United States Department of Agriculture

Forest 

Service

General Technical 

Report NRS-144

Northern 

Research Station February 2015

Accelerating the Development of 

Old-growth Characteristics in 

Second-growth Northern Hardwoods

Karin S. Fassnacht, Dustin R. Bronson, Brian J. Palik, 

Anthony W. D’Amato, Craig G. Lorimer, Karl J. Martin
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Manuscript received for publication 26 August 2014

Abstract

Active management techniques that emulate natural forest disturbance and stand development 

processes have the potential to enhance species diversity, structural complexity, and spatial 

heterogeneity in managed forests, helping to meet goals related to biodiversity, ecosystem 

health, and forest resilience in the face of uncertain future conditions. There are a number 

of steps to complete before, during, and after deciding to use active management for this 

purpose. These steps include specifying objectives and identifying initial targets, recognizing 

and addressing contemporary stressors that may hinder the ability to meet those objectives 

and targets, conducting a pretreatment evaluation, developing and implementing treatments, 

and evaluating treatments for success of implementation and for effectiveness after 

application. In this report we discuss these steps as they may be applied to second-growth 

northern hardwood forests in the northern Lake States region, using our experience with the 

ongoing managed old-growth silvicultural study (MOSS) as an example. We provide additional 

examples from other applicable studies across the region.

Cover Photos

Left: Standing dead wood in mesic forest, photo by Michele Woodford, Wisconsin DNR, used 

with permission. Right, top: Canopy gap, photo by Joshua Waukau, Wisconsin DNR, used with 

permission. Right, middle: large white pine in northern hardwoods, photo by J. Paul White, 

Wisconsin DNR, used with permission. Right, bottom: forest products from initial structural 

complexity harvest, photo by Karin Fassnacht, Wisconsin DNR, used with permission.

Quality Assurance

This publication conforms to the Northern Research Station’s Quality Assurance 

Implementation Plan which requires technical and policy review for all scientific publications 

produced or funded by the Station. The process included a blind technical review by at least 

two reviewers, who were selected by the Assistant Director for Research and unknown to 

the author. This review policy promotes the Forest Service guiding principles of using the 

best scientific knowledge, striving for quality and excellence, maintaining high ethical and 

professional standards, and being responsible and accountable for what we do.
1

INTRODUCTION

Maintaining compositional diversity1, structural complexity, and spatial 

heterogeneity in forests is increasingly being recognized as necessary for 

sustaining forest ecosystem health and resilience (Addis et al. 1995, Kern et al. 

2013, Mladenoff and Pastor 1993), particularly in the face of increased stresses 

from invasive species and climate change (Brang et al. 2014, Drever et al. 

2006, O’Hara and Ramage 2013, Rhemtulla et al. 2009). In the northern Lake 

States (Minnesota, Wisconsin, Michigan), forest complexity, heterogeneity, 

and compositional diversity were reduced as a result of extensive forest cutting 

and burning at the turn of the 19th century. These events largely eliminated 

formerly dominant conifer species such as eastern white pine (Pinus strobus) and 

eastern hemlock (Tsuga canadensis), increased the amount of aspen (Populus 

spp.) and maple (Acer spp.) (Mladenoff and Pastor 1993, Schulte et al. 2007), 

and significantly reduced the percentage of trees in medium and large size 

classes (i.e., >25 cm diameter at breast height [d.b.h.]) (Rhemtulla et al. 2009). 

Although much of the region has reforested, the composition and structure of 

these second-growth systems are quite different from the old-growth forests that 

once predominated (Schulte et al. 2007).

In northern hardwood forests of the region, traditional forest management has 

contributed to maintaining systems that are less diverse in species composition 

(Crow et al. 2002) and less complex and heterogeneous in structure (Crow et al. 

2002, Goodburn and Lorimer 1998) compared to the mature and old-growth 

forests (i.e., primary forest ≥120 yr old) which previously comprised almost 90 

percent of the northern hardwoods in this region (Frelich 1995) (Table 1). In 

particular, a common management approach for these forests has been singletree selection, with emphasis on removing defective, dead, and dying trees, as 

well as “less desirable” tree species (Arbogast 1957). As a result of these practices, 

structural components such as large standing dead wood (i.e., snags >45 cm 

d.b.h.) and large downed dead wood (>40 cm diameter) have been found to be 

significantly less abundant in managed uneven-aged and unmanaged even-aged

northern hardwood forests in northern Wisconsin and the Upper Peninsula of 

Michigan compared to old growth (Goodburn and Lorimer 1998). Studying 

similar forest types in the same region, Scheller and Mladenoff (2002) found that 

understory species richness in old growth was less than in the other two forest 

types, but that the variability of species composition in patches of understory 

vegetation (i.e., spatial heterogeneity) was greater, and the average size of those 

patches was smaller in the older forests. At the stand scale, the average size of 

understory vegetation patches is strongly correlated with both coarse woody 

debris (CWD) and variability in understory light availability (Scheller and 

Mladenoff 2002). In a related study, bird species richness and abundance were 

found to be similar in old-growth and managed uneven-aged stands, and no 

species was strictly limited to old growth, but the extensive old-growth forests 

did have some distinct assemblages of bird species (Howe and Mossman 1995).

The Authors

KARIN S. FASSNACHT is a 

research scientist with the Wildlife 

and Forestry Research Section of the 

Wisconsin Department of Natural 

Resources, Madison, WI 53703. 

DUSTIN R. BRONSON is a 

research scientist with the Wildlife 

and Forestry Research Section of the 

Wisconsin Department of Natural 

Resources, Rhinelander, WI 54501.

BRIAN J. PALIK is a research 

forest ecologist and team leader 

with the Northern Research Station 

of the United States Forest Service, 

Grand Rapids, MN 55744.

ANTHONY W. D’AMATO is an 

associate professor of applied forest 

ecology and silviculture with the 

Department of Forest Resources 

of the University of Minnesota – 

Twin Cities, St. Paul, MN 55108.

CRAIG G. LORIMER is a 

professor with the Department 

of Forest and Wildlife Ecology 

of the University of Wisconsin – 

Madison, Madison, WI 53706.

KARL J. MARTIN is director of 

the Community, Natural Resource 

and Economic Development 

(CNRED) program with the 

University of Wisconsin – 

Extension, Madison, WI 53706. 

At the time this research was 

conducted, he was chief of the 

Wildlife and Forestry Research 

Section with the Wisconsin 

Department of Natural Resources 

in Madison, WI.

1Words in bold are defined in the glossary that begins on page 32.
2

Table 1.—Comparison of selected characteristics of northern Lake States northern hardwoods among old-growth 

forests (OG), second-growth forests that are not managed (NM), second-growth forests under uneven-age management 

(UM), and second-growth forests under even-age management (EM)

Feature EM NM UM OG Example Reference

Large trees [percent of 

stems]

less – less more 50 cm class

UM = 3.2

OG = 7.9

≥60 cm class

UM = 3.2

OG = 5.7 (Hale)

≥39.1 cm

EM = 6.9

UM = 13.4

OG = 23.5

≥49.1 cm

EM = 1.2

UM = 4.1

OG = 11.3 (Angers)

Hale et al. (1999) – 

values derived from 

bar chart

Angers et al. (2005)

Standing dead wood – large 

d.b.h. [>45 cm G&L; ≥49.1 

cm Angers]

–

intermed* 

(but not 

signif)

less

–

less

less* (but not 

signif)

more

more (but not 

signif)

NM = 2/ha,

UM = 4/ha, 

OG = 17/ha (G&L)

EM = 4.9/ha

UM = 1.4/ha

OG = 7.6/ha

(Angers)

Goodburn and 

Lorimer (1998)

Angers et al. 

(2005) – *Year of 

last harvest for 

EM stand cut was 

~20 yr longer ago 

vs for UM stand

Downed dead wood – large 

[>40 cm]

– less intermediate more NM ~ 2 m3/ha

UM ~ 15 m3/ha

OG ~ 37 m3/ha

Goodburn and 

Lorimer (1998) – 

from graph

Gaps – sizes [m2] – smaller 

(but not 

signif)

intermediate 

(but not signif)

larger (but 

not signif)

NM = 9-11

OG = 37-49 (Dahir)

NM = 19.3

UM = 41.1

OG = 64.6 (Gdbrn)

Dahir and Lorimer 

(1996)

Goodburn (1996)

Gaps – diversity of sizes 

[range in m2]

– less intermediate more NM = <10 to 30

OG = <10 to 130 

(Dahir)

NM = 6 to 48

UM = 6 to 299

OG = 11 to 404 

(Goodburn)

Dahir and Lorimer 

(1996)

Goodburn (1996)

Gaps – number [No./ha] – less (but 

not signif)

more (but not 

signif)

intermediate 

(but not 

signif)

NM = 5.0

UM = 30.0

OG = 16.7

Goodburn (1996)

Diversity of tree sizes 

[range of d.b.h. classes]

– less less more NM = 4 to 60 cm

UM = 4 to 68 cm

OG = 4 to 84 cm

Goodburn (1996)

continued
3

Restoring or conserving old-growth northern hardwood 

forests could contribute to enhancing the biodiversity

of the northern Lakes States region. At the landscape 

scale, old-growth forest would contribute to the mosaic 

of forest ages and types, increasing the representation of 

a formerly abundant ecological condition. At the stand 

scale, the increased compositional diversity, structural 

complexity, and spatial heterogeneity of composition and 

structure naturally found in older forests would provide 

variability and habitat niches not found, or found to a 

lesser extent, in younger managed forests (Gilbert et al. 

1997, Howe and Mossman 1995, Lindner et al. 2006, 

Werner and Raffa 2000, Will-Wolf and Nelsen 2008) 

(Table 1). Particularly important in older forests are 

age-related structural features such as large dead wood 

(Fig. 1), large canopy gaps, supercanopy trees, cavity 

trees, and tip-up mounds (Mladenoff and Pastor 1993, 

Schaetzl et al. 1989).

