A

United States 

Department of 

Agriculture

Forest Service

Pacific Southwest 

Research Station

General Technical Report

PSW-GTR-220

(Second printing, 

 with addendum)

March 2009

An Ecosystem Management 

Strategy for Sierran MixedConifer Forests

Malcolm North, Peter Stine, Kevin O’Hara, William Zielinski,

and Scott Stephens
The Forest Service of the U.S. Department of Agriculture is dedicated to the principle of 

multiple use management of the Nation’s forest resources for sustained yields of wood, 

water, forage, wildlife, and recreation. Through forestry research, cooperation with the 

States and private forest owners, and management of the National Forests and National 

Grasslands, it strives—as directed by Congress—to provide increasingly greater service 

to a growing Nation.

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Authors 

Malcolm North is a research ecologist and Peter Stine is a program manager, 

U.S. Department of Agriculture, Forest Service, Pacific Southwest Research 

Station, Sierra Nevada Research Center, 1731 Research Park Drive, Davis, CA 

95618, mpnorth@ucdavis.edu, pstine@fs.fed.us. Kevin O’Hara is a professor 

of silviculture and Scott Stephens is an associate professor of fire sciences, 

Department of Environmental Science, Policy, and Management, University of 

California, Berkeley, CA 94720-3114, ohara@nature.berkeley.edu, stephens@

nature.berkeley.edu. William Zielinski is a research ecologist, U.S. Department 

of Agriculture, Forest Service, Pacific Southwest Research Station, Redwood 

Sciences Lab, 1700 Bayview Drive, Arcata, CA 95521, bzielinski@fs.fed.us.

Cover photos, clockwise from top: Lodgepole and white pine forest on the Lee 

Lake Trail, by Malcolm North; fisher, by Bill Zielinski; Aspen Valley mixed 

conifer, by Malcolm North; prescribed fire, by Malcolm North; and heavy fuel 

load, by Eric Knapp.

2nd printing, with addendum, February 2010
i

Abstract

North, Malcolm; Stine, Peter; O’Hara, Kevin; Zielinski, William; Stephens, 

Scott. 2009. An ecosystem management strategy for Sierran mixed-conifer 

forests. Gen. Tech. Rep. PSW-GTR-220. Albany, CA: U.S. Department of 

Agriculture, Forest Service, Pacific Southwest Research Station. 49 p.

Current Sierra Nevada forest management is often focused on strategically reducing 

fuels without an explicit strategy for ecological restoration across the landscape 

matrix. Summarizing recent scientific literature, we suggest managers produce 

different stand structures and densities across the landscape using topographic 

variables (i.e., slope shape, aspect, and slope position) as a guide for varying 

treatments. Local cool or moist areas, where historically fire would have burned 

less frequently or at lower severity, would have higher density and canopy cover, 

providing habitat for sensitive species. In contrast upper, southern-aspect slopes 

would have low densities of large fire-resistant trees. For thinning, marking rules 

would be based on crown strata or age cohorts and species, rather than uniform 

diameter limits. Collectively, our management recommendations emphasize the 

ecological role of fire, changing climate conditions, sensitive wildlife habitat, and 

the importance of forest structure heterogeneity.

Keywords: Climate change, ecosystem restoration, forest heterogeneity, 

forest resilience, topographic variability, wildfire.
ii

Addendum

After reading the first printing of this general technical report (GTR), forest 

managers have raised a number of issues that could use clarification and further 

detail. A second printing of the paper allows us to include this addendum 

addressing some of these issues, clarifying the paper’s intent, and adding 

some relevant recent publications.

A central concern has been that some of the concepts presented in the GTR 

would constrain a manager’s ability to design and implement forest management 

plans and practices based on local conditions. Our paper is not intended as a 

“standards and guides” that prescriptively dictates forest management. Prescriptive 

guidelines applied to the entire Sierra Nevada often frustrate best management 

practices, which need flexibility to respond to regional and local differences in 

climate, topography, soils, and forest conditions. We realize that managers already 

face many constraints when designing landscape-level treatments, some of which 

can be overcome more easily than others (Collins et al., in press). Rather than being 

prescriptive, the GTR’s intent is to provide a conceptual approach for managing 

Sierran forests, against which proposed management plans and practices can be 

evaluated. Managers already use many of concepts presented in the GTR, but it has 

been difficult to communicate how variable forest conditions are created without a 

research-based conceptual framework for landscape management. 

The GTR’s principal intent is to summarize the latest science on how land 

managers can treat forests to concurrently provide for fuels reduction, ecosystem 

restoration, and wildlife habitat. Varying treatments in response to site conditions 

and existing forest structure can often create such variability. For example, some 

areas do not have enough topographic relief to be categorized into different aspects, 

slope positions, or slope steepness. These conditions, however, need not frustrate 

efforts to apply the concepts presented in the GTR. The paper’s intent is offer alternatives to manage for different structural and fuel conditions while retaining extant 

habitat structures (i.e., hardwoods, large snags, groups of large trees). Keying off 

of these structures, the forest can be treated to provide both clusters of high canopy 

cover for wildlife and more open restoration conditions that favor rapid pine growth 

(Bigelow et al. 2009), better resilience to insects and pathogens, and with lower fire 

hazards. The GTR’s concepts might provide challenges but also present opportunities to design and implement new prescriptions and management practices. 
iii

Several specific issues have been repeatedly raised:

1. What are the silvicultural prescriptions that can be used to implement 

the concepts presented in the GTR? We have not developed specific prescriptions as the GTR presents concepts merely to guide resource specialists who best know their local forest conditions. The GTR’s concepts neither 

require nor preclude the use of any particular silvicultural prescription. 

Marking guidelines based on leaf area index have not yet been developed for 

the Sierra Nevada, but in the future may offer greater flexibility for designing silvicultural prescriptions grounded in more direct measures of physiological and ecological processes. 

2. In designing treatments to increase heterogeneity, what is the size of 

tree groups that should be retained and gaps that should be created in 

treated stands? Local stand conditions will often determine what size tree 

groups and gaps can be created. High canopy cover areas are usually defined 

by groups of larger trees, which at the smallest scale are generally less than 

10 individuals. This scale of tree clustering has been found both in unmanaged mixed conifer (E. Knapp unpublished data, North et al. 2004) and giant 

sequoia/mixed-conifer (Piirto and Rogers 2002) forests. Tree groups also 

occur at larger sizes such that several small-scale clusters are nested within 

a larger group. However, if a contiguous group of trees becomes too large, 

forest conditions can add to potential fire intensity. After a wildfire these 

large groups may no longer provide the high canopy cover habitat preferred 

by some sensitive species. Gaps impede fire spread (Agee and Skinner 2005) 

and therefore may reduce fire severity in forests where high canopy cover 

groups are retained. Stand structure reconstructions in mixed-conifer/giant 

sequoia stands suggest a wide range of gap sizes with most less than 0.5 

ac (Piirto and Rogers 2002). Locating gaps in areas with thinner soils or 

lower productivity may be logical to foster lower canopy cover since these 

areas historically supported lower tree densities and fuel loads (Meyer et 

al. 2007b). In the forest matrix between tree groups and gaps, frequent-fire 

forests generally consisted of widely spaced, large trees, most of which were 

pines. The relative proportion of these conditions (i.e., low density, dispersed 

large trees, and large and small gaps and tree groups) and their composition 

could be varied depending on existing forest conditions and topographic 

position.
iv

3. How should social and economic issues be addressed in applying the concepts presented in the GTR? The GTR does not address social or economic 

issues. We recognize that providing socioeconomic benefits, including the provision of a sustainable supply of timber, is part of the mandate of the USDA 

Forest Service. We also recognize that the economic viability of a project can 

affect whether the project will be implemented. We have not attempted to 

address these issues that are best resolved in the public arena. National forest staff, including interdisciplinary teams, which typically prepare National 

Environmental Policy Act (NEPA) documents for projects, are better qualified 

to address these issues. 

4. How should the concepts presented in the GTR be applied to the management of plantations? Some of the general principles outlined in the GTR 

can be applied to stand-level plantations, especially the ideas of increasing 

spatial heterogeneity. The high uniformity of plantations (both spatially and 

temporally) makes them vulnerable to catastrophic change from fire, insects, 

and disease (single-species plantations are the most vulnerable). Both precommercially thinned and unthinned plantations can experience high mortality 

in most wildfire conditions (Stephens and Moghaddas 2005b, Kobziar et al. 

2009). Modifying plantation tree density will not reduce the probability of 

mortality unless surface fuel loads are reduced and height to the base of the 

live crown is increased. Once a plantation reaches the stem-exclusion phase 

(Oliver and Larson 1996), silvicultural activities that produce gaps, increase 

the height to live crown (i.e. pruning, mastication, prescribed fire), and reduce 

surface fuels will increase plantation resiliency and resistance to fire. 

5. Are there situations where thinning intermediate-size pine trees would 

have ecological benefits for Sierra Nevada mixed-conifer forests? Yes. 

We overstated the need to avoid thinning pine trees, particularly larger pines 

(p. 24 “Thinned intermediate-size trees should only be fire-sensitive, shadetolerant species...”; p. 31 “silvicultural prescriptions would only remove 

intermediate-size trees when they are shade-tolerants on mid or upper slope 

sites.”). In general, leaving pine and thinning white fir, Douglas-fir, and 

incense-cedar will help restore historical species composition and increase 

the forest’s fire resilience. There are forests, however, where removing pine 

can reduce fuels, decrease the risk of drought or insect induced mortality, 

and accelerate the growth of the residual pine trees. We encourage thinning 

prescriptions that can be adaptable to existing stand structures and site 

conditions. 
v

6. What characterizes a “defect” tree that can provide wildlife habitat?

Although the importance of “defect” trees for wildlife habitat is widely 

acknowledged amongst managers, these trees are still marked for removal in 

many of the stands we’ve seen. Some silviculture technician classes may still 

be teaching thinning marks based on stand improvement with an emphasis on 

removing “defective trees.” Unfortunately, we are not aware of any quantitative or visual guide to identify tree sizes and defects associated with wildlife 

use (i.e., platforms, mistletoe brooms, forked tops and cavities) for the Sierra 

Nevada. Research is needed to determine the size and number of “defect” 

trees that may be required to maintain or improve wildlife habitat. We plan to 

develop such a guide shortly. In the interim, managers may wish to examine 

the Bull et al. (1997) guide to identifying important wildlife trees in the interior of the Columbia River basin.

