United States

Department of 

Agriculture

Forest Service

Northern 

Research Station

General Technical

Report NRS-19

Natural Disturbance and 

Stand Development Principles 

for Ecological Forestry

Jerry F. Franklin

Robert J. Mitchell

Brian J. Palik
Visit our homepage at: http://www.nrs.fs.fed.us/

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November 2007 Fax: (740)368-0152

Abstract

Foresters use natural disturbances and stand development processes as models for 

silvicultural practices in broad conceptual ways. Incorporating an understanding of natural 

disturbance and stand development processes more fully into silvicultural practice is the 

basis for an ecological forestry approach. Such an approach must include 1) understanding 

the importance of biological legacies created by a tree regenerating disturbance and 

incorporating legacy management into harvesting prescriptions; 2) recognizing the role of 

stand development processes, particularly individual tree mortality, in generating structural 

and compositional heterogeneity in stands and implementing thinning prescriptions that 

enhance this heterogeneity; and 3) appreciating the role of recovery periods between 

disturbance events in the development of stand complexity. We label these concepts, 

when incorporated into a comprehensive silvicultural approach, the “three-legged stool” of 

ecological forestry. Our goal in this report is to review the scientific basis for the three-legged 

stool of ecological forestry to provide a conceptual foundation for its wide implementation. 

Manuscript received for publication 1 May 2007
1

INTRODUCTION

Foresters use natural disturbances and stand development processes as 

models for silvicultural practices in broad conceptual ways. For example, 

even-aged harvest prescriptions are often described as analogs for 

stand-replacement disturbances, such as intense wildfi res or windstorms 

(Smith et al. 1996). Individual tree and group selection practices are 

modeled on patterns of disturbance and mortality that involve the death 

of individual or small groups of trees within otherwise intact stands. 

Silvicultural thinning (from below) is designed to capture densitydependent mortality before it occurs naturally. Hence, disturbance 

regimes and stand development processes are the conceptual foundation 

for the core of silviculture. However, silviculturalists have only recently 

begun to look beyond the type, intensity, and scale of disturbances to 

the specifi c ecological conditions created by natural disturbances and 

stand development and to more fully incorporate these conditions into 

silvicultural prescriptions (Kohm and Franklin 1997).

Incorporating an understanding of natural disturbance and stand 

development processes more fully into silvicultural practice is the basis 

for an ecological forestry approach. Implementing such an approach 

successfully requires that prescriptions be founded on a conceptual 

basis that links stand disturbance and dynamics to the development 

and maintenance of ecological complexity of stands, as expressed in 

structure, composition, and heterogeneity of these features in space 

and time. The implementation and expression of ecological forestry 

concepts will vary in practice based upon specifi c goals for management, 

characteristics of tree species and ecosystems, variation in starting 

conditions of stands and sites, and landscape context. However, our 

premise in this report is that some fundamental principles for ecological 

forestry transcend systems, conditions, objectives, and context, and can 

be applied in varying degrees in virtually all settings where melding of 

ecological and economic goals is an objective.

JERRY F. FRANKLIN is a 

professor of ecosystem analysis at 

the College of Forest Resources, 

University of Washington, 

Seattle, WA.

ROBERT J. MITCHELL is a 

senior scientist at the Joseph E. 

Jones Ecological Research Center 

in Newton, GA.

BRIAN J. PALIK is an ecologist 

and project leader with the U.S. 

Forest Service, Northern Research 

Station.
2

Three fundamental principles have 

emerged from research on natural 

disturbance regimes and stand 

development processes, which form the 

basis of an ecological forestry approach. 

These include 1) understanding the 

importance of biological legacies created 

by a tree-regenerating disturbance and 

incorporating legacy management into 

harvesting prescriptions; 2) recognizing 

the role of stand development processes, 

particularly individual tree mortality, in 

generating structural and compositional 

heterogeneity in stands and implementing 

thinning prescriptions that enhance this 

heterogeneity; and 3) appreciating the role 

of recovery periods between disturbances in 

the development of stand complexity. We 

label these concepts, when incorporated 

into a comprehensive silvicultural 

approach, the “three-legged stool” of 

ecological forestry (Fig. 1).

Our goal in this report is to review the 

scientifi c basis for the three-legged stool of 

ecological forestry to provide a conceptual foundation 

for its wide implementation. Specifi cally, we 1) review 

the concept of biological legacies; 2) present a conceptual 

model of natural disturbance regimes focused on the 

types and amounts of biological legacies they create, and 

contrast natural disturbances with their regeneration 

harvest counterparts with respect to biological legacies; 

3) review tree mortality processes during stand 

development with respect to generating structural 

heterogeneity and contrast stand development mortality 

with silvicultural thinning for their respective effects 

on heterogeneity; 4) review the importance of recovery 

periods for generating stand complexity and contrast 

this with the implementation of rotation periods; and 

5) provide principles and guidelines for incorporating 

natural disturbance and development concepts into 

silvicultural prescriptions that sustain or restore 

ecological complexity.

Sidebar 1.—The Three-Legged Stool of 

Ecological Forestry

A useful analogy for ecological forestry is a three-legged stool 

(Fig. 1). For the stool to function effectively each leg must 

contribute support to the seat. Remove one leg and the stool 

might support weight in a precariously balanced position for a 

time, but eventually the stool will fall. Remove two legs and it will 

fall sooner. By analogy, ecological forestry (the seat of the stool) 

depends on each of its three principles (legs of the stool) to 

fully succeed. These legs or principles for management include 

(1) retention of biological legacies at harvest; (2) intermediate 

treatments that enhance stand heterogeneity; and (3) allowances 

for appropriate recovery periods between regeneration harvests.

 

Ecological

Forestry

Legacy

Retention

Recovery

Periods

Intermediate

Treatments Figure 1.—The three-legged 

stool of ecological forestry.

BIOLOGICAL LEGACIES

Biological legacies as a concept emerged from studies 

that demonstrate that natural disturbances—even 

disturbances as intense as the Mount St. Helens eruptions 

(Franklin et al. 2000, Franklin and MacMahon 2000)—

rarely create the simple and homogeneous environment 

that is sometimes imagined and often emulated with 

even-aged silviculture (Fig. 2). While disturbances may 

dramatically disrupt the ecosystem and kill trees, limited 

amounts of organic matter are actually consumed or 

removed (Franklin et al. 1997). Much of the residual 

organic matter persists as structures—such as standing 

dead trees (snags) and tree boles and other woody 

debris on the ground—that provide critical habitat for 

organisms and fi ll other important functional roles in 

the ecosystem (e.g., Harmon et al. 2004, Hunter 1999, 

Maser et al. 1988). In addition, many live mature trees, 

seedlings, and seeds survive intense disturbances. As with 
3

many ecological constructs, the concept of biological 

legacies is not wholly new; rather, it is a revision and 

elaboration of Frederic Clements’ concept of organic 

“residuals” proposed nearly 100 years ago (Clements 

1916).

Biological legacies are defi ned as the organisms, organic 

matter (including structures), and biologically created 

patterns that persist from the pre-disturbance ecosystem 

and infl uence recovery processes in the post-disturbance 

ecosystem (Franklin et al. 2000). Legacies occur in varied 

forms and densities, depending upon the nature of both 

the disturbance and the forest ecosystem (Table 1).

Biological legacies play important roles in ecosystem 

reorganization and recovery following disturbance 

(Franklin et al. 2000, Franklin and MacMahon 2000). 

A generalized function of legacies is that of “lifeboating” 

or perpetuating genotypes and species in situ, which is 

particularly relevant to conserving biological diversity 

within heavily disturbed forest ecosystems (Lindenmayer 

and Franklin 2002). Specifi c mechanisms by which 

biological legacies “lifeboat” biological diversity include 

the following:

Perpetuating plant species, as surviving immature 

or mature individuals or as reproductive 

structures, such as seeds, spores, or vegetative 

parts with sprouting capability 

Perpetuating biota by providing habitat, 

supplying energy and nutrients, and by 

modifying microclimatic conditions

Providing habitat for recolonizing organisms, 

primarily by structurally enriching the 

developing young stand

Improving connectivity in the landscape for 

some organisms by providing protective cover 

within the disturbed area

•

•

•

•

Figure 2.—Biological legacies left after natural disturbances including: a) understory plant 

communities, tree seedling banks, snags and logs after volcanic eruption, Mount St. Helens, 

Washington; b) understory plant communities, tree seedling and sapling banks, and large 

volumes of down tree boles after stand-replacement wind disturbances, Bull Run River drainage, 

Mount Hood National Forest, Oregon; c) snags and associated woody debris after standreplacement fi re, Yosemite National Park, California.

a

b

c
4

These roles are particularly prominent where 

a large and intense (i.e., stand replacement) 

disturbance has taken place, but they also 

occur with smaller disturbances, such as within 

midsize to large canopy gaps in an otherwise 

intact stand.

A major part of the lifeboating function is 

typically provided by larger biological structures 

that persist following a disturbance, such as live 

trees, snags, and downed boles. These structures 

sustain organisms in the post-disturbance 

environment by providing necessary habitat 

(e.g., nesting sites and hiding cover) and energy, 

especially right after the disturbance. Live green 

plants have particular importance in sustaining 

high-quality energy fl ows to belowground 

organisms and food webs, as well as to 

aboveground herbivores. For example, residual 

trees in retention harvests in Douglas-fi r forests 

(Fig. 3) are associated with increased diversity 

of ectomycorrhizal fungi, which are important 

for seedling nutrition and survival (Louma 

et al. 2006). Residual structures also modify 

microclimate, often bringing it within the 

acceptable environmental range for organisms 

to survive.

Table 1.—Categories and examples of biological legacies

Legacy category Examples

Organisms Sexually mature and intact live trees

Tree reproduction (seedling and sapling banks)

Vegetatively reproducing parts (e.g., roots)

Seed banks

Shrub, herb, bryophyte species

Mature and immature animals and microbes

Organic matter Fine litter

Particulate material

Organically derived structures Standing dead trees

Downed trees and other coarse woody debris

Root wads and pits from uprooted trees

Organically derived patterns Soil chemical, physical, microbial properties

Forest understory composition and distribution

Figure 3.—Legacy Douglas-fi r left after a retention harvest in the 

Cascade Range.
5

Compositional legacies can be as important as structural 

legacies with regard to many aspects of ecosystem 

function (Palik and Engstrom 1999). Compositional 

legacies include the variety of organisms that survive 

a disturbance including trees, other plants, fungi, and 

animals. In forest ecosystems, tree-derived legacies are 

important in facilitating survival of organisms other 

than trees, but we must not forget that these organisms 

themselves play important roles in re-establishing a 

diverse and functional forest ecosystem (Dale et al. 

2005). Both structural and compositional legacies may 

also play a role in sustaining natural disturbance regimes. 

As an example, overstory tree legacies provide fuel for fi re 

in systems that depend on fi re, such as the longleaf pine 

(Pinus palustris Mill.), ponderosa pine (Pinus ponderosa

Laws.), and Great Lakes pine ecosystems (Allen et al. 

2002, Rebertus el al. 1989).

GENERAL MODEL OF DISTURBANCE 

AND LEGACY CREATION

Scientifi c understanding of disturbances and subsequent 

ecosystem recovery and development processes has 

increased dramatically during the last several decades. 

Several large and notable disturbances—the Mount 

St. Helens eruption of 1980 (Dale et al. 2005), the 

Yellowstone Fires of 1988 (Christensen et al. 1989), 

and Hurricanes Hugo (Walker et al. 1991) and Andrew 

(Pimm et al. 1994)—provided opportunities for ecosystem 

research teams to extend their studies of recovery processes 

to intensely disturbed ecosystems. Concurrently, there has 

been an increased understanding of the structural effects of 

smaller scale disturbance regimes and natural development 

processes in forest ecosystems (e.g., Frelich and Lorimer 

1991; Gray and Spies 1996; Kneeshaw and Bergeron 

1998; Runkle 1982, 1998).