Snags with loose bark plates – no diff no diff no diff No./ha

NM = 3.3

UM = 5.3

OG = 3.3

percent of snags

NM = 3.7

UM = 13.9

OG = 8.5

Goodburn and 

Lorimer (1998)

Tip-up mounds [percent 

area covered by recent pit + 

mound]

– no diff no diff no diff NM = 0.1

UM = 0.8

OG = 0.6

Goodburn and 

Lorimer (1998)

Vertical complexity [variance 

of canopy tree ht & depth of 

main canopy layer]

– less intermediate more pooled variance

NM = 7.1

UM = 19.9

OG = 44.8

depth of layer

NM = 10.3

UM = 12.4

OG = 15.3

Goodburn (1996)

Horizontal complexity [avg 

coefficient of variation for 

total crown area of understory 

(<6 m) for 100 m2 patch 

sizes] 

– no diff no diff no diff NM = 97 percent

UM = 68 percent

OG = 62 percent

Goodburn (1996)

Spp richness – overstory 

[No. spp]

no diff no diff no diff no diff EM = 13

NM = 16

UM = 11

OG = 16 (Crow)

NM = 5.8

UM = 5.7

OG = 5.2 (Gdbrn)

Crow et al. (2002) – 

avg 2 yr post-trt

Goodburn (1996) – 

avg 7 yr post-trt

Spp richness – understory 

[shrub and small tree]

no diff no diff no diff no diff EM = 14

NM = 14

UM = 10

OG = 10

Crow et al. (2002) – 

avg 2 yr post-trt

Diversity of light 

environments [avg variance 

in avail. seasonal solar 

radiation at 0.5 ha scale]

– – less more UM = 13 percent 

OG*= 132 percent

Hanson and 

Lorimer (2007) – 

*comparing mature 

and OG winddisturbed vs UM

Feature EM NM UM OG Example Reference

Table 1.—continued
4

Restoring old-growth conditions through passive 

management (i.e., permanently protecting areas 

from harvest) is desirable for some portion of the 

landscape (Bauhus et al. 2009, Mladenoff and Pastor 

1993). However, this technique alone will not provide 

significant increases in acreage with old-forest conditions. 

Reasons include the long time period needed to attain 

old-growth characteristics in this forest type in the 

absence of management (Lorimer and Frelich 1994) and 

the continued demand for forest products and a finite 

forest land area with which to produce them. As a result, 

goals of forest biodiversity, health, and resilience will not 

be met through passive management alone (Bauhus et al. 

2009, Mladenoff and Pastor 1993).

Active management techniques may offer an alternative. 

Recently, there has been increased discussion advocating 

the use of silvicultural techniques that better emulate 

natural forest disturbance and stand development to 

enhance biodiversity and structural characteristics in 

managed forests (Aplet and Keeton 1999, Crow and 

Perera 2002, Franklin et al. 2007, Palik et al. 2002, 

Zasada et al. 2004). While true old-growth forest cannot 

be restored through active management due to the 

extractive nature of such management, there is evidence 

that it is the structural features associated with old-growth 

forests, rather than the age of the forest itself, that is linked 

to the occurrence of many old-growth-associated species 

(Bauhus et al. 2009). Consequently, it may be possible 

to increase the benefits of old-forest characteristics in the 

broader forest landscape by accelerating the development 

of old-growth characteristics through active management 

if techniques can be found that do not substantially 

compromise other benefits derived from the forest.

In this report, we discuss issues to be considered when 

contemplating the use of active management to accelerate 

the development old-growth characteristics in secondgrowth northern hardwoods, as well as steps to be taken in 

the process of developing, implementing, and evaluating 

treatments. We use our experience with the ongoing 

managed old-growth silvicultural study (MOSS) (Fassnacht 

et al. 2013) as an example, and provide additional examples 

from other applicable studies across the region.

Figure 1.—Large (a) downed wood and (b) standing dead wood in mesic, nutrient-rich northern hardwood forests in 

northern Wisconsin. Photo (a) by J. Paul White, WDNR; photo (b) by Michele Woodford, WDNR; both used with permission.

a b
5

Background

Defining Old Growth

To successfully describe desired management goals 

in terms of structure and composition, it is necessary 

to have a definition for old-growth forest. Arguments 

have been made for definitions based on maintaining 

or restoring process rather than structure, while at the 

same time recognizing the need for criteria that can be 

identified in the field (Frelich and Reich 2003). Here we 

adopt the multilevel definition for old-growth northern 

hardwood forests provided by the Wisconsin Department 

of Natural Resources (WDNR) (Wisconsin Department 

of Natural Resources 2010). At the most general level, 

old-growth forests are defined as forests that are relatively 

old and relatively undisturbed by humans (Hunter 

1989). More specifically, these forests have diverse 

horizontal and vertical structure, including multiple 

canopy layers and canopy gaps in various stages of 

recovery. Plant and animal species are present that prefer, 

or are possibly dependent upon, old growth. These 

forests include a broad spectrum of canopy tree ages 

and have nutrient cycles and other processes potentially 

influenced by a shifting mosaic steady state at larger 

scales (Wisconsin Department of Natural Resources 

2010). Additional mid- and fine-level characteristics are 

described in Table 2.

For larger forest tracts, typically under public 

management, it may be desirable to consider the 

economic, social, and political questions regarding how 

much of the landscape should be in old growth (or 

managed primarily for old-growth structural attributes), 

and which areas are best suited to this management. Such 

discussions might take place as part of the old-growth 

definition process (e.g., Rusterholz 1996) or they might 

occur separately.

Table 2.—Criteria adding details to the general old-growth definition of “relatively old and relatively 

undisturbed by humans” (Hunter 1989). Adapted from Wisconsin Department of Natural Resources (2010).

Composition/succession variables

– presence of plant and animal species that prefer, or are possibly dependent upon, old-growth forest, such 

as fungi and lichens

– successional stage and the representation of late-successional tree species that establish in the understory 

and reach the canopy in tree-fall gaps

Structural development

– an abundance of large, old trees

– wide variation in tree sizes and spacing that reflect years of small natural disturbances

– diverse vertical structure that includes multiple layers, each reflecting a broad spectrum of ages

– large volumes of coarse woody debris (standing and down) representing a wide variety of decay classes

– tree-fall gaps of various sizes and ages

– randomly scattered pits and mounds

Functional variables

– species presence influenced by natural disturbance

– nutrient cycles influenced by long-term steady state

– undisturbed soils

– little evidence of human disturbance

Landscape/disturbance variables

– non-static; disturbance affects these stands

– shifting mosaic steady state

Minimum criteria for initial designation for northern hardwoods

– Even-aged/two-aged: ≥20 percent stand basal area in trees ≥170 yr old

– Uneven-aged: ≥18.4 m2/ha of basal area in trees ≥12.7 cm d.b.h.; ≥50 percent of stand basal area in trees 

≥45.7 cm d.b.h.; and 2.5 trees/ha >61.0 cm d.b.h.
6

Disturbance

Use of silvicultural systems that emulate natural 

disturbance requires an understanding of the particular 

regime associated with the forest type to be managed. 

Work by Frelich and Lorimer (1991) has shown that 

disturbance regimes in northern Lakes States northern 

hardwoods are complex. Frequent low-severity 

disturbances (10.0 to 19.9 percent stand-level canopy 

removal; 70-80 yr stand-scale natural rotation period) 

combine with intermediate-frequency moderate-severity 

disturbances and rare catastrophic disturbances (≥60 

percent canopy removal; ~1800 yr natural rotation 

period) to create stands that are uneven-aged, with an 

average of more than 10 age classes present. Background 

mortality and small disturbances are commonly related 

to the singular and interactive effects of insects, disease, 

senescence, drought, ice, and wind (Frelich 2002, 

Lorimer et al. 2001, Parshall 1995). Moderate-severity 

events are generally due to ice storms and wind (Hanson 

and Lorimer 2007), and catastrophic events are primarily 

due to wind. While some catastrophic wind disturbance 

is caused by tornadoes, these intense disturbances are 

more commonly caused by downbursts associated 

with thunderstorms (Canham and Loucks 1984). The 

frequency of surface fires in the period prior to European 

settlement is not well known, but point recurrence 

intervals for stand-replacing fires were generally very long 

(Frelich and Lorimer 1991, Lorimer and Frelich 1994, 

Schulte and Mladenoff 2005).

Despite their less-frequent occurrence, moderateseverity disturbances have important impacts on stand 

structure and composition. Frelich and Lorimer (1991) 

found more than a quarter of the combined plot area 

studied was occupied by trees recruited in decades 

with disturbance events removing ≥30 percent of the 

canopy. Given the increased amount of light reaching 

the forest floor after such events, and the diversity of 

light environments and microhabitats (Hanson and 

Lorimer 2007), moderate-intensity disturbances could be 

expected to allow mid-tolerant species, such as yellow 

birch (Betula alleghaniensis), to persist in stands (Dahir 

and Lorimer 1996, Webster and Lorimer 2002, Webster 

and Lorimer 2005), and also contribute to a diversity 

of habitat niches. Given the natural rotation period 

of 300 to 400 yr for disturbances removing 30 to 49 

percent of the canopy (Frelich and Lorimer 1991), most 

stands would be expected to be affected by moderateseverity disturbance during the maximum lifespan of the 

dominant tree species (300 to 500 yr) (Lorimer et al. 

2001).

Examples of Active Approaches

In the northern Lakes States, several modeling efforts 

have explored opportunities and limitations of alternative 

silvicultural systems for developing old-growth features 

in northern hardwoods. Choi et al. (2007) found 

through simulation that moderately-heavy thinning in a 

mature even-aged stand could reduce the time required 

to develop minimum desired characteristics (e.g., >80 

percent of basal area in trees ≥26 cm d.b.h., and >50 

percent of basal area in trees ≥46 cm d.b.h.) from 79 to 

36 yr. Similar treatment in an older uneven-aged stand, 

however, slowed development of desired characteristics 

due to the combined effects of natural mortality and the 

removal of medium-sized trees during harvest. Hanson et 

al. (2012) noted that the best balance between ecological 

and timber product objectives in simulated harvests was 

obtained using any cutting methods that retained eastern 

hemlock and yellow birch, had a maximum residual 

d.b.h. of 80 cm, or both. This was in comparison to 

other alternatives that considered various combinations 

of permanent legacy trees, CWD retention, and variable 

gap sizes (Hanson and Lorimer 2007).

In addition to modeling studies, there are ongoing field 

trials in northern hardwood forests testing hypotheses 

related to accelerating the development of old-growth 

characteristics through active management. A number of 

studies focus on gaps, including the impacts of retaining 

yellow birch seed trees in group selection openings 

(Klingsporn et al. 2012; Shields and Webster 2007; 

Shields et al. 2007a, b; 2008); the impacts of gap size 

on forest composition and structure (Kern et al. 2013); 

and the impact of gap size on sapling and stump-sprout 

growth (Dyer et al. 2009). Moderate- to broad-scale 

studies include the work of Keeton (2006) in Vermont 

and the Nature Conservancy in Michigan’s Upper 

Peninsula (Strand 2012). The Vermont study compares 

modified single-tree and group-selection methods with 
7

a “structural complexity enhancement” treatment (SCE) 

(Keeton 2006) in 2-ha experimental units. The SCE 

treatment increased dead wood, used a “rotated sigmoid” 

diameter distribution to guide marking, and had a 

90 cm maximum residual d.b.h. (Keeton 2006). The 

Nature Conservancy work created gaps of various sizes 

and retained dead wood as part of a commercial timber 

sale on a portion of ~9300 ha of Conservancy property 

(Strand 2012).

