Pacific Southwest 

Research Station

General Technical 

Report

PSW-GTR-241

February 2014

United States Department of Agriculture 

Forest Service

Fuel Reduction Practices and 

Their Effects on Soil Quality

Matt D. Busse, Ken R. Hubbert, and Emily E. Y. Moghaddas
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Authors

Matt D. Busse is a research soil scientist Ken R. Hubbert and Emily E. Y. 

Moghaddas were formerly research ecologists, Pacific Southwest Research Station, 

1721 Research Park Dr., Davis, CA 95618. Emily E. Y. Moghaddas is currently the 

Natural Resource Program Manager for the U.S. Air Force at the Kedena Air Force 

Base, Okinawa, Japan.

Cover photos: Upper left—Matt Busse, middle left—and lower left—Bob Carlson, 

right—Carol Shestak.
i

Abstract

Busse, Matt D.; Hubbert, Ken R.; Moghaddas, Emily E. Y. 2014. Fuel reduction 

practices and their effects on soil quality. Gen. Tech. Rep. PSW-GTR-241. 

Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest 

Research Station. 156 p.

Soils sustain our terrestrial ecosystems, help fuel plant growth, and govern key 

ecosystem services such as the storage and provision of clean water, degradation 

of toxic compounds, and regulation of atmospheric gases. Preserving the integrity 

of soil thus is an earnest responsibility of land stewardship in the United States. 

This report provides a synthesis of soil chemical, biological, and physical responses 

to various prescribed fire and mechanical thinning practices and offers practical 

considerations for use in fuel reduction planning. A wide range of current topics, 

identified in a nationwide survey of natural resource managers, is discussed in 

detail: (1) ecological consequences of prescribed fire on soil heating, water repellency, and soil nitrogen release; (2) whole tree harvesting and nutrient removal; (3) 

soil compaction; (4) masticated fuel beds; (5) pile burning; (6) cumulative effects 

of fire and thinning; (7) coarse woody debris; and (8) soil in a changing climate. 

We submit that with thoughtful planning and implementation, reducing fuels while 

proactively managing our soils can be complementary outcomes.

Keywords: Prescribed fire, mechanical thinning, compaction, soil fertility, soil 

productivity.
ii

Contents

1 Introduction

3 Wildfire Trends and Fuels Reduction Practices

9 National Survey of Land Managers—Soil Issues and Concerns

12 Prescribed Burning

14 Thinning

15 Thinning and Burning

18 Ecological Effects and Management Considerations

18 Prescribed Fire

20 Soil Heating 

36 Soil Water Repellency

39 Soil Nitrogen 

48 Repeated Burning

52 Pile Burning

61 Coarse Woody Debris

64 Mechanical Fuel Reduction Treatments

65 Whole Tree Harvesting and Nutrient Removal

74 Soil Compaction

81 Masticated Fuels—New Practices, New Concerns?

88 Masticated Fuels—To Burn or Not To Burn?

91 Thinning and Burning—Early Results From Long-Term Study Sites

96 Soil In a Changing Climate

102 Conclusion

105 English Equivalents

105 Literature Cited

141 Appendix: A Soil Quality Primer
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Fuel Reduction Practices and Their Effects on Soil Quality

Introduction

Soils are exceedingly complex. They contain a rich array of minerals and organic 

compounds, chemical reactions, physical states, and biological diversity and are 

responsible for countless ecological functions that belie their common appearance 

and colloquial namesake (dirt). Most importantly, soils sustain life by providing 

plants with essential nutrients, water, and physical support. They also play a major 

role in providing a clean water supply, degrading toxic compounds, supplying novel 

antibiotics for human health, and circulating greenhouse gases. Thus, properly 

functioning soils are a prerequisite of terrestrial ecosystems and society alike, as 

discovered by several ancient civilizations that failed to value their soils, to their 

eventual dismay and ruin (Diamond 2005).

But what of fuel management practices—are they of sufficient severity to alter 

soil functions? If so, should our soils be protected unilaterally, or do the benefits 

of fuel reduction practices outweigh possible impacts to soil? With the variety of 

management practices (from benign to intensive) and soils (from highly buffered to 

fragile), these questions are best answered on a site-by-site basis. A good starting 

point then is to rank the effects of fuel treatments relative to other global pressures 

on soil. Severe changes in soil properties result most often as a consequence of land 

use change (e.g., clearing of forests for agriculture production), acute disturbance 

(e.g., chemical contamination) or by the four main pathways identified by Lal 

(1997): (1) erosion from insufficient ground cover, (2) compaction from mechanized 

equipment, (3) nutrient depletion from unsustainable harvesting, and (4) chemical 

degradation from salt-affected irrigation water. Fuel reduction using best management practices is not a classic fit in these categories; however, it can affect soil 

erosion, compaction, or nutrient availability in certain circumstances (fig. 1) (Grigal 

2000, Neary et al. 2005). 

A fundamental sidebar in discussing the effects of fuel reduction practices is 

the knowledge that soil is not renewable in our lifetime, and it typically requires 

hundreds to thousands of years to develop (Jenny 1941, van Breeman and Buurman 

2002). Thus, the consequences of inappropriate management, whether inadvertent 

or intentional, can be severe and have pushed land agencies to promote a conservative or protective approach to soil management (see USDA FS 2005). However, 

soil scientists also recognize that many soils are fully capable of recovering from 

disturbance given sufficient time (Lal 1993). Griffiths and Swanson (2001), for example, found substantial changes in soil biological properties following harvesting 

in an eastern Oregon conifer forest, yet full soil recovery to preharvest conditions 

occurred once the developing forest reached 15 to 40 years of age. This underscores 
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general technical report psw-gtr-241

a primary challenge in managing soils: How do we blend a conservative approach 

that protects the natural resource while also acknowledging the resilient ability of 

many soils to recover from disturbance?

Two additional issues come into play when assessing the effects of fuel reduction practices on soils. First, reducing fuels and restoring landscapes is an iterative 

process that requires multiple treatments throughout the life of a stand. The effect 

of repeated treatments on soil is not well studied, which moderates our confidence 

to properly manage soil. Or, as stated by Davis et al. (2010), “Small changes in soil 

properties as a result of soil disturbance that might seem scientifically insignificant at one point may become significant when the soil disturbance is repeated in 

multiple rotations as an acceptable management practice.” Second, the objectives 

for managing soils need to be clarified within a societal framework. Is soil strictly 

a medium for forest or agricultural productivity, or is its greatest value related to 

Figure 1—Comparison of pile burn (A) and machanical treatments (B) and their effect on soil. Low soil impact from small, widely 

spaced burn piles (upper left); high soil impact from severe burning of closely spaced piles (upper right); low soil impact from past 

thinning on frozen soil (lower left); high soil impact from past thinning on wet soil (lower right). 

Matt Busse

Carol Shestak Matt Busse

Emily Moghaddas

(A) Pile burning

(B) Mechanical treatment

Treatment severity 
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Fuel Reduction Practices and Their Effects on Soil Quality

water quality, biodiversity, carbon (C) sequestration, or a combination of these services? Burger et al. (2010) point out that these services are not necessarily complementary and that the best soil management practices for maintaining biodiversity 

in some forests may not be the best for maximizing tree growth or for reducing 

greenhouse gas emissions. In this regard, we canvassed resource managers and 

field experts in a nationwide survey for their definition of “soil productivity.” Fewer 

than half of the respondents (55 out of 124) asserted that soil productivity relates to 

the ability to support plant growth, while the others believed that the term applies to 

a broader context that includes water quality, erosion, and biodiversity. 

Assessing the health of soils must be based on objective scientific analyses of 

changes in soil properties relative to pretreatment conditions. This task is made 

difficult by the wide variation in soil properties across the United States and, at 

times, by the unpredictable nature of responses of soil to fuel treatments. With this 

in mind, our purpose in this synthesis is to identify (1) what constitutes a healthy 

soil (one that is resilient and easily restored following single or multiple treatments) 

and (2) the environmental conditions, soils, and treatments that may lead to adverse 

damage.

Wildfire Trends and Fuel Reduction Practices

A century-long U.S. policy of fire suppression and exclusion has disrupted the 

natural fire regime of many ecosystems, resulting in excessive accumulation of 

dead vegetation, increased stand densities, and a shift to plant species that are not 

adapted to fire. Thus the paradox of successful fire exclusion: the more efficient at 

suppressing wildfires we become, the larger the wildfire problem becomes (Brown 

and Arno 1991) (fig. 2). Early logging practices have added to the problem by 

Figure 2—Wildfire burning through Wharton State Forest in southern 

New Jersey. 

Curt Hudson, The Associated Press
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general technical report psw-gtr-241

Figure 3—Total area burned by wildfire in the United States from 1960 through 2010. Data are 

from the USDA NIFC (2010).

removing large, fire-resistant trees, resulting in extensive forests of smaller trees 

(Agee and Skinner 2005). In the contiguous United States, 36 to 81 million ha (90 to 

200 million ac) are estimated to have unnaturally dense fuel accumulations (USDA 

GAO 2007). Additionally, warmer temperatures, lower precipitation, and reduced 

snowpack since 1986 have increased the duration and intensity of the wildfire 

season in the Western United States (Westerling et al. 2006). 

Area burned by wildfire peaked in the United States in 2006 (fig. 3), with the 

number of hectares burned generally greatest in Western and Southern States. 

Wildfire area from 2004 through 2007 reached 2.8 to 3.6 million ha (7 to 9 million 

ac) and suggests a baseline for fire activity that includes increased “megafires” and 

extreme wildland-urban interface (WUI) incidents. With climate change and urban 

expansion, this baseline may increase, and extreme WUI wildfires may become a 

more common occurrence. 

The extent of wildfire area differs greatly between the different regions of the 

United States and also between years in the same region. For example, the Rocky 

Mountain region approached 1.2 million burned hectares (3 million ac) in 2007, but 

dropped to >162 000 ha (400,000 ac) in 2008 (fig. 4). In the drought year of 2008, 

wildfires consumed >600 000 ha in the Texas/Oklahoma region as compared to 

about 40 000 ha in 2007. These year-to-year shifts across regions are indicative 

of rapidly changing fuel conditions that accompany periods of drought, unusual 

precipitation patterns, or recent fuel reduction practices and wildfires. 
5

Fuel Reduction Practices and Their Effects on Soil Quality

Most of the acreage burned in the United States occurs in just a few wildfires 

that burn under extreme conditions (The Brookings Institution 2005). These 

large “megafires,” along with extreme fires at the WUI, have resulted in a recent 

dramatic rise in fire suppression costs (fig. 5). Megafires often burn during severe 

fire conditions (periods of prolonged drought with large accumulations of dead 

and live biomass) and exhibit extreme fire behavior characteristics. Annual fire 

management activities (mainly fire suppression) accounted for only 13 percent of 

the U.S. Department of Agriculture Forest Service (USFS) total budget in 1991, but 

increased to 48 percent by 2009 (Ingalsbee 2010) as the annual cost of federal fire 

suppression surpassed $1.5 billion beginning in 2002 (fig. 5). In 2006, 20 wildfires 

on USFS land alone cost $500 million, or nearly one-third of the year’s expenditures on fire suppression (Ingalsbee 2010). Although megafires represent <1 percent 

of all wildfires, they account for about 85 percent of total suppression-related 

expenditures on federal lands (The Brookings Institute 2005).

Nationwide efforts to reduce the cost of fighting wildfires and to restore the 

function and health of our wildlands have relied heavily on the use of prescribed 

burning (understory burning, slash burning, pile burning), various mechanical 

Figure 4—Wildfire acreage compared across different regions in the United States for 2007 

and 2008. Data are from the USDA NIFC (2010).
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general technical report psw-gtr-241

Figure 5—Wildfire and fuel reduction area in the United States and their estimated costs from 2001 

through 2008. Data are from the USDA NIFC (2010). Costs include all federal agencies. Matt Busse Matt Busse

Matt Busse

Figure 6—Examples of fuel reduction methods: (top left) prescribed fire, (top right) piling and burning, (bottom left) 

thinning, and (bottom right) masticating.

Bob Carlson
7

Fuel Reduction Practices and Their Effects on Soil Quality

practices (thinning, masticating/chipping, mowing), and their combinations (fig. 6). 

Collectively, these treatments reduce wildfire hazard (fuel load and fuel continuity), limit fire intensity and rate of fire spread, restore forest health, protect life 

and property, provide defensible space that aids fire crews in protecting communities, and protect environmentally sensitive areas from catastrophic wildfire. Fuel 

reduction programs and the selection of preferred treatment practices differ among 

federal agencies as a consequence of their unique land stewardship missions. For 

example, both the USFS and the Bureau of Land Management use combinations of 

prescribed burning and thinning to support their multiple-use policy. In contrast, 

the mission of the Fish and Wildlife Service focuses on the conservation of wildlife 

habitat, and thus they primarily conduct prescribed burns to improve habitat. For 

like reasons and because fire is a natural process, prescribed burning is the preferred fuel treatment of the National Park Service. An alternative approach to these 

fuel reduction approaches is to allow fire to resume its natural role. Allowing wildfires to burn as a resource benefit, a policy formerly known as Wildland Fire Use 

(WFU) or the “Let Burn” policy, is the structured management of naturally ignited 

wildfires with the goals of restoring natural fire regimes and reducing heavy fuel 

accumulation (USDA and USDI 2005). In 2005, there were 96 000 ha (237,000 ac) 

of WFU across all agencies, a drop from 198 000 ha (489,000 ac) in 2005 (USDA 

NIFC 2010). The effects of this wildfire practice on soil resources have not yet been 

adequately studied and will not be a topic of this synthesis report. 

A comparison of federal lands treated by prescribed burning versus mechanical 

treatments is shown in figure 7. Fire is the most common fuel reduction practice 

throughout the United States, with the highest acreages in 2008 found across the 

country’s southern perimeter. Use of mechanical treatments was more common 

in the Western United States with >280 000 ha (>700,000 ac) treated in 2008. We 

presume year-to-year treatment of acreage is highly variable owing to changing 

weather conditions for fire prescriptions, budget constraints, and wildfire activity. 

Fuel reduction practices can affect a multitude of soil properties, which poses 

an interesting question for land managers: Is there a subset of soil properties that 

can be monitored that is affordable, easy to sample and analyze, and ecologically 

meaningful? This issue has been debated among soil scientists for decades without 

clear resolve (see the appendix for a more detailed discussion of soil quality principles). Further, soil properties most indicative of detrimental changes differ between 

fuel reduction practices, making comparisons among treatment types problematic. 

Indicators based on soil physical properties (porosity, water infiltration, soil 

strength, compaction) are most commonly used to identify soil changes following 
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mechanical treatment. Prescribed burning, on the other hand, results in soil chemical and biological effects that include oxidation of surface and soil organic material, 

changes in nutrient availability and pool size, changes in pH, and lethal heating to 

biota and fine roots. A simple, one-size-fits-all soil measurement that accurately 

documents change does not exist. 

Wildlands are not fireproof. Fuels will continue to accumulate and wildlands 

will burn. Climate change likely will alter the majority of ecosystems in the United 

States, dramatically changing their structure, composition, and distribution of 

species, and thus enhance the role wildfire will play in these systems (Chambers 

and Pellant 2008). Consequently, the need for fuel reduction programs will continue 

to grow, as will the importance of managing these programs with soil quality and 

conservation in mind. In this report, we offer an indepth examination of 12 soil-fuel 

reduction issues that were identified as key informational needs in a nationwide 

survey of resource managers. In addition, a detailed introduction to forest soil quality principles is presented in the appendix. 

Figure 7—Regional acreage treated by prescribed burning and mechanical thinning in 

2008, excluding state and private lands. Data are from the USDA NIFC (2010).
9

Fuel Reduction Practices and Their Effects on Soil Quality

National Survey of Land Managers—

Soil Issues and Concerns

We surveyed natural resource managers and practitioners to assess current perceptions of fuel treatments and their potential for causing detrimental soil disturbance. 

Survey questions appraised (1) whether soil productivity is an important consideration of managers when conducting fuel reduction treatments, (2) which soil 

properties are of greatest concern, and (3) what unanswered questions or informational needs managers have regarding soils. Our intentions were to capture current 

attitudes toward soil management and to identify perceived knowledge gaps, and 

then to use the information gained in selecting topics for discussion in the succeeding section on ecological effects and management considerations.

A survey of government personnel representing the U.S. Department of Agriculture Forest Service (USFS); U.S. Department of the Interior Bureau of Land 

Management (BLM), National Park Service (NPS), Fish and Wildlife Service 

(FWS), and Bureau of Indian Affairs (BIA) was conducted in 2008. Survey recipients were limited to federal employees (owing to legal constraints) involved either 

directly or indirectly in fuel reduction projects as identified using government 

databases, workshop and seminar lists, and environmental document reports. We 

used SurveyMonkey,1

 a Web-based online survey program (http://www.surveymonkey.com), to distribute the survey to federal employees across all regions of the 

United States. 

Of 1,146 individuals surveyed, 209 responded (18 percent). Forest Service 

employees accounted for 78 percent of the responses, followed by BLM (11 percent), NPS (8 percent), FWS (3 percent), and BIA (1 percent). Fire-related disciplines accounted for the largest portion of respondents, followed by “soil” and 

“silviculture,” with respondents in all disciplines averaging greater than 15 years 

of related work experience (table 1). Regional response was evenly distributed 

across the Western United States, while participation from the Southern, Eastern, 

and Alaska regions was noticeably lower. 

Survey questions asked are as follows:

Question 1. How important is soil productivity when conducting fuel reduction 

treatments?

Most respondents defined soil productivity as the long-term ability of the soil to 

grow native vegetation (produce biomass), to support plant growth and community 

1

 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.
10

general technical report psw-gtr-241

composition, and to meet the requirements of sustainable ecosystems. About 8 out 

of every 10 respondents agreed that they take soil productivity into considerationprior to and following fuel reduction treatments (fig. 8). This high percentage was 

consistent among regions and disciplines, indicating a fairly universal and positive 

attitude toward soil productivity. 

Few respondents ranked soil as a primary concern, however. We provided six 

criteria for selecting a fuel reduction treatment: cost, effectiveness, ease of use, soil 

quality, environmental factors (such as wildlife, air quality or invasive species), 

and legal factors (such as the National Environmental Policy Act [NEPA] or litigation), and asked the respondents to rank each criterion from most important to least 

important when selecting fuel treatments. Treatments included prescribed/broadcast 

burning (understory or scattered slash), mechanical thinning, and all fuel reduction 

practices (thinning, fire, pile burning, masticating, and their combinations). 

Respondents ranked treatment effectiveness (35 percent response) and treatment 

cost (31 percent response) as the most important criterion when selecting broadcast 

burning as a treatment (fig. 9). Few ranked soil quality (3 percent response) as the 

most important criterion. Treatment effectiveness was also the main criterion (55 

percent response) given for conducting mechanical thinning treatments. For all fuel 

treatments combined, effectiveness (43 percent response) again was the most important consideration. Nearly all respondents ranked soil quality of low to moderate 

importance when selecting fuel treatments (fig. 10). 

Table 1—Number of survey respondents by discipline and region

Total Years Number of

number of experience respondents 

Discipline respondents (mean) Region by region

Fuel managementa 53 17 Pacific Southwest 35

Fire managementb 46 18 Pacific Northwest 34

Soil 36 19 Intermountain 29

Silviculture 30 25 Rocky Mountain 26

Plant, wildlife ecology 13 25 Northern 24

Natural resources 10 24 Southwest 19

Fire ecology 9 16 Southern 11

Hydrology 6 19 Eastern 12

Other 6 11 Alaska 3

a Fuel reduction focus.

b

 Fire prevention, detection, and suppression operations.
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Fuel Reduction Practices and Their Effects on Soil Quality

Figure 8—Survey response to the question, “Is soil productivity a consideration when conducting fuel reduction treatments?” (n = 153).

Figure 9—Survey response to the question, “What is the most 

important factor when selecting a fuel reduction practice?” 

(n = 131). 
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general technical report psw-gtr-241

Figure 10—Survey response to the question, “How important 

are soil issues when selecting a fuel reduction treatment?” 

(n = 131). 

Question 2. Which soil properties are of greatest concern when applying 

treatments?

We asked “What is your level of concern for various soil factors when conducting 

fuel reduction treatments (rating from 1 = high concern to 5 = low concern), and 

provided a comment box to allow each respondent to clarify their selections as 

needed. We assigned ratings 1 and 2 = “high concern,” rating 3 = “medium concern,” and ratings 4 and 5 = “low concern.” Soil factors included compaction, erosion, forest floor depth, soil organic material (SOM), soil heating, nutrient status, 

soil pH, microbial and faunal health (soil biota), and mycorrhizae. Treatments were 

grouped into three categories: prescribed burning (broadcast and pile), thinning 

(mechanical, chain saw, mastication), and their combinations. 

Treatments

Prescribed Burning 

Forest floor depth was the greatest soil concern among respondents when applying 

fire, with 64 percent rating it high, followed by concerns for SOM, soil heating, 
13

Fuel Reduction Practices and Their Effects on Soil Quality

Figure 11—Percentage of survey respondents expressing “high 

concern” for the effects of fire on selected soil properties. 

erosion, and nutrient status (fig. 11). Concern for litter depth appeared to be closely 

related to nutrient protection, bare soil for seedling germination, and erosion 

control. Concern was relatively low for soil compaction, soil pH, and soil biota. 

Some respondents noted that prescribed fire is rarely hotter than a wildfire and that 

effects to soil properties are usually low. Others commented that prescribed fire 

generally will not have soil heating issues, but there should be awareness of postfire 

wind and water erosion. Both broadcast burning and jackpot burning (igniting 

concentrated fuels) were generally perceived as having positive ecological effects. 

Ninety percent of fuel managers answered “yes” to conducting pile burns. 

Many commented that they preferred to burn piles during the late fall or winter 

when there is at least 2 to 3 inches of snow on the ground to help prevent fire escape 

and reduce mop-up time. Several respondents noted that air quality concerns and 

poor smoke dispersion makes winter pile burning contentious at times, however. 

Summer was the least preferred season for pile burning because of the high risk of 

fire escape, residual heat and soil damage, and greater mop-up efforts required. 

Respondents noted that hand piles are typically smaller than 3 m (10 ft) in 

diameter and burn for about 8 hours or less, whereas mechanized piles are generally 

greater than 3 m in diameter and burn for 8 to 24+ hours. The largest piles reach 
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about 21 m (70 ft) in diameter and over 4.5 m (15 ft) tall, and are typically composed of whole-tree harvested material and burned at landing sites. 