Research on natural disturbances has largely emphasized 

variables such as the type, size, frequency, intensity, and 

impact of the disturbances (e.g., Pickett and White 1985, 

Turner et al. 1998). Disturbance scale has also received 

signifi cant attention, including a scientifi c synthesis 

of the distinctive features and effects of large intense 

disturbances (Turner et al. 1998). Predicting ecosystem 

responses to disturbances, however, is best understood 

by considering not only patterns of destruction or 

consumption, but also patterns and types of what remains 

following the disturbance, i.e., biological legacies.

Our general disturbance model incorporates scale and 

disturbance agent as the primary considerations in 

predicting kind, quantity, and spatial pattern of biological 

legacies (Table 2). Other widely discussed disturbance 

regime descriptors, i.e., intensity and frequency, are not 

treated directly, but are implicit in consideration of scale 

and agents.

Table 2.—Biological legacies associated with wind, fire, and bark beetle disturbances

Disturbance agent

Wind Fire* Beetle

Legacy Tree Gap Stand Tree Gap Stand Tree Gap Stand

Live, mature trees NA Few/

Absent

Few/Absent NA Few Few NA Species 

dependent

Species 

dependent

Seedling bank Possible Possible Possible No No/Rare Rare Possible Possible Possible

Intact understory Possible Yes Yes No Rare Rare Possible Yes Yes

Snags NA Few Few Yes Abundant Abundant Yes Abundant Abundant

Logs Yes Abundant Abundant No No Common No No No 

Uproots Yes Abundant Abundant No No No No No No

Mineral seedbed Yes Yes Yes Yes Yes Abundant No No No

*Fire at the tree and gap scales is largely surface fi re that spreads through a stand (with or without crowning) and may kill individual or small 

groups of trees.
6

Disturbance Scale

Spatial scale of dominant tree mortality is one dimension 

of our model of tree-regenerating disturbance (Table 

2). Disturbances are scaled from those involving 

individuals, to groups of overstory trees, and, fi nally, 

to large-scale mortality events, commonly described 

as stand-replacement disturbances. This gradient in 

size of individual disturbance events is correlated with 

proportion of the remaining intact forest and with degree 

of heterogeneity of structure and composition. Tree-scale 

and gap-scale mortality events leave the forest largely 

intact such that the forest matrix still dominates the postdisturbance environment (Fig. 4). Stand-level mortality 

events, on the other hand, destroy the dominant forest 

Figure 4.-Conceptual representation of spatial scale and distribution of overstory 

disturbance within a stand: (a) mature forest; (b) stand-replacement disturbance; 

(c) gap-scale disturbance; (d) partial canopy disturbance.

a

b

c

d
7

cover and shift the matrix to an open postdisturbance environment (Fig. 4). A condition 

intermediate to these two extremes results when 

multiple tree or gap events occur simultaneously 

(heavy, but partial canopy disturbance), leaving a 

stand that contains similar amounts of both open 

and intact canopy conditions (Fig. 4).

Stand-replacement events will generally homogenize 

(in a relative sense) the post-disturbance tree 

population structure of a stand by providing 

conditions for establishment or release of a new 

tree cohort and generating abundant coarse 

woody debris. Tree-scale and gap-scale events, 

on the other hand, tend to create or perpetuate 

stand structural heterogeneity, although in greater 

degrees with gap-scale events. Heavy partial canopy 

disturbance results in a high degree of structural 

heterogeneity and provides conditions for both new 

cohort establishment and release of residual trees (Fajvan 

and Seymour 1993).

Disturbance Agent

Disturbance agent is the other important dimension 

of tree-regenerating disturbances that directly affects 

biological legacies (Table 2). We use fi re, wind, and bark 

beetles to exemplify the effect of disturbance agent on 

biological legacies because these are common agents and 

relate to disturbance regimes in many well-known forest 

types. Other agents of disturbance, while not discussed 

in detail here, also create legacies (Sidebar 2). Wind, fi re, 

and bark beetles can kill trees across a range of spatial 

scales, from individual trees to entire stands, and these 

disturbance agents contrast greatly in their resultant 

biological legacies, irrespective of the scale of overstory 

tree mortality (Table 2).

Wind

Many forest types have a primary disturbance regime 

defi ned by wind-created gaps; even more forests 

incorporate this as a “secondary” regime (as discussed 

later). Legacies within wind- generated gaps typically 

include numerous boles on the forest fl oor and some 

large live trees and snags in larger gaps (Table 2). Cohorts 

of tree seedlings and saplings or seedling banks are often 

present, sometimes in high densities; these regeneration 

banks are typically referred to as advance regeneration 

by foresters. Understory vegetation and seed banks are 

typically left intact, except around the uprooted area 

and where they are buried beneath boles and other 

debris. Exposed mineral soil typically is limited to areas 

infl uenced by uprooting, i.e., tree-fall pits and mounds, 

which are also important structural legacies.

North American forest types characterized by a wind-gap 

disturbance regime include Pacifi c Coast rainforests of 

western hemlock (Tsuga heterophylla (Raf.) Sarg.), western 

redcedar (Thuja plicata Donn ex D. Don), Sitka spruce 

(Picea sitchensis (Bong.) Carr.) and Pacifi c silver fi r (Abies 

amabilis (Dougl. ex Loud.) Dougl. ex Forbes) (Lertzman 

et al. 1996); northern hardwood forests of the Great 

Lakes and New England regions (Frelich and Lorimer 

1991); and mid-Atlantic and southern hardwood forest 

ecosystems (Runkle 1982). In temperate South America, 

the lenga (Nothofagus pumilio (Poepp et Endl.) Krasser) 

forests of Tierra del Fuego provide an excellent example 

of a forest with a wind-gap disturbance regime (Arroyo et 

al. 1996, Rebertus et al. 1997).

Wind-generated disturbances at larger spatial scales (e.g., 

hurricanes, typhoons, straight-line winds, and tornadoes) 

are similar to wind-gap disturbances in the nature of 

biological legacies they produce (Table 2). Structural 

Sidebar 2.—Other Types of Disturbances 

and Legacies

Biological legacies can be tabulated for other types 

of disturbance regimes and forest types. For example, 

abundant biological legacies of various types are 

associated with such disturbances as floods (Michener 

et al. 1998), snow avalanches, outbreaks of defoliating 

insects (Hollings 1992), and diseases of various types, 

such as root rots or foliar diseases (Cogbill 1996). There 

are disturbances that leave few biological legacies, such 

as landslides or volcanic events that are dominated by lava 

flows or glowing avalanche deposits (Dale et al. 2005). 

Unusual species or life forms may also result in distinctive 

legacies; for example, the presence of coast redwood 

(Sequoia sempervirens (Lamb. ex D. Don) Endl.) in a stand 

adds a stump-sprouting, shade-tolerant coniferous species 

to some coastal forests of northern California and southern 

Oregon that are otherwise lacking in conifers having an 

significant ability to reproduce vegetatively.
8

legacies are primarily downed boles with some snags and 

a few larger live trees (e.g., Foster and Boose 1992, Palik 

and Robl 1999, Peterson and Pickett 1995). Some snags 

often persist, probably because they have less surface area 

(i.e., no canopy), providing less purchase for winds. In 

many temperate and tropical hardwood forests, broken, 

bent, and partially uprooted trees have the capacity 

to re-sprout, even though they are no longer part of a 

dominant overstory (Foster et al. 1997, Walker et al. 

1991); these damaged but live trees are an important 

structural legacy of the disturbance. The understory of 

affected stands remains largely intact, except for areas 

disrupted by uprooting or located directly under boles; 

hence, seed and seedling banks and understory plants are 

a major legacy of stand-scale wind disturbance. A stand 

of windthrown or jackstrawed trees (Fig. 5) also protect 

pre-disturbance and post-disturbance seedlings and other 

understory plants from browsing animals (Franklin and 

Dyrness 1973, Sharpe 1956).

Intense windstorms, including hurricanes, are an 

occasional disturbance for temperate hardwood forests 

in eastern North America (Canham and Loucks 

1984, Foster and Boose 1992), in subtropical and 

tropical hardwood forests in the Caribbean region 

Figure 5.—A stand of blowndown, jackstrawed (bent), and surviving red pine after an 

intense windstorm on the Huron-Manistee National Forest, Michigan. Photo credit: Linda 

Haugen, USDA Forest Service, www.forestryimages.org.

and, occasionally, in pine forests of the southeastern 

Coastal Plain (Myers and Van Lear 1998). Intense 

windstorms also periodically disturb areas of conifer 

forests along the Pacifi c Coast from northern California 

to Alaska. Portions of the Pacifi c Northwest dominated 

by Douglas-fi r (Pseudotsuga menziesii (Mirb.) Franco 

var. menziesii) forests are much better known for their 

stand-replacement fi re regimes, but both large-scale and 

more localized stand-replacement wind events do occur 

as evidenced by the 1962 Columbus Day windstorm 

(Orr 1963). Infrequent, but intense windstorms are 

also important in the Great Lakes region (Canham and 

Loucks 1984, Frelich and Lorimer 1991, Palik and Robl 

1999, Reich et al. 2001).

Fire

Fire kills overstory trees at either small (tree and gap) or 

large (stand) scales, but produces comparable legacies 

at all scales (Table 2). Typical legacies of fi re include a 

high density of snags and, sometimes, down boles on the 

forest fl oor, but relatively few large live trees if it is truly 

a stand-replacement disturbance. However, some large 

trees typically survive even intense fi res, as exemplifi ed 

by such diverse forest types as jack pine (Pinus banksiana 

Lamb.) in the Lake States (Abrams 1984) and Douglas-fi r 
9

on the Pacifi c Coast (Spies and Franklin 1991) (Fig. 6). 

As with all events that leave standing, dead, overstory 

trees, snags created by fi re will eventually disintegrate 

to provide coarse woody debris on the ground.

Understory seedling and seed banks, understory 

plants, and organic layers on the soil surface are all 

affected by fi re; however, some understory trees and 

plants typically survive either because of variable fi re 

intensity or adaptations to fi re, or both (Jacqmain 

et al. 1999). A tree seed bank may also persist as a 

result of ecological adaptations (serotinous cones) or 

fortuitous survival of portions of a current seed crop 

in the canopy (Larson and Franklin 2005). Abundant 

mineral soil seedbeds are an important legacy of fi re 

disturbances.

Gap-scale stand openings are often part of a 

frequent, low to moderate intensity fi re regime, as 

is characteristic of southeastern longleaf pine forests 

(Palik and Pederson 1996), some Great Lakes eastern 

white pine (Pinus strobus L.) and red pine (Pinus 

resinosa Ait.) forests (Heinselman 1973), and many 

ponderosa pine forests (Franklin and Van Pelt 2004). 

In these systems, surface fi res occasionally crown, 

killing patches of overstory trees and creating gaps in 

the process.

Fires may actually cover a large area, 

but kill few overstory trees and thus 

generate few gaps. In southern longleaf 

pine woodlands, many understory and 

ground cover plants are adapted to regular 

burning (Kirkman et al. 2004), while 

many potential canopy species, such as 

oaks, survive fi re, but do not grow tall 

(Jacqmain et al. 1999). In some of the 

forest types subject to this regime—such 

as longleaf pine—overstory tree mortality 

is caused primarily by lightning or bark 

beetles rather than by surface fi res (Fig. 

7; Palik and Pederson 1996), but these 

lighting strikes are an important ignition 

source for surface fi res, which are fueled 

by pine needles and fl ammable grasses 

(Williamson and Black 1981).