Northern Wisconsin Example

For the discussion that follows, we use MOSS (Fassnacht 

et al. 2013) as the primary example, with the simulation 

work and field trials described above providing additional 

examples. The MOSS is an ongoing, long-term study that 

compares the ability of six active treatments to accelerate 

the development of old-growth characteristics in 

second-growth northern hardwoods while still allowing 

sustainable timber harvests. The study is structured as an 

augmented split-plot design (Piepho et al. 2006) (Fig. 2) 

with three replicates (i.e., study sites) across northern 

Wisconsin (Fig. 3, Table 3). Active treatments combine 

harvest (i.e., canopy) treatments with coarse woody debris 

treatments. Canopy manipulation (whole plots; ~49 ha) 

includes a small gaps treatment (10.7-m diameter gaps 

plus thinning of the matrix), a large gaps treatment (18.3- 

and 24.4-m diameter gaps plus thinning; Fig. 4), and a 

novel irregular multi-cohort treatment (0.40-ha and 1.2-ha 

modified shelterwoods plus light-thin and heavy-thin 

zones; Fig. 5) based on the work of Hanson and Lorimer 

(2007). Each stand is split in half (split plot; ~24 ha) and 

receives two CWD treatments: low (no supplemental dead 

wood created) and high (supplemental dead wood created; 

Fig. 6). Control stands receive neither canopy treatment 

nor CWD treatment. All treatments are replicated at 

each study site. See Fassnacht et al. (2013) for more detail 

regarding the study sites, treatments, and treatment 

implementation.

SILVICULTURAL APPROACH

There are a number of steps to complete before, during, 

and after deciding to use active management to enhance 

the compositional diversity and structure of secondgrowth northern hardwoods in the northern Lake 

States region. These include specifying objectives and 

identifying initial targets, identifying and addressing 

contemporary stressors that may hinder the ability 

to meet those objectives and targets, conducting a 

pretreatment evaluation, developing and implementing 

treatments, and evaluating treatments for success of 

implementation and effectiveness after application. These 

and additional topics will be discussed.

Specifying Objectives and 

Identifying Initial Targets

The overall objective under discussion in this report 

is enhancing the compositional diversity, structural 

complexity, and spatial heterogeneity of secondgrowth northern hardwood forests through active 

management to accelerate the development of old-growth 

characteristics. To fully specify objectives for a particular 

stand, however, there are other factors to consider. For 

example: What should be used as a reference condition? 

To what degree should those reference conditions be 

met? What factors need to be manipulated to achieve 

desired targets? What are the tradeoffs (e.g., with timber 

production) of manipulating those factors? There are 

biological, economic, and social issues to contemplate 

when addressing these questions. Once objectives are 

specified, initial targets can be identified.

Reference Conditions

Within any given forest type, large differences can be 

expected among stands examined, reflecting a natural 

range of variability in structure and composition 

(Franklin et al. 2007). Examples from the scientific 

literature can be used to help gauge the range of 

structural and compositional characteristics that might 

be found in potential reference conditions. Information 

from the literature can be general or quite specific. 

Bauhus et al. (2009), for example, include “Large 

amount/mass of downed CWD” and “High spatial 

[variability] of tree distribution/irregular size and 

distribution of gaps” in a table of general old-growth 

structural attributes associated with different forest 

types. In contrast, Tyrrell and Crow (1994) include 

“dead wood >120-150 m3

/ha, with logs >80 m3/ha” and 

“canopy gaps occupying >10 percent of the stand, with 
8

Figure 2.—The six active treatments investigated in the managed old-growth silvicultural study (MOSS) as applied at the Highlands 

study site. Canopy treatments were applied to ~48.6 ha stands; coarse woody debris (CWD) treatments were applied to half of each 

stand. The ~48.6 ha control stand (not shown) received no canopy or CWD treatment. Canopy and CWD treatments are defined on 

page 7. Figure and most text from Fassnacht et al. (2013), used with permission.

Treatments Highlands study site

Entire stand = irregular 

multi-cohort

West = low CWD

East = high CWD

Entire stand = large gaps

West = low CWD

East = high CWD

Entire stand = small gaps

West = high CWD

East = low CWD

200 m

Stand boundary

CWD-treatment dividing line

Gap treatments (to scale)

Thinned

Cleaned + scarified

Cleaned

Minimally cleaned

Irregular multi-cohort trt

Heavy thin

Light thin

Modified shelterwd

High CWD treatment

New standing dead

New downed dead
9

WISCONSIN

Flambeau River State Forest 

(Flambeau)

Northern Highland –

American Legion State Forest

(Highlands)

Chequamegon – Nicolet

National Forest’s Argonne

Experimental Forest 

(Argonne) = location of research stand(s)

Figure 3.—Study sites of the managed old-growth silvicultural study (MOSS). Full site names are provided 

with the working site names shown below in parentheses. Working site names are used throughout the report. 

Figure from Fassnacht et al. (2013), used with permission.

Table 3.—Unweighteda pretreatment characteristics of the twelve study stands of the managed old-growth silvicultural study 

(MOSS). Data reported at the stand level. Standard errors provided in parentheses. Table and text modified from Fassnacht et al. 

(2013); used with permission.

Site Canopy treatmentb

Areac

[ha] Habitat typed

Mean 

live d.b.h.

[cm]

Mean 

snag d.b.h.

[cm]

Mean

live

stems/ha

Mean 

live BAe

[m2/ha]

Mean 

snag BA

[m2/ha]

Flambeau control 45.93 low AH (w) 24.6 (0.3) 20.7 (1.5) 493 (21) 28.0 (1.0) 1.4 (0.3)

small gaps 52.16 AH (w) 23.4 (0.3) 18.4 (0.9) 599 (22) 29.2 (0.8) 0.97 (0.1)

large gaps 50.22 ATD/ATM - ATD/AH (wm, w) 26.6 (0.6) 26.3 (1.4) 448 (18) 30.7 (0.8) 2.0 (0.2)

irregular multi-cohort 51.88 ATD (wm); ATAtOn (incl). 23.0 (0.3) 17.3 (0.6) 633 (17) 30.3 (0.5) 2.1 (0.3)

Highlands control 48.28 ATD 29.0 (0.6) 27.3 (1.5) 415 (13) 34.0 (0.7) 2.2 (0.3)

small gaps 47.83 ATD 25.2 (0.7) 25.1 (1.1) 464 (26) 29.1 (0.8) 1.8 (0.3)

large gaps 49.33 ATD 26.9 (0.7) 26.4 (1.3) 417 (21) 29.3 (0.8) 2.0 (0.3)

irregular multi-cohort 51.44 ATD 27.8 (0.6) 22.6 (1.1) 399 (16) 29.9 (1.0) 1.3 (0.2)

Argonne control 47.31 ATD 30.6 (0.6) 27.6 (1.1) 417 (17) 36.4 (0.8) 3.3 (0.3)

small gaps 49.17 AOCa (w) 29.7 (0.7) 28.5 (1.1) 352 (17) 29.7 (0.7) 3.6 (0.4)

large gaps 55.24 AOCa 29.9 (0.6) 24.9 (1.3) 398 (17) 33.1 (0.8) 3.8 (0.4)

irregular multi-cohort 46.34 AOCa (w) 31.7 (0.8) 27.7 (1.4) 351 (14) 32.6 (1.2) 4.3 (0.5)

a Values in this table are appropriate for characterizing the pretreatment conditions only and should not be used for comparison with post-treatment values 

(found in Fassnacht et al. 2013). 

b Canopy treatments are defined on page 7.

c

 1 ha = 2.47104393 ac

d Kotar et al. (2002); w = patches of wet, wm = patches of wet-mesic, incl = inclusions

e BA = basal area; 1 m2/ha = 4.356017426 ft2/ac
10

Figure 4.—A 24.4-m diameter gap in the fourth growing season 

after it was created at the Argonne site of the managed oldgrowth silvicultural study. Photo by Joshua Waukau, WDNR, 

used with permission.

Figure 5.—A 0.40-ha modified shelterwood in the second 

growing season after the establishment cut at the Highlands site 

of the managed old-growth silvicultural study. Photo by Brian 

Werner, WDNR, used with permission.

Figure 6.—Newly created (a) downed wood and (b) girdled tree at the Argonne site of the managed old-growth silvicultural study. 

The arrow in (b) points to the double girdle. Photos by Dean van Doren, WDNR, used with permisison.

a b

average gap size >50 m2...” among a list of characteristics 

shared by most of the older old-growth hemlock-hardwood 

stands they studied. Values such as those presented in the 

latter example can be used in developing specific targets 

once details of project objectives are finalized.

Deciding which reference to use within the range 

of variability observed, and the degree to which 

management should seek to achieve those reference 

conditions, is more of a social and economic decision 

than a biological one. Zasada et al. (2004) provide a 

framework for considering this question (see their Figure 

19.3). Factors shown to impact the degree to which a 

given stand approaches reference conditions include time 

since disturbance (determined by factors such as rotation 

length, frequency of entries, and natural disturbance 

events), degree of structural complexity sought, and 

management objectives (on a scale from production 
11

forestry to reserve). Questions to consider in deciding 

how closely to mimic a reference condition include: 

•	 What are the primary management objectives? 

•	 What is the operability of the harvest being 

considered? Is there a minimum yield needed? 

•	 What are the short-term and long-term costs as 

well as benefits of various options? 

•	 What is the social tolerance for disturbance 

frequency, the methods being used (e.g., amount 

of dead wood left), or both?

•	 What resources are available to undertake the 

management?

•	 What wildlife and plant species would likely be 

favored or hindered by old-growth characteristics 

in the stand(s)? 

•	 What interactions might there be between the 

proposed management and wildlife populations 

in the adjacent land cover types?

•	 What is the desired plant species composition?

•	 Are there exotic/invasive species or browsing 

pressure that may hinder or constrain restoration 

goals?

•	 What timeline is available to achieve the desired 

goals?