Soil heating was the major concern when selecting pile burning, with 70 

percent of the respondents rating it a high concern (fig. 11). Concerns for nutrient 

status, forest floor depth, and mycorrhizae followed with responses of 52, 48, and 

47 percent, respectively. Several respondents commented that pile-to-pile variability 

in burn intensity and duration played a significant role in the extent of soil heating. 

Although soil heating was a major concern, some respondents considered piles as a 

limited problem owing to the small area affected. However, others noted that they 

commonly have a large number of piles per acre. Postburn mitigation practices 

included (1) shoveling soil from outside the piles to recolonize the site and (2) using 

backpack blowers to spread the ash so that the site was colonized faster by native 

vegetation.

Thinning

Soil compaction was the greatest concern when using mechanical thinning, with 

71 percent of respondents rating it a high concern (fig. 12). Erosion received a high 

rating from 50 percent of the respondents. In contrast, concern for thinning effects 

on soil chemical and biological properties was generally low. Several respondents 

Figure 12—Percentage of survey respondents expressing “high concern” for the effects of thinning on selected soil properties. 
15

Fuel Reduction Practices and Their Effects on Soil Quality

commented that the size and type of equipment used when thinning are key considerations. Mitigating measures that were mentioned included (1) placing slash in the 

path of the equipment using cut-to-length forwarder/harvesters to lessen detrimental compaction, and (2) harvesting over snow to reduce soil impacts. 

There was little concern for detrimental soil effects owing to hand thinning 

(data not shown). The “low concern” response was expressed by about 70 to 80 

percent of respondents for all soil properties. Factors responsible for the lack of 

high concern included (1) no heavy equipment involved, (2) foot traffic is often on 

downed slash, and (3) most hand thinning is conducted on steep slopes.

Soil compaction was also a priority concern of 58 percent of the respondents 

for mastication treatments (fig. 12). Again, several respondents mentioned that the 

level of concern depended on the intensity of the treatment and size and type of 

equipment used. Most concerns focused on equipment and the amount of ground 

area covered when masticating vegetation. Mitigating measures that were mentioned included (1) using designated skid trails to reduce the amount of surface area 

affected, (2) masticating when soils were dry as soil moisture content greatly affects 

the impacts of compaction, (3) selecting the correct type of equipment for the job 

(4), using very low tire pressures, and (5) limiting the number of passes. There was 

some concern that deep residue layers, when present, would affect the soil carbon to 

nitrogen (C:N) ratio and related microbial processes such as nutrient mineralization.

Thinning and Burning

Soil properties that received the highest concern when combinations of thinning 

and burning were considered included compaction, erosion, forest floor depth, 

SOM, and soil heating (fig. 13). A number of respondents noted that fuel reduction 

treatments on federal lands must adhere to regulations specified by NEPA. They 

noted that resource specialists must evaluate the effects a proposed action will have 

on soils, and prescriptive treatments are recommended that meet local or regional 

soil standards and guidelines. Thus, treatment prescriptions typically focus on 

limiting soil compaction, soil erosion, litter depth, and achieving a coarse woody 

debris (CWD) target. 

Several respondents mentioned that improved methods for posttreatment validation monitoring are needed. In this regard, most respondents noted that posttreatment soil monitoring was conducted. Visual observation of ground cover following 

prescribed fire and soil compaction following mechanical thinning were the most 

popular monitoring measures. However, monitoring time periods were relatively 

short, most lasting less than 1 year. 
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Figure 13—Percentage of survey respondents expressing “high 

concern” for the effects of thinning and burning on selected soil 

properties. 

Question 3. What unanswered questions and information needs do you have?

To identify additional concerns that were not addressed in the survey questions, we 

asked, “Do you have any soil concerns specific to your region that have not been 

discussed?” Below is a summary list of responses by topic. In several cases, the 

information needs were repeated by respondents across regions.

Fire:

• What is the impact of pile burning on long-term soil productivity?

• Does fire sterilize soils? How much soil heating is acceptable?

• What soil temperatures during burning promote the germination of native 

plants and discourage the germination of invasive plants? 

• What temperatures result in increased water repellency?

• What is the effect of fire on soil fertility and its relationship to invasive 

plants?

• How do riparian soils respond to fire?

• What is the effect of burning on soil pathogens?
17

Fuel Reduction Practices and Their Effects on Soil Quality

Thinning:

• Does whole tree harvesting affect soil productivity? How much harvest 

slash should be retained to balance concerns for soil productivity versus 

those for wildfire hazard?

• How do soils differ in their response to compaction, rutting, or 

displacement?

• How severe is the loss of soil nutrients and carbon when thinning to presettlement conditions?

• What are the short- and long-term effects of mastication on soil temperature, moisture, and productivity? Can the depth of masticated residues 

affect soil productivity?

• Is harvesting in riparian areas detrimental to soil productivity?

• How damaging is harvesting on slopes >30 percent?

• What are the best mitigative practices to limit soil damage during 

harvesting?

General:

• What is the erosion potential of various soils following the removal of forest 

floor or understory vegetation cover? 

• Do fuel reduction treatments detrimentally affect water infiltration, storage, 

and transmission in soil?

• How much CWD is needed to maintain soil productivity?

• Are soil quality standards for fuel reduction treatments overly restrictive? 

Improved definitions and thresholds for detrimental soil disturbance are 

needed, as are improved mitigative measures to protect soil.

• What are the long-term effects of fire exclusion on soil productivity, 

organic matter content, and erosion?

• What are the best practices in landslide-prone areas?

• How can we best protect organic soils from excessive rutting when harvesting, from rapid oxidation when dry or exposed, or from consumption when 

burned?

In conducting this survey, we hoped to capture current managerial perceptions 

of fuel treatments and their potential for causing detrimental soil disturbance. We 

used this information and the questions put forward as the foundation of this report, 

and attempt to answer many (but not all) of these issues in the next section. 
18

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Ecological Effects and Management 

Considerations

Here we address the major soil and environmental concerns voiced in the survey 

results. Treatment impacts on soils are explored and input to help managers weigh 

the risks and benefits of fuel treatment options is provided. The topics range widely 

and are separated into two themes for simplicity—prescribed fire and mechanical 

fuel reduction—acknowledging that some overlap exists between themes. It is not 

our intent to provide a comprehensive assessment of all soil issues (see Neary et al. 

(2005) for a general review of fire and soil); instead we offer interpretive synthesis 

of research findings and also discuss important knowledge gaps associated with the 

following topics: 

Prescribed fire: 

• Soil heating 

• Soil water repellency 

• Soil nitrogen (N) 

• Repeat burning 

• Pile burning

• Coarse woody debris

Mechanical fuel reduction:

• Whole tree harvesting and nutrient removal

• Soil compaction

• Masticated fuels—new practices, new concerns?

• Masticated fuels—to burn or not to burn

• Thinning and burning–early results from the Fire and Fire Surrogate (FFS) 

study

• Soils in a changing climate 

Prescribed Fire 

Prescribed fire is an effective tool for reducing fire hazard and restoring forest 

health by replenishing available soil nutrients, stimulating plant growth, improving wildlife habitat, and maintaining biological diversity (Reinhardt et al. 2008). 

Examples include broadcast or underburning (surface fires of low to moderate fireline intensity that leave the overstory essentially intact), postharvest slash burning, 

and machine and hand pile burning (fig. 14). Dr. Harold Biswell helped reintroduce 

fire in the 1940s using controlled burning in longleaf pine (Pinus palustrus Mill.) 
19

Fuel Reduction Practices and Their Effects on Soil Quality

Bob Carlson

Figure 14—Prescribed fire examples; burning of understory fuels (top left), jackpot burning of slash (top right), pile burning of thinning slash (bottom left), burning of masticated fuels (bottom right). 

forests of the Southeastern United States (Biswell 1989). After moving to the west 

coast, he questioned the policies behind fire suppression and began promoting the 

use of prescribed fire in the oak and pine forests of northern California (van Wagtendonk 1995). However, his work remained underappreciated, and complete fire 

exclusion was practiced by land managers until the early 1960s (van Wagtendonk 

1995). Prescribed fire became more acceptable with the Wilderness Act of 1964, 

which identified the need for fire as part of a natural wilderness ecosystem. The 

necessity for prescribed fire became more apparent in the 1990s with the encroachment of communities into forests resulting in catastrophic Wildland-urban interface 

fires. Air quality concerns (smoke and haze) and risk of fire escape somewhat constrain the use of prescribed burns at present, especially in highly populated areas. 

Still, the use of prescribed burning greatly exceeds that of mechanical thinning as 

a fuel reduction treatment in the Southern United States, whereas total acreage for 

mechanical thinning surpasses that of prescribed burning on federal lands in the 

Western United States (see fig. 7). 

Montana Department of Natural 

Resources and Conservation 

Montana Department of Natural 

Resources and Conservation 

Eric Knapp
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Soil Heating

Key points:

Ecological effects Management considerations

• Mineral soil is a poor conductor of heat.

Soil damage (if any) is usually limited 

to the top few centimeters for all but 

high-severity burns.

• Encourage low- to moderate-severity

burning and incomplete forest floor

consumption to avoid mineral 

soil damage.

• Soil moisture is a critical factor

controlling heat transfer.

• Burn when soils are moist (>20

percent by volume) when possible 

to limit heat penetration.

• Comparatively low soil heating results

in fine root mortality. 

• Generally this is not a problem for

low- to moderate-severity burning as 

soil temperatures are not excessive

• Burn when forest floor layers are

moist (>65 percent) when possible or 

avoid burning if heavy root proliferation is detected in the duff layer.

• Alternatively, mixed-severity burning

with attending root mortality may 

be appropriate to reestablish forest 

heterogeneity.

• Smoldering of thick duff layers greatly

extends heat duration and may kill roots 

and affect tree vigor.

• Raking of litter and duff from the

base of old-growth trees may protect 

against fire-related damage.

• Soil micro-organisms are killed across a

wide range of temperatures. However, 

even severe burning does not sterilize soil

• Expect short-term changes in microbial function and diversity after 

severe burning. Long-term effects, 

although possible, are difficult to

prove and are not a driver of management decisions.

• Soil carbon and organic matter losses

from mineral soil are nominal unless 

burning is sever.

• Monitoring of soil chemical properties may be warranted if extensive 

areas of severe burning are anticipated or for nutrient-poor sites.

• Ephemeral increases in soil pH,

calcium, magnesium, and potassium 

result when fuel consumption is high.
21

Fuel Reduction Practices and Their Effects on Soil Quality

Why soil heating is of concern— 

All methods of prescribed burning generate soil heat which—under certain circumstances—can alter soil properties and functions. Changes in soil fertility, organic 

matter content, water infiltration, soil mineralogy, and nutrient availability are 

among the many potential responses to burning that may either benefit or degrade 

soil quality. And although only 8 to 10 percent of the heat produced during a fire is 

radiated downward to the soil (Hungerford 1989), soil temperatures can still surpass 

250 °C during broadcast burning (Haase and Sackett 1998) and approach 500 °C 

during pile burning (Roberts 1965). Indirectly, fire affects short-term microbial 

processes such as nutrient immobilization, mineralization, and nitrification by 

raising soil temperature and reducing soil moisture. Hillslope erosion and leaching 

losses of fire-released soil nutrients may further result during postfire rain events. 

Wind erosion and its ability to severely degrade soil quality is another indirect 

effect, although it is typically associated with wildfire and not fuel reduction burning (Bormann et al. 2008). Thus, there are many incentives for understanding and 

developing an awareness of soil heating effects (box 1).

Threshold temperatures offer insight into the potential problems associated 

with soil heating. These temperatures have been established in controlled experiments for many soil properties and functions (table 2), and are best thought of as 

“ballpark” values as they have not been exhaustively tested across a variety of 

soils. Nevertheless, the values show that several key biological properties such as 

micro-organism, root, and seed survival are vulnerable at reasonably low soil temperatures, whereas changes in soil physical and chemical properties require higher 

temperatures that are more typical of severe burning. In fact, the high temperature 

threshold for the volatilization of many nutrients (775 to 1960 °C) is rarely, if ever, 

reached in mineral soil for even the most severe of burns.

Note that some of the threshold values encompass a wide temperature range, 

making definitive statements about fire effects and soils difficult. For example, the 

lethal temperature for bacteria ranges tremendously owing to physiological differences among bacterial species (e.g., differences in cell wall structures and sporeforming abilities confer a variety of levels of heat tolerance). A wide temperature 

range is also reported for soil carbon (C) and N volatilization, although the majority 

of loss is thought to occur near the upper limit of their temperature ranges (Badia 

and Marti 2003, Guerrero et al. 2005). Additionally, the threshold values can vary 

considerably depending on soil water content at the time of burning as discussed 

below.
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Box 1

Does Severe Burning Sterilize Soil?

Soils often take on 

a moonscape-like, 

deathly appearance 

after severe burning. 

A reddish-orange 

coloring, symptomatic 

of extreme heating 

and oxidized organic 

matter, is not uncommon following burning 

of large slash piles 

(landing piles) or where 

large downed wood has 

been consumed slowly 

during wildfire. The damage is obvious to 

trained and untrained eyes alike. But are these 

soils sterile, without life? And if so, will life 

return? The answer to these questions lies in 

how we define the term sterile. To those meaning devoid of plant life, the answers are yes. 

Severe burning easily produces temperatures 

in surficial soil that kills the entire seedbank 

(>120 °C). Plant life will take several years 

to recolonize these soils, eventually aided by 

wind deposition of seeds from adjacent vegetation. Still, sterilization is relatively 

brief given this definition. A different answer results if we use the definition of 

“sterile” provided by the dictionary (“free from living organisms”). Countless studies 

have shown the dogged ability of some soil microbial species to survive severe 

burning (see Hebel et al. 2009), and suggest that the survivors may even flourish 

in the postfire environment. In fact, to kill all soil micro-organisms takes exacting 

conditions that are met typically by pressure cookers and laboratory autoclaves (121 

°C with steam or 650 to 1,000 °C with dry heat), but that are not attained in the soil 

profile even during the most severe of fires. Further, the recolonization process by 

surviving micro-organisms and by those organisms disseminated on the soil surface 

by wind or water begins almost immediately following fire.

Does severe burning sterilize soil? No. Life persists. But this is a technical 

answer and it does not speak to the multitude of soil changes that may be caused by 

severe burning. Loss of organic material, increased soil erosion, changes in surface 

physical and chemical properties, and alteration of long-term soil productivity are 

at risk. In the case of soil micro-organisms, severe burning does not eliminate their 

presence, but it does dictate which species remain and which species are consumed. 

Not sterile for sure, but of such concern that efforts to limit the damaging effects of 

severe burning are a long-standing objective of forest stewardship. Brett Goforth Brett Goforth

Dec. 2003 Dec. 2005 Brett Goforth
23

Fuel Reduction Practices and Their Effects on Soil Quality

Table 2–Examples of threshold temperatures for soil physical, chemical, and 

biological propertiesa

Soil property Threshold (°C) Source

Low

Microbial death:

Bacteria 50 to 400 Hungerford et al. 1991

Nitrifying bacteria 75 to 140 DeBano et al. 1977

Vesicular arbuscular mycorrhizae 94 Klopatek et al. 1988

Seed mortality 50 to150 Beadle 1940

Fine-root mortality 40 to 70 Zeleznik and Dickman 2004

Soil water loss 60 to 100 Hungerford 1991

Moderate

Particle aggregation 

(e.g., clay conversion to sand) 200 to 500 Terefe et al. 2008

Carbon and organic matter oxidation 200 to 500 Raison et al. 1985

Nitrogen volatization 300 to 500 Hungerford et al. 1991

Soil structure and aggregate 

stability loss 300

Amino acid loss 350 Hungerford et al. 1991

Water repellency >270 to 300 DeBano and Krammes 1966

Ectomycorrhizal fungi death 100 to 155 Dunn et al. 1985

High

Nutrient volatization:

Potassium 775 Raison et al. 1985

Phosphorus 775 Raison et al. 1985

Calcium 1240 to 1485 Raison et al. 1985

Manganese 1960 Raison et al. 1985

Magnesium 1107 DeBano 1991

Sulfur 375 to 900 Tiedemann 1987

Clay structure transformed 

or destroyed >550 Certini 2005, Douglas 1986, 

Ketterings et al. 2000

a

 These values are not absolute and can vary depending on to the heterogeneity of soils in natural systems.
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Applying the principles of thermal conductivity and heat capacity— 

Soil heat transfer during burning is a complex process involving a long list of soil 

physical properties (e.g., moisture, texture, porosity, density, particle contact), fuel 

characteristics (e.g., mass, moisture, surface area, structural arrangement), and 

fire attributes (e.g., rate of spread, fire weather). Scientific advances in predicting soil heating have been considerable, particularly through the development and 

validation of heat transfer models (e.g., Aston and Gill 1976, Campbell et al. 1995, 

Enninful and Torvi 2008, Massman and Frank 2004, Preisler et al. 2000, Steward 

et al. 1990). Most theoretically based models (e.g., FOFEM, http://frames.nbii.gov/

metadata/tools/FOFEM_5.7.html) rely heavily on mathematical derivations of soil 

thermal conductivity (ability to transfer heat) and heat capacity (amount of energy 

required to increase soil temperature). How these physical principles of heat transfer 

are influenced in practice by soil depth, moisture, and texture are discussed below. 

Soil depth—Soil is not a good conductor of heat. Consequently, both peak temperature and heat duration decline precipitously with soil depth during burning. Figure 

15A illustrates this, as a stair-step decline in the heat pulse is shown with increasing depth from the soil surface. Indeed, most studies show that damaging soil heat, 

even during severe burning, is usually limited to a fairly thin layer of surface soil 

(Busse et al. 2005, Massman and Frank 2004, Monsanto and Agee 2008). The reason for this lies in the extensive array of pores found throughout the soil matrix, 

which trap heated air and reduce heat penetration, analogous to home insulating 

materials. Heat transfer is thus relatively low in soil as the thermal conductivity of 

porous materials (like soil) is far lower than it is for solid materials. Smoldering 

fires in thick duff layers or large-diameter wood are important exceptions to this 

rule as trapped heat can lead to considerable soil heat penetration (Haase and 

Sackett 1998, Hebel et al. 2009, Hood 2010). 

Soil water—Soil water is considered the most important factor controlling heat 

transfer in soil (Frandsen and Ryan 1986, Hartford and Frandsen 1992). This is 

demonstrated in figure 15, where heat penetration is dramatically lower in moist 

soil than dry soil for a comparable fuel load and fuel consumption. Similar observations led Busse et al. (2010) to recommend burning when soil volumetric moisture 

content was 20 percent or greater as a means to limit soil heating beneath large fuel 

loads such as masticated fuels or slash piles. The basic principle is that soil water 

sharply increases the heat capacity of soil, resulting in a high-energy requirement to 

evaporate water prior to any substantial increase in soil temperature. Thus, although 

heat travels faster in moist soil than dry soil based on the principles of thermal 

conductivity (Jury et al. 1991), its movement is restricted by the energy-quenching 

effect of water. 
25

Fuel Reduction Practices and Their Effects on Soil Quality

Figure 15—Heat pulse in dry versus moist soil during burning of masticated fuels. Soil moisture contents are 5 percent for dry 

soil and 27 percent for moist soil. Only soil moisture was varied, as experimental conditions included the same fuel loading 

(169 Mg ha-1), fuel consumption, and soil type. Adapted from Busse et al. 2005.

Soil texture—Coarse-textured soils (sands) have greater thermal conductivity than 

finer textured soils because of their low porosity and high surface contact between 

particles (Aston and Gill 1976). Consequently, temperatures and heat penetration 

are theoretically greater in sandy soils than in loam or clay soils of similar rock 

content during burning. However, there is little experimental evidence to support 

this theory. In fact, Busse et al. (2010) found that heat transfer was similar across a 

range of soil textures during burning. Soil moisture far outweighed the role of soil 

texture as a determinate of heat transfer in their study. 

Organic matter content and physical compaction are confounding factors when 

assessing the role of texture in soil heat transfer. For example, the heat pulse into 

soils rich in organic matter from surface fires can be substantially reduced because 

of the high water-holding capacity and low thermal conductivity of organic matter. Organic soils (e.g., peat soils in the Southeast or Alaska) will burn intensively, 

however, if ignited when dry. Additionally, compaction can dramatically increase 

maximum soil temperatures and heat duration during burning (Busse et al. 2010). 

Compaction increases the thermal contact between soil particles and reduces the 

air-filled pore space, resulting in a gain in soil thermal conductivity.

How hot does it really get?— 

This is not an easy question to answer because variation is the rule of thumb. For 

example, Haase and Sackett (1998) showed that soil can surpass lethal temperatures 

for more than 20 hours when thick forest floor layers are completely consumed by 

prescribed burning. In comparison, Trammell et al. (2004) found that temperatures 

never reached 50 °C in the surface mineral soil even when temperatures in the litter 

layer exceeded 500 °C in hardwood forests of central Kentucky. Shea (1993) found a 
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similar lack of heat transfer during fuel reduction burning in young ponderosa pine 

(Pinus ponderosa Lawson & C. Lawson) stands as shown in figure 16. 

Many researchers have measured soil heat profiles during prescribed burning; 

their findings are summarized in table 3. Although the findings are far from complete, especially regarding the variety of fuel loadings and fuel moisture contents 

typically encountered when burning, several generalizations can be made:

• Forest underburning produces minimal soil heating except in areas where 

duff layers are completely consumed. Therefore, detrimental heat damage 

should not be expected in most cases.

• Small slash piles result in moderate soil heating in the surface 5 to 10 cm (2 

to 4 in). However, the range in reported temperatures does not suggest any 

major changes in soil properties with the exception of potential root, seed 

bank, and microbial mortality. See section on pile burning for more details.

• Large slash piles and, in particular, those containing a high proportion of 

large-diameter wood result in high soil temperatures and long heat durations. Detrimental heating effects on soil properties should be expected in 

the top 10 cm (4 in) or more. 

• Grassland fires produce nominal soil heating. The dominance of fine fuels 

in these systems ensures that burn duration time is generally low and soil 

temperatures are minimal.

