United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-238

June 2010

Mitigating Old Tree Mortality in 

Long-Unburned, Fire-Dependent 

Forests: A Synthesis

Sharon M. Hood

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Hood, Sharon M. 2010. Mitigating old tree mortality in long-unburned, firedependent forests: a synthesis. Gen. Tech. Rep. RMRS-GTR-238. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station. 71 p.

Abstract

This report synthesizes the literature and current state of knowledge pertaining 

to reintroducing fire in stands where it has been excluded for long periods and the 

impact of these introductory fires on overstory tree injury and mortality. Only forested 

ecosystems in the United States that are adapted to survive frequent fire are included. 

Treatment options that minimize large-diameter and old tree injury and mortality in 

areas with deep duff and methods to manage and reduce duff accumulations are 

discussed. Pertinent background information on tree physiology, properties of duff, 

and historical versus current disturbance regimes are also discussed.

Keywords: smoldering, fire effects, tree mortality, duff mounds, fire exclusion, 

fuel treatment, prescribed fire, monitoring, old-growth, heat injury, 

reintroduction of fire, duff consumption

Author

Sharon Hood is a Forester in the Fire, Fuels, and Smoke 

Program located at the Rocky Mountain Research 

Station’s Fire Sciences Laboratory in Missoula, MT. She 

completed a B.S. degree in Forestry from Mississippi 

State University and an M.S. degree in Forestry from 

Virginia Polytechnic Institute and State University.

Acknowledgments

I acknowledge funding from the Joint Fire Science 

Program under Project JFSP 07-S-09. I thank Morgan 

Varner, Michael Harrington, Dale Wade, Sally Haase, 

James Agee, Daniel Perrakis, Kevin Ryan, Donald Scott, 

James Reardon, Theresa Jain, Roger Ottmar, and Kevin 

Hiers for their helpful comments and insights during publication development. Reviews by Donald Scott, Morgan 

Varner, Michael Harrington, Sally Haase, Dale Wade, 

and Kevin Ryan greatly improved the manuscript. I also 

thank the survey respondents whose comments guided 

the content of this publication and provided valuable land 

management experiences about reintroducing fire into 

long-unburned forests (Appendix B).
Preface

Forest managers around the country have expressed 

concerns about large-diameter and old tree mortality when 

prescribed burning in long-unburned forests. The synthesis 

herein suggests recommendations for maintaining and 

perpetuating old trees in fire-dependent ecosystems. It 

expands on efforts funded by the Joint Fire Science Program 

(JFSP) to define the issues surrounding burning in fireexcluded forests of the United States that are adapted to 

survive frequent fire. When the JFSP initially funded this 

synthesis, two JFSP projects were examining the effect of 

raking on reducing old ponderosa and Jeffrey pine (subsequently published in Fowler and others 2010; Hood and 

The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.

others 2007a). Another JFSP project examined the effect 

of prescribed burning under different duff moisture conditions on long-unburned old longleaf pine mortality (Varner 

and others 2007). Two other syntheses were also recently 

published on this subject: Perpetuating old ponderosa 

pine (Kolb and others 2007) and The conservation and 

restoration of old growth in frequent-fire forests of the 

American West (Egan 2007). The scope of the synthesis 

herein focuses only on limiting overstory tree mortality 

in species adapted to survive frequent fire; therefore, the 

implications of fire suppression and fuel treatments on 

other ecosystem components are not discussed.

i
Contents

Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Organization and scope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Fire Impacts on Trees and Causes of Tree Death. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

First Order Tree Responses to Heat Injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Crown injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Cambium injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Root injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Delayed Tree Mortality From Bark Beetles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Properties of Soil and Duff. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Differences Between Duff Mounds and Typical Forest Floor Duff. . . . . . . . . . . . . . . 16

First-Entry Prescribed Burns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Factors Affecting Duff Consumption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Impacts of Moisture on Soil Heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Historical and Current Fire Frequencies and Stand Characteristics. . . . . . . . . . . . . . . . . 24

Red pine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Longleaf pine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Southwestern ponderosa pine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Pacific Northwest and California. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Giant sequoia-mixed-conifer forests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Mixed-conifer, ponderosa, and Jeffrey pine forests. . . . . . . . . . . . . . . . . . . . . . . . . 28

Interior West ponderosa pine, Douglas-fir, and western larch. . . . . . . . . . . . . . . . . . . 29

Black Hills ponderosa pine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Treatment Effects on Old Tree Resilience. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Radial Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Moisture Stress. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Resistance to Insect Attacks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Management Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

General Management Issues. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Treatment prioritizations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

No action alternative. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Defining prescribed burn and other management objectives. . . . . . . . . . . . . . . . . . 35

Landscape-Scale Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Landscape restoration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Wildland fire use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Stand-Level Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Prescribed burning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Thinning and prescribed burning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

ii
Treating Individual Trees. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Fire scars. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Raking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Snow well burning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Mixing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Hand lining. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Foam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Fire shelter wrap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Mechanized equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Management Implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Monitoring the Effects of Fire on Overstory Tree Mortality. . . . . . . . . . . . . . . . . . . . . . 54

What to Monitor?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

How to Monitor?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

How Long to Monitor?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Knowledge Gaps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Climate Change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Appendix A: Determining Duff Moisture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Gravimetric Versus Volumetric Moisture Content. . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Sampling Location and Collection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Duff Moisture Meter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Oven-Drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Moisture Probes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Large Scale Indices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Appendix B: Survey Respondents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

iii

USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 1

Introduction ____________________

Historically, many forested ecosystems in the United 

States burned frequently, both from lightning ignited 

fires and from Native American burning. Frequent fire 

maintained low fuel loadings and shaped forests composed 

of tree species adapted to survive low-intensity frequent 

fire. Longleaf pine (Pinus palustris Mill.) forests burned 

as frequently as every 2 to 8 years (Christensen 1981; 

Frost 1993), and historical records and dendrochronological studies provide evidence that ponderosa pine (Pinus 

ponderosa C. Lawson), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western larch (Larix occidentalis

Nutt.), giant sequoia (Sequoiadendron giganteum (Lindl.) 

J. Buchholz), red pine (Pinus resinosa Aiton), and many 

other forests also burned regularly. In the early 1900s, the 

United States government initiated a program to suppress 

all fires, both natural and anthropogenic. Many unintended 

consequences have resulted from over a century of fire 

suppression, such as increased tree densities and fuel, 

increased stress on older trees from competition, and 

greater risk of bark beetle attacks. These consequences 

are especially apparent in forests that historically burned 

frequently and have thus missed many fire cycles.

Maintaining old trees and perpetuating large-diameter 

trees is an increasing concern. Stands of old trees that were 

historically common across vast landscapes in the United 

States are now relatively rare on the landscape because 

of harvesting (Noss and others 1995). Though logging 

is no longer the principal threat to most old-growth forests, they now face other risks (Vosick and others 2007). 

Prescribed fire has become a major tool for restoring firedependent ecosystem health and sustainability throughout 

the United States and use will likely increase in the future. 

However, increased mortality of large-diameter and old 

trees following fire has been reported in many areas 

around the country, and there is increased concern about 

maintaining these on the landscape (Kolb and others 2007; 

Varner and others 2005). As early as 1960, Ferguson and 

others (1960) reported high longleaf pine mortality after 

a low-intensity prescribed burn consumed the majority 

of heavy duff accumulations around the base of the trees. 

Mortality of pre-settlement ponderosa pines in prescribed 

burn areas in Grand Canyon National Park was higher than 

in control plots (Kaufmann and Covington 2001). After 

beginning a forest restoration program that reintroduced 

fire by prescribed burning at Crater Lake National Park, 

excessive post-fire mortality of larger ponderosa pine was 

observed in the burn areas, and early season burns had an 

even higher mortality than late season burns (Swezy and 

Agee 1991). Both Swezy and Agee (1991) and McHugh 

and Kolb (2003) reported a U-shaped mortality distribution 

for ponderosa pine following wildfires, with smaller- and 

larger-diameter trees having higher mortality than middiameter trees.

Litter and duff accumulation around large-diameter 

trees has reached unprecedented levels in many areas as 

a result of 100+ years of fire exclusion. Abnormally high 

litter and duff accumulations after extended fire-free 

periods in fire-dependent ecosystems are well documented (Covington and others 1997; Dodge 1974; Haase 

and Sackett 1998; Sackett and Haase 1998; Sackett and 

others 1996; Swezy and Agee 1991; Varner and others 

2005). Even with mechanical thinning to reduce ladder 

fuels and thus, the likelihood of crown injury, deep duff 

mounds remain, as does the potential for stem and root 

injury. Because flames are not typically associated with 

smoldering duff, forest floor consumption after the flaming 

front passes draws little attention, and its consequences 

are easily overlooked. However, restoration burns in areas 

with deep basal duff around trees may result in greater 

duff consumption and higher soil temperatures than trees 

subject to periodic low-intensity fires with shallower basal 

duff (Swezy and Agee 1991).

Several studies have attributed large-diameter tree 

mortality to basal injury caused by duff mound smoldering. Long-term smoldering can cause extended high 

soil heating, frequently above 140 °F (60 °C), which is 

the temperature required to kill living tissue. Hartford 

and Frandsen (1992) reported soil temperatures under 

smoldering duff mounds of 750 °F (400 °C), with duff 

temperatures above 212 °F (100 °C) for over 16 hours, 

Mitigating Old Tree Mortality in 

Long-Unburned, Fire-Dependent Forests: 

A Synthesis

Sharon M. Hood
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

compared to soil temperatures of less than 176 °F (80 °C) 

and duff temperatures above 212 °F (100 °C) for 1 hour 

under burning slash. Temperatures in smoldering duff 

mounds exceeded 572 °F (300 °C) for 2 to 4 hours during 

a prescribed burn in Glacier National Park, resulting in 

45 percent mortality of the cambium sampled (Ryan and 

Frandsen 1991).

Basal tree injury from smoldering duff can attract bark 

beetles that can greatly influence post-fire delayed tree 

mortality. Rust (1933) reported for the 1928 Tubb’s Hill 

ground fire at Coeur d’Alene, ID, that bark beetles attacked 

a greater percentage of ponderosa pines containing severe 

basal injury (from long-duration smoldering of accumulated basal duff) than trees with severe crown injury. This 

type of fire injury, and subsequent insect attack, resulted 

in heavy losses in larger trees in more open stands (Rust 

1933). Bradley and Tueller (2001) stated that a burned tree 

was nearly 25 times more likely to be attacked by bark 

beetles than an unburned tree, and trees with deep soil 

charring were nearly 10 times more likely to be attacked 

than all other trees combined.

Although some work has been done on the effect of 

duff and litter removal from the base of trees (Covington 

and others 1997; Feeney and others 1998; Laudenslayer 

and others 2008; Swezy and Agee 1991), little is known 

about the factors that determine its success or failure as a 

practical management tool. Successful removal treatments 

could widen the window of opportunity for prescribed 

burning to achieve management goals already constrained 

by weather, fuel conditions, air quality concerns, societal 

constraints, fire resource availability, and concern about 

potential large-diameter tree mortality. Sample sizes in 

most of these studies were small and there were no controls or raking-only treatments. Fowler and others (2010) 

reported that raking reduced cambium kill at the bases of 

old ponderosa pine. However, the cambium kill did not 

result in tree mortality, and three years after burning, no 

trees, either raked or unraked, died in the study.

It is important to distinguish between first-entry, initial 

burns and subsequent maintenance burns in areas that 

have not burned for long periods. Long-unburned stands 

usually have substantially higher fuel loadings and greater 

duff to litter ratios than more frequently burned stands. 

Therefore, extra care must be taken when reintroducing 

fire to forests after decades of fire absence (Wade and 

Johansen 1986). In areas with deep basal duff mounds, the 

initial prescribed burn will likely have the greatest impact 

on tree mortality compared to subsequent burns. In these 

areas, utmost precaution to limit duff consumption at the 

base of large trees is required. It will likely take either 

multiple low-severity fires to slowly reduce fuel loadings 

to historical levels or mechanical removal methods such 

as raking to maintain the large tree component (Arno 

2000; Harrington and Sackett 1990). Fires that consume 

litter but little duff may create an environment that speeds 

up residual duff decomposition (Zeleznik and Dickmann 

2004; S. Haase, personal communication; T. Jain, personal 

communication). Because litter is consumed very quickly 

during passage of the flaming front, residence times are 

short and do not cause basal injury to trees with thick bark.

Organization and Scope

Maintaining large-diameter and old trees is only one 

aspect of forest restoration. Restoring and maintaining 

historic disturbance patterns and ecosystem functions 

is the larger goal of most restoration efforts. However, 

perpetuating the large-diameter tree component is key 

to restoring historical stand structure and many ensuing 

processes because old trees take longer than any other ecosystem component to replace (Kaufmann and Covington 

2001). The forest restoration literature discusses in detail 

these broader issues for several fire-frequent ecosystems in 

the United States (Apostol and Sinclair 2006; Clewell and 

Aronson 2008; Friederici 2003; Stanturf and Madsen 2004). 

This report focuses on maintaining existing large and/or old 

trees and perpetuating future old trees when reintroducing 

fire into long-unburned areas. Old-growth forest definitions abound and vary by ecosystem. Rather than trying 

to define and limit the volume to only old-growth forests, I 

discuss the impact of fire on all large-diameter and/or old 

trees for species that current knowledge leads us to believe 

survived frequent low-intensity surface fire regimes. Only 

tree species that are adapted to survive frequent low-tomoderate intensity surface fire are included. The majority 

of the literature focuses on ponderosa and longleaf pine, 

but many other pertinent species are discussed (Table 1).

Table 1—Trees species included in this volume. 

Mature trees listed historically survived under a frequent, low-intensity 

fire regime.

Species

Eastern U.S. Western U.S.

Red pine Ponderosa pine

Douglas-fir

Sugar pine

Western larch

Incense cedar

Southern U.S. White fir

Longleaf pine Red fir

Giant sequoia
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 3

This synthesis is organized into seven sections: (1) Fire 

Impacts on Trees and Causes of Tree Death; (2) Properties 

of Soil and Duff; (3) Historical and Current Fire 

Frequencies and Stand Characteristics; (4) Treatment 

Effects on Old Tree Resilience; (5) Management Options; 

(6) Monitoring the Effects of Fire on Overstory Tree 

Mortality; and (7) Knowledge Gaps. The first two sections provide a background for fire-related tree injury 

and ground fuels in fire-excluded stands. The third section contrasts historical and current stand conditions and 

disturbance regimes for fire-dependent forest types. 

The fourth and fifth sections provide information on 

treatment options at various scales based on pertinent 

studies and makes general treatment recommendations by 

forest type. Management Options also discusses defining 

treatment objectives, treatment prioritization, no action, 

and monitoring techniques. Differences between stand 

and individual tree monitoring, what variables to monitor, 

and appropriate monitoring time lengths are discussed in 

the Monitoring section. The last section identifies gaps in 

the scientific literature and recommends topics for future 

research.

Fire Impacts on Trees and Causes 

of Tree Death ___________________

When discussing fire effects on trees, it is helpful to 

first define some key terms. Injury and damage are not 

synonymous. Smith and Sutherland (2001) make the 

distinction: injury is an impairment or loss of function; 

damage involves loss of property, value, or usefulness. 

Damage is relative to management goals (Sutherland and 

Smith 2000), while injury is not.

The three basic types of fire are: crown fire, surface fire, 

and ground fire. Crown and surface fires consume surface 

and canopy fuels by active flaming. These two types can 

cause crown injury through convective and radiant heating, but typically cause little to no soil heating (Byram 

1959; Hartford and Frandsen 1992). Ground fires burn 

through duff and organic soils by smoldering combustion. 

Smoldering combustion is a much slower process with 

higher residence times, more smoke production, and lower 

temperatures than active flaming (Hartford and Frandsen 

1992). No flames are visible during smoldering and only 

wisps of smoke or a small, glowing front is visible during 

the glowing combustion phase (DeBano and others 1998). 

Ground fires and consumption of large-diameter surface 

fuels can cause root and basal stem injury by consuming 

fine roots growing in the duff layer and through long-term 

heating of the soil and cambium at the tree base (Hungerford 

and others 1994; Ryan and Frandsen 1991).

Trees can be killed immediately by fire, die several 

years post-fire from either direct fire injuries or indirectly 

though biotic or abiotic agents, or survive despite injuries 

(Loomis 1973). Fire injuries to a tree’s crown, cambium, 

and roots are first order fire effects. These injuries often 

lead to tree death, depending on the extent of injury, and 

influence the tree’s ability to withstand other factors such 

as post-fire bark beetle attacks and drought (Ryan 1982; 

Wade and Johansen 1986). These secondary factors that 

result from the interaction with the fire and the tree’s 

response to fire-caused injuries are called second order 

fire effects.

First Order Tree Responses to Heat Injury

Crown injury—Crown injury is typically cited as the 

most important factor determining post-fire tree survival 

(Fowler and Sieg 2004; Wagener 1961). Crown injury 

reduces a tree’s photosynthetic capacity by reducing the 

volume of the live crown. However, this reduction in 

photosynthetic capacity is not directly proportional to the 

percentage of crown volume lost because the lower onethird of the crown is less photosynthetically productive 

than the upper two-thirds (Ryan 1998). Wallin and others 

(2003) found that ponderosa pine trees with greater than 

50 percent crown scorch increased net photosynthetic 

rate, suggesting improved water relations in the remaining 

unscorched foliage after a prescribed burn in Arizona; 

Ryan (2000) also supported this finding.

Crown injury can be grouped into two major types: 

needle scorch and bud kill (Figure 1). Crown needle 

scorch is a measure of the amount of pre-fire crown 

where needles are killed by heated air (scorched) and 

can include areas with live and dead buds. Crown bud 

kill is the amount of pre-fire crown where buds are either 

killed by heated air or consumed by direct flame contact 

(Figure 2). These measurements are usually expressed as 

either percentage of pre-fire crown volume scorched/killed 

or percentage of pre-fire crown length scorched/killed.

For most species, the areas of the crown with needle 

scorch and bud kill are equal. However, the difference 

can be substantial for species with large buds, such as the 

southern pines, ponderosa pine, red pine, and Jeffrey pine 

(Pinus jeffreyi Balf.), or for those with protective features 

around the buds, such as the spur shoots on western larch 

branches. Larger buds require longer heat exposure to kill 

meristematic tissue; therefore, species with large buds are 

more resistant to crown injury. Long needles also form 

a protective sheath around the buds, offering additional 

protection (Ryan 1982; Wade and Johansen 1986). These 

species are capable of surviving very high levels of crown 
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Figure 1. Ponderosa pine showing the different types 

of crown injury. The uppermost, green portion of the 

crown was unaffected by the fire. The middle portion 

of the crown’s needles were scorched and killed, but 

the buds survived. The lower portion of the crown’s 

needles was scorched and both the needles and 

buds were killed.

Figure 2. Crown consumption from direct flame contact.

scorch if bud kill is minimal (Dieterich 1979; Wade and 

Johansen 1986; Wagener 1961). Burning during the dormant 

season may reduce bud kill more than burning during the 

growing season when buds are actively growing and ambient air temperature is lower (Ferguson 1955; Harrington 

1993; Wade and Johansen 1986). Southern pines also 

undergo multiple needle flushes during the growing season. This ability to quickly refoliate increases the chance 

of tree survival if high crown scorch occurs early in the 

season compared to the same level of injury occurring 

after the last flush of the year (Weise and others 1989).

Crown scorch is most easily determined several weeks to 

several months after the fire when the needles have turned 

brown but have not fallen (Ryan 1982). Immediately after 

the fire, scorched needles have a dull green appearance 

making it more difficult to determine scorch levels than 

when the needles are brown. Crown scorch cannot be 

reliably measured beyond 1 year post-fire because many 

scorched needles have already fallen and tree crowns 

may begin fading due to other factors, such as bark beetle 

attacks (Figure 3) or disease.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 5

Bud kill is most easily measured soon after bud break, 

during the first spring following the fire, or after the next 

needle flush for southern pines. At this time, new needle 

emergence is highly visible and the majority of scorched 

needles still remain in the crown. Areas of the crown with 

both needle scorch and bud kill retain scorched needles 

longer than areas where buds and branches are alive because 

the dead limbs cannot actively shed the dead needles. These 

scorched needles on dead branches eventually weather off 

with wind and precipitation. Areas of the scorched crown 

with little bud kill can abscise the dead needles, making 

the crown appear thin because of the lost scorched needles 

(Figure 4) (Ryan 1982).

Cambium injury—Cambium kill occurs when lethal 

temperatures reach the cambium layer between the bark and 

wood. The tree is girdled if cambium is killed around the 

entire tree circumference. Bark thickness is the principal 

factor determining the amount of heat transferred to the 

cambium during a fire (Martin 1963). Bark insulates the 

cambium from heat and is not easily combustible; therefore, 

thicker bark provides more protection. The rate of thickening differs by species, individual genetic differences, and 

environmental factors, and it influences how quickly this 

insulating layer forms (Hare 1965; Hengst and Dawson 

1994). Species that develop thick bark early in life become 

fire-resistant sooner and are adapted to survive frequent, 

low-intensity fire. Bark thickness generally increases 

linearly with tree diameter (Spalt and Reifsnyder 1962). 

Figure 3. Fading ponderosa pine tree from bark beetle attacks 1 year after fire. 

No needles on this tree were scorched by the fire.
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

However, Myers (1963) found old ponderosa pines in the 

Southwest had thinner bark than the younger ponderosa 

pines of equal diameter.

Species differences in bark and cambium moisture 

content, physical and thermal properties, and chemical 

composition also influence a tree’s resistance to cambium 

kill from fire (Jones and others 2004; Martin 1963; Spalt 

and Reifsnyder 1962). For example, Hare (1965) reported 

that for equal bark thickness, longleaf and slash pines 

(Pinus elliottii Engelm.) were nearly twice as resistant 

to cambium kill compared to sweetgum (Liquidambar 

styraciflua L.), American holly (Ilex opaca Aiton), and 

black cherry (Prunus serotina Ehrh.). However, Martin 

(1963) and Dickinson and Johnson (2004) conclude that 

differences in thermal tolerance among species and 

growing seasons were relatively small when compared to 

the effects of bark thickness. Peterson and Ryan (1986) 

estimated the length of time necessary to kill portions of 

the cambium based on tree diameter for several species 

in the northern Rockies (Figure 5).

Cambium is killed at approximately 140 °F (60 °C) 

(Dickinson and Johnson 2004). Lethal temperatures may 

be reached after a few seconds for species with very thin 

bark. However, this can take hours for species with thick 

bark. Long-term heating of this kind only occurs when 

there is a large amount of fuel burning near the tree, such 

as a stump, log, or deep duff. Basal cambium injury on 

tree species adapted to survive frequent, low-intensity 

fire is typically not a problem in low-intensity surface 

fires because these species have thick bark, and little litter 

and duff accumulates between fires. However, in longunburned areas, duff depth typically increases dramatically 

near the base of a tree, forming a basal mound. The longterm smoldering combustion of this fuel accumulation can 

increase cambium injury even for species with thick bark 

(Ryan and Frandsen 1991). This type of low-intensity, highseverity fire often produces a ring of charred, blackened 

bark near the groundline indicating the amount of duff 

that was consumed during the fire (Figure 6).

Figure 4. Tree crown in left foreground was scorched and buds were killed by fire. Center tree’s 

needles were scorched, but the buds survived. These buds flushed 1 year after the fire and the 

majority of scorched needles have already fallen.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 7

Figure 5. Critical time for predicted cambial kill as a function 

of tree diameter and species. Species are: Douglas-fir (DF), 

western larch (WL), ponderosa pine (PP), western hemlock 

(WH), grand fir (GF), Engelmann spruce (ES), western white 

pine (WP), western red cedar (WR), lodgepole pine (LP), and 

subalpine fir (SF). From Peterson and Ryan 1986.

Figure 6. Using a drill to sample the base of a ponderosa pine for cambium kill after a 

prescribed burn. The charred, blackened area around the tree indicates the amount of litter 

and duff consumption. Arrows indicate the top of the duff pins radiating out from the tree 

base that were level with the surface litter prior to burning.

Ryan and Frandsen (1991) developed a logistic regression model to predict the probability of ponderosa pine 

cambium death from prescribed burning. They measured 

duff consumption around old ponderosa pine trees during 

a prescribed burn in northwestern Montana. Cambium 

mortality was best predicted by the amount of duff consumed (Figure 7). This equation suggests that ponderosa 

pine duff depths deeper than 7 inches (18 cm) may cause 

substantial cambium mortality (>50 percent) if completely 

consumed. However, the results of this study are drawn 

from one research prescribed burn; additional research 

is needed to test this predictive model. Jones and others 

(2006) developed a physics-based stem heating model to 

predict cambium death during fire, but this model simulates 

stem heating and cambium death caused from flaming 

combustion and does not apply to long-term smoldering 

combustion.

