37 1.29 4.32 2.40 7.52 0.78

Shrub/herb 1.89 0.20 1.67 0.24 2.64 0.59 1.84 0.15

a

 Mixed oak: 71 plots; northern hardwoods: 72 plots; other: 12 plots.
45

nontree vegetation between the ground and 

standing trees. The average height for the other type 

is the tallest at 2.6 feet, followed by mixed oak at 

1.9 feet and northern hardwoods at 1.7 feet.

Pennsylvania’s northern hardwood forests contain 

the largest amount of CWD at nearly 5 tons 

per acre, followed by mixed oak (3.4) and other 

(2.5). CWD creates numerous ecological niches 

and serves as habitat for plants, animals, protists, 

bacteria, and fungi (Harmon and others 1986). 

The amount of CWD required to maintain a 

healthy ecosystem in not known (Densmore and 

others 2004), though large pieces of CWD provide 

the greatest habitat value (Lofroth 1998). In 

Pennsylvania, 87 percent of the CWD pieces are 

dominated by small diameters between 3 and 7.9 

inches (Fig. 48). Sixty percent of these pieces are in 

the more advanced stages of decay (decay classes 4 

and 5) (Fig. 49). Only 20 percent of Pennsylvania’s 

CWD pieces were estimated as being freshly fallen 

or within several years of recruitment.

With respect to stand-size class, the mean volume 

of CWD is statistically similar among the small, 

medium, and large stands (Fig. 50). However, 

the amount of FWD was significantly greater 

in small- and medium-size stands than in large 

stands (Fig. 51). The distribution of DWM across 

Pennsylvania’s landscape is shown in Figure 52.

3.0-7.9

87%

8.0-12.9

10%

13.0-17.9

2%

18+

1%

3.0-7.9

8.0-12.9

13.0-17.9

18+

Figure 48.—Percentage of coarse woody debris pieces per 

acre by transect diameter, Pennsylvania, 2001-04.

1

7%

2

13%

3

20%

4

27%

5

33% 1

2

3

4

5

Figure 49.—Percentage of coarse woody debris pieces 

per acre by decay class, Pennsylvania, 2001-04.

0

100

200

300

400

500

600

700

800

Small Medium Large

Stand-Size Class

Cubic feet/acre

Figure 50.—Distribution of coarse woody debris by stand-size class, 

Pennsylvania, 2001-04.
46

Coarse

Fine

Duff

Figure 51.—Distribution of fine woody debris 

loadings by stand-size class, Pennsylvania, 

2001-04.

Figure 52.—Distribution of down woody material 

by component, Pennsylvania, 2001-04.

0

1

2

3

4

5

6

7

Small Medium Large

Stand-Size Class

Tons/Acre l
47

Timber Products Output

The forest products industry in Pennsylvania is an integral part of the State’s 

economy, providing more than 100,000 jobs in the Commonwealth. (Pa. Dep. 

Conserv. and Nat. Resour. 2004). The industry is fragmented and therefore 

difficult to track. Small, farm-operated and family-owned mills have traditionally 

accounted for the largest percentage in terms of numbers of mills, but not the 

largest percentage in terms of production.

The most comprehensive accounting of sawmills was conducted in 1988 

(Wharton and Bearer 1994). By that time there were more than 1,500 operating 

facilities, most of which produced less than 50,000 board feet per year.

A Billion Board Feet and Beyond

A survey of Pennsylvania’s timber industries in 1988 (Wharton and Bearer 1994) 

showed that the total roundwood volume received by Pennsylvania’s primary 

forest-product-industry had reached nearly 1.5 billion board feet. More than 

1 billion board feet of this total were from sawlogs. Pennsylvania consistently 

exceeds more than a billion board feet of production annually. A study conducted 

for 1999 (Smith and others 2003a) 

revealed that the State’s sawmill 

output totaled more than 1.3 

billion board feet.

The harvest of sawlogs and 

veneer logs continues to dominate 

throughout the Commonwealth. 

About 71 percent of the total 

roundwood harvest is composed 

of sawlogs and veneer logs 

(Murphy 2006). However, there 

has been a decline in production 

of sawlogs and veneer logs due 

largely to a decline in sawlog 

production (Fig. 54).

The production of veneer from 

Pennsylvania’s timberland has 

increased in volume or at the very 

least accounts for a larger share 

of the market. The increase in veneer logs manufactured within the State has 

ranged from 31 billion board feet to 127 billion board feet since the late 1980s 

(Murphy 2006).

Hardwoods Predominate Production

No other state produces more hardwood sawlogs than Pennsylvania. Black 

cherry and oak, especially select oaks like northern red oak and white oak, 

Red maple logs.
48

predominate (Fig. 55). Black cherry, soft maples (red maple and similar species), 

and yellow poplar have increased in production. Other high-volume species 

in the resource base such as red oak and hard maples (sugar maple and 

similar species) have declined in importance to Pennsylvania’s forest-products 

industries. The consumption of white oaks has remained relatively constant but 

the consumption of red oak has declined sharply.

0

200

400

600

800

1000

1200

Sawlogs Veneer logs Total

Billion Board Feet .

1969 1988 2003

0

5

10

15

20

25

30

35

Red oak Black

cherry

White oak Soft

maples

Hard

maples

Yellow -

poplar

Ash Hemlock

Percent

1969 1988 2003

Figure 55.—Percent change in production by species, Pennsylvania, 1969, 1988, 

and 2003.

Figure 54.—Sawlog and veneer log production, Pennsylvania, 1969, 1988, and 2003.
49

chapter 4: Understory Conditions

(McWilliams and others 2002). Measurements are 

conducted on NRS-FIA sample plots during the leafon season (McWilliams and others 2003). To date, 80 

percent of PRS data plots have been measured. The data 

in this initial report provide an important baseline for 

future understory monitoring.

PRS measurements include a detailed tally of all tree 

seedlings down to a height of 2 inches and a survey of 

nontree vegetation. Data are analyzed using silvicultural 

guidelines for Pennsylvania (Marquis and others 1994). 

Sample plots are evaluated to gauge the capacity of 

advance tree seedling and saplings to regenerate the 

stand. Data on associated understory vegetation provide 

additional information on understory character and 

health.

PRS results are divided into five species groups. The 

high-canopy dominants species group comprises all 

species that currently contribute at least 2 percent of the 

total tree biomass in the State and that typically form 

a high canopy. The all high-canopy group includes all 

The understory is a key component of the forest 

ecosystem and forest-stand structure (Latham and others 

2005). It is the layer of vegetation below the dominant 

forest canopy layer, and consists of shrubs, grasses, sedges, 

wildflowers, other herbs, low-canopy trees, saplings, 

seedlings, fungi, mosses, and lichens. The presence of 

advance regeneration, that is, trees seedlings and saplings 

in position to replace high-canopy trees, frequently 

determines the capacity of the forest to reestablish 

following disturbance (Marquis 1994).

Measuring variables in the forest understory component 

is essential to understanding the overall condition of 

Pennsylvania’s forest. The Pennsylvania Regeneration Study 

(PRS) was developed to address the need for more detailed 

information on understory status in addition to existing 

NRS-FIA data. The PRS is the only large-scale regeneration 

and understory health study that covers the entire 

Commonwealth in a systematic and consistent fashion.

The PRS was implemented in 2001 following an 

intensive pilot study to evaluate sampling protocols 

Deer fence, Allegheny National Forest.
50

species with the ability to form a high canopy. The 

commercially desirable group consists of species that 

are the most preferred in timber management. The 

all commercial group includes all commercial species 

that provide a merchantable crop. The all-tree species 

group comprises all tree species, including those that 

do not form a high-canopy forest, e.g., striped maple 

and dogwood. Nonnative, invasive species also are 

part of this group.

The only difference between the commercially 

desirable and high-canopy dominants groups is that 

black birch and beech are included in the latter group. 

The impact of these two species on the understory is 

noticeable in the results.

The percentage of sample plots adequately 

stocked with advance tree-seedling and sapling 

regeneration (ATSSR) is shown in Figure 56. Results 

include sample plots with sufficient sunlight for 

the establishment and development of advance 

regeneration (40- to 75-percent stocked). Stands 

in this stocking range account for 57 percent of 

Pennsylvania’s forests, or about 1,600 samples.

Examining only species capable of producing a highcanopy forest, (high-canopy dominants and all high 

canopy), 48 percent of the sample plots contained 

adequate advance regeneration. This means that only 

about half of the State’s forests would regenerate to highcanopy status following significant overstory disturbance. 

When all commercial species are examined (commercially 

desirable and all commercial), results are similar; 47 

percent of the sample plots have adequate advance 

regeneration. When only the most desirable commercial 

timber species (commercially desirable group) are 

considered, only about 34 percent of the plots contain 

adequate advance regeneration.

Results by region for species that lead to canopy 

replacement are shown in Figure 57. Sample plots with 

adequate advance regeneration range from 40 to 54 

percent. Results for only the most desirable commercial 

timber species are shown in Figure 58. The percentage of 

Figure 56.—Percentage of samples adequately stocked with 

advance tree seedling and sapling regeneration for samples 

40- to 75-percent stocked, by species group and subgroup, 

Pennsylvania, 2004.

Rare wild azalea.

43

48

54

0

10

20

30

40

50

60

High Canopy Dominants All High Canopy All Tree Species

Species Group

Percent

34

47

54

0

10

20

30

40

50

60

Commercially Desirable All Commercial All Tree Species

Species Group

Percent

Canopy Replacement

Timber Management
51

sample plots with adequate advance regeneration ranged 

from 23 percent in the north-central region to 44 percent 

in the southwestern region.