Figure 16—Nominal heat penetration in soil during spring prescribed underburning in ponderosa pine forests. The burns 

were low to moderate intensity (0.6- to 1.0-m flame lengths) with fuel loads ranging from 88 to 113 Mg ha-1. Fuel consumption ranged from 40 to 60 percent on a mass basis. The coarse-textured pumice soils were moist (30 to 40 percent 

gravimetric moisture) at the time of burning. Data adapted from Shea (1993). Matt Busse
27

Fuel Reduction Practices and Their Effects on Soil Quality

Table 3—Maximum soil temperatures recorded during prescribed burning in different 

ecosystem and fire types

Ecosystem and Fuel Maximum soil

fire type consumption Soil depth temperature References

Centimeters °C

Forest Low to moderate 2 25 to 40 Bradstock and Auld 1995, 

underburn (25 to 75% percent 5 20 to 30 Penman and Towerton 2008, 

duff loss) Shea 1993, Trammell et al. 

2004, Vines 1968

Complete 2 280 Haase and Sackett 1998, 

4 to 5 113 to 200 Neary et al. 2005

Forest slash pile:

Hand pile Complete 2 to 3 80 to 150 Beadle 1940, Floyd 1966, 

10 35 to 80 Meyer 2009

Tractor pile Complete 2 235 to 400 Massman and Frank 2004,

5 150 to 350 Shea 1993

10 225

Wood pile Complete 2.5 500 Roberts 1965 

5 314 to 340

10 190 to 230

Grassland Complete 2 36 Scotter 1970 

4 27

Chaparral Complete 2.5 88 to 199 DeBano et al. 1979 

5 35 to 75

We offer the following short synopses on root mortality, ectomycorrhizal fungi 

and soil organic matter (SOM) loss, and changes in soil pH and cations as examples 

of soil properties affected by low-, moderate-, and high-threshold temperatures, 

respectively. Fire-induced changes in soil N are discussed in more detail later in this 

chapter.

Root mortality—Prolonged soil heating kills fine roots and can injure large structural roots (Guo et al. 2004, Swezy and Agee 1991, Zelznik and Dickman 2004). 

By convention, most studies consider 60 °C as the lethal soil temperature for root 

mortality (Preisler et al 2000, Varner et al. 2009) even though root destruction has 

been measured at temperatures as low as 52.5 °C when the duration of heating is 

sufficiently long (Zelznik et al. 2004).

Based on the range of soil temperatures observed in literature (table 2), temperature thresholds for root mortality are generally not exceeded in surface mineral 

soil layers during low- to moderate-severity fuel-reduction burning. In comparison, 

excessive root damage is an ongoing concern in old-growth forests that have 

developed thick duff mounds following decades of fire exclusion (fig. 17). Swezy 

and Agee (1991) were among the first to show that low-severity prescribed burning 
28

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Figure 17—Deep duff mounds can result in lethal heating when 

burned.

Carl Skinner

resulted in elevated mortality of old-growth ponderosa pine, and they hypothesized 

that root damage from the heat generated by smoldering duff was a contributing 

factor. In support, both Preisler et al. (2000) and Varner et al. (2009) showed that 

soil temperatures can exceed the root lethal threshold to considerable depths in the 

mineral soil beneath thick duff layers. Varner et al. (2009) further reported a link 

between the duration of lethal soil heating and carbohydrate drain in coarse roots, 

suggesting that prolonged smoldering of the duff exacerbates postfire tree stress. 

These findings support a model of postfire stress where injured trees use available 

carbohydrate storage to replenish killed or injured fine roots, depleting carbohydrate reserves and thereby compromising the tree. Of course, many other factors in 

addition to soil heating, particularly bark beetle attack, are also known to contribute 

to postfire tree stress and mortality (Fettig et al. 2006, Kolb et al. 2007, Perrakis 

and Agee 2006). 

Raking duff mounds away from the base of large trees is a recommended 

management practice for lowering fire intensity, lethal soil heating, and, consequently, root and tree mortality (Hood 2010, Kolb et al. 2007). However, supporting 

evidence for the effectiveness of raking has been inconclusive to date (Hood 2010, 

Knapp et al. 2011, Laudenslayer et al. 2008, Swezy and Agee 1991). These findings 

led Kolb et al. (2007) to suggest that “the effects of raking treatment may be site 

specific” and acknowledge that tree mortality is a function of numerous site factors 

(crown scorch, tree species and age, postfire insects, drought), not just soil heating 

effects. 

In discussing options for limiting soil heating when burning duff mounds, 

Hood (2010) emphasized that duff moisture content is the critical determinate of 

duff ignition and smoldering duration. She suggested that a moisture content greater 
29

Fuel Reduction Practices and Their Effects on Soil Quality

than 65 to 85 percent is sufficient to restrict duff smoldering and thus limit excessive soil heating and root damage. Her recommendations for managers included: 

• For large-scale operations where thick duff layers (>12 cm) are prevalent, 

burn during the dormant season when duff moisture content is high, or 

within 2 days after a significant rain event in the Southeast. 

• For protecting individual trees in small stands, burn when the lower duff 

layer is moist and either (1) lightly rake or blow some, but not all, of the 

duff away from the tree base, (2) burn within snow wells at the base of trees 

at least 1 year prior to broadcast burning, or (3) mop-up with water during 

burning. 

• For protecting individual trees with visible fire scars, rake all litter and duff 

from the base and apply snow, if available, to the scar before burning.

Perrakis et al. (2011) further suggested that managers use a gradual approach 

for restoring forests with excessive duff accumulation by first attempting to 

improve the vigor of large trees prior to an initial entry of extremely low-intensity 

burning. They found that mortality was greatest for trees showing poor crown 

structure and minimal radial growth in recent years, and suggested that managers 

consider thinning nearby trees to increase preburn tree vigor.

These recommendations offer a gentle, conservative approach for reintroducing 

fire into old-growth systems when the survival of high-value trees is paramount. 

In other cases, mixed-severity burning that kills roots and low- and mid-canopy 

trees may be appropriate to restore natural variability in forest conditions. For 

example, Collins et al. (2011) found that moderate-severity burning in old-growth, 

mixed-conifer stands in Yosemite National Park conserved large trees and produced 

variable stand structures that resembled historical conditions. In comparison, they 

found that low-severity burning was mostly ineffective at restoring stand densities 

or structural heterogeneity, key ecosystem components for maintaining resilience 

against modern stressors such as disease, wildfire, and climate change (Stephens et 

al. 2010). Using prescribed fire to reestablish forest heterogeneity should also lead 

to patchy (mosaic) forest floor layers, ranging from bare soil to unburned areas, 

which may be most appropriate for meeting desired fuel conditions while safeguarding soil quality (Knapp and Keeley 2006).

Ectomycorrhizal fungi—Ectomycorrhizal (ECM) fungi are found naturally in soils 

where they colonize roots and form a mutualistic relationship with conifers and 

some shrub species. Plants support ECM by providing carbon (C) (20 percent of a 

plant’s photosynthate may be allocated to support ECM fungi). In return, ECM 

fungi provide plants with nutrients, water, and protection from plant pathogens 
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(Smith and Read 1997). Fungal hyphae effectively increase the surface area of 

roots, enabling plants to better thrive in water and nutrient-limited conditions. 

In addition, ECM fungi release exudates that help bind soil particles into aggregates, thereby improving soil structure, aeration, and water infiltration (Miller and 

Jastrow 2000). 

Because ECM fungi require living roots to survive, any fuel reduction treatment that alters forest vegetation may have profound effects on ECM. Soil compaction from heavy machinery can harm the mycelium (a matlike mass of fungal 

hyphae), reduce mushroom productivity (Wiensczyk et al. 2002), and reduce ECM 

root tip abundance and diversity (Amaranthus et al. 1996). Prescribed broadcast 

burning can reduce fungi populations indirectly by increasing soil pH, inducing a 

nutrient flush, and reducing litter and duff levels, or directly when the heat pulse is 

excessive (Kipfer et al. 2010, Stendell et al. 1999). Repeated burning has also been 

shown to significantly reduce ECM biomass (Hart et al. 2005a) and species diversity (Tuininga and Dighton 2004). 

Season of burn is an additional concern for ECM survival and function in many 

Western forests (Cairney and Bastias 2007). Fall burning when soils are dry can 

result in higher burn severity and ECM damage compared to moist spring burning. 

For example, Trappe et al. (2009) found that only fall burning resulted in changes in 

ECM fungal productivity and fruiting patterns. Smith et al. (2004) reported that fall 

prescribed burning in dry ponderosa pine stands significantly reduced duff depth, 

live root biomass, and ECM species richness compared with low-severity spring 

underburning. Thus spring burning when soil and duff moisture is comparatively 

high appears to be a good alternative for maintaining ECM populations. Additional 

forest management strategies to maintain ECM fungi diversity and health are listed 

in table 4.

Soil organic matter—Surface organic matter is oxidized by fire as temperatures 

exceed 200 °C (Johnson et al. 2004), releasing carbon dioxide (CO2

) and associated 

gases, charcoal, and ash. Although this a seemingly straightforward combustion 

process, Tinoco et al. (2006) stated that the effects of fire on surface and mineral 

soil organic matter “is one of the most complex subjects in systematic research on 

environmental impacts.” In practice, mineral soil temperatures rarely exceed 200 

°C during low- to moderate-severity burning, let alone reach higher temperatures 

(400 to 500 °C) where the majority of soil organic matter is lost (Guerrero et al. 

2005). Indeed, Rau et al. (2009) actually found an increase in soil C immediately 

following low-severity burning that was likely due to the incorporation of ash, charcoal and partially burned organic matter into the mineral soil (box 2). 
31

Fuel Reduction Practices and Their Effects on Soil Quality

 Table 4—Forest management strategies to maintain ectomycorrhizal (ECM) 

 fungi diversity and healtha

Strategy Management practice

Provide a source of ECM fungal • Retain live trees

inoculum • Retain areas of old-growth forest across the landscape

Provide differing habitats or • Avoid uniform prescribed burns of high severity

microsites to promote fungal • Minimize disturbance to forest floor during treatment

community diversity • Retain standing and downed coarse woody debris 

Maintain a diverse and robust • Avoid restoration projects that involve grass seeding

ECM fungal population • Encourage regeneration of a mixture of tree species

Conserving ECM fruiting • Minimize soil compaction from heavy machinery and

body production trampling 

• Minimize disturbance to forest floor 

• Retain standing and downed coarse woody material

a

 Source: adapted from Wiensczyk et al. 2002.

Box 2

Soil Organic Matter (SOM) as a Key Indicator of Soil Health

• Provides a reservoir of nutrients that is gradually released to the soil

• The equivalent of 1 percent SOM can release 9 to 14 kg (20 to 30 lb) of available nitrogen per year (Vigil et al. 2002)

• Regulates biotic activity, providing a carbon and energy source for soil 

microbes

• Absorbs and holds up to 90 percent of its weight in water, with most of the 

water being available to plants

• Contributes to the formation of stable soil aggregates

• Improves soil permeability and aeration 

• Contributes up to 20 to 80 percent of the cation exchange capacity

of a soil

Most studies show relatively few short-term changes in total mineral soil C 

owing to burning. For example, Johnson and Curtis (2001) found no overall effect 

of prescribed fire or wildfire on soil C in a meta-analysis of 48 observations from 

13 publications, with relatively few outliers showing significant soil C loss (e.g., 

Grier 1975). Surprisingly, they found that prescribed fire alone resulted in a small 

decline in soil C (<6 percent), whereas wildfire generally resulted in higher soil C 

content. They attributed the increase in soil C after wildfire to the incorporation 

of unburned residues and deposition of charcoal (box 3). In addition, they noted 
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Box 3

Black Carbon: Do Charcoal Additions Improve Soil Quality?

The dark, rich terra preta soils in the Amazon Basin offer an unmistakable example of 

how continual additions of fire-derived charcoal can greatly improve soil productivity. 

Along with providing a reservoir of essential plant nutrients, the stable charcoal in 

these soils contains a high concentration of carbon (C) that improves soil water-holding 

capacity, nutrient retention, detoxification of plant- and microbial-inhibiting compounds, and soil-warming capability. These soils developed in localized areas centuries 

ago where charcoal from cooking fires was repeatedly mixed with soil. Today, there is 

growing interest in the role of charcoal not only in terra preta soils, but in soils exposed 

to forest and grassland burning (wildfire or prescribed fire) and in soils receiving 

soil biochar amendments from bioenergy projects. Recent review articles point to the 

importance of soil charcoal in improving soil quality and stabilizing the global C cycle 

(DeLuca and Aplet 2008, Forbes et al. 2006, Preston and Schmidt 2006); however, they 

stop short of confirming these claims because of an acknowledged scarcity of supporting scientific data. So we ask the question, where along the perceived gradient from 

“no impact” to “critical soil constituent” is charcoal found? Here is an abbreviated 

summary of the current state-of-the-knowledge of soil charcoal:

• Charcoal is a common biproduct of fire that is generated when the combustion 

process becomes oxygen-limited. 

• The aromatic ring structure of charcoal confers high internal porosity and 

surface area, resulting in a high capacity for water, nutrient, and chemical 

sorption (Preston and Schmidt 2006). 

• Charcoal is relatively inert. Estimates of its longevity range from 650 to 

>10,000 years depending on exposure to oxygen and possible consumption in 

succeeding fires (Ohlson et al. 2009).

• The production of charcoal differs considerably depending on fire regime, 

fuel type, loading, moisture, temperature, and fire intensity (Ohlson et al. 

2009). However, useful information on prescription conditions for fuel reduction burning that maximize charcoal production is lacking. Carol Shestak

Charcoal accumulation in the surface 5 cm 

of soil after pile burning.

Carol Shestak

Comparison of postburn (left) and preburn 

soil (right) from beneath a pile burn.
33

Fuel Reduction Practices and Their Effects on Soil Quality

Box 3 (continued)

• Approximately 0.7 to 3 percent of burning organic matter is converted to charcoal, although the degree of spatial variability in soil charcoal content is high 

within a fire perimeter (Czimczik et al. 2003, Ohlson et al. 2009). This suggests that charcoal may act primarily at localized “hotspots” to improve soil 

quality. 

• Most estimates of soil charcoal content come from wildfire sites. We found no 

quantitative information on soil charcoal produced by fuel reduction treatments. Only DeLuca and Aplet (2008) provided a rough estimate on soil charcoal from a hypothetical 200-year prescribed fire cycle (a modest addition of 4 

Mg ha-1). 

Charcoal offers numerous benefits to soil chemical and physical properties. 

However, uncertainties about prescription conditions for optimal production and the 

high level of spatial variability following burning underscore the fact that insufficient 

knowledge exists to assess the practical importance of soil charcoal derived from fuel 

reduction practices. Or as pointed out in an indecisive statement by Zachrisson et al. 

(1996), “the levels of charcoal in.... forest soils are sufficient for them to have a possible ecological effect.”

an average 10 percent increase in soil C for those studies that collected data for a 

minimum of 10 years after fire. But unlike short-term changes in soil C, definitive 

evidence of long-term soil C changes after fire is clouded as it depends on many 

site-specific variables (fire frequency, fire severity, vegetation structure and composition, postfire presence of N-fixing vegetation, charcoal production, damage from 

previous entries). 

Soil pH and cation exchange capacity—Soil pH is a critical attribute that affects nutrient availability and the toxicity of elements like aluminum and iron. 

Generally, soils with neutral or near-neutral pH (about 5.5 to 7.7) are considered the 

most chemically balanced to support the extensive variety of soil processes critical to all ecosystems (Fisher and Binkley 2000). During combustion, base cations 

are mineralized from surface fuels leaving behind a nutrient-rich ash layer. The 

first rains leach the cation-laden ash into the soil, providing a flush of nutrients for 

new growth. These cations increase soil pH by displacing the H and Al ions adsorbed on the negative charges of the soil colloids. Increased soil pH after fire is a 

well-documented phenomenon whose ecological effects depend, in part, on preburn 

conditions. For example, a postfire increase in pH may be very beneficial for a low 

pH soil because of is positive effect on nutrient availability, whereas fire-induced 

changes may be biologically nominal in neutral pH soils (Franklin et al. 2003).
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Two factors help determine the extent of soil pH change: (1) amount of fuel 

consumed, and (2) soil buffering capacity, or a soil’s ability to resist change as a 

function of its total cation content, cation exchange capacity, and reserve acidity. In 

effect, greater fuel consumption leads to greater release of cations in ash and thus 

greater pH change, whereas highly buffered soils are capable of absorbing postfire 

increases in available cations without concomitant changes in pH. Typically, fertile 

clay soils have higher buffering capacity than sandy soils and are more resistant to 

pH change. The examples presented in table 5 show that low-severity burning (e.g., 

broadcast burning) results in only minor gains in soil pH, whereas high-severity 

pile burns result in considerably larger increases. In a meta-analysis of fire and fire 

Table 5—Examples of pH changes in the top 0 to 5 cm of soil following 

broadcast underburning (BB) or pile burning (PB) 

Soil pH

Ecosystem and study Type of Burn Pre- Post- Potential

location burn severity burn burn ecological effect

Ponderosa pine,

Arizonaa BB Low 5.27 5.49 Minor

Ponderosa pine,

Montanab BB Low 5.00 5.30 Minor

Ponderosa pine/white fir,

Oregonc BB Low 5.78 5.93 Minor

Oak,

Kentucky/Tennesseed BB Low 4.55 4.81 Minor

Oak/pine woodlands, 

Alabamae BB Low 4.80 4.70 Minor

Oak hardwoods,

Massachusettsf BB Low 4.20 4.43 Minor

Loblolly/longleaf pine, 

South Carolinag BB Low 4.60 4.30 Minor

Mixed conifer, 

Californiah BB High 5.19 5.83 Moderate

British Columbia 

Canadai PB High 3.48 5.22 Large

Ponderosa pine, 

Arizona j PB High 5.9 7.0 Moderate

a

 Grady and Hart (2006).

b

 Gundale et al. (2005).

c

 Trappe et al. (2009).

d

 Franklin et al. (2003).

e

 Nobles et al. (2009).

f

 Neill et al. (2007).

g

 Binkley et al. (1992).

h

 Moghaddas and Stephens (2007).

i

 Arocena and Opio (2003).

j

 Korb et al. (2004).
35

Fuel Reduction Practices and Their Effects on Soil Quality

surrogate treatments, Boerner et al. (2009) noted that significant increases in soil 

pH occurred where fire severity was greatest or where pretreatment base status was 

the lowest. They also affirmed the commonly held knowledge that changes in soil 

pH are ephemeral (Neary et al. 1999, Sherman and Brye 2009), as they remained 

high during the first year following fire but not in subsequent years.

Like soil pH, postfire increases in plant-available cations are often short-lived. 

For example, Franklin et al. (2003) noted that significant postfire increases in soil 

nutrient concentrations had returned to preburn levels within 1 to 3 years. Soil 

nutrient reserves may even be depleted if fire substantially reduces the cation 

exchange capacity (CEC), which is a measure of a soil’s ability to adsorb and 

release plant-essential cations. Cation exchange capacity is closely tied to soil 

texture and SOM content: both clay particles and SOM have high surface areas and 

negatively charged sites that confer high CEC. Decreases in CEC are often noted 

after severe burning when high temperatures consume SOM and eliminate cation 

exchange sites (St. John and Rundel 1976, Seymour and Tecle 2005). Nobles et al. 

(2009) observed lower retention of soil potassium (K) and sodium (Na) after fire 

due to leaching losses, whereas less mobile cations like calcium (Ca) remained 

unchanged in the surface horizon. Boerner et al. (2009) noted that effects of low- to 

moderate-severity fire on cation availability were modest and were limited mainly 

to increased Ca concentrations during the first year following fire at several sites 

across the United States. Thus, similar to the effects of fire on soil pH and organic 

matter, changes in plant-available cations will likely be moderate, at most, except 

with high-severity burning. 
36

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Soil Water Repellency

Key points:

Ecological effects Management considerations

• Water repellency results from the 

presence of naturally formed hydrophobic coatings on soil particles. 

Hydrophobic compounds are volatilized during fire and move downward 

through soil along a temperature gradient before recondensing.

• Some degree of soil hydrophobicity 

soils is likely unavoidable following prescribed fire. Burning thick 

duff, particularly when soils are dry, 

will increase repellant conditions. 

Managing for a diversity of fuel consumption (allowing some unburned, 

some complete consumption) may 

create an acceptable mosaic of repellant conditions.

• Repellency intensifies at soil temperatures from 175 to 200 °C. These 

temperatures are readily reached by 

burning heavy fuels when soil is dry, 

but infrequently attained when soil is 

moist.

• Expand the use of mosaic burning 

patterns and uneven forest floor consumption to limit the spatial extent 

of hydrophobicity.

• Repellency may be destroyed if soil 

temperatures exceed 270 °C, such as 

under burn piles. Definitive evidence 

is inconclusive, however.

• Hydrophobic soils tend to remain 

water repellant until moisture content 

increases above 10 to 13 percent. 

Postfire erosion may be high during 

rain events when hydrophobic soils 

are dry. 

• Vegetation or forest floor cover 

is crucial to limit erosional losses 

in the first year after fire. Limit 

extent of complete forest floor 

consumption.

Resistance to wetting, or soil water repellency, results from hydrophobic 

coatings on soil particles, which reduce the affinity between soil and water and can 

lead to localized surface erosion (Doerr and Thomas 2000, Hallet 2007). These 

water-repellent compounds are produced from decomposing plants, root exudates, 

some fungal species, surface waxes from plant leaves, and decomposing duff 

and litter (Fogel and Hunt 1979, Hallett 2007). Evergreen plants (e.g., eucalyptus, 

pines, and Mediterranean shrublands) are most commonly associated with water 

repellency, particularly trees and shrubs with high foliar resin, wax, or aromatic oil 

contents (Doerr et al. 2000). The magnitude and persistence of repellency is further 

influenced by soil textural differences, as sandy soils usually exhibit higher repellency than clayey soils because their small total surface area is more easily coated 

by hydrophobic substances than are finer sized particles with high total surface area 

(Doerr et al. 2006, Woche et al. 2005). 
37

Fuel Reduction Practices and Their Effects on Soil Quality

Prescribed burning of vegetation and litter can substantially alter soil water 

repellency given the right heating conditions. Hydrophobic compounds, when 

volatilized by fire, move downward through soil along a temperature gradient and 

condense on cooler soil particles (DeBano 1981), usually at a soil depth between 0 

to 8 cm (Huffman et al. 2001). Water repellency generally intensifies at soil temperatures between 175 to 200 °C owing to a strengthening of bonding between waterrepellent substances and soil particles (Doerr et al. 2000, Savage 1974). Above 270 

to 300 °C, hydrophobic substances are thought to be irreversibly destroyed (DeBano 

et al. 1976). However, recent observations (Hubbert, unpublished) show repellency 

occurring below pile burns at soil temperatures well above 300 °C.