Factors to be Manipulated to Achieve Targets and 

the Potential Tradeoffs

Some of the questions from the list above can be 

answered by considering the factors to be manipulated 

to achieve potential targets of interest. For example, 

increasing the amount of large standing and downed 

dead wood typically involves leaving some trees on site 

that would otherwise be harvested. Trees can be left as 

designated permanent legacy trees (Bauhus et al. 2009) 

or as new dead wood created through girdling (Fassnacht 

et al. 2013) or mechanically tipping over (Keeton 2006) 

trees. Steele and others2

 are currently evaluating the 

opportunity costs associated with foregoing revenue 

associated with new dead-wood trees. For the amount of 

CWD being created in MOSS (Fassnacht et al. 2013), 

the opportunity costs appear to be minimal (~3 percent 

of volume, ~1 percent of value).3

In addition to dead wood, other common factors 

manipulated to enhance compositional diversity and 

structure include quantity, size, and spatial arrangement 

of overstory trees to be removed (or retained). Leaving 

larger trees, some permanent legacy trees, or both, 

harvesting trees in groups or small shelterwoods, and 

removing less volume may all impact the economics of a 

harvest. Simulation and field-based trials have evaluated 

the impacts of combined aspects of natural-disturbancebased silviculture in northern hardwoods. For example 

in a simulation study, Hanson et al. (2012) found that 

prescriptions incorporating varying aspects of structural 

complexity enhancement reduced timber volume produced 

compared to standard single-tree selection by highly 

variable amounts (9 to 55 percent) depending on the 

techniques included in the prescriptions. In a field trial, 

Keeton and Troy (2006) reported that treatments focused 

on structural complexity enhancement had the potential to 

be profitable on high-quality sites and with good market 

conditions, though they were less profitable compared to 

modified single-tree and group-selection alternatives.

Example

In MOSS, objectives were determined through a 

combination of public input and state-level policy 

priorities. The study is part of the second phase of oldgrowth research initiated by the WDNR in cooperation 

with other regional partners (e.g., University of 

Wisconsin–Madison, U.S. Forest Service Northern 

Research Station, University of Minnesota–Twin Cities, 

and University of Wisconsin–Green Bay) in response 

to public concerns that forest management practices 

were not maintaining biodiversity.4

 Public concern, 

2 Manuscript in preparation: Steele, T. W.; Knoot, T. G.; 

Fassnacht, K. S.; Martin, K. J. The opportunity cost of 

enhancing structural complexity and species diversity in 

second-growth, even-aged northern hardwoods.

3 T. Knoot, Wisconsin Department of Natural Resources, 

personal communication.

4 Introduction to a report titled “A comparison of old-growth 

and managed forests in the Great Lake states.” Introduction by 

G. A. Bartelt. Report on file with Nicholas Anich at Wisconsin 

Department of Natural Resources, 2501 Golf Course Road, 

Ashland WI 54806.
12

and Wisconsin’s commitment to sustainable forest 

management, resulted in the development of a longterm old-growth research program. The first phase of 

this research compared old-growth with managed and 

unmanaged second-growth northern hardwood and 

hemlock-hardwood forests (e.g., Goodburn and Lorimer 

1998, Howe and Mossman 1995, Lindner et al. 2006, 

Miller et al. 2002, Werner and Raffa 2000, Will-Wolf 

and Nelsen 2008). The second-phase research is seeking 

to determine what modifications in silvicultural practices 

have the potential to promote biodiversity while still 

allowing for timber production.

Specific forest harvest targets for MOSS were set based 

on state guidelines and references from the scientific 

literature. For the canopy treatments, the gap size (i.e., 

10.7-m diameter) and density of gaps for the small 

gaps treatment were based on values recommended for 

conversion from even-age to uneven-age management in 

northern hardwoods (Wisconsin Department of Natural 

Resources 2006). For the large gaps treatment, the size 

and density of the 18.3-m diameter gaps was based on 

recommendations in the same document for conversion 

to uneven-age management while promoting midtolerant species. The size of the 24.4-m diameter gaps in 

this treatment was based on work by Strong et al. (1998) 

which identified gaps of intermediate size as providing 

the optimum balance between slowing gap closure rates 

and minimizing gap capture by Rubus species. Finally, 

structural targets for the irregular multi-cohort treatment 

were based on the values for wind-disturbed stands as 

reported by Hanson and Lorimer (2007).

For the CWD treatments, the target value was initially 

chosen to be 70 percent of the larger, less decayed dead 

wood volume (or stem density for snags) calculated 

for Sylvania Wilderness Area in the Upper Peninsula 

of Michigan, based on data collected for the WDNR 

phase I old-growth research5

 (Goodburn and Lorimer 

1998 for snags). A target less than 100 percent was 

chosen to help maintain economic viability of the 

treatments in recognition that some opportunity costs 

would be incurred by leaving wood on site. In addition, 

the target was chosen to be greater than 60 percent, 

the approximate proportion of dead wood found in 

northern hardwoods managed using the selection system 

in the WDNR phase I old-growth studies. Further 

modifications of the 70 percent target were needed 

once treatment development and implementation 

were undertaken, as will be described in the sections 

“Treatment Development” and “Treatment 

Implementation” (see pages 16 and 19).

Contemporary Stressors

In developing management goals, it is important to 

identify and attempt to factor in contemporary stressors 

that may impact the ability of planned harvests to meet 

long-term objectives related to restoring old-growth 

compositional and structural conditions. For northern 

hardwood forests of the Lake States, such stressors include 

herbivores—especially white-tailed deer (Odocoileus 

virginianus)—invasive species, and climate change.

Deer

The ability of land managers to achieve structural targets 

in their managed stands can be made more difficult when 

a site is experiencing, or has experienced, moderate to 

high browsing pressure (Horsley et al. 2003, Kern et al. 

2012, Reuling 2014). Negative impacts of deer browsing 

on the stem density and growth of tree regeneration 

have been noted for preferred browse species (e.g., 

eastern hemlock, yellow birch) compared to species that 

are not preferred (e.g., ironwood [Ostraya virginiana]) 

or which better tolerate browsing pressure (e.g., sugar 

maple [Acer saccharum]) (Dahlberg and Guettinger 

1956, Frelich and Lorimer 1985, Horsley et al. 2003, 

Webb et al. 1956). Preferred species, which include 

many of the historically more abundant species in Lakes 

States northern hardwood forests, are less able to take 

advantage of purposely created canopy gaps (Horsley et 

al. 2003, Kern et al. 2012) due to the lack, or decreased 

vigor, of advanced regeneration resulting from recent 

browsing (Reuling 2014, Salk et al. 2011, Stoeckeler 

et al. 1957) (Fig. 7). There also may be a lack of seed 

source for these species as a result of historical browsing 

or land-use practices (Horsley et al. 2003, Mladenoff 

5 For downed woody debris—unpublished data from R. Howe 

and M. Mossman. On file with Michael Mossman, Wisconsin 

Department of Natural Resources, 2801 Progress Road, 

Madison, WI 53716.
13

and Stearns 1993, Tanentzap et al. 2011). Suitable 

microsites for germination (e.g., well-decayed downed 

wood for hemlock and yellow birch) are also rendered 

less important if overbrowsing prevents seedling growth 

on these features (Frelich and Lorimer 1985, Witt and 

Webster 2010).

The best approach to addressing deer impacts is 

unclear given the complex relationship between deer 

population density and forest vegetation (e.g., Horsley 

et al. 2003) and the many other factors contributing to 

establishment and growth of various species (Kern et al. 

2012, Mladenoff and Stearns 1993). Approaches might 

include activities that directly impact deer numbers, 

such as increased levels of deer hunting (Alverson et al. 

1988, Tanentzap et al. 2011, Witt and Webster 2010), 

as well as activities designed to protect plants from 

browsing (Alverson et al. 1988, Tanentzap et al. 2011, 

White 2012, Witt and Webster 2010) and to increase the 

likelihood of successful establishment (White 2012, Witt 

and Webster 2010), including the use of fencing or tree 

shelters. If these activities are affordable, they may help 

reduce impacts of browsing over time and in selected 

locations. Nonetheless, other factors contributing to 

species decline at a landscape or regional scale (e.g., 

land-use history, climate, life-history requirements; 

Mlandenoff and Stearns 1993) may prevent successful 

re-establishment of historically important species even 

if browsing impacts can be reduced (Mlandenoff and 

Stearns 1993).

Invasive Species

The effectiveness of management strategies to enhance 

species diversity may be seriously compromised 

by invasive species. For example, composition and 

successional trajectories can be impacted through changes 

in forest floor thickness (Hale et al. 2005); mycorrhizal 

communities (Frelich et al. 2006, Rodgers et al. 2008); 

and nutrient dynamics associated with earthworms 

(Bohlen et al. 2004, Suárez et al. 2003), herbaceous 

plants (Rodgers et al. 2008) or exotic plant-eating insects 

(Gandhi and Herms 2010). In addition, canopy gaps 

installed to increase representation of mid-tolerant species 

(e.g., white ash; Fraxinus americana) may be rendered 

Figure 7.—Regeneration within a deer exclosure located within a gap at the Highlands 

site of the managed old-growth silvicultural study in the fourth growing season after the 

gap was created. The white bracket denotes the area of the exclosure. Notice the lack of 

advanced regeneration in the gap outside of the exclosure. Photo by WDNR staff, used 

with permission.

{
14

ineffective if desired species are no longer providing 

seed sources or successful regeneration after having been 

severely reduced or eliminated from a stand (Frelich et 

al. 2006, Gandhi and Herms 2010, Holdsworth et al. 

2007). Other barriers to gap colonization by desired 

species can include soil conditions altered by invasive 

species (Frelich et al. 2006, Rodgers et al. 2008) and 

competition from more aggressive native or exotic species 

such as Pennsylvania sedge (Carex pensylvanica), or garlic 

mustard (Alliaria petiolata) (Holdsworth et al. 2007, 

Rodgers et al. 2008) (e.g., Fig. 8). Invasive shrubs may 

also potentially impact which species, if any, are able 

to succeed in the created gaps (Fagan and Peart 2004, 

Gorchov and Trisel 2003).

Structural features of a stand can also be impacted 

by invasive species (Castello et al. 1995, Dukes et al. 

2009). Small scattered gaps can be created throughout 

the stand due to mortality associated with exotic insect 

infestation (Gandhi and Herms 2010) or disease 

(Dahir and Lorimer 1996). While these deaths may 

increase variability of horizontal stand structure, the gap 

dynamics associated with insect- or disease-created gaps 

differ from those created by windthrow or tree senescence 

in that pest-associated gaps may result from the death of 

only a single tree species, and resulting snags may remain 

standing longer than trees killed by wind or senescence 

(Dahir and Lorimer 1996, Gandhi and Herms 2010). 

Insect infestations can also alter dead wood dynamics 

by increasing the amount and species composition of 

standing and down dead wood as trees are killed by the 

invaders (Gandhi and Herms 2010).

The approach to managing invasive species is complex. 