Figure 6.—Individual Douglas-fi r and western larch (Larix 

occidentalis Nutt.) trees survived this intense, stand-replacement 

fi re on the Colville National Forest, Washington.

Figure 7.—Dominant longleaf pines are susceptible to injury and mortality 

from lightning strikes, such as this tree at the Jones Ecological Research 

Center in southwest Georgia. Lightning strikes are important ignition sources 

for surface fi res. Lightning injuries can induce declines in vigor in otherwise 

healthy trees, leading to attacks by various bark beetles.
10

High intensity, stand-replacement fi re is a common 

disturbance regime for many important forest types in 

North America. These include coastal Douglas-fi r forests 

(Spies et al. 1988), jack pine and red pine forests in the 

Great Lakes region (Heinselman 1973, Van Wagner 

1971), lodgepole pine (Pinus contorta Dougl. ex Loud.) 

forests throughout much of western North America 

(Romme and Knight 1981), and subalpine and boreal 

spruce (Picea spp.) forests in North America (Galipeau et 

al. 1997). Highly variable fi re intensities are characteristic 

of these large fi res; consequently, high levels of spatial 

heterogeneity in burn intensity and type and density 

of biological legacies are common. This heterogeneity 

is found at all spatial scales, from within stands to 

landscapes, resulting in patches of varying burn intensity 

as well as unburned patches.

Bark Beetles

Bark beetle disturbances can occur at a variety of spatial 

scales, with specifi cs depending upon the beetle and 

host species. The Douglas-fi r bark beetle (Dendroctonus 

pseudotsugae Hopkins) typically kills small groups of 

mature to old Douglas-fi rs, producing a gap (Franklin 

et al. 2002). Similarly, western pine beetle (D. brevicomis

LeConte) kills individual or small patches of ponderosa 

pine in old-growth stands (Johnson et al. 2003). Where 

extensive pure stands of host species exist, several 

species of bark beetles are capable of large-scale standreplacement disturbances. Notable examples are the 

southern pine beetle (D. frontalis Zimmermann) in 

plantations of southern pines (McNulty et al. 1998), the 

mountain pine beetle (D. ponderosae Hopkins) in young 

to mature stands of ponderosa pine and lodgepole pine as 

well as in extensive old-growth stands of lodgepole pine 

(Romme et al. 1986), and the spruce beetle (D. rufi pennis

(Kirby)) in spruce-dominated forests throughout 

subalpine and boreal forest regions in North America 

(Veblen et al. 1991). Snags and, eventually, boles and 

other coarse woody debris on the forest fl oor are primary 

biological legacies of beetle outbreaks (Table 2). Intact 

understory communities, including seedling and seed 

banks and undisturbed forest fl oors are also legacies.

Interactions between Disturbance 

Scales and Agent

Scale and agent can interact to determine the specifi cs 

of a disturbance regime. One of the most signifi cant 

interactions involves forest types that are characterized by 

both stand-replacement events (wind, fi re) and gap-scale 

events (wind, beetles). Forest developmental sequences 

initiated by catastrophic disturbances are invariably 

subject to gap-based disturbance processes that typically 

operate throughout stand development; the longer the 

developmental timespan, the more infl uential the gapbased processes become (Franklin et al. 2002).

Coastal Douglas-fi r stands are a case in point. In these 

forests, time intervals between stand-replacement 

wildfi res may exceed 400 years (Agee 1993, Franklin 

et al. 2002, Hemstrom and Franklin 1982). Assuming 

that succession begins with destruction of an old-growth 

forest stand, a spatially patchy, multi-aged stand is 

replaced with a relatively even-aged cohort of trees by 

the initiating disturbance. Where live-tree structures are 

concerned, the stand-replacement fi re can be viewed as 

a homogenizing event, although it can create landscapelevel heterogeneity (Delong and Kessler 2000). During 

the youthful competitive-exclusion stage of development, 

density-dependent competitive mortality among the 

young conifer cohort contributes further to structural 

homogeneity within the stand (Fig. 8), because the 

mortality is concentrated in smaller tree sizes and is most 

intense in the densest portions of the stands (Franklin et 

al. 2002). As the Douglas-fi r cohort matures, mortality 

shifts to such density-independent agents as wind, bark 

Figure 8.—Competitive-exclusion phase in a 45-year-old 

Douglas-fi r plantation on the Upper Cowlitz District of the 

Gifford Pinchot National Forest, Washington. Photo credit: 

Andrew J. Larson.
11

beetles, and root and bole diseases, which affect trees 

in the largest diameter classes and are typically spatially 

aggregated. This shift in causes and patterns of mortality 

results in the development of canopy gaps within the 

stand and, consequently, regeneration and release of 

shade-tolerant associates, such as western hemlock and 

western redcedar. Hence, gap-based disturbances come to 

dominate within a maturing of the Douglas-fi r forest sere 

and continue to do so until a stand-replacement event 

erases this spatially complex stand structure.

A similar interaction of disturbance scale and agent is 

seen in aspen ecosystems in the Great Lakes region of 

the United States. Many aspen (Populus grandidentata 

Michx., P. tremuloides Michx.) stands are characterized 

by largely single-cohort structure, initiating after standreplacing disturbances, particularly fi re or logging 

(Graham et al. 1963). As the stand matures, aspen 

begin to die from localized wind disturbance and 

disease, opening gaps that provide opportunities for 

establishment or release of later successional species, 

including tolerant northern hardwoods on richer sites or 

eastern white pine, white spruce (Picea glauca (Moench) 

Voss), and other conifers on poorer sites (Frelich and 

Reich 1995; Palik and Pregitzer 1993a, 1994). In either 

case, structurally complex, multi-cohort and mixedspecies stands develop.

This pattern of gradual modifi cation of homogenous, 

even-aged stands, by gap-level disturbance events appears 

to be repeated in most forest types subject to standreplacement disturbances, whether initiated by fi re or 

other agents of mortality. Some of these types may not 

reach the level of spatial complexity characteristic of 

old-growth stands, due to lack of longevity, limited tree 

growth potential, and insuffi cient intervals between 

stand-replacement disturbances. Nevertheless, the pattern 

whereby gap-based disturbances gradually modify the 

homogenizing infl uence of the stand-replacement event is 

common.

CONTRASTS BETWEEN NATURAL 

DISTURBANCES AND TRADITIONAL 

REGENERATION SYSTEMS

Biological legacies typically left after traditional 

regeneration harvest practices and those created by 

natural disturbances can be distinctly different. The 

contrasts can be signifi cant whether silvicultural 

approaches involve even-aged, two-aged, or uneven-aged 

regeneration harvesting techniques (Table 3).

Stand-Replacement Disturbances vs. 

Even-Aged Management Systems

Clearcut, seed tree, and shelterwood are the standard 

even-aged regeneration approaches. In their purest forms, 

Table 3.—Biological legacies associated with common regeneration harvest methods as traditionally applied

Method

Even-aged Two-aged Uneven-aged

Legacy Clearcut with

site prep

Seed tree with

site prep

Shelterwood with 

site prep1

Shelterwood 

with reserves 

and site prep

Group selection Single-tree

selection

Live, mature 

 trees

No Few/No No Yes Few/No (in group) n.a.

Seedling bank No No Yes Yes Possible Possible

Intact understory No No No Possible Possible Possible

Snags No No No No No (in group) n.a.

Logs Few/No Few/No Few/No Few/No Few/No (in group) No

Uproots No No No No No No

Mineral 

seedbed2

Yes Yes Yes Yes Possible Possible

1

Following fi nal removal of overstory.

2

Assuming ground-based harvesting.
12

they do not provide for any long-term retention of live or 

dead trees from the harvested stand (Table 3). Typically, 

overstory trees retained following a seed tree cut and an 

initial shelterwood harvest entries are eventually removed 

after successful establishment of regeneration (Smith et 

al. 1996). The option of retaining live trees indefi nitely is 

explicitly mentioned only in seed tree cutting (Smith et 

al. 1996) and in modifi cation of even-aged systems, i.e., 

clearcut or shelterwood with reserves.

The lack of signifi cant structural legacies is a major 

difference between these traditional even-aged harvest 

methods and natural stand-replacement disturbances, 

whether by fi re, wind, or insects. Most prominent among 

the missing legacies are remnant live trees, snags, and 

downed boles (Fig. 9), with associated pit and mounds in 

the case of windthrow. Stand-scale harvest disturbances 

also tend to homogenize subsequent stand structure 

(Bergeron et al. 1999), whereas natural disturbances 

that occur at the stand-replacement scale typically 

generate substantial spatial heterogeneity of biological 

legacies (Eberhart and Woodard 1987).

Other important contrasts between stand-replacement 

disturbances and traditional even-aged silviculture 

relate to spatial and temporal patterns in regeneration 

(Sidebar 3) and differences in size and shape of 

openings. Clearcuts are often smaller and have simpler 

shapes than areas affected by stand-replacement 

disturbances (Bergeron et al. 2002, Seymour et al. 

2002), unless they have been specifi cally designed to 

minimize visual impacts. Clearcut boundaries are also 

abrupt rather than feathered like boundaries of many 

natural disturbances.

Gap-Creating Disturbances vs. 

Uneven-Aged Management Systems

Single-tree selection and group selection are the 

traditional uneven-aged regeneration systems (Smith 

et al. 1996). Theoretically, selection practices are 

modeled closely on individual tree or gap-based natural 

disturbance regimes. In practice, these approaches 

can be highly formalized, such as where selection of 

trees to harvest is driven by efforts to create balanced 

diameter distributions (Sidebar 4), even though 

most natural stands fail to exhibit such regularity 

or balance (Matthews 1989, O’Hara 1996). The 

ecological problem with this approach is that most 

selection prescriptions remove different sizes of trees 

and in different spatial patterns than small-scale 

natural disturbances (Seymour and Hunter 1999), 

with signifi cant consequences for biological legacies 

(Table 3).

As with even-aged management, multi-aged 

management regimes can also result in homogenizing 

of structure (Seymour and Hunter 1999). For example, 

traditional selection systems for northern hardwood 

ecosystems in the Great Lakes region effectively drive 

overstory composition to sugar maple (Acer saccharum

Marsh.) dominance (Strong et al. 1997), whereas 

unmanaged mature and old-growth stands often 

support four or fi ve species in abundance and 10 or 

more species in total (Curtis 1959).

Figure 9.—Signifi cant legacies in the form of snags, fallen 

trees, and live residual trees are left after an intense 

windstorm on the Chequamegon-Nicolet National Forest in 

northern Wisconsin. The level of complexity in managed evenaged stands is substantially reduced relative to this condition.
13

Sidebar 3.—Pattern Legacies: 

Tree Regeneration after Stand-Scale Disturbance

Even-aged management regimes and natural stand-replacement disturbances can differ 

substantially in the dynamics of tree regeneration. Natural regeneration following wildfire is 

typically variable in time and space, depending upon such factors as proximity of seed sources 

and occurrence of large seed crops (e.g., Wahlenburg 1946) and heterogeneity of the fire, which 

can leave patches of advance regeneration intact. In such situations, establishment of dense tree 

regeneration and subsequent tree canopy closure may take many years or even decades (Tappeiner 

et al. 1997), particularly on sites where the environment is severe or which have been subjected 

to multiple burns (e.g., Franklin and Hemstrom 1981, Palik and Pregitzer 1991). In other cases, 

regeneration may occur quite rapidly following wildfire (e.g., Isaac and Meagher 1936, Larson and 

Franklin 2005). Establishment of regeneration by planting—as is often practiced with even-aged 

management regimes—is designed to consistently produce rapid and uniform re-establishment 

of forest cover. This difference in spatial and temporal patterns of regeneration, resulting from 

heterogeneity of fire and legacies (i.e., advance regeneration, seed sources, or site conditions) is a 

significant difference between natural and silvicultural stand-replacement disturbance.