Most prescribed fires produce a mosaic of non-, low, moderate, and high 

repellency across the landscape (fig. 18). The mosaic pattern is a function of 

nonuniformity of fire temperature and duration, forest floor consumption, vegetation type, and soil moisture and texture (Hubbert et al. 2006). Therefore, one should 

not expect the spatial distribution of soil water repellency to be uniform across 

a broad landscape, or even at smaller scales in steep watersheds (Robichaud and 

Miller 1999).

Water repellent soils typically alternate between repellent and nonrepellent 

states in response to postfire rainfall patterns (Dekker et al. 1998, Doerr and 

Thomas 2000, Shakesby et al. 2000). In most cases, soil water repellency increases 

in dry soil and either decreases or vanishes following periods of precipitation (fig. 

19) (Crockford et al. 1991, Ritsema and Dekker 1994). In fact, several studies show 

that repellency is greatly reduced when soil moisture content exceeds 10 to 13 

percent by volume (Dekker et al. 2001, Hubbert and Oriol 2005, MacDonald and 

Huffman 2004). 

Because soil repellency is greatly increased when soils are dry, we would 

expect pronounced overland flow and subsequent erosion when storm events follow 

prolonged dry periods (Doerr et al. 2000). However, Imeson et al. (1992) noted that 

although water-repellent soils can contribute to high overland flow rates locally, any 

effects at the hillslope or catchment scale are dwarfed by high spatial variability 

in infiltration rates. In fact, most studies have only inferred a causal link between 

water repellency and erosion, and have failed to isolate the erosional impacts of 

water repellency from the confounding effects of losses in vegetation cover, litter 

cover, or soil aggregate stability (Doerr et al. 2000, Hubbert et al. 2006, Scott 1993). 

The degree of postfire reduction in water infiltration is dependent on the spatial 

distribution of hydrophobicity on the landscape (Shakesby et al. 2000). Local areas 

of repellency will reduce infiltration, but may pond water and enhance preferential 

flow down cracks and root channels. In this case, preferential flow can move both 
38

general technical report psw-gtr-241

Figure 18—Effects of fire on spatial and temporal patterns of water repellency at the soil surface. 

Repellency increases from green to red in the legend. Green and light green represent no repellency 

and very low repellency. Yellow represents low repellency. Brown represents moderate to high repellency. Red represents very high repellency. Adapted from Hubbert et al. (2008).

Figure 19—Soil water repellency is generally greatest when soils are dry. Data were collected 

following the September 2002 Williams Fire in the San Gabriel Mountains, California. Adapted 

from Hubbert and Oriol 2005 and Hubbert et al. 2012. 

vertically and laterally through the subsurface, removing water that would otherwise contribute to overland flow. On the other hand, enhanced transfer of water to 

the subsoil may result in mass failures in clayey soils prone to landslides (Booker 

et al. 1993). Using simulated rainfall, patterns of localization of overland flow were 

observed by Cerda et al. (1998) in southwest Spain and by Imeson et al. (1992) in 

North America. Both studies found that the extent of overland flow was limited, 

however, because of the interspersed presence of hydrophilic soils.
39

Fuel Reduction Practices and Their Effects on Soil Quality

Soil Nitrogen

Key points:

Ecological effects Management considerations

• Loss of nitrogen (N) during fire varies widely, from nominal to a large 

proportion of the ecosystem total. 

• Greater fire severity leads to greater 

duff consumption, and, consequently, 

N loss.

• Estimate N loss for prefire planning 

or postfire accounting as 10 kg ha-1

for every 1 Mg ha-1of duff consumed.

• Develop simple accounting procedures to document cumulative N 

losses from repeated fuel reduction 

treatments.

• Nearly all loss of N is from litter and 

duff.

• Impacts to mineral soil N are not 

a strong consideration for burn 

programs.

• N loss is strongly influenced by the 

mass and moisture content of the 

forest floor. 

• Burning of moist duff layers (> 65 

percent moisture) limits duff consumption and N loss.

• Whether N losses are damaging to 

soil or site productivity depends on 

how fertile a site is. Nutrient-limited 

sites are at risk of detrimental N loss 

from severe burning. Conversely, 

large N losses may be inconsequential on sites with large N reserves.

• Avoid severe burning and complete 

duff consumption at nutrient-poor 

sites.

• Consider mechanical treatments for 

extremely poor sites. 

• A flush of plant-available N results 

after fire because of downward 

movement and condensation of firevolatilized N. 

• Low- to moderate-severity burning 

probably does not release enough 

inorganic N to noticeably benefit 

plant growth. 

• Postfire plant responses are more 

likely a function of reduced plant 

competition and favorable seedbed 

conditions.

Prescribed burning can result in two major changes in soil N: (1) loss of total N 

to the atmosphere during the consumption of litter and duff layers, and (2) a pulse 

of plant-available N owing to downward movement and condensation of firevolatilized inorganic N. This is a double-edged sword—simultaneous loss of total 

N and gain of inorganic N—with the two processes linked such that burns resulting 

in a high loss of total N also yield the greatest increase in plant-available N (Wan 

et al. 2001). From there, a cascade of related soil N transformations may follow suit 
40

general technical report psw-gtr-241

when burning is sufficiently hot (e.g., changes in microbial N pools, mineralization 

rates, nitrification, N fixation, enzyme activity). Unfortunately, predicting the exact 

quantity of total N lost, inorganic N gained, or changes in other N properties is 

difficult given the variable findings from literature, themselves reflecting the range 

of site characteristics and burn prescriptions used throughout the United States. 

Acknowledging the importance of N to forest function and the wide range of 

reported soil N responses to prescribed fire leads us to ask the following questions: 

• How extensive are changes in total soil N and inorganic N? 

• What site conditions and burn prescriptions lead to the greatest changes in 

soil N? 

• Are N losses damaging to ecosystem health? If so, will N eventually be 

replenished?

These questions are addressed here from the standpoint of a single burn, recognizing that repeated burning is a more complicated and often preferred component 

of fuel reduction programs. The following information then is intended as a foundation for understanding the effects of repeated burning on soil N, which is discussed 

later in the chapter. 

How extensive are changes in total soil N and inorganic N?—

Total soil N—In a classic review of fire and soil N transformations, Raison (1979) 

stated that “in any one fire, nutrient loss is usually but not universally small relative 

to the total soil and biomass reserve.” If true, then the assumption for modern-day 

prescribed burning is there should be little concern about soil N loss, regardless of 

fire severity. But what does “usually but not universally” mean? And how small is 

“small”? As a counterpoint, Johnson and colleagues (Caldwell et al. 2002; Johnson 

et al. 1998, 2009; Murphy et al. 2006) argued that awareness of soil N loss is one of 

the most important aspects of fire-soil dynamics. Table 6 articulates these contrasting views by showing published examples of N loss that range from minor to severe 

during a single prescribed fire. Clearly, any blanket statement about the effects of 

prescribed burning on soil N loss is too simplistic without attention given to preburn soil conditions and fire severity. However, it is important to recognize that N 

losses can be severe in certain circumstances and that preventative efforts to limit 

N loss may be needed or encouraged.

Nearly all N lost from the studies reported in table 6 originated from litter and 

duff, not mineral soil. For example, Moghaddas and Stephens (2007) found no 

significant changes in mineral soil N content even though up to 723 kg ha-1 N was 

volatilized from the forest floor (fig. 20). Similarly, in a meta-analysis of fire and 
41

Fuel Reduction Practices and Their Effects on Soil Quality

Table 6—Nitrogen (N) loss during prescribed burning from selected published studies 

across the United States

Extent of N loss N loss Region Fire type Study

 kg ha-1 

Low to moderate 40 to 88 SE UB Bell and Binkley 1989

(0 to 200 kg ha-1) 39 to 194 SE UB Hough 1981

11 to 40 SE UB Richter et al. 1982

55 W CTLB Gundale et al. 2005

55 SE UB Hubbard et al. 2004

54 SE UB Kodama and Van Lear 1980

150 SW UB Covington and Sackett 1984

56 to 61 W UB Caldwell et al. 2002

94 W SB Jurgensen et al. 1981

1 to 134 SE UB Vose and Swank 1993 

119 W UB Klemmedson et al. 1962

114 W WTB Murphy et al. 2006

Moderate to severe 252 W CTLB Murphy et al. 2006

(200 to 750 kg ha-1) 200 to 600 W SB Little and Ohmann 1988

225 to 571 W SB Little and Klock 1985

347 to 435 W CTLB, WTB Shea 1993

362 W UB Caldwell et al. 2002

353 W UB Klemmedson et al. 1962

551 to 723 W UB, CTLB + Moghaddas and Stephens 2007

masticate

Regions include SE = Southeast, SW = Southwest, W = West. Fire types are UB = underburn, WTB = whole tree thin and 

burn, CTLB = cut-to-length thin and burn, and SB = slash burn.

Figure 20—High-severity burning with near-complete consumption of surface residues resulted in large losses of forest floor 

nitrogen (N), but little change in mineral soil N (Moghaddas and Stephens 2007). Scott Stephens

Scott Stephens
42

general technical report psw-gtr-241

soil N studies, Wan et al. (2001) found that prescribed burning had essentially no 

effect on mineral soil N content. These findings clarify that soil N loss is primarily 

a surface organic matter phenomenon and suggest that efforts to limit N loss—

when deemed necessary—focus on conservation of the duff layer.

Inorganic N—Soil solution ammonium (NH4

) and nitrate (NO3) are the primary 

inorganic N compounds assimilated by plants. Without fire, their concentrations 

are usually low and vary with seasonal trends in plant uptake, leaching loss, and N 

mineralization, nitrification, and immobilization rates (Fisher and Binkley 2000). 

Fire acts to jumpstart the soil inorganic N pool as a portion of the volatilized N 

gas travels downward and condenses as NH4 in the forest floor or mineral soil. 

Ammonium concentrations are usually elevated immediately after burning and 

are then converted to NO3 by nitrifying bacteria within several months. Wan et al. 

(2001) found in their meta-analyses that prescribed fire resulted in more than a doubling of the peak concentrations of NH4 and NO3 on average. They also found that 

the increase in both NH4 and NO3 was greatest in the surface 2.5 cm of mineral 

soil and that the increase was ephemeral, lasting for 1 to 2 years before returning to 

preburn levels. 

Most fire studies report inorganic N gains on a part per million or concentration basis, making ecological interpretations difficult. Increases in N concentration 

may be statistically significant, but whether they affect plant N uptake and growth 

is often ambiguous. Fortunately, a few studies have also estimated the inorganic 

N flush on an area basis, which provides a more complete picture of the biological significance of this process (Covington and Sackett 1992, Hart et al. 2006, 

Jurgensen et al. 1981). For example, Covington and Sackett (1992) found that high 

severity prescribed burning in old-growth ponderosa pine consumed nearly 100 Mg 

ha-1 of forest floor material and released a modest 34 kg ha-1 of plant-available N. 

Their low- to moderate-severity burns in younger stands released only 4 to 15 kg 

ha-1 of inorganic N, which is in general agreement with the findings of Jurgensen et 

al. (1981) and Hart et al. (2006). These few studies suggest that prescribed burning 

will not release enough inorganic N to noticeably benefit plant growth unless burning is fairly severe or is conducted annually or fairly frequently. Instead, postfire 

responses by vegetation are more likely due to improvements in seedbed properties 

(lower forest floor depth, greater light availability, less plant competition for water 

and nutrients, seed scarification) than to a flush of inorganic N.
43

Fuel Reduction Practices and Their Effects on Soil Quality

What site conditions and burn prescriptions lead to the greatest changes 

in soil N?— 

Site conditions that lead to high forest floor content such as (1) high litterfall rates in 

dense forest stands, (2) slow decomposition rates in cold or dry climates, and (3) a 

lack of recent forest floor disturbance from burning or harvesting contribute to the 

greatest potential for N loss. Actual N loss then is a function of the forest floor N 

content and the burn prescription that either encourages or limits duff consumption. 

Not surprisingly, duff moisture content is the primary prescriptive variable explaining duff consumption (Brown et al. 1985, Hille and Stephens 2005). Burning of 

moist duff layers results in low consumption (fig. 21) and presumably low N loss. 

It is possible to use duff consumption as a surrogate for estimating N loss. For 

example, studies from the Southeast United States have shown that forest floor consumption on a weight basis is an excellent predictor of total N loss in pine forests 

(Hough 1981, Schoch and Binkley 1986). We extended this concept to forests across 

the United States by plotting N loss versus forest floor consumption for the studies 

listed in table 6. A strong, positive relationship was found among all sites, showing 

that 10 kg ha-1 N loss resulted for every 1 Mg ha-1 of forest floor consumed (fig. 22). 

Interestingly, low N losses were most common for studies from the Southeast 

where warmer annual temperatures and consistent seasonal precipitation patterns 

suggest higher duff decay rates, lower duff accumulation, and wetter duff layers 

compared to western forests. Fire planners and soil scientists can use the predictive relationship from figure 22 to help set burn prescriptions when limiting N 

losses from fire is recommended, or as a means to estimate N losses when burning. 

Consider an example from young ponderosa pine forests in central Oregon where 

total ecosystem N is approximately 2500 kg ha-1 (Little and Shainsky 1995). If loss 

of no more than 15 percent of the site N, or 375 kg ha-1, is recommended, then burn 

prescriptions and fuel moisture contents can be selected using a fire model such as 

Consume 3.0 (http://www.fs.fed.us/pnw/fera/research/smoke/consume/consume30_

users_guide.pdf), which targets forest floor consumption of 37.5 Mg ha-1 or less. 

Alternatively, a more severe burn in this forest type that hypothetically consumes 

90 percent of the forest floor (60 Mg ha-1 of duff) will volatilize 600 kg ha-1 N, or 

24 percent of the total ecosystem N.

Are N losses damaging to ecosystem health? If so, will N eventually 

be replenished?— 

We are not aware of any empirical evidence that a single prescribed burn has ever 

altered plant composition or growth in healthy ecosystems through its effect on 
44

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Figure 21—High duff moisture content at the time of prescribed burning limits duff consumption. Data are from several conifer stands in the northern Rocky Mountains (Brown 

et al. 1985). Dashed lines are one standard error from the regression line. Duff moisture 

content refers to the average moisture content in the lower one-half of the duff.

Figure 22—Predicted nitrogen (N) loss as a function of forest floor consumption. Data are 

from studies conducted in the Western and Southeastern United States, as listed in table 6. 

total soil N. Certainly the range of N loss for low- to moderate-severity burns (0 to 

200 kg ha-1) represents a minor proportion of the total N in most ecosystems and 

should have little impact on essential functions. Whether larger N losses associated 

with severe burning are ultimately damaging to ecosystem health depends largely 

on the size of the total N reservoir at a given site. For example, the loss of 723 kg 

ha-1 N measured by Moghaddas and Stephens (2007) in a productive mixed-conifer 

forest is quite dramatic, yet it represents only about 5 percent of their site’s total N 

reservoir2 and presumably will not affect site functions. A similar loss from a less 

productive forest may have different results, however. Shea (1993) found that about

2

 Powers, R.F. 2010. Personal communication. Research forester, retired. Pacific Southwest 

Research Station, 3644 Avtech Parkway, Redding, CA 96002.
45

Fuel Reduction Practices and Their Effects on Soil Quality

50 percent consumption of the forest floor in dry, nutrient-poor ponderosa pine forests volatilized 435 kg ha-1 N, or about 18 percent of the total N reservoir. Although 

this N loss has not resulted in a change in plant diversity or productivity to date 

(Busse et al. 2009a), it represents a sizable deficit that will not be easily replenished 

and will likely become exacerbated by future treatment. 

A quick procedure to estimate the percentage of the total ecosystem N lost by 

burning is to (1) know your site—estimate the total N reservoir in soil, forest floor, 

and vegetation using published results or professional experience; (2) estimate the 

total N loss from fire by assuming 10 kg ha-1 N loss for every 1 Mg ha-1 of forest 

floor consumed; (3) divide the N loss in step 2 by the total N reservoir estimated in 

step 1. Although crude, this method offers a rapid assessment of potential changes 

in soil quality. This information can also be used as a starting point for developing a 

running total of a site’s N budget following additive treatments.

Any reductions in total N resulting from fire may be effectively offset if 

postfire increases in N mineralization rates (microbial release of NH4) result. This 

can occur if fire reduces the forest floor depth and results in warmer soil temperatures, greater availability of NH4 substrate, and, consequently, greater microbial 

activity and N turnover (Schoch and Binkley 1986). However, there is no consistent 

evidence in literature to support this sequence of events. Studies of postfire N mineralization have found no effects (Hart et al. 2006, Hubbard et al. 2004, Kaye and 

Hart 1998, Moghaddas and Stephens 2007), positive effects (Schoch and Binkley 

1986), detrimental effects (Bell and Binkley 1989), and variable effects (Gundale et 

al. 2005, White 1986) of fire on soil N mineralization. In fact, DeLuca and Zouhar 

(2000) found contrasting short-term increases and longer term declines in mineralizable N when comparing a chronosequence of burns in western Montana forests. 

Therefore, any expectations that increases in N mineralization will partially offset 

the loss of N may be unrealistic.

Finally, we ask the academic question of how long it would take to replenish 

N losses resulting from fire by a combination of atmospheric N deposition and N 

fixation by free-living soil bacteria or N-fixing plants (the assumption here is that N 

needs to be fully replenished, which may not be valid in many cases). Table 7 highlights the tremendous range in time that would be required to balance ecosystem N 

following fire. For example, millennia are needed to replace N loss from a low- to 

moderate-severity prescribed fire if the only N source is from free-living, N-fixing 

soil micro-organisms. In contrast, N losses of 100 kg ha-1 can be offset in as few as 

3 years in moist forests near urban centers by a combination of N deposition and 

symbiotic N fixation. The variation in atmospheric N deposition with proximity to 
46

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Table 7—Years required to offset nitrogen (N) losses from prescribed fire by

atmospheric deposition, free-living N fixation, or symbiotic N fixationa

 Years required 

 to offset N loss of:

 Estimated

N process Condition N fixation rate 100 kg ha-1 400 kg ha-1

 (kg/ha/yr)

Atmospheric Remote forest 1 100 400

deposition

Near urban center 15 7 27

Free-living Cool, moist forest 0.2 500 2,000

N fixation

Symbiotic Dry forest, 1 100 400

N fixation Purshia understory/ 

30 percent cover

Dry forest, 10 10 40

Ceanothus understory/ 

30 percent cover

Moist forest 20 5 20

Ceanothus understory/ 

30 percent cover

a

 N fixation rates are estimates from Weathers and Lynch (2011) for atmospheric deposition, Hendrickson (1990) 

for free-living N fixation, and Busse (2000) for symbiotic N fixation.

urban centers is attributable to the strong influence of localized air pollution and 

N emissions (Weathers and Lynch (2011). In fact, deposition rates near large cities 

are sufficiently high to surpass critical loads for N saturation, leading to possible 

disruption of key ecosystem functions (Fenn et al. 2010). Here, the use of repeated 

prescribed fire to volatilize N is a potential management option for releasing excess 

N and avoiding N saturation conditions (Fenn et al. 2010). 

If we assume that N gains from atmospheric deposition are balanced in natural 

(remote) ecosystems by N losses from leaching, then postfire recruitment of N-fixing plants becomes an important mechanism for replacing N loss. As an example, 

Johnson et al. (2004) predicted a complete recovery of postfire N by the presence of 

the N-fixing species snowbrush (Ceanothus velutinus [Douglas x Hook.]). However, 

the success of this process will differ considerably depending on which N-fixing 

species are present and the amount of ground cover they occupy. Slow-growing 

shrubs and small trees (e.g., bitterbrush, mountain mahogany [Cercocarpus Kunth]) 

likely fix enough N to meet their own limited demands but contribute little to the 
47

Fuel Reduction Practices and Their Effects on Soil Quality

Figure 23—Postfire recruitment of nitrogen (N)-fixing shrubs like snowbrush can facilitate the 

recovery of site N losses. In this example, snowbrush cover (about 30 percent) will fully replenish N losses from a low- to moderate-severity fire in about 10 years. 

Matt Busse

soil N budget (Busse 2000). Faster growing shrubs, legumes, and trees can provide 

much higher inputs of fixed N if their presence is widespread (fig. 23) (Domenach 

et al. 1989, McNabb and Cromack 1983). In some cases, burn prescriptions can be 

tailored to enhance recruitment of N-fixing plants, particularly for fire-adapted species like snowbrush that are capable of vegetative sprouting or for species that have 

a well-established seed bank. Because these are site-specific processes, we recommend talking with local plant ecologists about the potential for recruiting N-fixing 

plants following fire.
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Repeated Burning

Key points:

Ecological effects Management considerations

• Soil effects are controlled by burn 

severity and fuel consumption for 

both individual and repeated burns.

• Prescribe burn conditions to meet 

fuel and soil objectives.

• Fire exclusion causes nutrients to 

accumulate in forest floor fuels and 

dead organic matter. Large amounts 

of nutrients can be lost when heavy 

fuel loads are burned.

• Use low- to moderate-severity burning and leave a sufficient portion 

of the duff layer intact to temper 

nutrient loss. Estimate the quantity 

of nutrients you want to retain, or 

how much you are willing to lose, 

and select burn conditions and forest 

floor consumption levels to match 

your objectives.

• The more frequently a stand is 

burned, the greater the nutrient loss 

from the forest floor compared to 

long-unburned controls. 

• Increase the interval between burns 

or design repeat burns to be heterogeneous, leaving areas of forest floor 

unburned, when this is a concern.

• Repeat burning can greatly reduce 

forest floor fuels and nutrients, but 

there are no consistent effects on 

mineral soil characteristics. 

• Impacts to mineral soil nutrients may 

not be a strong factor when deciding to repeat burn, except at sites 

with specific nutrient deficiencies or 

concerns.

• Repeat burning may stimulate nitrogen (N)-fixing understory plants 

in some vegetation types, helping 

replenish lost N. 

• Take note of understory development 

following prescribed fire. Where 

fuel objectives are not compromised, 

allow postfire N-fixing plants to 

establish.

• Nutrient-limited sites are at risk of 

productivity impacts from repeat 

burning.

• Consider mechanical methods to 

reduce fuels.