A tree will likely die if the majority of its circumference is girdled by fire, or if additional injury to the crown 

also occurs. A girdled tree may take several years to die 

because the xylem is intact and can continue to transport 

water to support the crown, but photosynthate cannot be 

transported down to roots (Figure 8). The root system 

is eventually depleted of stored carbohydrate reserves 
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Figure 7. Logistic regression model for predicting the probability of cambium mortality in mature ponderosa pine resulting from 

duff consumption. Pm is the probability of cambium mortality (0 to 1) and duff consumed is the amount of duff depth reduction 

(cm). From Ryan and Frandsen 1991.

Figure 8. Cross section of tree showing functional 

parts of a tree stem.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 9

and stops producing fine roots, which absorb soil water. 

Therefore, the tree dies from water stress (Greene and 

Shilling 1987; Michaletz and Johnson 2007).

Partial cambium kill produces a fire scar. Although trees 

can be scarred numerous times from reoccurring fires and 

survive, exposed fire scars are particularly susceptible to 

further injury in subsequent fires, especially if decayed 

wood is present. The leeward side of the tree, usually the 

uphill side in sloped areas, is most susceptible to cambium 

injury because of increased residence times, flame lengths, 

and temperatures as flames wrap around the backside of 

the tree and create vortices (Gill 1974; Gutsell and Johnson 

1996). Guyette and Stambaugh (2004) found 90 percent 

of fire scars were located on the uphill side of the tree in 

mixed oak-shortleaf pine (Quercus spp. - Pinus echinata 

Mill.) forests in Arkansas.

Ryan (1982) developed bark char codes to help indicate 

stem injury after fire (Table 2). However, bark char on species with thick bark is not a definitive indicator of actual 

cambium kill (Breece and others 2008; Hood and others 

2008). Hood and others (2008) evaluated these codes for 

many western conifer species and found that deep char 

usually indicated underlying dead cambium for species 

with thinner bark (Table 3). However, moderate char was 

not clearly associated with either live or dead cambium for 

species with thick bark. Breece (2006; Breece and others 

2008) also reported an ambiguous relationship between 

bark char codes and cambium kill for ponderosa pine.

Cambium kill is most easily determined several months 

after a fire by removing a small portion of the bark at 

groundline (Figure 6 and 9). It is important to sample as 

close to the groundline as possible, as this is where heat 

injury to the cambium is most likely to occur (Hood and 

others 2007b). Samples are most easily obtained with a 

hatchet, or alternatively, a drill with a hole-saw attachment, an increment borer, or an increment hammer can 

also be used (Lentile and others 2005). Drills provide an 

exposed area of uniform size and go through thick bark 

quickly, but are heavy and cumbersome to use in most 

field situations. Increment borers and hammers are time 

consuming, reveal a very small portion of the cambium, 

and are easily damaged when used on charred, resinous 

bark (S. Hood, personal observation, 2002; Lentile and 

others 2005).

Once cambium is exposed, its status can be determined 

visually or tested using a vital stain (Ryan 1982). Live tissue will feel moist, soft, and spongy and will be a light 

pink or salmon color. Live cambium is pliable and is 

usually easily peeled away from the wood and bark. 

Dead cambium either will be hardened, with a dark, 

shiny appearance or will feel sticky, with a darker color 

Table 2—Bark char codes and description of bark appearance 

(adapted from Ryan 1982).

Bark char code Bark appearance

Unburned No char

Light Evidence of light scorching; can 

still identify species based on bark 

characteristics; bark is not completely 

blackened; edges of bark plates charred

Moderate Bark is uniformly black except possibly in 

inner fissures; species bark characteristics 

still discernable

Deep Bark has been burned into, but not 

necessarily to the wood; outer bark 

species characteristics are lost

Table 3—Recommended management guidelines for using 

Ryan’s (1982) bark char codes as a surrogate for 

direct cambium sampling after fire. Species/code 

combinations not listed are not clearly associated with 

either live or dead cambium and should be sampled 

directly to determine injury.

Species Bark char code

Probable 

cambium 

status

Lodgepole pine

Whitebark pine

Western white pine

Western red cedar

Engelmann spruce

Subalpine fir

Light, moderate, 

or deep

Dead

White fir

Incense cedar

Ponderosa pine

Douglas-fir

Sugar pine

Light Alive

White fir

Incense cedar

Ponderosa pine (wildfire)

Douglas-fir (wildfire)

Sugar pine

Deep Dead

Ponderosa pine

(prescribed fire)a

Moderate or 

deep

Alive

Douglas-fir 

(prescribed fire)a

Moderate Alive

Western larch Light, moderate, 

or deep

Alive

aIf pre-fire duff mound depths are high and most duff is consumed in 

fire, then the probability of cambium mortality is higher.
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

and a sour smell. Sometimes the resin may have dried 

and have a whitish cast (Figure 9). Dead cambium will 

not easily separate from the wood and bark (Hood and 

others 2007b; Ryan 1982).

Root injury—Root mortality is a function of temperature and duration of heat. Instantaneous tissue death 

occurs at approximately 140 °F (60 °C). However, the effect 

of long-term elevated temperatures below 140 °F (60 °C) 

on tissue is poorly understood. Smoldering ground fires 

can kill roots growing near the soil surface or in the duff 

either directly as duff is consumed or from heating the 

soil to lethal temperatures. Literature on root location, 

production, and turnover is sparse for most tree species, 

although it is commonly accepted that the majority of 

fine roots occur in the upper mineral soil horizon in most 

areas (Persson 2000). Soil is a poor conductor of heat, 

but studies have shown that deep, smoldering duff can 

heat soil to over 140 °F (60 °C) at least 8 inches (20 cm) 

deep in ponderosa pine (Sackett and Haase 1998) and 

longleaf pine (Varner and others 2009). In mixed-conifer 

forests of California, lethal temperatures were common 

to 4 inches (10 cm) deep in mineral soil during prescribed 

burns (Haase and Sackett 1998). In that study, forest floor 

depths beneath tree canopies where soil temperatures were 

measured ranged from 2.4 to 10.9 inches (6 to 28 cm) and 

duff consumption was complete. Increasing the time that 

soil was heated above 140 °F (60 °C) caused a steep decline 

in the amount of coarse root non-structural carbohydrates 

when burning a fire-excluded longleaf stand (Varner and 

others 2009). The authors hypothesized that fire-injured 

trees were using available carbohydrates stored in coarse 

roots to replenish heat-killed fine roots, thus compromising 

the trees and making them vulnerable to second order fire 

effects, such as bark beetles, diseases, or climatic stress. 

Another study in long-unburned longleaf pine found that 

trees were 20 times more likely to die when basal duff 

consumption exceeded 30 percent compared to trees 

with less basal duff consumption (J. O’Brien, personal 

communication). Tree mortality was also attributed to 

mortality of fine roots located in the accumulated duff 

layer that were consumed during the fire.

Figure 9. Douglas-fir cambium. The upper portion of exposed cambium is alive, and the lower portion is dead.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 11

Studies of root location in the soil and in proximity to 

the bole are conflicting. Location likely varies by tree 

species, site productivity, tree age, duff depth, and drought 

conditions. Average fine root content and concentration 

was higher at the dripline than halfway between the 

dripline and tree bole for large Douglas-fir (>17 inches; 

>43 cm diameter at breast height (DBH)) and ponderosa 

pine (>19 inches; >48 cm DBH) in Idaho (Dumm 2003). 

However, the relationship was the opposite for small trees. 

The majority of their roots were located in mineral soil, 

rather than in the forest floor. Duff at this study site was 

relatively shallow, with a mean depth of 1.4 inches (3.5cm), 

and differences in depth between sample locations were not 

reported. In contrast, there were no significant differences 

in fine root biomass by tree age or by distance from bole 

in ponderosa pines growing in central Oregon (Andersen 

and others 2008). This study compared root biomass 

among 15 to 20, 50 to 60, and greater than 250-year-old 

trees. Sampling occurred at 50, 100, and 150 percent of 

the distance between tree bole and dripline. Fine root 

lifespan was greater than 1 year. However, no roots were 

sampled from the forest floor layers, and duff depths or 

time since fire was not reported. Curtis (1964) excavated 

the root system of a 16.9-inch (42.9-cm) DBH, 60-year-old 

ponderosa pine growing on the Boise Basin Experimental 

Forest, ID. He found 24 percent of fine roots were located 

within 5 inches (12.7 cm) of the tree bole.

It is speculated that fire exclusion has allowed fine roots 

to grow up into accumulated duff on some sites where 

frequent fire would typically limit duff development and 

contain roots mostly to the mineral soil horizons (Jain and 

Graham 2004; Wade 1986). The presence of fine roots in 

the duff is an important observation when determining 

potential tree mortality from prescribed burning. Gordon 

and Varner (2002) found no significant differences in biomass of longleaf pine roots <0.08 inches (<2 mm) diameter 

in the forest floor and upper 9.8 inches (25 cm) of mineral 

soil at the base of old trees in a fire-excluded stand. Old 

ponderosa pine tree mortality has been attributed to firecaused injury to fine roots located in the duff in Oregon 

(Swezy and Agee 1991) and in shallow volcanic soils in 

Arizona (Fulé and others 2002b). Duff consumption near 

the tree bole during a prescribed fire was significant in 

predicting white fir (Abies concolor (Gord. & Glend.) Lindl. 

ex Hildebr.), sugar pine (Pinus lambertiana Douglas), and 

ponderosa pine mortality (Stephens and Finney 2002). 

Pre-burn duff moisture averaged 15.8 percent, resulting 

in almost complete duff consumption.

Detecting root kill after a fire is much more difficult 

than detecting crown or cambium injury. Ground char 

codes exist to provide a general assessment of potential 

root damage (Table 4) (Ryan 1982; Ryan and Noste 1985), 

but they have not been tested to determine if there is a 

relationship between increasing ground char and root kill. 

Table 4. Ground char codes and descriptions (Ryan 1982; Ryan and Noste 1985).

 Ground 

char code Ground appearance

Unburned • Not burned

Light • Litter charred to partially consumed

• Upper duff layer may be charred but the duff is not altered over the

entire depth

• Surface appears black

• Where litter is sparse, charring may extend slightly into soil surface, but 

soil is not visibly altered

• Woody debris partially burned

• Logs are scorched or blackened but not charred

• Rotten wood is scorched to partially burned

Moderate • Litter mostly to entirely consumed, leaving coarse, light colored ash (ash 

soon disappears, leaving mineral soil)

• Duff deeply charred, but not visibly altered

• Woody debris is mostly consumed

• Logs are deeply charred and burned out stump holes are evident

Deep • Litter and duff completely consumed, leaving fine white ash (ash soon

disappears leaving mineral soil)

• Mineral soil charred and/or visibly altered, often reddish

• Sound logs are deeply charred, and rotten logs are completely 

consumed
12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Average ground char rating was significantly higher for 

dead ponderosa pine trees than live trees (McHugh and 

Kolb 2003; Thies and others 2006), but it was not significant in predicting either ponderosa pine or Douglas-fir 

tree mortality (Hood and Bentz 2007; McHugh and Kolb 

2003; Thies and others 2006).

Delayed Tree Mortality From Bark Beetles

Many bark beetle species are attracted to burned areas 

and cause post-fire tree mortality beyond what is expected 

from fire injury alone (Breece and others 2008; Hood 

and Bentz 2007; Hood and others 2007c; Lombardero 

and others 2006; McHugh and others 2003; Perrakis and 

Agee 2006; Ryan and Amman 1996). The influence of 

this secondary interaction between fire and bark beetles 

on delayed tree mortality varies with beetle population 

levels and host availability, but the widespread influence of bark beetles on post-fire delayed tree mortality 

is evident from previous studies. Of the 41 studies that 

have examined the effects of fire on old or large-diameter 

tree mortality in historically fire-frequent forests in the 

United States, 22 reported bark beetles caused additional 

mortality post-fire (Table 5). This secondary interaction 

is further documented in other forest types and younger 

stands (Fowler and Sieg 2004; Negrón and others 2008).

Bark beetles can have a major influence on the timing and 

amount of post-fire delayed tree mortality. Many studies 

have reported little additional mortality beyond the second 

post-fire year (Fowler and Sieg 2004). However, others have 

observed considerable tree mortality occurring over much 

longer periods where bark beetles attacked fire-injured 

trees (Hood and Bentz 2007; Sackett and Haase 1998; 

Weatherby and others 2001). Differences in the influence of beetle attacks on fire-injured trees among studies 

may, in part, be a function of the length of time trees are 

monitored post-fire. Fifteen of the 41 existing studies on 

post-fire old or large-diameter mortality in fire-dependent 

U.S. forests monitored mortality and bark beetle attacks 

for 5 years or longer; the remaining studies only report 

post-fire mortality for 3 years or fewer (Table 5). Records 

of the long-term effects of bark beetles and fire on mortality are even scarcer, only three studies have reported 

results from 10 years or more post-fire (Table 5). There 

is clearly a need for longer-term monitoring in order to 

fully understand the effect of bark beetle attacks and other 

secondary interactions on post-fire tree mortality. This 

is discussed further in the Monitoring the Effects of Fire 

on Overstory Tree Mortality section of this publication.

Bradley and Tueller (2001) found significant correlations between prescribed burning and bark beetle attacks 

on Jeffrey pine in the Lake Tahoe Basin, CA. Beetles 

attacked 24 percent of trees in burned plots compared to 

less than 1 percent of unburned plots. A burned tree 

had a 24.81 times greater chance of being attacked than a 

similar unburned tree. The majority of attacks were from 

red turpentine beetles (Dendroctonus valens LeConte). 

Though it is not typically considered a tree-killing beetle, 

it seemed to be attracted to fire-injured trees and to predispose the burned trees to attacks by more aggressive 

Dendroctonus beetle species. All burned trees attacked by 

Jeffrey pine beetle (Dendroctonus jeffreyi Hopkins) were 

also attacked by red turpentine beetles. No red turpentine 

beetle attacks were observed in the control plots.

Prescribed burning an old-growth mixed-conifer forest 

in the Sierra Nevada, CA, increased bark beetle attacks and 

subsequent large-diameter sugar and Jeffrey pine mortality 

compared to unburned units (Maloney and others 2008). 

No crown kill was observed in the burned units, leading 

the authors to speculate that a combination of bark beetle 

attacks and basal injury killed the trees. However, another 

study in the Sierra Nevada found no difference in probability of sugar pine mortality after mountain pine beetle 

(Dendroctonus ponderosae Hopkins) attack between 

small and large trees after prescribed burning (Schwilk 

and others 2006).

Western pine beetles (Dendroctonus brevicomis

LeConte), alone or in conjunction with fire injury, were 

the largest mortality agent after prescribed burns in oldgrowth ponderosa pine units in Crater Lake National 

Park, OR (Perrakis and Agee 2006). In a separate study 

in Crater Lake National Park, 2 percent of large ponderosa 

pine died from insects or pathogens over 5 years in the 

control units compared to 6 percent in spring burn units 

and 13 percent in fall burn units (Agee and Perrakis 2008). 

Thomas and Agee (1986) reported that the proportion 

of beetle-killed ponderosa and sugar pines greater than 

27.6 inches (70 cm) DBH was higher in prescribed burned 

areas than in unburned areas in Crater Lake.

Douglas-fir beetles (Dendroctonus pseudotsugae

Hopkins) are attracted to large-diameter, moderately fireinjured Douglas-fir trees (Furniss 1965; Hood and Bentz 

2007; Rasmussen and others 1996). Large Douglas-fir 

trees that initially survive fire injuries are susceptible to 

Douglas-fir beetle attack, which can cause high levels of 

delayed tree mortality. In areas with nearby Douglas-fir 

beetle populations, Hood and Bentz (2007) estimated that 

the Douglas-fir beetle was responsible for an additional 

25 percent of observed mortality of Douglas-fir trees 

greater than 9 inches (23 cm) DBH 4 years after wildfire 

in Montana and Wyoming. Ryan and Amman (1996) found 

beetle attacks increased with increasing Douglas-fir basal 

girdling after wildfires in Yellowstone National Park.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 13 Table 5—Studies that have examined the effects of fire on old or large-diameter tree mortality in historically fire-frequent forests in the United States. Studies are listed in alphabetical order. Main reference is the most recent report of mortality. Additional references listed report older or other pertinent results from the same study. Reference Years monitored Primary speciesa Crown injury assessed Basal injury assessedb Root injury assessed Basal duff depth assessed Bark beetles influenced mortality Additional references Additional comments Agee 2003 13 Ponderosa pine Douglas-fir White fir Yes Thomas and Agee 1986 Causal agent of mortality identi- fied, but tree level fire injury not assessed Agee and Perrakis 2008 5 Ponderosa pine Yes Perrakis and Agee 2006 Causal agent of mortality identi- fied, but tree level fire injury not assessed Breece and others 2008 3 Ponderosa pine x x Yes Campbell and others 2008 1, 2 Longleaf pine Yes Causal agent of mortality identi- fied, but tree level fire injury not assessed Fettig and others 2008 2 Jeffrey pine Ponderosa pine White fir Yes Causal agent of mortality identi- fied, but tree level fire injury not assessed Fowler and others 2010 3 Ponderosa pine x x x No Fulé and others 2007 5 Ponderosa pine

x x No

Fulé and others 2002b; Roccaforte and others 2008; Waltz and others 2003

Duff around all presettlement trees was raked prior to burning

Fulé and others 2005 5 Ponderosa pine x x No Fulé and others 2002a Duff around presettlement trees raked in some treatmentsGanz and others 2003 1 Jeffrey pine

Ponderosa pineIncense cedarWhite fir

x x x Yes

Haase and Sackett 1998 9 Giant sequoia

Sugar pine x x x x No Root injury inferred from soil tem- perature measurements

Hanula and others 2002 2 Longleaf pine

Slash pine x x Yes

Henning and Dickmann 1996

2, 7 Red pine x No

Hood and Bentz 2007 4 Douglas-fir x x Yes

Hood and others 2007b 2, 3, 4 Jeffrey pine

Ponderosa pineIncense cedarWhite firRed fir

x x Yes

Hood and others 2007a 2, 3 Jeffrey pine

Ponderosa pine x x x Yes
14 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 Table 5 (CONTINUED) Reference Years monitored Primary speciesa Crown injury assessed Basal injury assessedb Root injury assessed Basal duff depth assessed Bark beetles influenced mortality Additional references Additional comments Jerman and others 2004 2 Ponderosa pine x x x No Kaufmann and Covington 2001 3, 7 Ponderosa pine x No Kobziar and others 2006 1 Douglas-fir Ponderosa pine Incense cedar White fir Tan oak Black oak x x No Kolb and others 2001 6 Ponderosa pine No Covington and others 1997; Feeney and oth- ers 1998; Stone and others 1999; Wallin and others 2004 Duff removed from site prior to burning Lambert and Stohlgren 1988 2 to 8 Giant sequoia x Laudenslayer and others 2008 6 Jeffrey pine Ponderosa pine Sugar pine x Yes Loomis 1973 Not reported Black oak White oak Post oak Scarlet oak x No Maloney and others 2008 3 Jeffrey pine Sugar pine Incense cedar White fir Red fir x Yes McHugh and Kolb 2003 3 Ponderosa pine x x Yes Menges and Deyrup 2001 7 South Florida slash pine x x x Yes Methven 1971 1 Red pine Eastern white pine x No Miller and Patterson 1927 3 Ponderosa pine x x Yes
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 15 Reference Years monitored Primary speciesa Crown injury assessed Basal injury assessedb Root injury assessed Basal duff depth assessed Bark beetles influenced mortality Additional references Additional comments Mutch and Parsons 1998 5 Giant sequoia Jeffrey pine Sugar pine Incense cedar White fir Red fir California black oak x Yes van Mantgem and others 2003 Outcalt and Foltz 2004 1 Longleaf pine Slash pine South Florida slash pine x Regelbrugge and Conard 1993 2 Ponderosa pine Incense cedar California black oak Canyon live oak x No Rust 1933 5 Ponderosa pine x x Yes Ryan and Frandsen 1991 1 Ponderosa pine x x x No Sackett and Haase 1998 19, 20 Ponderosa pine x x x No Sackett and others 1996 Santoro and others 2001 2 Red pine x x Yes Schwilk and others 2006 2 Sugar pine White fir Incense cedar x x Yes Knapp and others 2005 Sullivan and others 2003 3 Longleaf pine x Yes Otrosina and others 2002 Swezy and Agee 1991 4, 10 Ponderosa pine x x x Yes Varner and others 2007 2 Longleaf pine x x x Yes Willard and others 1994 4 Ponderosa pine x No Williams and others 2006 1 Longleaf pine x x No Wright and others 2003 2, 3, 6, 7 Ponderosa pine x No aTan oak (Lithocarpus densiflorus (Hook. & Arn.) Rehder); California black oak (Quercus kelloggii Newberry); Canyon live oak (Q. chrysolepis Liebm.); Black oak (Q. velutina Lam.); White oak (Q. alba L.); Post oak (Q. stel- lata Wangenh.); Scarlet oak (Q. coccinea Münchh.). bBasal injury assessed directly or by noting bark char presence. Table 5 (CONTINUED)
16 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

No differences in trap captures of southern pine beetles 

(Dendroctonus frontalis Zimmermann) or eastern fivespined ips (Ips grandicollis Eichhoff) were found between 

prescribed burn units and unburned, control units in 

40-year-old longleaf pine stands in South Carolina. 

However, almost all dead trees in the burned units were 

attacked by the two bark beetles, suggesting that burned 

trees are more susceptible to attack than unburned trees 

(Sullivan and others 2003).

Bark beetles and pathogens can interact to cause extensive 

mortality after the first post-fire year. Most longleaf pine 

mortality after prescribed burns was not observed until 

2 to 3 years post-fire (Otrosina and others 2002; Sullivan 

and others 2003). Fires that consumed more duff and killed 

fine roots resulted in the highest mortality. The authors 

hypothesized that root pathogenic fungi on the site may 

have opportunistically infected the trees through injured 

fine roots, predisposing the trees to subsequent bark beetle 

attacks (Sullivan and others 2003). The combination of root 

disease, root injury, and secondary bark beetle attacks could 

have resulted in the significant mortality observed after the 

low-intensity prescribed burns. The authors recommended 

dormant-season heading fires that consume litter but not 

duff to reduce longleaf pine mortality.

Concerns abound that bark beetle populations will 

increase after fire and attack neighboring unburned areas. 

Whether this phenomenon occurs regularly, however, is 

unclear. Miller and Patterson (1927) found western pine 

beetle attacks increased the first 2 years after fire, but 

dropped to pre-fire levels by year 3. Brood production 

decreased in the attacked, burned trees and no spillover 

effects were observed. Furniss (1965) expressed concern 

about Douglas-fir beetles spreading to adjacent unburned 

forests, but reported decreased brood production in 

attacked, burned trees. Douglas-fir beetles attacked adjacent unburned trees 2 years after wildfires in Yellowstone 

National Park (Amman and Ryan 1991). However, beetle 

populations were increasing before the fires occurred, 

making it difficult to conclude if fire further increased the 

populations. Beetles did not spread to adjacent unburned 

areas after wildfires or prescribed fires in south Florida 

slash pine (Pinus elliottii Engelm. var. densa Little & 

Dorman) (Menges and Deyrup 2001). According to Jenkins 

and others (2008), the likelihood of bark beetles attacking 

adjacent, unburned areas depends on: (1) susceptibility of 

stands in both burned and unburned areas before and after 

the fire, (2) extent and severity of fire damage, (3) local 

bark beetle populations before the fire, and (4) weather 

conditions before and after the fire.

Properties of Soil and Duff ________

The forest floor includes all litter and decomposing 

organic layers above the mineral soil surface (Pritchett 

and Fisher 1987). The forest floor typically consists of 

three layers: litter, fermentation, and humus (Figure 10), 

although some layers may not be found or may not be 

distinguishable on all forested soils. The fermentation and 

humus layers are often collectively called duff and may 

be bound by mycelium to form a mat-like structure. The 

following is a detailed description of the layers:

Litter layer (L) or Oi: consists of unaltered, recently cast 

organic matter, such as leaves, needles, twigs, bark flakes, 

cones, and animal scat. This is the uppermost layer of the 

forest floor. The origin of the material is easily identifiable.

Fermentation layer (F) or Oe: consists of fragmented, 

partially decomposed organic material. This layer is found 

immediately below the L layer. The material is discolored, 

but the origin is still identifiable. This is also referred to 

as the upper duff layer. It has a higher bulk density and 

mineral content than litter.

Humus layer (H) or Oa: consists of well-decomposed, 

amorphous organic matter and possibly some mineral soil. 

This layer is found between the F layer and mineral soil. 

This is also referred to as the lower duff layer. It has a 

higher bulk density and mineral content than the F layer.

Sackett and Haase (1996) described the forest floor in 

terms of fire behavior called the fire intensity (FI) and fire 

severity (FS) layers (Figure 11). The FI layer consists of 

the L layer and upper portions of the F layer. These surface 

fuels burn by flaming combustion. The FI layer is often 

highly combustible because of its surface position and low 

bulk density, and it is a major component in determining 

rates of spread. The FS layer consists of the lower, denser 

portion of the F layer and the entire H layer. This layer 

is ground fuel that burns as smoldering combustion after 

the main flaming front has passed.