Sidebar 4.—The B-D-q Method

Traditionally, selection systems (single-tree and group) are implemented quantitatively using the 

B-D-q method (O’Hara 2001). The structural outcomes of this approach differ in several significant 

ways from the outcomes of single-tree or gap-based natural canopy disturbances. With the B-D-q 

approach, a target residual basal area (B) and maximum diameter (D) are specified a priori, while 

the distribution of trees across diameter classes is determined by the diminution quotient (q), a 

value that reflects the ratio of the number of trees in diameter class a, to the number in diameter 

class a+1 (Smith et al. 1996). With this approach, the resultant diameter distribution is a negative 

exponential, often referred to as a reverse J distribution.

Specification of a target residual basal area is not necessarily in conflict with a natural disturbancebased approach to uneven-aged management; it is reasonable to expect a manager to select this 

target basal area based on an understanding of stand responses to disturbance and the silvics 

of component species. However, many natural canopy disturbances often result in substantial 

horizontal variation in stand structure including basal area and canopy openness. In contrast, 

selection systems based on the B-D-q method may deliberately or inadvertently minimize horizontal 

variation in stand structure in an attempt to reach a constant target basal area throughout the stand.

Both the selection of maximum tree diameters and q values have the potential to move stands 

structurally in directions having little or no natural analog. The theoretical goal of the q quotient is 

to create “balanced” all-aged stands that will sustain timber yield; that is, stands with uniform ratios 

between successive diameter distributions across the full range of diameters. In reality, there are 

few examples of natural forests that exhibit such balanced distributions (Matthews 1989).

In practice, when sustained yield is the goal, high q values are selected because they allow for 

greater numbers of smaller (regeneration) trees, relative to low q values. High q values also select 

against larger diameter trees. These larger trees, often the oldest in the stand, are the hallmark of 

multi-cohort, old-growth forests and should be retained in some reasonable abundance if managing 

for ecological objectives has primacy over, or even equal priority with, timber objectives.

It is not likely that this dilemma can be overcome through careful choice of a q value, as neither 

field research nor computer simulation has found a value that is relevant to both economic and 

ecological objectives (Hann and Bare 1979, O’Hara, 2001). More importantly, it has been shown that 

stable multi-cohort tree populations can be achieved by diameter distributions other than a negative 

exponential (Goff and West 1975, Goodburn and Lorimer 1999, James et al. 2004, Leak 1996, 

Lorimer and Frelich 1984). Simulations also suggest that a negative exponential distribution may 

not maximize yields or optimize economic returns (Adams and Ek 1974, Bare and Opalach 1988, 

Erickson et al. 1990, Kaya and Buongiorno 1989).
14

Individual Tree Selection

Typical individual tree selection contrasts signifi cantly 

with tree-scale natural disturbances in terms of the types, 

amounts, and pattern of trees removed, and, the nature 

of resultant biological legacies (Table 3). For various 

reasons, most selection systems involve the systematic 

removal of at least some of the very large and old trees 

(Moser et al. 2002). Many of these trees are the most 

valuable from a timber perspective and are viewed as no 

longer contributing signifi cantly to stand growth based 

on a presumption that large old trees are overmature 

and likely to die in the near future. Thus, their removal 

is driven more by economic, rather than ecological, 

considerations. However, studies of tree demography in 

many forest types and for many tree species show that 

large trees actually have lower rates of mortality—i.e., 

greater probability of survival—than smaller trees, at 

least until individuals get very large (Lorimer et al. 2001, 

Monserud and Sterba 1999). Moreover, large old trees 

have tremendous ecological value, providing habitat 

in the form of cavities, deeply fi ssured bark, and large 

limbs.

In practice, most single-tree selection prescriptions do 

not suffi ciently recognize the ecological importance of 

decadent trees and their derivatives, snags and down 

boles (Fig. 10). The numerous and important roles of 

dead wood, both snags and logs, are well known (see 

e.g., Harmon et al. 2004, Maser et al. 1988). Most 

individual-tree selection systems involve the systematic 

removal of decadent, diseased, or poorly formed trees, 

although some practitioners explicitly retain such trees 

for ecological purposes, especially as wildlife habitat 

(Mitchell et al. 2000). Additionally, windthrows 

generate localized soil disturbance (Fig. 11), which is not 

emulated with selection systems.

Figure 10.—A snag and downed log left after a small-scale 

canopy disturbance in an old-growth northern hardwood 

forest. Photo credit: Steven Katovich, USDA Forest Service, 

www.forestryimages.org.

Figure 11.—Localized soil disturbance in the form of a tippedup root wad is characteristic of many tree blowdowns.
15

Group Selection

Traditional group selection harvest practices contrast in 

their biological legacies with multi-tree, gap-based natural 

disturbance regimes. Natural multi-tree gap disturbances 

leave behind signifi cant biological legacies, including 

snags or downed boles or both (Fig. 12). Depending 

upon the disturbance agent, there may be living legacies, 

such as intact understory layers (including seedling 

banks) and pits-and-mounds in the case of wind-created 

gaps. Few silvicultural prescriptions based upon group 

selection have explicitly incorporated such structural 

legacies (Table 3).

Contrasts in opening size between multi-tree gap 

disturbance and traditional group selection also are 

common. Group selection prescriptions generally use larger 

(sometimes several times larger) openings on average than 

the size of gaps created by natural disturbance processes. 

For example, group selections proposed as the primary 

harvest method in mixed-conifer types on national forests 

in the Sierra Nevada involve 1- to 2-ha openings. A “gap” 

of this size is many times larger than the size of natural 

gaps (typically 0.08- to 0.10-ha) that are typically found 

within these forests (e.g., Knight 1997).

Natural gaps generally exhibit a range in sizes typifi ed by 

a distribution heavily skewed toward smaller openings 

(e.g., Rebertus et al. 1997). Factors infl uencing foresters 

to deviate from this distribution and select larger than 

natural average opening sizes under group selection 

prescriptions include concerns about growth rates of 

reproduction, cost and operational diffi culties associated 

with harvesting in small areas, and generation of greater 

revenues during an entry.

Heavy Partial Disturbances vs. TwoCohort Management Systems

Heavy partial disturbances that create stands with two 

tree cohorts provide a middle ground along the gradient 

from even-aged to uneven-aged management (Palik 

et al. 2002). Heavy partial canopy disturbances are 

characteristic of a wide array of forest types including 

Acadian mixed conifer forests (Fajvan and Seymour 

1993, Seymour and Hunter 1999) and Great Lakes 

red pine-white pine forests (Heinselman 1981). Such 

disturbances result in legacies in the form of dead 

and down trees, live residual canopy trees, patches of 

understory plant populations, as well as horizontal 

heterogeneity in stand structure.

Figure 12.—A disturbance gap within a Sierra Nevada mixed conifer forest in Yosemite 

National Park, California; note the biological legacies of snags and downed boles, as 

well as natural regeneration of ponderosa pine and associated species within the gap. 
16

Heavy partial disturbances often result in higher levels 

of retention than is characteristic of stand-replacement 

disturbances, but also remove more of the overstory in 

any one event than is characteristic of individual tree 

or gap disturbances. Initially the stand will consist of 

two distinct age cohorts after regeneration is established 

(Fig. 13).

In the nomenclature of traditional silviculture, a 

shelterwood with retention (e.g., retention exceeding 20 

percent but less than 60 percent) is an analog to heavy 

partial canopy disturbance (Palik and Zasada 2002, 

Seymour and Hunter 1999). However, such systems 

often are implemented in ways that create homogeneity 

in stand structure, resource availability, and competitive 

environments, and pay limited attention to retention 

of biological legacies (Table 3). For instance, large live 

trees may be retained more for their economic value, 

rather then their potential to contribute substantially to 

structural complexity and wildlife habitat.

The same concerns about lack of legacy management in 

even-aged systems generally are concerns for two-aged 

systems. Residual trees may be dispersed uniformly across 

the stand, creating fairly uniform environments for new 

regeneration. A primary consideration when retaining 

overstory trees in such systems has been their expected 

growth and yield potential and, hence, fi nancial return. 

There also may be interest in providing adequate seed 

sources for a new cohort. For both reasons, a single tree 

species may be selected preferentially for retention. As 

with selection systems, two-cohort management may 

not explicitly consider the ecological importance of large 

decadent trees. Such trees, including dying, diseased, or 

poorly formed trees, generally are considered liabilities 

and are removed during harvest operations.

TREE MORTALITY PROCESSES 

DURING STAND DEVELOPMENT

Various conceptual models have been used to describe the 

processes and outcomes of stand or patch development 

following tree-initiating disturbances, particularly 

large-scale disturbance. These include relatively simple 

four-stage models that focus largely on live tree cohort 

establishment and development of vertical patterns 

(Oliver and Larson 1996) or those based on ecosystem 

processes, such as biomass accumulation and nutrient 

cycles (Bormann and Likens 1979). A recent, more 

complex model of stand development (Franklin et al. 

2002) specifi cally considers the role of tree growth 

and decline, plant competition and competitioninduced mortality, and small-scale disturbances during 

Figure 13.—An early 20th century two-cohort stand of eastern white pine and red pine, 

after heavy partial canopy disturbance on the Chippewa National Forest, Minnesota.
17

stand development as generators of complexity and 

heterogeneity in structure and composition.

Consideration of these developmental processes, 

particularly those related to tree mortality, and their 

structural outcomes is essential to the formulation of 

comprehensive ecological forestry prescriptions that 

include intermediate silvicultural treatments. The 

complex structures and spatial heterogeneity that are 

distinctive features of mature and old-growth forest are 

often the result of mortality processes occurring during 

stand development (Fig. 14), i.e., after the stand initiation 

(sensu Oliver and Larson 1996) or stand disturbance and 

legacy creation stage (sensu Franklin et al. 2002).

Competitive Tree Mortality

Competitive tree mortality, or natural thinning, 

results from competition for light and soil resources 

among trees in a stand or patch (Oliver and Larson 

1996). Although often viewed as a continuous process, 

competitive mortality may occur in pulses over the 

course of stand development (Horsley et al. 2000, Palik 

and Pregitzer 1993b). As trees grow in size, they occupy 

more growing space and compete for limited resources. 

Many trees in the stand may decline in growth as 

competition intensifi es; however, inferior competitors 

decline in growth and vigor at a faster rate than superior 

competitors, and the former eventually die. This 

mortality frees growing space and resources, which in 

turn allows growth rates of vigorous trees to increase until 

the next period of competitive interaction and resultant 

mortality.

A key feature of competitive exclusion is that trees generally 

die from below; that is, the smallest, weakest trees die, 

leaving larger, more vigorous individuals to use liberated 

resources (Oliver and Larson 1996). The result of this 

process is the development of larger trees, as well as some 

size variation within the stand, particularly when multiple 

tree species of different shade tolerance are present (Nyland 

2002). Additional results include the development of 

associated structural features, such as large branches and 

extensive heartwood. Competitive tree mortality also 

generates snags and dead wood on the ground, but this 

material is typically small and thus of limited value as 

habitat and a modifi er of microclimate (Fig. 15).

Figure 15.—Dead wood generated by competitive tree 

mortality in a stand in the Cedar River Watershed, King 

County, Washington. This material is typically small in size 

and not persistent and consequently has limited habitat 

value. Photo credit: James A. Lutz.

Figure 14.—Cross-section of a 650-year-old stand of western red cedar, Douglas-fi r, and 

western hemlock (Cedar Flats Research Natural Area, Washington), illustrating the mosaic 

of structural patches characteristic of old-growth stands in the Pacifi c Northwest. This 

mosaic is the consequence of centuries of development, including small-scale canopy 

disturbance, within a stand that was initially of even structure and age. Drawing courtesy 

of Robert Van Pelt.
18

In forests generated from stand-replacing disturbances, 

competitive tree mortality continues to occur over time, 

but is increasingly concentrated in spatially distinct 

patches as the stand transitions into the maturation 

phase of development (Franklin et al. 2002). Of course, 

in forests characterized by tree-scale and gap-scale 

disturbance, competitive thinning always occurs within 

discrete patches of trees that have established or been 

released in large canopy gaps (Fig. 16).