Many fire-adapted ecosystems prevailed for thousands of years with frequent 

fire. Reintroduction of fire, on some periodic basis, is often recommended to restore 

or maintain these systems and curtail hazardous fuel accumulations. Perhaps the 

most extensive research on the forest impacts of long-term, frequent prescribed fire 

programs has come from the U.S. southeastern Coastal Plain. The pine woodlands 

in this area, often dominated by longleaf or loblolly pine (Pinus taeda L.), are 

among the most fire-adapted forests in the United States. Fires historically burned 

at 1- to 4-year intervals (Brockway and Lewis 1997, Wade et al. 2000), and forest management practices often include frequent prescribed fire as a silvicultural 

tool to reduce fuels and help maintain these southern pine systems (Stanturf et al. 
49

Fuel Reduction Practices and Their Effects on Soil Quality

2002). Related research has also been conducted in the Northeast (Neill et al. 2007, 

Tuininga and Dighton 2004), Southwest (Covington and Sackett 1986, Wright and 

Hart 1997), and Northwest (Busse and Riegel 2009, Busse et al. 2009a). 

Repeat fire reduces forest floor biomass and nitrogen with limited

impacts on soil total nitrogen—

Numerous soil studies have examined impacts following 20 to 65 years of annual 

burning, as well as comparisons with fires every 2, 3, 4, or more years in southern 

pine forests (Bell and Binkley 1989, Binkley et al. 1992, Brockway and Lewis 1997, 

Jorgensen and Hodges 1970, McKee 1982, Waldrop 1987). The consistent results 

from these studies show reductions in forest floor biomass and the concomitant loss 

of nutrients in the combusted material. Some portion of the nutrients may be lost to 

the atmosphere and moved offsite, whereas others may be moved downward into 

the mineral soil or redistributed locally as fine ash. Studies typically show N losses 

from the forest floor. The more frequent the burns, the greater the cumulative reduction in forest floor N content. For example, following 30 years of prescribed fire 

treatments in the Coastal Plain of South Carolina, forest floor N mass was reduced 

by 29, 60, 60, and 85 percent following fires every 4, 3, 2, and 1 years, respectively, 

relative to the 480 kg N ha-1 in the control stand (Binkley et al. 1992). McKee (1982) 

also reported increased N losses from the forest floor with increasing fire frequency 

at Coastal Plain sites. At a Florida site, annual burning reduced forest floor N by

95 percent, whereas prescribed fires every 4 years led to a 72 percent reduction, 

relative to the 131 kg N ha-1 in the control. At a South Carolina site, annual burning reduced forest floor N by 68 percent, and periodic fire every 7 years resulted in 

a 32 percent loss of N relative to the 408 kg N ha-1 in the control (McKee 1982).

Despite dramatic reductions in forest floor mass and nutrients following 

decades of repeat burn treatments, there is no clear and consistent effect of longterm repeat burning on total N contained in mineral soil. Impacts from prescribed 

fires are typically limited to the uppermost soil layers, and the following examples 

show data from the top 10 to 15 cm of mineral soil. For example, soil N decreased 

by 110 kg N ha-1, or 7 percent, following 20 years of biennial burning in southwestern pine stands (Wright and Hart 1997). During 30 years of prescribed fire, soil N 

both increased and decreased at a South Carolina Coastal Plain site, depending on 

burn frequency and season (McKee 1982). Comparing measurements at 10 and 30 

years of treatment, the winter burns increased soil N by 3 and 10 percent, respectively, after periodic (about 7-year intervals) and annual burns, whereas summer 

burns decreased soil N by 9 and 22 percent, respectively, for the same burn frequencies. During the same 20-year period, soil N in the control stand increased by 4 

percent. The author suggested that seasonal influences may be related to the growth 
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of N-fixing plants following burns (McKee 1982). After 30 years of repeat burning in a loblolly/longleaf pine forest, Binkley et al. (1992) measured minor gains 

of 1 and 6 kg N ha-1 in the surface 10 cm of mineral soil following burn intervals 

of 1 and 3 years, respectively, and losses of 25 and 1 kg N ha-1 after 2- and 4-year 

intervals, respectively, but none of these were significantly different than the 1280 

kg N ha-1 contained in the unburned control. Prescribed fires typically burn at low 

to moderate intensities, and the resulting degree and duration of heating generally 

retain most soil N (Certini 2005, Johnson et al. 2009).

Covington and Sackett (1986) proposed that repeated use of prescribed fire can 

enhance forest productivity by releasing periodic pulses of available, inorganic N in 

the form of NO3 and NH4. However, following 20 years of biennial burning of ponderosa pine stands in the Southwest, inorganic N levels did not differ between burn 

plots and controls (Wright and Hart 1997). Using restoration treatments designed 

to emulate historical vegetation and fuel conditions in the inland West, Hart et al. 

(2005b) showed that the N released from frequent prescribed fires was less than a 

third of the plant demand for N at the site. They argued that, rather than directly 

cycling N to maintain productivity, repeat fires are more important in maintaining 

open stand conditions that allow N-fixing herbs and microbial N mineralization to 

supply N for plant uptake. Following 9 years of annual burning in a longleaf pine 

stand, Greene (1935) found that legume density was more than twice that measured 

in unburned areas. He suggested that accumulating plant debris in the unburned 

stand had a smothering effect on leguminous plants, and would lead to reduced soil 

N compared to the frequently burned area. In contrast, Busse and Riegel (2009) 

reported that frequent fires in Oregon ponderosa pine forests likely hindered 

reproduction of N-fixing bitterbrush shrubs in favor of grasses. Fire suppression 

there has increased the abundance of bitterbrush, which acts as a ladder fuel in dry 

forests. These conflicting reports indicate that each forest system responds uniquely 

to frequent fire. Taken together, we surmise that while prescribed fires often release 

available N, this may not occur in all forest types. The role of N-fixing plants in 

increasing soil N following prescribed fire is also site specific. Where N-fixers are 

stimulated by prescribed fire, they may be an important, though not sole, source for 

replenishing some of the N lost through combustion.

Prescribed fire frequency in contemporary, managed landscapes—

Once an initial prescribed burn has been implemented (with or without thinning), 

how do we decide if, or when, to burn again? Considering that many forests 

historically burned with very short return intervals (one to two decades or less), 

repeat burning is often proposed or justified in the context of historical fire regimes. 

Forest managers recognize the historical role of frequent fires as a fundamental
51

Fuel Reduction Practices and Their Effects on Soil Quality

disturbance agent that influences plant succession, nutrient cycling, and other ecosystem processes. Presumably, frequent fire kept fuel loads in check, and nutrients lost during fires did not reduce forest productivity and soil quality because, 

over time, they were replenished by inputs from plants, the atmosphere, or mineral 

weathering. Management activities including fire suppression, timber harvesting, grazing, recreational use, and urbanization have tremendously altered forest 

stand conditions, disturbance regimes, and associated ecological processes (fig. 24). 

Knowledge of historical fire regimes can provide an important frame of reference 

when designing forest management strategies (Mutch and Cook 1996), but impacts 

to nutrient pools, availability, and cycling should not be overlooked when considering restoration of fire regimes on today’s landscapes.

Fire exclusion has altered nutrient cycling and storage by allowing organically 

bound nutrients (unavailable for plant uptake) to accumulate in organic matter on 

the forest floor. Substantial amounts of N can be lost when heavy fuel accumulations burn during repeat prescribed fires. For example, heavily stocked ponderosa 

pine stands in central Oregon were burned in 1991 and 2002, and the resulting N 

loss from the two fires was 550 kg ha-1, or 22 percent of the total ecosystem N of 

the site (Busse and Riegel 2005). This forest is relatively dry and infertile compared 

to more productive stands in moister climates. The frequent use of prescribed fire 

in this or similar forests may affect long-term site productivity by volatilizing 

greater and greater proportions of the total site N. Forests with greater overall N 

stores and nutrient cycling rates should be more resilient to N losses from frequent 

fire because of their increased ability to buffer against nutrient changes. 

Figure 24—Mixed-conifer stands characterized by frequent low- to moderate-severity historical fire regimes. Open stand in the 

Sierra San Pedro Martir, Mexico, has experienced little to no fire suppression (left). Dense, ingrown stand in the Sierra Nevada, 

USA, has experienced aggressive fire suppression for more than a century (right), but has remnant large trees with fire scar evidence 

of frequent historical fires (right, inset).

Emily Moghaddas

Scott Stephens
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Pile Burning

Key points:

Ecological effects Management considerations

• The amount of soil heating during pile burning varies greatly 

depending on fuel composition. 

In particular, piles dominated by 

large-diameter wood (>25 cm) can 

produce extreme soil heating. 

• When possible, avoid burning piles 

that contain a high percentage of large 

wood. However, extreme soil heating 

may be of little concern if these piles 

are widely spaced and occupy a small 

percentage of the land surface.

• Piles containing a mix of fuel sizes 

(e.g., precommercial thinning slash—

poles, limbs, tops) will generally not 

produce excessive soil temperatures.

• The diameter and arrangement of 

hand piles (numerous small piles vs. 

fewer large piles) appears to have 

little effect on soil quality. 

• Other considerations (cost, fire risk, 

human safety) are probably more 

important than potential soil effects 

when planning pile size specifications.

• The amount of land surface covered by piles is a critical soil 

consideration. 

• Determine the amount of ground 

coverage occupied by piles of all 

types. Visual estimates, small plots, or 

transects can provide rapid estimates 

and help when assessing cumulative 

effects of repeated treatments.

• Expect some changes in postburn 

soil properties such as (1) lower total 

carbon (C) and nitrogen (N), root biomass, microbial community size and 

function, and water infiltration; (2) 

increased calcium (Ca), magnesium 

(Mg), potassium (K), pH, inorganic 

N, and tree growth from the “ash-bed 

effect.”

• Where ground coverage is high (>15 to 

30 percent) and large-diameter fuel is 

prevalent, consider leaving some piles 

unburned for several years or scattering some of the wood to decay on site.
53

Fuel Reduction Practices and Their Effects on Soil Quality

Soil heating during pile burning can be extreme. Massman and Frank (2004) 

measured soil temperatures of 400 °C beneath a large slash pile, with temperatures 

remaining elevated for several days. Significant changes in soil physical, chemical, 

and biological properties are likely in this circumstance (see table 2). But not all 

pile burns result in extreme soil temperatures or soil damage (Busse et al. 2013, 

Meyer 2009). The severity of an individual burn depends on a complexity of fuel, 

soil, and climatic conditions at the time of ignition, making it difficult to generalize 

about the effects of pile burning on soils (fig. 25). Here we offer a brief background 

on observed soil responses to pile burning, along with (1) results from a case study 

from the Lake Tahoe Basin that compare the relative importance of pile size, fuel 

composition, and distance from streams on soil and site quality, and (2) an outline 

of factors to consider when specifying preferred pile conditions.

Background— 

Studies have reported a variety of changes in surface soils from pile burning. 

Common responses include volatile loss of organic C and N, transient increases in 

plant-available N and phosphorus, reduced microbial activity, and changes in soil 

mineralogy (Covington et al. 1991, Jiménez Equilín et al. 2007, Loupe et al. 2007, 

Massman and Frank 2004, Miller et al. 2005). Ulery and Graham (1993) noted redder hues in surface-burned soils, significantly reduced organic C contents, and the 

formation of sand-sized aggregates that altered the soil texture. In addition, they 

observed the collapse of interlayer spacing of secondary clay minerals and the dehydroxylation of iron-bearing phyllosilicates (Ulery et al. 1996). Goforth et al. (2005) 

observed a significant increase in calcium carbonate and soil pH in surface soils 

where logs had thoroughly combusted. At temperatures that normally occur under 

pile burns, Hubbert et al. (2006) noted an increase in soil bulk density and a decline in soil porosity, factors that could lead to reduced water infiltration. Decreased 

infiltration in the surface soil may also occur owing to soil water repellency formed 

from condensation of organic compounds in the soil profile (DeBano 1981). 

Pile burning is also responsible for the so-called “ash-bed effect” in which the 

release of nutrients (particularly N, Ca, Mg, K) from organic materials can temporarily augment soil fertility (Knoepp et al. 2005). As an example, York et al. (2009) 

found 10-year height and diameter growth of conifer seedlings as high as 49 percent 

greater within pile burn perimeters compared to adjacent, unburned ground. 

Because the study area was generally free of competing vegetation, they surmised 

that the tree response was due to improved soil N availability following burning. 
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Pile burning in the Lake Tahoe Basin: a case study— 

Lake Tahoe straddles the borders of California and Nevada in the Sierra Nevada 

Mountains, and is treasured for its deep blue waters and picturesque scenery. 

Extensive timber harvesting in the 1800s, grazing, fire exclusion and suppression, 

drought-induced insect outbreaks, and urban development have resulted in substantial changes in the composition and structure of the surrounding forests in the Lake 

Tahoe Basin (LTB). Today’s forests have higher fuel loads, stand densities, and 

presence of shade-tolerant tree species than were found prior to settlement (Taylor 

2004). Consequently, LTB planners and managers are balancing their efforts to 

reduce fuel hazards and restore forest condition while minimizing the input of sediments and nutrients into Lake Tahoe. 

Pile burning of forest residues is now commonly practiced throughout the LTB 

as part of restoration efforts to reduce wildfire hazard and improve forest condition. Carol Shestak USDA Forest Service Ken Hubbert Ken Hubbert

Figure 25—Piling, curing, and burning of slash piles.
55

Fuel Reduction Practices and Their Effects on Soil Quality

To better understand the effects of pile burning on soil properties and processes, we 

installed a series of research plots to address the following questions:

• How hot does soil get beneath pile burns? 

• To what extent does pile size or fuel composition affect soil heating? 

• Does burning within riparian zones result in unwanted release of nutrients 

to streams?

• What are the short- and moderate-term effects of burning on selected soil 

physical, chemical, and biological properties?

• What is the range in ground coverage occupied by piles within the LTB? 

Soil heating was recorded at several depths in the soil profile beneath piles 

ranging from 1.8- to 6.1-m (6- to 20-ft) diameter with dominant fuel composition 

ranging from small woody slash to large-diameter bole wood. Burning was conducted in late fall, with fuel consumption essentially complete. Nutrient release in 

surface and subsurface water and changes in soil chemical, physical, and biological 

properties were monitored for two growing seasons after burning. Findings were as 

follows:

• Fuel composition was the primary driver of soil heating. Piles containing large wood from insect-killed trees reached extreme temperatures for 

extended durations, whereas piles from small-diameter thinning operations 

had shorter heat durations and moderate temperatures (fig. 26).

• Pile size had little effect on maximum soil temperatures or heat duration 

except when large-diameter wood was present in high amounts.

• Soil temperature declined rapidly with soil depth (fig. 26). The highest 

temperatures and durations were measured in the surface 5 cm, suggesting 

that the heating effects during pile burning are limited to surface soil. Also, 

soil heating was greatest beneath the pile center and declined considerably 

toward the pile edge.

• Nutrient release in surface and subsurface water was relatively low in the 

initial year after burning. 

• Short-term changes in physical, chemical, and biological properties were 

found in the surface 5- to 10-cm of mineral soil. Mixed responses in the 

initial year included loss of fungal biomass, increased soil C content (from 

ash), and inconsistent changes in soil water repellency, total soil N, inorganic N, microbial biomass, and nutrient content.

• Ground coverage occupied by piles averaged 8 percent, with a range from 1 

to 35 percent (n = 71 sites). Only a few sites, those with high tree mortality 

and, consequently, high fuel loads, exceeded 15 percent ground cover.
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Figure 26—Soil heat pulse during pile burning in the Lake Tahoe Basin varies considerably depending on fuel 

type. (adapted from Busse et al. 2013). 
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Fuel Reduction Practices and Their Effects on Soil Quality

Factors to consider when selecting pile specifications–

Factors to consider when selecting pile specifications include pile conditions, pile 

size, pile spacing, and repeated pile burning needs.

1. Pile conditions:

• Fuel composition. Fuel composition differs from site to site as a function 

of pretreatment fuel loading, stand conditions, and associated management 

objectives. For example, piling of precommercial thinning slash in conifer 

stands typically results in an assortment of small- and medium-sized fuels 

(<23 cm diameter), whereas piles built in areas with substantial tree mortality may contain high amounts of large woody material (fig. 27). Burning 

of large bolewood can have a tremendous effect on heating dynamics 

(Monsanto and Agee 2008) and postburn soil properties (Hebel et al. 2009). 

Results from the Tahoe case study further suggest that wood size is the primary factor controlling soil heating during pile burning. A simple precaution in cases where the consumption of large wood is anticipated would be 

to ensure that the percentage of ground coverage occupied by piles is well 

under 15 percent. 

Figure 27—A worst-case scenario. Preparing for pile burning of an entire stand of beetlekilled lodgepole pine (Pinus contorta Douglas ex Loudon). Ground cover in this example is 

near 30 percent.

Matt Busse
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Figure 28—Flaming and upward movement of heat.

USDA Forest Service, Deschutes National Forest.

• Fuel moisture. Reasonably dry and well-cured fuels lower the risk of a 

smoldering fire and, thus, lower the potential for high soil heating. 

• Pile quality. Excessive mixing of mineral soil within piles can restrict 

the upward flow of heat and presumably result in excessive soil heating. 

However, to our knowledge, no published study has quantified a threshold 

for soil content within piles that translates to excessive trapping of heat. 

Modern forestry practices (e.g., use of brush rakes, operator awareness in 

limiting soil additions during tractor piling) are well suited to promote the 

construction of machine piles with a minimum of soil mixing. 

2. Pile size:

Large piles with high fuel loads (e.g., machine piles) obviously generate more heat 

than smaller piles (e.g., hand piles) assuming near-complete fuel consumption. But 

this factor alone does not dictate that the highest soil temperatures or the greatest damage will be found beneath the largest piles. Most heat energy rises during 

burning (fig. 28) (Hungerford et al. 1991), particularly if fuels are well cured and 

dry, suggesting that the degree of soil heating is not proportional to the size of a 

pile. In support of this claim, Seymour and Tecle (2005) showed little effect of pile 

size on postfire soil physical properties, and Busse et al. (2013) found no significant 

relationship between pile size and maximum soil temperature or heat duration for 

piles ranging from 1.8- to 6.1-m (6- to 20-ft) diameter in the LTB case study. 
59

Fuel Reduction Practices and Their Effects on Soil Quality

Figure 29—Amount of ground coverage occupied by piles within a treatment 

area as a function of pile diameter and total fuel volume (high = 1300 m3

 ha-1, 

moderate = 700 m3

 ha-1, low = 200 m3 ha-1).

Pile size can influence the amount of ground disturbance within a treatment 

unit. Numerous small piles result in greater ground coverage compared to fewer, 

large piles, assuming a common fuel loading per hectare. Thus the potential exists 

for more damage on an area basis if smaller piles are constructed. However, the 

differences in ground coverage resulting from pile size are not great, particularly 

for low and moderate fuel loads (fig. 29). This suggests that decisions regarding 

optimal size and number of piles per treatment unit can be made in most cases 

based on factors such as cost effectiveness, fire risk, and operator safety, rather than 

potential soil effects. 

3. Pile spacing:

Extreme soil heating may be of little concern if it occurs beneath widely spaced 

piles that occupy little of the total land surface. Conversely, site and soil quality 

damage will occur when soil heating is extreme and the treatment unit has a high 

density of piles or plans for repeated treatments. Figure 30 identifies the number 

of piles per hectare required to exceed 15 percent ground cover for piles ranging 

in diameter from 1.8 to 6.1 m (6 to 20 ft). For example, it takes nearly 620 piles per 

hectare (250 piles per acre) to exceed 15 percent ground cover when the average pile 

diameter is 1.8 m (6 ft). Determining ground coverage and ensuring that it does not 

exceed 15 percent, when practicable, is an easy step that can be estimated from a 

simple field measurement of the number of piles per hectare within a treatment unit.

4. Repeated pile burning:

The cumulative effects of pile burning are not well understood; we could find 

no scientific literature on the topic. Whether the effects on soil are additive and 

therefore become increasingly detrimental to soil functioning with successive 
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Figure 30—Estimating ground coverage occupied by piles for a range of pile sizes.

treatment, or, alternatively, whether soils effectively recover between burn cycles 

is untested. Certainly any cumulative effects will vary from site to site depending 

on burn severity, soil resilience, extent of ground coverage, and the length of time 

between burns. Frequent burning of wood piles on low fertility sites, for example, 

may be detrimental, whereas pile burning of mostly small slash material every 10 

to 15 years on more productive sites is likely of little concern. But beyond these 

simple examples, it is impossible to make any definitive statements without relying 

on unproven assumptions. Again, a prudent approach to deal with this unknown 

is to document pile burn conditions for each entry. Qualitative assessments of 

fuel type, ground coverage, and burn severity can be made rapidly and entered in 

a database format to document site history for use in planning additional burns. 

Simple quantitative measures of soil quality, such as organic matter content or 

water infiltration rate, may provide additional planning insight for those sites with 

unique soil concerns.
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Fuel Reduction Practices and Their Effects on Soil Quality

Coarse Woody Debris

Key points:

Ecological effects Management considerations

• Coarse woody debris (CWD) contributes to wildlife habitat, erosion 

protection, and fire hazard in many 

ecosystems. However, little definitive 

evidence exists proving or disproving 

the essential role of CWD in maintaining soil quality.

• Determining appropriate CWD retention levels remains a guessing game. 

Consider developing site-specific 

guidelines that set the upper limit 

based on fire hazard concerns and 

the lower limit by wildlife habitat 

needs or by erosion control on steep 

ground. Any possible contributions 

to soil quality can then be viewed as 

fortuitous.

• Higher CWD loadings may be 

appropriate where larger log sizes 

dominate because of their benefit to 

wildlife habitat.

• Spatial variability in CWD is often 

pronounced at broad landscape 

scales. Meeting rigid targets may be 

neither feasible nor desirable on a 

treatment-area basis.

Coarse woody debris (CWD; woody material >7.5 cm diameter) is well recognized as a fundamental structural and functional attribute in many forests (Harmon 

et al. 1986). Fire, insects, ice storms, and windstorms can introduce large quantities 

of CWD to the forest floor as trees fall and break. Woody debris, in turn, provides 

habitat and protection for wildlife, acts as an erosion control barrier, and serves 

as an energy source for numerous fungal and insect species (Harmon et al. 1986). 

Decaying wood also provides available summer moisture as well as habitat for fine 

roots, micro-organisms, and mycorrhizal fungi. During summer dry periods, fungal 

hyphae penetrate CWD and extract water and nutrients for critical sustenance. A 

food chain develops as invertebrates feed on the microbes, and in turn, mammals, 

reptiles, and birds feed on invertebrates. Nitrogen is released back into the soil 

through microbially mediated nonsymbiotic N fixation that is common in CWD. 