Differences Between Duff Mounds and 

Typical Forest Floor Duff

In the absence of fire, litter and duff from bark and 

needle shedding accumulate rapidly around the bases of 

trees (first 3 to 4 ft (1 m) horizontally) to depths that can 

cause injury to the roots and stems when burned. This 

is especially evident in dry forest types that have slow 

decomposition rates. The forest floor is usually deepest 

at the bases of large, older trees that have higher crown 

masses (Figure 12). Depth then decreases rapidly from 

the bole to the dripline (Figure 13) (Gordon and Varner 

2002; Ryan and Frandsen 1991; Swezy and Agee 1991).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 17

Figure 10. Long-unburned longleaf pine forest floor profile. 

Photo by Morgan Varner.

Figure 11. Forest floor profile showing fire intensity and fire 

severity layers. Photo by Morgan Varner.

Composition of duff also changes in proximity to the 

tree base. Duff at the base of trees typically has a much 

higher proportion of bark flakes than duff away from 

tree bases (Gordon and Varner 2002). As bark sloughs off 

trees, it is deposited directly at the tree base and is largely 

responsible for the mound of fuel accumulation often seen 

around large trees.

Duff is generally drier under tree crowns than between 

them due to crown interception of precipitation and radiation (Hille and den Ouden 2005; Miyanishi and Johnson 

2002). Tree crowns intercept precipitation, reducing 

moisture input directly beneath the crowns. At night, 

tree crowns reduce terrestrial radiational cooling at the 

ground surface, which limits dew formation (Miyanishi 

and Johnson 2002). Litter moisture changes diurnally and is 

more variable than duff moisture because it wets and dries 

more quickly than the lower duff layers due to its surface 

exposure and lower bulk density. A study in longleaf pine 

found litter moisture content increased sharply after all 

rain events, but moisture in deep duff layers at the base of 

mature longleaf pine trees only increased after heavy and 

sustained rain events (>0.8 inches (>20mm) precipitation 

in 24 hours) (Ferguson and others 2002). The average time 

lag for ponderosa pine duff 3.4 inches (8.7 cm) deep was 

50 hours (Fosberg 1977). See Appendix A for a description 

of methods used to measure duff moisture.
18 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Figure 12. Deep duff mound at base of ponderosa pine after 100+ years of fire suppression. Large pine 

cones at base are vectors that increase the likelihood of duff ignition and consumption.

Figure 13. Mean and standard deviation of duff depth (cm) as a 

function of the distance (cm) from the base of the tree. From Ryan 

and Frandsen 1991.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 19

Duff depth varies greatly throughout a stand, and temporal changes in duff are the hardest to predict of all fuel 

components (Hall and others 2006). Duff consumption 

is often reported and modeled as a unit average, assuming uniform consumption throughout the burned area. In 

reality, duff consumption can be patchy, ranging from 

completely burned areas to unburned or scarcely burned 

areas (Miyanishi and Johnson 2002). The percentage of duff 

consumption is often much higher in the duff mounds 

than in the ambient duff found away from tree bases 

(Hille and den Ouden 2005). This is likely because duff 

moisture varies throughout the stand and because deep 

duff can sustain smoldering at higher moisture contents 

(Miyanishi and Johnson 2002). In numerous prescribed 

burns in ponderosa pine in Arizona, almost all duff and 

litter was consumed to mineral soil around trees greater 

than 18 inches (45.7 cm) DBH to the dripline. In contrast, 

the forest floor was consumed to mineral soil for only a 

few inches around pole-size trees, 4 to 11 inches (10.2 to 

27.9cm) DBH. And in doghair thickets, only the litter layer 

was consumed and very little mineral soil was exposed 

(Sackett and Haase 1998). Prescribed burns in Montana 

reduced average stand-level duff depths from 17 to 30 percent, but 100 percent of duff was consumed around large 

tree bases (Kalabokidis 1992). Hille and Stephens (2005) 

found the probability of duff existence after a fire goes 

from near 0 percent at the tree base to over 60 percent in 

tree gaps in mixed-conifer forests of north-central Sierra 

Nevada, CA (Figure 14). This variability in duff depth 

highlights the importance of measuring forest floor 

depths at different locations throughout a stand before 

and after prescribed burning, as fire effects can differ 

significantly among points (Covington and Sackett 1992).

Figure 14. Spatial variation of duff remaining in the moist () and the dry (O) prescribed fires, related to absolute (a) and 

relative (b) distance from dominant sugar 

and ponderosa pine trees. The probability 

of duff remaining (y-axis) is calculated 

from the percentage of data points where 

duff survived the fire. The relative distance 

from the stem base is expressed as ratio 

to the crown perimeter. A relative distance 

of “1” marks the edge of the crown (Hille 

and Stephens 2005). Copyright 2005 by 

Society of American Foresters. Reproduced with permission of Society of 

American Foresters in the format Journal 

via Copyright Clearance Center. 

(a)

(b)
20 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

First-Entry Prescribed Burns

Long-unburned stands usually have greater duff to litter ratios than more frequently burned stands. Duff in old 

stands without recent disturbance is deeper and weighs 

more per unit area than younger stands (van Wagtendonk 

and others 1998), and duff bulk density generally increases 

with depth (Stephens and others 2004; van Wagtendonk 

and others 1998). Duff depth is deepest at the tree base 

and the high percentage of duff to total forest floor depth 

makes large trees more susceptible to fire injury during 

first-entry burns if the duff is consumed. Reducing basal 

duff consumption, either slowly through multiple burns 

under high duff moisture contents or by physical removal, 

is imperative to reduce cambium injury and possible tree 

mortality in areas with deep basal duff. Subsequent treatments normally do not require this intensive treatment 

because duff depths and, therefore, residence times are 

lower.

Factors Affecting Duff Consumption

There are two main questions concerning the probability 

of duff consumption during a fire:

(1) What factors influence duff ignition?

(2) What factors influence continued duff smoldering 

after ignition begins?

Initiation of smoldering is influenced by fuel moisture 

and surface fuel load. Surface fires spread over the forest 

floor and ignite woody fuels and cones as the fire burns 

through fine fuels. Burning large surface fuels increases 

residence times, dries out underlying duff, and increases 

the likelihood of duff ignition (Harrington 1987; Hille 

and den Ouden 2005; Sandberg 1980; Valette and others 

1994). Therefore, areas with higher surface loads will 

likely lead to increased duff ignition. Large pine cones 

can also smolder for long periods, up to 74 minutes for 

Jeffrey pine and 49 minutes for longleaf pine (Fonda and 

Varner 2004). Long-duration heating from large cones 

or long-burning surface fuels act as duff ignition vectors 

and greatly increase the chance of igniting the underlying 

duff (Figure 12). The continued influence of surface fuels 

on duff consumption once smoldering begins is unclear. 

Some studies have shown that surface loading strongly 

influences duff consumption (Hille and den Ouden 2005; 

Norum 1977). Others have found little to no relationship 

between surface loading and consumption after duff 

moisture and depth are accounted for (Brown and others 

1985; Reinhardt and others 1991b).

After duff is ignited, the fire may spread laterally 

and downward in the duff layer through smoldering 

combustion (Figure 15). As smoldering progresses, 

an insulating ash layer develops that traps heat and 

helps support combustion at high duff moisture contents 

(Frandsen 1987; McMahon and others 1980). Smoldering 

ground fires move up to three orders of magnitude slower 

than the slowest spreading surface fire (1.2 to 4.7 inches/

hour; 3 to 12 cm/hour) (Frandsen 1987). Very little heat 

is transferred to the mineral soil during the passage of a 

surface fire because of the fast rate of spread through the 

litter layer and the insulating duff layer. However, smoldering fires that consume most of the duff layer can transfer 

large amounts of heat into the mineral soil because of the 

slow movement, long duration, and direct soil contact.

Figure 15. Diagram of the smoldering process. (A) Shows 

the ignition point where smoldering is initiated by a passing 

surface fire. (B) Shows the lateral and downward spread as 

duff is consumed from the initial point. (C) Shows the pyrolysis 

and drying zones ahead of the glowing zone. The ash cap 

helps to trap heat and sustain smoldering. From Hungerford 

and others 1995.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 21

Propagation of smoldering combustion is influenced 

by duff moisture, mineral content, bulk density, and 

depth (Table 6). The likelihood of sustained smoldering decreases as duff moisture, mineral content, or bulk 

density increase (Hartford 1989). Duff depth influences 

smoldering propagation by trapping heat inside and conserving convective heat lost at the surface layer. Therefore, 

thicker duff layers can burn under higher moisture contents 

(Miyanishi and Johnson 2002).

Duff moisture is widely considered the most important 

factor controlling duff consumption (Brown and others 

1985; Hille and Stephens 2005; Sandberg 1980). However, 

moisture of extinction limits vary widely and there is 

great variability in the data (Hungerford and others 1995; 

Reinhardt and others 1991a). The moisture of extinction is 

the fuel moisture content at which a fire will not spread, or 

will spread only sporadically and unpredictably (Jenkins 

2005). Reinhardt and others (1991b) evaluated 24 duff 

consumption equations using data from 449 prescribed 

fires in short and long needle conifer forests in the western 

United States and Canada. The authors found that at average duff moistures above 175 percent, less than 15 percent 

of duff was consumed, and at duff moistures below 50 

percent, more than half of duff was consumed. This finding did not support the rule-of-thumb suggested by Brown 

and others (1985) and Sandberg (1980) that most duff is 

consumed at moistures less than 30 percent and little is 

consumed above 120 percent (Figure 16) (Reinhardt and 

others 1991b). However, the 120 percent upper level was 

confirmed in laboratory experiments on Scots pine (Pinus 

sylvestris L.) duff (Hille and den Ouden 2005). In this study, 

duff consumption below 120 percent was best modeled 

as a parabolic curve; above 120 percent, consumption 

was negligible and was best modeled as a linear function 

(Figure 17). Van Wagner (1970, 1972) reported that red 

pine duff consumption is minimal above 60 percent 

moisture content and ceases to burn around 140 percent.

Lower duff moisture was the most important variable 

in predicting duff consumption when prescribed burning 

mixed-conifer forests in the northern Sierra Nevada of 

California (Kauffman and Martin 1989). In this study, 

experimental burns were conducted during early spring, 

late spring, early fall, and late fall on three sites to test 

the relationship between burning and consumption under 

a wide range of moistures. Average pre-burn forest floor 

depths at the sites ranged from 6.3 to 8.8 inches (15.9 to 

22.3 cm). Burns conducted when lower duff moisture was 

less than 50 percent always consumed at least 70 percent 

of the duff. One burn consumed 70 percent of duff even 

with average duff moisture of 120 percent (Figure 18). 

However, regression models developed to predict duff 

consumption from duff moisture only explained 51 percent of the variability. Site specific equations were more 

accurate, leading the authors to conclude that general 

equations to predict duff consumption may not be useful. 

Reinhardt and others (1991b) reached the same conclusion 

after evaluating 24 duff consumption equations using 

independent data. Ferguson and others (2002) approximated the moisture of extinction for longleaf pine in the 

panhandle of Florida as 3 to 8 percent for litter and 16 to 

19 percent for duff based on volumetric moisture content. 

Volumetric moisture values are generally lower than the 

more commonly reported gravimetric moisture values 

(see Gravimetric versus volumetric moisture content in 

Appendix A). The authors concluded that current forest floor moisture was almost completely explained by 

the previous day’s moisture content and precipitation in 

longleaf pine forests.

Table 6—Factors influencing duff consumption by smoldering combustion. 

Compiled from Frandsen 1987; Hartford 1989; Miyanishi and Johnson 

2002; Otway and others 2007.

Impact on

smoldering as

 Duff Variable Influence variable increases

Moisture content Heat sink ↓

Mineral content Heat sink; increases space ↓

between burnable particles

Bulk density Increases packing ratio ↓

Depth Traps heat more effectively; less ↑

heat is lost to surface convection
22 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Figure 16. Percentage duff depth reduction versus lower duff moisture 

content. Equations 8 and 9 (solid lines) and Norum’s (1977) curves (dashed 

line) are graphed. From Brown and others 1985.

Figure 17. Humus consumption (weight percentage of humus remaining) at 

different humus moistures. Different fuel loads were simulated with one (×), 

two (∆), or three (O) bars of charcoal lighter as an ignition source. The lines 

show the parabolic relation for humus moisture <120 percent and an almost 

constant humus consumption percentage for higher humus moisture. 

© International Association of Wildland Fire 2005. Reproduced with 

permission from the International Journal of Wildland Fire 14(2): 153-159 

(Marco Hille and Jan den Ouden) http://www.publish.csiro.au/nid/114/paper/

WF04026.htm. Published by CSIRO PUBLISHING, Melbourne Australia.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 23

Figure 18. Percentage of forest floor fuel loading consumed and corresponding lower duff moisture for experimental 

prescribed burns in Sierra Nevada mixed-conifer forests. Adapted from Kauffman and Martin 1989.

Duff mineral content is another important factor in duff 

consumption (Frandsen 1987; Reardon and others 2007). 

Inorganic matter in duff absorbs heat, which causes a net 

loss to the propagation of smoldering. Frandsen (1987, 

1997) estimated the limits of smoldering combustion in 

peat moss by varying the moisture and inorganic ratios 

during a series of laboratory burns (Figure 19). Samples 

stopped smoldering when the inorganic ratio increased 

while the moisture ratio was held constant. He attributed 

different field estimates of smoldering combustion limits 

to differences in duff mineral contents that were not 

accounted for in other studies. More recent work in thick 

organic soil horizons in North Carolina substantiated 

Frandsen’s earlier work that found the limits of sustained 

combustion were a function of moisture and mineral 

content (Reardon and others 2007).

Another factor that influences duff consumption is duff 

bulk density. As bulk density increases, the surface-tovolume ratio decreases, which causes heat exchange to 

decline (Otway and others 2007). The authors questioned 

whether sustained smoldering is possible in shallow duff 

layers less than 2 inches (5 cm) deep if bulk density is high. 

Duff bulk density is strongly correlated with mineral 

content. Mineral soil weighs much more than duff given 

the same sample volume. This is likely a contributing 

factor to observations of increasing bulk density in 

lower duff layers that are closer to the mineral soil layer.

There are many equations to predict duff consumption 

during fire (Brown and others 1985; Harrington 1987; 

Reinhardt and others 1991b; Sandberg 1980). All of these 

models, however, were developed to predict average stand 

duff consumption using relatively shallow duff layers as 

samples. Hood and others (2007a) compared consumption of deep duff at the base of old Jeffrey and ponderosa 

pine trees to values predicted by the First Order Fire 

Effects Model (FOFEM) (Reinhardt and others 1997). 

FOFEM underpredicted duff consumption, and the 

authors concluded that the model was not appropriate for 

use in areas of deep duff.
24 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

A general trend emerges from these studies of duff 

consumption. Complete duff consumption independent 

of adjacent burning fuels does not usually occur above 

120 percent moisture content in relatively shallow duff. 

In areas of very deep duff, moisture contents greater than 

150 percent are required to limit duff consumption. Duff 

consumption below these levels is extremely variable. 

These values reflect levels of reduced duff smoldering 

after ignition begins and do not pertain to the probability 

of duff igniting.

Impacts of Moisture on Soil Heating

Wet duff limits heat transport to the mineral soil 

(Frandsen and Ryan 1986). Burning duff under wet soil 

conditions reduces soil temperatures and heat duration 

(Busse and others 2005; Frandsen and Ryan 1986; Valette 

and others 1994) (Figure 20). In an experimental study, 

Frandsen and Ryan (1986) monitored soil heating and 

duration when burning under different combinations of 

wet and dry peat moss and wet and dry sand. Burning wet 

moss over dry sand greatly reduced sand temperatures 

0.8 inches (2 cm) below the surface compared to burning 

dry moss over dry sand, even though all moss was consumed in both experiments. Burning wet moss over wet 

sand consumed 0.4 of the 0.8-inch (1 cm of 2-cm) moss 

layer and drastically reduced sand temperatures compared 

to the other moisture regimes (Figure 21). Burning dry 

moss over wet sand was not tested, and the burns were 

not replicated.

Historical and Current Fire 

Frequencies and Stand 

Characteristics__________________

Reconstruction of historical fire regimes and stand 

characteristics provide insight to what forests may have 

looked liked before European settlement and to how 

disturbance shaped those characteristics. The historical 

record is best used to create a historical range of variation, 

not to recreate forest conditions from one point in time 

(Swetnam and others 1999).

0 

20 

40 

60 

80 

100 

120 

0 10 20 30 40 50 60 70 80 90 100 

Moisture content (%) 

Inorganic content (%) 

Burn 

No Burn 

Figure 19. Ignition limit from Frandsen (1987, 1997). The line is the ignition limit based on moisture content and inorganic 

mineral soil content at an organic bulk density of 12.5 tons/acre/inch (110 kg/m3) from laboratory experiments burning 

peat moss. Successful ignitions are accomplished only when moisture content and inorganic content are within the 

triangle bounded by the axes and the ignition limit.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 25

Figure 20. Heat duration exceeding the plant lethal temperature of 140 °F 

(60 °C) when burning different depths of wood mulch in (a) dry and (b) moist 

soil. Bars are means (n = 3) plus standard errors for four depths in the soil 

profile. From Busse and others 2005. 

Figure 21. Temperature histories in uncovered dry sand (– – –), dry sand 

covered with wet peat moss (—), and wet sand covered with wet peat moss 

(····). Two curves are shown for each profile. The upper curve is the temperature 

at the sand surface; the lower curve is 0.8 inches (2 cm) below the surface. 

From Frandsen and Ryan 1986.
26 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

The species discussed in this synthesis are fire-climax 

communities. Frequent low-intensity surface fires perpetuated their existence. These fires perpetuated open conditions, created mineral seedbeds for seedlings, kept fuel 

loadings low, and reduced fire-intolerant species establishment. Implementation of a national fire suppression policy 

and removal of Native Americans significantly reduced 

the amount of land that burned, resulting in these forests 

succeeding to more shade-tolerant, fire-intolerant species.

Red Pine

Red pine occurs in a narrow zone about 1500 miles 

(2400 km) long and 500 miles (800 km) wide around the 

Great Lakes and the St. Lawrence River, most of it within 

or closely adjacent to the area glaciated during the late 

Pleistocene period. In the United States, red pine extends 

from Maine westward to southeastern Minnesota and 

eastward to Wisconsin, Michigan, northern Pennsylvania, 

northern New Jersey, Connecticut, and Massachusetts. It 

also grows locally in northern Illinois and eastern West 

Virginia (Burns and Honkala 1990). Prior to settlement, red 

pine/eastern white pine (P. strobus L.) forests comprised 

97.7 million acres (39.5 million hectares). Today, red pine/

eastern white pine forests cover approximately 20.5 million acres (8.3 million hectares), of which approximately 

509,000 acres (206,000 hectares) are old-growth, late 

seral red pine/eastern white pine forests (Hauser 2008).

Red pine forests developed with both frequent lowintensity surface fires and infrequent, stand-replacement 

fires (Burgess and Methven 1977; Hauser 2008; Spurr 

1954). A fire history study in a red pine dominated area 

in upper Michigan determined that the historic fire regime 

was characterized by frequent, low-severity, nonstandreplacing fires (Drobyshev and others 2008b). Fire return 

interval (FRI) was between 23 and 33 years, with a fire cycle 

(time required for all the study area to burn) of 150 years 

for sand ridges and 50 years for glacial outwash channels. 

More than half of fires were late-season (53 percent), and 

these fires burned a larger portion of the study area than 

the smaller, early-season fires. Two main cohort-initiation 

periods corresponded with large fire years and were likely 

due to higher-intensity, stand-replacement fires. Drobyshev 

and others (2008a) found duff depth and fine wood fuels 

were lower in stands that burned regularly. This frequent 

fire regime limited the establishment of other more firesensitive tree species, created multi-cohort stands, and 

maintained low fine fuel loadings and shallower duff 

depths (Burgess and Methven 1977; Drobyshev and others 

2008a; Drobyshev and others 2008b).

Engstrom and Mann (1991) studied fire history of red 

pine in Vermont and also found evidence for a historically 

frequent, low-severity fire regime interspersed with small 

but high-intensity stand-replacing fires that allowed red 

pine regeneration to establish. The mean FRI was 37 years, 

but some stands had fires as frequent as every 3 to 5 years.

Seedling establishment is dependent on high-intensity 

surface or crown fire to create a mineral seedbed and 

full sun conditions. Red pine then develops thick bark 

by about 40 to 60 years of age that is resistant to bole 

injury (Henning and Dickmann 1996; Van Wagner 1970). 

Burgess and Methven (1977) reported that the majority of 

30-year-old red pine survived a wildfire, even when over 

50 percent of the cambium was killed. Trees can survive 

high levels of crown scorch. After prescribed burning two 

red pine stands on May 31 and June 15, Methven (1971) 

reported mortality of trees larger than 9 inches (22.9 cm) 

DBH began with 46 to 50 percent crown scorch, rose to 

50 percent mortality with 81 to 85 percent scorch, and 

was 100 percent for trees with 96 to 100 percent scorch. In 

another study, trees survived much higher levels of crown 

scorch from an early season wildfire (April 12) due to little 

bud kill (Sucoff and Allison 1968). In that study, only 40 

percent of trees with greater than 95 percent crown scorch 

and little to no bud kill were killed. Van Wagner (1970) 

found no tree mortality from basal cambium injury alone.

 In the absence of fire, red pine declines dramatically 

(Spurr 1954; Van Wagner 1970). Today, most red pine in 

the Lake States are even-aged stands planted in the 1930s 

and 1940s after most of the native forests were clearcut 

(Palik and Zasada 2003). Without fire to create favorable 

seedling establishment conditions, red pine in the understory is now rare throughout much of its range (Burgess 

and Methven 1977; Engstrom and Mann 1991).

Longleaf Pine

Once the dominant tree species of the southeast, stretching from Virginia down the coast to Florida and westward 

to eastern Texas, the longleaf pine ecosystem is now one 

of the most endangered ecosystems in the United States 

(Noss and others 1995). Approximately 2.7 million acres 

(1.1 million hectares; 3 percent) remain of the estimated 

91.4 million acres (37 million hectares) of pre-settlement 

longleaf pine forest (Frost 1993; Landers and others 1995). 

Old-growth longleaf pine is even more imperiled— only 

12,600 acres (5,095 hectares) are estimated to exist (Varner 

and Kush 2004). Therefore, historical structure and fuel 

conditions are extremely limited to historical accounts, 

photographs, and a few remnant stands. Frost (1993) 

estimated that 80 percent of the pre-settlement longleaf 

pine forests were dominated by longleaf pine, while the 

remaining 20 percent were mixed species stands with a 

large longleaf pine component.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 27

Dendrochronology is rarely used to reconstruct historical longleaf fire regimes because so few stands 

remain (Bhuta and others 2008), but historical accounts 

and reconstructions estimate the forests burned very 

frequently by low-intensity lightning-caused fires or 

fires deliberately set by Native Americans. Frequency 

depended on the area, but most accounts agree forests 

burned every 1 to 5 years (Christensen 1981; Frost 1993). 

This frequent fire regime created open forest conditions. 

Most stands were uneven-aged and were clustered into 

small, even-aged groups (Platt and others 1998). In the 

southern extent of longleaf pine’s range, tree density was 

very low, with a savanna appearance.

Heyward and Barnette (1936) described frequently 

burned longleaf pine forest floors as having, at most, 

3 years of pine needle accumulation and dead grasses 

and no duff layers. The authors described a compact 2- to 

3.5‐inch (5- to 8.9-cm) thick uppermost mineral soil layer 

(A1)intermixed with organic matter that was more typical 

of grassland than forests. They described this A1 layer as a 

“fire climax” layer because it was formed and maintained 

by frequent fires. Their research occurred when fire suppression was just becoming widespread in the region. 

After 10 years of fire suppression, they reported that the 

“luxuriant ground cover” was replaced by a developing 

traditional forest floor consisting of true L, F, and H layers 

(Heyward and Barnette 1936).

Longleaf pine is extremely tolerant of fire, except for a 

brief period of time between the grass and sapling stages. 

Adaptations to frequent fire include large buds, protective 

dense needle clusters around buds, and thick bark that 

develops at an early age (Chapman 1932). Chapman (1923) 

noted that longleaf saplings as young as 3 to 4years survive 

complete crown scorch from winter and early spring fires. 

Longleaf and slash pine can tolerate high crown scorch 

with little to no mortality if bud kill is kept to a minimum 

(Wade 1986). Storey and Merkel (1960) reported no mature 

longleaf or slash pine mortality unless a portion of the 

crown was consumed, even with 100 percent crown scorch. 

Mortality was only high (87 percent) when more than 50 

percent of the crown was consumed. They hypothesized 

that no mortality occurred among trees with high needle 

scorch and no crown consumption because low ambient 

air temperature at the time of the burn limited bud kill.