In natural forests, competitive tree mortality does not 

progress uniformly throughout a stand, due to spatial 

variation in initial tree density, microenvironment, 

vigor, and species composition. Consequently, different 

portions of the stand will thin at different rates and 

the residual trees will grow at different rates. The result 

is some small degree of horizontal variation in stand 

density, tree sizes, and vigor; however, this horizontal 

variation for the most part develops as a result of 

noncompetitive tree mortality, as described below. 

Noncompetitive Tree Mortality

Competition-induced tree mortality is augmented 

by small-scale canopy disturbances (Fig. 17). Single 

tree and small gap mortality can occur throughout 

the life of a forest stand, although the probability 

of occurrence increases with stand age, becoming 

particularly important after the initial period 

of exponential stand growth and competitive 

mortality (Franklin et al. 2002). A key element 

of this mortality is that it is not directly the result 

of competition, although trees weakened from 

competition may be more susceptible to exogenous 

disturbance.

Agents of small-scale mortality include root rots, 

wind, bark beetles, lightning, ice damage, and surface 

fi re (Franklin et al. 1987, Franklin and DeBell 1988, 

Harcombe and Marks 1983, Kneeshaw and Bergeron 

1998, Palik and Pederson 1996). Many of these same 

agents could be part of a stand or patch initiating 

regeneration disturbance, as described previously. We 

distinguish small-scale mortality from the latter based on 

their cumulative spatial infl uence in the stand; smallscale disturbances may occur infrequently enough and 

at younger stand ages, such that a signifi cant amount of 

Figure 17.—Single-tree canopy disturbance in an 

old-growth hardwood forest. Toumey Forest, East 

Lansing, Michigan.

Figure 16.—A dense patch of longleaf pine 

regeneration developing within a canopy gap 

at the Jones Ecological Research Center, 

Georgia. Competitive tree mortality occurs 

among trees within this gap.
19

regeneration of future overstory trees is not necessarily 

an outcome. In reality, small-scale mortality events grade 

into gap-based regeneration disturbances.

Outcomes characteristic of small-scale disturbance 

include formation of small canopy gaps and generation 

of large dead wood, including snags and downed boles. 

Moreover, the opening of small gaps, either above or 

below the ground, will increase resource availability 

locally. Neighboring trees may capture these resources 

and increase growth accordingly (similar to competitioninduced mortality), closing the gap laterally (Parsons 

et al. 1994). Alternatively, gaps may result in new 

establishment or proliferation of understory plant 

populations, including herbs, shrubs, and understory 

trees (McGuire et al. 2001).

Small-scale canopy disturbances are patchy in occurrence 

across a stand and variable in frequency over time. As a 

consequence, the development of signifi cant structural 

heterogeneity, both vertically and horizontally, within 

a stand is a fundamental outcome of such canopy 

disturbances. This heterogeneity in canopy conditions 

results in spatial variability in many stand attributes, 

including abundances of tree, shrub, and herbaceous 

plant populations, snag and downed wood loadings, tree 

size distributions, forest fl oor conditions, and mineral soil 

exposure.

COMPARISONS TO TRADITIONAL 

THINNING TREATMENTS

Contrasts between tree mortality during stand 

development and traditional applications of silvicultural 

thinning center on the creation of heterogeneous 

versus homogeneous structural and compositional 

stand conditions (Table 4). In a relative sense, natural 

tree mortality, particularly noncompetitive mortality, 

generates heterogeneous stand conditions, whereas 

Table 4.—Contrasts between the outcomes of tree mortality processes and traditional thinning treatments

Unmanaged stand Managed stand

Process Cause Outcomes Treatment Purpose Outcomes

Competitive 

tree mortality

Resource 

competition

-Larger trees retained

-Competitively superior 

trees favored regardless 

of species

-Shift toward uniform 

tree size distribution, but 

variability occurs

-Tree quality and form 

will vary

Silvicultural 

thinning

-Free growing 

space for crop trees

-Capture 

economically 

valuable wood 

before mortality

-Larger trees 

favored

-Commercial 

species favored

-Strong shift 

toward uniform tree 

size distribution

-Poor quality trees 

removed

Small-scale 

canopy 

disturbance

Exogenous agents 

(ice, wind, fi re, 

insects, disease) 

-Dominant individuals 

removed

-Creation of canopy 

openings

-Canopy closure from 

adjacent trees 

-Height recruitment of 

existing regeneration 

-Establishment of 

regeneration

-Establishment or growth 

of shrub and herbaceous 

plants 

-Generation of snags or 

large wood on the ground

Few 

silvicultural 

analogs 

implemented 

as an 

intermediate 

treatment, as 

opposed to a 

regeneration 

treatment
20

thinning, as traditionally implemented, tends to 

homogenize the structure and composition of a stand.

Silvicultural thinning, whether pre-commercial or 

commercial, often has a basic goal of removing trees 

that are likely to die in the near future, thereby directing 

resources to and concentrating growth on remaining 

(crop) trees. In the case of commercial thinning, the 

harvested trees are of a size to be economically valuable; 

hence the operation is designed to capture mortality of 

usable wood before death actually occurs.

Thinning bypasses the process of competitive mortality 

and, in so doing, can be a valuable tool to facilitate the 

development of large trees. While competitive mortality 

as a process does tend to result in more homogeneous 

distributions of trees than were present before its initiation, 

silvicultural thinning carries this homogenizing process 

to the extreme. Thinning is often distributed across a 

stand specifi cally to create a uniform distribution of crop 

trees, effi ciently distributing access to resources to those 

individuals that will eventually be harvested (Fig. 18).

Thinning is also used to improve and standardize tree 

quality and form (Smith et al. 1996). Poor quality trees, 

as measured from an economic standpoint— e.g., those 

with cavities, large branches, or decay pockets—may be 

preferentially removed (Graves et al. 2000). The longterm goal is to retain only healthy, ostensibly genetically 

superior trees. Only recently have the contributions that 

“non-standard” trees make to biodiversity and wildlife 

habitat been considered in thinning prescriptions (e.g., 

Carey 1995) (Fig. 19).

Figure 18.—Unthinned (left) and 

thinned (right) stands of lodgepole 

pine. Photo credit: USDA Forest 

Service - Rocky Mountain Region 

Archives, www.forestryimages.org.

Figure 19.—Sugar maple wildlife tree created by pileated 

woodpeckers (Dryocopus pileatus (L.)) on the Argonne 

Experimental Forest, Wisconsin. Photo credit: Terry Strong.
21

The contrasts between small-scale canopy 

disturbance (non-competitive tree mortality) 

occurring during stand development and 

typical thinning practices are signifi cant, 

largely because the latter typically do not 

include an analog for small gap-creating 

disturbances, unless they are specifi cally part 

of a gap-based regeneration prescription. 

Traditionally, timber management specifi cally 

seeks to avoid any disturbance-based 

mortality, especially when it kills dominant 

trees and tends to be spatially aggregated, 

i.e., creates canopy gaps. Such mortality has 

typically not been viewed as a part of natural 

or, at least, acceptable stand development 

processes. As a consequence, applications 

of silvicultural thinning to create structural 

heterogeneity have been limited.

RECOVERY PERIODS 

BETWEEN DISTURBANCE 

EVENTS

Stand-Replacement 

Disturbance

Stand-initiating natural disturbances vary 

not only in their type, scale, intensity, and 

legacies they produce, but also in their 

frequency. Often, but not always, the return 

interval between stand-initiating events is 

long enough that structural complexity, 

particularly as a result of noncompetitive 

mortality processes, can develop (Fig. 20). 

For example, the predicted return interval 

for Class 4 stand-replacing hurricanes in 

Coastal Plain pine forests in South Carolina 

is 260 years (Myers and van Lear 1998). 

Between these major disturbances, small-scale 

tree mortality events occur with increasing 

frequency as stands age (Palik and Pederson 

1996), leading to development of complex 

heterogeneous stand conditions.

The importance of recovery period for development of 

structural complexity is well illustrated by the dynamics 

of large dead wood in Pacifi c Northwest Douglasfi r forests. Spies et al. (1988) illustrate the temporal 

a

b

Figure 20.—Conceptual representation of the development of stand-scale 

ecological complexity. (a) A young post-disturbance stand begins with 

simplifi ed structure and composition and over time develops signifi cant 

structural complexity due primarily to small-scale canopy disturbance. (b) In 

many traditional forestry approaches, the stand is harvested before signifi cant 

compositional and structural complexity has had time to develop.

dynamics of large wood after stand-initiating fi res (Fig. 

21). They show that initial high loadings of dead wood 

before disturbance increase dramatically after a fi re, 

primarily due to fi re mortality, with lesser contributions 
22

from carryover from the previous stand and, in the case 

of partial burns, contributions from the residual stand 

after the fi re. Wood levels then decline dramatically 

within the fi rst 150 to 200 years of stand development, 

falling well below initial post-disturbance values. Several 

centuries are required before the amount of large, 

persistent dead wood increases to values approaching 

pre-disturbance levels, primarily as a result of small-scale 

canopy disturbances and mortality in the developing 

stand (Agee 1993, Franklin et al. 2002, Hemstrom 

and Franklin 1982). While the time period required to 

develop comparable levels of complexity is probably not 

as long in most forest types as in Douglas-fi r, the general 

premise still applies; it takes a signifi cant period of time 

between major disturbances for small-scale mortality 

processes to create a spatially complex and heterogeneous 

forest.

Single Tree and Gap Disturbances

The concept of recovery period is clearly applicable to 

stand-replacement disturbance regimes. However, there 

is value in considering this concept in the context of 

gap-replacement regimes as well. Forests that develop 

primarily as a result of gap-disturbance regimes, e.g., 

hardwood forest in eastern North America (Frelich and 

Lorimer 1991, Runkle 1982), are spatially heterogeneous 

and structurally complex precisely because gaps occur 

infrequently in space and time, creating a patch mosaic 

of forest in different stages of development. At the stand 

scale, this mosaic of recovering patches includes recent 

canopy openings in which dead wood is recruited, 

resource availability is increased, and new cohorts of trees 

and other plants are established. Although gaps might 

recur at or near the same location by chance, there often 

is a long recovery period between gap events at any one 

location. This allows for the development of large old 

trees, cavities, and ultimately a source of large snags and 

dead wood on the ground.

CONTRASTS BETWEEN RECOVERY 

PERIODS AND ROTATION LENGTH

In commodity-oriented forestry, stands invariably are 

terminated before signifi cant structural complexity has 

developed, even though substantial biomass may have 

accumulated (Fig. 20). Consequently, commercially 

managed stands typically lack trees of very large 

diameter, signifi cant amounts of coarse wood, and 

trees with unique structures (e.g., cavities, large limbs, 

heartwood, and brooms). Moreover, they often are low 

in tree species diversity because the time between harvest 

cycles is insuffi cient for tolerant species to establish or 

to advance into intermediate and co-dominant canopy 

positions in stands. This problem is compounded when 

stands are deliberately or inadvertently simplifi ed during 

establishment, e.g., if no legacies are retained or only a 

single species is planted. An example of this is seen in 

Great Lakes aspen forests. Most of these forests originated 

in the early part of the 20th century after widespread 

Figure 21.—The development of structural complexity 

over time as illustrated by the dynamics of large dead 

wood in Pacifi c Northwest Douglas-fi r dominated forests 

after partial stand-replacement fi re. Dead wood is 

derived from four sources, including carryover from the 

pre-disturbance stand, mortality from the fi re, mortality 

from the post-disturbance residual stand, and the newly 

Stand age (years) developing stand. Redrawn from Spies et al. 1988.