Additionally, nutrients and water are provided to conifers through associations with 

ectomycorrhizal fungi that inhabit decaying CWD. 

Guidelines for CWD retention have been developed in many regions to safeguard the long-term presence of downed wood. For example, Graham et al. (1994) 

used ectomycorrhizae as a bio-indicator and recommended the following amounts 
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of CWD to be left onsite in Montana: 11 to 20 Mg ha-1 (5 to 9 tons ac-1) (Douglasfir/ninebark), 27 to 54 Mg ha-1 (12 to 24 tons ac-1) (Douglas-fir/pine grass; subalpine fir/bear grass; subalpine fir/twinflower), 16 to 31 Mg ha-1 (7 to 14 tons ac-1) 

(grand fir/bear grass), and 18 to 40 Mg ha-1 (8 to 18 tons ac-1) (subalpine fir/blue 

huckleberry). In Arizona, they recommended 11 to 29 Mg ha-1 (5 to 13 tons ac-1) 

for ponderosa pine/fescue and Gambel oak habitat types. These values were later 

adopted by Brown et al. (2003) in their recommendation for CWD levels in inland 

West forests even though they remain to be rigorously tested or validated. 

Recruitment of CWD to meet forest guidelines, including the provision of 

multiple decay classes (fig. 31), requires thoughtful planning. This is particularly 

true for fuel reduction treatments, as they can potentially modify CWD mass and 

distribution across the landscape. For example, prescribed fire can consume CWD, 

particularly logs in the later stages of decay (sound CWD is typically left unconsumed by underburning), whereas thinning operations may fragment decaying logs 

and disrupt their functional integrity. Alternatively, both practices may boost levels 

of small-diameter CWD if killed trees are retained onsite. 

How much CWD to leave after prescribed burning or thinning does not come 

with a refined answer. Differing climates, forest types, and fire-return intervals all 

suggest the necessity for site-specific CWD guidelines. Adding to this uncertain 

equation is the fact that the spatial distribution of CWD is typically quite variable 

at landscape scales. For example, Rubino and McCarthy (2003) found that slope 

position strongly influenced the distribution and abundance of CWD. Lower slope 

positions had greater accumulations of CWD as a result of transport from upslope 

positions. Similar observations of spatial variability led Ganey and Vojta (2010) to 

suggest that maintaining average CWD loadings on every piece of ground is neither 

feasible nor desirable. They encouraged the use of malleable, landscape-scale 

targets that account for the many ecological roles of CWD while also addressing the 

reality of spatial heterogeneity and fire hazard concerns.

In this context, it is intriguing to ask whether CWD serves as an important contributor to soil productivity. Removing woody biomass from the forest floor rather 

than letting it decompose onsite may affect soil chemistry, which in turn influences 

soil fertility and plant growth. Further, CWD is an active source of nonsymbiotic 

N fixation, accounting for up to 50 percent of the total N fixed in some forests 

(Jurgensen et al. 1997). However, the relative amount of N fixed in CWD is exceedingly small compared to the total pool of ecosystem N. And although decomposing 

wood helps replenish soil nutrients, the net benefit is thought to be small because of 

the exceedingly low nutrient content of CWD. As an example, Prescott and Laiho 

(2002) noted that CWD did not contribute significantly to C, N, or phosphorus (P) 
63

Fuel Reduction Practices and Their Effects on Soil Quality

Figure 31—Photo series showing the five coarse woody debris decay classes: top left—decay classes 1 and 3; top right—decay 

class 2; bottom left—decay class 4; and bottom right—decay class 5. Ken Hubbert

Ken Hubbert

Ken HubbertKen Hubbert

cycling in the coniferous forests of the Rocky Mountains. At fir sites, they reported 

that CWD released 5 percent of the N, as compared to pine sites where the contribution of CWD to N and P release was ≤2 percent. In coniferous forests of central 

Oregon, Busse (1994) also found that the role of CWD in nutrient cycling was 

small. Adding to these findings are observations that CWD often occupies only a 

small fraction of the ground area in fire-adapted forests (Ganey and Vojta 2010, 

Stephens et al. 2007). Collectively, these observations suggest that CWD may not 

appreciably improve soil productivity or soil quality in many systems, and that 

managers may first consider other ecosystem components such as wildlife habitat 

or erosion control rather than soil nutrient cycling when identifying desired conditions for CWD. 
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Mechanical Fuel Reduction Treatments

Treating hazardous fuels in densely stocked stands often requires removing or processing many small-diameter stems per hectare. Conventional harvest techniques 

are being modified to reduce crown and ladder fuels, but few equipment options 

have been specifically designed to treat these fuels (Rummer et al. 2005). Equipment innovation will surely prompt questions about soil impacts. For example, 

novel use of in-woods chippers for fuel reduction can chip trees up to 60 cm (2 ft) 

diameter directly into a box mounted on a forwarder. By eliminating the need to 

skid the trees to a landing, soil compaction or displacement may be reduced, but the 

vehicle tire pressure and soil conditions will also play a role in minimizing compaction. Increasingly, whole-tree harvest methods are employed to treat fuels because 

smaller trees, including nonmerchantable materials, are targeted for removal and 

few slash treatments are required. Mechanical fuel treatments do not always include 

the harvest or removal of forest products, however. Onsite treatments can reduce 

ladder or crown fuels by shredding or crushing them at the stump and rearranging 

them on the forest floor, or cutting and piling them into burn piles. 

Forest soil scientists have measured impacts of heavy forestry equipment and 

skid trails for more than 50 years, using physical parameters including bulk density, 

permeability, and macroporosity (Dyrness 1965, Steinbrenner and Gessel 1955). 

Although skidding equipment and the forest products they remove have changed 

substantially during this time, skidding impacts are largely unchanged—skid 

trails increase bulk density and reduce permeability and porosity compared to 

surrounding areas. To limit the deleterious conditions caused by skidding, much 

of the research supports the notion of creating a designated skid trail system and 

reusing existing skid trails (Ampoorter et al. 2007, Han et al. 2009, McIver et al. 

2003, Olsen and Seifert 1984). However, the previous transportation network may 

be poorly suited for the current harvest. For example, landings may be located in 

sensitive areas such as riparian zones or near seeps or springs. If a landing will not 

be used, it is highly unlikely that the skid trails servicing that landing could be used 

either. Multiple treatments are often necessary to meet fuel management objectives 

(Reinhardt et al. 2008). Mechanical harvests can leave a surface fuel bed of slash, 

or regeneration of brush and trees over time can create an undesirable fuel load that 

may warrant additional treatments. Without careful planning, repeated stand entries 

can increase the skid trail density to cover up to 80 percent of the stand (Froehlich 

1981).
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Fuel Reduction Practices and Their Effects on Soil Quality

Whole Tree Harvesting and Nutrient Removal

Key points

Ecological effects Management considerations

• Whole tree harvesting transports more nutrients offsite than 

removing only tree stems.

• In most cases, the site reserves will 

remain adequate to supply nutrients to 

remaining vegetation.

• Gain perspective by comparing estimated levels of nutrient removal with the 

amount held in the soil or whole ecosystem. For a ballpark estimate of the nutrients removed in crown material:

(1) Estimate crown fuel biomass, based on 

measurements or literature.

(2) Estimate crown nutrient content, based 

on measurements or species-specific 

or site-specific literature.

(3) Determine the percentage of crown to 

be removed.

 Nutrients removed = (1) × (2) × (3).

• Nutrient-poor sites are most sensitive to whole tree harvesting:

▪ Glacial outwash sands

▪ Shallow soils

▪ Many coarse-textured soils

• Options to minimize nutrient loss:

▪ Harvest deciduous stands when dormant, so foliar nutrient losses are minimized.

▪ Backhaul slash and redistribute within 

the harvest area, being mindful of fuel 

loads.

▪ Extend the reentry period to allow more 

time for nutrient inputs.

▪ Fertilize.

• More nutrients are removed by 

whole tree harvesting productive 

sites.

• Productive sites are more resilient to 

nutrient removal. They tend to have 

greater rates of nutrient input and cycling, 

so have more to give.

Why whole tree harvesting is of concern— 

Whole tree harvesting removes the entire aboveground portion of a tree (fig. 32). 

This raises concerns about nutrient loss and long-term site productivity because 

branches and foliage are removed along with the tree stems (Janowiak and Webster 

2010). Although the foliage makes up a relatively small proportion of tree biomass, 

typically ranging from about 5 to 15 percent, depending on species and diameter 

(Ter-Mikaelian and Korzukhin 1997), it can contain more than half the nutrients 

stored in a tree (Little and Shainsky 1995, Phillips and Van Lear 1984). Two to three 

times the nutrients are removed from a site when the crowns are harvested along 
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with the boles (Alban et al. 1978, Patric and Smith 1975). With time, any harvesting 

of nutrients can deplete the soil if they are not balanced by inputs from precipitation, dry deposition, N fixation, and mineral weathering.

Many whole tree harvest studies document clearcutting effects— 

Concerns over whole tree harvest on site nutrients have existed for decades (e.g., 

Kimmins 1977, Mälkönen 1976), and until recently, most research in U.S. forests 

has focused on clearcut harvests. Recent studies found that whole tree clearcuts led 

to only slight or insignificant reductions in tree growth in the regenerated stands 

relative to bole-only clearcuts. Following clearcuts in the U.S Pacific Northwest, the 

whole tree treatment resulted in a minor decrease in basal diameter growth, about 

1 cm, compared to the bole-only treatment at 5 years of age (Ares et al. 2007). 

Concentrations of foliar N did not differ between the treatments, and the authors 

suggest that the small growth differences may have been caused by changes in 

microclimate between the treatments. A similar diameter decrease of 1.5 cm was 

measured in a 23-year-old plantation in North Wales following whole tree clearcutting (Walmsley et al. 2009). At this site, there were no significant differences in 

stand height, basal area, or density between whole tree and bole-only treatments at 

this age. In the Southeastern United States, Johnson et al. (2002) found no effect of 

clearcut residue treatments on tree biomass after 15 years at a site in Tennessee. In 

the same study, biomass was reduced 17 percent following whole tree clearcutting 

after 18 years at a site in South Carolina. The relative increased growth following 

bole-only harvesting was attributed to both greater nutrient uptake by trees and soil 

enrichment from the logging residues left onsite. 

Figure 32—Feller bunchers are commonly used to harvest whole trees during fuel treatments. (A) A swing boom 

feller buncher can reach out to harvest fuels, eliminating the need to drive to each tree. (B) Whole tree harvest 

material is often processed at a landing.

Seth Bigelow

Seth Bigelow
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Fuel Reduction Practices and Their Effects on Soil Quality

In addition to comparing vegetative growth following whole tree harvesting, 

studies often analyze soil nutrient capital to assess productivity impacts. A metaanalysis of data from 13 publications showed that whole tree clearcuts reduced soil 

C and N by an average of 6 percent relative to pretreatment data or controls (Johnson and Curtis 2001). A number of the included studies removed the forest floor in 

addition to logging residues. In soils under 10-year-old plantations, C concentrations declined where both forest floor and harvest slash was removed, but total C 

pools remained unchanged relative to the bole-only clearcut treatment (Powers et 

al. 2005). Powers et al. (2005) emphasized that the drop in C concentration was due 

to the loss of the forest floor rather than removal of harvest slash. Studies of stands 

greater than about 15 years old suggest that whole tree clearcut impacts to soil C 

and N stocks diminish with time (Jandl et al. 2007, Johnson et al. 2002, Jones et al. 

2008, Walmsley et al. 2009). 

Fuel treatments are often designed as stand thinnings, and remove far less 

biomass than a clearcut prescription. For example, fuel reduction thinnings in dense 

Sierra Nevada stands removed an average of 12 percent of the standing live volume 

(Collins et al. 2007), which in this case was equivalent to 21 percent of the basal 

area (Stephens and Moghaddas 2005). Fuel reduction treatments in dense stands 

typically reduce basal area by 20 to 45 percent (Boerner et al. 2008a) while retaining a majority of the standing volume onsite. Fuels are often thinned from below, so 

that understory, suppressed and intermediate trees are thinned before codominant 

or dominant ones. These lower crown positions have proportionately less canopy 

biomass (Reinhardt et al. 2006), and therefore fewer canopy nutrients, than the 

dominant overstory. Relying on whole tree clearcut studies to infer nutrient loss 

impacts following fuel reductions would grossly overestimate effects to soil nutrient 

pools and stand productivity.

Most research on whole tree thinning comes from Nordic countries— 

Few whole tree thinning studies were found for U.S. forests, let alone fuel reduction 

thinning. However, in a fuel reduction study in dry forests of central Oregon, Busse 

et al. (2009a) compared the effects of whole tree harvest, bole-only removal, and 

thinning without biomass removal on vegetation responses. With periodic measurements spanning 17 years following the treatments, they found no differences among 

treatments in tree growth, shrub cover, or herbaceous biomass. When compared to 

the other residue treatments, whole tree harvest did not reduce the site potential or 

soil nutrient status of the relatively infertile sites they studied (Busse and Riegel 

2005). 
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Whole tree thinning studies are most prevalent in forests of Nordic countries 

(i.e., Denmark, Finland, Norway, and Sweden), and have shown varying and 

inconsistent impacts on site productivity. Nearly all studies examined treatments in 

Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies [L.] Karst.) stands, 

two dominant forestry species in this region. In an analysis of 15 thinned stands 

of young Norway spruce and Scots pine, Jacobson et al. (1996) found, on average, 

no growth differences between whole tree and conventional (bole-only) harvests 

5 years after treatment. However, they did observe both significant increases and 

decreases in growth in individual whole tree harvested stands. Ten years after these 

thinnings, they found that the whole tree harvest reduced stand growth an average 

of 5 to 6 percent compared to bole-only harvesting (Jacobson et al. 2000). Again, 

they also observed significant growth increases and decreases in individual stands. 

Other studies have shown only short-term growth reductions or no productivity 

impacts following whole tree thinnings. A Danish study of early whole tree thinnings of Norway spruce reported growth reductions for four growing seasons after 

thinning, but no significant growth differences over the next 6 years (Nord-Larson 

2002). In addition, several studies in Sweden found no significant growth reductions 5 to 10 years after whole tree thinning treatments (Egnell and Leijon 1997, 

Mård 1998). In studies where reduced tree growth was observed, the effect was 

generally attributed to nutrient removals during the whole tree harvest. Some 

authors (e.g., Jacobson et al. 2000) allow that reduced growth may also be the indirect effect of altered microclimate and mineralization rates resulting from thinning, 

or to greater competition with ground vegetation. Where no growth reduction was 

measured, study authors suggested that atmospheric deposition of nutrients may 

have compensated for losses during the harvest.

Nutrient balance sheets: how much is removed from the existing reservoir?— 

Balance sheets are useful to compare nutrient inputs, outputs, and onsite reserves 

(Smith et al. 1986). We can estimate the amount of nutrients removed during whole 

tree fuel reduction by first taking stock of what is held in the crowns and boles and 

then asking how much material is removed. For example, canopy fuels in dense 

stands with high fire hazard conditions in the Western United States ranged from 

9 to 21 Mg ha-1 (Reinhardt et al. 2006). Other extensive studies examining conifer 

crown fuel loads in the Western United States also fell within this range (Cruz et 

al. 2003, Fulé et al. 2001). With a few assumptions about the N content of the leaves 

and amount of crown removed, we determined that 65 to 150 kg N ha-1 might be 

removed as crown material during fuel treatment in these stands (fig. 33). Assuming 

an equal portion of N is stored in the wood, 65 to 150 kg N ha-1 may also be removed as bole material. For many U.S. forests, 130 to 300 kg N ha-1 represents only 
69

Fuel Reduction Practices and Their Effects on Soil Quality

a few percent of the N stored in the soil, and even less of ecosystem N onsite. Of 

course, the actual amount of N removed will differ by stand and thinning treatment. Remember that N may be chronically added to forest stands by deposition, or 

lost by leaching, the levels of which vary by geographic region. By estimating local 

deposition or leaching rates, one can consider harvest N removals in terms of how 

quickly they will be replenished (table 8). Fertile sites with deep, rich soils are more 

resilient to whole tree harvests than poor sites, such as shallow soils over bedrock or 

coarse textured soils (Raulund-Rasmussen et al. 2008). More nutrients are generally exported during whole tree harvests from fertile sites compared to poor ones, 

Figure 33—Conceptual approach to estimate N losses resulting from whole tree fuel reduction thinning. To 

evaluate the impact of a single harvest removal, it is important to consider annual N deposition into the system, 

leaching out of the system, and the standing pool stored in the soil.
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but higher levels of nutrient inputs and cycling rates often allow for rapid replacement of the lost nutrients. Busse and Riegel (2005) estimated that whole tree fuel 

reduction in central Oregon removed 4 percent of ecosystem N, whereas bole-only 

harvest removed 1 percent. A whole tree clearcut in coastal Washington removed 

only about 5 percent of ecosystem N, compared to 3 percent for the bole-only harvest (Ares et al. 2007). In both these studies, differences in subsequent vegetative 

growth between bole-only or whole tree harvests were minimal. 

Before developing a nutrient balance sheet, it can be useful to consider the 

greatest risk factors for nutrient loss from whole tree harvest: soil depth, site 

class, texture, and nutrient buffer. These are highlighted in the decision tool below 

(fig. 34). 

Existing guidelines for whole tree harvesting and biomass or slash removal— 

Because of the increased focus on fuel treatments and removals of nonmerchantable biomass as an energy resource, a number of states have developed management 

guidelines or recommendations that address the impacts of forest removals on site 

nutrients, soil productivity, and long-term sustainability. Although biomass harvests 

may not be designed to reduce wildfire hazard, they often remove whole trees and 

slash from forest stands, and can lead to similar concerns regarding soil nutrient 

Table 8—Example soil nitrogen (N) capital and balance accounts for a site 

thinned by whole tree harvest 

Credit/debit N kg ha-1 Explanation

Soil pool 9800 The soil acts as a large reservoir to store N. Most soil 

N occurs in organic form, which is not readily available for plant uptake and cannot be easily leached. 

Plant-available inorganic N is slowly released 

through decomposition and mineralization pathways.

Harvest exporta -200 Whole tree thinning removes some of the N capital 

from the site. The time required to replenish lost N 

depends on rates of inputs and outputs. Harvesting 

alters soil microclimate, which can increase or decrease the amount of N available for plant uptake by 

altering rates of decomposition and N mineralization.

Annual deposition 7 Nitrogen is continually added to terrestrial systems 

as both dry and wet deposition.

Annual leaching -2 Nitrogen leaching losses typically occur as plantavailable nitrate.

a

 The harvest removal represents less than 3 percent of the soil N pool. Assuming that deposition and leaching rates 

remain constant, N is added in this example at a rate of 5 kg ha-1 yr-1. The N removed from the whole tree thinning 

treatment would be replenished in about 40 years, or sooner if the abundance of N-fixing vegetation increases after 

treatment.
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Fuel Reduction Practices and Their Effects on Soil Quality

Figure 34—Whole tree harvest decision tool.

loss as whole tree harvested fuel treatments. Several states maintain guidelines for 

whole tree harvesting, and generally recommend avoiding whole tree methods only 

in shallow, nutrient-poor soils or in sensitive ecosystems (table 9).

Mitigating nutrient impacts from whole tree harvesting— 

There are numerous options to compensate for nutrient losses that result from whole 

tree harvesting. More nutrients can be kept onsite by harvesting in the fall or winter. Wood becomes more brittle during this time and is more likely to break during 

thinning activities. Leaving broken branches or tops in the stand will reduce the 
72

general technical report psw-gtr-241 Table 9—State-level recommendations for whole tree harvesting and biomass or slash removal under differing forest conditions Site condition Management considerations and recommendations State General harvest sites Current guidelines, allowing for slash and biomass removal, are deemed adequate to protect forest resources Oregona Moderate to deep loamy soil development; good nutrient status Bole-only thinning preferred, but whole tree probably okay Montanab Moderate to deep soil development; moderate nutrient status Bole-only thinning recommended, but whole tree may be okay Select for trees with moderate to low nutrient demands Montanab Poor soil development with low-nutrient status Bole-only thinning recommended, but whole tree may be okay for thinning from below or other light thinning 

Select for trees with low nutrient demands

Montanab

Aspen or hardwood on well-drained sandy soil, or Aspen or hardwood on shallow soil (≤ 20 cm) over bedrock

Avoid whole tree harvest Retain/redistribute slash

Avoid short rotationsExtend rotation ageFertilize

Minnesotac

Brush lands and open lands Nutrients usually replenished within 10 years MinnesotacGeneral harvest sites Retain fine wood < 10 cm diameter created by incidental breakage 

Retain/scatter fine woody material from 10 percent of trees removed

Wisconsind

Sensitive ecosystems or areas with high conservation needs 

(old growth, vernal pools, large bogs, seeps, caves), or 

Shallow soil (≤ 50 cm) over bedrock, orDry, nutrient-poor sandy soil, or Wetland soil with ≥ 40 cm organic matter

and are nutrient poor with low pH (dysic Histosols)

Do not remove fine woody material < 10 cm diameter Wisconsind

Low fertility site, shallow to bedrock soil, coarse sandy soil, 

poorly drained site, or erosion prone site

Minimize removal of fine woody material < 10 cm diameter Mainee

a Oregon Department of Forestry (2008).b Garrison-Johnston and Johnson (2006).c Minnesota Forest Resources Council (2005).d Herrick et al. (2009).e Benjamin (2010).
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Fuel Reduction Practices and Their Effects on Soil Quality

nutrients exported offsite. Harvesting trees during their dormant period will also 

reduce nutrient exports, as leaves retranslocate their nutrients to the roots and other 

components at senescence (Nambiar and Fife 1991). Thinning deciduous trees after 

leaf drop will reduce foliar export out of the stand. Marking guidelines that account 

for species nutrient requirements can also help offset nutrient removals. Thinning 

trees with high nutritional needs while retaining more frugal species can reduce the 

overall nutrient demand in the residual stand. For example, Garrison-Johnston and 

Johnson (2006) ranked forest species in Montana from highest to lowest nutritional 

needs, but unfortunately did not provide information on the relative levels of nutrients required: grand fir > Douglas-fir > white pine > ponderosa pine > lodgepole 

pine > western larch. Where species exhibit great differences in nutrient needs, 

such rankings could become a consideration in marking guidelines for fuel reduction, in addition to crown spacing, shade tolerance, and fire behavior characteristics. Five years after whole tree thinning to reduce fuels and restore stand structure, 

units where trees were retained based on species preference had different levels 

of available N in the soil and total N in the forest floor compared with units where 

retention trees were based on size (Miesel et al. 2008). If these effects persist for decades, the nutrient differences could further affect stand productivity and resilience.