7. How is ecological restoration defined in the GTR? In the face of changing 

climate conditions, our focus is on increasing ecosystem resiliency. This focus 

is consistent with that described in USDA Forest Service Manual 2020.5, 

which defines ecological restoration as: “The process of assisting the recovery 

of resilience and adaptive capacity of ecosystems that have been degraded, 

damaged, or destroyed. Restoration focuses on establishing the composition, 

structure, pattern, and ecological processes necessary to make terrestrial and 

aquatic ecosystems sustainable, resilient, and healthy under current and future 

conditions.” 

8. In the face of changing climate conditions, how can managers improve 

forest resiliency? One measure of resilience is that disturbance produces 

mortality patterns consistent with the dynamics under which the forest 

evolved. Mixed-conifer resilience might be best ensured by (1) reducing 

fuels such that if the forest burned, the fire would most likely be a lowseverity surface fire (Hurteau et al. 2009, Stephens et al. 2009b) and (2) 

producing a forest structure that keeps insect and pathogen mortality at 

low, chronic levels. In some fire-suppressed forests, mortality from bark 

beetles has shifted to large-scale, episodic occurrences (Fettig et al. 2008). 

One method of changing this pattern is to reduce tree moisture stress and 

subsequent bark beetle activity by reducing stand density with mechanical 

thinning and prescribed fire (Negron et al. 2009). Evidence suggests tree dieoffs may be increasing in some forests (Breshears et al. 2009, Lutz et al. 2009, 

van Mantgem et al. 2009). The mechanisms behind such die-offs are complex, 
vi

but in fire-dependent forests often there is a cascade of effects linking 

water stress and bark beetle attacks (Ferrell and Hall 1975, Ferrell et al. 

1994, McDowell et al. 2008, Stephens and Fule 2005). Drought can produce 

prolonged periods of stomata closure preventing carbon dioxide absorption 

needed for growth, or “carbon starvation” (McDowell et al. 2008). Carbonstarved trees may not be able to mount sufficient defenses to ward off insects 

(Breshears et al. 2005, Gower et al. 1995). In dense, fire-suppressed stands, 

thinning can significantly reduce the amount of transpiring leaf area often 

leading to decreased transpiration and increases in soil water content (Ma et 

al., in press, Zou et al. 2008). Even when no difference is detected in soil water 

content between thinned and control stands, it is common to detect improved 

water uptake and metabolic function of the remaining trees. This may be 

due to increased per-tree water availability (Brodribb and Cochard 2009). 

Fuels reduction thinning has reduced water stress, as measured by predawn 

water potential, in many ponderosa and Jeffrey pine stands (McDowell et al. 

2003, Sala et al. 2005, Simonin et al. 2006, Walker et al. 2006). The thinning 

intensity needed to reduce moisture stress and the associated risk of bark 

beetle infestations will differ based on local site, soil, and stand conditions 

(Meyer et al. 2007b). 

Prescribed fire can also reduce tree density, but some studies have found an 

immediate, short-term (1 or 2 years) increase in bark beetle damage and tree 

mortality following fire (Fettig et al. 2008, Youngblood et al. 2009). After 

that initial mortality increase, however, surviving tree growth rates can be 

higher than in untreated stands (Fajardo et al. 2007). Prescribed burning has 

also been found to increase soil moisture (Zald et al. 2008, Soung-Ryoul et 

al. 2009), although the increase may not be detected until several years after 

treatment (Feeney et al. 1998; Wallin et al. 2008).

Mixed-conifer forests have persisted in the Sierra Nevada through more 

severe droughts (Cook and Krusic 2004) than they are currently experiencing. 

These forests, however, are not adapted to the high densities and fuel loads 

now commonly found in many stands. Much is unknown about the potential 

long-term effects of a warming and/or drying climate. In the near term, however, reducing surface fuels and the densities of small-diameter stems may be 

the best means of creating more resilient forests. 
vii

9. When is cutting 20- to 30-inch diameter at breast height (dbh) trees ecologically appropriate? The ecological benefits of and rationale for cutting 

20- to 30-in dbh trees depend on site-specific conditions. In many locations, 

there may be no benefit to cutting such trees because most ecological restoration will result from treating surface fuels and smaller diameter trees. In other 

locations, removing such trees may genuinely serve an ecological goal. For 

example, in some locations, where intermediate-size trees are abundant, they 

may present a fire and fuels risk, especially when live crowns are continuous 

to the forest floor. Typically overstory fuels are a small component of the fire 

hazards in mixed-conifer forests (Stephens and Moghaddas 2005b), however 

removal of trees in larger size classes can improve firefighter safety in some 

instances (Moghaddas and Craggs 2007). In other locations, intermediate-size 

trees contribute to overly dense stands that are moisture stressed and at risk 

of bark beetle attacks. And in yet other locations, intermediate-size conifers 

may be invading stands of at-risk species, like aspen, jeopardizing restoration 

efforts. These are just a few stand examples of situations where the removal of 

intermediate-size trees may be warranted from a strictly ecological perspective. Given the overall deficit of large trees in the Sierra Nevada, however, the 

removal of trees in the 20- to 30-in dbh class needs to be balanced against the 

desired development of more large trees and the future recruitment of large 

snags.

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ix

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mixed-conifer forest. Ecological Modelling. 220: 2460–2468. 

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reduction treatments in a Sierra Nevada pine plantation. International Journal of 

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decline of large-diameter trees in Yosemite National Park, California; USA. 

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density reductions. Plant, Cell and Environment. 26: 631–644.

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xii

Contents

1 Introduction

2 Recent Scientific Information

3 Fuel Dynamics and Current Management Practices

5 Ecological Restoration Using Fire

7 Climate Change

10 Sensitive Wildlife

11 Management of Large Structures

12 Other Key Structures and Habitats

15 Importance of Heterogeneity

16 Within-Stand Variability

19 Landscape-Level Forest Heterogeneity

22 Revising Silvicultural Prescriptions

22 Importance of Tree Species

22 Retention of “Defect” Trees

22 Revising the Desired Diameter Distribution

23 Groups of Large Trees

24 Managing the Intermediate Size Class

26 Allocation of Growing Space

26 Conclusion

28 Summary Findings

31 Research Needs

33 Acknowledgments

33 Metric Equivalents

33 Literature Cited
1

An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

1 See definition in http://www.whitehouse.gov/infocus/healthyforests/

Healthy_Forests_v2.pdf.

Introduction

In recent years, there has been substantial debate over Sierra Nevada forest management. All perspectives on this debate inevitably cite “sound science” as a necessary 

foundation for any management practice. Over the dozen years since publication of 

the last science summary, the Sierra Nevada Ecosystem Project (SNEP 1996), many 

relevant research projects have published findings in dozens of scientific journals, 

yet these have not been synthesized or presented in a form that directly addresses 

current land management challenges. 

Current management usually cites a “healthy forest”1 as a primary objective. It 

is difficult, however, to define forest “health,” and, as a broad concept, “a healthy 

forest” provides few specifics to guide management or assess forest practices. 

Various constituencies have different ideas of forest health (i.e., sustainable timber 

production, fire resilience, biodiversity, etc.) making forest health unclear as an 

objective (Kolb et al. 1994). A premise of silviculture is that forest prescriptions 

can be tailored to fit a wide variety of land management objectives, once those 

objectives are defined. We attempt to define some of the key management objectives on National Forest System lands in the Sierra Nevada and how they might be 

approached through particular silvicultural prescriptions.

In this paper, we focus on summarizing forest research completed at different scales and integrating those findings into suggestions for managing forest 

landscapes. Although many experiments and forest treatments still occur at the 

stand level, ecological research and recent public input have emphasized the need 

to address cumulative impacts and coordinate management across the forest landscape. We believe our synthesis has some novel and highly applicable management 

implications. This paper, however, is not intended to produce new research findings 

for the academic community; rather it is an effort to provide managers of Sierran 

forests with a summary of “the best available science.” Some of the suggestions in 

this paper are already used in different Forest Service management practices. 

There are several aspects of forest management that this paper does not address, 

but we would like to particularly note two omissions. The USDA Forest Service is 

charged with multiple-use management, which can include more objectives (e.g., 

socioeconomic impacts) than our focus on ecological restoration of Sierran forests. 

Restoration practices need both public and economic support to be socially and 

financially viable. Also, we do not specifically address the issues of water yield and 

quality in this paper, although water is one of the Sierra’s most important resources. 

Over the dozen years 

since publication of the 

last science summary, 

the Sierra Nevada 

Ecosystem Project, 

project findings have 

not been synthesized 

or presented in a 

form that directly 

addresses current 

land management 

challenges.
2

GENERAL TECHNICAL REPORT PSW-GTR-220 

Although our focus is on forest conditions, the suggested management practices 

may also make forests more resilient to disturbances including climate change. 

Management practices that help restore the forest headwaters of Sierran watersheds 

will benefit water production and quality for downstream users. 

Recent Scientific Information

Current Sierra Nevada forest management is often focused on landscape strategies 

intended to achieve immediate fuel reduction (e.g., strategically placed area 

treatments [SPLATs] [Finney 2001], defensible fuel profile zones [DFPZs], and 

defense zones) (SNFPA 2004). Fire scientists have developed effective models for 

the strategic placement of these fuel treatments across forest landscapes accounting 

for practical limitations of how much area can actually be treated in the coming 

decades (Finney 2001, Finney et al. 2007). These models have been particularly 

valuable for optimizing and prioritizing fuel treatment locations, and comparing 

likely fire behavior between treated and untreated landscapes (Ager et al. 2007, 

Bahro et al. 2007, Finney et al. 2007, Stratton 2006). Although these models have 

assisted managers in the strategic placement of fuel treatments, they don't have 

the capacity to evaluate ecosystem responses to treatments. Treatments often rely 

upon various diameter limits for mechanical tree removal and treat only a portion 

of the landscape, roughly 20 to 30 percent, relegating most of the forest matrix 

to continued degradation from the effects of fire suppression. With a focus on 

evaluating fire intensity and spread, these fuel strategies do not explicitly address 

how forests might be ecologically restored or wildlife habitat enhanced. Without 

addressing these issues, treatments often face legal challenges resulting in fueltreated acres falling far behind Forest Service goals (e.g., approximately 120,000 

ac/yr in the Sierra Nevada [Stephens and Ruth 2005]).