An important part of the PRS is measuring and 

monitoring nontree understory vegetation, such as 

shrubs, ferns, grasses, and other herbs. The understory 

composition for all stands can be described by ranking 

42 41

53

54

54 40

32 23

44

37

39 34

Figure 57.—Percentage of samples adequately stocked with advance tree seedling and 

sapling regeneration for canopy replacement species and samples 40- to 75-percent 

stocked, by ecopolitical region, Pennsylvania, 2004.

Figure 58.—Percentage of samples adequately stocked with advance tree seedling and 

sapling regeneration for desirable timber species and samples 40- to 75-percent stocked 

by ecopolitical region, Pennsylvania, 2004.

species/species groups by their contribution to the total 

vegetative cover sampled (Fig. 59). Grass/forb and 

rhizomous fern dominate understory communities, 

accounting for nearly one-third of the total vegetative 

cover sampled. Other common species include blueberry, 

Rubus spp., and mountain-laurel. Data collection for 

these understory communities is in the early phases. 

Future reports will include more thorough analyses.
52

0 5 10 15 20 25

Other herb

Barberry

Vine honeysuckle

Gray-stemmed dogw ood

Bush honeysuckle

Maple-leaved viburnum

Huckleberry

Greenbrier

Poison ivy

Common spicebush

Virginia creeper

Other fern

Grape

Rose

Witch-hazel

Mountain laurel

Brier, bramble, dew berry

Blueberry

Rhizomous fern

Grass/forb

Total Cover (%)

Figure 59.—Percentage of total cover by species/species group for top 20 species/species groups, 

Pennsylvania, 2004.

Dry oak-heath woodland (Fike 1999).
53

chapter 5: Forest Health 

by NRS-FIA. Some of the data can serve as a baseline for 

future monitoring.

Drought, Insects, and Disease

Drought is a significant stressor that frequently 

precedes or contributes to the onset of major outbreaks 

of damaging insect pests and diseases. Seasonal or 

prolonged droughts are common in Pennsylvania. 

Since the 1989 NRS-FIA inventory, severe droughts 

have occurred in 1991-92, 1995, 1999, and 2002 (Fig. 

60). Alternatively, some of the wettest years on record 

were in 1994 and 2003-2004. Storm damage due to 

wind, ice, and excessive precipitation can contribute 

Forest health can be assessed at scales ranging from 

individual trees to entire landscapes. A healthy forest 

can renew itself, recover from disturbances, and retain 

ecological resiliency while meeting the current and 

future demands of people for products and services 

(USDA For. Serv. 2003).

The health and condition of forests are influenced 

by stressors that include drought, flooding, cold 

temperatures or freeze injury, nutrient deficiencies, soil 

properties, pollutants, insects and diseases, exotic and 

invasive species, and human disturbance. This chapter 

includes results for the forest-health indicators identified 

Gypsy-moth pupae.

Figure 60.—Drought conditions in Pennsylvania, 1989-2005. 

Source: http://www.cpc.noaa.gov/products/monitoring_and_data/drought.shtml

Months With Severe

or Extreme Drought (%)

0 - 4

5 - 9

10 - 15
54

Table 3.—Percentage of dead standing basal area for 

selected tree species, Pennsylvania, 1989 and 2004

Dead standing basal area

Species 1989 2004 Change

--------------------Percent--------------------

Black locust 28.1 24.9 -3.2

Beech 7.3 12.6 5.3

White pine 10.3 10.3 0.0

Chestnut oak 19.0 8.5 -10.5

White ash 12.1 7.1 -5.0

Northern red oak 10.0 6.5 -3.4

Black oak 14.3 6.2 -8.0

Sugar maple 4.8 6.2 1.4

Black birch 8.8 5.5 -3.3

Hemlock 5.0 5.4 0.4

Black cherry 6.1 5.2 -0.9

White ash 4.8 4.4 -0.4

Red maple 4.3 3.6 -.08

Yellow-poplar 1.6 2.1 0.6

Blackgum 2.2 0.9 -1.3

Table 4.—Percentage of basal area by crown-dieback category 

for selected tree species, Pennsylvania, 2004

Basal area

0-20% 25-45% 50-95% 100%

Species dieback dieback dieback dieback

Beech 92.4 2.3 5.3 0.0

Hemlock 94.9 1.2 3.9 0.0

Black oak 95.5 4.1 0.4 0.0

White ash 95.6 3.9 0.5 0.0

Red maple 97.8 1.3 0.8 0.1

Black locust 98.2 1.2 0.6 0.0

Black cherry 98.5 0.8 0.6 0.0

White pine 98.7 1.3 0.0 0.0

White oak 98.8 0.0 1.2 0.0

Northern red oak 99.1 0.9 0.0 0.0

Sugar maple 99.2 0.7 0.2 0.0

Chestnut oak 99.4 0.4 0.2 0.0

Black birch 99.6 0.0 0.2 0.0

Yellow-poplar 100.0 0.0 0.0 0.0

Blackgum 100.0 0.0 0.0 0.0

to disease outbreaks, and the initiation and collapse of 

insect outbreaks, and exacerbate damage associated with 

droughts.

Exotic insects, diseases, and invasive plant species 

threaten the productivity and stability of forest 

ecosystems around the world (Liebhold and others 1995; 

Vitousak and others 1996; Pimentel and others 2000). 

Invasions by alien species have resulted in a numerous 

direct and indirect effects at the community level, 

including changes in plant species diversity and richness, 

community structure, vegetation dynamics, and plantanimal interactions (McDonnell and Roy 1997).

Over the last century, Pennsylvania’s forests have been 

damaged by well known exotic and invasive agents such 

as chestnut blight, gypsy moth, hemlock woolly adelgid, 

and beech bark disease (Mattson 1997). Insect pests and 

diseases, both native and exotic, continue to damage and 

kill trees, though most species have shown a decrease in 

the percentage of standing dead basal area since 1989 

(Table 3), and tree crowns generally are healthy for most 

species across the State (Table 4). Only beech and 

eastern hemlock have more than 5 percent of the 

basal area showing crown dieback of at least 25 

percent.

Certain tree species are facing additional risks from 

newly introduced insects and diseases. The following 

analysis by species highlights the major insects, 

diseases, and other stresses that threaten forest health 

in Pennsylvania.

Oaks

Insect defoliations have played an important 

role in shaping Pennsylvania’s extensive oak 

forests. Recurring gypsy moth (Lymantria dispar) 

defoliations have affected every region by altering 

productivity, species composition, and stand 

structure. Gypsy moth has defoliated millions of 

acres each decade since 1975, according to the 

Bureau of Forestry (Figs. 61-62). Many areas, 

especially those threatened with repeated severe 

defoliation, were treated with aerial applications 
55

Figure 61.—Number of acres defoliated by and sprayed for gypsy moth, Pennsylvania, 

1975-2005.

0

500,000

1,000,000

1,500,000

2,000,000

2,500,000

3,000,000

3,500,000

4,000,000

4,500,000

5,000,000

1975197719791981198319851987198919911993199519971999200120032005

Acres 

Acres Defoliated

Acres Sprayed

of insecticide, often the biological insecticide Bacillus 

thuringiensis, to avoid higher rates of tree mortality and 

decline. The influence of gypsy moth on oak mortality is 

evident from mapped estimates of percent standing dead 

basal area for each oak species (Fig. 63).

Widespread gypsy moth outbreaks in the 1970s and 

1980’s are reflected in the mean percentage of standing 

dead basal area from the 1989 inventory results. These 

percentages decreased in the current inventory and likely 

correspond with the decrease in gypsy moth defoliation, 

salvage harvesting, and natural tree fall.

Also causing significant defoliation are periodic outbreaks 

of oak leaftier (Croesia semipurpurana), oak leafroller 

(Archips semiferana), orange striped oak worm (Anisota 

senatoria), and walking stick (Diapheromera femorata).

A potential threat to the oak resource in Pennsylvania 

is sudden oak death (SOD) caused by the pathogen 

Phytophthora ramorum. Over the past 10 years, certain 

oak species in California and Oregon have been dying 

as a result of P. ramorum, which also causes leaf blight 

and shoot dieback on many other woody and nonwoody 

plant species. Surveys of nurseries and general forest 

communities were conducted in Pennsylvania in 

2003-06 as part of efforts to detect P. ramorum. This 

pathogen has since been transported to several eastern 

states via infected ornamental nursery stock, specifically 

camellias, the source of inoculum from which introduced 

Rhododendron cultivars, native Rhododendron, and 

other susceptible horticultural and native hosts may be 

exposed. In 2006, several rhododendron plants exhibiting 

blight symptoms associated with P. ramorum were 

detected in a nursery in southeastern Pennsylvania. The 

infected and exposed plant materials were destroyed and 

a monitoring program near the infected nursery location 

was established. Should P. ramorum become established, 

the oak resource in association with understory hosts 

such as Rhododendron, Vaccinium, and Kalmia will be at 

risk. The occurrence of oak overstory, understory hosts, 

and their co-location are shown in Figure 64.
56

1980-1984

1985-1989

1990-1994

1995-1999

2000-2002

1975-1979

Figure 62.—Frequency of gypsy moth defoliation, Pennsylvania, 1975-2002.

1980-1984

1985-1989

1990-1994

1995-1999
57

Chestnut oak

White oak

Black oak

Northern red oak

Figure 63.—Percent standing dead basal area, Pennsylvania, 2004.
58

Overstory and Understory

Understory

Overstory

Figure 64.—Probability of presence of sudden oak 

death hosts, Pennsylvania.