0 100 200 300 400 500

Amount of Large Dead Wood (Mg/ha)

0

100

200

300

400

500

600

Input from new stand

Accumulation from remnant stand

Accumulation from fire mortality

Carryover
23

logging followed by intense slash fi res (Graham et 

al. 1963). Many managed aspen stands are devoid 

of any signifi cant biological legacies and are spatially 

homogeneous (Fig. 22). Moreover, the traditional 

management prescription calls for clearcut harvesting 

of stands at 60 to 80 years of age—typically before 

signifi cant structural and compositional complexity has 

developed.

The motivation for terminating stands before 

development of structural complexity is often driven by 

economic factors. On many timberlands managed for 

commodities the prevailing practice is to base rotation 

age on fi nancial calculations, particularly discounted 

present net value (Davis et al. 2001); long rotations are 

an anathema using this criterion. Government resource 

agencies (e.g., USDA Forest Service) use culmination of 

mean annual growth increment in which rotation age is 

determined by the growth rate of crop trees. A problem 

with this approach is that in unthinned stands, time to 

growth culmination can be quite short, relative to tree 

lifespans. For instance, volume growth of site index 

170 Douglas-fi r culminates at around 65 years (as cited 

in Daniel et al. 1979)—well before much structural 

complexity has developed in this particularly long-lived 

forest type.

FORMULATING AN ECOLOGICAL 

FORESTRY APPROACH

Prescriptions incorporating classical silvicultural systems 

provide a comprehensive plan for regeneration and 

tending of forest stands through time. The classic 

systems encompass all stages of stand development 

from regeneration to harvest of mature trees (Barrett 

1994, Burns 1983, Nyland 2002, Smith et al. 1996). 

Silvicultural approaches that incorporate an understanding 

of natural disturbances and stand development processes, 

i.e., an ecological forestry approach, need to be similarly 

comprehensive. Although we do not address all 

components of an ecological forestry prescription, it is our 

view that at a minimum, relevant management activities 

for ecological forestry must incorporate an understanding 

of tree-regenerating disturbance events that create 

biological legacies, less intense disturbances and mortality 

that affect the structural development of the established 

stand, and the importance of recovery periods between 

disturbance events as a process leading to greater ecological 

complexity within a stand. In this section, we provide 

some guiding principles to develop such prescriptions, in 

the context of the three-legged stool of ecological forestry 

(Sidebar 1). For examples of comprehensive silvicultural 

prescriptions that incorporate ecological concepts, we 

refer the reader to specifi c case studies (e.g., Beese 1995, 

Mitchell et al. 2000, Pecore 1992, Seymour et al. 2006).

An essential fi rst step in applying principles from 

natural disturbance and stand development to 

ecological forestry is understanding that creating and 

perpetuating appropriate structural, functional, and 

compositional attributes is often a primary management 

goal. “Appropriate” here means attributes that achieve 

defi ned goals, which in ecological forestry always 

include ecological objectives. We are not suggesting that 

silvicultural systems should be precisely modeled on 

natural disturbances and stand development processes, 

because this is not possible (Palik et al. 2002). Rather, the 

objective is to understand natural processes and resultant 

patterns and draw upon this understanding to design 

silvicultural approaches that achieve ecological and other 

management goals.

Figure 22.—A regenerating aspen stand in northern 

Michigan. Except for the older eastern white pine, the 

stand is devoid of biological legacies from the previous 

stand and is simplifi ed and homogeneous in structure. 

Photo credit: Kurt Pregitzer.
24

Three principles form the basis of an ecological forestry 

program (in our analogy, the three legs that support the 

stool of ecological forestry): 

Incorporating biological legacies into harvest 

prescriptions

Incorporating natural stand development 

processes, including small-scale disturbance, into 

intermediate treatments 

Allowing for appropriate recovery periods between 

regeneration harvests

We review each of these principles in more detail below.

Principle 1: Incorporating Biological 

Legacies into Harvest Prescriptions

Incorporating management of biological legacies into 

regeneration harvest prescriptions is the fi rst fundamental 

principle of ecological forestry. Referencing our earlier 

discussion, we know that legacies include important 

structures from existing stands, such as large healthy 

trees, decadent trees, snags, and boles, and other coarse 

woody on the forest fl oor. Such structures typically 1) 

persist as legacies even through the most intense standreplacement disturbances; 2) play critical roles as habitat 

and modifi ers of the physical environment; and 3) are 

diffi cult or impossible to re-create once removed from 

managed stands, hence the need to carry them over from 

the pre-disturbance stand. Directly and indirectly, such 

structural legacies lifeboat many elements of biodiversity, 

whether these elements are explicitly identifi ed or not, 

and where a stand-replacement disturbance regime 

prevails, structurally enrich the new stand.

Retention of compositional legacies (e.g., various 

tree species and other plants) also is an important 

consideration. For example, retention of certain species 

is often an explicit element of a retention prescription. 

Sometimes this will actually be done in order to retain 

structures or structural conditions of a specifi c type, 

such as with retention of a hardwood component in 

an otherwise conifer-dominated stand. Compositional 

retention should consider both commercial and 

noncommercial species; historically, noncommercial 

species often are removed despite their ecological value. 

1.

2.

3.

For instance, non-commercial hardwood species have 

routinely been removed in Finno-Scandinavian conifer 

forests, potentially reducing populations of lichens that 

are unique to these species (Kuusinen 1994). Retention 

may also be prescribed to maintain species with special 

functional capabilities, such as nitrogen fi xation.

Increasingly, retention of biological legacies, particularly 

structural legacies, is being incorporated into harvest 

prescriptions throughout the temperate forest regions 

of the world (Franklin et al. 1997, Lindenmayer and 

Franklin 2002, Palik and Zasada 2002, Vanha-Majamaa 

and Jalonen 2001). This approach has been labeled 

“variable retention harvesting” (VRH) and is defi ned as:

“. . . an approach to harvesting based on the 

retention of structural elements or biological 

legacies (trees, snags, logs, etc.) from the harvested 

stand for integration into the new stand to achieve 

various ecological objectives. . . Major variables 

are types, densities, and spatial arrangements of 

retained structures” (Helms 1998).

In British Columbia the approach has been formalized 

legally as the “Variable Retention Harvest System” as 

follows:

“Retention system means a silvicultural system that 

is designed to: (a) retain individual trees to maintain 

the structural diversity over the area of the cut block 

for at least one rotation; and (b) leave more than 

half of the total area of the cut block within one 

tree height from the base of a tree or group of trees, 

whether or not the tree or group of trees is within 

the cut block” (Mitchell and Beese 2002).

VRH prescriptions must address three fundamental issues 

including what to retain, how much to retain, and what 

the spatial pattern of retention should be—e.g., spatially 

dispersed or aggregated in the harvest unit. Decisions on 

these three variables obviously must be related to specifi c 

management goals, priorities among goals, and type 

of silvicultural system used (e.g., stand replacing, twocohort, or selection); consequently, a broad spectrum of 

specifi c prescriptions is possible (Fig. 23) (Franklin et al. 

1997, Lindenmayer and Franklin 2002).
25

Incorporating Legacies into Stand-Replacement 

Systems

Incorporating biological legacies into traditional evenaged harvest practices is the most obvious way to better 

approximate the outcomes of natural stand-replacing 

disturbances. In fact, VRH has most often been used 

to modify traditional even-aged management in forest 

types and sites characterized by stand-replacement 

disturbances. Legacies typically selected for retention are 

large structures--including large old and often decadent 

trees, snags, and downed boles--that fulfi ll many 

important ecological functions (Fig. 24) and often take 

decades to centuries to develop in new stands.

The spatial pattern of retention in stand-replacement 

systems is an interesting issue because some ecological 

objectives are best met by dispersing retained structures 

and others are best served by concentrating the retention 

Figure 23.—Variable retention harvesting 

represents a continuum of possibilities; (a) 

which traditional forest harvest systems 

have only partially exploited; (b) but which 

can be exploited to respond to a range of 

management goals (c).
26

(Aubry et al. 2004, Franklin et al. 1997) (Table 5). 

For example, aggregate retention in stand-replacement 

systems (and in two-cohort systems as described below) 

is an obvious way to provide some protection from 

harvest disturbance to understory plants and the forest 

fl oor, an important consideration in ecological forestry 

(Kirkman et al. 2004). Different patterns of retention 

also may have profoundly different effects on growth 

and productivity of regenerating trees (Palik et al. 2003, 

2005). As the name suggests, one common goal of 

VRH systems is incorporating spatial heterogeneity of 

retention within a harvest unit, ranging from dispersed 

to aggregate within the same harvest unit (Fig. 25).

Modifying clearcut and shelterwood prescriptions to 

incorporate wildlife trees, snags, and logs is widely 

practiced today and represents a fi rst step towards more 

effective legacy management. Even-aged harvesting with 

“reserves” is a more formalized approach to incorporating 

some legacies into traditional harvest methods. However, 

this approach rarely explicitly recognizes the multiple 

ecological roles played by such “reserved” structures or the 

nearly infi nite array of structural retention prescriptions 

possible, such as spatially variable patterns of retention.

Confusion or misunderstanding can also arise from 

use of the term “reserves.” When signifi cant structural 

Table 5.—Hypothesized effects of spatially dispersed and aggregate structural retention on ecosystem characteristics 

and objectives

Characteristic or objective Spatial pattern of retention

Dispersed Aggregate

Microclimate modifi cation Less: generalized over harvest area More: localized within harvest area 

Infl uence of geohydrological processes Same as above Same as above

Maintenance of root strength Same as above Same as above

Retention of diverse tree sizes, species, 

and conditions

Low probability High probability

Retention of large-diameter trees More emphasis Less emphasis

Retention of multiple vegetation layers Low probability High probability

Retention of snags Diffi cult Readily accomplished

Retention of areas of minimal forest fl oor 

and understory disturbance

Limited possibilities Yes 

Retention of structurally intact forest 

habitat patches

Not possible Possible

Distributed sources of coarse woody debris Yes No

Distributed sources of arboreal energy to 

maintain belowground processes

Yes No

Windthrow hazard of residual trees Average wind fi rmness greater (strong 

dominants), but trees are isolated

Average wind fi rmness less, but 

trees have mutual support 

Residual tree damage High probability Low probability

Tree form and geometry Uniform Variable

Distribution of fi ne fuels Uniform Variable

Regeneration growth (intolerant species) Lower (impacts generalized over 

harvest area)

Higher (impacts are localized)

Regeneration growth (tolerant species) Higher Lower (outcompeted between 

aggregates)
27

Figure 24.—Typical structures retained on a 

harvest unit on public lands in the Douglasfi r region, Pacifi c Northwest, include large, 

decadent live trees, large snags, and large 

downed boles, all of which are impossible to recreate in stands managed under even moderate 

(e.g., 100-year) rotations.

Figure 25.—Variable retention harvesting in Oregon. Spatial pattern of retention includes both 

dispersed and aggregate within the same harvest unit.
28

legacies are retained in an area identifi ed as a clearcut, 

most observers will be confused because the area is clearly 

not a clearcut. Using terms like “reserve shelterwood” 

or “seed tree” is misleading when the primary goal for 

retaining large trees is sustaining ecological services, 

rather than regeneration potential.

In marking structures for retention, attention should be 

given to retaining a diversity of tree species. “Cleaning” 

stands by eliminating noncommercial species may have 

signifi cant negative impacts on biological diversity 

(Palik and Engstrom 1999). Similarly, emphasis should 

be given to retaining trees across a range of size classes 

and levels of decadence. Different types of disturbance 

tend to remove trees predominantly from one end of 

the size range or the other; for example, wind tends to 

thin from above while surface fi re thins from below. 