In many cases, whole trees are skidded to a landing where processors such as 

delimbers remove the nonmerchantable material from the bole. Rather than chipping or burning the slash, skidders could backhaul some or all of it into the unit to 

redistribute the nutrients onsite. This would allow for efficient harvesting equipment such as feller bunchers to fell trees, while reducing the nutrient losses. Rich 

(2004) addresses some operational constraints and practices to help make backhauling of slash a feasible option. Consideration should also be given to limit fuel loads 

in the redistributed slash, however.

An additional consideration to help prevent unsustainable mining of the site is 

to extend the reentry period between treatments to allow greater nutrient accumulation from deposition, N fixation, mineral weathering, and mineralization. In some 

cases, the best mitigation for nutrient removals may be to apply fertilizers. This 

requires knowledge of which nutrients should be replaced and in what proportions. 

In all cases, the reduction of nutrients from whole tree thinnings can be offset by 

practical management practices. These treatments should be site specific, based on 

the unique characteristics of each stand.
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Soil Compaction

Key points:

Ecological effects Management considerations

• Soils are easily compacted. Drastic 

changes in soil and site productivity 

may follow.

• Adhere to best management practices 

during fuel reduction harvesting:

▪ Keep machinery off moist soils (harvest when soils are dry or frozen).

▪ Use low ground pressure equipment 

if possible.

▪ Use designated skid trails or limit the 

number of random passes.

▪ Harvest on top of deep slash layers 

for high-risk soils.

▪ Consider subsoiling or scarification to 

restore compacted soils.

• One size does not fit all. 

Detrimental effects of compaction 

are site-specific. 

• Identify high-risk soils:

▪ Clay and silt-sized particles are most 

prone to detrimental compaction. 

▪ Rock fragments, soil organic matter, 

and forest floor cover help reduce 

compaction potential.

▪ Avoid harvesting on poorly drained 

soils when possible.

• Ten-year results from the LongTerm Soil Productivity study do not 

show clear evidence of detrimental 

changes in site productivity owing 

to severe compaction. 

• Compaction can increase soil water 

availability on sandy soils, leading to 

improvements in vegetation growth.

• Subboreal aspen was an exception to 

the rule—soil compaction substantially reduced stand biomass.

• Confounding effects of soil mixing and 

soil displacement were not tested.

• The results apply only to well-drained 

soils.

Few topics in forest soil science have received more attention than the effects 

of harvesting on soil compaction. More than a half century of research findings, 

anecdotal observations, and management intervention have been directed at 

understanding, assessing, and preventing soil compaction. From early findings that 

identified detrimental impacts of tractor logging on soil physical properties (Munns 

1947, Steinbrenner and Gessel 1955) and associated tree growth (Froehlich 1981) 

to recent advances in logging systems that are more “soil friendly,” considerable 
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Fuel Reduction Practices and Their Effects on Soil Quality

time and effort have gone into developing practical improvements for limiting soil 

damage. The list of critical functions at risk in compacted soil is great (e.g., soil 

aeration, water infiltration, erosion, nutrient and water availability, root growth, site 

productivity) and it underscores the prolonged attention given to the topic. Several 

key points have emerged during this period:

• Most soil types are easily compacted by machinery. However, predicting 

the extent of compaction or its effect on site productivity is quite difficult 

because of the extreme diversity of forest soils, vegetation types, climatic 

regimes, and logging systems. One size does not fit all. 

• Soil moisture content at the time of harvest is a primary determinate of soil 

compaction. Countless studies have shown that soil moisture content near 

field capacity (i.e., wet, but not saturated) is most conducive to compaction (see Greacen and Sands 1980, Howard et al. 1981, McNabb et al. 2001). 

Saturated soils compact less but are at great risk of detrimental, deep rutting by harvesting equipment. Consequently, it is now common to include 

specifications or special clauses in harvesting contracts that require low 

to moderate soil moisture conditions (or frozen soil) before entry. Poorly 

drained soils, riparian soils, and soils that remain wet for much of the year 

(e.g., udic and perudic moisture regimes in the coastal Pacific Northwest 

and Southeast Coastal Plains) are particularly vulnerable to severe rutting 

and thus require additional preventative or mitigative measures—if harvesting is even appropriate—such as fully suspended logging, skyline harvesting, cut-to-length harvesting on top of thick slash beds, use of low ground 

pressure equipment with limited entry, or site preparation techniques such 

as bedding to mitigate damage where water tables are high. 

• Soil compaction is substantially affected by relatively few passes by 

ground-based equipment (McNabb et al. 2001, Parker 2007, Williamson and 

Neilsen 2000). Because of this, the use (and reuse) of designated skid trails 

has been highly encouraged and practiced as an option for limiting the 

aerial extent of site disturbance. 

• Use of low-ground-pressure equipment with wide tires or tracks that avoid 

slippage is well establish in literature and practice as a primary means to 

minimize soil damage. 

• Heavy slash placed on the forest floor during cut-to-length harvesting or 

mastication operations can reduce soil compaction (Ampoorter et al. 2007, 

Moghaddas and Stephens 2008). However, the effect is not universal. 

Benefits to soil physical properties typically require deep slash-beds, large-
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diameter slash, and relatively few passes by harvesting equipment (Han et 

al. 2009, McDonald and Seixas 1997). 

• Soil texture (percentage of sand, silt, and clay) alone is not a good predictor of a soil’s vulnerability to become compacted (Howard et al. 1981). 

Although silt-sized particles are most prone to compacting (Greacen and 

Sands 1980), complicating interactions with other soil properties (organic 

matter content, rock fragments, clay mineralogy, structure and drainage 

properties, initial bulk density, moisture content) prevent straightforward 

predictions.

• Soil recovery is generally a long-term process, particularly for compacted 

layers at soil depths of 10 to 15 cm and greater that develop beneath skid 

trails (Greacen and Sands 1980, Labelle and Jaeger 2011, Page-Dumroese et 

al. 2006). Surface layers may recover faster if soils are exposed to freezethaw cycles or if they contain shrink-swell clay.

• Subsoiling and scarification are effective at accelerating restoration of compacted sites, but are not always practical (e.g., wet or rocky sites) and may 

damage roots in the residual stand.

• The effect of compaction on site productivity is complicated. Tree-growth 

studies have shown short-term responses ranging from detrimental to neutral to positive (Ares et al. 2005, Froehlich et al. 1986, Gomez et al. 2002, 

Miller et al. 2001, Parker et al. 2007, Ponder et al. 2012). The net response 

depends on (1) the degree of compaction, as controlled by many soil properties and machinery specifications; (2) the aerial extent of compaction; 

(3) soil texture—e.g., compaction may improve soil water availability and 

vegetation growth in sandy soils; (4) soil resilience, or the time required for 

recovery; and (5) thinning effects on factors other than compaction such as 

root severing, bole scarring, and conifer release. As stated by McNabb et 

al. (2001), “the consequences of compaction on ecosystem processes...is 

ecosystem specific.”

Lessons learned from fuel-reduction thinning studies— 

Most information on soil compaction comes from studies of clearcut logging. 

However, there is a growing body of literature that addresses the effects of thinning 

on soil compaction (see Ampoorter et al. 2007, Labelle and Jaeger 2011, Landsberg 

et al. 2003, Moghaddas and Stephens 2007, Parker 2007, Parker et al. 2007, Sidle 

and Drlica 1981). Drawing an overall conclusion from these studies is difficult, 

however, because of the variety of harvesting methods and equipment, thinning 

prescriptions, soils, and techniques used to measure compaction. For example, 

Moghaddas and Stephens (2008) found that commercial harvesting by chain saw 
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Fuel Reduction Practices and Their Effects on Soil Quality

felling and grapple skidding followed by mastication of understory trees resulted 

in relatively minor changes in soil properties, whereas Parker (2007) found sharp 

increases in compaction when testing a variety of harvesters and grapple skidders. Landsberg et al. (2003) also tested a variety of ground-based equipment in 

an operational thinning study and found a wide range in degree of compaction and 

surface disturbance, from 17 to 57 percent on an aerial basis, yet they were unable 

to attribute the differences in compaction to specific equipment types. 

Ground-based thinning operations compact soil and result in accompanying 

soil mixing and displacement when trees are skidded to landings. This is accepted 

knowledge. A more immediate concern then is whether such changes are detrimental to site productivity. Of relevance here, Parker et al. (2007) examined site productivity on thinned plots that were either compacted (random grapple skidding) or not 

(no equipment traffic). He found that compaction led to reduced growth of individual trees within 9 m of the most severely affected ground (soil strength about 2,500 

kPa). However, when scaled up to the plot level, there was no significant decline 

in total tree growth or site productivity in 17 years following thinning (Busse et 

al. 2009a, Parker 2007). The compacted zones in effect served as “hotspots” of 

reduced tree growth within a much larger unaffected area. This finding suggests 

that site productivity can be maintained if the aerial extent of severe compaction 

is kept to a minimum, or, alternatively, if a sufficient network of old root channels, 

shrinkage cracks, or soil faunal activity exists within the compacted zone to allow 

for root proliferation (Batey 2009, Nambiar and Sands 1992). Recent findings from 

the Long-Term Soil Productivity study (see below) support this conclusion, as 

numerous research sites throughout Canada and the United States showed minimal 

change in forest productivity even when nearly 100 percent of the ground surface 

was compacted. 

Gaps exist in our knowledge of soil compaction and its effect on site productivity and function. Until the gaps are filled, Parker (2007) outlined several practical, 

simple steps for fuel reduction thinning practices frequently used by managers. 

• Establish realistic, obtainable harvesting goals that address concerns for the 

local soils and their susceptibility to compaction using the best available 

science and local experience. Avoid applying regional generalizations about 

soils and management criteria.

• Include contract specifications and special clauses for acceptable limits of 

compaction, along with incentives for meeting contract requirements and 

penalties for noncompliance. This includes selection of appropriate harvesting methods (e.g., cut-to-length harvesting, whole tree harvesting), soil 

moisture restrictions, and maximum ground pressure that best meet all 
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environmental, operational, and economic goals.

• Meet on the site with all personnel involved before the thinning operation begins to discuss project goals, concerns, soils issues, and contract 

specifications.

• Use designated skid trails (echoed by most studies).

• Establish a plan for contract inspectors to repeatedly monitor progress 

onsite.

Does soil compaction alter site productivity? Recent results from the North 

American Long-Term Soil Productivity Study— 

The Long-Term Soil Productivity (LTSP) program is a large-scale research network of more than 100 forested sites throughout the United States and Canada that 

examines the short- and long-term effects of soil compaction and surface organic 

matter retention on site productivity (Powers 2006). Common LTSP treatments, 

including combinations of soil compaction (none, moderate, severe) and forest floor 

mass (high, medium, none), were installed following clearcut harvesting of residual 

forest stands beginning with the first site installation in Louisiana in 1990. The collective findings from the study offer an insightful and somewhat surprising view of 

compaction and its impact on site productivity. Among the key findings are that soil 

compaction (1) improved productivity and water-holding capacity of sandy soils in 

summer-dry climates (Gomez et al. 2002), (2) dramatically reduced productivity of 

aspen stands regenerating naturally from root suckers (Ponder et al. 2012), and, (3) 

when averaged across the entire network of sites using meta-analysis procedures, 

resulted in greater tree growth compared to uncompacted controls (Ponder et al. 

2012).

A brief background is helpful to understand these findings. Among the original 

objectives of the LTSP study were to determine (1) if changes in soil porosity (compaction) have a lasting effect on forest vegetation, (2) whether compaction effects 

differ by climatic zone or soil type, and (3) whether detrimental impacts, when 

found, are reversible with time (Powers 2006). All LTSP sites were clearcut harvested, then treatments were applied to 0.5-ha plots prior to planting tree seedlings 

(or natural regeneration in the case of aspen). Logs were fully suspended during 

harvesting to ensure that compaction, soil displacement, and soil mixing did not 

occur prior to treatment. Compaction treatments were applied by repeated driving 

on the soil surface with heavy equipment (e.g., asphalt roller, bulldozer, grappler 
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Fuel Reduction Practices and Their Effects on Soil Quality

depending on local availability and preference) or by compressing with high ground 

pressure equipment uniformly across entire plots (Page-Dumroese et al. 2006b). No 

soil displacement or mixing occurred, and thus the results are specific to the effects 

of soil compaction alone. Severe compaction was targeted at 80 percent of the 

restrictive bulk density or soil strength for root penetration at each site. Moderate 

compaction approached the midpoint between severe and no compaction. Interestingly, soils at the 12 LTSP sites throughout California could not be compacted 

without first removing all surface organics down to bare soil (the forest floor was 

later replaced on the soil surface), demonstrating the difficulty of compacting some 

soils. 

The LTSP sites span boreal, temperate, Mediterranean, and subtropical climates 

(fig. 35). Corresponding forest types range from black spruce (Picea mariana Mill.), 

jack pine (Pinus banksiana Lamb.), aspen (Populus tremuloides Michx.), mixed 

conifer, ponderosa pine, mixed hardwood–pine, and southern pine–hardwoods, 

with an equally diverse range in soil parent material, soil texture, soil organic matter content, and site productivity (Ponder et al. 2012, Powers 2006). 

In summarizing the LTSP results at year 10, Ponder et al. (2012) stated, “Perhaps the most notable and unexpected result to date is the marked planted conifer 

growth response to compaction.” Essentially they found positive responses in tree 

biomass on severely compacted compared to uncompacted plots in their analysis of 

46 LTSP installations. Biomass gains were particularly significant at several sites 

in the southeast and in California. Exceptions to these observations were a strong 

decline in biomass production for aspen sites in subboreal climates and a moderate 

decline in productivity for a mixed-conifer site in California growing in a moderately fine-textured soil. 

Ponder et al. (2012) offered several explanations for these findings that have relevance to the discussion of fuel reduction thinning and site productivity. Foremost, 

the majority of LTSP sites are located on well-drained soils. Thus, any negative 

effects on soil aeration, anaerobic conditions, or nutrient supply that are common 

in poorly drained soils were presumably inconsequential. As already mentioned, 

poorly drained or wet soils are a particular concern during thinning operations and 

require special consideration to avoid soil damage. In addition, about 60 percent 

of the LTSP sites have sandy to coarse loamy soils, which likely benefitted from 

improved water-holding capacity when compacted. The authors also reiterated that 

the results were valid for compaction forces without any additive effects of soil 
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Figure 35—Location of Long-Term Soil Productivity installations. Core 

sites (yellow circles) have the complete combination of compaction and 

organic matter treatments; affiliated sites (red triangles) lack some treatments, although most include the soil compaction.

displacement, mixing, or rutting, and that the use of skidding equipment should 

increase both the intensity and aerial extent of compaction (Han et al. 2009). A 

final caveat pointed out by the authors was that the findings reflect the short-term 

response by vegetation to compaction. Whether any long-term losses in site productivity will result are yet to be determined. 
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Fuel Reduction Practices and Their Effects on Soil Quality

Masticated Fuels—New Practices, New Concerns?

Key points:

Ecological effects Management considerations

• What is vertical becomes horizontal– canopy, ladder, and standing surface fuels are rearranged into woody 

surface fuel bed dominated by small 

(1- to 100-hour) fuel classes.

• There is an initial increase in potential surface fire behavior until masticated residues settle and decay.

• Nutrients are retained onsite.

• Soil disturbance and compaction can 

be minimized during mastication.

• Use a boom-mounted rather than 

rigid-mounted cutting head.

• Use low-ground-pressure equipment.

• Operate when soils are dry, frozen, or 

under deep snow.

• Limit equipment to designated trails.

• Masticated residues act as a mulch 

layer.

▪ Soil moisture retention is increased.

▪ Soil heating is reduced.

▪ Seedling establishment may be

inhibited extending fuel treatment 

longevity.

• The net mulch effect is difficult to 

predict as it depends not only on the 

residue layer depth and compactness, but on the opposing effects 

of increased solar radiation and air 

temperature that result following 

thinning.

• Short-term microbial response is 

relatively insensitive to mastication.

• Based on existing knowledge, shortterm impacts to microbial communities and processes are not primary 

drivers for management decisions 

regarding mastication.

• Long-term studies may reveal true 

biological implications. 

• Contrary to expectations, nitrogen 

(N) immobilization has not been 

observed following mastication. 

• Concerns that mastication will 

reduce N availability and site productivity are not substantiated by existing research.

There are so few studies of mastication impacts on soil resources that it is difficult to interpret long-term ecological consequences. There is no natural analogue 

to mastication. The masticated debris is unlike the natural forest floor in terms of 

particle size, composition, bulk density, and moisture regime, so there are few easy 

comparisons to natural wildland systems or processes. Short-term studies have 

assessed mastication impacts on limited physical, chemical, and biological properties of soils, and found few detrimental effects. These are reviewed and discussed 

in the following sections. Long-term consequences or indirect effects from mastication remain largely unstudied.
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What is mastication?— 

Mastication is a mechanical fuel treatment applied to shred, grind, mulch, mow, or 

chunk woody understory vegetation and small trees. The resulting material is typically broadcast away from the machine as it operates and left onsite. As a fuel treatment, mastication serves to convert canopy, ladder, and standing surface fuels into 

a woody surface fuel bed—essentially rearranging fuels from vertical to horizontal 

by shredding and dispersing them on the forest floor.

Mastication equipment is quite variable, and usually consists of a cutting head 

that rotates on either a vertical or horizontal shaft (fig. 36) (Harrod et al. 2009). The 

cutting head can be mounted on an excavator, tractor, skid steer, all-surface vehicle, 

or other heavy equipment, for use in wildland environments. Boom-mounted cutting heads allow the operator to reach up to 9 m in any direction, thereby reducing 

the need for equipment turns and limiting the associated ground disturbance (Harrod et al. 2009). Rigidly mounted cutting heads require more equipment maneuvering to process fuels, potentially resulting in greater soil disturbance (Coulter et al. 

2002).

Physical characteristics of masticated fuel beds: particle size, depth, fuel 

load— 

Characteristics of a masticated fuel bed will differ depending on the project objectives; equipment used; treatment intensity; and the size, age and amount of the fuels 

(fig. 37) (Kane et al. 2006). As a result, the size of masticated particles may differ from a predominance of fuels in the 1- and 10-hour time lag classes (≤0.64 cm 

and >0.64 cm to ≤2.54 cm diameter, respectively) (Kane et al. 2006), to predominance of 10- and 100-hour fuels (>2.54 to ≤7.64 cm diameter) (Reiner et al. 2009), 

to football- or basketball-size chunks (Jain et al. 2008). Masticated biomass is often 

dispersed unevenly across a site, with some areas receiving deep accumulations of 

materials while other areas remain unaffected. For example, percentage of cover 

of residues averaged 8 percent and 17 percent in two ponderosa pine-dominated 

mastication treatments in the southern Sierra Nevada (Reiner et al. 2009). Harrod 

(2009) stated that masticated materials can add as much as 5 cm to the forest floor. 

Maximum depths reported for masticated materials range from 7 cm in a pinyon-juniper woodland (Owen et al. 2009) to 24 cm from a dense shrub understory within a 

ponderosa pine plantation (Busse et al. 2005), but average depths are typically from 

3 to 7 cm. Fuel loadings reported in the literature for masticated sites range from 15 

Mg ha-1 (7 tons ac-1) of woody fuels (Kane et al. 2006) to 78 Mg ha-1 (35 tons ac-1) 

of masticated biomass (Busse et al. 2005), both at northern California sites. Over 

time, the masticated layer will settle and decay, and decrease in both depth and 

mass. Chips, which tend to be more uniform than masticated materials, were found 
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Fuel Reduction Practices and Their Effects on Soil Quality

Figure 36—Orientation of masticator cutting heads. Many more examples are available in equipment guides and 

catalogs. (A) horizontal shaft, (B) vertical shaft. 

Gregg Riegel

Richy Harrod

Eric Knapp

Scott Stephens

Eric Knapp

Figure 37—Masticated debris ranges in size and continuity depending on the vegetation, cutting head, and time spent working the 

material. (A) fine debris, (B) coarse debris, (C) masticated brush.

to settle 18 percent during the first year after a chipping operation of ponderosa pine 

biomass in Colorado (Wolk and Rocca 2009). 

Masticating without displacing and compacting the soil— 

Ground disturbance to the forest floor and mineral soil will depend on the vehicle 

specifications, soil conditions, and operator experience. Masticator cutting heads 

requiring higher machine power necessitate the use of heavier equipment (Harrod et 

al. 2009). Depending on the type of machine used and operating conditions, maneuvers such as turning can displace and churn the organic and mineral horizons. 

Mastication trials to treat fuels in the Northwest used nine different equipment 

scenarios at four sites, with vehicles ranging from a 3900-kg (8,500-lb) all-surface 

vehicle to an 36 000-kg (80,000-lb) excavator to power a variety of cutting heads 

(Coulter et al. 2002). At each trial, soil disturbance was gauged using a visual assessment method. The greatest soil disturbance was caused by operations designed 
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to incorporate masticated materials into the soil. Other operations caused soil 

disturbance to increase from “slight,” prior to treatment, to “some” or “moderate” 

following treatment. In these cases, which included the 36 000-kg excavator, soil 

conditions changed from “virtually undisturbed” to showing some displacement of 

the forest floor and evidence of compaction such as platiness or lack of structure. At 

three sites, no increases in soil disturbance were recorded, owing to operations over 

deep snow or recent site disturbance from forest thinning that was not increased by 

the mastication activities.