Overstory and Understory

Beech

Beech bark disease (BBD), which continues to spread 

slowly throughout Pennsylvania’s forests is an insectfungus complex involving the beech scale insect 

(Cryptococcus fagisuga) and the exotic canker fungus 

Neonectria coccinea var. faginata or the native Neonectria 

galligena. The scale insect creates small wounds in bark 

tissue that serve as entry points for the fungal pathogens. 

Small cankers may coalesce over time. As cankers become 

more numerous, the host tree is weakened and declines, 

and may die.

Three phases of BBD are recognized: 1) the advancing 

front, or areas recently invaded by scale populations; 

2) the killing front, or areas where fungal invasion has 

occurred (typically 3 to 5 years after the scale insects 

appear but sometimes as long as 20 years) and tree 

mortality begins, and 3) the aftermath forest, or areas 

where the disease is endemic (Shigo 1972, Houston 1994).

In Pennsylvania, beech scale was first observed in 1958 

at Promised Land State Park near Gouldsboro, PA. In 

1969, bark cankers caused by N. coccinea were detected 
59

west of the park. The advancing front has continued 

across the northern tier counties and is proceeding slowly 

in a southerly migration (Fig. 65). The killing front 

has advanced similarly as there are significant amounts 

of damaged and standing dead beech in much of the 

northern portion of the State. In fact, the amount of 

standing dead beech basal area has increased by 5 percent 

since the 1989 inventory. In the aftermath forest, many 

areas exhibit excessive root sprouting that creates a beech 

brush understory.

Other major stress factors associated with beech include 

the combined effects of drought and defoliation by elm 

spanworm (Ennomos subsignanus), gypsy moth, and fall 

cankerworm (Asophila pomentania). In the early and mid-

1990s, these stressors along with BBD caused significant 

tree damage and mortality in the northern-tier counties 

(Figs. 66-67).

Figure 66.—Percentage of beech basal area with observed 

damage, Pennsylvania, 2004.

Figure 65.—Spread of the advancing front of beech-bark disease 

in Pennsylvania.

Figure 67.—Percentage of standing dead beech basal area, 

Pennsylvania, 2004.
60

Sugar Maple

Since the mid-1990s, forest tent caterpillar 

(Malacosoma americanum), fall cankerworm (Alsophila 

pometaria), and related insects have defoliated sugar 

maple to varying degrees across the Commonwealth. 

Cool, moist conditions in 1994 resulted in an 

outbreak of anthracnose (Discula campestris) on sugar 

maples and red maples previously damaged by the 

forest tent caterpillar and elm spanworm, respectively. 

Many trees that were defoliated by an insect pest 

in the spring of 1994 were severely damaged by 

anthracnose during the refoliation phase. The affected 

trees showed high levels of crown dieback and many 

died within 5 years.

A decline in the health of sugar maple stands 

throughout the unglaciated region of the Allegheny 

Plateau has been documented since the late 1970s. 

Researchers have been examining a variety of stress 

factors associated with the decline. Stressors that have 

been identified include poor availability of select base 

cations (e.g., calcium, aluminum, and manganese) 

in soil solution, excessive amounts of antagonistic 

cations (aluminum and manganese) that increase 

availability under acid soil conditions, and defoliation 

insects (Long and others 1997, Horsley and others 

2000, 2002, Bailey and others 2004, 2005, Hallet and 

others 2006).

There are numerous damaged and standing dead 

sugar maple across the northern tier of Pennsylvania 

(Figs. 68-69). The percentage of standing dead sugar 

maple has increased slightly since the 1989 inventory. 

However, despite substantial decline in this species, 

many live sugar maple trees are healthy with little 

crown dieback.

Eastern hemlock

Many hemlock stands across Pennsylvania are 

vulnerable to two important insect pests. Elongate 

hemlock scale (Fiorinia externa), an exotic pest 

introduced from Japan and first observed in the 

Eastern United States in 1908, and hemlock woolly 

adelgid (HWA) (Adelges tsugae), an exotic pest 

introduced from Asia and first reported in the United 

States in 1951, cause significant foliage damage to 

Figure 69.—Percentage of standing dead sugar maple, 

Pennsylvania, 2004.

Figure 68.—Percentage of sugar maple basal area with observed 

damage, Pennsylvania, 2004.

Hemlock woolly adelgid. Forestry images.org. Pennsylvania 

Department of Conservation and Natural Resources.
61

hemlocks of all ages. In recent years, drought, insect 

infestations, and a needle blight disease (Fabrella 

tsugae) have been associated with needle cast, poor 

crown conditions, and tree mortality.

HWA has the greatest potential to cause significant 

mortality and threaten the sustainability of hemlock 

species. It was first detected in Pennsylvania in 

1979 in Montgomery County and has since spread 

westward across the State (Fig. 70). Concentrations of 

mortality are evident across the northern half of the 

Commonwealth (Fig. 71).

Ash

The two major ash diseases in Pennsylvania are ash 

yellows (a Phytoplasma bacteria) and ash decline 

(a complex). Along with drought, these likely are 

responsible for most of the standing dead ash in the 

State (Fig. 72). A significant threat to all ash species 

in Pennsylvania is posed by the emerald ash borer 

(EAB) (Agrilus planipennis), an exotic insect pest 

first detected in Butler County in 2007. Monitoring 

plots were established in western Pennsylvania in 

2005 and will detect the eventual spread of this pest, 

which causes ash mortality in both urban and forested 

landscapes.

Figure 71.—Percentage of standing dead eastern hemlock, 

Pennsylvania, 2004.

Figure 70.—Spread of hemlock woolly adelgid (HWA), 

Pennsylvania.

Figure 72.—Percentage of standing dead white ash, 

Pennsylvania, 2004.

Emerald ash borer. Forestry images.org. David Cappaert.
62

Exotic-Invasive Plants

The range of exotic-invasive plants continues to 

expand across Pennsylvania. Much like other exoticinvasive organisms, these plants typically have some 

advantage over native plants, e.g., prolific seed 

production and dispersal. Native forest ecosystems 

have limited ability to compete with these invaders, 

which affect both canopy-dominant species and 

understory plant communities. Of primary concern 

is the impact of exotic-invasive plants on native 

forest composition, health, structure, function, 

resource productivity, and overall sustainability. 

These plants are especially threatening because little 

is known about the complex interactions between 

introduced invasive species and native 

systems, and because both data sources 

and monitoring are minimal to date.

In 2003, NRS-FIA added a list of exoticinvasive plants for collection on 

Pennsylvania Regeneration Study 

samples1to gain a better understanding 

of their distribution and impact on the 

forest ecosystem. The list was compiled 

by researchers with the Forest Service, 

Pennsylvania Department of 

Conservation and Natural Resources, 

Bureau of Forestry, The Nature 

Conservancy, Morris Arboretum, 

Pennsylvania State University, 

Pennsylvania Biodiversity Partnership, 

and Allegheny National Forest. The exotic-invasive 

plant survey covers 19 shrubs, 8 vines, 12 forbs and 

grasses (Fig. 73) along with Norway maple, tree-ofheaven, and other tree species.

The exotic-invasive survey includes more than 500 

sample plots across Pennsylvania. As additional data 

are collected, future reports will address questions 

related to the distribution of exotic-invasive plants 

species and their impact on the State’s forests.

Figure 73.—List of exotic-invasive shrubs, vines, and forbes/grasses sampled.

1

Northeast invasive plants identification field guide (a 

compilation of online fact sheets), on file at Northern 

Research Station, Forest Inventory and Analysis, 

Newtown Square, PA.

Japanese stiltgrass, Valley Forge National Park.

Tree-of-heaven under an oak canopy.

Vines

Fiveleaf akebia

Porcelain-berry

Oriental bittersweet

English ivy

Japanese honeysuckle

Mile-a-minute vine

Kudzu

Common periwinkle

Shrubs

Japanese barberry

European barberry

Russian olive

Autumn olive

Winged Euonymous

Border privet

Common privet

Bell’s honeysuckle

Amur honeysuckle

Morrow’s honeysuckle

Standish honeysuckle

Tartarian honeysuckle

Common buckthorn

Glossy buckthorn

Multiflora rose

Wineberry

Japanese spiraea

Linden viburnam

Guelder rose

Forbes and Grasses

Garlic mustard

Spotted knapweed

Canada thistle

Bull thistle

Crown-vetch

Giant hogweed

Purple loosestrife

Japanese stilt grass

Reed canary grass

Common reed

Japanese knotweed

Giant knotweed
63

Abundant understory exotics include 

multiflora rose, Russian/autumn olive, 

garlic mustard, Japanese stiltgrass, 

and bush honeysuckles. Data from 

established tree-inventory plots show that 

tree-of-heaven has expanded to the point 

where there is an average of one stem for 

every forested acre in the State. Tree-ofheaven stems have increased by about 

80 percent since 1989. The distribution 

of tree-of-heaven based on detection at 

sample plots is shown in Figure 74. This 

species is most common in regions where 

the density of human population is high.

Forest Soils

Soil is an important component of 

forest ecosystems and significantly 

influences productivity and species 

composition (Pritchett and others 

2000). Forest soils are highly variable 

in physical and chemical properties. 

This is reflected in the variability of 

forest vegetation across the landscape. 

Inventorying and assessing forest soils 

provide critical baseline information 

on forest health and productivity 

and provide an opportunity to detect 

changes in soil nutrition.