We have learned much in recent years that can contribute to how forests are 

managed within strategically placed fuel treatments and throughout the landscape 

matrix. The Forest Service is already using many ideas in this paper. In other 

instances, litigation, limited funding, and regulations have fostered practices, 

such as thinning to a diameter limit or limited use of prescribed fire, that no 

one is happy with. We hope this science summary contributes to revising and 

removing some of these restraints.

In this paper, we first summarize recent science findings on fuel dynamics 

that might improve current fuel treatment practices. Even with these changes, 

however, Sierra Nevada forest management still lacks an explicit strategy for 

enhancing forest resilience and wildlife habitat, or managing the majority of the 

forested landscape outside fuel-treated areas. To incorporate these goals into 

Fuel strategies do 

not explicitly address 

how forests might be 

ecologically restored 

or wildlife habitat 

enhanced. Without 

addressing these 

issues, treatments 

often face legal 

challenges.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

3

current management, we then examine recent research on the ecological role of fire, 

forest resilience under changing climate conditions, and habitat requirements of 

sensitive wildlife. Research in all of these areas stresses the ecological importance 

of forest heterogeneity. Knowing the restoration importance of fire, we determined 

the pattern and stand structures for implementing this heterogeneity based on 

how fuel and fire dynamics varied topographically. We discuss how these variable 

forest conditions could be implemented with revised silvicultural practices. Finally 

we summarize the paper’s content in short bullet points, distilling the applied 

management implications and listing research needed to improve and modify 

implementation. 

Fuel Dynamics and Current Management Practices

Forest fuels are usually assessed in three general categories: surface, ladder, and 

canopy bulk density (Agee et al. 2000). Fuel treatments often focus on ladder 

fuels (generally defined to be variably sized understory trees that provide vertical 

continuity of fuels from the forest floor to the crowns of overstory trees [Keyes and 

O’Hara 2002, Menning and Stephens 2007]). Some studies and models, however, 

suggest a crown fire entering a stand is rarely sustained (i.e., sustained only under 

extreme weather conditions) if understory fuels are too sparse to generate sufficient 

radiant and convective heat (Agee and Skinner 2005, Stephens and Moghaddas 

2005). Surface fuels merit as much attention as ladder fuels when stands are treated. 

Prescribed fire is generally the most effective tool for reducing surface fuels. 

One approach to developing fuel prescriptions, similar to current Forest Service 

procedures, is using modeling software to understand how the load of different fuel 

sizes and weather conditions affect predicted fire intensity. For example, Stephens 

and Moghaddas (2005) have modeled fire behavior and weather using Fuels 

Management Analysis (FMA) (Carlton 2004) and Fire Family Plus software (Main 

et al. 1990), respectively. The FMA uses two modules, Dead and Down Woody 

Inventory (data supplied by the Brown 1974 fuel inventory) and Crown Mass (data 

supplied by inventories of trees by species, size, height, and crown ratio), to model 

a stand’s crowning and torching indices (the windspeed needed to produce an active 

and passive crown fire, respectively), scorch height, and tree mortality. All four 

outputs can be controlled by changing surface and ladder fuels, giving managers 

an opportunity to interactively develop target fuel conditions for a desired fire 

behavior. Fuels can be reduced until the crowning and torching indices are higher 

than conditions that are likely to occur even under extreme weather events (e.g., 

Stephens and Moghaddas 2005). 

Surface fuels merit 

as much attention 

as ladder fuels when 

stands are treated. 

Prescribed fire is 

generally the most 

effective tool for 

reducing surface fuels. 
4

GENERAL TECHNICAL REPORT PSW-GTR-220 

In addition to ladder and surface fuels, managers have been concerned with 

reducing canopy bulk density in DFPZs and the defense zone of wildland urban 

interaces (WUI). Overstory trees are commonly removed, and residual trees are 

evenly spaced to increase crown separation. The efficacy of canopy bulk density 

reduction in modifying fire behavior is largely a function of weather conditions. 

Research has suggested there is often limited reduction in crown fire potential 

through overstory thinning alone, without also treating surface fuels (Agee 2007, 

Agee and Skinner 2005, Agee et al. 2000, Stephens and Moghaddas 2005). However, some field observations (JoAnn Fites Kaufmann, Forest Service Enterprise 

Team, Steve Eubanks, Tahoe National Forest) suggest that under severe weather 

conditions (e.g., sustained high winds) or on steep slopes, crown separation may 

reduce the risk of crown fire spread. Fire behavior under extreme conditions is 

still difficult to model, and, furthermore, what constitutes “extreme” (because 

many wildfires occur under hot, windy conditions) has not been defined (for the 

Southwest see Crimmins [2006]). In forests adjacent to homes or key strategic 

points, managers may want to reduce canopy bulk density to reduce potential fire 

severity under all possible weather scenarios. Outside of those cases, the value of 

crown separation in preventing crown fire spread may be limited (Agee et al. 2000, 

Stephens and Moghaddas 2005). 

A concern with the widespread use of canopy bulk density thinning in defensible fuel profile and defense zones is the ecological effects of the regular tree spacing 

(fig. 1). In the Sierra Nevada, historical data (Bouldin 1999, Lieberg 1902), narratives (Muir 1911), and reconstruction studies (Barbour et al. 2002, Bonnicksen and 

Stone 1982, Minnich et al. 1995, North et al. 2007, Taylor 2004) indicate mixedconifer forests were highly clustered with groups of trees separated by sparsely 

treed or open gap conditions. This clustering can be important for regenerating 

shade-intolerant pine (Gray et al. 2005, North et al. 2004, York and Battles 2008, 

York et al. 2003), increasing plant diversity and shrub cover (North et al. 2005b), 

moderating surface and canopy microclimate conditions within the tree cluster 

(North et al. 2002, Rambo and North 2009), and providing a variety of microhabitat 

conditions for birds (Purcell and Stephens 2006) and small mammals (Innes et al. 

2007, Meyer et al. 2007a). Studies in Baja’s Sierra San Pedro del Martir (SSPM) 

forests also indicate forest structures (live trees, snags, logs, and regeneration) are 

highly clustered (Stephens 2004, Stephens and Fry 2005, Stephens and Gill 2005, 

Stephens et al. 2007a). This forest in Mexico shares many characteristics of mixedconifer forests found in the Sierra Nevada but has had little fire suppression and has 

not been harvested. Although these Baja forests have a different weather pattern 

than California’s Sierra Nevada (Evett et al. 2007), they can provide some insight 

Mixed-conifer forests 

were highly clustered 

with groups of trees 

separated by sparsely 

treed or open gap 

conditions.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

5

into the structure and ecological dynamics of a mixed-conifer forest with an active 

fire regime. A recent study of stressed SSPM Jeffrey pine/mixed-conifer forests 

where a 2003 wildfire was preceded by a 4-year drought, found spatial heterogeneity was a key feature in forest resiliency (Stephens et al. 2008). A clumped tree 

distribution, where groups are separated by gaps, might also slow crown fire spread 

(fig. 2), but we do not know of any studies that have examined this idea. Studies 

in other mixed-conifer forests (e.g., Klamath Mountains and eastern Washington) 

imply this heterogeneity may be an important characteristic of frequent fire’s effect 

on mixed-conifer forests (Hessburg et al. 2005, 2007; Taylor and Skinner 2004). 

Fuel treatments that produce uniform leave tree spacing reduce this ecologically 

important spatial heterogeneity.

Managing surface fuels and reducing the use of regular leave-tree spacing 

can improve current fuel treatments. These changes, however, have not addressed a 

fundamental public concern that current forest management lacks explicit strategies 

for ecological restoration and provision of wildlife habitat. 

Ecological Restoration Using Fire

Fire plays a pivotal role in reshaping and maintaining mixed-conifer ecosystems. 

Fire was once very common in most of the Sierra Nevada and has been a primary 

force shaping the structure, composition, and function of mixed-conifer forests 

(Fites-Kaufman et al. 2007, Franklin and Fites-Kaufman 1996, McKelvey et al. 

Figure 1—Regular spacing of “leave” trees in a defensible fuel profile zone. 

Eric Knapp
6

GENERAL TECHNICAL REPORT PSW-GTR-220 

1996, Stephens et al. 2007b). Management strategies need to recognize that, in 

many situations, fire is both a viable fuel-treatment tool (Agee and Skinner 2005; 

Stephens et al. 2009) and an important jumpstart for many ecosystem processes 

stalled by accumulating surface fuels and the absence of frequent burning (North 

2006). The main effect of low-intensity fire is its reduction of natural and activity 

(i.e., resulting from management activities) fuels, litter, shrub cover, and small 

trees. These reductions open growing space, provide a flush of soil nutrients, and 

increase the diversity of plants and invertebrates (Apigian et al. 2006, Knapp et 

al. 2007, Moghaddas and Stephens 2007, Murphy et al. 2006, Wayman and North 

2007). By reducing canopy cover, fire also increases habitat and microclimate 

heterogeneity at site, stand, and landscape levels (Chen et al. 1999, Collins et al. 

2007, Concilio et al. 2006, Falk et al. 2007, Hessburg et al. 2007, Miller and Urban 

1999). Fire is an indispensable management tool, capable of doing much of the work 

to restore ecological processes (Bond and van Wilgen 1996, Covington et al. 1997, 

North 2006, Stephenson 1999, Sugihara et al. 2006). 

By itself, prescribed fire will be difficult to apply in some forests owing to fuel 

accumulations, changes in stand structure, and operational limitations on its use. 

Figure 2—An example of the clumped tree distribution and canopy gaps produced by an active fire 

regime. The photograph is an aerial view of the Beaver Creek Pinery, which has experienced very 

little fire suppression.