The majority of longleaf pine forests was harvested for 

timber, cleared for agriculture, or destroyed for naval stores 

production beginning in the 1800s. Many areas failed to 

regenerate after harvesting due to lack of fire and the 

introduction of open range hogs (Sus scrofa L.) that ate 

the roots of longleaf seedlings (Frost 1993). Slash pine and 

loblolly pine (Pinus taeda L.) naturally regenerated much 

of the cutover longleaf pine forests (Frost 1993; McCulley 

1950). In the 1900s, much of the second-growth longleaf 

forests were converted to loblolly and slash pine plantations (Brockway and Lewis 1997).

Longleaf pine establishment quickly declines in the 

absence of fire, allowing a midstory layer of hardwoods, 

primarily oaks, to develop (Gilliam and Platt 1999). Deep 

forest floor layers develop and herbaceous diversity drastically declines (Varner and others 2005). Brockway and 

Lewis (1997) reported forest floor accumulations of 5.9 

to 9.8 inches (15 to 25 cm) in a second-growth longleaf 

pine stand that had not burned in 40 years.

Southwestern Ponderosa Pine

Southwestern ponderosa pine forms almost a continuous belt for 400 miles (644 km) diagonally from northern 

Arizona southeastward across the Mogollon Rim to the 

Gila and Black Range Wildernesses in southwestern New 

Mexico (Kaufmann and others 2007). Historical stands 

were uneven-aged and had a clumpy or random distribution. A combination of good seed crop years once every 

3 years, summer drought conditions, cone predation, and 

high fire frequency limited tree regeneration and maintained an open forested savanna (Bailey and Covington 

2002; Schubert 1974; White 1985). Most reconstructed 

pre-settlement forests show low tree densities, averaging 

around 22.8 trees/acre (56.3 trees/hectare) (Covington and 

others 1997) to 24.9 trees/acre (61.5 trees/hectare) (Waltz 

and others 2003). However, densities as high as 74 trees/

acre (183 trees/hectare) are also reported (Abella 2008). 

Basal area was concentrated in large ponderosa pine trees 

and averaged 17 to 57 ft2/acre (4 to 13 m2/hectare) (Waltz 

and others 2003).

Frequent surface fires typified the historical fire regime. 

No accounts of crown fires in Arizona exist before 1900, 

and surface fires rarely killed large trees (Cooper 1960). 

At Fort Valley Experimental Forest near Flagstaff, AZ, 

Dieterich (1980) determined that, prior to settlement, 

low-intensity surface fires occurred every 2 to 4 years.

Ponderosa pine is very tolerant of fire. Adaptations to 

survive surface fires include open crowns, self-pruning 

branches, thick bark, thick bud scales, high foliar moisture, 

a deep rooting habit, and tight needle bunches that enclose 

and protect meristems, then open into a loose arrangement 

that does not favor combustion or propagation of flames 

(Howard 2003).Ponderosa pine is able to survive high levels 

of crown scorch if little bud kill occurs (Dieterich 1979).

Today’s southwestern ponderosa pine forests are much 

denser with heavier fuel loadings, largely due to fire 

suppression and grazing. Abundant research has been 

conducted and several syntheses have been written about 
28 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

restoration of southwestern ponderosa pine forests (Egan 

2007; Friederici 2003; Kolb and others 2007). Covington 

and others (1997) found tree density increased from 22.8 

trees/acre (56.3 trees/hectare) in 1876 to 1253.5 trees/acre 

(3096 trees/hectare) in 1992 in an unlogged ponderosa pine 

forest near Flagstaff, AZ. At the nearby repeated burn 

study site at Fort Valley Experimental Forest, tree density 

was also high before treatments in 1976, with 993 trees/

acre (2454 trees/hectare) (Covington and Sackett 1984).

Considerable within-stand variation of density can exist 

and can be separated into five conditions dominated by different size classes (sub-stands): sapling (doghair thickets), 

pole stands, mature, old-growth groves, and open areas in 

the groves without crowns overhead (Sackett and Haase 

1996). Fuel loadings can differ dramatically within a stand 

depending on where sampling occurs.

As early as 1960, Cooper identified that 40 years of fire 

exclusion in the southwest had increased the potential of 

destructive wildfires by allowing excessive fuel buildup 

on the forest floor, thereby lowering the average crown 

base height of the trees making crown fire more likely and 

permitting the formation of dense stands of saplings over 

wide areas. Continued fire suppression has only exacerbated the conditions observed by Cooper and has created 

conditions far different than pre-settlement ponderosa pine 

forests (Covington and Moore 1994).

Pacific Northwest and California

Giant sequoia-mixed-conifer forests—Giant sequoiamixed-conifer forests are found at mid-elevation (4,920 to 

7,545 ft (1,500 to 2,300 m)) on the west slope of the Sierra 

Nevada in California. These forests are dominated by giant 

sequoias in the upper canopy at heights of 147 to 246 ft 

(45to 75 m), with a secondary canopy layer of giant sequoia, 

sugar pine, and white fir at 98 to 180 ft (30 to 55 m). On 

drier sites, incense cedar (Calocedrus decurrens (Torr.) 

Florin), ponderosa pine, or Jeffrey pine may be present. 

Red fir (Abies magnifica A. Murray) is likely on higher, 

cooler sites. Frequent, low-intensity surface fires were 

common, interspersed with some small, patchy crown 

fires likely occurring at longer intervals (Kaufmann and 

others 2007). Kilgore and Taylor (1979) reported a historic 

FRI of 3 to 35 years in sequoia-mixed-conifer forests, with 

a mean of 10 years on southwest aspects and 15 to 18 years 

on southeast aspects. These estimates are similar to the 

historic mean FRI of 2 to 3 years during drought periods 

and 10 to 25 years during cool periods that Kaufmann and 

others (2007) found.

Giant sequoia seedling and saplings are highly susceptible to fire. As giant sequoia age, they quickly become 

very fire-tolerant and exhibit the following adaptations 

to fire: rapid growth, thick bark, elevated canopies and 

self-pruned lower branches, latent buds, and serotinous 

cones (Habeck 1992). Frequent fire maintained relatively 

open conditions dominated by giant sequoia and ponderosa and Jeffrey pine and limited fuel accumulations 

(Kaufmann and others 2007). Periodic small crown fires 

created mineral seedbeds favorable to giant sequoia and 

pine establishment.

Sheep were introduced into the area in the 1860s. Heavy 

grazing reduced the fine fuel continuity and disrupted the 

frequent surface fires. Grazing and fire suppression have 

caused a significant increase in white fir tree density, with 

little giant sequoia regeneration (Kaufmann and others 

2007). The increased density of shade-tolerant tree species creates more ladder fuels that increase the potential 

for crown fires (Stephenson 1999).

Mixed-conifer, ponderosa, and Jeffrey pine forests—

Mixed-conifer and pine forests cover a broad area of 

California, Oregon, and Washington, occurring at elevations from 2,950 ft to 8,500 ft (900 to 2,600 m). Most 

precipitation occurs as snow during the winter months, 

with very little precipitation during the growing season 

(Kaufmann and others 2007).

Historically, the FRI was slightly longer and fires burned 

later in the season in the fire-frequent forests of the Sierra 

Nevada and Cascade Ranges, moving from south to north 

(Agee 1993; Beaty and Taylor 2008; McNeil and Zobel 

1980). Prior to settlement, median fire occurrence was every 

2.5 years, with a range of 1 to 13 years in a mixed-conifer 

forest in the San Jacinto Mountains of southern California 

(Everett 2008). The majority of these fires burned during mid to late summer. Mean historic FRI was 6.3 and 

9.3 years for Jeffrey pine-dominated forests of Yosemite 

and Sequoia/Kings Canyon National Parks, respectively 

(Collins and Stephens 2007). Mixed-conifer forests on the 

west side of Lake Tahoe, CA, primarily burned during the 

dormant season, on average every 8 to 17 years (Beaty 

and Taylor 2008). Lower-elevation Jeffrey pine forests at 

Lassen Volcanic National Park, CA, burned an average 

of every 4 to 6 years (Taylor 2000). Fire history studies 

estimate the FRI in eastside Douglas-fir forests was 7 to 

11 years in the Wenatchee Valley to 10 to 24 years in the 

Okanogan National Forest, WA. In the Blue Mountains of 

Oregon, the FRI averaged 10 years. Most fires were low 

intensity, although some higher-severity fires did occur 

(Agee 1993). Historic FRI in ponderosa pine-dominated 

forests in the Pacific Northwest averaged 7 to 20 years and 

likely burned frequently over small areas. In the ponderosa 

pine-white fir forests in Crater Lake National Park, mean 

FRI ranged from 9 to 42 years (McNeil and Zobel 1980).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 29

Jeffrey pine/mixed-conifer forests in the Sierra San 

Pedro Martir, Mexico, have never been harvested or 

experienced widespread fire suppression. The vegetation 

is similar to forests of the southern Sierras and allows 

insight into historic forest structure, fire regime, and 

fuel loadings of the southern Sierra. Stephens (2004) 

reported an average surface fuel loading (1 to 1000 hour 

fuels) of 6.4 tons/acre (1.4 kg/m2). Litter depth averaged 0.6 inches (1.6 cm) with an average loading of 

3.5 tons/acre (0.8 kg/m2

); no duff was present. Fine 

fuel loading (1 to 100 hour fuels) was low at 0.87 tons/

acre (0.2 kg/m2

). Snags averaged 2 trees/acre (5 trees/

hectare), with 85 percent over 11.8 inches (30 cm) DBH. 

Snag distribution was patchy. Both snag distribution 

and fuel loadings were highly variable across the forest.

A reconstruction of forest structure of an old-growth 

mixed-conifer forest on the Teakettle Experimental 

Forest in California estimated tree density in 1865 was 

27 stems/acre (67 stems/hectare) and a quadratic mean 

diameter of 19.5 inches (49.5 cm) (North and others 

2007). The average historic FRI was 17 years, and the 

last widespread fire occurred in 1865. More than 140 

years without fire had increased tree density to 190 

stems/acre (469 stems/hectare), decreased quadratic 

mean diameter to 7.7 inches (19.6 cm), and altered species composition. Historically, shade-tolerant species 

comprised approximately 51 percent of the stems, mostly 

as white fir and incense cedar, and shade-intolerant 

Jeffrey and sugar pine comprised the remainder. Current 

forest structure revealed a dramatic decline in pine to 

only 14 percent of stems. White fir increased from an 

estimated 33.7 percent in 1865 to 67.2 percent of stems 

currently (North and others 2007).

Fire suppression has increased white fir density, 

created more homogeneous forests, and increased fuel 

loadings (Beaty and Taylor 2008; Knapp and others 

2005; Stephens 2004). Frequent fire regulated tree density by killing seedlings and saplings still susceptible 

to fire because their bark was not yet thick enough to 

prevent cambium injury. Over 100 years of fire suppression has allowed many of these white firs to become 

resistant to low-to-moderate-intensity fires (Collins 

and Stephens 2007; Kilgore 1972; Thomas and Agee 

1986). Youngblood and others (2004) suggested a tree 

density of 20 ± 1.5 trees/acre (50 ± 3.5 trees/hectare) and 

a mean diameter of 23.6 inches ± 0.6 inches (60.0 cm ± 

1.55 cm) DBH as reference goals when restoring eastside 

ponderosa pine forests to mimic historic old-growth 

conditions in northern California and Oregon.

Interior West Ponderosa Pine, Douglas-Fir, 

and Western Larch

Ponderosa pine in the Interior West stretches from 

Montana to the Colorado Front Range. An estimated 

21 percent of the ponderosa pine/Douglas-fir forests in 

the inland northwest historically consisted of large (11.8 to 

23.6 inches (30 to 60 cm) DBH), widely spaced (approximately 101 trees/acre (250 trees/hectare)) ponderosa pine 

(Jain and Graham 2004). Currently, only 5 percent of the 

landscape consists of mature, open ponderosa pine forests 

(Hann and others 1997).

The low-severity ponderosa pine fire regime in the 

southwest was not as ubiquitous in the Interior West, where 

many ponderosa pine/Douglas-fir forests historically 

developed under a mixed-severity fire regime, although 

many areas did burn as low-severity (Kaufmann and 

others 2004). Ponderosa pine at lower-elevation, drier 

sites in Montana typically burned under a low-severity 

fire regime, with average fire-free intervals from 5 to 

20 years. Maximum fire-free intervals ranged from 

21 to 30 years, with minimum intervals of 3 to 4 years 

(Arno 1980). Lake sediment cores in northwestern Montana 

indicate a slightly longer historic FRI of 30 years for lowelevation ponderosa pine (Power and others 2006), but 

more fires may go undetected with this method than with 

dendrochronological methods. While these forests experienced frequent low-intensity surface fires, infrequent 

high-severity fires also occurred (Baker and others 

2007; Pierce and Meyer 2008).

Above the drier ponderosa pine zone, in cooler and 

moister climates, forests were still dominated by ponderosa pine and western larch, mixed with Douglas-fir 

and lodgepole pine (Pinus contorta Douglas ex Louden). 

Historically, fire regimes were mixed-severity. Mean 

FRI ranged from 15 to 30 years with maximum fire-free 

intervals of approximately 35 to 60 years (Arno 1980). 

A study of 11 ponderosa pine and western larch stands 

in western Montana showed all stands had historically 

experienced frequent low-intensity fires, while three of the 

western larch stands also had occasional stand-replacing 

fires (Arno and others 1995; Arno and others 1997). In the 

absence of fire, forests in this zone are seral to Douglas-fir.

Sherriff and Veblen (2007) estimated approximately 

20 percent of ponderosa pine forests along the Colorado 

Front Range burned as low-severity fires, with the remaining burning as mixed-severity fires. Sites below 6,900 ft 

(2,100 m) likely burned approximately every 10 to 30 years 

as low-intensity surface fires. At higher-elevation sites, 

the majority of ponderosa pine forests along the Colorado 
30 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Front Range had fire-free intervals of 30 to 100+ years. 

Fires here burned as mixed-severity, with large areas of 

stand-replacement fire, which helped create very patchy, 

open ponderosa pine forests. Kaufmann (2007) estimated 

that 90 percent of the historical landscape had a canopy 

cover of 30 percent or less.

Widespread harvesting of ponderosa pine in the late 

1800s and early 1900s contributed to the decline in pine 

dominance in the Interior West. Fire suppression further 

contributed to changes by increasing fuel loadings, tree 

densities, and Douglas-fir dominance in these dry forests (Arno and others 1997). In the Selway-Bitterroot 

Wilderness, MT, these changes have resulted in a shift from 

equal proportions of surface and stand-replacement fires to 

45 percent surface fires and 55 percent stand-replacement 

fires (Brown and others 1994). Large, stand-replacing fire 

frequency is also increasing in other areas of the Interior 

West (Kaufmann and others 2007).

Black Hills Ponderosa Pine

The Black Hills of southwestern South Dakota and 

northeast Wyoming support an isolated ponderosa pine 

forest covering almost 6000 miles2

 (15,540 km2) that is 

surrounded by the Great Plains (Shepperd and Battaglia 

2002). The majority of the Black Hills were logged within 

the past 100 years. While the extent of ponderosa pine 

forest remains relatively unchanged, Symstad and Bynum 

(2007) estimate that approximately only 5,130 acres 

(2,076 hectares) of old-growth ponderosa pine remain on 

public lands in the Black Hills.

Fire history studies and historical accounts show fire was 

a frequent disturbance agent in the area. Fire frequencies 

ranged from 10 to 13 years on the lower-elevation, warmer, 

and drier sites to 20 to 24 years at the higher-elevation 

sites (Shepperd and Battaglia 2002). The historical fire 

regime in the Black Hills was likely mixed-severity 

(Lentile and others 2005). Low-intensity surface fires that 

maintained ponderosa pine dominance, while controlling 

tree densities, were common. However, there is evidence 

of higher-intensity fires that killed large patches of trees 

and led to even-aged stand structures in areas (Shinneman 

and Baker 1997).

A century of fire suppression and livestock grazing 

has led to higher stand densities, fuel accumulations, and 

fuel continuity (Lentile and others 2005; Shepperd and 

Battaglia 2002). Favorable growing conditions and good 

seed crops often coincide, leading to abundant seedling 

establishment and dense stands in the absence of fire 

(Battaglia and others 2008; Shepperd and Battaglia 2002). 

These changes have led to several recent, large-scale fires 

in the Black Hills. Between 2000 and 2004, seven fires 

burned over 148,000 acres (60,000 hectares) (Keyser and 

others 2006). The 2000 Jasper Fire was approximately 

25 percent larger than any recorded fire in Black Hills 

history (Lentile and others 2005).

Treatment Effects on Old Tree 

Resilience______________________

Old trees generally grow slower than young trees (Yoder 

and others 1994). This reduction in rate of wood production 

is not fully understood, but it is often attributed to increased 

maintenance respiration costs as living biomass increases 

(Ryan and others 1997). However, other reasons may 

better account for reduced growth rates, such as reduced 

photosynthetic rates and increased energy investment in 

fine root production (Grier and others 1981; Kaufmann 

and Ryan 1986; Ryan and others 1997). Yoder and others 

(1994) found net photosynthesis averaged 14 to 30 percent 

lower in foliage from old ponderosa and lodgepole pine 

trees, while growth efficiency (wood growth and leaf 

area) of old trees averaged 41 percent less than younger 

trees. The authors hypothesized that reduced hydraulic 

conductance in old trees because of their greater height 

and longer branches cause stomata to close about 2 hours 

earlier in the day than younger trees, thus reducing photosynthetic rates and, in turn, growth efficiency. Kaufmann 

and Ryan (1986) also found growth efficiency declined 

with age for lodgepole pine, Engelmann spruce (Picea 

engelmannii Parry ex Engelm.), and subalpine fir (Abies 

lasiocarpa (Hook.) Nutt.) trees.

Older trees in dense stands are often in competition 

with younger, more vigorously growing trees. Mortality of 

large-diameter (>39.4 inches (>100 cm) DBH) white fir, red 

fir, incense cedar, and sugar pine trees was significantly 

higher than expected in the Teakettle Experimental Forest, 

CA (Smith and others 2005). Mortality was also significantly higher in denser stands than in more open stands. 

Jeffrey pine was the only species for which mortality in 

the larger diameter classes was not higher, and these trees 

grew primarily in the more open, drier ridgetops. Teakettle 

Experimental Forest is a mixed-conifer, old-growth forest 

with a very limited logging history. The historic FRI was 

17 years, but the last recorded widespread fire was in 1865. 

Fire suppression has resulted in significant increases in 

tree density on the forest. The authors hypothesize that 

this increase in density and competition is accelerating 

large-diameter, old tree mortality.

Silvicultural treatments to reduce stress may increase 

vigor of old trees and improve their resilience to fire, bark 

beetle attacks, and drought. Van Mantgem and others 

(2003) related pre-fire growth rates and crown injury to 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 31

tree survival between burned and unburned stands. They 

determined that the majority of white fir with greater than 

50 percent crown volume scorched and radial growth 

greater than 0.2 inches/year (5.0 mm/year) survive; the 

majority of those growing less than 0.2 inches/year 

(5.0 mm/year) died. Thinning understory Douglas-fir 

increased branch production of old ponderosa pine and 

western larch in Montana (Sala and Callaway 2004). Kolb 

and others (2007) provide a review of studies examining 

the effect of management treatments to stimulate old ponderosa pine vigor. They concluded that careful thinning 

can increase resource uptake and growth of old ponderosa 

pines by reducing water stress and can cause increases 

in constitutive resin defenses against bark beetle attacks.

Radial Growth

The few studies that have examined the effects of 

silvicultural treatments on old tree growth rates in firedependent forests primarily involve ponderosa pine. All 

but one study reported increased growth of old ponderosa 

pine trees after either thinning or thinning followed by 

burning. In Oregon, thinning stands dominated by old 

ponderosa pine significantly increased basal area increment 

(BAI) compared to old trees in unthinned stands for up to 

15 years (McDowell and others 2003). Old trees in thinned 

units had decreased water stress and increased stomatal 

conductance, which improved carbon assimilation and 

growth. Treatment response time may take longer in old 

trees than in younger trees. In another Oregon study, lag 

time between thinning and increased growth varied by 

site and species for old trees, but growth rates for many 

of the trees did not increase until more than 5 years after 

treatment (Latham and Tappeiner 2002).

No differences in radial growth of pre-settlement ponderosa pine were observed compared to control trees 

3 years after thinning from below and 1 year after burning an old-growth ponderosa pine stand near Flagstaff, 

AZ (Skov and others 2005). Measurements in this study 

may have been made too soon after treatment for the presettlement trees to respond. In contrast, pre-settlement 

trees on the Gus Pearson Natural Area, AZ, had increased 

BAI 3 years after thinning and thin/burn treatments 

(Feeney and others 1998). Prior to burning, litter (Oi) on 

the study plots was raked aside, and the duff layers (Oe and 

Oa ) were removed. The litter and dried native grass foliage 

were then rescattered over the plots to mimic historical 

forest floor loadings and to reduce potential injury to tree 

bases and roots (Covington and others 1997). Zausen and 

others (2005) compared long-term changes in ponderosa 

pine tree physiology among unmanaged stands, stands 

thinned 8 to 16 years ago, and similarly thinned/burned 

stands in Arizona. Mean BAI was significantly greater in 

thinned/burned ponderosa pine stands than in unmanaged 

stands. BAI was intermediate in thinned-only stands, and 

was not significantly different from either the unmanaged 

or the thinned/burned stands.

Thinning increased BAI in old ponderosa pines in 

Montana (Fajardo and others 2007). BAI of the 10 years 

following treatment was significantly higher for presettlement trees in thinned treatments compared to 

controls. BAI of pre-settlement trees in the thinned and 

burned treatments was intermediate between the thinnedonly and control treatments, but not significantly different 

from the other two treatments.

Moisture Stress

Thinning, both with and without burning, has been shown 

to reduce moisture stress in old ponderosa and Jeffrey pine 

trees. A second-growth Jeffrey pine stand (100+ years old) 

on the Tahoe National Forest, CA, was thinned to release 

dominant and codominant stems. Thinning reduced moisture stress over the 3-year study period, even though the 

treatment coincided with an extended drought (Walker and 

others 2006). Additionally, burning caused no detrimental 

effects on predawn water potential. Average flame length 

was 2.3 ft (0.7 m), although no descriptions of tree injury, 

such as crown scorch or cambium kill, were reported that 

could impact tree moisture stress. Thinning 90 percent of 

post-settlement trees on the Gus Pearson Natural Area, AZ, 

increased water uptake and foliar nitrogen concentration 

on old ponderosa pine trees the first year after thinning. 

Thinning also increased needle length and bud size of presettlement trees (Stone and others 1999), increased needle 

toughness (a measure of resistance to the defoliator pine 

sawfly (Neodiprion spp.)) and increased BAI compared 

to control pre-settlement trees (Feeney and others 1998; 

Wallin and others 2004) (see Radial growth for additional 

information). These positive treatment effects continued 

3 to 7 years after thinning. Predawn water potential was 

significantly higher in ponderosa pine stands thinned 

8 to 16 years ago, both with and without burning during the peak of the dry season (Zausen and others 2005). 

Thinning and thinning followed by prescribed burning 

caused a long-term decrease in water competition and 

improved growth rates across the northern Arizona study 

sites.

Resistance to Insect Attacks

Increased resin production is thought to be a measure 

of a tree’s resistance to bark beetle attacks. Many studies 

have found increased resin production after burning. 
32 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Ponderosa pine resin flows were significantly higher than 

control trees after both spring and fall burns (Perrakis and 

Agee 2006). Burned, unattacked Virginia pine (Pinus 

virginiana Miller), table mountain pine (P. pungens 

Lambert), pitch pine (P. rigida Miller), and eastern white 

pine in North Carolina produced more resin than unburned, 

unattacked trees for up to 18 months after fire (Knebel and 

Wentworth 2007). Resin production was lower 5 months 

after prescribed fire in Arizona for ponderosa pine trees 

with greater than 51 percent crown scorch compared to 

trees with less crown scorch, and only trees with higher 

crown scorch were attacked by bark beetles (Wallin and 

others 2003). Red pine resin production initially decreased 

after prescribed burning, then returned to pretreatment 

levels within 7 to 10 days, and then increased to twice 

that of control trees 55 days post-fire (Lombardero and 

others 2006). Agee and Perrakis (2008) found this same 

trend of decreased resin production after fire, followed by 

increased production in the subsequent 4 years post-fire. 

These results suggest that fire-injured trees may increase 

constitutive and induced resin defenses to help deter successful bark beetle attacks.