Pennsylvania’s forests are largely 

underlain by inceptisols, ultisols, alfisols, 

and entisols (Fig. 75). Inceptisols are 

a diverse soil occurring across a range of climates and 

vegetative communities. They are characterized by 

the combination of water available to plants and the 

development of one or more soil horizons, but lack the 

noticeable movement of soil material, e.g., clay. Ultisols 

are characterized by the presence of an illuvial clay 

horizon and low base status. Both of these properties 

result from the movement of water through the soil 

profile, so these highly weathered soils with low native 

fertility typically are found in stable, older, unglaciated 

environments. Ultisols generally form under forests 

(Brady 1990).

Figure 74.—Phase 2 sample plots with at least one tree-of-heaven, 

Pennsylvania, 2004.

water

developed

agriculture

forest

Figure 75.—Soil orders and limit of glaciation, Pennsylvania.

Alfisols are fertile soils generally with an illuvial clay 

horizon and a base status that is medium to high 

(USDA Nat. Resour. Conserv. Serv. 1999). Most 

alfisols developed under deciduous forest (Brady 1990). 

Entisols are characterized by the absence of soil horizons, 

which may be the result of insufficient time for soil 

development or resistant parent material (USDA Nat. 

Resour. Conserv. Serv. 1999).

Southern limit of Wisconsin glaciation

0 50 Miles

Soil order

Alfisol

Entisol

Inceptisol

Ultisol
64

Calcium/aluminum ratios closely follow observed 

patterns of acid deposition (Fig. 76). Pennsylvania 

receives some of the highest levels of acid 

deposition of any state, though there has been 

a downward trend over the past 10 years (Nat. 

Atmos. Deposition Prog. 2007) (Fig. 77). The 

deposition is heaviest in western Pennsylvania 

where much of the electricity is generated by 

coal-fired power plants (Fig. 78). An essential 

macronutrient, calcium is particularly important 

for plant structure (Marschner 1986). Acid 

deposition alters the interactions between critical 

soil minerals and has been linked to calcium 

leaching (Driscoll and others 2001). Toxic in 

high concentrations, aluminum is increasingly 

mobilized in these environments (Driscoll and 

others 2001, McBride 1994). The inhibition of 

root growth is one of the initial responses to aluminum 

toxicity (Marschner 1986). The lower the calcium/

aluminum ratio (Fig. 76), the higher the amount of 

aluminum that is in the soil and thus a higher probability 

of tree stress or toxicity.

The Soil Quality Index (SQI) combines the distinct 

physical and chemical properties of the soil into a single, 

integrative assessment.2 Particular chemical or physical 

properties of the soil may be important for specific forest 

Figure 76.—Calcium:aluminum ratios, Pennsylvania, 2001-03. (Source: National Atlas of the 

USA; National Atmospheric Deposition Program; USDA Forest Service; U.S. Geological Survey)

Figure 77.—Mean hydrogen wet deposition by year, Pennsylvania, 

1994 to 2004.

0.55

0.50

0.45

0.40

0.45

0.60

0.65

0.70

Ca:Al ratio (0-10 cm)

> 1.50

1.01 - 1.50

0.51 - 1.00

0.21 - 0.50

0.01 - 0.20

H+ dep. (kg/ha)

0 50 Miles

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

Mean Hydrogen Ion Deposition .

assessments, but the SQI combines 19 properties into a 

single value that describes overall soil health. As such, it 

facilitates the documentation of trends in forest health. 

Soil quality is average to above average in the unglaciated 

northeast and the developed alfisols of the southwest 

(Fig. 79). This is particularly apparent in the deeper soil 

(10 to 20 cm).

Lichens

Monitoring lichen communities can aid in assessing 

the impact of air pollution and identifying trends in 

biodiversity. There is a close relationship between lichen 

communities and air pollution, particularly acidifying 

2

Amacher, M.C.; O’Neill, K.P. Soil vital signs: a new index for 

assessing forest soil health. In preparation.
65

Figure 78.—Mean hydrogen wet deposition, Pennsylvania, 1994 to 2004.

(Kg/ha)

Figure 79.—Soil quality index (SQI) patterns, Pennsylvania, 2001-03.

0 50 Miles

SQI (10-20 cm)

Below average

Average

Above average

SQI (0-10 cm)

Below average

Average

Above average
66

or fertilizing nitrogen- and sulfur-based pollutants. 

Lichens are ideal as indicators of air quality because 

they rely on the atmosphere for nutrition (McCune 

2000).

Eighty-one lichen species in 34 genera were sampled 

on plots in Pennsylvania (Table 5). Although lichens 

are found on many substrates, e.g., rocks, sampling was 

restricted to standing trees or recently fallen branches 

and twigs. The most common lichen genera, Punctelia

and Physcia, were present on nearly 15 percent of the 

plots (Table 6). Of the species sampled, the most (14) 

were in the genus Cladonia.

Species diversity is used to describe the number of 

different species present in an area and the distribution 

of individuals among species. The easiest way to 

measure species diversity is to count the number of 

species at a site. The resulting value, species richness, 

does not provide a complete picture of diversity in 

an ecosystem because it ignores abundance. Richness 

values were low to medium across Pennsylvania. Mean 

species richness of the mixed oak forest-type group was 

slightly higher than that of the northern hardwoods 

group; the mean for both groups was considerably 

higher than for softwood types (Table 7).

Table 5.—Summary table for lichen communities, 

Pennsylvania, 2000-03

Number of plots surveyed 141

Number of plots by species richness category

0-6 species (low) 80

7-15 species (medium) 60

16-25 species (high) 1

Median 6

Range of species richness score by plot (low-high) 1-22

Average species richness score per plot (alpha diversity) 6.3

Standard deviation of species richness score per plot 3.2

Species turnover rate (beta diversity) 16.7

Total number of species per area (gamma diversity) 81

Note: Beta diversity is calculated as gamma diversity divided by alpha 

diversity.

Table 6.—Percentage of specimens and number of 

species for lichen genera sampled, Pennsylvania, 2000-03

Genus All specimens All species

Punctelia 14.7 5

Physcia 14.6 6

Phaeophyscia 12.6 3

Cladonia 12.2 14

Parmelia 11.8 4

Flavoparmelia 11.1 3

Myelochroa 4.3 4

Cetraria 3.4 2

Melanelia 2.5 2

Hypogymnia 2.4 2

Candelaria 1.8 2

Parmotrema 1.4 4

Flavopuctelia 1.0 2

Pyxine 0.9 2

Imshaugia 0.7 1

Physconia 0.5 1

Hypotrachyna 0.4 2

Parmelinopsis 0.4 2

Bulbthrix 0.3 1

Heterodermia 0.3 3

Physciela 0.3 1

Usnea 0.3 2

Anaptychia 0.2 1

Bryoria 0.2 1

Canoparmelia 0.2 1

Hypocenomyce 0.2 1

Lobaria 0.2 2

Evernia 0.1 1

Hyperphyscia 0.1 1

Menegazzia 0.1 1

Parmeliopsis 0.1 1

Pseudevernia 0.1 1

Rimelia 0.1 1

Umbilicaria 0.1 1

Total 100.0 81

Table 7.—Number of FIA Phase 3 plots and richness and 

diversity scores for lichen species by broad forest-type 

group, Pennsylvania, 2000-03

Broad forest-type group

Number 

of plots Richness Diversity

Softwoods 12 4.1 1.3

Mixed oak 63 6.9 1.8

Northern hardwoods 62 6.3 1.7
67

Lichen species richness also increased with stand size 

(Table 8). The mean number of lichen species was 

lowest for small stands and higher for large stands, 

which reflects the lag time for lichen recolonization after 

harvest or disturbance. The spatial distribution of lichen 

species-richness scores are shown in Figure 80. Species 

richness generally is higher in central Pennsylvania. The 

richness and diversity scores reported here will serve as 

baseline estimates for future monitoring at the state and 

regional levels.

Due to the sensitivity of many lichen species to 

airborne pollution, it is useful to examine levels 

of acid deposition. Showman and Long (1992) 

reported that mean species richness for lichens 

was significantly lower in areas where sulfate 

deposition is high than in low-deposition areas 

as found in north-central Pennsylvania. Levels of 

sulfate deposition have been relatively high in the 

Commonwealth and surrounding states in recent 

years (Fig. 81). Estimated lichen species richness 

in the Northeastern United States in 2000-03 and 

the location of pollution-sensitive lichen species 

in Pennsylvania are shown in Figures 82 and 83, 

respectively.

Ground-Level Ozone Injury

Ozone (O3) biomonitoring is used to monitor the 

potential impact of tropospheric O3 (smog) on forests. 

O

3

pollution is a byproduct of industrial processes 

that facilitate the production and accumulation of O3

which forms when nitrogen oxides and volatile organic 

compounds react in the presence of sunlight (Brace and 

others 1999). Ground-level O3 pollution might reduce 

tree growth, alter species composition, and predispose 

trees to attack by insects and disease.

Certain plant species exhibit visible, easily diagnosed 

foliar symptoms of O3 stress and thus are as important 

indicators of O

3 pollution. A national system of 

monitoring sites is used to asses the impact of O3on 

forests (Smith and others 2003b).3

 These sites are not 

Table 8.—Number of FIA Phase 3 plots and richness 

and diversity scores for lichen species by stand-size 

class, Pennsylvania, 2000-03

Stand-size class

Number 

of plots Richness Diversity

Small 12 4.1 1.3

Medium 63 6.9 1.8

Large 62 6.3 1.7

Figure 80.—Estimated lichen species richness, 

Pennsylvania, 2000-03.