Carl Skinner

Fire is both a viable 

fuel-treatment tool and 

an important jumpstart 

for many ecosystem 

processes stalled by 

accumulating surface 

fuels and the absence 

of frequent burning 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

7

Mechanical treatments can be effective tools to modify stand structure and influence subsequent fire severity and extent (Agee et al. 2000, Agee and Skinner 2005) 

and are often a required first treatment in forests containing excessive fuel loads. 

Prescribed fire is generally implemented very carefully, killing only the smaller size 

class trees (Kobziar et al. 2006). In some cases, it is ineffective for restoring resilience, at least in the first pass (Ritchie and Skinner 2007). For example, prescribed 

fire may not kill many of the larger ladder-fuel or co-dominant true fir trees that 

have grown in with fire suppression (Knapp and Keeley 2006, North et al. 2007). In 

many stands, mechanical thinning followed by prescribed fire may be necessary to 

achieve forest resilience much faster than with prescribed fire alone (Schwilk et al. 

2009, Stephens et al. 2009). 

Some forests cannot be prescription burned, at least as an initial treatment, 

because of air quality regulations, increasing wildland home construction, and 

limited budgets. Yet restoration of these forests still depends on modifying fuels 

because it reduces wildfire intensity when a fire does occur (Agee and Skinner 

2005) and can produce stand conditions that simulate some of fire’s ecological 

effects (Innes et al. 2006, Stephens and Moghaddas 2005, Wayman and North 

2007). Mechanical control of fuels allows fire, both wildland fire and prescribed 

fire, to be more frequently used as a management tool. 

Climate Change

Forest restoration has often examined past conditions, such as the pre-European 

period, as a basis for developing management targets. With climate change, however, is restoring forests to these conditions even an appropriate goal? Returning 

to a pre-European condition, is unlikely to be feasible, because in addition to 

climate, livestock grazing and Native American ignitions have changed (Millar 

and Woolfenden 1999, Millar et al. 2007). Rather than strive for restoration of a 

fixed presettlement condition, managers could increase tree, stand, and landscape 

resiliency. 

Research suggests global mean minimum temperatures may have already 

begun to rise (Easterling et al. 1997). One effect of this change for western forests 

would be earlier spring melt of mountain snowpacks. An analysis of Western U.S. 

fire season length over the last 50 years suggests that during the last two decades, 

fires begin earlier in the spring and occur later in the fall possibly owing to this 

trend in elevated nighttime minimum temperatures (Westerling et al. 2006). An 

analysis of fire severity and size in California has found an increase in both, along 

with a regional rise in temperature (Miller et al. 2009). Climate change effects on 

precipitation have been more difficult to predict with models suggesting regional 

Forest restoration 

has often examined 

past conditions, 

such as the preEuropean period, as a 

basis for developing 

management targets. 

With climate change, 

however, is restoring 

forests to these 

conditions even an 

appropriate goal?
8

GENERAL TECHNICAL REPORT PSW-GTR-220 

differences. For example, some models predict an increase in precipitation for 

northern California, some predict a decrease, and others suggest little change 

(Hayhoe et al. 2004, Lenihan et al. 2003). Most models predict the southern Sierra 

will receive less precipitation, with a higher percentage of it occurring as rain rather 

than snow (Miller et al. 2003). Climate models suggest there will be more frequent 

and stronger shifts between El Niño and La Niña events making changes in average 

precipitation difficult to predict. Perhaps one point of consensus is that most modelers agree the climate will become more extreme, suggesting oscillations between 

wet and drought conditions will be more common. 

The potential effects of these changes on vegetation, fire, and wildlife are 

largely speculative (Field et al. 1999, Skinner 2007). Studies of past vegetation 

communities under a range of climates show unique plant assemblages without 

modern analogs (Millar et al. 2007, Williams and Jackson 2007). This suggests 

species will not simply shift up in elevation or latitude in response to warming 

conditions. Some general predictions that grouped species by functional categories 

have predicted an increase in broad-leaved over needle-leaved species, a general 

increase in ecosystem productivity (i.e., total biomass), and a decrease in forest 

and an increase in shrub and grasslands (Lenihan et al. 2003). Changes in forest 

understory may vary depending on existing vegetation and the synergistic effects 

of increasing nitrogen enrichment from pollution and increased herbaceous 

fuels affecting burn intensity and frequency (Hurteau and North 2008). If Sierra 

precipitation decreases or experiences more frequent, intense La Niña events, 

forests are likely to become more drought stressed. One study examining several 

decades of mixed-conifer demography trends (van Mantgem and Stephenson 2007) 

suggests a recent increase in mortality may be related to increased drought stress 

from a warming climate. This drought stress would make current, high-density, 

Sierra forests more susceptible to pest and pathogen mortality, particularly from 

bark beetles (Ferrell 1996, Fettig et al. 2007, Maloney et al. 2008, Smith et al. 2005). 

Managing forests under these conditions will be challenging. In the face 

of uncertainty, Millar et al. (2007) have suggested managers consider adaptive 

strategies focused on three responses: resistance (forestall impacts and protect 

highly valued resources), resilience (improve the capacity of ecosystems to return 

to desired conditions after disturbance), and response (facilitate transition of 

ecosystems from current to new conditions). All of these strategies acknowledge 

the influence of climate change and suggest management may fail if focused on 

re-creating past stand conditions using strict structural targets. 

Species will not simply 

shift up in elevation or 

latitude in response to 

warming conditions.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

9

Although historical forest conditions may not provide numerical guidelines, the 

past still has lessons for managing Sierran forests. Historical forests can provide 

a better understanding of the ecological processes that have shaped mixed-conifer 

forest and the habitat conditions to which wildlife have adapted (Falk 1990, Society 

for Ecological Restoration 1993). All reconstruction studies, old forest survey data 

sets, and 19th-century photographs (Gruell 2001, McKelvey and Johnson 1992) 

suggest that frequently burned forests had very low tree densities. For example, in 

the early 20th century, Lieberg (1902) estimated that stem density in the northern 

California forests he surveyed was only 35 percent of its potential because of 

mortality from frequent fire. Studies reconstructing pre-European conditions all 

indicate that forests had a greater percentage of pine, a clustered pattern with highly 

variable canopy cover, and a high percentage of the growing stock in more fireresistant, large-diameter classes. These past conditions give general guidance but 

should not be taken as strict numerical targets for density or diameter distribution 

in silvicultural prescriptions. What these reconstructions do provide is inference 

about the cumulative process effects of fire, insects, pathogens, wind, and forest 

dynamics on stand structure and composition, producing forests resilient to most 

disturbances, including wildfire. A modeling comparison of different stand structures grown over 100 years, including those produced by fuel treatments (Hurteau 

and North 2009), found a low-density forest dominated by large pines was most 

resilient to wildfire, sequestered the most carbon, and had the lowest carbon dioxide 

(CO2) emissions and thus contributed less to global warming. An analysis of carbon 

emissions and storage from different fuel treatments, found understory thinning 

followed by prescribed fire produced the greatest reduction in potential wildfire 

severity without severely reducing carbon stocks (North et al., in press). As climate 

changes, managing the process or behavior of fire (i.e., manipulating fuels to influence burn intensity) may produce more resistant and resilient forests than managing 

for a desired number and size of trees.

An important benefit of forest management focused on affecting fire behavior is 

that in areas of wildland fire and prescribed burning, forest structure and composition are allowed to reestablish to modern dynamic equilibrium by adapting to fire 

that occurs under current climate and ignition conditions (Falk 2006, Stephenson 

1999). A recent analysis of fire severity data by 10-yr periods in Yosemite’s mixedconifer forest (Collins et al. 2009) revealed a fair degree of stability in the proportion of area burned among fire severity classes (i.e., unchanged, low, moderate, 

high). This suggests that free-burning fires, over time, can regulate fire-induced 

effects across the landscape. 

As climate changes, 

managing the process 

or behavior of fire may 

produce more resistant 

and resilient forests 

than managing for a 

desired number and 

size of trees.
10

GENERAL TECHNICAL REPORT PSW-GTR-220 

Sensitive Wildlife

A strategy for mixed-conifer ecological restoration will conserve wildlife and minimize habitat impacts for both the broader animal community as well as the specific 

needs for a subset of species of concern. For over 15 years, Sierran forest management devoted significant effort to meeting the needs of old-forest-associated species, particularly the California spotted owl (Strix occidentalis occidentalis) (Verner 

et al. 1992) and the Pacific fisher (Martes pennanti). Sound wildlife management 

strategies balance species needs (both sensitive and common) at a variety of spatial 

(microsite to foraging landscape) and temporal (immediate to long-term population 

viability) scales (Noss et al. 1997). 

Managing for owl and fisher viability needs to account for a few shared characteristics of these top tropic species, including territoriality, large home range size, 

strong associations with late-seral forest structures, and long-distance travel for 

foraging contribute to improved owl and fisher viability. Both species are strongly 

associated with Sierran forest stands characterized by large trees and dense canopy 

cover (Verner et al. 1992, Zielinski et al. 2004b). These features are consistently 

selected by spotted owls for nesting (North et al. 2000), and by fishers for denning 

and resting sites in the Sierra Nevada (Mazzoni 2002; Zielinski et al. 2004a, 2004b) 

and elsewhere. Fishers use cavities in living and dead conifers and hardwoods 

(particularly California black oak [Quercus kellogii Newb.]) as daily refuges, and 

tend to select the largest individual trees in dense canopy stands (fig. 3). Individual 

trees are rarely reused as rest structures, at least consistently from night to night 

(Zielinski 2004b), so many different large trees are required. This behavior makes 

provision of resting habitat critical to fisher conservation (Zielinski 2004b). Spotted owls also use many different large trees within their home range for roosting 

(Verner et al. 1992). Large decadent trees are less common in the Sierra Nevada 

than they once were (Bouldin 1999), and providing for this structure requires 

protecting existing large trees, managing for their future development, and reducing 

major threats (i.e., high-severity fire and pest mortality). 