Early detection and removal is critical in many cases 

(Webster et al. 2006). Consequently, invasive species 

should be evaluated as part of every site assessment to 

help identify problems early. In addition, collaborative 

efforts across ownerships are often necessary due to the 

landscape-scale nature of the problem (Webster et al. 

2006). Once invasive species are established, eradication 

can be expensive and difficult (Webster et al. 2006), if it 

is even possible, and impacts may take a very long time 

to overcome (Frelich et al. 2006).

Figure 8.—European swamp thistle (Cirsium palustre) and Rubus in a gap at the 

Highlands site of the managed old-growth silvicultural study in the fourth growing season 

after the gap was created. Photo by WDNR staff, used with permission.
15

Climate Change

Climate change has the potential to create challenges 

with respect to meeting many types of management 

objectives, including increasing plant diversity and 

forest complexity/heterogeneity. For example, forest 

community composition may be altered by climate 

change (Iverson et al. 2011, Webb 1992) because 

individual tree species may be affected by, and react 

differently to, changing conditions (Scheller and 

Mladenoff 2008). As a result, successional trajectories 

may become difficult to predict (Swanston et al. 2011), 

thereby increasing the uncertainty in the outcomes of 

silvicultural activities6

. Impacts will likely be greatest on 

species currently at the edge of their range (Davis and 

Shaw 2001, Swanston et al. 2011). Regeneration may 

also become more difficult6

 and require active measures 

in areas where natural regeneration was previously 

sufficient. Furthermore, large older trees may be more 

susceptible to increased frequency and magnitude of 

wind disturbance due to their larger size (Hanson and 

Lorimer 2007), complicating efforts to increase large-tree 

representation within a stand.

Uncertainty regarding future climate conditions provides 

a substantial challenge in determining the best approach 

to incorporating potential impacts into management 

plans. Choosing prescriptions that incorporate 

flexibility and robustness are generally recommended 

(D’Amato et al. 2011, Dukes et al. 2009, Janowiak and 

Swanston 2012, U.S. Forest Service 2011). Swanston 

and Janowiak (2012) provide a clear framework for 

this decisionmaking process, with a menu of potential 

strategies and approaches and a workbook to help land 

managers incorporate potential climate change impacts 

into existing policies and plans. They emphasize the need 

to consider that, under climate change, management 

objectives may need to be changed or adjusted if 

challenges to meeting current objectives significantly 

reduce the feasibility of meeting those objectives. 

Peterson et al. (2011) suggest that instead of focusing on 

outcomes with a particular composition and structure, 

plans incorporating adaptation to climate change focus 

more on ecosystem services and ecological processes.

Example

Incorporation of climate change into forest management 

plans is being undertaken with several demonstration 

projects in northern Wisconsin in collaboration with 

the Shared Landscapes Initiative.7

 In upland hardwood 

areas of demonstration projects of the Bayfield Regional 

Land Conservancy8

 and The Nature Conservancy9, 

possible adaptation actions include creating refugia for 

eastern hemlock by maintaining high quality patches 

of the species where they occur, increasing species and 

age diversity using larger gap sizes in areas with active 

management, favoring eastern white pine and other 

better-adapted species where they occur either by 

releasing advanced regeneration or planting, and using 

shelterwoods or large group cuts to increase northern red 

oak (Quercus rubra) representation where there is already 

regeneration. These approaches are generally compatible 

with management strategies aimed at increasing 

compositional and structural complexity/heterogeneity, 

and a blending of these objectives (i.e., adaptation and 

complexity enhancement) may be necessary to address 

future uncertainties.

Pretreatment Evaluation

After determining the forest management goals for 

a stand, the next step in the process involves gaining 

information about the stand of interest. Inventories of 

forest stands have long been performed by foresters to 

gain information helpful for management. Timber cruises 

are an obvious example. When contemplating using 

active management to enhance structural complexity 

and compositional diversity, many of the forest 

characteristics measured are the same as in a traditional 

cruise; however, there are additional characteristics that 

6 Wisconsin initiative on climate change impacts, forestry 

working group report. Unpublished report. Available at http://

www.wicci.wisc.edu/report/Forestry.pdf (accessed March 19, 

2014).

7 http://www.sharedlandscapes.org/adaptation-projects.html 

(accessed March 10, 2014)

8 http://forestadaptation.org/node/224 (accessed March 10, 

2014)

9 http://climateframework.org/node/193 (accessed March 10, 

2014)
16

may be of interest. Volume, size, and species of dead 

wood, number and sizes of newer pre-existing gaps (i.e., 

younger gaps that might appear/function similarly to any 

gaps created through management), and diversity of light 

environments are some examples. The exact variables 

to measure depend on specific objectives for the stand, 

what information may be available for comparison from 

reference stands (as discussed above), and amount of 

time available. Characteristics noted in Table 1 provide 

some examples for consideration. In addition, if there 

are specific wildlife objectives, such as increasing habitat 

for northern flying squirrels (Glaucomys sabrinus), then 

surveys of particular habitat features—such as the number 

of cavity trees, presence of mast species (such as northern 

red oak), or percentage of basal area in conifers (Wisconsin 

Department of Natural Resources 2013)—may be desired.

Characteristics measured in the pretreatment inventory 

will provide information from which initial treatments 

can be developed, and which will help determine the 

feasibility of meeting desired objectives. In addition to 

collecting information on features that are directly related 

to objectives, such as those listed above, collecting data 

related to deer browsing (e.g., Brose et al. 2008, Frelich 

and Lorimer 1985, Witt and Webster 2010), presence 

and prevalence of invasive species, and any special notes 

related to susceptibility or opportunities associated with 

climate change may also be advisable.

Example

In MOSS, the pretreatment evaluation included 

measurement of overstory vegetation, canopy 

characteristics, pre-existing gaps, site index, habitat type, 

understory characteristics, and downed woody debris.

Measurements made of overstory vegetation included 

species, d.b.h., and suppression class (suppressed or not 

suppressed) of all live trees ≥10 cm d.b.h. In addition 

species, d.b.h., snag height class, and snag decay class 

were noted for all self-supporting dead trees that were at 

least breast height tall and ≥10 cm d.b.h. The presence 

of cavities and hollows were also recorded. From 

these variables, the following stand characteristics were 

determined: stand basal area, basal area of individual 

species, diameter distribution of the overstory, average 

d.b.h. of live trees, average d.b.h. of snags, live standing 

tree volume, and the stem density (i.e., trees/area) of live 

trees, snags, cavity trees, and trees with hollows.

Canopy characteristics measured included percentage of 

canopy closure and light transmission (direct, diffuse, 

and total). The size class and frequency of newer preexisting gaps were also inventoried using line transects 

(Table 4). Site index was determined for sugar maple 

only, as it was the dominant tree species. Site index 

for other species was determined from sugar maple site 

index using conversion equations from Carmean (1979). 

Habitat type for each site was characterized according to 

Kotar et al. (2002). Understory measurements included 

number of stems per area by species in three size classes 

(0.10 to 0.49 m tall, 0.5 m tall to 1.9 cm d.b.h., and 

2.0 to 9.9 cm d.b.h.) for woody vegetation. In addition, 

percentage of cover for all herbaceous understory 

vegetation together was measured, as was percentage of 

cover for six individual native and exotic invasive species 

of special interest. Measurements for large downed 

woody debris included volume of stumps and root flares, 

and volume of large downed woody debris other than 

stumps and root flares (Fig. 9).

Methods used, and summaries of values determined, 

from the MOSS pretreatment evaluation are reported in 

Fassnacht et al. (2013).

Treatment Development

Once targets are determined and an initial pretreatment 

survey has been completed, development of treatments 

can begin. A comparison of values measured in the 

inventory with targets from reference sites will highlight 

how far current stand conditions are from the desired 

goal. If current and future desired conditions are 

substantially different, the time to achieve goals may 

need to be extended, targets revised, or other alterations 

made to initial plans, such as number of entries required 

to achieve desired goals. Furthermore, data collected 

during the inventory can identify additional activities 

that may be necessary to improve chances of success, 

such as measures to control or remove invasive species or 

to reduce deer populations or impacts.
17

Treatment development also includes a consideration of 

finer details associated with desired targets. For example, 

if ~10 percent of the stand area is to be converted to new 

gaps, what size gaps should be used and how should they 

be distributed across the stand? Given a desired residual 

basal area, how will this target be met? That is, what 

types of trees should be marked and in what size classes? 

If a certain volume of dead wood is an objective, what 

does that translate into with respect to numbers and sizes 

of trees?

Previous research efforts can help address some of 

these questions and provide insights into special 

considerations. For example, a number of studies have 

evaluated gap size with respect to effects on species 

diversity, some specifically examining impacts on 

mid-tolerant species such as yellow birch (Bolton and 

D’Amato 2011; Kern et al. 2013; Shields et al. 2007b; 

Webster and Lorimer 2002, 2005). These studies suggest 

that site quality and current species composition are 

important in determining the effectiveness of canopy 

gaps for increasing representation of mid-tolerant tree 

species. They also give guidance on the most appropriate 

size of gaps and the need for additional management to 

support objectives.

On moderately-rich, mesic sites, which are less likely to 

be sugar maple dominated, larger gaps appear to have 

the potential to increase representation of mid-tolerant 

species such as yellow birch. For example, working in a 

hemlock-hardwood forest, Webster and Lorimer (2002) 

found that gaps >400 m2

 (~23-m diameter if circular) 

favored yellow birch over eastern hemlock, with yellow 

birch generally making up more than 40 percent of 

dominant and codominant gap saplings for gaps 16 to 

55 yr old (Webster and Lorimer 2005). Sugar maple was 

present in these gaps but contributed less to relative stem 

densities compared to yellow birch.

Table 4.—Number of pre-existing gapsa by stand and size 

class (modified from Fassnacht et al. 2013)

Site/canopy treatmentb

Sizec [m]

10.7 to 18.3 18.3 to 24.4 24.4+

Flambeau 

 Control 3 0 0

 Small gaps 3 1 0

 Large gaps 1 0 0

 Irregular multi-cohort 2 0 0

Highlands 

 Control 0 0 2

 Small gaps 4 0 0

 Large gaps 2 1 1

 Irregular multi-cohort 10 1 0

Argonne 

 Control 16 1 0

 Small gaps 34 3 2

 Large gaps 17 3 1

 Irregular multi-cohort 24 3 0

a For a gap to be included in the survey, woody regeneration in the 

gap was required to average ≤2 m tall.

b Canopy treatments are defined on page 7.

c Diameter ranges of round gaps (35 to 60 ft, 60 to 80 ft, and 80+ ft 

in original units). Diameters were allowed to have somewhat smaller 

minor axes in the case of elliptical gaps. Dimensions for elliptical 

gaps to qualify to be a “10.7-m diameter” gap were 9.14 x 12.5 m; 15.51 

x 21.64 m for a “18.3-m” gap; and 21.03 x 28.35 m for a “24.4-m” gap. 