Figure 81.—Mean sulfate ion wet deposition, 1994 to 

2002, Northeastern United States (source: USDA Forest 

Atmospheric Deposition Program).

3

Smith, G.C.; Smith, W.D.; Coulston, J.W. Statistics band 

sampling and estimation document; ozone indicator. In 

preparation.
68

co-located with NRS-FIA samples. O3 plots 

are chosen for ease of access and optimal size, 

species, and plant counts. As such, O3plots do 

not have set boundaries and vary in size. At 

each plot, 10 to 30 individual plants of three 

or more indicator species are evaluated for O3

injury. Each plant is rated for the proportion of 

leaves with O

3 injury and the mean severity of 

symptoms, using break points that correspond 

to the human eye’s ability to distinguish 

differences. A biosite index is calculated based 

on amount and severity ratings.

O

3

plot data from 1998 to 2004 (Table 9) show 

a high degree of year-to-year variation. Twentyeight percent of the sampled plants showed 

symptoms of O3 injury in 1998, though most 

of the injury was on less than 25 percent of the 

foliage. By contrast, less than 10 percent of the 

sampled plants showed symptoms in 2002, and 

less than 5 percent showed symptoms in 1999, 

2001, 2003, and 2004. It should be noted that 

O

3

injury is influenced by soil moisture. Moist 

conditions facilitate theentry of O3 into leaves 

during the growing period; dry conditions tend 

to block O

3 entry into leaves.

The number of plants sampled by species is 

shown in Table 10. Blackberry had the highest 

occurrences of O

3 damage, with 55 percent of 

sampled plants showing symptoms of damage in 

1998 versus less than 25 percent in other years. 

Nearly 30 percent of the black cherry plants 

sampled showed injury symptoms in 1998, but 

less than 10 percent showed symptoms between 

Figure 82.—Estimated lichen species richness, Northeastern United 

States, 2000-2003.

Lichen Species Richness

Pollution-Sensitive Lichen 

Species

Figure 83.—Presence of pollution-sensitive lichen species, Pennsylvania, 2004.

Table 9.—Percentage of sampled plants with ozone injury, Pennsylvania, 1998-2004

Sampled plants (%)

Year

No. plots

evaluated

No. plots

sampled No injury

1-6% 

leaf injury

7-25% 

leaf injury

26-50% 

leaf injury

51-75% 

leaf injury

76+% 

leaf injury

1998 96 2,285 71.6 7.4 8.6 6.4 3.7 2.4

1999 122 6,719 98.0 0.5 0.9 0.4 0.1 0.1

2000 95 7,117 89.5 3.3 3.2 1.7 1.6 0.7

2001 98 10,487 96.4 0.8 1.1 1.0 0.6 0.2

2002 40 5,040 92.1 1.7 2.9 1.9 1.2 0.2

2003 42 10,372 98.2 0.8 0.8 0.1 0.1 0.0

2004 40 4,493 96.3 2.5 0.6 0.3 0.2 0.0

Note: Rows and columns may not add to 100 due to rounding.
69

1999 and 2004. Damage was minimal to other 

species sampled except in 1998 when more 

than one-third of the sampled white ash and 

nearly 20 percent of the yellow-poplar showed 

injury symptoms. Even for species with high 

occurrences of O

3 injury, severity was low.

A large portion of Pennsylvania, particularly 

in the central part of the State, has a moderate 

to high risk of O3 damage. The biosite index 

in Figure 84 was created using block kriging, 

a geospatial mapping procedure, to determine 

which tree species in the Northeastern United 

States are sensitive to O

3 (Coulston and others 

2003). 

A typical pattern of O3in summer for the 

Northeastern United States is shown in Figure 

85 (USDA For. Serv. 2002a). The term 

SUM06 is defined as the sum of all valid hourly 

O

3

concentrations that equal or exceed 0.06 

part per million. Controlled studies have found 

that high O3levels (shown in orange and red) 

can lead to measurable growth suppression 

in sensitive tree species (Chappelka and 

Samuelson 1998). Smith and others (2003) 

reported that even when ambient O3 exposures 

are high, the percentage of injured plants can 

be reduced sharply in dry years. Differences 

in the amount of O

3

injury between years are 

more likely due to precipitation levels than to 

ambient O

3 exposure levels.

Table 10.—Number of plant specimens examined (percentage injured in parentheses), by selected species, 

Pennsylvania, 1998-2004

Species 1998 1999 2000 2001 2002 2003 2004

Big leaf aster 0 (0) 218 (0) 13 (0) 105 (0) 0 (0) 180 (0) 30 (0)

Black cherry 254 (29) 1,117 (2) 1,207 (6) 2,041 (3) 1,093 (7) 2,216 (1) 960 (3)

Blackberry 717 (54) 842 (6) 1,303 (25) 1,763 (13) 974 (20) 2,028 (1) 937 (4)

Milkweed 837 (13) 1,810 (1) 1,640 (8) 1,900 (1) 824 (10) 1,970 (4) 738 (13)

Pin cherry 16 (0) 19 (0) 0 (0) 25 (0) 30 (0) 124 (0) 30 (0)

Sassafras 39 (0) 452 (0) 661 (7) 1,034 (0) 412 (0) 980 (0) 432 (1)

Spreading dogbane 286 (0) 1,456 (0) 1,024 (2) 1,830 (0) 692 (6) 1,056 (2) 360 (0)

White ash 88 (36) 775 (0) 975 (8) 1,383 (3) 875 (1) 1,434 (1) 789 (1)

Yellow-poplar 48 (19) 60 (0) 294 (1) 406 (0) 140 (1) 384 (1) 217 (4)

Figure 84.—Estimated surface of ozone biosite index, Pennsylvania 

(Coulston and others 2003).

Figure 85.—Typical ozone exposure rates in the Northeastern United 

States (source: USDA Forest Service, Biomonitoring Project).
70

Forest Carbon

Forest land accounts for a significant portion of carbon (C) sequestered in terrestrial 

ecosystems. The net accumulation of C in forests partially mitigates the increase in 

carbon dioxide in the atmosphere. The exchange of C between the atmosphere and 

forests is affected by photosynthesis, growth, mortality, decomposition, disturbances 

such as fires or pest outbreaks, weather, and edaphic processes, as well as 

anthropogenic activities such as harvesting, thinning, clearing, and replanting.

Carbon is sequestered in growing trees, principally as wood (as cellulose, hemicellulose, and lignin) in the tree bole. Accrual in forest ecosystems also depends 

on the accumulation of C in dead wood, litter, and soil organic matter. When 

wood is harvested and removed from the forest, not all C flows immediately to the 

atmosphere. The portion of harvested C sequestered in long-lasting wood products 

or landfills may not be released to the atmosphere for years or even decades. Thus, 

C stored in harvested wood products is an important part of complete accounting of 

forest C.

Carbon Estimates

Estimates are first developed as 

stocks, or total mass of C for a defined 

area and component of the forest 

ecosystem. Six ecosystem C pools and 

two pools of C remaining in harvested 

wood products are used to segment 

estimates. Forest structure provides 

a convenient modeling framework for 

assigning C to distinct forest-ecosystem 

pools: live trees, standing dead trees, 

understory vegetation, down dead 

wood, forest floor, and soil organic 

C. These pools are consistent with 

guidelines developed by the Intergovernmental Panel on Climate Change. The fate of 

C in harvested wood is summarized as C remaining in products in use or in landfills.

With the exception of soils, forest-ecosystem C stocks are based on the individual 

C pool estimators in FORCARB2, a forest C accounting model (Heath and others 

2003, Smith and others 2004, 2007). FORCARB2 is essentially a national empirical 

simulation and C accounting model that produces stand level inventory-based 

estimates of C stocks.

Estimates of C sequestered in wood products depend on information on the 

quantities of wood harvested and processed every year. Each end-use product can 

be characterized according to its expected lifespan and ultimate disposal of the 

product. Carbon that does not remain in products in use or in landfills is emitted to 

the atmosphere. The amount of C emitted depends on the initial quantity harvested, 

characteristics of the wood products, and time since harvest and initial processing. 

American chestnut down woody material.
71

Thus, C stocks and net annual flux of C in wood products are subject to continuous 

change over time.

Carbon Stocks

The distribution of C stocks by ecosystem pool for 1989 and 2004 is shown in Figure 

86. Live trees and forest floor/soils account for 48 percent and 45 percent of total 

C, respectively. Standing dead, downed dead, and understory vegetation make 

up the remaining C stocks. Average C density for live trees on forest land in the 

Northeastern United States and Pennsylvania is shown in Figure 87.

Figure 86.—Carbon (total tons) by forest ecosystem pool, Pennsylvania, 1990 and 2004.

Figure 87.—Carbon density for live trees on forest land by county, Northeastern United States.

0

200

400

600

800

1000

1200

1400

1989 2004

Million tons .

Live Trees

Standing Dead

Understory

Dow n Dead

Floor

Soil

Unavailable 

0-20 

20-40 

40-60 

60-80 

80-100 

> 100

Tons per hectare Tons/Hectare
72

Chapter 6: Sustainability Issues and Knowledge Gaps

it. This block of land periodically shifts back and forth 

between forest and nonforest. Often, abandoned pasture 

with sufficient stocking of young tree needs only be 

mowed to be reclassified as pasture in a subsequent 

inventory. This conversion of land from agricultural 

to forest (and vice versa) makes it difficult to fully 

understand and illustrate real gains and losses in forest 

land. Additional regional analyses in future reports will 

help to pinpoint changes occurring in the forest-land 

base.