The effect of thinning in combination with burning on 

bark beetle attack success is unclear. In an Arizona study, 

bark beetle attack rates and colonization success were lower 

in thinned and burned units than in the control unit (Wallin 

and others 2008). Every ponderosa pine tree baited with 

pheromone lures in the control was attacked, compared 

to 50 percent and 7 percent of baited trees in the full and 

partial restoration treatments, respectively. Of the attacked 

trees, beetle success rates were 100 percent in the control, 

33.3 percent in the partial restoration, and 3 percent in the 

full restoration. The partial restoration treatment removed 

35 percent of the basal area by thinning post-settlement 

trees. The full restoration treatment removed 58 percent 

of basal area. Beetle populations were low in the study 

area, and no unbaited trees were successfully attacked. 

However, study results support thinning ponderosa pine 

stands to reduce the potential for bark beetle attacks. 

Thinning likely aids in pheromone plume dispersal and 

burning increases resin production that may reduce successful beetle attacks.

Bark beetle attacks increased in thinned units following 

burning at the Blacks Mountain Experimental Forest in 

northeastern California (Fettig and others 2008). Two-year 

post-fire mortality was low at 5 percent, and the authors 

attributed 28.8 percent of mortality to bark beetle attacks. 

No differences in attack rates were found among diameter 

classes, and large-diameter (>23.5 inches (>59.7 cm) DBH) 

Jeffery and ponderosa pine mortality was not higher than 

smaller diameter classes.

Bark beetle attack success may be influenced by the 

timing of prescribed burns and beetle flight. Lombardero 

and others (2006) cautioned against burning during peak 

Ips beetle flight due to decreased resin production the 

first several days after prescribed burning. The authors 

hypothesized that red pine may be particularly susceptible 

to bark beetle attack during this short window. Ips abundance doubled in May following an April prescribed fire 

in an old-growth red pine forest and returned to control 

levels by late summer. Ips attack preference was not related 

to pre-burn tree growth rates, but seemed to prefer charred 

areas on the lower bole. Half of attacked trees died within 

one year (Santoro and others 2001).

Management Options ____________

Management options relating to minimizing largediameter and old tree injury and mortality when reintroducing fire into fire-dependent forests vary greatly 

depending on the scale of the treatment area. Landscapelevel, stand-level, and individual tree-level treatments are 

all necessary to meet the multitude of resource management 

objectives for a given land area. Many small stand- or 

individual tree-level projects can be embedded within 

larger landscape-level projects. Regardless of scale, successful restoration of fire-dependent ecosystems typically 

includes reducing tree density and ladder fuels to reduce 

crown fire risk, protecting large trees from significant 

injury, restoring surface fires, and increasing native herbaceous ground cover and biodiversity levels (Allen and 

others 2002). Reducing tree density also reduces competition around large trees, which may improve vigor, and 

returns forest structure, and perhaps composition, closer 

to historical levels.

Our knowledge of appropriate treatment options in 

fire-dependent, old-growth ecosystems is limited by the 

relatively few existing studies on the subject (Table 5). 

Only a handful of these are long-term studies with prescribed fire treatments (Table 7).

General Management Issues

Treatment prioritizations—Reducing the risk of 

high-intensity fire, including crown fire, should be 

the first treatment priority when restoring forests that 

historically burned frequently. Other key considerations 

in fuels management are a forest’s proximity to communities and important watersheds, protection of oldgrowth and areas with sensitive species, and strategic 

placement of treatments to break-up continuous fuels 

(Allen and others 2002).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 33 Table 7—Long-term restoration ecology studies that include fire effects on old-growth trees in forests that historically burned frequently. Study Establishment year Location Dominant tree species Treatments Selected References Gus Pearson Natural Area 1992 Arizona Ponderosa pine Control Thinning Thinning with area-wide forest floor removal, litter addition, and burning Covington and others 1997; Feeney and others 1998; Stone and others 1999; Wallin and others 2004 Chimney Springs, Fort Valley Experi- mental Forest 1976 Arizona Ponderosa pine Control Burning every year Burning every 2 years Burning every 4 years Burning every 6 years Burning every 8 years Burning every 10 years Sackett and others 1996; Sackett and Haase 1996, 1998 Limestone Flats, Long Valley Experimental Forest 1977 Arizona Ponderosa pine Control Burning every year Burning every 2 years Burning every 4 years Burning every 6 years Burning every 8 years Burning every 10 years Sackett and others 1996; Sackett and Haase 1996, 1998 Mt. Trumbull 1997 Arizona Ponderosa pine Control Thinning, raking pre-settlement trees and snags, and burning Fulé and others 2007; Waltz and others 2003 Blacks Mountain 1997 California Ponderosa and Jeffrey pine, white fir, incense cedar Control Burn only Low structural diversity (thinning from above ) High structural diversity (thinning from below) Low structural diversity and burning High structural diversity and burning Fettig and others 2008; Zhang and others 2008 Wade Tract 1982 Georgia Longleaf pine Annual summer burning Noel and others 1998; Platt and others 1998 Tiger Corner 1958 South Carolina Longleaf pine Unburned control Annual burning Biennial burning Triennial burning Quadrennial burning Glitzenstein and others 2003 Osceola 1958 Florida Longleaf pine Unburned control Annual burning Biennial burning Quadrennial burning Glitzenstein and others 2003 Lick Creek 1991 Montana Ponderosa pine Control Thinning followed by spring burning Thinning followed by fall burning Thinning Fajardo and others 2007; Sala and others 2005; Smith and Arno 1999
34 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

In the context of prioritizing treatments to perpetuate 

and develop areas of old-growth, Fiedler and others (2007) 

suggest classifying forests into one of three categories:

(1) Forests that currently feature old-growth structural 

components,

(2) Forests with developing old-growth structural 

components, or

(3) Forests lacking old-growth structural components.

Managers commonly identify category (1) forests as most 

likely to benefit from restoration treatments. However, 

treatments that foster the development of old-growth 

structural and functional conditions in categories (2) and 

(3) are also necessary to perpetuate old-growth on the 

landscape over a longer time period.

No action alternative—Choosing the no action 

alternative and letting nature take its own course is 

an intentional management decision (Cole and others 

2008). However, there is a conflict between the no 

action philosophy and continued attempted fire exclusion. Often, the no action alternative may be more of a 

threat to old and large trees than restoration activities 

(Noss and others 2006). Even in some protected areas 

such as National Parks and wilderness areas there is 

growing concern that human-perceived valuable ecosystem conditions cannot be maintained without natural 

disturbance or human-implemented treatments (Cole 

and others 2008). It is important to recognize that some 

large-tree mortality must be anticipated when implementing restoration treatments. This mortality creates 

structural diversity and important habitat by creating 

large snags and woody debris that are often scarce on 

the landscape (Allen and others 2002).

Choosing the no action alternative can leave forests more 

susceptible to high-intensity wildfire. In fire-dependent, 

low-elevation forests, the likelihood of fire is very high 

because of long, dry seasons and high potential for 

natural and human ignitions. We cannot predict when or 

where these wildfires will occur, but research studies and 

numerous anecdotal examples show that fuel reduction 

treatments in these forests can often reduce fire severity, thus reducing overstory tree mortality from wildfire. 

For example, mortality of trees greater than 11.8 inches 

(30 cm) DBH was nearly 100 percent after a wildfire in an 

untreated ponderosa pine stand on the Blacks Mountain 

Experimental Forest, CA, compared to almost no mortality within adjacent thinned and burned units. Mortality 

from wildfire in thinned-only units was less than 20 

percent. Treatments were implemented within 6 years of 

the wildfire; prior to that, the areas had not burned in over 

100 years (Ritchie and others 2007).

It is helpful to know typical background mortality in 

the absence of large-scale disturbance in order to set reasonable and attainable burn objectives when conserving 

large or old trees. Background mortality is the expected 

mortality if the no action alternative is chosen and no 

large-scale disturbance occurs. Ten-year mortality of 

ponderosa pine greater than 8.7 inches (22 cm) DBH 

was less than 10 percent in unburned areas of Crater 

Lake National Park, OR (Swezy and Agee 1991). This 

area was dominated by ponderosa pine and white fir, 

with scattered lodgepole pine, sugar pine, and other 

conifers, on well-drained soils derived from Mount 

Mazama deposits. In a separate study in Crater Lake 

National Park, 5-year mortality of ponderosa pine larger 

than 7.9 inches (20 cm) DBH from insects or pathogens 

was 2.3 percent in the control units (Agee and Perrakis 

2008). Most mortality occurred in low vigor trees. 

Mortality rates were 2.1 and 8.6 percent, respectively, 

for trees rated C and D (low vigor) using Keen’s vigor 

classes (Perrakis and Agee 2006). No trees in the A or 

B vigor classes (high vigor) died. Thirteen years after 

another prescribed burn at Crater Lake (Thomas and 

Agee 1986), mortality of trees greater than 7.9 inches 

(20 cm) DBH in unburned areas was 10 percent for 

sugar pine, 4 percent for white fir, and 14 percent for 

ponderosa pine (Agee 2003). Sugar pine and white fir 

mortality was significantly higher in the burned areas 

at 36 and 25 percent, respectively. This was not the case 

for ponderosa pine, which had 17 percent mortality.

When established in 1934, the Blacks Mountain 

Experimental Forest, CA, was unlogged and dominated 

by ponderosa and Jeffrey pine, white fir, and incense 

cedar. A comparison of forest structure between a 

1933/1934 tree census and another conducted in the late 

1990s showed large trees had declined by about half 

and small tree density increased more than four-fold 

(Ritchie and others 2008). Basal area of ponderosa and 

Jeffrey pine declined, and white fir and incense cedar 

concomitantly increased. Fifty-four to 61 percent of 

the remaining living, large trees in the untreated units 

were rated as high risk, compared to 15 percent in the 

thinned unit and 17 percent in the thinned and burned 

unit. Five-year periodic mortality rate of the large 

tree component was 6 to 19 percent in untreated areas 

(Ritchie and others 2008). The authors predicted that the 

control would experience similar declines in the large 

tree component over the next 15 years. In this area, a no 

action alternative is leading to the eventual loss of all 

large trees, with little opportunity to replace ponderosa 

and Jeffrey pine. Lutz and others (2009) also reported 

decreases in large-diameter Jeffrey, ponderosa, and 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 35

sugar pine trees in unburned, low-elevation portions 

of Yosemite National Park, CA, whereas plots that 

had burned in the 20th century retained large-diameter 

ponderosa pine.

In another study of background mortality, the loss of 

old ponderosa pine in untreated areas increased from 

0.2 trees/acre (0.5 trees/hectare) per decade in the 1920s 

to 2.4 trees/acre (5.9 trees/hectare) per decade in the 1970s 

at the Gus Pearson Natural Area, AZ (Mast and others 

1999). This increased mortality rate was attributed to lack 

of fire that allowed numerous post-settlement ponderosa 

pine trees to establish and increase competition.

Defining prescribed burn and other management 

objectives—There are many appropriate objectives 

when planning prescribed burns and other fuel reduction treatments. Each objective should be measurable 

on a stated scale over a stated timeframe. Appropriate 

treatment options will vary by scale and forest type. For 

instance, small, remnant stands of old-growth can warrant intensive restoration efforts that are not economically 

or physically feasible on the landscape-scale. Objectives 

that pertain to maintaining large-diameter and old trees 

should first include descriptions of both the untreated 

forest structure and the desired post-treatment forest 

structure. This should include stand density, diameter 

distribution, age distribution, species composition, and 

spatial arrangement. These are all measurable objectives 

that will determine if immediate treatment goals were 

met (Fiedler and others 2007).

When using prescribed fire, both first order and second 

order fire effects should be included in the objectives. First 

order fire effects may include immediate mortality targets, 

duff and surface fuel consumption, acceptable crown 

scorch levels, or desired area burned under specified severity levels. These should be assessed within 1 year of the 

fire to determine if objectives were met. Second order fire 

effects and resource management objectives may include 

acceptable longer-term delayed tree mortality and changes 

in tree vigor, regeneration, or understory species diversity. 

Maintaining or reestablishing historical forest disturbance 

processes (for example, fire frequency, severity, intensity, 

season, and insect attack levels) may be other important 

objectives when prescribed burning (Agee 2003). Process 

and structural objectives should be considered together, 

as process goals can help create and perpetuate desired 

structural conditions. Monitoring programs that revisit 

treated areas are essential for determining if immediate 

and long-term treatment goals are being met. The only 

way to determine if goals are met is through the stated 

measurable objectives.

Identifying ecosystem components of interest is key 

to developing burn objectives and monitoring plans to 

assess fire effects beyond hazard reductions. Also, treatment results should be tied to the identified objectives. For 

example, if large-diameter tree retention is an objective 

for a prescribed burn, it is important to assess and report 

duff depth and moisture near the base of these large trees, 

in addition to the interspaces. Duff consumption is usually highly variable across a burned unit. This variability 

largely results from microsite changes in fuel moistures 

due to differences in canopy thickness and changes in 

surface fuel consumption. The tree crown shelters the 

forest floor immediately underneath it; therefore, fuels 

are deeper and drier there than in the areas between tree 

crowns. For example, an experimental prescribed burn in 

ponderosa pine at Chimney Spring, AZ, reduced forest 

floor fuel loadings by 63 percent. However, consumption 

was 100 percent from the bole to the dripline around all 

the large trees where the fuel loading was also the highest 

(Sackett and others 1996). In this case, reporting only the 

average stand fuel consumption would make it difficult to 

determine a possible cause of the later observed mortality 

in the larger-diameter trees.

Landscape-Scale Options

Landscape restoration—Noss and others (2006) provide several recommendations for successful widespread 

restoration of southwestern ponderosa pine ecosystems 

that pertain to retaining old trees when reintroducing 

prescribed fire into long-unburned areas. Many of these 

recommendations are also suitable for other ecosystems 

that evolved with frequent fire.

(1) Think big. Plan conservation and restoration projects on landscape and regional scales. Restoring a 

natural fire regime can result in a heterogeneous 

landscape with multiple stands conditions that are 

likely to provide suitable habitat for a wide range 

of species. Focusing on just a small project area 

can be contentious and may make it hard to justify 

potentially adverse treatment impacts on threatened 

or endangered species. Planning restoration treatments on a landscape-scale can restore a frequent 

fire regime, while allowing for areas of denser stands 

and differences in fire severity in order to provide 

diverse structural conditions.

(2) Recognize that protected areas may require active 

management. Protected areas should not be automatically excluded from consideration for restoration treatments in areas that historically burned 

frequently. Though treatments in high-elevation 
36 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

protected areas are probably not suitable, treatments 

in lower-elevation areas that have experienced fire 

exclusion may foster the qualities for which they 

were originally protected.

(3) Restoration strategies should encompass both 

wildlands and the wildland-urban interface (WUI). 

Restoration treatments should include both the WUI 

and the extended landscape away from human 

development. Treatment prescriptions will likely 

differ based on location to nearby communities, but 

restoration efforts are necessary across the whole 

landscape.

Wildland fire use—In some areas, naturally ignited 

fires are allowed to burn with minimal to no suppression 

efforts to meet resource benefit objectives. Federal 

agencies have used various terms to describe this activity since the program was first implemented in the late 

1960s, including “let burn,” “prescribed natural fire,” and 

“wildland fire use” (van Wagtendonk 2007). Beginning 

in 2009, fires are no longer categorized by the latest term 

“wildland fire use for resource benefit” (WFU). Fires 

are now designated as wildfires or prescribed fires based 

on whether the ignition was planned. Based on the existing land management plan, wildfires will be managed 

for a combination of protection and resource objectives 

(U.S. Department of Agriculture, U.S. Department of the 

Interior 2009). WFU is used in this document, though it 

is outdated, because it is the term used by the references 

discussed in this report.

Differences in ponderosa pine forest structure and fire 

occurrence were compared in the Gila Wilderness, NM, 

and Saguaro Wilderness, AZ (Holden and others 2007). 

Fires in the Saguaro Wilderness have been managed as 

WFU fires since 1971 and in the Gila since 1975 (van 

Wagtendonk 2007). In both wilderness study areas, tree 

density was significantly lower in areas that had burned one 

or more times than in unburned areas. Small tree density 

was significantly lower in burned areas versus unburned 

areas in the Saguaro Wilderness. In the Gila Wilderness, 

small tree densities were lower in areas that had burned 

two or more times compared to less frequently burned 

areas. Average DBH was also significantly higher in the 

burned areas compared to unburned areas in the Gila, but 

no differences were found in the Saguaro. No differences in 

basal area or large tree density were found between burned 

and unburned areas in either wilderness, except for areas 

in the Saguaro wilderness that had burned two or more 

times and also in a pre-WFU fire. In these areas, large 

tree density was significantly higher than less frequently 

burned areas. These results suggest that in areas where 

fire can safely be allowed to burn, forest structure may 

more closely mimic historical forests while maintaining 

large tree density. However, it may take multiple fires to 

achieve these results.

Fulé and Laughlin (2007) investigated the effects of 

WFU fires on the North Rim of the Grand Canyon, AZ. 

They were able to compare changes in forest structure and 

fuels between burned and unburned plots established prior 

to the WFU fires. The low-elevation sites dominated by 

ponderosa pine historically burned every 3.2 to 5.5 years 

before livestock introduction and fire suppression caused 

large fire cessation in 1879. Before the 2003 WFU fire, 

these sites had burned three times since 1879, in 1892, 

1924, and 1987 (Fulé and others 2003). The mid- and highelevation sites had not burned since 1879. The WFU fire 

decreased tree density by 36 percent on the low-elevation 

site, with small trees (≤7.9 inches (≤20 cm) DBH) comprising 95 percent of mortality 2 years after the fire. Across all 

sites, large (≥14.8 inches (≥37.5 cm) DBH) ponderosa pine 

tree mortality constituted 7 percent of the total mortality 

(Fulé and Laughlin 2007). This study suggests that the 

WFU fires on the North Rim of the Grand Canyon moved 

the sites closer to historical reference conditions and are 

a viable management alternative.

Keane and others (2006) investigated the effects of 

a 2003 WFU fire on old ponderosa pine in the Bob 

Marshall Wilderness, MT. Before 1930, the area burned 

every 20 to 30 years, but no fires had occurred since then. 

During this fire-free period, thick duff layers of 6 to 15 inches 

(15 to 38 cm) accumulated at the base of the pre-settlement 

ponderosa pine, and Douglas-fir and lodgepole pine trees 

became established. The WFU fire was primarily a lowintensity surface fire. One year after the fire, 16 percent 

of trees greater than 20 inches (51 cm) DBH had died, and 

an additional 18 percent were noted as dying. Some dead 

and dying trees had relatively high levels of crown scorch, 

but the authors attributed much of the mortality to basal 

girdling from complete duff consumption and mountain 

pine beetle attacks. Many trees in this area were peeled 

by Native Americans for food and are living cultural 

artifacts (Figure 22). The fire killed approximately half 

of the historic bark-peeled trees. This study suggests that 

high mortality may occur after WFU fires in old-growth 

ponderosa pine stands in the northern Rockies that have 

missed several fire cycles. However, using other treatments in wilderness areas is greatly restricted, and not 

allowing fire at all is worse for the long-term perpetuation 

of ponderosa pine.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 37

Stand-Level Options

Prescribed burning—It took decades of no fires 

to develop the thick basal duff layer that now exists in 

many stands. It will also likely take multiple treatments 

and broadcast burns to reduce fuel loadings in order to 

limit overstory tree mortality. This is especially true in 

the absence of other individual tree duff reduction treatments (for example, raking). Each consecutive prescribed 

broadcast burn should aim to remove only a portion of 

the accumulated duff around the bases of overstory trees.

Fire intensity, or fireline intensity, is the rate of energy 

or heat release per unit length of fire front and is mathematically related to flame length (Byram 1959). Fireline 

intensity or flame length is a good choice for predicting 

crown scorch; however, it is not a good indicator of the 

amount of heat transferred down into the soil (DeBano 

and others 1998; Wade 1986). Fire severity is the effect of 

the fire on an ecosystem (Ryan and Noste 1985). It relates 

to how the fire affects plant survival and consumption of 

forest floor material or surface fuels (in other words, the 

depth of burn). Therefore, fire severity is a better indicator of the amount of heat transferred into the soil (Wade 

1986). It is possible to have a low-intensity, high-severity 

fire with little crown scorch but high duff and/or large 

surface fuel consumption. This type of fire may cause 

considerable stem and root injury through long-term 

smoldering combustion.

Manipulating fire intensity through ignition patterns 

is effective in achieving the desired above-ground fire 

effects. Heading fires have longer flame lengths, faster 

rates of spread, and higher fire intensities than backing 

fires. Backing fires have longer residence times than heading fires, which can increase the chance of duff ignition 

and lead to more smoldering (DeBano and others 1998). 

Backing fires consumed significantly more duff than 

heading fires in experimental prescribed burns in longleaf 

pine, but the amount of litter consumed was not affected 

by ignition pattern (Sullivan and others 2003).

Heading fires are effective at killing small-diameter, 

shorter trees by scorching most of their crowns. For thinning doghair thickets, lighting a spot fire in the center 

of the thicket, followed by a ring fire around the thicket 

is effective. Areas where flanking fires merge or heading and backing fires merge also increase fire intensity 

(Sackett and Haase 1998). Backing fires generally do not 

kill small-diameter trees, unless their bark is very thin.

Prescribed burning long-unburned longleaf pine 

forests—Introduction of growing season fires to longunburned longleaf pine forests may cause delayed 

Figure 22. Native American peeled ponderosa pine tree in the Bob Marshall Wilderness, MT. 

Photo by Robert Keane.
38 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

mortality of the older trees. Several dormant season prescribed burns should be applied when lower duff is very 

moist to gradually reduce duff layers before switching to 

growing season burns (Brockway and others 2004; Kush 

and others 2004). Burns conducted under lower ambient 

air temperatures help to reduce crown scorch because more 

heat is required to reach 140 °F (60 °C), the lethal level 

for living tissue (Wade and Johansen 1986). Removing 

midstory hardwoods either by mechanical thinning or by 

applying herbicides also reduces fire intensity, crown injury 

to overstory trees, and competition. Thinning followed 

by prescribed burning promotes understory grasses and 

forbs while reducing hardwood sprouts, which promotes 

future low-intensity prescribed fires (Brockway and others 

2004; Kush and others 2004). In small restoration burns, 

generous amounts of water can be applied during and after 

prescribed burns at the bases of trees with heavy duff 

accumulations to stop smoldering (Kush and others 2004).

Mortality of large-diameter longleaf pines after prescribed burning old-growth stands in the Florida Panhandle 

was directly related to duff consumption and duff moisture (Varner and others 2007). In this study, burns were 

conducted under three different gravimetric duff moisture 

conditions: dry (55 percent), moist (85 percent), and wet 

(115 percent) (note that moistures are not volumetric as 

reported in Varner and others 2007; R. Ottmar, personal 

communication). Stands had not burned for approximately 

30 years prior to treatment (M. Varner, personal communication). Duff consumption was highly related to lower duff 

moisture content (R2

 = 0.78). Duff consumption around 

mature tree stems greater than 5.6 inches (15 cm) DBH 

was significantly higher in the dry burns than in the moist 

and wet burns at 46.5, 14.5, and 5 percent, respectively. 

Overstory tree mortality averaged 20.5 percent in the 

dry burns, with no significant differences in mortality 

among the wet, moist, and control units. Mortality rates 

increased with increasing tree diameter in the dry burns 

(Figure 23). Crown scorch and stem char height were not 

significantly different across burn treatments. Mortality 

did not begin until 12 to 18 months after the burns, and all 

pines that died were attacked by black turpentine beetle 

(Dendroctonus terebrans) and ambrosia beetle (Playypus 

flavicornis). The moist prescription is now widely used 

at Eglin Air Force Base, FL, to achieve moderate basal 

duff consumption with little overstory longleaf mortality 

when burning fire-excluded stands (M. Varner, personal 

communication). This condition is achieved by burning 

within 2 days of a 1-inch (2.5-cm) rain event (K. Hiers, 

personal communication).

Dale Wade, retired USDA Forest Service scientist at the 

Southern Research Station, recommends the following 

Figure 23. Diameter distribution of overstory longleaf pine 

trees (>6 inches (15 cm) DBH) killed following experimental 

prescribed fires of various duff moisture prescriptions in longunburned longleaf pine forests in northern Florida. No overstory 

pines died in the unburned control treatment. From Varner and 

others 2007.

prescription when reintroducing fire into long-unburned 

southern forests that have a dense midstory of hardwoods. 

Under most conditions, the midstory creates a humid, 

sheltered environment and prevents the litter layer from 

drying quickly after rain, yet the lower duff layers are 

moist enough to limit consumption. Therefore, the primary 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 39

criterion for a successful first-entry prescribed burn is a 

very steep forest floor moisture gradient caused by the 

passage of two cold fronts in quick succession that wet 

the duff layer but bring wind to dry out fine surface fuels 

and push the fire quickly through the stand. The first cold 

front must bring precipitation and must be followed by 

a second, dry cold front. Ignition should occur within a 

few hours after the second front passes to utilize the high 

winds and to ensure the litter is wet enough to not carry 

a backing fire. Because of the sheltered conditions in the 

stand, the burn prescription is very narrow in level terrain 

and conditions may materialize only once or twice a year. 