Foraging habitat, unlike resting habitat, is much easier to provide for spotted 

owls and fishers. The fisher’s diet is very diverse and includes a variety of small 

mammals, birds, reptiles, fruits, and insects (Zielinski et al. 1999). Owls have a 

somewhat more specialized diet. In most locations they tend to prey on woodrats 

(Neotoma spp.), northern flying squirrels (Glaucomys sabrinus), and deer mice 

(Peromyscus maniculatus), at least during nesting season (Forsman et al. 2004, 

Williams et al. 1992). Although our current knowledge of fisher and owl foraging 

habitats is fairly limited, we do know that their array of prey species are associated 

with a variety of forest conditions suggesting that habitat heterogeneity at different 

Prey species are associated with a variety 

of forest conditions 

suggesting that habitat heterogeneity at 

different spatial scales 

across the landscape 

may be desirable for 

sustaining adequate 

food supplies.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

11

spatial scales across the landscape may be desirable for sustaining adequate food 

supplies (Carey 2003, Coppeto et al. 2006, Innes et al. 2007, Meyer et al. 2005). A 

cautious strategy would be emulating patterns created by natural disturbance to 

provide a heterogeneous mix of forest habitat across a managed landscape (Lindenmayer and Franklin 2002, North and Keeton 2008). 

Management of Large Structures

Much of the public apprehension over forest management practices stems from 

possible impacts to old-forest-associated species such as the Pacific fisher, California spotted owl, and northern goshawk (Accipiter gentilis). All three of these 

sensitive species depend on a forest structure usually dominated by large trees, 

snags, and downed logs, which provide suitable substrate for nesting, denning, 

and resting sites. Retaining these large snags and logs may increase fire hazard in 

these favorable habitat microsites, particularly in warming climate conditions. In 

some stands that have been depleted of larger trees, the best available structures 

may be intermediate-sized trees, generally defined as the 20- to 30-inch size class 

for conifers. In these stands, retaining conifers of this size is important not only for 

immediate wildlife needs, but also because they will become the next generation of 

large trees, (and eventually) snags and logs. Fisher rest structures include live trees 

Figure 3—Pacific fisher resting on a limb of a large black oak. Although this picture is from the 

Klamath Mountains, it is typical of the stand conditions associated with fisher resting locations.

Rebecca Green
12

GENERAL TECHNICAL REPORT PSW-GTR-220 

(e.g., cavities, broken tops), snags (e.g., cavities, broken tops, stumps), platforms 

(nests, mistletoe growths, witches’ brooms), logs, and ground cavities (Zielinski 

et al. 2004b). We do not yet have a good understanding of how best to distribute 

potential rest sites or how many are needed. 

Other Key Structures and Habitats

Other forest features that may be important to sensitive species as well as the 

broader wildlife community include hardwoods, shrubs, “defect” trees, and riparian corridors. Hardwoods, particularly black oak, are increasingly regarded as an 

important species for providing food and cavities. Many small and large mammals 

and birds use acorns as a food source (McShea 2000), particularly in large masting 

years (Airola and Barrett 1985, Morrison et al. 1987, Tevis 1952). Oaks often have 

broken tops and large cavities from branch breakage, and are frequently used for 

resting and nesting sites by small mammals (Innes et al. 2007), forest carnivores 

(Zielinski et al. 2004b), and raptors (North et al. 2000, Richter 2005). In many 

areas, hardwoods are in decline because they have become overtopped and shaded 

by conifers. The larger oaks likely germinated and had much of their early growth 

in more open forest than exists today (Zald et al. 2008). Provisions are needed 

to create open areas within stands to facilitate hardwood recruitment. Thinning 

around large oaks that are prolific seed producers creates open conditions that favor 

oak regeneration. However, thinning around large, cavitary oaks that are currently 

shaded is a difficult decision. It is important to balance thinning to prolong the 

life of the oak against the possibility that reducing the canopy around the oak will 

decrease the overall habitat value of the rest structure. Managers might consider 

thinning around some, but not all, cavitary oaks if several are present within 

a stand.

In fire-suppressed forests, shrubs are often shaded out (Nagel and Taylor 2005, 

North et al. 2005b) reducing their size, abundance, and fruit and seed production in 

low-light forest understories. Anecdotal narratives (Lieberg 1902, Muir 1911), a 

forest reconstruction (Taylor 2004), and a few early plot maps2

 suggest shrub cover 

in active-fire conditions might have been much higher than in current forests, 

mostly owing to large shrub patches that occupied some of the gaps between tree 

clusters (fig. 4). In SSPM’s active-fire Jeffrey pine/mixed-conifer forests, Stephens 

et al. (2008) found shrub cover was highly spatially variable, and often occurred in 

high-density patches. Some birds (Robinson and Alexander 2002) and small 

mammals, including spotted owl prey such as the woodrat (Coppeto et al. 2006, 

2 Eric Knapp. 2008. Personal communication. Research ecologist, USDA Forest Service, 

Silviculture Laboratory, 3644 Avtech Parkway, Redding, CA. 96002.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

13

Figure 4—Photograph taken in 1929 of mixed-conifer forest before thinning and approximately 40 

years after the last fire, near Pinecrest, California. Note the extensive cover of understory shrubs, 

particularly under the canopy gap in the foreground. 

Robert Dunning
14

GENERAL TECHNICAL REPORT PSW-GTR-220 

Innes et al. 2007), are associated with these habitat patches. We also know that 

species of Ceanothus are an important source of available nitrogen (Erickson et al. 

2005, Johnson et al. 2005, Oakley et al. 2006) that persists even after the shrubs 

have been removed by fire (Oakley et al. 2003). In forests where shrubs are 

currently rare, it is important for managers to consider protecting what shrubs 

remain and increasing understory light conditions for shrub establishment and 

patch expansion. Patch size and configuration of such habitat should vary (see 

discussion on habitat heterogeneity in the next section).

Forest management practices have sometimes removed decadent, brokentopped, or malformed trees that are actually some of the most important features of 

habitat for many wildlife species (Mazurek and Zielinski 2004, North et al. 2000, 

Thomas et al. 1976, Zielinski et al. 2004b). These “defect” trees are some of the 

rarest structures in current forest conditions, often rarer than large trees. Successful management strategies might consider incorporating a means of preserving 

what remains and adding more of these features across the landscape. The Green 

Diamond Resource Company developed a guide for identifying and ranking the 

potential habitat quality of these forest structures in the Klamath Mountains. 

Developing a similar guide for Sierra forests would be extremely useful. 

Connecting habitat within a landscape using corridors has been extensively 

studied, but results often indicate that suitable forest conditions within the corridor 

and the optimal distribution of corridors differs by species (Hess and Fischer 2001). 

In the Sierra Nevada, with its extended summer drought, riparian forests may be 

particularly important habitat and movement corridors for many species. Owing 

to greater soil development and moisture retention, these corridors usually provide 

more vegetative cover, have greater plant and fungal abundance and diversity 

(Meyer and North 2005), and a moderated microclimate (Rambo and North 2008). 

Many small mammals are found in greater abundance in riparian areas (Graber 

1996, Kattelmann and Embury 1996, Meyer et al. 2007a), and some of these species 

are selected prey of old-forest-associated species. Initial observations of fisher 3

(Seglund 1995, Zielinski et al. 2004a) suggest that riparian areas may be preferred 

movement corridors. Riparian corridor width would be better defined by overstory 

and understory vegetation than the set distances of 150 and 300 feet that are 

specified in the Sierra Nevada Forest Plan Amendment (SNFPA 2004).

3 Katherine Purcell. 2008. Personal communication. Research wildlife biologist, USDA 

Forest Service, Forestry Sciences Laboratory, 2081 E Sierra Ave., Fresno, CA 93710.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

15

Riparian forests are less moisture limited than upland areas, are highly 

productive, and now have some of the heaviest ladder and surface fuel loads of 

any Sierran forest communities (Bisson et al. 2003, Stephens et al. 2004). Recent 

Western U.S. research suggests that although reduced, fire is still a significant 

influence on riparian forest structure, composition, and function in forests with 

historically frequent, low-intensity fire regimes (Dwire and Kauffman 2003, 

Everett et al. 2003, Olson 2000, Pettit and Naiman 2007, Skinner 2003). Although 

fire in Sierran riparian areas was probably less frequent than in surrounding 

uplands, we do not yet know what its historical frequency, intensity, and extent 

was in stream corridors. When inevitable wildfires burn these corridors, they are 

likely to be high-severity crown fires that can denude riparian areas of vegetation 

(Benda et al. 2003). Any management activity in riparian areas, including no 

action, has risks. We suggest that riparian corridors be treated with prescribed fire 

in spring or late fall (after rains) to help reduce surface fuels (Beche et al. 2005). 

In moist conditions, some observations4 suggest that low-intensity prescribed fire 

can reduce fuels while maintaining high canopy cover and large logs if fuels have 

high moisture content. 

Importance of Heterogeneity

A management strategy that includes methods for increasing forest heterogeneity 

at multiple scales will improve habitat quality and landscape connectivity. 

Creating vertical and horizontal heterogeneity in forests with frequent fire, 

however, has been a challenge. Multilayered canopies, often associated with 

Pacific Northwest old-growth forests (Spies and Franklin 1988), are not the 

best model for Sierran mixed-conifer forests because when adjacent trees are 

multilayered, the continuity of vertical fuels can provide a ladder for surface 

fire into the overstory canopy. Horizontal heterogeneity, however, used to be 

relatively common in Sierran mixed-conifer forests (Franklin and van Pelt 2004, 

Knight 1997, Stephens and Gill 2005). All of the Sierran reconstruction studies 

(Barbour et al. 2002, Bonnicksen and Stone 1982, Minnich et al. 1995, North et 

al. 2007, Taylor 2004) suggest mixed-conifer forests, under an active fire regime, 

had a naturally clumped distribution containing a variety of size and age classes. 

4 Dave McCandliss, 2008. Personal communication. Fire management officer, USDA Forest 

Service, Sierra National Forest, 1600 Tollhouse Road, Clovis, CA 93611.