Figure 9.—Measuring downed woody debris. Photo by Brian 

Werner, WDNR, used with permission.
18

On richer sites, which are more likely to be sugar maple 

dominated and to have abundant sugar maple advanced 

regeneration, it is not clear that creating canopy gaps 

is effective in increasing representation of mid-tolerant 

species without supplemental management (Bolton 

and D’Amato 2011, Kern et al. 2013, Shields et al. 

2007b). In one study, regeneration failure for all species 

was reported for larger gaps (i.e., >10-m diameter), 

where larger gaps were dominated by Rubus species 12 

yr after gap establishment (Kern et al. 2013). Other 

studies have reported increased stem densities of yellow 

birch seedlings, saplings, or both, in gaps compared to 

closed-canopy forest, but yellow birch was substantially 

less abundant than sugar maple (Bolton and D’Amato 

2011, Shields et al. 2007b). Yellow birch also did not 

necessarily differ in density among gap sizes (Shields et al. 

2007b) and likely had insufficient densities to maintain 

its abundance in the overstory at historical levels (Shields 

et al. 2007b).

Factors found to be associated with yellow birch 

occurrence on these richer northern hardwood sites 

included microsite factors (e.g., exposed mineral soil 

and coarse woody debris, especially large, well-decayed 

conifer coarse woody debris; Bolton and D’Amato 2011, 

Shields et al. 2007b) and location within the gap (more 

birch at the edge; Shields et al. 2007b). Competition 

with advanced regeneration of shade-tolerant species also 

appeared to be an important barrier to increasing yellowbirch representation (Bolton and D’Amato 2011).

On these richer northern hardwoods sites, consequently, 

additional management activities may be necessary, 

beyond simply creating canopy gaps, to improve the 

likelihood of increasing representation of mid-tolerant 

species. Examples include creation of gaps around seed 

trees; scarification or pulling over of trees to expose 

mineral soil and create tip-up mounds; addition of 

conifer downed wood; periodic enlarging of gaps; 

removal of advanced regeneration from competing 

shade-tolerant species such as sugar maple; underplanting 

desired species in gaps; application of herbicides 

or physical removal of competing shrubs to release 

established regeneration; and erecting fencing to exclude 

deer (Bolton and D’Amato 2011, Kern et al. 2013, 

Shields et al. 2007b, Webster and Lorimer 2002).

Prior to finalizing treatments, land managers should 

consider modifications that may be necessary due to 

the contemporary stressors discussed previously. For 

example, for sites near the southern edge of the range of 

underrepresented species such as yellow birch, it may be 

prudent to adjust a management plan such that targeted 

mid-tolerant species include those better adapted to 

projected future climate conditions and stresses, such 

as northern red oak or black cherry (Prunus serotina) 

(Iverson et al. 2011).

Example

Information gained during the pretreatment evaluation 

phase of MOSS resulted in several adjustments to the 

initial targets for new CWD creation. As described 

above, the initial target for the MOSS CWD treatment 

was 70 percent of the larger, less-decayed standing and 

downed dead wood found in Sylvania Wilderness Area 

(the reference site used for dead wood). In addition, 

all trees to be girdled, or felled and left on site, initially 

were to be ≥40.6 cm d.b.h. Based on data from the 

pretreatment stand inventories, it became apparent that 

there were not enough large trees that could be marked 

to meet proposed CWD targets while still permitting the 

removal of sufficient volume to ensure economic viability 

of the harvest. Economic viability was an important 

aspect of the prescriptions being developed in order 

to increase the likelihood of broader adoption of the 

methods should desired ecological outcomes be achieved. 

To accommodate the concerns raised by the pretreatment 

assessments, three modifications were made to the 

original targets. First, desired CWD targets were reduced 

from 70 to 65 percent of the larger, less decayed dead 

wood found in the Sylvania Wilderness Area reference 

site. Second, the size of trees to be felled and left, or 

girdled, was modified to include smaller trees. The 

assessment found that two sites had very few trees 

≥40.6 cm d.b.h.; consequently the desired size class for 

CWD was reduced to ≥30.5 cm d.b.h. for those sites. In 

addition, required sizes of newly added CWD for all sites 

were spread over three size classes per site rather than just 

one (i.e., ≥40.6 cm d.b.h. or ≥30.5 cm d.b.h., depending 

on stand), with the new smallest size class being 25.4 

to 30.2 cm d.b.h. for all sites. The third adjustment 
19

allowed girdled trees to be counted toward both standing 

dead wood quotas as well as downed dead wood quotas 

(since the newly created snags would eventually fall and 

become part of the downed wood pool). This adjustment 

had ecological, economic, and logistic advantages. 

Ecologically, counting girdled trees toward our downed 

wood total would allow us to effectively add downed 

wood over time, rather than in one large pulse. With 

this approach there would be a smaller initial pulse of 

downed wood added with the trees that were felled and 

left on site, and then additional wood would be added 

over time, in various states of decay, as the girdled trees 

fell. Economically, this adjustment reduced the total 

number of trees needed to meet added CWD targets, 

thus reducing the opportunity costs associated with 

this treatment. From a logistical perspective, it was 

more straightforward to have CWD-tree targets evenly 

split between standing and downed wood at each site, 

than to have very different requirements for downed 

and standing dead wood, potentially varying in relative 

proportion among sites. To facilitate marking in the 

field, desired downed wood volumes were converted to 

number of trees per area by applying a single volume-totree conversion factor to calculated volumes of downed 

wood needed to meet targets.

Information regarding the development and 

implementation of other MOSS treatments can be found 

in Fassnacht et al. (2013).

Treatment Implementation

There are many potential questions associated with 

treatment implementation. How will the location of gaps 

be determined in a stand? How will gaps be marked? 

How will “CWD trees” be distributed within a stand? 

How will CWD trees be selected? These questions may 

be best addressed using trials in a small portion of the 

stand to be managed. Different methods can be tried and 

compared to determine which best meets project needs 

and objectives. Initial implementation of treatments can 

also serve as a trial of sorts, potentially highlighting areas 

of the management plan still in need of modification. 

Over time, experience gained by a land manager or by 

others, with results reported in the literature, will reduce 

the need for trials and in-process modifications.

Example

Developing the CWD treatment in MOSS was an 

iterative process. After the adjustments described above 

had been made, one additional adjustment was required 

after marking had started. For two treatment areas, the 

total amount of new CWD to be created under the 65 

percent-of-reference target was simply more than could 

be accommodated while maintaining an economically 

viable timber sale. These two sites had the lowest amount 

of pre-existing CWD, consequently they required the 

largest amount of wood to be added to reach target 

dead-wood quantities (Fassnacht et al. 2013). The last 

CWD-treatment adjustment, made for these two sites 

only, was to extend the time period over which the 65 

percent-of-reference target was to be met to include more 

than one entry. As a result of this adjustment, amounts 

of new dead wood required for the two treatment areas 

was changed for this initial entry to be more similar to 

volumes required for the other study treatment areas 

(Fassnacht et al. 2013).

Treatment Evaluation

Once treatments have been implemented, evaluation 

and monitoring are important steps to determine 

progress toward desired targets. Monitoring allows for 

an assessment of current management practices and has 

the potential to highlight the need for alterations in 

the management plan. Important questions to consider 

include: How does one define success? What kind of 

monitoring is necessary? What variables need to be 

monitored? What time frame should be considered 

for monitoring and reaching targets? Are there any 

unanticipated consequences of the active management?

Treatments can be evaluated for successful 

implementation as well as for effectiveness. Success 

in implementation refers to the degree to which 

targets are met for variables directly manipulated by 

management. For example, in the Vermont study of 

structural complexity enhancement (SCE) (Keeton 

2006), post-treatment diameter distributions were 

compared to the target rotated sigmoid distribution 

and found to be statistically indistinguishable (Keeton 

2005). Implementation of this aspect of the treatment, 

therefore, could be considered successful. Effectiveness, 
20

in contrast, refers to the degree to which targets are met 

for variables indirectly impacted by variables manipulated 

by management. For example, if manipulating forest 

structure as prescribed is intended to improve the habitat 

for high priority bird species or bird species that serve 

as indicators of old-growth forest, and increases in those 

bird species are observed, the prescribed management 

may be considered to be effective.

Implementation monitoring is valuable because if 

eventual outcomes do not meet expectations, it is possible 

to know whether the problem was that the treatment 

was not implemented as prescribed or was not effective 

as prescribed. Effectiveness monitoring is critical to 

improving management strategies when outcomes are not 

as expected. For example, Bolton and D’Amato (2011) 

found that that creating canopy gaps within the range of 

variability of those found in older northern hardwood 

systems was insufficient on its own to increase tree species 

diversity and promote mid-tolerant tree species such as 

yellow birch on nutrient-rich, sugar-maple-dominated 

sites in Minnesota. Effectiveness monitoring allowed the 

authors to provide suggestions for additional management 

activities that might improve the prescription, including 

the addition of large conifer downed wood, scarification, 

and removal of advanced regeneration of competing 

shade-tolerant species such as sugar maple.

Variables to monitor include those that were manipulated 

directly, as well as the indirect outcomes identified as 

objectives for the project. The time frame for monitoring 

will depend on the type of monitoring (implementation 

or effectiveness) and objectives. Implementation 

monitoring will be relatively short term, i.e., the same 

time frame as treatment implementation. Effectiveness 

monitoring may take place over years, decades, or 

potentially centuries, depending on objectives.

Effectiveness monitoring done soon after treatment 

implementation may be of interest but may not 

demonstrate the full effectiveness of the treatments 

since plant and animal populations can take much more 

time to respond. For example, field trials in New York 

evaluating regeneration responses to shelterwoods cuts 

using a chronosequence found yellow birch seedlings on 

less than a third of the plots measured in a stand 2 yr 

after treatment, and among none of the tallest saplings 

measured (Kelty and Nyland 1981). In contrast, in 

stands 6 and 10 yr after treatment, yellow birch was 

found on more than three quarters of the plots, and was 

the tallest on 40 percent of them.

If objectives are more economically focused, the 

effectiveness of some aspects of treatment can be evaluated 

on the scale of 10 to 15 yr. For example, Kern et al. (2013) 

considered the gap treatments installed on a nutrientrich, sugar maple-dominated site to be regeneration 

failures after 12 yr due to the high levels of Rubus species 

and unacceptably low levels of tree saplings in the gaps. 