The needles on the lower branches of the overstory pines 

may scorch, but residence times are short and little duff is 

consumed. This prescription topkills many small-diameter 

stems and widens the prescribed fire window for the next 

burn, which should occur in 1 to 2 years. The steep duff 

moisture gradient is necessary for ensuing prescribed 

burns until duff depths are greatly reduced. Brockway 

and others (2005) further describe this prescription and 

others appropriate for restoring longleaf pine ecosystems.

The chevron ignition technique proceeding down from 

the ridges is recommended in sloped areas. Igniters should 

never light from the bottom of the slope. The steep moisture gradient is the key to success, rather than season of 

the year. Hand removal of some understory and midstory 

fuels helps reduce fireline intensity and allows more wind 

into the stand, but cut material should not be left on site 

to burn. For the same reasons as thinning, herbicides also 

help to expand the prescribed fire burn window (D. Wade, 

personal communication).

Fire was successfully reintroduced into an old-growth 

longleaf pine forest in Flomaton, AL, using many of the 

techniques described above (Wade and others 1998). This 

65-acre (26-hectare) tract had not burned in 45 years before 

the first prescribed burn was initiated in 1995. The area 

was burned when the duff layer was very moist and brisk, 

persistent winds were present to quickly push strip headfires through the stand. In this way, much of the hazardous 

midstory and understory layers were consumed, but not 

the duff layer. Two subsequent burns and thinning of the 

midstory have continued to slowly reduce fuel accumulations by 25 to 35 percent with limited overstory longleaf 

pine mortality (Kush and others 2004; Varner and others 

2005; Varner and others 2000; Wade and others 1998).

Presence of fallen longleaf pine cones and mast years 

must be considered when scheduling burns in fire-excluded 

longleaf stands or duff consumption will be much higher 

than expected. At Eglin Air Force Base, FL, longleaf pine 

cones encountered along planar intercept fuel transects 

(Brown 1974) are recorded as 100-hour fuels because 

of their long burnout times (Fonda and Varner 2004; 

K. Hiers, personal communication). Longleaf pine typically produces heavy crops of cones every 8 to 10 years 

(Maki 1952). It is best to burn these areas before the 

heavy cone crop falls to the ground because cones take 

about 10 years to decompose. Mast years are known 1 year 

in advance, which gives the manager some leeway to 

complete the burn prior to cone fall (K. Hiers, personal 

communication).

Prescribed burning long-unburned red pine forests—

A variety of mostly low-intensity surface fire with small 

patches of higher-intensity fire that causes some overstory 

mortality should both perpetuate overstory red pine and 

promote red pine seedling establishment. Van Wagner 

(1970) recommends the following when burning in red 

pine stands:

(1) a sparse but adequate stocking of mature red pines,

(2) a good red pine seed year,

(3) a considerable period of dry weather to promote 

mineral soil exposure,

(4) a summer surface fire of 200 to 500 Btu/ft/sec 

(690 to 1730 kW/m) and average flame lengths 

of 5 to 8 ft (1.6 to 2.4 m) to keep crown scorch of 

mature pine below 75 percent,

(5) satisfactory post-fire weather for germination and 

early growth, and

(6) no fire for several decades to allow red pine seedlings 

to become fire-resistant.

It should be noted that the stands Van Wagner studied 

likely did not have heavy basal duff accumulations caused 

from years of fire exclusion; therefore, these recommendations may be more suitable to promoting red pine seedling 

establishment than to limiting overstory red pine mortality. 

Henning and Dickmann (1996) support Van Wagner’s recommendation to limit red pine crown scorch to less than 

75 percent to minimize mortality, but suggest maximum 

flame lengths of 2 ft (60 cm).

Prescribed burning long-unburned ponderosa pine 

forests—Using prescribed fire to incrementally reduce 

basal duff depths in ponderosa pine forests seems 

more difficult and variable than in other forests types 

(M.Harrington, personal communication). This may be 

due to the generally dry summer conditions that rarely 

fully moisten lower duff layers.

Jain and Graham (2004) developed a method for incrementally reducing ponderosa pine basal duff depths in 

western Idaho by burning snow wells. They recommend 

burning when lower duff moisture is greater than 100 

percent and temperatures are low (<28 °F; <-2 °C). This 
40 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

prescription consumes the litter and some upper duff, while 

leaving lower duff intact. These conditions occur in the 

early spring when snow is usually still present throughout 

the stand but the areas at the bases of trees are clear. The 

authors stressed the need for repeated prescribed burns to 

slowly reduce duff depths around tree bases and to force 

fine roots to grow back down into the mineral soil layers 

rather than in the lower duff.

With the exception of southwestern ponderosa pine, early 

season prescribed burning seems to be the best option 

when reintroducing fire to an area that has excessive fuel 

accumulations due to past suppression activities. Burning 

under higher moisture conditions, such as in spring after 

snow melt, often results in patchier burns and reduced 

fuel consumption compared to burning under drier conditions. Therefore, spring burning can moderately reduce 

fuel build-up while limiting injury to vegetation from 

long-term heating.

Perrakis and Agee (2006) found higher mortality 

after fall prescribed burns compared to spring burning. 

Mortality was highest in the lowest vigor classes, and low 

vigor was associated with lower growth rates (Agee and 

Perrakis 2008). All burns in this study were low-intensity 

and caused little crown scorch. Fall burns consumed 51.8 

percent of total dead fuels versus 17.9 percent consumed 

in spring burns. Four years post-fire, mortality of old 

ponderosa pine was 2.3 percent in the control units, 6.1 

percent in the spring burn units, and 16.4 percent in the 

fall burn units. Only 24 of the 139 dead trees were due to 

direct fire effects or windthrow. Bark beetle attacks were 

the largest cause of mortality (Agee and Perrakis 2008; 

Perrakis and Agee 2006), but the authors did not report 

differences in bark beetle activity between spring and fall 

burns. The higher mortality observed in the fall burns 

was attributed to “intense burning at the root collar” that 

caused stem injury and breakage and led to bark beetle 

attacks (Perrakis and Agee 2006).

In the southwest, Sackett and others (1996) recommend 

conducting initial prescribed burns in ponderosa pine 

stands in the fall due to the monsoons the area experiences. 

Historically, most fires occurred at the beginning of the 

monsoon season, just after spring. The first few storms 

are usually dry and accompanied by lightning. Therefore, 

burning dense stands with high fuel loadings in the spring 

when fuel moisture is low and the fire season is approaching is riskier than doing so in the fall. Weather and fuel 

moisture conditions are more moderate in the fall than in 

the spring, and high winds are not as likely. Spring burning 

is a good option after the first or second burn, after fuel 

loadings have been reduced (Sackett and others 1996).

Sackett and others (1996) established an experimental 

prescribed burn study on ponderosa pine in Arizona in 1976 

to determine the optimal burn rotation to restore stands to 

near pre-settlement conditions, decrease wildfire intensities, and maintain low fuel loadings. Treatment plots were 

burned at 1-, 2-, 4-, 6-, 8-, or 10-year intervals. Conditions 

in the 1- and 2- year burn interval units had enough fuel 

to carry fire, but burning was often not possible due to 

marginal weather conditions. The most effective rotation 

was to burn every 4 years. At this timeframe, fuel loadings 

were kept to a minimum, making the prescribed burns 

easier to implement, and optimal weather conditions usually occurred. Heading fires ignited on the 4-year interval 

plots did not cause overly high crown scorch. The 6-, 8-, 

and 10-year rotations all allowed sufficient fuel accumulations to cause undesirable fire intensities and crown injury 

to overstory trees.

Prescribed burning long-unburned giant sequoiamixed-conifer forests and mixed-conifer forests—Haase 

and Sackett (1998) found sugar pine was more susceptible to cambium and root injury than giant sequoia from 

prescribed fires in California. Eight research prescribed 

burns in Sequoia-Kings Canyon National Park recorded 

lethal cambium and soil temperatures around giant sequoia 

and sugar pine. Sixty-seven percent of sugar pine died 

after the prescribed burns compared to no giant sequoia 

mortality (Haase and Sackett 1998). In addition, park 

personnel had not observed giant sequoia mortality that 

could be attributed to the prescribed burn program during the previous 26 years. During the research prescribed 

burns, in which fire ignited and carried over the surface 

litter layer, complete duff consumption occurred with 

duff moisture contents ranging from 7 to greater than 200 

percent. High duff moisture slowed the combustion rate 

but did not stop consumption. Average forest floor depths 

at the base of the sampled giant sequoia where cambium 

temperatures were measured ranged from 3.2 to 21.4 inches 

(8.1 to 54.4 cm) and 2.0 to 13.3 inches (5.1 to 33.8 cm) for 

sugar pine. Lethal temperatures varied around each tree. 

The authors noted that a substantial portion of the giant 

sequoia root system appears to be located directly under 

the tree base where it is protected from soil heating. Sugar 

pine also had proportionally more beetle attacks after fire 

than ponderosa pine or white fir in Crater Lake National 

Park, OR (Thomas and Agee 1986). In that study, mountain pine beetles killed 25 percent of sugar pines greater 

than 7.9 inches (20 cm) DBH 2 to 4 years after the fire. 

The authors recommended burning in late spring when 

soil and duff are wet but surface fuels are dry enough to 

carry a low- to moderate-intensity fire (<87 Btu/ft/sec 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 41

(<300 kW/m)) and flame lengths (<3.5 ft (<1 m)) with low 

duration in order to minimize large pine mortality.

Early season burns consumed significantly less fuel and 

were patchier than fall burns in a Sierra Nevada mixedconifer forest. The authors concluded that the resulting 

tree mortality was related to fire intensity rather than 

tree phenology (Knapp and others 2005; Schwilk and 

others 2006).

Thinning and prescribed burning—Thinning can be 

an effective fuels treatment to reduce crown fire potential and to lower fire intensity by removing ladder fuels 

and creating gaps in the overstory canopy. Thinning is 

also commonly used to achieve restoration objectives 

by returning stands closer to historical stand structures. 

Under extreme weather conditions, such as in wildfires, 

thinning can reduce crown fire potential and, therefore, 

lower the probability of immediate tree death from crown 

scorch. However, thinning creates activity fuels and does 

not raise individual tree crown heights or control shrubs, 

seedlings, and small trees (O’Hara and Waring 2004). 

Thinning alone can also increase the intensity of a subsequent wildfire if activity fuels are not treated (Stephens 

and Moghaddas 2005). Thinning also does not solve the 

problem of accumulated fuel around the bases of old 

trees. These treatments are best used in conjunction with 

prescribed burning to meet restoration goals.

Mechanical thinning to reduce understory tree density 

is often much easier than using fire alone to thin smaller 

trees. In long-unburned areas, understory trees may also 

develop thick bark and become more fire-tolerant, making them difficult to kill with fire alone. After 18 years 

of prescribed burning to attempt to “manipulate fuels and 

tree density in an overstocked post-settlement ponderosa 

pine stand so that it would survive a wildfire that would 

otherwise be stand-replacing,” Sackett and others (1996) 

concluded that achieving post-settlement conditions by 

prescribed fire only would be difficult. This and other 

research has led to the conclusion that a combination of 

thinning and prescribed burning to reduce stand densities 

and fuel loadings would better meet restoration goals in 

southwestern ponderosa pine forests (Covington and others 1997). In southern and eastern forests that historically 

burned frequently, fire suppression has also allowed many 

hardwood species that are easily killed by fire when young 

to become fire-resistant (Abrams 2006). Thinning this 

in-growth of now fire-tolerant trees is likely necessary 

before prescribed burning, as fire alone will not return 

the stands closer to historical forest structures (Harmon 

1984; Hutchinson and others 2008). Modeling efforts 

also suggest that careful thinning of northern hardwood 

stands may shorten the time required to reach structures 

more typical of old-growth forests (Choi and others 2007).

Fulé and others (2002a; 2005) compared 1-year and 

5-year post-treatment effects of two levels of thinning 

and burning (FULL, MIN), burning only (BURN), and a 

control on ponderosa pine stand structure near the Grand 

Canyon South Rim, AZ, where many of the larger, mature 

trees had been harvested earlier in the past 100 years. The 

FULL thinning and burning treatment removed the majority of post-settlement trees to restore the pre-settlement 

pattern of tree species and spatial arrangement. The MIN 

thinning and burning treatment removed post-settlement 

trees growing in the immediate vicinity of old trees to help 

protect them from wildfire. In both the FULL and MIN 

treatments, the forest floor was raked approximately 12 

inches (30 cm) from all old trees. Old trees in the BURN 

and control treatments were not raked. No significant differences in crown scorch or bole char were found across 

the burned treatments. Large tree (>14.8 inches (>37.5 cm) 

DBH) mortality was low across all treatments. Five years 

post-treatment, 1 large ponderosa pine tree (3 percent) 

died in the control, 1 tree (9 percent) died in the FULL, 

no trees died in the MIN, and 2 trees (13 percent) died in 

the BURN.

Modeling scenarios predicted the FULL treatment would 

return the area most quickly to near pre-settlement forest structure conditions and would best mitigate against 

future wildfire. The FULL treatment was also predicted 

to remain effective against reducing fire intensity for at 

least 40 years post-treatment. Disadvantages to the FULL 

treatment are high costs and the necessity of roads for 

heavy equipment. Over larger areas or where road access 

is limited, the MIN and BURN treatments may be more 

feasible. In more remote areas with remaining old-growth, 

thinning treatments similar to the MIN treatment offer 

extra protection for these trees against future wildfire 

(Fulé and others 2002a).

Fire behavior was modeled under 80th, 90th, and 97.5th

percentile weather conditions after fuel treatments 

in mixed-conifer forests in California (Stephens and 

Moghaddas 2005). Treatments included: (1) thinning from 

below followed by mastication (mechanical), (2) mastication followed by a prescribed fire (mechanical+fire), 

(3) prescribed fire only (fire), and (4) untreated control. 

Treatments did not affect predicted large tree (>20 inches 

(>51 cm) DBH) mortality under 80th percentile weather 

conditions. Under 90th percentile weather conditions, 

only the mechanical+fire treatment lowered the predicted 

mortality of trees 20 to 30 inches (51 to 76 cm) DBH, but 

not for trees larger than 30 inches (76 cm) DBH. Predicted 

large tree mortality was significantly lower for all fuel 

treatments compared to the control under 97.5th percentile 

weather conditions.
42 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Thinning and a combination of thinning and burning 

stabilized the large (>23.6 inches (>60 cm) DBH) ponderosa 

and Jeffrey pine tree component at a study at the Blacks 

Mountain Experimental Forest, CA (Ritchie and others 

2008). Five years after treatment, the mortality rate of 

the large tree component was 1.2 percent in the thinned 

units, 8.4 percent in the thinned and burned units, and 

6 to 19 percent in the controls. The burn-only treatment 

did not reduce the risk of large tree mortality compared to 

the control units. Fifty-four to 70 percent of the large trees 

in the control and burn-only units had a high risk rating 

compared to 15 to 17 percent in the thinned units. The 

authors predicted that the 15-year change in tree density 

estimated similar declines in the large tree component for 

the control and burn-only units, whereas in the thinned 

and thinned and burned units, small increases in large tree 

density were forecasted. While more large trees died in 

the thinned and burned stand than in the thin only stand, 

prescribed burning lowered fuel loadings, which could 

increase resiliency to wildfire in the long-term. The authors 

predicted that in similar forests to the Blacks Mountain 

Experimental Forest study area, untreated stands will likely 

eventually lose most large ponderosa and Jeffrey pine trees 

if current mortality rates continue. In a separate study 

on the Blacks Mountain Experimental Forest, a high 

structural thinning treatment both with and without 

prescribed burning did not cause any mortality of old 

dominant trees (≥30 inches (≥76.2 cm) DBH) 5 years 

after treatment (Zhang and others 2008). However, a 

complete census of the experimental units in the study 

showed western pine beetle attacked and killed 6 percent 

of the large trees (>23.5 inches (>59.7 cm) DBH) in one 

thinned and burned unit (Fettig and others 2008). Fuel 

consumption and post-fire tree injury were not reported. 

Large trees continued to grow after treatment application, 

and the thinned treatments appeared to have enhanced 

late seral attributes compared to untreated stands without 

significantly increasing mortality rates.

Treating Individual Trees

Accumulated duff creates significantly greater potential 

for tree injury than deep litter when reintroducing fire. 

Deep litter layers will burn quickly during the flaming 

front without causing basal injury in thick-barked trees. 

Deep duff can smolder for many hours after the flaming 

front has passed. Smoldering basal duff, stumps, and logs 

near the tree bole typically result in basal and root injury; 

therefore, the decision to treat individual trees because 

of heavy fuel accumulations must focus on the duff layer 

around the base of trees.

Fire scars—Trees with exposed fire scars, or cat-faces, 

are especially vulnerable to prescribed fire. The scars are 

dry and often covered with pitch so they are easily ignited 

(Figure 24). Once ignited, the potential is high for the 

tree’s heart wood to burn out, which usually kills the tree 

standing or burns it over (Figure 25). Duff adjacent to 

trees with fire scars should be mitigated before prescribed 

burning in order to minimize tree mortality. During snow 

well burning, filling cat-faces with snow before and during duff mound ignition reduces the potential of the scar 

igniting (Graham and Jain 2007; R. Taplin, personal communication). An alternative is to physically remove duff 

from the scar prior to burning using a rake or leaf blower.

Giant sequoias with fire scars seem to be the exception. 

Lambert and Stohlgren (1988) found giant sequoias with 

exposed fire scars did not have higher mortality rates 

than unscarred trees after prescribed burns in Sequoia 

and Kings Canyon National Parks. In that study, giant 

sequoia mortality rates in units prescribed burned between 

1979 and 1984 were compared with unburned units. No 

Figure 24. Exposed ponderosa pine fire scar burning.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 43

differences were found in mortality between trees with 

“extremely heavy ground fuels” removed around trees 

with large fire scars and those without fuel removal, as 

mortality was low for all treatments. Haase and Sackett 

(1998) also found low-intensity prescribed fire did not 

cause high rates of mortality in giant sequoia, even if fire 

scars were present.

Raking—Raking is a treatment to reduce the amount of 

litter and duff at the tree base in order to reduce potential 

bole and root injury from long-term smoldering (Figure 

26). Researchers have reported mixed results, but treatment 

implementation and burn severity has differed greatly 

among studies, and only two studies (Fowler and others 

2010; Hood and others 2007a) have specifically investigated the effectiveness of raking as a viable treatment 

to reduce old tree mortality.

In one of the first studies to examine raking, Swezy and 

Agee (1991) removed the litter layer around 3 trees before 

a prescribed burn in Crater Lake National Park, OR. The 

duff layer was left intact. These trees were compared to 

three unraked, burned trees and to three unraked, unburned 

trees. One high, moderate, and low vigor tree was chosen 

for each treatment. Four years post-fire, the low vigor 

raked and burned tree died after attack by western pine 

beetle. Duff moistures were not collected before the burn, 

but the authors hypothesized that raking only the litter 

layer dried the duff layer more quickly than duff around 

unraked trees. Perrakis and Agee (2006) also raked litter 

3.3 to 6.5 ft (1 to 2 m) away from ponderosa pine trees 

greater than 7.9 inches (20 cm) DBH in a nearby area of 

Crater Lake National Park. Burn season and vigor were 

the only significant predictors of tree mortality; raking 

had no effect.

At Mt. Trumbull in northern Arizona, all pre-settlement 

ponderosa pine trees were raked after thinning and prior 

to burning to test the effectiveness of compressing slash 

prior to burning to reduce fire intensity (Jerman and 

others 2004). The majority of forest floor material was 

removed 1.6 to 3.3 ft (0.5 to 1 m) from the boles, but the 

areas were not raked to mineral soil. One unit was then 

broadcast burned with thinning slash intact. In a second 

Figure 25. Living ponderosa pine tree with exposed fire scar that ignited during a prescribed burn and fell over. 

Photo by Michael Harrington.
44 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Figure 26. Infrared image of smoldering duff around an (a) unraked tree and (b) raked tree. 

Note the expanded black area around the base of the black tree indicating low temperatures. 

(a)

(b)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 45

treatment, a D-6 bulldozer compressed residual thinning 

slash throughout the unit before it was broadcast burned. 

No pre-settlement trees died within two growing seasons 

after burning in the slash compression treatment. In the 

burn-only treatment, raked pre-settlement tree mortality 

was 14 percent (3 of 22 trees) versus the 35 percent in 

post-settlement, unraked trees. Because raking was done 

as a precautionary measure around all pre-settlement trees 

instead of as a specific treatment, no comparison between 

raked and unraked pre-settlement trees was possible. Slash 

compression reduced fire intensity and crown scorch. 

The authors concluded that slash compression and raking 

treatments appear to reduce pre-settlement ponderosa pine 

mortality from broadcast prescribed burning in Arizona.

In a separate area of the Mt. Trumbull site, Fulé and others 

(2007, 2002b) observed high mortality of old ponderosa pine 

trees after burning. Accumulated duff and litter was raked 

12 to 24 inches (30 to 60 cm) away from pre-settlement 

trees in the burned units prior to burning. This study was 

also not designed to test raking as a treatment; therefore, 

all trees in the burned units were raked. The high mortality 

only occurred in areas of shallow volcanic soils, leading 

the authors to hypothesize that the burns killed more 

fine roots that were growing near the soil surface and led 

to high rates of old-growth mortality (Fulé and others 

2002b). Five years after treatment, an average of 4 large 

ponderosa pine trees/acre (>14.8 inches (>37.5 cm) DBH); 

10 trees/hectare) had died in the thinned and burned units 

compared to 1.9 large ponderosa pine trees/acre (4.6 trees/

hectare) in the control units. There was no difference in 

crown volume scorched between pre-settlement trees that 

died or survived (both were 30 percent) in the burned 

units (Fulé and others 2007). Prescribed burning on lava 

soils is currently suspended in Arizona due to concerns 

of excessive pre-settlement tree mortality, regardless of 

whether or not trees are raked (Waltz and others 2003).

Laudenslayer and others (2008) sampled 30 large 

(>24.0 inches (>61 cm) DBH) and 20 small (<24 inches 

(<61 cm) DBH) ponderosa, Jeffrey, and sugar pine trees 

in two units at Lassen Volcanic National Park in northern 

California. Half of the trees were raked around the first 

3.3 ft (1 m) of the bole to mineral soil prior to prescribed 

burning. Trees were raked in September 1996 and prescribed burned in October 1996. Six years post-fire, 2 of the 

10 large raked trees had died compared to 6 of 10 unraked 

trees in the Lake burn unit. In the Lost Creek burn unit, no 

raked trees (5 total) had died compared to 3 of 5 unraked 

trees. Pre-fire basal litter and duff depths, post-fire scorch 

heights, and bark beetle activity were greater in dead trees 

than in live trees, and the authors reported that most dead 

trees had heavy surface fuel loadings near the tree.

Fowler and others (2010, 2007) designed a study specifically to examine the effectiveness of raking treatments on 

ponderosa pine mortality in northern Arizona in 2004. 

Trees were assigned one of four treatments in both burned 

and unburned units: (1) no removal (unraked), (2)rake forest floor 9 inches (23 cm) away from bole, (3) rake forest 

floor 3.3 ft (1 m) away from bole, and (4) blow forest floor 

9 inches (23 cm) (with leaf blower) from bole. All forest 

floor material was removed to mineral soil within 30 days 

of fall prescribed burns. Study trees were ≥ 18.1 inches 

(46 cm) DBH and had at least one measure of litter and 

duff ≥ 5 inches (13 cm) deep within 9 inches (23 cm) of 

bole before treatment. There were no woody fuels greater 

than 3 inches (8 cm) diameter around the first 3.3 ft (1 m) 

of the tree and trees showed no evidence of bark beetles, 

fire scars, dwarf mistletoe, or broken tops.

Forest floor removal by either raking or blowing around 

tree bases was effective at preventing cambium kill on the 

bole, and no differences were found among the removal 

treatments. Seventeen percent of the unraked, burned trees 

had areas of dead cambium, but there was no difference in 

mortality between raked and unraked trees 3 years postfire, as only 3 trees had died. Two of these were struck by 

lightning and the third, an unraked burned tree, was mass 

attacked by western pine beetle. This study demonstrates 

that removing duff and litter as little as 9 inches (23 cm) 

away from the bole is as effective as greater removal 

distances at preventing cambium kill during fire. While 

duff removal was effective at reducing cambium kill, it 

had no effect on tree mortality within 3 years post-fire. 

The authors recommended that duff removal efforts in 

northern Arizona be limited to large trees (>18 inches 

(46 cm) DBH) to prevent cambial kill or to protect high 

fire-risk trees, such as those with rotten fire scars, pitch 

seams, or large nearby stumps, as well as those growing 

in droughty microsites.

Hood and others (2007a) also studied the effectiveness 

of raking treatments on old ponderosa and Jeffrey pine 

tree mortality. The authors chose sites dominated by 

ponderosa and Jeffrey pine sites in northern California 

that had not burned in over 100 years (Taylor 2000). 