Forest Ownership

Private entities and individuals own 71 percent of 

Pennsylvania’s forests. Major public owners include the 

Department of Conservation and Natural Resources, the 

Game Commission, and the Forest Service (Allegheny 

National Forest). Families and individual owners 

account for more than half of Pennsylvania’s forest-land 

base, making them an important component of future 

policies and outreach efforts. Families and individuals 

value their forests for many reasons. They own relatively 

small tracts of land, and often do not seek formal or 

planned assistance from resource professionals when 

making decisions about their properties. This lack of 

This report has focused on a range of data and indicators 

concerning the condition of Pennsylvania’s forests. 

This chapter highlights key findings related to forest 

sustainability and also identifies important knowledge 

gaps associated with the existing NRS-FIA inventory 

data. This information is provided to aid management 

and policy decisions, and to improve and enhance future 

analyses.

Changing Forest Land Base

Although Pennsylvania’s forest-land base is stable at 16.6 

million acres, the impact of urbanization, parcelization, 

and fragmentation poses a serious threat to forest 

sustainability in certain regions of the Commonwealth. 

Since 1989, the State has lost 663,000 acres of forest 

land, with nearly two-thirds of this loss—or 28,000 acres 

a year—to some type of development. Urbanization, 

parcelization, and fragmentation generally result in 

permanent changes to the landscape that adversely affect 

forest character, biodiversity, ecosystem function, and 

resource availability.

The statewide forest-land base appears stable because 

nearly 350,000 acres of agricultural land shifted into 

Oak regeneration within a deer fence.
73

management planning is especially troublesome because 

many privately owned forests are becoming financially 

mature, a stage when numerous management options 

are available. Without effective planning, many of the 

opportunities to improve forest conditions or ensure 

adequate regeneration are lost when the trees are 

harvested. Another important consideration of family 

and individual owners is that 18 percent have indicated 

that they plan to sell their forest land or pass it on to 

heirs within 5 years. This is important because harvesting 

and other decisions often are made at the time of or 

shortly following the transfer.

The Forest’s Changing Structure and

Composition

Pennsylvania’s forests are aging as evidenced by increases 

in the percentage of stands dominated by large trees and 

a slowing in increases in inventory volume. Although 

information on harvest levels is not yet available from 

the inventory, decreases in the fully and overstocked 

stands indicate expanding harvest levels. There is a clear 

difference between conditions on public and private 

forest land as the shifts in stocking levels are most evident 

on private lands.

Changes in species composition and stand structure 

also point to aging forests in Pennsylvania. The number 

of smaller trees is decreasing while the number of large 

trees is increasing. This trend is apparent across all 

regions. Relative differences between species indicate 

that hemlock, sugar maple, and the oaks will be less 

prominent in the future. Although increases in red 

maple inventory have slowed, red maple will continue to 

dominate for many years. Black birch is now positioned 

to become one of the dominant species as its influence on 

species composition and stand structure will increase in 

coming years.

Pennsylvania now supports the highest sawtimber volume 

since the turn of the century. High volumes (averages 

in excess of 5,000 board feet per acre), high quality 

(over half the inventory in tree grades 1 or 2), and high 

global demand for the State’s hardwoods make harvesting 

financially attractive in many areas. Landowner decisions 

must consider residual stand conditions and regeneration 

for the next forest as trees are harvested.

Forest Health

Exotic diseases and insects are very serious threats to 

Pennsylvania’s forests. Oaks continue to be at risk from 

gypsy moth and SOD could devastate oaks because 

understory hosts are prevalent in mixed-oak forests. 

The killing front of BBD continues to expand and 

combined with other stressors could increase beech 

mortality. Threats to the oaks and beech are especially 

important because they are the most important sources 

of hard mast for wildlife. HWA is beginning to affect 

the hemlock resource, so continued monitoring will 

be critical to research and management efforts aimed 

at understanding and ameliorating the impact of this 

insect pest. The emerald ash borer has entered the State 

and should be considered a serious threat to the ash 

resource. The Asian long-horned beetle is another exotic 

pest that could cause considerable harm to the maple 

resource should it enter the State. Many native diseases 

and pests could cause severe defoliation and mortality as 

intense outbreaks in local areas, or when they occur in 

conjunction with other stressors, e.g., drought and acidic 

precipitation.

Information on the occurrence and spread of introduced 

exotic-invasive plants is just becoming available but it 

is clear from the existing sample that these plants pose 

a threat to Pennsylvania’s forests. Exotic-invasive plants 

occur in all of the major life forms—trees, shrubs, 

vines, herbs, etc. As such, this issue has moved from a 

concern over single-species invasion to forest-community 

invasions, particularly in urbanized areas. Understory 

species appearing in the inventory include multiflora 

rose, Russian/autumn olive, garlic mustard, Japanese 

stiltgrass, and the bush honeysuckles. Tree-of-heaven has 

expanded in more urbanized areas and is positioned to 

replace native species following stand disturbance.

Information on soil conditions shows that this critical 

feature of the forest ecosystem has been affected by acid 

deposition. Historical information reveals high levels 

of acid deposition in the western half and northeastern 

corner of Pennsylvania. Low calcium/aluminum ratios 

were found across the northern-tier counties. This 

means aluminum toxicity likely is affecting healthy 

root development and thus, overall forest productivity. 
74

Additional research is needed to 

determine the impacts of varying levels 

of acid deposition on the forest as 

well as interactions with disturbances 

such as timber harvesting and deer 

browsing.

Lack of Forest 

Regeneration

The most disturbing finding from 

the forest inventory is the general 

lack of understory plants and 

tree regeneration across much of 

Pennsylvania. Understory plant 

communities are particularly 

problematic north of Interstate 

80. Fencing to account for deer 

browsing and herbicide applications 

to control competing vegetation 

often are necessary to ensure the establishment and 

growth of tree seedlings. Changes in soil chemistry due 

to acid precipitation and other potential influences 

such as invasive species increase the complexity of 

understanding soil-plant-deer interactions. Harvesting 

trees without considering the complex suite of soil-plantdeer interactions could significantly affect the future 

composition of plant species as well as tree stocking levels 

in Pennsylvania’s forests.

Sustainability Issues Summary

Forests offer a wide array of resources, uses, and values 

to Pennsylvanians. The current forest originated about 

100 years ago following large-scale harvests at the turn 

of the last century. Trees in the forest have grown to 

the point where stands of large trees predominate and 

inventory volume growth is beginning to slow. Although 

forests are both gained and lost through agricultural 

land conversions, Pennsylvania loses about 28,000 acres 

of forest each year to development. The most glaring 

issue, a general lack of understory plants and trees over 

large areas, requires expensive management options 

and is certain to dramatically affect forest composition 

and health if not addressed. The array of exotic plants, 

diseases, and insects constitute a significant threat 

especially when considered along with native stressors 

and other disturbances. There is considerable evidence 

that managing for a more even distribution of forest 

land by seral stage could improve the overall health and 

resiliency of the future forest. There is scant information 

on the compositional and structural development of 

Pennsylvania’s forests during the first half of the century, 

and little is known about how today’s stands will evolve. 

The opportunity to study reference stands in various 

seral stages will aid in understanding forest dynamics, 

and developing late-succession forests for the major forest 

community types would fill an important scientific void. 

The challenge lies in understanding and overcoming 

the various threats and their interactions on forest 

development. Forest managers and policymakers are at a 

critical juncture in planning Pennsylvania’s future forests.

Knowledge Gaps

The following knowledge gaps are hindering efforts to 

manage Pennsylvania’s forest resource effectively. Filling 

these gaps will greatly aid scientists, landowners, and 

policymakers plan for and manage the Commonwealth’s 

future forests.

The current NRS-FIA annual inventory has not 

provided statistically reliable estimates of change 

components, e.g., net growth, removals, and 

mortality. This report has included numerous 

surrogate indicators but there is no substitute 

•

Stump from previous logging.
75

for understanding harvest levels in relation 

to growth and mortality rates. Estimates of 

components of change will greatly increase our 

ability to gauge sustainability.

Improving and expanding the suite of resources 

that NRS-FIA addresses will further aid in 

understanding the real wealth of Pennsylvania’s 

complex forest. Carbon storage has become an 

important measure of the forest’s role in climate 

change. Other examples include nontraditional 

forest products, contributions to water quality, 

and relationships between forests and human 

health and well-being.

Wood resource availability is another gap that 

needs to be filled. The complexity of this issue 

will require integrating physical, biological, 

economic, legal, and social considerations.

Once the gaps related to change components 

and wood resource availability are filled, there 

will be a valuable opportunity to examine 

prospective future trajectories. Application of 

•

•

•

projection models could aid in understanding 

how the forest can be managed to meet the many 

goals and objectives of Pennsylvania’s residents.

Additional information is needed to fully 

understand the complex interrelationships that 

underlie forest-ecosystem processes. For example, 

it is apparent that acid deposition is affecting soil 

chemistry; however, the impact on native plant 

communities, forest pests/diseases, and invasive 

plants is not fully understood. Understanding 

how these interrelationships affect future forest 

conditions will enable scientists, landowners, and 

policymakers to make more informed decisions.

Gaps in our knowledge, e.g. a lack of statistically sound 

information on net growth, removals, and mortality, will 

be filled as the NRS-FIA annual inventory progresses 

through the second 5-year cycle. Additional research 

on wood availability, ecosystem services, and resource 

projection models also will aid in understanding resource 

dynamics.