However, most natural disturbances leave trees across a 

range of sizes (e.g., Elmqvist et al. 2001, Palik and Robl 

1999, Peterson 2004), unlike traditional shelterwood and 

seed tree applications. Retaining trees in various states 

of decadence is important for providing critical habitat 

features, such as cavities, and to ensure a sustained source 

of large dead wood.

Based on these guidelines, an important question when 

judging the ecological success of retention is whether 

the retained species and structures (e.g., snags) represent 

the range of sizes and physical conditions that typically 

exist in the stand following a natural disturbance. If the 

answer to this question is no, then the success of the 

retention harvest from an ecological standpoint may be 

in doubt. If retained trees consist primarily of the poorly 

formed, small, and noncommercial elements, then the 

prescription is certainly inappropriate.

Incorporating Legacies into Two-Cohort Systems

Two-cohort management systems provide excellent 

opportunities to incorporate concepts of legacy 

management into silviculture with only minor 

adjustments (O’Hara 2001). The key adjustment is 

to incorporate greater heterogeneity in structural and 

compositional conditions in the post-harvest stand. For 

example, retention of overstory trees in two-cohort stands 

can range from relatively dispersed to relative aggregated 

within the same harvest unit, resulting in a variety of 

resource and competitive environments (Palik and Zasada 

2002).

An important and broadly applicable principle in 

managing two-cohort stands is to sustain some desired 

population of large old trees (Fig. 26) and the derivative 

large snags and large downed boles they provide. This is 

one goal implicit in most VRH prescriptions, whether 

Figure 26.—Overstory retention of eastern 

white pine, with a new cohort of pines 

developing below it, after partial harvest 

in a Great Lakes mixed-pine ecosystem, 

Minnesota. A goal in this stand is to retain 

a population of large old pines for extended 

periods for both ecological and economic 

reasons. Photo credit: Elizabeth Jacqmain.
29

in even-, two-, or uneven-aged stands. However, 

perpetuating a specifi c tree population obviously goes 

beyond the immediate goal of lifeboating organisms and 

processes and makes explicit the need to manage the 

remainder of the stand to provide for replacement trees.

Incorporating Legacies into Selection Systems

Structural and compositional retention is also relevant 

to silvicultural prescriptions for multi-cohort stands, 

i.e., for forest types subject to tree- or gap-based 

disturbance regimes. Selection silviculture can be 

modifi ed to incorporate biological legacies at the 

tree and gap scale, along with stand-level goals that 

provide for compositional richness and structural 

complexity characteristic of gap-driven systems. Such 

an approach maintains the landscape matrix in a mature 

forest condition, which is one of the most important 

environmental aspects of uneven-aged forestry. Marking 

guidelines can explicitly incorporate goals of maintaining 

old and large trees indefi nitely and as a consequence 

as source for large snags and downed boles. Examples 

of such approaches are being applied in several regions 

(Keeton 2005, Mitchell et al. 2000, USDA Forest 

Service 2004).

Prescriptions based on group selection can and usually 

should explicitly incorporate retention of trees, snags 

and downed boles, particularly at larger gap sizes 

(Fig. 27). Retention within the gaps can be augmented 

by permanently reserving a certain percentage of the 

stand from any future harvest, thereby ensuring that 

some completely intact structural patches will persist 

indefi nitely. Such prescriptions are being applied to 

such diverse forest types as coast redwood and eastern 

hardwood forests. Group selection prescriptions in 

mixed-conifer types on national forests in the Sierra 

Nevada provide for structural retention by requiring that 

all trees >30 inches diameter at breast height (d.b.h.) 

be retained; the intention is to retain these structures in 

perpetuity (Verner et al. 1992).

Opening sizes and shapes prescribed under group 

selection should also vary to more closely match the sizes 

and shapes of gaps created by natural disturbances, rather 

than using a standard Varying group size to match the 

variability in opening ized (“cookie cutter”) opening. 

sizes observed in natural stands is likely to result in a 

distribution skewed towards smaller average opening 

sizes. Although tree growth rates, particularly of shadeintolerant species, may be less than those achievable 

in clearcuts, reduced growth rates should not be a key 

concern when practicing ecological forestry. In many 

cases, establishment and early survival of regeneration are 

likely to be much better with partial shade than in the 

open conditions associated with larger openings, even 

for species highly intolerant of shade, such as Douglas-fi r 

(e.g., Isaac 1943) and longleaf pine (e.g., McGuire et al. 

2001).

Incorporating variable structural objectives within 

individual stands should be considered for individual 

tree and small group selection systems (Mitchell et al. 

2000). As an example, silvicultural systems that create 

(if not already present) and maintain a population 

of large old trees is appropriate for many forest types 

subject to gap- or tree-based disturbance regimes. North 

American mixed conifer forests characterized by low 

severity fi re regimes are an example. Large old trees and 

their derivative structures (large snags and downed boles) 

were constant and important structural elements of such 

forests (Covington and Moore 1994, Noel et al. 1998). 

A management goal in many such forests is to restore 

and maintain a target population of large old trees, but 

without requiring that this goal be met on every hectare 

(e.g., Johnson et al. 2003).

Figure 27.—Structural retention with a group selection 

opening on the Chippewa National Forest, Minnesota.
30

Principle 2: Intermediate Treatments 

that Enhance Heterogeneity

Managing established stands to sustain or restore 

structural and compositional heterogeneity is the second 

important principle of ecological forestry. The primary 

way this is accomplished is through innovative uses of 

thinning. For an ecological forestry approach, the goal 

of thinning is to create structural and compositional 

heterogeneity throughout the stand, rather than to 

concentrate growth on selected trees and create spatially 

uniform stands, as in a traditional forestry approach.

As discussed previously, thinning treatments are generally 

modeled on natural decline and mortality of trees during 

stand development. Our focus here is on thinning 

treatments that are carried out in ways that create and 

maintain structural and compositional complexity and 

heterogeneity including:

Thinning to stimulate development of larger 

trees

Variable density thinning to stimulate 

development of horizontal heterogeneity 

Small gap creation to develop vertical and 

horizontal heterogeneity and opportunities for 

establishing and releasing regeneration and other 

understory components

Standard silvicultural thinning is intended to anticipate 

natural competition-induced mortality by removing 

suppressed trees before they die from resource limitations 

(thinning from below). An important consequence 

of such thinning is the release of growing space and 

resources for the remaining trees. Appropriately 

implemented thinning from below accelerates the 

development of large-diameter and high-quality trees at 

rates faster than would occur naturally. The ecological 

benefi ts of large trees is refl ected in their use as habitat 

for nesting, roosting, and avoidance of predators, their 

infl uence on microenvironment, and their contribution 

of food resources and fi ne fuels. Moreover, large trees are 

important for their derivatives, large snags, wood on the 

ground, tip-up mounds, and root wads.

•

•

•

There are some fundamental differences in the way 

thinning is implemented for ecological versus economic 

goals, even though they have some comparable 

objectives, such as creation of large diameter trees. 

Standard silvicultural thinning is intended to create an 

evenly distributed population of crop trees, all having 

similar access to light, water, and soil nutrients. In 

contrast, natural stands undergoing competitive thinning 

often display some spatial variation in tree densities, 

growth rates, and tree sizes.

As discussed previously, competitive-based mortality is 

augmented by small-scale canopy disturbances, from 

wind, lightning, insects, or fi re. Small-scale disturbances 

are a fundamental feature of natural stand development 

and contribute greatly to the development of spatial 

heterogeneity in stand structure (Franklin et al. 2002). 

We argue that appropriate silvicultural analogs should 

specifi cally plan for thinning treatments that replicate the 

small-scale disturbance or gap-forming processes, along 

with competition-based mortality.

Within-stand variation in competitive tree mortality, as 

well as incorporation of small-scale canopy disturbance 

as an intermediate treatment, can be accomplished 

simultaneously using an approach known as variable 

density thinning of VDT (Sidebar 5, Fig. 28). VDT 

approaches emulate the natural variation that results 

from small-scale canopy disturbances and competitionbased mortality (Carey 2001). VDT prescriptions 

provide for unthinned areas (sometimes referred to as 

“skips”) and heavily thinned patches (“gaps”), along 

with intermediate levels of thinning and residual 

density through the bulk of the stand (Franklin and 

Lindenmayer 2002, Harrington et al. 2005). The result 

is much greater spatial variability in stand densities 

and, consequently, greater structural complexity 

and heterogeneity of structure (Fig. 29). Ecological 

benefi ts of VDT include development of large trees, 

opportunities for release or new establishment of woody 

and herbaceous species, and creation of spatially variable 

microclimatic and habitat conditions.
31

Sidebar 5.—Variable Density Thinning (VDT)

In practice VDT is implemented by envisioning 

a grid pattern in the stand that is to be thinned 

(Harrington et al. 2005). It may be useful to actually 

lay out a grid on the ground (Fig. 28). Thinning 

treatments are assigned to each cell, including gaps 

(removal of all or most trees), skips (no thinning), 

and thinned. Grid cell size should be defined by 

characteristics of the forest type (e.g., sizes of 

small canopy gaps) and objectives. In applications, 

cell sizes of 0.04 to 0.10 ha for gaps and 0.10 to 0.30 

ha for skips have been used, with 10 to 20 percent 

of cells assigned to gaps, 10 to 20 percent assigned 

to skips, and the remainder assigned to some level 

of standard thinning, which can also incorporate 

variability in resultant density from cell to cell.

When assigning cells to gaps or skips, the 

practitioner should look for opportunities to tie 

cell treatments to existing features of the stand. 

For instance, gap cells might be placed in areas 

that already have a natural opening or advanced 

regeneration of a desired species. Similarly, 

skip cells might be assigned to protect sensitive 

features, such as a large snag, small wetland, or 

patch of understory vegetation. When selecting 

cell size and the distribution of treatments, it is 

important to remember that the goal of VDT is 

creation of distinctly heterogeneous conditions 

across the stand. Very small cell sizes may not 

result in distinctly different resource and habitat 

conditions across the stand; similarly, large cell 

sizes can result in gaps that approach the size of 

small clearcuts within the stand.

Figure 28.—Grid approach for implementing 

variable density thinning.

-0.10 ha grid scale 

-Vary thinning by 0.10 ha units 

-20% skips (black) 

-20% gaps (light gray) 

-60% thinned (gray) 

Figure 29.—Stylized representation of variable density thinning: (a) unthinned stand; (b) thinned stand displaying 

horizontal variation in stand density including gaps, skips (unthinned areas), and lightly thinned matrix.
32

Sidebar 6.—Intermediate Treatments that Enhance Structural 

Heterogeneity of a Stand

Thinning is perhaps the most important intermediate treatment that can be used to enhance 

structural complexity and heterogeneity in developing stands. However, some additional 

treatments or approaches can also be used to pursue this objective. Although discussing these 

activities in detail is beyond the scope of this report, we mention several below.

Decadence creation

This technique involves deliberate killing of living 

trees or injuring them to induce decline, with 

the goal of creating dead wood in the form of 

snags or logs on the ground. Felling trees and 

girdling trees are examples of techniques used 

to accomplish this. A more creative approach 

in systems prone to lightning strikes involves 

injuring trees in a way that induces a decline and 

ultimately death, analogous to what might happen 

after a lighting strike. One technique used to do 

this involves placing explosive charges within the 

crown of the tree above the lowest live branches. 

The resultant explosion will remove most, but not 

all, of the live crown, followed by death of the 

tree over one or more years (Fig. 30).