From a purely ecological perspective, in contrast, if 

sufficient numbers of trees were able to overtop the Rubus

species after 20 or 30 yr and eventually capture the gap, 

the gaps could potentially still be considered effective.

Evaluation of treatments also includes monitoring 

changes taking place in forest structure to determine the 

timing of future treatment manipulations. For example, 

with MOSS the regeneration in modified shelterwood 

areas is being monitored to determine the appropriate 

time to remove a portion of the remaining overstory.

Example

For MOSS, an initial post-treatment cruise of areas 

receiving different cutting intensities was conducted 

to determine residual basal area. Comparing observed 

and target residuals, some substantial differences were 

noted (Table 5). Additional implementation monitoring 

found that, in most cases, error was incurred primarily 

in the marking phase of implementation rather than 

in the harvesting of marked trees.10 Further analyses 

of data associated with the harvest is estimating logger 

error rates for trees receiving different prescriptions 

and is evaluating potential contributing factors to those 

errors.10 In another implementation evaluation effort11, 

treatments are being assessed with respect to their success 

10 Manuscript in preparation: Steele, T.W.; Fassnacht, K.S. 

Comparison of prescribed versus observed outcomes of timber 

harvests in northern hardwood forests.

11 Manuscript in preparation: Fassnacht, K.S.; Steele, T.W. 

Snag dynamics in northern hardwood forests under different 

management scenarios.
21

in enhancing standing dead wood, as well as determining 

potential factors related to snag longevity (for natural 

snags) and the transition between live and standing dead, 

and standing dead and down (for girdled trees only).

Early effectiveness monitoring has been initiated for 

MOSS as well. One study has compared tree regeneration 

and groundlayer abundance and diversity in the different 

treatment areas (Reuling 2014). Results from this study 

suggest that, from this early perspective, there may be 

some challenges in meeting study objectives, particularly 

with respect to species diversity. While stem densities in 

smaller size classes (<2.0 cm d.b.h.) increased in treatments 

compared to controls, the largest gains were in shadetolerant species, providing little progress towards species 

diversity goals. The study also found that groundlayer 

vegetation was richer in small gaps due to contributions 

from forest interior species. In treatments creating 

more open canopies, however, enhanced diversity and 

richness was due to greater numbers of exotic invasives 

and disturbance-related species, both of which may have 

negative impacts on desired tree regeneration. In addition, 

comparison of fenced and unfenced areas found that while 

regeneration stem density generally was not impacted by 

deer browsing 3 to 4 yr after treatment, yellow birch did 

benefit from deer exclusion. This suggests that browsing 

may have the potential to hinder the ability of treatments 

to meet species diversity objectives for some species.

In a second study, habitat use and nesting patterns 

of southern flying squirrels (Glaucomys volans) were 

compared among treatments in the first 2 yr after harvest 

(Steinhoff 2010, Steinhoff et al. 2012). Results showed 

that all management strategies provided adequate squirrel 

habitat. There were some differences in nest use among 

treatments, however, with more nest switching in the 

Table 5.—Comparison of managed old-growth silvicultural study (MOSS) target residual basal 

areas with values derived from an initial post-treatment cruise for different canopy treatments, 

cutting zones, or both, within canopy treatments

Canopy treatment/site

No. sample 

points

Observed 

basal area

Target basal 

area 

Absolute 

difference

Percentage 

difference

------------------------- m2/ha -----------------

Small gaps

 Flambeau 38 18.8 19.5 -0.7 -4

 Highlands 36 20.0 19.5 0.5 2

 Argonne 41 22.0 20.0 2.1 10

Large gaps

 Flambeau 38 24.6 19.5 5.1 26

 Highlands 34 21.6 19.5 2.1 11

 Argonne 38 23.4 20.0 3.4 17

Irregular multi-cohort

Light thin

 Flambeau 11 23.9 20.1 3.2 16

 Highlands 14 25.3 22.3 3.0 13

 Argonne 9 23.4 23.0 0.5 2

Heavy thin

 Flambeau 24 23.7 18.4 5.3 29

 Highlands 13 21.1 20.2 0.9 5

 Argonne 14 23.4 20.7 2.8 13

Shelterwood

 Flambeau 36 14.0 13.8 0.2 2

 Highlands 37 10.6 13.8 -3.2 -23

 Argonne 36 14.9 13.8 1.1 8
22

small gaps treatment compared to the irregular multicohort treatments. Nest switching in the irregular multicohort treatment did not differ from that in the control

stands, however. Moderate levels of nest switching are 

expected for southern flying squirrels to help reduce 

nest parasite loads and to access resources at other nest 

sites (Steinhoff et al. 2012). Low levels of switching can 

indicate low parasite loads, abundant resources at the 

nest site location, or a lack of suitable nest trees. The 

latter reason was suspected as a factor in the lower nest 

switching rate of the irregular multi-cohort treatment, 

with heavy cutting in some areas reducing number of 

available nest sites. Additional studies over time will need 

to evaluate habitat use and nesting patterns as the treated 

stands continue to develop.

Timeline

One of the considerations in the decision to use active 

forest management to accelerate the development of oldgrowth characteristics is the time available to achieve the 

desired conditions. How long will it take a given stand to 

achieve target conditions in the absence of management, 

and how does that compare to what could be achieved 

through active management? How far might the initial 

harvest move a stand toward project goals?

In a modeling study, Choi et al. (2007) found that a 

77-yr-old even-aged northern hardwood stand receiving 

no management took an additional 79 yr (stand age 

156 yr) to reach minimum old-growth criteria. Multiple 

entries of moderately-heavy thinning in this stand 

reduced the time to meet old-growth criteria to 36 yr 

(stand age 113 yr). In contrast, a second-growth northern 

hardwood stand that was already uneven-aged was 

projected to reach old-growth thresholds in only 12 yr in 

the absence of management. Moderately-heavy thinning 

increased the time to reach the old-growth threshold 

to 20 yr due to the need to remove larger trees to meet 

the thinning targets. Using moderate thinning instead 

of moderately-heavy thinning in this second example 

reduced the time to reach old-growth criteria compared 

to the no-treatment alternative by only 2 yr. These results 

suggest that the time to achieve structural goals will not 

necessarily be decreased with active management for all 

northern hardwood stands. Furthermore, they suggest 

that somewhat younger, even-aged stands may offer 

the best opportunity to shorten the time to meet goals 

through active management.

Example

The MOSS treatment stands were evaluated before and 

after treatment relative to basic minimum structural criteria 

required to attain old-growth characteristics as defined 

by Choi et al. (2007) (Table 6). Values were calculated 

individually for each treatment area (i.e., half stands for 

active treatments, whole stands for controls) to illustrate the 

degree to which results might vary depending on initial 

conditions. Results showed that none of the treatment 

areas would meet minimum thresholds before treatment, 

although several were close. After the initial harvest, 5 

of the 21 treatment areas met minimum conditions. 

Additional analyses (not shown) suggest that for three of 

the five areas, artifacts of the sampling strategy may have 

increased calculated basal areas in large trees somewhat, 

contributing to perhaps a premature determination that 

the thresholds were reached. Furthermore, analyses using 

only slightly different thresholds (defined by Hanson 

et al. 2012; not shown) resulted in two stands meeting 

criteria before treatment and six treatment areas meeting 

criteria after treatment, highlighting the influence that 

differences in definitions of old-growth characteristics 

can have on outcomes. Nonetheless, using either set of 

criteria, the harvest treatments moved stands closer to 

meeting old-growth conditions, and in some cases helped 

stands meet basic minimum structural conditions after a 

single entry. With stands meeting basic minimum criteria 

identified, additional variables can be evaluated (e.g., 

volume of downed wood, measures of vertical structure), 

to determine whether these stands have attained a more 

complete suite of characteristics associated with oldgrowth forests.

Levels of Implementation

The concepts described in this report can be applied to 

forest stands under a variety of ownerships, both public 

and private, and with a range of primary management 

objectives, from ecological to economic. Because 

techniques to increase forest structural complexity include 

a number of different strategies, the degree to which they 

may be implemented can vary with landowner objectives 
23

and available resources. Even forests managed largely for 

timber production can incorporate some techniques (e.g., 

retaining some legacy trees) ( Zasada et al. 2004) which 

can increase ecological complexity with relatively minor 

losses in timber production (Hanson et al. 2012).

Modeling studies, such as those by Hanson et al. (2012), 

can provide useful data for informing decisions regarding 

how to best balance ecological and economic objectives. 

In this study, modeled results from 22 different 

manifestations of ecological forestry techniques were 

compared to single-tree selection and to a no-treatment 

control. The ecological forestry techniques ranged 

from simple actions, such as increasing the maximum 

residual tree diameter, to more complex combinations of 

treatments, such as multi-cohort harvests in combination 

with retention of all yellow birch and eastern hemlock 

and an increased maximum residual tree diameter. 

Results showed varying degrees of impact on timber 

production, from a 9 percent long-term reduction in 

yield associated with permanently retaining 7 trees/

ha, to a 55 percent reduction in yield associated with 

Table 6.—Comparison of stratum-weighted managed old-growth silvicultural study stand characteristics 

with minimum criteria for old-growth status as defined in Choi et al. (2007) before and after initial treatments. 

Highlighted cells have surpassed minimum criteria.

Pretreatment Post-treatment

Canopy treatmenta/

site – CWD trta

Total BAb

[m2/ha]

Percent BA in 

trees ≥26 cm 

d.b.h.

Percent BA in 

trees ≥46 cm 

d.b.h.

Total BA 

[m2/ha]

Percent BA in 

trees ≥26 cm 

d.b.h.

Percent BA in 

trees ≥46 cm 

d.b.h.