•

Bald eagle.
76

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80

Appendix

estimation unit. Stratified estimation produces more 

precise estimates than simple random sampling. 

Phase 2

Field measurements are conducted at sample locations 

distributed systematically about every 3 miles across 

the landscape. Sample locations are situated within 

individual cells of a national hexagonal grid laid across 

Pennsylvania. Each Phase 2 sample represents about 

6,000 acres depending on the Phase 1 stratification of 

forest land. The new national design also incorporates 

a change to a four-subplot cluster (USDA For. Serv. 

2002). At each location, a suite of variables is measured 

that characterizes the land and trees associated with the 

sample (Fig. 88). Each year, 20 percent of the sample 

locations are measured, that is, it takes 5 years to 

complete the inventory. Each year’s sample is referred to 

as an “inventory panel.” The overall design is referred to 

as an “interpenetrating design” because no two cells are 

adjacent to one another within each inventory panel. As 

a result, each panel provides an unbiased representation 

of conditions across the State. Each completed panel 

Annual Inventory Overview

The annual inventory system combines features of the 

periodic system with a new systematic grid of sample 

plots and incorporates forest-health parameters. The 

inventory consists of three phases.

Phase 1

Phase 1 procedures reduce variance associated with 

estimates of forest-land area. A statistical estimation 

technique is used to classify digital satellite imagery and 

stratify the land base as forest or nonforest to assign a 

representative acreage to each sample plot. Source data 

are from Landsat Thematic Mapper (30-m resolution) 

imagery that ranged from 1999 to 2001. An image 

filtering technique is used to classify individual pixels 

using the 5- by 5-pixel region that surrounds each 

pixel that contains the sample plot. The resulting 26 

classes are collapsed for each estimation unit (county 

or supercounty; the latter is a combination of small 

counties). Stratified estimation is applied by assigning 

each plot to one of these collapsed strata and by 

calculating the area of each collapsed stratum in each 

Forester measuring tree height.
81

is combined with existing panels to produce 

the most precise estimates possible. This report 

is based on the first five panels measured in 

Pennsylvania using the new annual inventory 

protocols. After the next panel is complete, the 

set of panels used for estimation “moves” to the 

most recent five panels (2nd through 6th year). The 

moving average approach ensures that the most 

current and complete inventory data are used.

Phase 3

More extensive forest-health measurements are 

collected during a 10-week period in summer 

on a subset of Phase 2 sample locations. The 

measurements are grouped into six general 

categories of indicators: crown condition, 

understory vegetation, down woody material, 

soil condition, lichen communities, and ozone 

damage. The intensity of the Phase 3 sample is 

one sample location per 96,000 acres of land. 

The relatively small number of Phase 3 samples 

does not provide detailed analyses in some cases. 

For example, breaking down tree damage for a 

particular species by region reduces the number of 

samples and yields a high sampling error (SE).

Statistical Significance

This report contains a wealth of statistical 

estimates that are compared over time and among 

numerous variables. Changes in estimates are 

discussed in terms of direction and magnitude. 

All mention of “significant” changes are based on 

comparing 95-percent confidence intervals for the 

various estimates. If confidence intervals overlap, 

there has been no real change in a statistical 

sense. When confidence intervals do not overlap, 

significant change has occurred.

Phase 3 Phase 2

Figure 88.—Phase 2 and 3 sample-plot design, Pennsylvania, 2000-04.
82

Definition of Terms

Basal area . The cross-sectional area of a tree stem at 

breast height, expressed in square feet.

Board foot. A unit of lumber measurement 1 foot 

long, 1 foot wide, and 1 inch thick, or its equivalent. 

International 1⁄4 inch rule is used as the USDA Forest 

Service standard log rule in the Eastern United States. 

Commercial species. Tree species currently or 

prospectively suitable for industrial wood products; 

excludes species of typically small size, poor form, or 

inferior quality, e.g., hawthorn and sumac.

Condition. A delineation of a land area based on land 

use, forest type, stand size, regeneration status, reserved 

status, tree density, and owner class.

Cropland. Land that currently supports agricultural 

crops including silage and feed grains, bare farm fields 

resulting from cultivation or harvest, and maintained 

orchards. Includes cropland used for cover crops and soil 

improvement.

Cull tree. A rough tree or a rotten tree.

Diameter at breast height (d.b.h.). The diameter outside 

bark of a standing tree measured at 4‐1/2 feet above the 

ground.

Dry ton. A unit of measure of dry weight equivalent to 

2,000 pounds or 907.1848 kilograms.

Dry weight. The weight of wood and bark as it would be 

if it had been oven dried; usually expressed in pounds or 

tons.

Forest land. Land that is at least 10 percent stocked with 

trees of any size, or that formerly had such tree cover 

and is not currently developed for a nonforest use. The 

minimum area for classification of forest land is 1 acre. 

The components that make up forest land are timberland 

and all noncommercial forest land.

Forest type. A classification of forest land based on the 

species that form a plurality of live-tree stocking.

Forest-type group. A combination of forest types that 

share closely associated species or site requirements are 

combined into forest-type groups.

Growing-stock trees. Live trees of commercial species 

classified as large or medium size; that is, live trees of 

commercial species except rough and rotten trees.

Growing-stock volume. Net volume, in cubic feet, of 

growing-stock trees 5.0 inches and larger in from a 1‐foot 

stump to a minimum 4.0-inch top diameter outside bark 

of the central stem, or to the point at which the central 

stem breaks into limbs. Net volume equals gross volume 

less deduction for cull.

Hardwoods. Dicotyledonous trees, usually broad-leaved 

and deciduous.

International 1/4-inch rule. A log rule or formula 

for estimating the board-foot volume of logs. The 

mathematical formula is:

Board-foot volume = (0.22D2 - 0.71D)(0.904762)

for 4-foot sections, where D = diameter inside bark at 

the small end of the log section. This rule is used as the 

USDA Forest Service standard log rule in the Eastern 

United States.

Land area. (a) Bureau of Census: The area of dry land 

and land temporarily or partly covered by water, such 

as marshes, swamps, and river flood plains; streams, 

sloughs, estuaries, and canals less than 200 feet wide; and 

lakes, reservoirs, and ponds less than 4.5 acres in area; 

(b) Forest Inventory and Analysis: same as (a) except that 

the minimum width of streams, etc. is 120 feet, and the 

minimum size of lakes, etc. is 1 acre.

Land use. A classification of land that indicates the 

primary use at the time of inventory. Major categories are 

forest land and nonforest land. 
83

Large-size stand. A stand-size class of forest land that is at 

least 10 percent stocked with live trees of which half or 

more of such stocking is in medium- or large-size trees or 

both, and in which the stocking of large trees is at least 

equal to that of medium trees.

Large-size tree. A live tree of commercial species at least 

9.0 inches d.b.h. for softwoods or 11.0 inches d.b.h. for 

hardwoods.

Live tree. Live trees at least 1.0-inches d.b.h. and larger, 

including growing-stock, rough, and rotten.

Medium-size stand. A stand-size class of forest land that 

is at least 10 percent stocked with live trees of which 

half or more of such stocking is in medium or large-size 

trees or both, and in which the stocking of medium trees 

exceeds that of large trees.

Medium-size tree. A live tree of commercial species 

meeting regional specifications of soundness and form 

and at least 5.0 inches d.b.h. but smaller than a large tree 

(9.0 inches d.b.h. for softwoods and 11.0 inches d.b.h. 

for hardwoods)

Merchantable stem. The main stem of the tree between a 

1‐foot stump height and a 4-inch top diameter (outside 

the bark), including the wood and bark.

Mortality. The estimated net volume of trees at the 

previous inventory that died from natural causes before 

the current inventory (divided by the number of growing 

seasons between surveys to produce average annual 

mortality).

Net dry weight. The dry weight of woody material less 

the weight of all unsound (rotten) material.

Net growth. The change, resulting from natural causes, 

in volume during the period between surveys (divided 

by the number of growing seasons to produce average 

annual net growth). Components of net growth are 

ingrowth plus accretion, minus mortality, minus cull 

increment, plus cull decrement.

Noncensus water. Streams/rivers 120 to 200 feet wide 

and bodies of water 1 to 4.5 acres in size. The Bureau of 

the Census classifies such water as land.

Noncommercial species. Tree of typically small size, poor 

form, or inferior quality that usually are unsuitable for 

industrial wood products.

Nonforest land. Land that has never supported forests, 

or land formerly forested but now in nonforest use, e.g., 

cropland, pasture, residential areas, marshes, swamps, 

highways, industrial or commercial sites, or noncensus 

water.

Nonsalvable dead tree. A dead tree with most or all of its 

bark missing that is at least 5.0 inches d.b.h. and at least 

4.5 feet tall.

Nonstocked area. A stand-size class of forest land that is 

less than 10 percent stocked with live trees.

Pasture land. Includes pasture land other than cropland 

and woodland pasture. It can include lands that have had 

lime fertilizer or seed applied, or that had been improved 

by irrigation, drainage, or control of weeds and brush.

Relative stand density. A stocking classification procedure 

that reflects species, stage of development, and the 

characteristics of the trees present in a stand.

Removals. The net volume harvested or killed in logging, 

cultural operations (such as timber stand improvement) 

or land clearing, and the net volume neither harvested 

nor killed but now growing on land that was reclassified 

from timberland to noncommercial forest land or 

nonforest land during the period between surveys. This 

volume is divided by the number of growing seasons to 

produce average annual removals.