Three units (two burned and one unburned) were located 

on the Lassen National Forest (LNF) and two units (one 

burned and one unburned) were on Lassen Volcanic 

National Park (LVNP). Ponderosa and Jeffrey pine trees 

greater than 25 inches (63.5 cm) DBH with no sign of 

insect attack were chosen randomly throughout the 

units. One tree in each pair was then randomly selected 

for raking to mineral soil in the first approximate 2 ft 

(60 cm) around the tree base unless a fire scar was 
46 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

present. In this case, the tree with the fire scar was 

designated for raking. Raked material was spread out 

away and around the tree so as not to form a mound 

(Figure 27). Trees were raked in the late summer/fall 

of 2003 on the LNF units and in the late summer/fall 

of 2004 on the LVNP unit. The LVNP burn unit was 

prescribed burned in June 2005. Both LNF burn units 

were prescribed burned in October 2005. Raked areas 

were cleared of any newly fallen material since the first 

raking just prior to burning.

0 

5 

10 

15 

20 

25 

30 

0 5 10 15 20 25 30 35 

Time to Rake (minutes) 

Duff Depth (cm) 

Predicted 

lower CI 

upper CI 

Tr = 10.023 + (0.197*brush) + (0.396*duff depth) 

Average time to rake the duff away from the tree bole 

was 16 minutes/person. A crew of two or three people 

could clear duff and shrubs in approximately 6 minutes/

tree. The time required was dependent on the depth 

of litter and duff at the tree base (p<0.0001) and the 

amount of shrubs in the duff removal area (p = 0.0001) 

(Figure28). The presence of shrubs increased the amount 

of time necessary to clear the area to mineral soil by 

up to 10 minutes/tree.

Figure 27. Crews raking ponderosa pine basal litter and 

duff prior to prescribed burning.

Figure 28. Predicted time and 

upper and lower confidence 

intervals (CI) for one person to 

rake duff and litter to mineral soil 

2 ft (60 cm) away from ponderosa 

and Jeffrey pine tree boles when no 

shrubs are present in the removal 

area. Tr is raking time; brush is 

percent circumference of the raked 

area occupied by shrubs; duff 

depth is average duff depth (cm) 

immediately adjacent to tree base. 

From Hood and others 2007a. 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 47

Duff mound consumption was almost 100 percent in 

both LNF burned units (Figure 29). On the LVNP burned 

unit, duff consumption around the sample trees was 

lower and much more variable, with a median of 45 to 

70 percent consumed around the first 4 ft (1.2 m) of the 

tree bole (Figure 29). Average duff mound moisture at 

burn time for the LNF units was 24 percent compared 

to 101 percent for the LVNP unit.

Raking reduced the probability of red turpentine beetle 

attacks in the burned units. Though the number of trees 

attacked by western pine beetle or Jeffrey pine beetle was 

low, most of the attacked trees in the burned units had 

previously been heavily attacked by red turpentine beetle. 

This seems to indicate that burned trees with numerous 

red turpentine beetle attacks were susceptible to attacks 

by primary bark beetles. This finding was also reported 

by Bradley and Tueller (2001). It is unclear if the red turpentine beetles were attracted to the charring of the tree 

bole, the cambium injury, or both.

Raking decreased cambium injury by limiting heating at the base of the trees in the burned units. However, 

tree mortality was very low (13 of 380 trees; <4 percent) 

and there was no difference in tree mortality among the 

Figure 29. Average litter and duff consumption for unraked trees by study site. The Lassen National Forest (LNF) burned sites 

were combined for simplicity due to very similar results. LVNP= Lassen Volcanic National Park. Solid bars in boxes are median 

values and dots are 5th and 95th percentile outliers.

treatments 3 years post-fire (LNF units) and 4 years postfire (LVNP units). Raking did not appear to significantly 

reduce tree mortality in the study, but the authors cautioned 

that additional mortality could still occur and that trees 

need to be assessed for a longer period. They concluded 

that the decision to rake should be based on the management objectives for large trees in the prescribed fire area, 

current bark beetle activity, amount of duff around the 

large trees, and duff moisture prior to burning.

Five-year post-raking tree growth as determined by 

basal area increment (BAI) was compared between raked, 

unburned trees and unraked, unburned trees on the LNF 

unburned site (Noonan-Wright and others, in review). 

No differences in 5-year BAI (1.32 inches2

 unraked, 

1.30 inches2 raked (8.5 cm2 unraked, 8.4 cm2 raked); 

p = 0.871) or tree age (265 years for unraked, 267 years 

for raked; p = 0.525) were found between treatments. This 

suggests that raking alone does not stress the trees enough 

to influence growth or mortality.

Injury to raked and unraked old western larch trees 

was compared after a low-intensity fall prescribed burn 

in western Montana (Michael Harrington, unpublished 

data). Basal duff was very deep on unraked trees, and 
48 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

the fire consumed 100 percent of basal duff at 35 percent 

duff moisture content on all but one tree, with virtually no 

crown scorch. All unraked trees had some cambium kill, 

while raked trees had none. No tree mortality occurred 

within 5 years of burning. Large western larch have 

extremely thick bark, and cambium was killed primarily 

in bark fissures even though a significant portion of the 

bark was consumed. Also, western larch is not susceptible 

to any primary Dendroctonus beetles. These features, in 

addition to being deciduous, make western larch highly 

fire-resistant. This study suggests that removing deep basal 

duff around western larch trees may not be necessary prior 

to fall prescribed burning.

Longleaf pine trees with red-cockaded woodpecker 

(Picoides borealis (Vieillot)) cavities were raked to test 

the effectiveness of reducing tree mortality from fire at 

Eglin Air Force Base, FL (Williams and others 2006). 

Red-cockaded woodpecker colonies are typically in 

stands where fire has not been excluded, so basal duff 

accumulation is usually not a concern (K. Hiers, personal 

communication). Cavity trees have copious amounts of 

sap streaming down the bole, and the goal when burning 

is to not ignite the sap, which increases the probability of 

tree death. Therefore, treatments were designed to reduce 

fire intensity around cavity trees, not necessarily to reduce 

duff loadings. Trees received one of six treatments:

(1) clearing with hand tools and light raking,

(2) mechanical clearing only (performed with a DR®

mower),

(3) mechanical clearing and light raking,

(4) mechanical clearing and raking to mineral soil,

(5) burning out from the tree base prior to the actual 

burn, and

(6) control (burn-only).

Light raking consisted of removing all vegetation and 

the litter layer with fire rakes while leaving the organic 

duff layer intact. Deep raking removed all vegetation, 

litter, and the duff layer down to mineral soil. Protection 

treatments were applied from the bole to the dripline, an 

average of 10 ft (3 m) from the base of the tree. Mortality 

was significantly lower for treated trees (2.70 percent) 

compared to the burn-only treatment (6.18 percent) 1 year 

after burning. Among the protection treatments, mortality 

was lowest (0.86 percent) for trees receiving treatment (1), 

hand clearing and light raking, and highest (4.46 percent) 

for those receiving treatment (3), mechanical treatment and 

light raking. However, there was no significant difference 

in mortality among the protection treatments (Figure 30). 

Treatment implementation times varied from a minimum 

of 30 minutes/tree on average for mechanical clearing only 

to a maximum of 65 minutes/tree for burning around the 

tree prior to the broadcast burn (Figure 30).

Light and deep raking around longleaf pine were compared at the Ordway Biological Station, FL (M. Varner, 

personal communication). Light raking consisted of only 

removing litter approximately 3.3 ft (1 m) out from the 

tree base 1 to 2 weeks prior to prescribed burning. Deep 

raking removed all material to mineral soil from the bole 

out to the dripline. Mortality was low for both treatments; 

however, the light rake was much easier and quicker to 

implement. The prescribed fire only singed the remaining 

duff around the lightly raked trees and caused very little 

bark char (Figure 31).

Kolb and others (2007) and Perrakis and Agee (2006) 

recommend raking 1 or 2 years before burning if fine 

roots are growing in the duff. This may offset the immediate loss of fine roots by raking before any further loss 

occurs from prescribed burning, and it may encourage 

new roots to grow in mineral soil rather than in the lower 

duff. Ponderosa and Jeffrey pine stands were burned 1 and 

3years after raking with little tree mortality within 3 years 

post-fire (Hood and others 2007a). However, Fowler and 

others (2010) and Laudenslayer and others (2008) raked 

duff within approximately 1 month of prescribed burning 

and very little mortality occurred. More research about 

the timing of raking to burning treatments is needed to 

determine if waiting to burn is necessary after raking.

Raking around old ponderosa pine trees is becoming a 

common practice in some areas of the Blue Mountains, 

OR. Managers there are advised to rake orange, smoothbarked, ponderosa pine greater than 21 inches (53 cm) 

DBH and duff greater than 5 to 6 inches (13 to 15 cm) 

deep (D.Scott, personal communication). Large-diameter, 

young trees do not produce the exfoliating bark scales like 

old trees and typically do not have deep duff accumulations at the base, thus raking these is not usually a concern. 

Recommendations include raking trees to mineral soil, 

about 3 ft (1 m) out from the boles, using care to not create 

a berm of raked material around the trees. More recent 

recommendations state that leaving a couple of inches of 

duff at the tree base is satisfactory if the majority of the 

material is removed. A resting period of 1 to 2years between 

raking and burning is also advised (Scott 2002, 2005).

Leaf blowers are an alternative to removing duff accumulations around tree bases. Raking and blowing both 

clear litter and duff to mineral soil and produce no differences in treatment effects (Fowler and others 2010). Leaf 

blowers ease the task of material removal, disperse it more 

effectively, and often require less time to clear away the 

accumulated forest floor than raking, but shrubs in the 

duff mound area can reduce the leaf blower effectiveness 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 49

Figure 30. Comparison of protection effort requirement and 

percent mortality (for all trees) among five red-cockaded 

woodpecker cavity tree fire protection methods on Eglin 

Air Force Base, FL, 1 year post-fire. MC/DR = mechanical 

clearing/deep raking, HC/LR = hand clearing/light raking, MC/

LR = mechanical clearing/light raking, MC/NR = mechanical 

clearing/no raking. Different letters above treatment bars 

indicate significant differences at the alpha ≤0.05 level. From 

Williams and others 2006.

Figure 31. Longleaf pine with light raking treatment (litter removed approximately 3.3 ft (1 m) away 

from bole) after a prescribed burn on the Ordway Biological Station, FL. The fire barely carried over 

the raked surface and consumed no duff. Photo by Morgan Varner. 
50 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

(M. Harrington, personal communication). Two people 

are necessary to effectively use a leaf blower in areas of 

very deep duff. One person should first loosen the duff 

with a pitch fork, while a second person follows operating 

the blower (Figure 32).

Time required to rake duff away from trees increases 

with forest floor depth, heavy fuels, and shrubs (Figure 

28). Though raking is intensive and potentially costly to 

implement, it is a one-time cost. Raking pre-settlement 

trees is not necessary in subsequent burns. Scott and 

Spiegel (2007) reported an average raking cost of $16 to 

$20/tree for ponderosa pines in the Blue Mountains of 

Oregon. Basal duff depth and time to walk between trees 

will also impact treatment costs.

Duff removal around large-diameter and/or old trees 

allows managers to burn under a wider range of duff 

moisture scenarios without concern that the duff removal 

treatment alone will cause tree death. This is important 

because of the difficulty in predicting duff consumption 

in duff mounds based on pre-fire duff moistures.

Figure 32. (a) Loosening basal duff with a pitchfork and (b) using a leaf blower to clear 

loosened litter and duff away from the tree base and to spread material out before a 

prescribed burn. Photos by Michael Harrington.

(a)

(b)
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 51

In areas of deep duff, where the potential for basal 

cambium injury is high, raking minimizes injury to the 

tree bole near groundline from long-term duff smoldering (Fowler and others 2010; Hood and others 2007a). 

By reducing the residence time of the fire, the chance of 

cambium injury is reduced. Most studies examined duff 

mounds raked to mineral soil; however, this degree of 

removal may not be necessary. Raking the majority of the 

duff, but not to mineral soil, will prevent long residence 

times, reduce potential injury to roots in the mineral 

soil, and reduce the time required to rake. However, the 

exception is where fire scars exist on large-diameter or 

old trees. These scarred trees typically ignite with any 

flame contact, so duff removal to mineral soil adjacent 

to the scar is important. Raking is a viable option when 

there is concern that burning will cause large-diameter, 

old ponderosa, Jeffrey, and longleaf pine mortality. The 

effect of season on raking is not known. Most researchers 

have raked basal litter and duff in the fall when fine root 

growth is less active to minimize injury to roots growing 

in the lower duff layers.

The following raking techniques are recommended to 

remove forest floor accumulations from the bases of trees 

based on the available literature and expert knowledge:

• Rake the majority of litter and duff away from the 

tree base. Raking litter only is not advisable in areas 

with low summer precipitation because the duff 

will dry significantly and ignite without litter. In the 

southeastern United States, raking only litter prior to 

burning will reduce potential for fire spread around 

raked trees, thereby reducing basal injury; however, 

it will not ameliorate duff accumulation. Raking to 

mineral soil is not necessary except around fire scars. 

For trees without fire scars, leaving 2 to 3 inches (5 to 

8 cm) of duff is acceptable in the West, and leaving 

1 to 2 inches (3 to 5 cm) of duff is acceptable in the 

South.

• Remove litter and duff at least 9 inches (23 cm) away 

from the tree base. Expand raking to 3 ft (1 m) if shallow supporting roots are present. It is not necessary 

to remove material all the way to the dripline.

• Take care to spread raked material away from the 

tree in order to not create a new fuel mound around 

the tree.

• Rake during the fall or winter when fine root growth 

is minimal.

• If possible, allow at least one growing season between 

raking and burning to encourage new fine root development in the mineral soil on sites with numerous 

fine roots growing in the lower duff.

Snow well burning—Snow wells are areas immediately adjacent to trees that are free of snow, while snow is 

still present farther away from the tree base. Snow wells 

are usually about the size of the width of the tree crown 

(sometimes larger on steeper slopes and south and west 

aspects) compared to areas outside the tree wells that 

usually have a foot or more of snow. Snow well burning 

in the early spring when temperatures are below 40 °F (4 

°C) and lower duff moistures are greater than 100 percent 

can consume litter and upper duff layers while leaving 

the lower duff intact. This treatment has been tested in 

ponderosa pine stands in Idaho (Graham and Jain 2007). 

The required burning conditions in Idaho usually occur 

when snow is still present throughout the stand but when 

the forest floor around the tree bases is clear. However, duff 

moistures and temperatures during the burn are the key 

factors, not the presence of snow (T. Jain, personal communication). Snow well burning around old ponderosa 

pine has been successful at Ponderosa State Park, ID, 

since 1995 (R. Taplin, personal communication).

The primary objective of snow well burning is to 

gradually reduce duff depths in order to force fine roots 

growing in the lower duff layer to migrate back down into 

the mineral soil. The area can be broadcast burned once the 

amount of fine roots in the duff layer is reduced—usually 

1 to 2 years after the snow well burn. To burn the well, ring 

the tree with fire (Figure 33). Flame lengths are typically 

6 inches and burn for approximately 2 to 3 minutes before 

going out, depending on upper duff moisture (T. Jain, 

personal communication).

Figure 33. Snow well burning around ponderosa pine 

trees on the Boise Basin Experimental Forest, ID. Photo by 

Theresa Jain.
52 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Old ponderosa pine trees in the Boise Basin Experimental Forest, ID, were burned using the snow well 

technique followed by broadcast burning. This area 

had not burned in 100+ years. An unpublished fire 

history study of the area estimated a historic FRI of 

8 to 30 years (T. Jain, personal communication). The 

snow wells around selected trees were first burned in 

2002, and half of these were burned a second time in 

2004. Snow well burning reduced root concentrations 

in the duff layer compared to unburned control trees. 

The unit was then broadcast burned in the spring of 

2005 (Graham and Jain 2007). To date, approximately 

11 old trees in the 90-acre unit have died (approximately 

3 percent) (T. Jain, personal communication). In these 

cases, tree death was due to bark beetles and ignited fire 

scars that burned out the center of the tree (Graham and 

Jain 2007). A comparison was not made between snow 

well treated trees and broadcast-burn-only trees.

At Ponderosa State Park, burns are conducted when the 

duff is very moist, so little duff is consumed beyond the 

first 6 inches (R. Taplin, personal communication). Trees 

with fire scars are either packed with snow or are raked to 

remove duff from the scar area but not from the whole tree 

base. Snow well burning in the park reduces duff around 

trees by approximately 3 to 6 inches (7.5 to 15 cm). More 

consumption occurs on trees with larger snow wells, trees 

on south and west aspects, and during spring seasons with 

longer drying periods. Fire scars on unraked trees often 

ignite but can be extinguished by packing available snow 

in the scar area. Duff around the trees continues to break 

down or decompose over the summer, possibly due to 

warmer temperature from the blackened surface, nutrient release, or some combination of the two. Sometimes 

another 6 or more inches (15 cm) of duff decomposes 

after the initial fire treatment. Multiple snow well burns 

have reduced duff accumulations around some large 

ponderosa pine trees in the state park from over 20 inches 

(51 cm) down to mineral soil. Old ponderosa pine trees 

with very deep basal duff accumulations usually require 

two snow well burning treatments before conducting a 

broadcast prescribed burn, during which the remaining 

duff is expected to be consumed. Snow well burning in 

Ponderosa State Park is also used as a precautionary treatment to increase the probability of old tree survival in the 

event of a wildfire.

Benefits of snow well burning are: (1) only a few people 

are needed to conduct the burn, (2) firelines and suppression activities are not necessary, and (3) fire fighter 

safety concerns are minimal. The main drawbacks are 

the short window of adequate environmental conditions 

and uncertainty about the level of duff consumption. At 

Ponderosa State Park, adequate conditions occur every 

spring for about 4 to 7 days (R. Taplin, personal communication). Burning snow wells to gradually reduce duff 

layers should be tested in other areas to determine the 

treatment’s applicability outside of Idaho. For example, 

Haase and Sackett (1998) reported that for prescribed 

burns in giant sequoia-mixed-conifer forests, 100 percent of 

duff consumption usually occurs if fire can carry through 

surface litter, even if duff moisture exceeds 200 percent. 

The prescribed burns described by Haase and Sackett 

(1998) were not conducted to test snow well burning prescriptions. However, many of the burns were conducted 

under the high duff moisture conditions recommended 

for snow well burning, and high duff moisture slowed the 

combustion process, but did not stop it.

Mixing—Mixing the litter and duff layers around 

tree bases may increase decomposition rates by allowing 

moisture and heat to penetrate through the forest floor 

(Graham and Jain 2007). The goal of mixing is to speed 

decomposition and to train fine roots to grow into the 

mineral soil, while broadening the prescribed burning 

window. Once the presence of fine roots in the duff layer 

is reduced, usually 1 to 2 years after mixing, the area 

can be broadcast burned. To mix, a hoe is used to break 

up and aerate the forest floor layers around a tree base, 

but material is left in place (Figure 34). This treatment 

was tested in ponderosa pine stands in the Boise Basin 

Experimental Forest, ID, in conjunction with the snow well 

treatments previously described. Mixing was applied to 

selected trees in 2002. Half of those were mixed a second 

time in 2004, followed by a broadcast burn in spring 2005 

(Graham and Jain 2007). Mixing required approximately 

Figure 34. Mixing duff mounds around a ponderosa pine 

tree on the Boise Basin Experimental Forest, ID. Photo by 

Theresa Jain.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 53

3 to 5 minutes/tree at the study site (T. Jain, personal communication) and reduced root concentrations in the duff 

layer compared to unburned control trees. Eleven old trees 

in the 90-acre unit have died (approximately 3 percent) in 

four growing seasons post-fire, due to fire scars igniting 

and bark beetles (Graham and Jain 2007; T. Jain, personal 

communication). A comparison between mixed trees and 

broadcast-burn-only trees was not made.

Hand lining—Digging a fireline, or lining, is commonly used around snags or large jackpots of fuel to 

prevent them from burning. However, lining around trees 

to prevent the forest floor from burning does not solve the 

problem of deep duff accumulations. Therefore, it is not 

recommended as a way to reduce large tree injury and 

mortality from prescribed burning.

Foam—Spraying fire-retardant foam is sometimes 

cited as a technique to reduce old tree mortality during 

prescribed fire (Arno and others 2008). It has more typically been applied to create temporary fire breaks during 

prescribed burns. The only known study to examine the 

effectiveness of this treatment in reducing cambium mortality was conducted by Ryan and Steele (1989) on leave 

trees during prescribed burns in mixed-conifer shelterwood harvests on the Priest River Experimental Forest, 

ID. Though foam statistically reduced cambium mortality, 

the difference in mortality rates between foam-treated, 

burned trees and untreated, burned trees was less than 6 

percent. A key deficiency is that foam is only effective for 

a relatively short time after application, usually less than 

one hour depending on weather conditions (Schlobohm 

1995). Therefore, timing between application and ignition is critical and may be logistically difficult (Ryan and 

Steele 1989). If spraying, foam should be applied on the 

duff mound around the tree, not on the bole. The problem 

of heavy duff accumulations is not abated by spraying 

foam around individual trees. It may only prevent fire 

from burning surface fuels around selected trees during 

the broadcast prescribed fire. Therefore, it is not recommended as a way to reduce large tree injury and mortality 

from prescribed burning.

Water—Water has been successfully used to extinguish 

smoldering duff at the base of trees in small stands of 

longleaf pine (Kush and others 2004). Copious amounts 

are required to stop smoldering—enough to thoroughly 

saturate the area surrounding the tree base, because once 

duff begins to smolder it can be extremely difficult to 

stop. Duff can still smolder, even when no smoke is visible. Therefore, this method requires intense monitoring 

after the burn to make sure enough water is applied and 

smoldering is extinguished. Another concern is that fire 

fighters must be within the burn perimeter shortly after 

ignition, which exposes them to heavy smoke and the 

potential of falling snags.

This treatment is extremely labor-intensive and is probably only applicable to small areas or around unique trees 

with good accessibility. For example, hundreds of gallons 

of water were applied around the Nation’s largest western 

larch tree in Montana (http://www.americanforests.org/

resources/bigtrees/index.php) prior to burning the stand in 

order to limit forest floor consumption (S. Hood, personal 

observation, 2003). Another example is old-growth longleaf 

pine stands. These are very rare, and efforts to increase 

tree survivorship during fire can be justified that are not 

feasible in other areas.

Fire shelter wrap—Fire shelters wrapped around 

the bases of old trees is another technique that has been 

proposed to reduce old tree mortality during prescribed 

fire (Arno and others 2008). However, no studies exist 

regarding the effectiveness of this treatment, and it is not 

recommended as an option for reducing old tree mortality. Fire shelter material is expensive and wrapping tree 

bases is time-intensive. This treatment could prevent bole 

heating during passage of surface flames, but that is not 

normally an issue for fire-adapted thick barked trees. 

Fire shelters would do nothing to prevent lower stem 

heating from smoldering ground fires unless all duff was 

cleared to ground level during shelter placement. Unless 

the shelter material was placed in contact with mineral 

soil, deep duff accumulations could still girdle the tree 

directly at the ground line. In deeply fissured trees, shelters 

can create air currents between the tree and shelter that 

cause fire columns to funnel up through the shelter. This 

may actually increase stem heating (M. Varner, personal 

communication).

Mechanized equipment—Mowers were tested around 

red-cockaded woodpecker cavity trees prior to burning 

as a protection treatment in Florida (Williams and others 

2006; see Raking for additional detail). Red-cockaded 

woodpeckers typically are not found in long-unburned 

longleaf pine forests, and the trees in this study did not have 

much duff accumulation at the bases (K. Hiers, personal 

communication). The mowing treatment was intended 

to reduce fire intensity by reducing midstory oaks and 

other hardwoods around a specific cavity tree so that the 

sap on the tree bole would not ignite. Mowing alone does 

not mediate basal duff accumulations. Trees were mowed 

either using a DR® mower alone or in conjunction with a 

raking treatment. One year after burning, burn-only trees 

had higher mortality rates than those receiving a protection 

treatment. However, no difference in mortality was found 

among the different protection treatments (Figure 30). 
54 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

The authors concluded that mowing alone was the most 

cost effective treatment. It took 30 minutes/tree on average, 

required fewer personnel, and was faster than other protection treatments; all of which reduced costs and allowed 

more trees to be treated prior to burning. Mowing in late 

autumn remained effective for 6 months. When mowing, 

it is important to avoid damage to the bole of trees with the 

mower head or to fine roots by setting the blade too low. 

Mowers are not appropriate for use in wetland areas or in 

areas inhabited by the flatwood salamander (Ambystoma 

cingulatum Cope) (Williams and others 2006). Mowing 

around cavity trees is now the preferred preparation treatment at Eglin Air Force Base, FL. Managers there prepare 

approximately 2000 red-cockaded woodpecker trees a year 

on the largest tract of longleaf pine in existence. Mowing 

to reduce fire intensity is an appropriate option in longunburned stands if used in conjunction with other duff 

abatement treatments (for example, burning under high 

duff moisture, raking, or blowing).