We suggest that 

riparian corridors be 

treated with prescribed 

fire in spring or late 

fall (after rains) to help 

reduce surface fuels.
16

GENERAL TECHNICAL REPORT PSW-GTR-220 

Within-Stand Variability

At the stand level, vertical heterogeneity can still be provided by separating groups 

of trees by their canopy strata (fig. 5). For example, a group of intermediate-size 

trees that could serve as ladder fuels might be thinned or removed if they are growing under large overstory trees. The same size trees in a discrete group, however, 

might be lightly thinned to accelerate residual tree growth or left alone if the group 

does not present a ladder fuel hazard for large, overstory trees. These decisions 

could be made using the revised silvicultural markings proposed (see “Allocation 

of Growing Space” section), where growing space is allocated by leaf area index 

among trees in different height strata. This strategy would produce within-stand 

vertical heterogeneity, albeit in discrete tree clusters, which will contribute to 

horizontal heterogeneity. 

Figure 5—Transect of a mixed-conifer forest in Yosemite National Park’s Aspen Valley, which has experienced three understory burns 

within the last 50 years. Note that the stand has vertical heterogeneity but that trees in different canopy strata tend to be spatially 

separated.

Drawing courtesy of Robert van Pelt.

To increase horizontal heterogeneity, we suggest using microtopography as 

a template (Sherlock 2007) (fig. 6). Wetter areas, such as seeps, concave pockets, 

and cold air drainages, may have burned less frequently or at lower intensity 

(fig. 7). Limiting thinning to ladder fuels in these areas is suggested because with 

their potentially higher productivity and cooler microclimate, they can support 

greater stem densities, higher canopy cover, and reduced fire effects. A concern 

with current uniform fuel reduction is that these microsite habitats associated 

with sensitive species would be eliminated. Surface fuel loads at these microsites 

should still be reduced to lower their vulnerability to high-intensity fire. 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

17

Figure 6—Stand-level schematic of how forest structure and composition would vary by small-scale topography after treatment. Cold air 

drainages and concave areas would have high stem densities, more fir and hardwoods. With increasing slope, stem density decreases and 

species composition becomes dominated by pines and black oak.

Mature tree drawiings courtesy of Robert van Pelt
18

GENERAL TECHNICAL REPORT PSW-GTR-220 

In contrast, upslope areas, where soils may be shallower and drier and where 

fire can burn with greater intensity, historically had lower stem densities and 

canopy cover (Agee and Skinner 2005) (fig. 8). On these sites, thinning might 

reduce the density of small or, where appropriate, intermediate trees and ladder 

and surface fuels toward a more open condition. In some circumstances this thinning may reduce water stress, accelerating the development of large residual trees 

(Kolb et al. 2007, Latham and Tappenier 2002, McDowell et al. 2003, Ritchie et al. 

2008). Within a stand, varying stem density according to potential fire intensity 

effects on stand structure would create horizontal heterogeneity. 

Figure 7—Mixed-conifer stand structure at Aspen Valley, Yosemite National Park, produced by frequent, low-intensity fire. Note the 

higher stem density and hardwoods in the seep drainage in the background.

Malcolm North

Within a stand, 

varying stem density 

according to potential 

fire intensity effects 

on stand structure 

would create horizontal 

heterogeneity. 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

19

Figure 8—Upslope stand conditions where thinner soils and rock outcrops are often associated with 

drier conditions, and lower density forests, which burned frequently.

Landscape-Level Forest Heterogeneity

Landscapes with an active fire regime are highly heterogeneous. In Baja’s activefire Jeffrey pine/mixed-conifer forests, Stephens et al. (2007a) found that “average” stand characteristics such as snag density, large woody debris, tree density, 

basal area, and surface fuel loads were rare (approximately 15 to 20 percent of the 

sampled stands) and varied by an order of magnitude among localized (0.25-ac) 

plots. Studies in the Sierra Nevada (Fites-Kaufman 1997, Urban et al. 2000) and 

Klamath Mountains (Beaty and Taylor 2001, Taylor and Skinner 2004) found that 

mixed-conifer structure and composition varied by fire patterns that were controlled 

by landscape physiographic features (fig. 9). Fire intensity, and consequently a more 

open forest condition, increased with higher slope positions and more southwesterly 

aspects. In eastern Washington mixed-conifer forests, Hessburg et al. (2005, 2007) 

also found a heterogeneous historical forest landscape shaped by topographic influences on fire behavior. Cumulatively these studies suggest that forest landscapes 

varied depending on what structural conditions would be produced by topography’s 

influence on fire frequency and intensity.

Malcolm North

These studies 

suggest that forest 

landscapes varied 

depending on what 

structural conditions 

would be produced by 

topography’s influence 

on fire frequency 

and intensity.
20

GENERAL TECHNICAL REPORT PSW-GTR-220 

We suggest creating landscape heterogeneity in the Sierra Nevada by mimicking the forest conditions that would be created by the fire behavior and return 

interval associated with differences in slope position, aspect, and slope steepness 

(Sherlock 2007). In general, stem density and canopy cover would be highest in 

drainages and riparian areas, and then decrease over the midslope and become lowest near and on ridgetops (fig. 10). Stem density and canopy cover in all three areas 

would be higher on northeast aspects compared to southwest. Stand density would 

also vary with slope becoming more open as slopes steepen. 

Figure 9—Landscape variation in burn intensity on the Moonlight Fire (2007), Plumas National Forest.

Eric Knapp

We suggest creating 

landscape heterogeneity in the Sierra 

Nevada by mimicking 

the forest conditions 

that would be created 

by the fire behavior 

and return interval 

associated with 

differences in slope 

position, aspect, and 

slope steepness.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

21

Figure 10—Landscape schematic of variable forest conditions produced by management treatments that differ by topographic factors 

such as slope, aspect, and slope position. Ridgetops have the lowest stem density and highest percentage of pine in contrast to riparian 

areas. Midslope forest density and composition varies with aspect: density and fir composition increase on more northern aspects and 

flatter slope angles.

Mature tree drawiing courtesy of Robert van Pelt
22

GENERAL TECHNICAL REPORT PSW-GTR-220 

Revising Silvicultural Prescriptions

A new silviculture for Sierran mixed-conifer forest that balances ecological restoration and wildlife habitat with fuel reduction can meet multiple forest objectives. By 

necessity, recent Sierran silviculture has first been focused on reducing fire severity through fuel reduction. For many reasons, including maintaining or restoring 

resilient forests, public safety, and property loss, fuel reduction remains a priority. 

We suggest that, with some modification, wildlife and ecological objectives can also 

be met.

Importance of Tree Species

Diameter-limit prescriptions applied equally to all species will not remedy the 

significant deficit of hardwoods and pines in current forests (Franklin and FitesKaufmann 1996, SNFPA 2004). Prescriptions that differ by species can retain 

hardwoods, which are important for wildlife, and favor pines that can increase the 

forest’s fire resilience. Given their current scarcity in many locations, there are few 

instances that warrant cutting either hardwoods or pines in mixed-conifer forests. 

Retention of “Defect” Trees

Given the wildlife habitat value of large trees with multiple tops, rot, cavities, etc., 

managers may want to retain them whenever possible. These growth forms often 

result from disease or injury (e.g., from lightning, wind breakage, and being struck 

by adjacent falling tree), and are important structural features for many wildlife 

species. Disease incidence does not necessarily indicate that a tree is genetically 

more susceptible and therefore should be “culled” (Tainter and Baker 1996). 

Modern Sierran forests have a significant shortage of these “decadent” but 

essential habitat structures (McKelvey and Johnson 1992). 

Revising the Desired Diameter Distribution

The proposed silvicultural approach is a multiaged-stand strategy driven by the 

need for wildlife habitat, fire-resistant stand structures, and restoration of stand 

and landscape patterns similar to active-fire conditions in mixed-conifer forests. 

Although we use the term multiage, we are most interested in size and structure, 

and their associated ecological attributes. Multiaged stands are a flexible means of 

including variable stand structures with two or more age classes and integrating 

existing stand structures into silvicultural prescriptions. More traditional forms 

of uneven-age silviculture were heavily reliant on achieving a reverse-J diameter 

distribution that reduced large-tree retention (O’Hara 1998). Past silviculture has 

often changed the slope of this line (i.e., adjusting the q-factor [Smith et al. 1996]) 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

23

in response to different forest types and stand conditions, but has not fundamentally 

changed the shape of the curve or its allocation of growing space. The reverse-J 

diameter distribution prescribes a stand structure with a surplus of small trees and 

limited space for large trees. Such a distribution is inconsistent with historical Sierran mixed-conifer forests where fire reduced small-tree abundance while retaining 

fire-resistant, large-diameter trees (North et al. 2005a, 2007) (fig. 11). Research 

suggests that fire-prone forests rarely had reverse-J diameter distributions (Bouldin 

1999; O’Hara 1996, 1998; Parker and Peet 1984). 

Figure 11—Density of live trees (stems per hectare) in seven size classes for seven conditions in the 

Teakettle Experiment. The five fuel-reduction treatments (prescribed burn only, understory thin, 

understory thin and burn, overstory thin, and overstory thin and burn) retain the same reverse-Jshaped diameter distribution as the pretreatment (fire-suppressed old growth) and do not approximate 

the reconstruction of the diameter distribution in 1865 active-fire conditions. Reconstruction methods 

probably underestimate the number of small stems in 1865 active-fire conditions, but even a three- to 

fourfold increase would not produce a reverse-J distribution (reprinted from North et al. 2007). 

240

200

160

120

80

30

20

10

0

0 25 50 75 100 125 >150 

Density (stems/ha)

Diameter at breast height (cm)

Pretreatment

Burn/no thin

Unburned/understory thin

Burn/understory thin

Unburned/overstory thin

Burn/overstory thin

1865 reconstruction

Groups of Large Trees

Clusters of intermediate to large trees (i.e., >20 inches diameter at breast height 

[d.b.h.]) are sometimes marked for thinning with the belief that they are overstocked 

and thinning would reduce moisture stress. Some evidence, however, suggests these 

groups of large trees may not be moisture stressed by within-group competition 

Research suggests 

that fire-prone forests 

rarely had reverse-J 

diameter distributions. 
24

GENERAL TECHNICAL REPORT PSW-GTR-220 

because they have deep roots that can access more reliable water sources including 

fissures in granitic bedrock (Arkley 1981, Hubbert et al. 2001, Hurteau et al. 2007, 

Plamboeck et al. 2008). Reconstructions of Sierran forests with active fire regimes 

(Barbour et al. 2002, Bonnicksen and Stone 1982, Minnich et al. 1995, North et 

al. 2007, Taylor 2004) have consistently found large trees in groups. These groups, 

however, can be at risk if intermediate and small trees grow within the large tree 

groups. Thinning these small and intermediate trees will reduce fire laddering. 