Underplanting

Successional processes in developing stands 

often involve the establishment of more shadetolerant tree species from local or introduced 

seed. When such seed sources are lacking due to 

past management activities, it may be of interest 

to underplant the appropriate species with the 

goal of reintroducing the missing compositional 

element in the stand. For example, eastern white 

pine might be underplanted in Great Lakes aspen 

forests, where past management or disturbances 

have removed most local seed sources. 

Underplanting might be coordinated with variable density thinning, targeting mid-tolerant species 

in gaps and more tolerant species in lightly thinned or unthinned portions of the stand.

Prescribed fi re

Prescribed surface fire is an underused intermediate treatment in many fire-dependent forest 

types and one that lends itself to goals of sustaining or restoring understory heterogeneity. 

Natural surface fires are inherently variable, both in the pattern and size of unburned patches 

within a burned mosaic, and in the intensity and energy released in the flaming front. As such, 

surface fires can be used to enhance spatial heterogeneity of understory conditions in burned 

stands. This variability is in turn a strong driver of high plant diversity that is often a characteristic 

of fire-dependent ecosystems. Longleaf pine forests of the Southeastern U.S. are well known for 

this relationship. These systems begin to lose plant diversity on the most productive sites when 

the burn interval increases beyond once every 3 years and on the less productive sites when 

the burn interval extends longer than once every 5 years. Frequent surface fire is also important 

for maintaining characteristic stand structures and reducing catastrophic fire risk in systems 

dependent on regular burning. For instance, without sufficient burning frequency, fire regimes 

in ponderosa pine and mixed conifer forests of the Western U.S. shift from low/mixed intensity 

fires to crown fires, due to dense regeneration providing ladder fuels to connect surface fires to 

canopy fuels. 

Figure 30.—A red pine snag created by 

removing most of the crown through use of 

small explosive charges, Chippewa National 

Forest, Minnesota. Photo credit: Harvey Tjader.
33

Many other intermediate treatments can be used to 

enhance stand-scale complexity and heterogeneity 

(Lindenmayer and Franklin 2002). Discussing these 

in detail is beyond the scope of this report. However, 

several of these approaches, including decadence creation, 

underplanting, and prescribed fi re, deserve consideration 

(Sidebar 6, Fig. 30).

Principle 3: Allowing for Appropriate 

Recovery Periods

The third principle of ecological forestry is to allow 

for appropriate recovery periods between management 

entries, especially regeneration harvests (in which case the 

recovery period is traditionally known as the rotation), 

to allow complexity to develop (Fig. 20). In ecological 

forestry, rate of development of desired structures or 

structural conditions largely determines the length of 

the recovery period, although these goals do have to be 

integrated with social and economic objectives. Moreover, 

it may be desirable to use silvicultural treatments during 

stand development, such as variable density thinning, 

to increase the rate at which desired structural features 

develop (e.g., Carey 2001, Tappeiner et al. 1997).

The principle of allowing appropriate ecological recovery 

periods between regeneration harvests is potentially a 

contentious issue because these periods will almost 

always be much longer than rotations based on 

economic factors and probably longer than rotations 

based on growth factors, such as culmination of 

annual increment. However, culmination of growth 

increment can be delayed for extended periods of time, 

through periodic thinning (Curtis 1995, Williamson 

1982). This suggests that ecological objectives linked 

to large old trees can be achieved alongside economic 

objectives, if the latter involve development of largediameter, high-quality logs. For example, the mean 

annual volume increment curve for a 140-year-old 

red pine stand in northern Minnesota shows extended 

periods of continued growth, due to periodic growth 

increases after repeated thinnings which began at 

age 85 (Fig. 31). In this stand, the quality and value 

of wood for sawtimber, large poles, and cabin logs 

continue to increase, as does the ecological signifi cance 

associated with growing large old red pines.

In short, for ecological forestry, there is little reason to 

set a rotation age of a stand based solely on fi nancial or 

growth factors. Rather, the primary determination of 

harvest age should be the development of acceptable 

levels of structural complexity, compositional diversity, 

and within-stand heterogeneity.

ECOLOGICAL FORESTRY: HOW 

CLOSE TO A NATURAL MODEL?

Mimicking natural disturbance regimes and stand 

development processes with silviculture is a challenge, 

if for no other reason than the conditions created by 

natural processes can never be fully achieved in forests 

that are also managed for wood production (Palik et 

al. 2002). However, failure to precisely duplicate these 

conditions is not a shortfall of the approach but, rather, a 

consequence of the desire to achieve multiple objectives, 

i.e., to maintain ecological values and produce some 

wood for extraction.

The challenge then is to develop approaches that 

lead to maintenance (or restoration) of ecological 

complexity, along with opportunities to meet certain 

timber management goals. The balance may shift toward 

one goal or the other at different times and different 

Age (years)

85 95 105 115 125 135 145

Volume Growth (ft3/ac)

10

20

30

40

50

Figure 31.—Volume increment curves for red pine in the 

Cutfoot Experimental Forest, Minnesota. The forest was 

thinned seven times, to a growing-stock basal area of 60 ft2

/ac 

beginning at a stand age of 85 years. The dashed line is mean 

annual increment; the solid line is periodic annual increment 

(B. Palik, unpublished data).
34

locations, depending upon priorities among management 

objectives. In many cases, the ultimate objective is to 

facilitate implementation of silviculture based upon 

natural disturbance and stand development models 

without completely ignoring economic objectives. If the 

latter is ignored, then the former will never be widely 

implemented at scales needed to achieve signifi cant 

change in the landscape.

A useful framework for assessing how well the goals 

of ecological forestry are being met in stands managed 

for wood production is to quantify discrepancies in 

complexity between these stands and reference conditions 

(Sidebar 7, Fig. 32). While the nature of the reference 

condition is debated (Hunter 1996), and trying to defi ne 

it is beyond the scope of our discussion, it is important 

to recognize that for any specifi c forest type the reference 

condition actually consists of a domain of conditions 

that are naturally variable, i.e., a natural range of 

variation (Landres et al. 1999). As such, reference targets 

best represent ranges, rather then central tendencies. 

Moreover, the reference condition includes not only 

old-growth stands, but also those in various stages of 

post-disturbance recovery, including very young stands, 

which typically display high levels of structural and 

compositional complexity, even after a stand-replacing 

disturbance (Franklin et al. 1997). Thus, comparisons of 

managed stands to reference conditions should be made 

at all stages of development.

When making these comparisons, managers should 

determine how similar to the reference condition a stand 

needs to be to achieve ecological forestry goals. The 

answer is driven by objectives. When managing ecological 

reserves, the goal probably will be to get as close to 

the reference condition as possible. When managing 

for maximum fi ber yield, reducing the disparity with 

the reference condition perhaps is not a consideration. 

When managing for outcomes within the bulk of the 

forest landscape, reducing the disparity to the reference 

condition is likely an important consideration, but in 

many instances it will be done within the real-world 

constraints of managing for wood production. In 

this case, the objective is to devise innovative ways to 

incorporate the three-legged stool of ecological forestry 

into silvicultural prescriptions, while still maintaining 

some level of timber production as another objective.

Simberloff (1999) suggests that any proposed 

silvicultural system designed to maintain biodiversity and 

produce timber should be treated as a hypothesis, due 

to the limited number of empirical studies to support 

or refute the approach. The comparative framework 

we outline can be used to pose such hypotheses and 

then design silvicultural experiments to test them. For 

instance, one might hypothesize that as managed systems 

move closer to the reference condition, productivity 

of bole wood declines linearly, exponentially, or not 

at all. Alternatively, one could test the hypothesis that 

the amount of complexity in managed stands can 

be increased while maintaining a fi xed level of wood 

production.

The challenge many foresters and forest management 

organizations now face is the need to develop silvicultural 

systems that result in stand conditions that incorporate 

ecological complexity and heterogeneity in much greater 

degrees than have been considered before (i.e., moving 

stands closer to reference conditions). Moreover, there 

is a need to implement these systems on large portions 

of the forest landscape, i.e., most of the forest where 

managing for ecological objectives along with wood 

production is a driving concern. We have intended to 

facilitate both development and implementation of 

such systems by providing an overview of the scientifi c 

underpinnings of ecological forestry and by synthesizing 

these underpinnings into generally applicable principles, 

i.e., the three-legged stool of ecological forestry. Our 

intent is not to provide a cookbook for developing 

such systems nor to provide comprehensive silvicultural 

prescriptions. This is an impossible task given the 

diversity of ecosystems, objectives, and conditions that 

must be considered. Moreover, an attempt to do so 

would be contrary to the creative intent of silviculture 

as a discipline. Rather, we hope that by distilling key 

concepts into practical guidelines we can facilitate the 

development of practices that are adaptable to meet the 

varied needs and conditions in a wide array of forest 

ecosystems.
35

Sidebar 7.—Practical Considerations for Ecological Forestry: 

How Close to a Reference Condition?

A useful framework for addressing ecological forestry in stands managed for wood is a 

conceptual model that arrays stands along gradients of (1) management objectives, ranging 

from production forestry to reserve management, with ecological or matrix management 

between these endpoints; (2) time or age since disturbance; and (3) variability in degrees 

of complexity among stands (Fig. 32). In this model, the area in gray represents the 

domain of natural variability in reference conditions for the system in question, as defined 

by combinations of time since disturbance and degree of structural and compositional 

complexity. In other words, this domain represents the array of possible reference 

conditions for stands in that system; some of which are naturally more complex and some of 

which are naturally less complex (relative to each other). The dashed rectangles are domains 

of variability in complexity for management scenarios that differ in the degree to which 

they achieve multiple objectives of sustaining ecological complexity and wood productivity. 

Domain A might represent a plantation of an exotic species managed intensively for fiber; 

note that the range of variability in complexity is not only narrow but also lies outside the 

range of natural variability. Domain B might include systems managed for large-diameter 

saw logs and structural complexity, perhaps using two-cohort stands. Notice that the range 

of variability is narrow relative to the reference condition, but still within the domain of 

natural variability. Domain C might represent a system managed for maximum similarity to a 

reference condition, using limited harvesting of lightning- or wind-killed trees, for instance. 

Figure 32.—Three-dimensional conceptual model for judging disparity in 

ecological complexity between managed forests and reference conditions.
36

ACKNOWLEDGMENTS

Funding for this project was generously provided by 

the National Council for Science and Sustainable 

Forestry. Support also came from the USDA Forest 

Service Northern Research Station, the Robert Woodruff 

Foundation, the USDA Forest Service Pacifi c Northwest 

Research Station, and McIntyre-Stennis funding.

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Franklin, Jerry F.; Mitchell, Robert J.; Palik, Brian J. 2007. Natural disturbance and 

stand development principles for ecological forestry. Gen. Tech. Rep. NRS-19. 

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

Research Station. 44 p.

Foresters use natural disturbances and stand development processes as models 

for silvicultural practices in broad conceptual ways. Incorporating an understanding 

of natural disturbance and stand development processes more fully into silvicultural 

practice is the basis for an ecological forestry approach. Such an approach must 

include 1) understanding the importance of biological legacies created by a tree 

regenerating disturbance and incorporating legacy management into harvesting 

prescriptions; 2) recognizing the role of stand development processes, particularly 

individual tree mortality, in generating structural and compositional heterogeneity 

in stands and implementing thinning prescriptions that enhance this heterogeneity; 

and 3) appreciating the role of recovery periods between disturbance events in the 

development of stand complexity. We label these concepts, when incorporated into a 

comprehensive silvicultural approach, the “three-legged stool” of ecological forestry. 

Our goal in this report is to review the scientific basis for the three-legged stool of 

ecological forestry to provide a conceptual foundation for its wide implementation.

KEY WORDS: ecological forestry, silviculture, biological diversity, stand complexity, 

biological legacies, ecological management
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Capitalizing on the strengths of existing science 

capacity in the Northeast and Midwest to attain a more 

integrated, cohesive, landscape-scale research program