Reserved productive forest land. Forest land sufficiently 

productive to qualify as timberland but withdrawn from 

timber utilization through statute or administrative 

designation; land exclusively used for Christmas tree 

production.
84

Rotten tree. A live tree of commercial species that 

does not contain at least one 12‐foot sawlog or two 

noncontiguous sawlogs, each 8 feet or longer, now or 

prospectively, and does not meet regional specifications 

for freedom from defect primarily because of rot; that 

is, more than 50 percent of the cull volume in the tree is 

rotten.

Rough tree. The same as a rotten tree except that a rough 

tree does not meet regional specifications for freedom 

from defect primarily because of roughness or poor form; 

also a live tree of noncommercial species.

Salvable dead tree. A tree at least 5.0 inches d.b.h. 

that has died recently and still has intact bark; may be 

standing, fallen, windthrown, knocked down, or broken 

off.

Sampling error. A measure of the reliability of an 

estimate, expressed as a percentage of the estimate. The 

sampling errors given in this report are calculated as the 

square root of the variance, divided by the estimate, and 

multiplied by 100. Indicated in statistical tables as SE.

Sapling. All live trees 1.0 to 4.9 inches d.b.h.

Sapling/seedling stand. A stand-size class of forest land 

that is at least 10 percent stocked with live trees of which 

half or more of such stocking is in saplings or seedlings, 

or both.

Sawlog. A log meeting regional standards of diameter, 

length, and freedom from defect, including a minimum 

8-foot length and a minimum top diameter inside bark 

of 6 inches for softwoods and 8 inches for hardwoods 

(see specifications under Tree-Grade Classification).

Sawlog portion. The part of the bole of a large tree 

between the stump and the sawlog top.

Sawlog top. The point on the bole of a large-size tree 

above which a sawlog cannot be produced. The minimum 

sawlog top is 7.0 inches diameter outside bark (d.o.b.) for 

softwoods and 9.0 inches d.o.b. for hardwoods.

Sawtimber volume. Net volume in board feet 

(International 1/4-inch rule) of sawlogs in large trees. Net 

volume equals gross volume less deductions for rot, sweep, 

and other defects that affect use for lumber.

SE. See Sampling error.

Seedling. A live tree at least 6.0 inches tall for softwoods 

and 12.0 inches for hardwoods.

Seral stage. A series of ecological communities formed in 

ecological succession.

Small-size tree. A live tree of commercial species less than 

5.0 inches d.b.h.

Small-size stand. A stand-size class of forest land that is at 

least 10 percent stocked with live trees of which stocking of 

small-size trees exceeds medium- and large-size trees.

Snag. Standing dead tree with most or all of its bark 

missing that is at least 5.0 inches d.b.h. and at least 4.5 feet 

tall (does not include salvable dead).

Softwoods. Coniferous trees, usually evergreen and having 

needles or scalelike leaves.

Sound-wood volume. Tree volume of the central stem from 

a 1-foot stump to a minimum top diameter outside bark 

or a point at which the stem breaks into limbs. Sound cull 

portions are included and rotten cull portions are excluded; 

most often expressed in cubic feet for live trees.

Stand. A group of forest trees growing on forest land.

Stand origin. An indication of how the measured stand 

originated: 100 percent natural, 100 percent artificial, or 

a combination of both.

Stand-size class. A classification of forest land based on 

the size class of the stocking of live trees in the area. 

Stands are classified as small (previously referred to 

as sapling/seedling), medium (poletimber), and large 

(sawtimber).
85

State lands. Lands owned by the state or leased to the 

state for 50 years or more.

Stocking. The degree of occupancy of land by trees 

relative to the growth potential utilized by a site. It is 

expressed as a percent of the “normal” value presented 

in yield tables and stocking quides. Two categories of 

stocking are traditionally used in FIA reports: live-tree 

and growing-stock trees. The relationships between 

the classes and the percentage of the stocking standard 

are: nonstocked (0 to 9), poorly stocked (10 to 34), 

moderately stocked (35 to 59), fully stocked (60 to 100), 

and overstocked (100+). 

Stump. The main stem of a tree from ground level to 1

foot above ground level, including the wood and bark.

Timberland. Forest land producing or capable of 

producing crops of industrial wood (more than 20 cubic 

feet per acre per year) and not withdrawn from timber 

utilization by statute or administrative designation. The 

statutes and designations apply to publicly owned land 

only. Timberland was formerly known as commercial 

forest land. Timberland may be “nonstocked” so long 

as no natural condition or human activity prevents or 

inhibits the establishment of tree seedlings.

Timberland includes the following components:

a) Rural. The historical and traditional acreage classified 

as timberland in previous inventories. 

b) Other forest land. Defines a subset of forest land that 

is producing, or capable of producing, crops of industrial 

wood, but is associated with or part of a nonforest land 

use. In the past, these areas would have been treated as 

inclusions in the nonforest land use because they were 

considered part of a development. The minimum area 

for classification as other forest land is 1 acre. These 

strips of timber must have a crown width of at least 120 

feet. Examples of land that could be classified as other 

forest land are forested portions of city parks, forested 

land in highway medians and rights-of-way, forested 

areas between ski runs, and forested areas within golf 

courses. Generally, although surrounded by nonforest 

development, these areas have not been developed and 

exhibit natural, undisturbed understories.

c) Urban timberland. A subset of forest land that now 

is grouped into timberland. Includes land that except 

for its location would be classified as rural timberland. 

This land is nearly (surrounded on three sides) or 

completely surrounded by urban development, whether 

commercial, industrial, or residential. This land meets all 

the criteria for timberland, that is, at least 1 acre capable 

of producing at least 20 cubic feet per acre per year of 

industrial wood, is not developed for other than timber 

production, and is not reserved by a public agency. It is 

highly unlikely that such land would be used for timber 

products on a continuing basis. Such land may be held 

for future development, or scheduled for development. 

(The timber that is present may be utilized only at the 

time of development.) The land may be undeveloped due 

to periodic flooding, low wet sites, steep slopes, or their 

proximity to industrial facilities that are unsuitable for 

residential development. Forested areas within city parks 

are not urban forest land; it may be other forest land if all 

requirements are met. City parks cannot be classified as 

urban timberland as it is currently defined.

Timber products. Roundwood (round timber) products 

and manufacturing plant by-products harvested from 

growing-stock trees on timberland, from other sources, 

e.g., cull trees, salvable dead trees, limbs, tops, and saplings, 

and from trees on noncommercial forest and nonforest 

lands.

Timber removals. The volume of trees removed from the 

inventory for roundwood products, plus logging residues, 

volume destroyed during land clearing, and volume of 

standing trees on land that was reclassified from timberland 

to noncommercial forest land.

Top. The wood and bark of a tree above the merchantable 

height (or above the point on the stem 4.0 inches in 

diameter outside bark); generally includes the uppermost 

stem, branches, and twigs but excludes the foliage.

Tree class. A classification of the quality or condition of 

trees for sawlog production. Tree class for large trees is 
86

based on current condition. Tree class for medium trees is 

a prospective determination--a forecast of potential quality 

when they become large (11.0 inches d.b.h. for hardwoods, 

9.0 inches d.b.h. for softwoods).

Tree grade. A classification of large-tree quality based 

on guidelines for tree grades for hardwoods, white pine, 

and southern pine. (note: red pine was graded using the 

guidelines for southern pine).

Trees. A woody perennial plant, typically large, with a 

single well-defined stem carrying a more or less definite 

crown; sometimes defined as attaining a minimum 

diameter of 3 inches (7.6 cm) and a minimum height of 

15 feet (4.6 m) at maturity.

Unproductive forest land. Forest land that is incapable 

of producing 20 cubic feet per acre per year of industrial 

wood under natural conditions.

Upper stem portion. That part of the main stem or fork 

of a large tree above the sawlog top to a diameter of 4.0 

inches outside bark, or the point at which the main stem 

or fork breaks into limbs.
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The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on 

the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial 

status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because 

all or part of an individual’s income is derived from any public assistance program (Not all prohibited 

bases apply to all programs.) Persons with disabilities who require alternate means for communication 

of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at 

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of Civil Rights, 1400 Independence Avenue, S.W., Washington, DC 20250-9410, or call (800)795-3272 

(voice) or (202)720-6382 (TDD). USDA is an equal opportunity provider and employer. 

McWilliams, William H.; Cassell, Seth P.; Alerich, Carol L.; Butler, Brett J.; Hoppus, 

Michael L.; Horsley, Stephen B.; Lister, Andrew J.; Lister, Tonya W.; Morin, 

Randall S.; Perry, Charles H.; Westfall, James A.; Wharton, Eric H;. Woodall, 

Christopher W. 2007. Pennsylvania’s Forest, 2004. Resour. Bull. NRS-20. 

Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern 

Research Station. 86 p.

Pennsylvania’s forest-land base is stable, covering 16.6 million acres or 58 percent 

of the land area. Sawtimber volume totals 88.9 billion board feet, an average of 

about 5,000 board feet per acre. Currently, only half of the forest land that should 

have advance tree seedling and sapling regeneration is adequately stocked 

with high-canopy species, and only one-third has adequate regeneration for 

commercially desirable timber species. Several exotic diseases and insects threaten 

the health of Pennsylvania’s forests. Stressors such as drought, acidic deposition, 

and ground-level ozone pollution are adversely affecting the State’s forests.

KEY WORDS: forest inventory, forest health, sustainability, tree regeneration

Manuscript received for publication 16 April 2007
Silverthread Falls. Photo from the Pennsylvania Department of 

Conservation and Natural Resources.