Managing the Intermediate Size Class

Many studies have documented the importance of large tree structures in forests 

for many ecological processes and their value for wildlife habitat (see summaries 

in Kohm and Franklin 1997, Lindenmayer and Franklin 2002). However, “large” 

varies with forest type and site productivity, and there is no set size at which a tree 

takes on these attributes. We only address this question of 20- to 30-in d.b.h. trees 

because it is so pivotal in the current management strategies for Sierran forests and 

is driving much of the discussion around fuel treatment thinnings. 

What is achieved by thinning intermediate sized (20- to 30-in d.b.h.) trees? 

Some research suggests that for managing fuels, most of the reduction in fire severity is achieved by reducing surface fuels and thinning smaller ladder-fuel trees (see 

summaries in Agee et al. 2000, Agee and Skinner 2005, Stephens et al. 2009). What 

is considered a ladder fuel differs from stand to stand, but typically these are trees 

in the 10- to 16-in d.b.h. classes. If trees larger than this are thinned, it is important 

to provide reasons other than for ladder-fuel treatment. These may include additional fuel reduction such as thinning canopy bulk density in strategic locations. 

Or it could be other ecological objectives such as restoration of an active-fire stand 

structure, managing for open habitat that includes shrubs, or accelerating the 

development of large leave trees. Although large trees are often old, studies have 

found diameter growth increases significantly when high densities of adjacent small 

stems are removed (Das et al. 2008, Latham and Tappeiner 2002, McDowell et al. 

2003, Ritchie et al. 2008, Skov et al. 2004). There may be socioeconomic purposes 

for harvesting intermediate-sized trees such as generating revenue to help pay for 

fuel treatment or providing merchantable wood for local sawmills (Hartsough et al. 

2008). Clear statement of the objectives for thinning intermediate-sized trees will 

help clarify management intentions. 

Under what conditions could intermediate trees be thinned? We suggest the 

following criteria but stress that these criteria are based on working hypotheses. 

The first selection criteria is species. Thinned intermediate-size trees should only 

be fire-sensitive, shade-tolerant species such as white fir (Abies concolor (Gord. 

If trees larger than 

10 to 16 inches in 

d.b.h. are thinned, it is 

important to provide 

reasons other than for 

ladder-fuel treatment.

Under what conditions 

could intermediate 

trees be thinned? 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

25

& Glend.) Lindl. ex Hildbr.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) 

and incense-cedar (Calocedrus decurrens (Torr.) Florin). In mixed-conifer forest, 

attempt to keep intermediate-size pines and hardwoods because of their relative 

scarcity and importance to wildlife and fire resilience. A second criterion would 

be tree growth form. Some intermediate-size trees can still function as ladder fuel, 

particularly those that were initially grown in more open conditions (fig. 12). These 

trees can have live and dead limbs that extend down close to the forest floor providing a continuous fuel ladder. A third condition is middle to upper slope topographic 

position. In these slope positions, some thinning of intermediate-size trees may 

help accelerate the development of large “leave” trees. We suggest, however, that 

these criteria not be applied to riparian areas, moist microsites often associated with 

deeper soils, concave topography, or drainage bottoms because these areas may 

have supported higher tree densities and probably greater numbers of intermediatesize trees (Meyer et al. 2007b). 

Figure 12—Fire suppressed stand in the Teakettle Experimental Forest with white fir (Abies concolor) 

20- to 30-in d.b.h. with ladder fuel potential.

Malcolm North
26

GENERAL TECHNICAL REPORT PSW-GTR-220 

Allocation of Growing Space

We propose a form of multiaged silviculture for Sierra mixed-conifer forest that is 

flexible to meet diverse forest objectives, that would retain existing large trees and 

promote recruitment of more large structures, and that provides for sustainability. 

The silvicultural system is based on leaf area representing the occupied growing 

space of trees and stands. By segmenting stand-level leaf area index among canopy 

strata, we can develop tools to allocate growing space and provide flexibility 

for creating heterogeneous stand structures and meet ecological objectives (fig. 

13) (O’Hara 1996, O’Hara and Valappil 1999). For example, leaf area could be 

allocated primarily to larger trees in one stand where these large trees are present 

and important structural components. In other stands, large trees may be absent 

and leaf area is allocated to developing cohorts to expedite development of large 

structural features. Trees are harvested and timber is an output, but the silvicultural 

system’s focus is on retained stand structures, not what is removed for harvest. On 

the ground, this system provides for a diverse stand structure with both vertical 

(in discrete groups) and horizontal heterogeneity. It is prescribed one stand at a 

time and creates landscape-level heterogeneity by varying the stocking regime. 

Treatments are intended to create a mixture of structures sustained throughout 

the period between active management entries.

The proposed silvicultural system recognizes canopy strata as the primary unit 

for allocation of growing space. Within these strata, space is allocated to species or 

species groups. A resulting stocking matrix might consist of three canopy strata and 

three species groups (e.g., pines, white fir and incense-cedar, and others) providing for a stocking matrix with nine cells. This approach generally simplifies the 

marking of trees and also can modify species composition (O’Hara et al. 2003). 

This silvicultural revision will, however, require a new research project to adapt the 

MultiAge Stocking Assessment Model (MASAM) to Sierra Nevada mixed-conifer 

forests.

Conclusion

A central premise of this paper is that the risks of carefully considered active forest 

management are lower than the risks of inaction and continued fire suppression 

in the Sierras’ fire-prone forest types. We recognize the need to address specific 

management priorities (e.g., sensitive species) while developing practical and 

ecologically sound silvicultural guidelines. Many of the ideas contained within 

this ecosystem management strategy are not new, but their implementation will 

require some innovations, and they may provide a greater range of management 

options than do current practices. Our scientific understanding of mixed-conifer 

The risks of carefully 

considered active 

forest management are 

lower than the risks of 

inaction and continued 

fire suppression in 

the Sierras’ fire-prone 

forest types. 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

27

Figure 13—MultiAged Stocking Assessment model of a three-strata (or three-cohort) Oregon ponderosa pine (Pinus 

ponderosa Dougl. ex Laws.) stand. Growing space can be allocated in a variety of patterns providing flexibility in stand 

structure design (from O’Hara et al. 2003). (— = not calculated.)

Cohort 4

0

0

Cohort 4

0

 0

0

0

0

0

0

0

0

0

0

TOTAL Leaf area index (LAI)

Number of trees/cohort/acre

Percent of LAI/cohort

Leaf area index/cohort ECC

Leaf area index/cohort BCC

Leaf area/tree (ft2

) ECC

BA/cohort (ft2/ac) ECC

BA/cohort (ft2/ac) BCC

Average volume increment/tree (ft3/yr) ECC

Average volume increment/CC (ft3/ac/yr)

Quadratic mean DBH/cohort (in) ECC

Tree vigor (in3/ft2/yr)

Stand density index ECC

Stand density index BCC

6

Cohort 1

25

50

Cohort 1

3.0

1.3

5,227.2

72.5

29.7

1.02

20.7

23.1

0.408

95.2

46.6

Cohort 2

40

35

Cohort 2

2.1

0.7

2,286.9

47.6

14.4

0.64

16.3

14.8

0.460

74.6

28.7

Ponderosa pine MASAM — OREGON 

Cohort 3

55

15

Cohort 3

0.9

0

712.8

19.8

0

0.18

5.0

8.1

0.440

39.4

0

Diagnostic Information

User-Specified Variables

TOTAL

120

100

TOTAL

6.0

2.0

—

139.8

44.1

—

42.0

—

—

209.2

75.3
28

GENERAL TECHNICAL REPORT PSW-GTR-220 

ecosystems is rudimentary, and, therefore, it is important to continue learning from 

these strategies as they are applied. We have tried to identify information that is 

supported by many studies, that is suggested by fewer but often recent studies, and 

that we can only infer from lines of evidence or observation but do not yet know 

with any degree of certainty. In the “Research Needs” section below, we identify 

some of the topics raised in this paper that need further investigation, although 

management will also be improved by trying some of the proposed strategies and 

learning what works and what fails.

This project began at the invitation of Forest Service Pacific Southwest Region 

managers who asked if we could develop a summary of current research to inform 

mixed-conifer management. In bringing together authors representing different 

key disciplines affecting Sierran forests, we did not know whether recent fire 

science, forest ecology, and wildlife biology research would provide contrasting or 

complementary management concepts or whether the concepts could be translated 

into silviculture practices. It was soon clear that each discipline’s research findings 

coalesced around the importance of variable forest structure and fuels conditions 

for ecological restoration, forest resilience, and resulting diversity of wildlife habitat. We know that fire was the most important process influencing these ecosystems 

and that fire behavior was influenced by topography. This suggests managers could 

use localized site conditions and landscape position as a guide for varying forest 

treatments. The various treatments can be based on flexible thinning guidelines 

using tree species and canopy position to vary retention by site conditions. In sum, 

our management strategy is based on emulating forest conditions that would have 

been created by low-intensity, frequent fire throughout the forest matrix. 

Summary Findings

Sierra Nevada mixed-conifer forests could benefit from a new management strategy 

that goes beyond short-term fuel treatment objectives and incorporates long-term 

ecological restoration and habitat improvement into forestry practices. This strategy 

is compatible with current landscape fuel treatments (i.e., SPLATs, DFPZs, and 

WUI defense zones), but strives to incorporate ecological restoration and wildlife 

habitat needs that have not been explicitly addressed. This strategy can be implemented using a multiage silvicultural system to meet fuel reduction, ecosystem 

restoration, and wildlife habitat objectives. Important facets of the strategy include:

• Mechanical fuels management: When stands cannot be burned, reducing 

fuels to moderate fire behavior is still a key priority because wildfire is 

likely to burn the area eventually. A few of the ecological benefits of fire are 

achieved with mechanical fuel reduction, but thinning is not an effective 

In bringing together 

authors representing 

different key disciplines affecting 

Sierran forests, 

we did not know 

whether they would 

provide contrasting 

or complementary 

management concepts; however, each 

discipline’s research 

findings coalesced 

around the importance 

of variable forest 

structure and fuels 

conditions.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

29

substitute for fire in affecting ecosystem processes. Reducing surface fuels 

is as important as reducing ladder fuels.