Min. OG thresholdc >20 >80 >50 >20 >80 >50

Control

 Flambeau 30 67 20 32 71 22

 Highlands 35 82 39 35 84 40

 Argonne 37 85 35 37 86 38

Small gaps

 Flambeau - low 29 63 3 24 76 5

 Flambeau - high 31 65 10 25 77 14

 Highlands - low 32 76 31 25 83 42

 Highlands - high 32 71 16 25 78 20

 Argonne - low 30 85 49 24 89 56

 Argonne - high 31 83 32 25 86 41

Large gaps

 Flambeau - low 33 80 42 28 86 53

 Flambeau - high 34 83 46 29 88 55

 Highlands - low 31 75 16 22 79 25

 Highlands - high 32 83 45 24 87 54

 Argonne - low 34 79 30 24 88 40

 Argonne - high 34 88 33 23 94 46

Irregular multi-cohort

 Flambeau - low 32 63 8 25 77 12

 Flambeau - high 31 58 8 24 71 10

 Highlands - low 28 77 31 23 82 39

 Highlands - high 33 79 35 25 86 45

 Argonne - low 36 90 47 26 95 61

 Argonne - high 31 82 27 24 89 41

a trt = treatment; CWD = coarse woody debris; canopy and CWD treatments are defined on page 7. 

b BA = basal area

c

 Min. = minimum; OG = old-growth
24

a combination of CWD creation (75 percent of oldgrowth levels), increased maximum residual diameter (to 

80 cm d.b.h.), and group selection. Results also showed 

varying degrees of success with respect to achieving 

ecological goals. For example, none of the treatments 

provided similar volumes of CWD compared with old, 

unmanaged forests. The CWD retention treatments, 

however, did double CWD volume over single-tree 

selection, and treatments retaining all yellow birch and 

eastern hemlock performed similarly to CWD retention 

treatments after approximately 110 yr. Likewise, while 

most treatments, including group selection and multicohort management, performed poorly relative to species 

diversity goals, treatments including retention of all eastern 

hemlock and yellow birch significantly increased basal area 

representation of these species compared to alternatives.

With information regarding tradeoffs from modeling 

studies as described above, or from field trials, as described 

briefly earlier, discussions can be undertaken regarding 

the levels of implementation appropriate for a given 

property. For example, large blocks of forest on national 

forests may be the most suitable for implementing the 

full suite of techniques to enhance forest structure. Other 

public lands under state or county ownership might be 

well-suited to incorporating an intermediate level of 

implementation, allowing for a moderate level of ecological 

benefits while increasing revenue from forest products. On 

private ownerships focused on timber production, or for 

small parcels of land, incorporating a few, more modest, 

ecological forestry techniques may represent the best 

balance of considerations.

SUMMARY

Active management techniques that emulate natural 

forest disturbance and stand development processes have 

the potential to enhance species diversity and structural 

complexity in managed forests, helping to meet goals 

related to biodiversity, ecosystem health, and forest 

resilience in the face of uncertain future conditions. In 

considering the use of active management to enhance 

composition and structure in northern hardwood forests, 

it is important to clearly define what reference conditions 

are to be used, and the degree to which management 

will seek to meet those reference conditions. Specific 

initial targets can then be identified. Modification 

of these targets may be necessary to accommodate 

contemporary stressors including herbivory, invasive 

species, and climate change. A pretreatment evaluation 

of the stand(s) being considered for management will 

provide information from which initial treatments 

can be developed, and which will help determine the 

feasibility of meeting desired objectives. Additional 

adjustments to targets may be needed at this time. Pilot 

implementation of targets being considered can be 

useful in refining techniques and identifying additional 

adjustments to targets that may be necessary prior to full 

implementation of the prescription. Finally, evaluation 

and monitoring of the treatments, both for success of 

their implementation as well as for effectiveness, is critical 

for learning what works and what does not, allowing 

future treatments to be modified as part of an adaptive 

management process. It is our belief that the process 

described in this report can be used to determine which 

techniques, from the suite that are available to enhance 

forest structure, might be appropriate for a given forest 

and set of management objectives, thereby helping to 

increase the ecological complexity of managed northern 

hardwood forests.

ACKNOWLEDGMENTS

Funding for the managed old-growth silvicultural study 

(MOSS) was provided by the U.S. Fish and Wildlife 

Service Wildlife Restoration Fund (Grant W-160-P); the 

Wisconsin Department of Natural Resources, Division 

of Forestry; the U.S. Department of Agriculture, Forest 

Service, Northern Forest Research Station; the University 

of Wisconsin-Madison; McIntire-Stennis Fund 

(WIS04684); the Connor Hardwood Research Fund; 

and the University of Minnesota-Twin Cities Natural 

Resources Science and Management Graduate Programs 

and Department of Forest Resources. The authors wish 

to acknowledge Tom Steele for his contributions to 

MOSS study design, data analysis, and evaluation. The 

authors thank Brad Hutnik for stimulating discussion 

about basic minimum structural criteria required to 

attain old-growth characteristics, and Sarah Herrick, 

Drew Feldkirchner, and two anonymous reviewers for 

providing helpful comments on earlier versions of the 

manuscript.
25

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Chapter 19.
32

GLOSSARY

active management – management consisting of any 

purposeful activity in a stand or area

basal area – cross sectional area of an individual tree at 

breast height (tree basal area), or the cumulative cross 

sectional area at breast height of trees in a specified size 

range within an area of interest (plot basal area, stand 

basal area, etc.); stand basal area is commonly reported in 

ft2

/ac or m2/ha; see diameter at breast height below for a 

definition of breast height

biodiversity – biological diversity (see definition for 

diversity below)

coarse woody debris [CWD] – large standing and 

downed woody debris

cohort – group of trees which became established moreor-less at the same time

diameter at breast height [d.b.h.]– the diameter of 

a tree measured 1.37 m (4.5 ft) above the point of 

germination; in Canada, breast height is 1.3 m above the 

point of germination

diversity – the condition of having or being comprised 

of differing elements12

even-aged – stand of trees composed of a single age class 

in which the range of ages is usually not more than 20 

percent of the potential rotation age

hollow – as defined for the managed old-growth 

silvicultural study, a cavity that touches the ground, is due 

to rot, provides overhead protection from precipitation, 

and is ≥10 cm wide and ≥10 cm deep (Fassnacht et al. 

2013)

legacy tree – a tree retained (permanently or for a long 

period) in a stand after harvest; these trees increase 

structural and age diversity of a stand harvest, can act as a 

seed source, and can act as a refuge from which plant or 

animal species can either repopulate a stand after harvest or 

maintain populations despite harvest (e.g., northern flying 

squirrel; Carey 1995) (Lindenmayer and Franklin 2002)

maximum residual d.b.h. – diameter at breast height of 

the largest tree(s) to be left in a stand after harvest; all trees 

larger than the maximum residual d.b.h. may be harvested

mid-tolerant tree species – tree species requiring 

intermediate levels of light for adequate survival and 

growth

natural rotation period – “Mean time needed to disturb 

an area equivalent to the study area (the study area is 

arbitrarily defined; some sites may be disturbed several 

times in this period and others not at all – thus ‘study area’ 

must be explicitly defined)”.13 Natural rotation period is 

most often not equivalent to “return interval,” which refers 

to the average interval between consecutive events on the 

same site. Natural rotation period equals return interval 

only in the case where the magnitude of the event is 

catastrophic, such that it disturbs an area equivalent to the 

entire study area in a single event. In all other cases (i.e., 

where events disturb an area smaller than the equivalent 

of the entire study area), more than one event will be 

required in order to disturb an area equivalent to the 

study area, and therefore natural rotation period > return 

interval.

niche – a combination of biotic (such as plant species 

composition, and structure) and abiotic (such as 

temperature and moisture) factors making up the 

environment required by an organism

northern hardwood forest – forest dominated by a 

combination of sugar maple, American basswood (Tilia 

americana), yellow birch, white ash, and American beech 

(Fagus grandifolia); associated species may include red 

maple (Acer rubrum), eastern hemlock, balsam fir (Abies 

balsamea), northern red oak, and eastern white pine 

(Wisconsin Department of Natural Resources 2006)

12 www.merriam-webster.com/dictionary/diversity (accessed 

February 12, 2014)

13 www.biol.ttu.edu/faculty/mncintyre/Landscape%20Ecology/

disturb_defns_scales.pdf (Accessed February 12, 2014)
33

passive management – management consisting of 

purposeful inactivity in an area (i.e., leaving it alone)

shifting mosaic steady state – a condition of a forest 

stand or landscape is composed of a mosaic of cohorts

of different ages, with each cohort occupying an 

approximately equal area; while each cohort changes over 

time, the mosaic itself is stable with respect to properties 

such as size distribution, species composition, and biomass 

(see definition for steady state below)

silvicultural techniques – forest management practices 

that manipulate the establishment, growth, composition, 

quality, and health of forest stands

spatial heterogeneity – in relation to species 

composition and structural elements, reflects their 

non-uniform spatial arrangement in a stand, creating a 

diversity of conditions and habitats that vary across space 

steady state – a state, such as composition, structure, or 

biomass which does not change substantially over time

structural complexity – characterized by having many 

parts in an intricate arrangement14

structure – “the vertical and horizontal arrangement of 

plants, dead and alive”15; in reference to stand structure 

– pertaining to the array of elements directly or indirectly 

derived from trees that occur in a diversity of sizes (e.g., 

diameters, downed logs), conditions (e.g., cavities, tip-up 

mounds) and configurations (e.g., multiple age cohorts); 

best conceived relative to a simplified condition (e.g., 

narrowly even-age, lacking large deadwood, etc.).

supercanopy trees – trees extending above the main 

canopy of a stand; these trees are typically remnants 

of the stand that existed before the stand-replacing 

disturbance which initiated the current stand

tip-up mounds – mounds left by the decaying root wads 

of trees that have fallen over due to uprooting; associated 

with pits, next to the mounds, from where the root wads 

used to be

uneven-aged – a forest stand with at least three distinct 

age classes, either intimately mixed or in small groups

14 http://en.wikipedia.org/wiki/Complexity (accessed February 

12, 2014)

15 http://northernwoodlands.org/articles/article/what-is-foreststand-structure-and-how-is-it-measured/ (accessed February 

12, 2014)
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Fassnacht, Karin S.; Bronson, Dustin R.; Palik, Brian J.; D’Amato, Anthony W.; Lorimer, 

Craig, G.; Martin, Karl J. 2015. Accelerating the development of old-growth 

characteristics in second-growth northern hardwoods. Gen. Tech. Rep. NRS-144. 

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

Station. 33 p.

Active management techniques that emulate natural forest disturbance and stand 

development processes have the potential to enhance species diversity, structural 

complexity, and spatial heterogeneity in managed forests, helping to meet goals related 

to biodiversity, ecosystem health, and forest resilience in the face of uncertain future 

conditions. There are a number of steps to complete before, during, and after deciding 

to use active management for this purpose. These steps include specifying objectives 

and identifying initial targets, recognizing and addressing contemporary stressors that 

may hinder the ability to meet those objectives and targets, conducting a pretreatment 

evaluation, developing and implementing treatments, and evaluating treatments for success 

of implementation and for effectiveness after application. In this report we discuss these 

steps as they may be applied to second-growth northern hardwood forests in the northern 

Lake States region, using our experience with the ongoing managed old-growth silvicultural 

study (MOSS) as an example. We provide additional examples from other applicable studies 

across the region.

KEY WORDS: compositional diversity, structural complexity, spatial heterogeneity, 

Wisconsin, old-growth forest, active management, Lake States

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