Using dozers to remove fuels around tree bases is not 

recommended. This treatment is hard to apply without 

causing injury to roots growing near the soil surface 

and duff interface from the dozer blade. Dozers have 

caused extensive mortality around longleaf pine redcockaded woodpecker cavity trees (D. Wade, personal 

communication).

Management Implications

Management options included in this section provide 

background information, guidance, and precautions for 

prescribed burning areas with deep basal duff. Basal duff 

less than 2 inches (5 cm) deep in the southeastern United 

States (K. Hiers, personal communication) and less than 

5 inches (13 cm) deep in the western United States at the 

base of mature trees are generally not considered hazardous. In these cases, prescribed burns can be conducted in 

accordance with locally accepted methods, objectives, 

and prescriptions.

Management options exist for areas with deep duff, but 

are limited. The provided decision key (Figure 35) highlights the best available treatment options based on project 

scale. Options are: (1) burning when the basal duff layer 

is very moist during the dormant season and (2) reducing 

basal duff around individual trees by physical removal. 

The available burning window is narrow for prescribed 

burning when basal duff is very moist. It is also much 

more difficult to predict actual duff consumption, making it harder to achieve burn objectives. More overstory 

tree mortality should be expected if no individual tree 

treatments are implemented before broadcast burning. 

All individual tree treatments are labor-intensive and 

require extra time to implement before broadcast burning. 

However, individual tree treatments widen the broadcast 

burn prescription window and can be completed years 

before broadcast burning. They should be considered 

additional tools in the manager’s toolbox when concerns 

exist that standard prescribed burning techniques will not 

meet objectives.

Thinning from below, followed by activity fuel treatments, will also reduce competition and increase water 

and nutrient resources available to old trees. Mechanical 

thinning can quickly manipulate stand structures to more 

closely resemble historical stand conditions and to be more 

resilient to future disturbances.

These efforts must be couched in the larger perspective 

of the importance of maintaining and perpetuating old trees 

on the landscape and with the realization that no action 

will likely result in significant tree mortality in forests 

that historically burned frequently. Acceptable levels 

of old tree mortality will vary by location and species. 

Places where little mortality is acceptable will warrant 

more intensive treatments. The high value of old trees on 

certain landscapes, especially at historically significant and 

high-use recreation sites, and the length of time required 

to produce large and old trees merit strong consideration 

of using the unconventional burning and individual tree 

treatments previously described. Some may hesitate to 

use a novel strategy to reduce overstory tree mortality 

when reintroducing fire to long-unburned areas because 

of the increased treatment costs or logistical difficulties. 

But remember that an individual tree treatment need only 

occur once to initially reduce the deep duff layer, and then 

regularly scheduled maintenance burning can be conducted 

without supplemental treatment.

Monitoring the Effects of Fire on 

Overstory Tree Mortality __________

Monitoring is fundamental to successful land management programs. Without monitoring and treatment 

documentation, it is extremely difficult to understand the 

relationship between treatments and subsequent effects. 

This understanding leads to adaptive management, under 

which treatments are continually refined to better achieve 

management objectives (Mayfield and Smith 2008). It is 

important to distinguish between a record of post-burn 

observations and a true monitoring program. Monitoring 

requires pre-fire and during-fire measurements in order 

to relate the pre-fire conditions and silvicultural treatment 

to post-fire outcomes. When the primary objective is to 

reduce overstory tree mortality, long-term monitoring is 

key, as mortality may not occur until several years post-fire.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 55

Figure 35. Decision key of treatment options when reintroducing fire to long-unburned forests to reduce overstory tree 

mortality. Treatment options apply to forests that historically burned frequently. See text for detailed treatment descriptions. 

This section is not a how-to guide to build and implement 

a monitoring program, and it does not describe the different sampling methods. Many such guides are available 

(Elzinga and others 1998; Lutes and others 2009; Lutes 

and others 2006; U.S. Department of the Interior, Fish and 

Wildlife Service 1999; U.S. Department of the Interior, 

National Park Service 2001). This section provides general 

information about appropriate monitoring timeframes and 

sampling techniques pertinent to the objective of limiting overstory tree mortality from prescribed burning in 

historically fire-frequent forest types.

What to Monitor?

Pre-burn and post-burn fuel loadings, weather and 

moisture conditions at the time of burn, fire behavior, 

tree characteristics, post-burn tree injury, resident insect 

populations and post-burn attack densities, and tree status 

should be evaluated during monitoring sessions to determine if tree mortality related objectives are met. Without 

one of these pieces, the burn and subsequent fire effects 

on tree mortality cannot be accurately connected. The 

following variables should be documented to determine 

tree mortality from first-entry prescribed burns:

• Fuel loadings: pre- and post-fire duff depth at base 

of trees desired for retention

• Weather: temperature, relative humidity, windspeed, 

time since last precipitation

• Fuel moisture: percent litter, duff, and fine fuel moisture at base of trees desired for retention
56 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

• Fire behavior: flame length, rate of spread

• Tree characteristics: DBH, species, condition

• Tree injury: percent crown volume or length killed. 

Basal injury is also helpful if feasible to assess.

• Post-fire insect attacks: species, intensity of attacks 

(mass or stripped)

• Tree status: alive or dead

How to Monitor?

Monitoring overstory mortality from fire where heavy 

duff accumulations exist calls for slightly different strategies than standard monitoring techniques. The specific 

trees of interest must be evaluated and monitored over 

time in order to know if objectives are being met. Fixed 

area tree plots are best, but often include few large or old 

trees because of generally wide spacing between these 

trees. Three options to resolve this issue in order to most 

efficiently collect data on the trees of interest are: (1) install 

more plots, (2) install larger area plots, and (3) monitor 

individual trees.

Installing more or larger plots will increase the likelihood of capturing a greater number of larger and/or older 

trees. Fixed area plots will most accurately describe forest stand conditions, as tree density and basal area of all 

size classes are assessed. To save time, apply appropriate 

break-point diameters and plot sizes to most effectively 

sample the trees of interest. For example, if an objective 

is to limit mortality of trees >20 inches (>50 cm) DBH to 

10 percent, set a break-point diameter of 20 inches (50 cm) 

DBH and only sample trees larger than this in the largest plot. The plot size should include at least one tree of 

interest per plot. If this criterion is not met, the plot size 

should be increased. Data on smaller trees can be collected 

on a smaller plot nested inside the larger plot. This way 

data on all size classes are collected, but less time is spent 

collecting data on the smaller trees.

Alternatively, individual trees can be monitored. Tagging 

and tracking individual trees ensures that information 

on the trees of most interest is collected, and it is a good 

way to increase sample size of these trees. However, it 

does not provide data on forest structural conditions. A 

mix of fixed area plots supplemented with individual tree 

monitoring may often be the most efficient way to collect 

the most pertinent data related to the identified prescribed 

burn objectives.

Fuels data are most commonly collected using the planar intercept method (Brown 1974). Though appropriate 

for describing stand-level fuels, site-level fuel transects 

will not provide information about fuels near large or old 

trees. Measure duff depths and collect moisture samples 

near the base of the sample trees (identified during plot 

establishment) to best describe conditions affecting individual trees. Document where data was collected to help 

clarify stand-level versus individual tree-level effects.

How Long to Monitor?

Mortality from low-intensity, high-severity prescribed 

burns may not occur until several years after the burn and 

is often from the secondary effect of bark beetle attacks 

(Swezy and Agee 1991). Most studies of post-fire mortality have reported less than 5 years of results (Table 5), but 

long-term studies show that significant mortality may 

go unreported if not monitored for longer periods. For 

example, after prescribed burning old ponderosa pine in 

Oregon, 75 percent of the trees died 2 to 4 years post-fire 

(J. Agee, personal communication) and 35 percent died 3 

to 4 years post-fire (Agee and Perrakis 2008). Mortality in 

the spring burns had returned to the control unit levels 4 

years post-fire. Mortality in the fall burns was still slightly 

elevated, but was steeply declining. In another study in 

Crater Lake National Park, most small-diameter and 

younger trees died within the first year after a prescribed 

fire, while the majority of larger and older ponderosa and 

sugar pine died between 4 and 8 years post-fire. Larger, 

older white fir died mostly 8 to 13 years after the fire 

(Agee 2003; Thomas and Agee 1986). Drought and bark 

beetles were thought to significantly influence long-term 

post-fire tree mortality; however, it was not possible to 

know the exact impact of these second order fire effects 

and fire-caused tree injury. Mortality of old ponderosa 

pines at the Chimney Spring study site in Arizona began 

approximately 1.5 years after an initial prescribed burn 

in a stand that was long-unburned. Twenty years after the 

initial burn and other subsequent burns, 39 percent of the 

old trees had died compared to 16 percent in the control 

(Sackett and others 1996). The researchers attributed the 

higher mortality to the heavy fuel consumption at the base 

of the trees during the initial burn.

The above examples highlight the importance of longterm monitoring to determine prescribed fire effects on 

overstory tree mortality. In these cases, the fact that the 

prescribed burn tree mortality exceeded objectives would 

have gone undetected had monitoring been conducted for 

only a few post-burn years.

So, how long should prescribed burns be monitored? 

The first evaluation should occur within 1 year of the prescribed fire to capture first order fire effects. The timing 

of subsequent evaluations to capture bark beetle attacks, 

delayed tree mortality, and other second order fire effects 

will vary by location and forest type. In general, 3-year 

and 5-year post-burn monitoring is recommended. Filip 

and others (2007) recommended monitoring for at least 
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 57

5 years to determine post-fire delayed tree mortality. If 

mortality has not stabilized between the 3- and 5-year 

assessments, then monitoring should continue. Developing 

an appropriate monitoring schedule should be an adaptive 

management process, whereby monitoring programs are 

continually evaluated to determine if they are capturing 

the treatment effects of interest.

It was only by repeated monitoring and adaptive management that delayed tree mortality was observed in some burn 

units after prescribed burning at Grand Canyon National 

Park, AZ. One of the park management’s prescribed 

burning objectives is to limit overstory tree mortality 

to 20 percent as measured 5 years post-fire (Kaufmann 

and Covington 2001). Selected prescribed burns in the 

park appeared to meet this objective, but Kaufmann and 

Covington (2001) advised longer-term monitoring due to 

sustained higher levels of pre-settlement tree mortality in 

burned areas compared to unburned areas.

Knowledge Gaps ________________

This synthesis reports the current state-of-knowledge 

for reintroducing fire into long-unburned forests while 

limiting overstory tree mortality. However, there are still 

many unknown answers to the multitude of managerial 

questions on this topic. A literature search for research 

on old or large-diameter tree mortality in fire-dependent 

U.S. forests yielded 41 studies on the topic (Table 5). A 

cursory review of the table shows how limited the research 

is on relating post-fire tree mortality to first order fire 

injuries, pre-fire fuel loading, and bark beetle attacks. 

Many of these studies had primary objectives other than 

overstory tree mortality; however, Table 5 makes clear the 

need for long-term studies that document pre-fire forest 

and fuel conditions, fire and silvicultural treatments, and 

post-fire effects.

Research provides managers with statistically tested, 

peer-reviewed results. In the absence of applicable research, 

the expertise of experienced managers is often the best 

resource for deciding what treatments work on the local 

level. As these managers retire or change positions, local 

monitoring programs become even more important for 

determining appropriate treatment actions. Monitoring 

programs help bridge the gap between scientific research, 

local experience, and operational land management programs, especially where little to no applicable research 

exists for a given topic or region.

Abundant room exists for more research on limiting 

overstory tree mortality from prescribed fire in longunburned forests. Research topics include but are not 

limited to the following:

• Define the relationship of time-temperature profiles 

of soil heating to actual root mortality.

• Characterize deep duff moisture-of-extinction limits 

for all fire-dependent forests.

• Determine the feasibility and parameters of reducing only a portion of deep basal duff layers during 

prescribed burns in the western United States.

• Determine critical microsite characteristics and 

parameters that affect basal duff consumption and 

potential cambium injury for fire-dependent species 

and overstory size classes.

• Determine if season of raking or timing of raking to 

prescribed burning affects tree mortality.

• Correlate level of cambium injury to insect attack 

level.

• Determine the horizontal and vertical distribution 

and abundance of fine roots adjacent to the tree bole 

for a variety of sites and species.

• Conduct long-term studies on the effects of fire on 

old trees and other ecosystem components.

Climate Change

Restoring fire to fire-adapted ecosystems is important 

even in the face of climate change. Expert consensus suggests that fire frequency will increase as temperatures rise. 

Therefore, improving ecosystem resilience to fire makes 

sense. Restoration efforts that increase forests’ resistance 

to wildfire, insects, pathogens, and invasive species will 

also increase resilience to these stresses under a changing 

climate (Fulé 2008).

The future effects of climate change on fire-adapted 

forests are certainly open to conjecture. Tree mortality 

across the western United States has increased over the past 

50 years, likely due to increased temperatures and subsequent drought stress associated with climate change (van 

Mantgem and others 2009). However, fire and other disturbances have shaped these forests for millennia. Forests 

that more closely resemble historical reference conditions 

seem most likely to survive increased fire intensities and 

frequencies (Fulé 2008). Therefore, carefully restoring fire 

to long-unburned forests that historically burned frequently 

will reduce accumulated fuel and duff, retain old trees, 

and perpetuate these fire-dependent forests.
58 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

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USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 67

Appendix A: Determining Duff Moisture _________________

Duff moisture is probably the most important predictor of duff consumption; therefore, knowing 

duff moisture before prescribed burning helps determine if conditions are right to meet objectives. Tracking duff moisture levels prior to burning and then monitoring duff consumption after 

the burn allows for better prediction of future burn outcomes. Unfortunately, obtaining duff 

moisture is not nearly as straightforward as measuring other fuel components. Duff moisture 

meters for estimating volumetric duff moisture exist; however, currently there is no accurate 

way to immediately estimate gravimetric duff moisture in the field. 

Gravimetric Versus Volumetric Moisture Content

Gravimetric moisture content is the mass of water per unit mass of dry soil. This is most frequently used by the fire management community because it is the easiest, most direct measurement of moisture content. Woody fuel moistures are always reported as gravimetric moisture 

content. The calculation is as follows:

×!"" − =

'() !"#$%&

*&+!"#$%& '() !"#$%&

-("./#&+(/, *&/-0+

*&/-0+ *&/-0+ '344 #1/!+3(&,12+&2+ (eq. 1)

Volumetric moisture content is the volume of water per unit volume of soil. Bulk density of 

the sample must be calculated to determine volumetric water content. Bulk density of the duff 

sample is calculated as follows:

!"#$%&

'() !"#$%&

*+%,#&

-&./01 3,%4'&2!.1) = (eq. 2)

Moisture probes and meters calculate only volumetric moisture content. Gravimetric moisture content can be calculated from volumetric moisture content and bulk density using the 

following equation:

!"#$%&

'(%)#&*+,-

.+"',#&*+,- 2)%3/&0!,*1

/)44 #(,!*)+&-(0*&0* /)44 #(,!*)+&-(0*&0* = (eq. 3)

Sampling location and collection

It is important to sample duff near the base of large trees where heavy fuel has accumulated 

and tree mortality is a concern. In deep duff, collect the sample from the lower portion of the 

profile, being careful to not include mineral soil. A large handful of material is sufficient. For 

oven-drying, place the sample into an airtight container for transport to drying and weighing 

facilities. Duff samples taken to determine bulk density should also be collected at the base of 

large trees. For bulk density samples, measure duff depth around a sampling frame and then 

collect all duff within the frame. Volume is calculated from the dimensions of the frame and 

duff depth. 

Duff moisture meter 

The duff moisture meter (DMM600) is a portable, battery-powered device that gives an immediate reading of volumetric duff moisture content (Robichaud and others 2004). The DMM600 

consists of a cylinder that houses the electronics, sample chamber with a compression knob, 

and LCD readout (Figure 36). To measure duff water content, place the sample in the sample 

chamber and turn the compression knob until an audible indicator signals the sample is properly 
68 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

compressed and the measurement is complete. Total time for measurement is about 30 seconds. 

Readings are displayed in real-time only; measurements are not stored. The included sieve fits 

in the opening of the sample chamber and helps to break up large fragments and improve measurement accuracy in duff materials with a large range of fragment sizes. The meter’s standard 

calibration converts the output of the measurement circuit to volumetric water content. This 

factory-supplied calibration is derived from laboratory measurements of duff moisture content 

from four forest cover types: Douglas-fir, western larch, lodgepole pine, and Engelmann spruce/

subalpine fir. User-derived calibrations are possible using the included PCDMM software. If 

gravimetric moisture content is desired, the bulk density of several duff samples should be 

calculated first (eq. 2). Using the average bulk density, gravimetric moisture content can easily 

be calculated in the field from the duff moisture meter readings (eq. 3) (Robichaud and others 

2004). A duff moisture meter costs $1,950.

Advantages: quick; easy; determines moisture in field

Disadvantages: gives volumetric moisture content; more expensive than oven-drying; requires 

calculation of bulk density to determine gravimetric moisture content

Oven-drying

Oven-drying duff samples provides the most accurate measurement of gravimetric moisture 

content. Weigh sampled duff before drying to determine the weight of the wet sample. Place 

duff samples in paper bags or in other permeable, oven-safe containers and dry samples for at 

least 24 hours, or until weight stabilizes in a 212 oF (100 oC) oven. Weigh oven-dried sample and 

calculate gravimetric moisture content using equation 1. Norum and Miller (1984) provide more 

detailed instructions for collecting, drying, and weighing fuel to determine moisture content. 

Basic drying ovens cost around $400 and balances are around $200. Ovens stuffed full will 

not completely dry in 24 hours. 

Advantages: most accurate measurement; gives gravimetric moisture content

Disadvantages: slow—requires 1 to 2 days to calculate; sample is invalid if any precipitation 

has occurred between sampling and the prescribed burn

Figure 36. Duff moisture meter (DMM600).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 69

Moisture probes

Time domain reflectometry (TDR) moisture probes give an immediate reading of volumetric 

moisture content. There are several different designs based on either two or three probes that 

are inserted into the duff to provide a moisture reading. These probes were originally designed 

to estimate soil moisture. They do not work well in non-homogenous, low bulk density materials such as duff (Ferguson and others 2002; Robichaud and others 2004). Ferguson and others 

(2002) used moisture probes to generate a moisture index of prescribed burn units prior to burning. They cautioned that moisture probes should be inserted with minor disturbance, remain in 

place, and be calibrated once installed when determining moisture in organic soils. Calibration 

requires repeated sampling of a known volume of duff, then drying and weighing the samples to 

calculate volumetric moisture content. This setup is more appropriate for research burns when 

detailed moisture trends and values are needed. Miyanishi and Johnson (2002) used a simpler 

setup of a three-pronged moisture probe connected to a portable, battery-powered meter (Delta-T 

Devices, Ltd. Cambridge, U.K.) to determine multiple duff moisture readings in a unit prior to 

burning. However, no calibrations using oven-dried samples were made to determine accuracy 

of this sampling method. Moisture probes are not recommended to estimate duff moisture for 

typical field use because they are extremely dependent on contact along probe length and it is 

hard to achieve consistent readings.

Large scale indices

The National Fire Danger Rating System (NFDRS) (Deeming and others 1977) and the KeetchByram Drought Index (KBDI) (Keetch and Byram 1968) are tools to predict fire intensity and 

fuel availability on a large scale. The NFDRS is a risk rating of fire potential throughout the 

United States. NFDRS areas are typically greater than 100,000 acres (40,468 hectares) using 

weather observations from one time during the day at one location. The system was designed 

for low-resolution, medium-to-large scale applications (Bradshaw and others 1984). KBDI uses 

measures of evapotranspiration and precipitation to estimate cumulative moisture deficiency in 

deep duff or upper soil layers that relates to the flammability of organic material in the ground. 

The purpose of the drought index is to provide managers with a continuous scale of reference 

for estimating deep-drying conditions in areas where such information may be useful in fire 

planning and preparedness levels (Keetch and Byram 1968). Some duff consumption studies 

have related the NFDRS 1000-hour fuel moisture estimate to duff consumption (Brown and 

others 1985; Sandberg 1980). While these rating systems provide valuable information about 

fire potential, general trends, and long-term drought, they are not appropriate to predict localized duff mound moisture and consumption.
70 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010

Appendix B: Survey respondents ______________________

John K. Agar Wenatchee National Forest, WA

James K. Agee University of Washington, WA

Andy Aldrich Stanislaus National Forest, CA

James Bennington Camp Grafton Training Center-Army, ND

Bernie Bornong Bighorn National Forest, WY

Tim Brickell Gallatin National Forest, MT

Beth Buchanan National Forests in North Carolina, NC

Larry Burd Sequoia National Forest, CA

Jason Butler Boise National Forest, ID

Kelly Cagle Uwharrie National Forest, NC

Carol Carlock Humboldt-Toiyabe National Forest, NV

Jonathan L. Casebeer Department of Military Affairs, IL

Gerald Chonka Gunnison National Forest, CO

Chris Church Boise National Forest, ID

J. Allison Cochran National Forests in Alabama, AL

Blaine Cook Black Hills National Forest, SD

Diane Cote Manti-La Sal National Forest, UT

Scott Dailey Tahoe National Forest, CA

Dennis Divoky Glacier National Park, MT

Gabe Dumm Umpqua National Forest, OR

Rich Fairbanks California Nevada Region, The Wilderness Society, CA

Calvin Farris National Park Service, Klamath Network, OR

Roger D. Fryar Ozark-St. Francis National Forest, AR

Peter Fulé Northern Arizona University, AZ

Bill Gabbert Sagacity Wildfire Services LLC, SD

Allen Gallamore Colorado State Forest Service, CO

Sarah Gallup Colorado State University, CO

Todd Gardiner San Juan National Forest, CO

Bruce Greco Coconino National Forest, AZ

Steve Hanna Sequoia and Sierra National Forest, CA

Joseph Harris Red Lake Department of Natural Resources, Forestry, MN

Hylton Haynes MTC - Fort Pickett, VA

Reed Heckly Umatilla National Forest, OR

Jennifer Hensel Lassen National Forest, CA

Dan Huisjen BLM, Montrose Interagency Fire, CO

Theresa Jain U.S Forest Service, Rocky Mountain Research Station, ID

Jason Jerman Idaho Panhandle National Forest, ID

Dale Johnson BLM and Inyo National Forest, CA

Kim M. Johnson Bitterroot National Forest, MT

Mike Johnson Wallowa-Whitman National Forest, OR

Jeffrey Kane Humboldt State University, CA

Tobin Kelley U.S Forest Service, Rocky Mountain Research Station, MT

Alan Kelso Cibola National Forest, NM

Eric E. Knapp U.S. Forest Service, Pacific Southwest Research Station, CA

Thomas E. Kolb Northern Arizona University, AZ

Mike Landram U.S. Forest Service, Region 5, CA

George M. Libercajt Fremont-Winema National Forest, OR

W. Scott MacDonald Mt. Hood National Forest, OR

Steve Martin Lewis and Clark National Forest, MT

Rob Martinez Helena National Forest, MT
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-238. 2010 71

Bob Means Wyoming BLM, WY

Anne Mileck Modoc National Forest, CA

Lauren B. Miller Wasatch-Cache and Uinta National Forests, UT

Dave Mills Kaibab National Forest, AZ

Paul S. Minow Rio Grande National Forest, CO

Steve Mooney Fremont-Winema National Forest, OR

Caroline Noble National Park Service, Southeast Region, GA

Shilow T. Norton Apache-Sitgreaves National Forest, AZ

Tonja Opperman Bitterroot National Forest, MT

Kara Paintner Fire Management Program Center and Natural Resource Program

Center, National Park Service, CO

Ed Paul Prescott National Forest, AZ

Todd Pechota Black Hills National Forest, SD

Stephen Pietroburgo Little Pend Oreille National Wildlife Refuge, WA

Alicia Reiner U.S. Forest Service, Enterprise team, CA

Michele Richards Fort Custer Training Center, MI

Peter R. Robichaud U.S Forest Service, Rocky Mountain Research Station, ID

Kevin Ryan U.S Forest Service, Rocky Mountain Research Station, MT

Kristen Sanders BLM, ID

Richard Taplin Ponderosa State Park, ID

B. Walker Thornton Coconino National Forest, AZ

Meg Trebon Okanogan and Wenatchee National Forests, WA

Eric Trimble Colville National Forest, WA

Jamie Tripp-Kahler Flathead National Forest, MT

Russ Truman Kaibab National Forest, AZ

Phillip van Mantgem U.S. Geological Survey, Sequoia and Kings Canyon Field Station, CA

Morgan Varner Humboldt State University, CA

Scott Wagner San Juan National Forest, CO

Jon Warder Bighorn National Forest, WY

Gary A. Weber U.S Forest Service, Coeur d’Alene Dispatch Center, ID

Scott Weyenberg Mississippi National River and Recreation Area, MN

Andrew White San Isabel National Forest, CO


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