• Limit use of crown separation in fuel treatments: Sparingly apply 

canopy bulk density reduction and increased tree crown separation only in 

key strategic zones. More research is needed, but current models suggest its 

effects on reducing crown fire spread are limited, and the regular leave-tree 

spacing does not mimic tree patterns in active-fire-regime forests. 

• The ecological importance of fire: Prescribed fire can help reduce 

surface fuels and restore some of the ecological processes with which 

mixed-conifer forests have evolved. 

• Treatments focused on affecting fire behavior: Efforts to restore preEuropean forest conditions are likely to fail in the face of climate change 

and also do not provide flexible prescriptions that adapt to different site 

conditions. Focus treatments on affecting potential fire behavior by manipulating fuel conditions, thereby allowing forests to equilibrate to fire under 

modern conditions and increasing forest heterogeneity.

• Retention of suitable structures for wildlife nest, den, and rest sites:

Trees providing suitable structure for wildlife include large trees and trees 

with broken tops, cavities, platforms, and other formations that create structure for nests and dens. These structures typically occur in the oldest trees. 

Develop and adopt a process for identifying, and thus protecting, such trees 

for use by inventory and prescription-marking crews. 

• Stand-level treatments for sensitive wildlife: Areas of dense forest and 

relatively high canopy cover are required by California spotted owls, fishers, and other species. Identify and manage areas where, historically, fire 

would have burned less frequently or at lower severity owing to cooler 

microclimate and moister soil and fuel conditions for the higher stem and 

canopy densities that they can support. 

• Large trees and snags: Given their current deficit in mixed-conifer forest 

and the time necessary for their renewal, protect most large trees and snags 

from harvest and inadvertent loss owing to prescribed fire. 

• Landscape-level treatments for prey of sensitive wildlife: In the absence 

of better information, habitat for the prey of owls and fishers may best be 

met by mimicking the variable forest conditions that would be produced by 

frequent fire. Reductions in stem density and canopy cover would emulate 

the stand structure produced by local potential fire behavior, varying by a 

site’s slope, aspect, and slope position.
30

GENERAL TECHNICAL REPORT PSW-GTR-220 

• Retain hardwoods and defect trees and promote shrub patches: 

Hardwoods (particularly black oak) and defect trees (i.e., those with cavities, broken tops, etc.) are valued wildlife habitat and should be protected 

whenever possible. Increasing understory light for shrub patch development, can increase habitat for some small mammals and birds.

• Riparian forest fuel reduction: Prescribed burning of riparian forest will 

help reduce fuels in these corridors that are also important wildlife habitat. 

• Spatial dispersion of treatments: Trees within a stratum (i.e., canopy 

layers or age cohorts) would often be clumped, but different strata would 

usually be spatially separated for fuel reasons. Give particular attention to 

providing horizontal heterogeneity to promote diverse habitat conditions. 

• Spatial variation in forest structure: “Average” stand conditions were 

rare in active-fire forests because the interaction of fuels and stochastic fire 

behavior produced highly heterogeneous forest conditions. Creating “average” stand characteristics replicated hundreds of times over a watershed 

will not produce a resilient forest, nor one that provides for biodiversity. 

Managers could strive to produce different forest conditions and use topography as a guide for varying treatments. Within stands, important stand 

topographic features include concave sinks, cold air drainages, and moist 

microsites. Landscape topographic features include slope, aspect, and slope 

position.

• Stand density and habitat conditions vary by topographic features: 

Basic topographic features (i.e., slope, aspect, and slope position) result in 

fundamental differences in vegetation composition and density producing 

variable forest conditions across the Sierra landscape. Drainage bottoms, 

flat slopes, and northeast-facing slopes generally have higher site capacity, 

and thus treatments retain greater tree densities and basal areas.

• Tree-species-specific prescriptions: Hardwoods and pines, with much 

lower densities in current forests compared with historical conditions, 

would rarely be thinned. Thinning would be focused on firs and incensecedar. Address pine plantations separately. 

• Silvicultural model and strategy: Tree diameter distributions in activefire forests vary but often have nearly equal numbers in all diameter size 

classes because of periodic episodes of fire-induced mortality and subsequent recruitment. Stand treatments that significantly reduce the proportion 

of small trees and increase the proportion of large trees compared to current stand conditions will improve forest resilience.
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

31

• Treatment of intermediate-size trees: In most cases, thinning 20- to 30-in 

d.b.h. trees will not affect fire severity, and, therefore, other objectives for 

their removal should be provided. Where those objectives are identified, 

silvicultural prescriptions would only remove intermediate-size trees when 

they are shade-tolerants on mid or upper slope sites. 

• Field implementation of silvicultural strategy: Modify marking rules 

to ones based on species and crown strata or size and structure cohorts (a 

proxy for age cohorts) rather than uniform diameter limits applied to all 

species.

• Allocation of growing space: A large proportion of the growing space 

would be allocated to the largest tree stratum.

• Assessment of treatment effects: Emphasis is on what is left in a treated 

stand rather than what is removed.

Research Needs

Some of our management recommendations are currently based on inferences from 

studies in other forest types. There are many aspects of Sierra Nevada ecosystems 

that are still poorly understood. The list below is focused on research needed to 

investigate and refine some of the suggested management practices. These studies 

and implementing the suggested strategy will undoubtedly raise new questions. 

Working together, forest managers and researchers can exchange information and 

identify unknowns as they develop. 

1. Quantify the leaf area and growth relationships needed to develop stocking control relationships for Sierra Nevada mixed-conifer forest. This will 

allow completion of a Sierra Nevada MASAM for the Kings River Project 

(KRP) area or any other area in the Sierra where this approach could be 

implemented. This tool will allow the design and assessment of a variety 

of multiaged-stand structures that include, among others, older residual 

trees, development of resilient structures, and accommodation of prescribed 

burning regimes. 

2. Develop and implement an adaptive monitoring strategy to assess the efficacy of a multiaged strategy at both the stand and landscape scales. This 

information will include both on-the-ground monitoring of treated stands 

and simulations using Sierra Nevada MASAM. This input will be used to 

refine the strategy over time and make large-scale assessments of landscape 

patterns for wildlife habitat, potential fire behavior, and general diversity of 
32

GENERAL TECHNICAL REPORT PSW-GTR-220 

vegetation patterns. A multiaged strategy would be adjusted pending results 

of monitoring efforts to accommodate other resource objectives such as 

wildlife, fire, or forest restoration. 

3. Assess the potential outcomes of this proposed silvicultural approach on 

vegetation response and wildlife habitat features of interest. This could be 

combined with a comparison to other possible silvicultural strategies to 

evaluate the similarities and differences of approaches. Research would 

also assess the effects of any treatment on predicted fisher resting habitat using either a predictive microhabitat model (Zielinski et al. 2004b) or 

a habitat model based on Forest Inventory and Analysis (FIA) protocols 

(Zielinski et al. 2006). 

4. More closely examine the distribution of tree size and canopy density 

characteristics within female fisher home ranges to establish the means 

and variances of tree number/density by size class, for both conifers and 

hardwoods. This would require overlaying the boundaries of female 

fisher home ranges, which have been estimated on the Sierra and Sequoia 

National Forests (Mazzoni 2002, Zielinski et al. 2004a), and then using 

both remotely sensed and ground-based methods to described the vegetation within these areas. Once we have estimates of the average number of, 

say, white fir between 20 and 30 in d.b.h. per acre within the average female 

home range, we will be able to compare this and other characteristics 

with the average number of this species and size class predicted to occur 

as residuals after proposed treatments. If the selected tree size or density 

characteristic, when measured after treatment, is significantly lower than 

what occurs in female home ranges, then the proposed management activity 

would not be consistent with fisher conservation.

5. Determine fire histories of riparian areas to identify fire frequency, intensity, and extent. How far does the riparian influence for dampening fire 

extend away from the stream? What stream characteristics (i.e., bank slope, 

stream size, etc.) affect the size of the riparian influence zone? What were 

historical fuel loads in these forests? How can riparian systems be managed 

to reduce adverse fire effects while maintaining wildlife habitat? In current 

wildfires, are riparian forests typically experiencing high-intensity crown 

fires, or are moister fuels and microclimate still dampening fire behavior? 
An Ecosystem Management Strategy for Sierran Mixed-Conifer Forests

33

6. Determine how forest structure and composition varied by topographic 

feature under an active-fire regime in the Sierra Nevada. There have 

been studies in the Klamath Mountains and eastern Washington, but no 

information is available for California forests. The research would identify 

which topographic features matter, and stand structure and fuels loads 

associated with different physiographic areas.

Acknowledgments

We acknowledge the following reviewers who contributed valuable insights and 

written comments that helped us shape this paper: Sue Britting, Steve Eubanks, 

Chris Fettig, Mark Finney, Jerry Franklin, Julie Gott, Dave Graber, Steve Hanna, 

Chad Hanson, Paul Hessburg, Matthew Hurteau, Jerry Jensen, Bobette Jones, 

JoAnn Fites Kaufmann, John Keane, Eric Knapp, Mike Landram, Dave McCandliss, 

Connie Millar, Joe Sherlock, Carl Skinner, Kim Sorini-Wilson, Nate Stephenson, 

Craig Thomas, Don Yasuda, and three anonymous reviewers. Thanks to Robert van 

Pelt, University of Washington, and Steve Oerding, Academic Technology Services, 

University of California, Davis for providing graphics. We are also grateful to 

Maichi Phan for her help with formatting the manuscript.

Metric Equivalents

When you know: Multiply by: To find:

Acres (ac) 0.405 Hectares

Inches (in) 2.54 Centimeters

Feet (ft) .305 Meters

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