Compaction resulting from mastication operations can be largely curtailed 

by selecting appropriate equipment and site-specific operating conditions. Limiting masticators to designated trails or using low-ground-pressure equipment can 

reduce the extent and intensity of physical soil disturbance. The British Columbia 

Ministry of Forests and Range (2008) defines low ground pressure as equipment 

that exerts less than 43.4 kPa (6.3 pounds per square inch [psi]) of total ground 

pressure. Thinning followed by mastication of mixed-conifer forest in the Sierra 

Nevada with a 31-kPa (4.5-psi) masticator did not increase the extent of detrimental 

compaction in heavily managed stands (Moghaddas and Stephens 2008). The 

boom-mounted masticator was not limited to designated trails, but compaction may 

have been limited by the cushioning effect of the masticated debris bed. Hatchett et 

al. (2006) measured few compaction impacts in a masticated fir stand of the Tahoe 

basin in the Sierra Nevada, after treatment with a 37.9-kPa (5.5-psi) masticator. Any 

compaction from the operations was limited to a narrow range of soil depth, and 

detectable only when the soil beneath the masticator travel path was compared with 

areas far from the machine traffic and were much less disturbed. As in the Sierra 

Nevada studies, mastication fuel treatments in a Colorado pinyon-juniper woodland 

did not cause soil compaction (Owen et al. 2009). Although machine specifications 

were not provided, operations were conducted over snow and frozen ground, which 

help resist compaction by heavy equipment. 

These few studies suggest that compaction can be avoided during mastication 

operations, but, in fact, little research has been dedicated to this topic. The impacts 

of heavy equipment on skid trail compaction have been demonstrated for more than 

50 years (Dyrness 1965, Steinbrenner and Gessel 1955), but many mastication fuel 

treatments are fundamentally different than extractive forestry practices. During 

harvests, impacts tend to be concentrated on trails that receive multiple passes as 

materials are transported to landings. In contrast, a masticator may track over broad 
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Fuel Reduction Practices and Their Effects on Soil Quality

areas to treat fuels, particularly if the cutting head is rigidly mounted rather than 

boom-mounted. Consequently, a large proportion of the treated area may be subject 

to a single machine pass. The masticated fuel bed can buffer against soil compaction, similar to a slash mat in cut-to-length harvest treatments, but the debris depth 

required to offset various levels of machine ground pressure for a variety of soil 

moisture or texture conditions is not known. Prudence dictates use of low-groundpressure equipment, particularly on sensitive sites. Windel and Bradshaw (2000) 

catalogued many equipment options for fuel treatment, including very low-groundpressure (less than 20.7-kPa or 3.0-psi) machines.

Mastication can substantially modify soil temperature and moisture regimes— 

Mastication activities leave a residue of woody debris that acts as mulch on the 

forest floor. The woody mulch insulates against heat gain and moisture fluctuations in the soil. A variety of bark and chip mulches reduced the soil temperature 

and increased moisture during the growing season in a maple plantation (Iles and 

Dosmann 1999). Under masticated residues 10 cm deep, Massman et al. (2006) also 

recorded cooler soil temperatures at 15-cm depth than in control plots. They found 

that during summer months, the mulch reduced radiant heat gain in the soil and 

temperatures were about 1 to 9 °C cooler than controls. In late fall and early winter, 

however, the mulch served to impede cooling, and soil temperatures remained up 

to 4 °C higher than controls. A different summer mulch effect was observed in a 

Sierra Nevada study, where Busse (unpublished) measured maximum daily soil 

temperatures about 5 °C higher for surface-applied masticated residues compared 

to the control (fig. 38). In this case, the mastication treatment effectively opened 

the stand and resulted in greater solar radiation and air temperatures at the forest 

floor surface compared to the densely vegetated control plots. Incorporating masticated fuels into the soil increased soil temperatures. In a study of Rocky Mountain 

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), Gower et al. (1992) observed 

that wood chips applied to the forest floor significantly increased forest floor moisture content, and that the layer of wood chips remained frozen as much as 2 weeks 

longer than the natural forest floor. While rainfall patterns and woody mulch depth 

play important roles in soil moisture dynamics, masticated debris acts as a barrier against both water infiltration into the soil and evaporative losses from the soil 

(Massman et al. 2006). 
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Figure 38—Soil temperature differences among stands of untreated control areas, masticated understory, and masticated 

understory that has been incorporated into surface soil (Busse, unpublished data). 

Mastication has limited short-term effects on soil microbial communities— 

Forest thinning can allow more solar radiation to the forest floor, causing soil 

heating. This may enhance microbial activity in the spring, when soils are moist. 

This effect will be more pronounced in northern forests compared to subtropical 

or tropical areas closer to the equator. In areas that experience summer drought, 

thinning can also hinder microbial populations by drying soils earlier in the season. Thinning by mastication, however, adds a woody layer that can act as a mulch, 

buffering soil heating and moisture changes, with potential impacts to soil microbial processes (Miller and Seastedt 2009). Despite increased substrate inputs following mastication in a ponderosa pine plantation, Kobziar and Stephens (2006) did 

not detect any resulting changes in mean annual soil respiration rates. They surmise 

that mastication may have reduced root respiration by shredding vegetation, and 

that heterotrophic respiration from soil organisms may have increased to make up 

the difference, but this remains unconfirmed. In a study focused on soil carbon
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Fuel Reduction Practices and Their Effects on Soil Quality

sequestration, Busse et al. (2009b) found fungal and bacterial biomass responses 

relatively insensitive to forest mastication and mastication followed by incorporation of residues into the mineral soil. They attribute this result to the influence of 

residue fragment size and differences in decay rates at their study sites. Owen et al. 

(2009) studied the response of arbuscular mycorrhizal fungi to mastication thinning 

in a pinyon-juniper woodland. Treatments were designed to reduce overstory 40 to 

60 percent, but the masticated mulch served to reduce temperatures and increase 

moisture in the soil. Despite these environmental changes, mastication did not 

result in differences in abundance, species richness, or community composition of 

arbuscular mycorrhizal fungi 2.5 years after treatment. 

Soil nitrogen not always immobilized after mastication— 

Mastication and the associated addition of woody residues are commonly thought 

to reduce N availability. By adding mulch that is generally low in N and high in C, 

such as woody fragments, it is often speculated that microbes will assimilate available inorganic N from the soil in order to decompose the added material. If true, 

this N immobilization could reduce the amount of soil N available for plant growth. 

Few studies have examined N transformations and dynamics following mastication, 

however. In a Sierra Nevada comparison of fuel treatments, commercial thinning 

followed by mastication did not significantly alter available N or net nitrification 

rates 2 years after treatment, compared to untreated control stands (Moghaddas 

and Stephens 2007). In a pinyon-juniper mastication study, Owen et al. (2009) 

unexpectedly found increased available N (ammonium) 2.5 years after mastication. Similarly, Miller and Seastedt (2009) found no change in N availability in the 

first 2 years after thinning and wood chip application in a Colorado pine stand, but 

a 33-percent increase in available soil N in the third growing season. Contrary to 

common expectations, these studies did not measure N immobilization following 

mastication treatments. 

Mastication is a relatively recent and novel fuel reduction practice. Although 

many thousands of hectares are being treated with mastication across the United 

States, particularly in the WUI, relatively little is known about its short- or longterm impacts to soils. Most research examining mastication effects on soils has 

been conducted in the Western United States, and in California in particular. 
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Masticated Fuels—To Burn or Not To Burn?

Key points:

Ecological effects Management considerations

• Expect greater fire intensity and 

aboveground severity than burns in 

unmasticated stands.

• Allowing masticated residues to 

settle and compact for 1 or more 

years can reduce fire behavior.

• A prolonged waiting period may also 

allow emerging understory vegetation to accumulate, increasing the 

burn effectiveness at reducing surface fuels. 

• Soil heating can kill plant roots 

and soil organisms, but burn prescriptions can limit maximum soil 

temperatures.

• Burning when soil is moist (at least 

20 percent moisture by volume) will 

limit most soil heating to the top 2 

to 5 cm.

• The deeper the masticated residues, 

the more likely heat will penetrate 

further into the soil.

• Spatial variability in residue depth 

will lead to a range in soil heating, 

from areas of possibly no heating to 

hotspots below deep accumulations.

• Soil texture is unlikely to affect 

depth of soil heating.

• Burning masticated residues does not 

mean more bare mineral, soil.

• Burn prescriptions can be designed 

to result in similar levels of bare 

ground for masticated fuel beds or 

natural fuels.

• Soil nitrogen (N) content and transformations are similar in burned 

stands, with and without prior 

mastication.

• High variability in fuel consumption 

and N movement after burning will 

produce variable results.

Along with mastication operations, many fuel reduction projects also propose 

to use prescribed fire as a followup treatment. Mastication can effectively reduce 

ladder fuels and height to the live crown base in forested stands (Stephens and 

Moghaddas 2005), which enhances the feasibility of prescribed fire operations. 

Masticated fuel beds differ from natural fuel beds in their moisture and temperature regimes (Massman et al. 2006), surface area to volume ratios (Kreye and 

Varner 2007), particle shape, and size class composition (Kane et al. 2009), making 

fire behavior and effects in masticated residues difficult to predict. For example, 

Knapp et al. (2011) observed that fire-severity measures in northern California 
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Fuel Reduction Practices and Their Effects on Soil Quality

masticated fuel beds exceeded model predictions by nearly a factor of four. Because 

mastication redistributes large fuels into finer classes, there is potential for more 

extreme fire behavior immediately after the treatment. For example, a study of prescribed fire in the Klamath Mountains of northern California found that mastication 

increased both fire intensity and aboveground severity compared to nonmasticated 

plots (Bradley et al. 2006).

Burning masticated residues with limited soil heating— 

Despite the potential for increased fire effects in masticated materials, several 

northern California studies have shown limited soil heating during prescribed 

fire in masticated stands by burning residues during spring, when soils are moist. 

Bradley et al. (2006) found that despite greater fire behavior measures in their masticated stand, no changes in heating at the soil surface were detected between masticated and nonmasticated prescribed fire plots. These spring burns were conducted 6 

months after mastication, and the loosely arranged fuel bed was considered a contributing factor to fire behavior. It was expected that, over time, the masticated residues would settle and compact, potentially lowering fire intensity. Similarly, Knapp 

et al. (2011) observed long flaming duration and high heat content of the masticated 

woody fuel bed, but soil temperatures at 5-cm depth remained below 60 °C at 

nearly all thermocouple locations. In this study, prescribed fires were conducted in 

late spring, 2 to 3 years after mastication (fig. 39). Several studies suggest that over 

time, masticated residues will compact, leading to reduced fire behavior (Knapp et 

al. 2011, Kobziar et al. 2009).

Constructed fuel beds burned under controlled moisture conditions can allow 

researchers to develop guidelines for soil conditions or fuel loadings that may limit 

soil heating. Busse et al. (2005, 2010) conducted several studies on soil heating in 

which the masticated depth, fuel load, soil moisture, or soil texture were controlled 

Figure 39—Minimal soil heating occurred during burning of these masticated fuels in a study reported by 

Knapp et al. (2011).

 Eric Knapp

Carol Shestak
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in constructed test plots to determine effects on the depth of lethal temperatures. 

They found that soil moisture is the primary factor in limiting soil heating. In 

dry soils (4 percent volumetric moisture), temperatures exceeded 60 °C from the 

surface to 10 cm soil depth when the mulch layer was deeper than 2.5 cm. Moist 

soils (16 percent volumetric moisture), however, required a very deep mulch layer 

(12.5 cm) to cause lethal soil heating below 2.5-cm soil depth (Busse et al. 2005). 

Additionally, burning when volumetric soil moisture exceeds 20 percent (common 

during moist spring conditions) should limit damaging soil heating to the top 2.5 to 

5 cm soil depth (Busse et al. 2010). Although differences in soil texture can affect 

heat transfer, temperature trials in sand, loam, and clay soils did not respond differently to prescribed fire treatments (Busse et al. 2010).

Within a treatment unit, masticated fuels are highly variable in depth, fuel loading, and continuity. This heterogeneity can result in patchy prescribed fires with 

spatially variable soil heating effects. A wide range of soil heating was measured 

during fall burns of masticated units in the Cascade Range of northern Washington 

(Harrod et al. 2008). These were implemented within 6 months of mastication, and 

took place during cool, moist conditions. As a result, fire intensity was relatively 

low, with flame lengths generally <1 m. Although many thermocouples did not 

record any temperature increase resulting from the patchy fires, the duration of 

lethal heating extended as long as 10 hours at some locations. 

Burning masticated stands does not expose more bare soil than untreated 

areas— 

Compared to untreated areas, areas treated with mastication increase in amount and 

continuity of fuels on the forest floor, which can lead to increased fire intensity, but 

this does not necessarily mean that more bare mineral soil will be exposed during 

prescribed fire operations. When comparing fuel treatments including prescribed 

fire and mastication followed by prescribed fire, Moghaddas and Stephens (2007) 

and Kobziar (2007) both found no statistical difference in the amount of bare soil 

exposed after burning. Fuel moisture plays a large role in fuel consumption and 

subsequent soil exposure. Prescribed burning of masticated fuels during the typical 

fire season, or summer drought period, resulted in 49 percent (Kane et al. 2010) and 

56 percent (Moghaddas and Stephens 2007) cover of bare soil. Following 2 cm of 

precipitation, bare soil exposure was less than 3 percent (Kobziar 2007). Burning 

masticated residues during moist spring conditions led to only modest duff consumption, where less than 30 percent was consumed (Bradley et al. 2006).
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Fuel Reduction Practices and Their Effects on Soil Quality

Prescribed fire yields similar nutrient and microbial responses in masticated 

and nonmasticated stands— 

Fires rapidly oxidize and decompose organic matter, releasing nutrients such as N 

in available, inorganic form. However, in a comparative study of fuel treatments 

in the Sierra Nevada, Moghaddas and Stephens (2007) found no significant differences in inorganic N concentrations (ammonium or nitrate), net N mineralization, 

or nitrification rates between burned stands with or without prior thinning and mastication. High within-treatment variability in fuel consumption and nutrient movement likely influenced the extent and intensity of these N alterations.

In another Sierra Nevada study, Kobziar (2007) measured soil respiration rates 

in pine plantations that had been burned, with and without mastication. The relative contribution of heterotrophic respiration (from soil organisms) increased in 

both types of treatments. This appeared linked to a decline in the importance of 

root (autotrophic) respiration, owing to root mortality and reduced photosynthetic 

capacity caused by the prescribed burning. The two prescribed fire treatments did 

not clearly distinguish themselves in terms of heterotrophic versus autotrophic 

respiration rates.

Thinning and Burning—Early Results From Long-Term Study 

Sites

Key points:

Ecological effects Management considerations

• Combinations of mechanical thinning and prescribed fire applied in 

a variety of U.S. forests using best 

management practices produced 

modest short-term changes in soil 

properties.

• Mechanical removal of 20 to 45 percent 

basal area exposed more bare soil and 

increased bulk density at a few study 

locations, but did not substantially alter 

soil quality across the national network 

of sites.

• Prescribed fire effects were limited to 

short-term reductions in forest floor 

mass and a modest pulse of available nitrogen that increased with fire 

severity.

• Combined treatments have not substantially affected soil quality any differently than have single treatments yet.

• These are early findings from a longterm national study. Caution in extending the results beyond the current 

treatment longevity and range of sites 

is advised.
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Mechanical fuel treatments allow managers to alter forest structure and emulate 

the effects of fire on the size and species distribution within a stand. Silvicultural 

treatments, however, may not mimic the ecological functions driven by fire. To 

address this issue, the Fire and Fire Surrogate (FFS) study compares prescribed 

fire treatments with mechanical “fire surrogates” across a national network of sites, 

utilizing a common study design and set of response variables. Study locations 

represent forests with hazardous fuel loads, but historically short-interval, low- to 

moderate-severity fire regimes. Sites include western coniferous forests, ranging 

from the Pacific Northwest to the Southwest, as well as Appalachian hardwood, 

Piedmont pine, and southern pine forests of the Coastal Plain. Control plots and 

three manipulative fuel treatments were established at the sites: mechanical treatment, prescribed fire, and combined mechanical and prescribed fire treatments. 

Site conditions and treatment intensities were quite variable among FFS sites. 

For example, helicopters yarded logs in Washington, trees were whole tree harvested in northern California, nonmerchantable trees and brush were masticated 

at another California site, and mowers and choppers mechanically treated fuels 

in Florida. Prescribed fire treatments were also highly variable. On average, burn 

treatments in a Rocky Mountain site in Montana consumed 12 percent of the duff 

layer (Gundale et al. 2005), whereas those in a California Sierra Nevada locale 

reduced forest floor mass and depth by more than 80 percent (Moghaddas and 

Stephens 2007). Prescribed fire was typically applied following mechanical operations, but to meet fuel reduction objectives in the Florida flatwoods, stands were 

first burned then mechanically chopped (Onokpise et al. 2007). Despite these broad 

differences in treatment implementation, soil responses in the initial 2 to 3 years 

after treatment were generally similar across the FFS sites. 

In a meta-analysis of 12 sites, Boerner et al. (2009) reported that FFS treatments resulted in only minor, ephemeral impacts to soil physical, chemical, and 

biological properties compared to the control treatment (table 10). Treatments 

involving fire significantly increased bare mineral soil. In the first year following 

treatment, bare soil ranged from 0 to 46 percent cover for prescribed fire only and 

from 0 to 56 percent cover for mechanical thinning plus fire (Boerner et al. 2009, 

Moghaddas and Stephens 2007). After 1 to 2 years of litter fall, bare soil exposure 

at all reporting sites and treatments was less than 10 percent cover (Boerner et al. 

2009). Although bare soil facilitates germination of many pines, shrubs, and herbaceous species, there was no clear treatment effect on seedling density across the 

FFS network (Schwilk et al. 2009). At some study sites, more seedlings were lost as 

a result of the treatments than were gained through germination. At a Sierra Nevada 

site where mineral soil exposure exceeded 50 percent, the fire treatments, both with 
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Fuel Reduction Practices and Their Effects on Soil Quality

Table 10—Short-term impacts of the Fire and Fire Surrogate treatments on soil properties

Soil property Mechanical Fire

Mechanical 

+ fire Notes

Bare mineral soil –– ↑ ↑ Greater fuel consumption led to greater soil exposure. In year 1, bare mineral soil exceeded 

15 percent at three of eight sites. Exposed soil 

was <10 percent 1 to 2 years later.

Bulk density –– –– –– Operations highly variable, including conventional harvest, whole tree harvest, helicopter 

yarding, and mastication.

pH –– –– ↑ Significant pH increases were transient, and limited to sites with greatest fuel consumption.

Available calcium (Ca), 

magnesium, potassium

–– –– –– No significant changes at network scale, but Ca 

was increased in Oregon 1 year after treatment, and in Oregon and Ohio 1 to 2 years 

later. 

Available Phosphorus –– –– –– No significant changes at network or individual 

site scale.

Total soil nitrogen (N) –– –– –– No significant changes at network or individual 

site scale.

Total inorganic N (TIN) ↑ ↑ ↑ On average, TIN increased by 1 to 6 kg ha-1 in 

year 1, but changes were not significant 1 to 2 

years later.

Rate of N mineralization –– –– –– Greater variation in N mineralization at western 

sites compared to eastern sites.

Soil carbon (C) –– –– –– Most soil C impacts are limited to top few 

centimeters. By looking at top 15 cm, this 

study did not measure any significant soil C 

changes.

Soil C:N ratio –– –– –– Modest effects on C and N led to nonsignificant 

changes in C:N ratio at network scale.

Forest floor C –– ↓ ↓ Prescribed fire consumes C-rich surface litter.

“––” indicates no significant change in effect size relative to control treatment at network scale.

“↑” indicates significant increase in effect size relative to control treatment at network scale.

and without mechanical operations, increased the density of Douglas-fir seedlings 

(Moghaddas et al. 2008). Despite this increase, no significant differences in understory plant abundance were detected among treatments (Collins et al. 2007). The 

FFS treatments did reduce species richness of native plants relative to the control 

in the first year at this site, but the decline was modest, about one fewer species on 

average (Collins et al. 2007).
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The meta-analysis also showed that prescribed fire, mechanical, and combination treatments increased inorganic N in the mineral soil compared to the control 

(table 10). This was not unexpected as (1) forest floor-bound N is typically released 

in inorganic form during burning, and (2) thinning plus burning may alter the soil 

microclimate, enhancing N mineralization rates and, consequently, the release of 

inorganic N. The measured spike in inorganic N in the first year after treatment 

was fairly moderate, however, and short lived. 

Prescribed fire, alone and combined with mechanical treatment, significantly 

reduced total C storage in the forest floor by an average of 7.3 and 6.5 Mg C ha-1, 

respectively. These losses were mostly due to litter consumption and typically 

returned to pretreatment levels in a short period (Boerner et al. 2008a). In comparison, C stored in soil organic matter (SOM) to 30 cm depth was not significantly 

different than control values, with soil C changing less than 10 percent across the 

network (Boerner et al. 2008a). Short-term patterns of total N loss varied by treatment and region (fig. 40). On average, 84 percent of N lost from prescribed fire was 

from the forest floor pool, whereas 92 percent of N lost from mechanical treatment 

was from vegetation. Nitrogen losses resulting from combined mechanical and fire 

operations were closely split among these pools, with 55 percent from forest floor 

and 45 percent from vegetation. Overall, the FFS treatments removed less than 10 

to 15 percent of the total N in these stands, primarily from forest floor and vegetation layers. Mineral soil storage, which accounts for more than 80 percent of total 

N at these sites (Boerner et al. 2008b), was unaffected and will likely provide an 

adequate buffer against soil quality or productivity impacts. 

Cumulative effects research— 

Results from the FFS study are in their infancy. Still, they offer early evidence 

of soil resilience to both single and combined treatments when applied using best 

management practices. As additional entries and burns are planned for most FFS 

sites, the study will ultimately provide long-term evidence of either soil tolerance, 

resilience, or susceptibility to cumulative practices across forest types and regions. 

In support of the FFS results, other studies of more limited geographical scope 

have demonstrated only modest changes in soil or site quality following repeated 

fuel reduction treatments (Binkley et al. 1992, Busse et al. 2009a, Hart et al. 2005a, 

McKee 1982). Still, our knowledge of cumulative soil effects from fuel reduction 

and forest restoration practices is incomplete and does not allow for broad-sweeping 

conclusions. Will repeated harvesting traffic eventually result in extensive damage 

to soil structure and greater soil erosion? Will repeated fire or whole tree harvesting operations deplete nutrients to suboptimal levels? Will the indirect effects of 
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Fuel Reduction Practices and Their Effects on Soil Quality

Figure 40—Net changes in total nitrogen (N) following fuel reduction treatments. Study site 

location: Fire and Fire Surrogate (FFS) = network average, East = Eastern United States, 

West = Western United States. Adapted from Boerner et al. (2008b). 

repeated treatments (modified microclimate, variable forest floor depths, heterogeneity in forest structure) have a greater impact on long-term soil quality than any 

pulse effects of fire or harvesting? And which soil types and site conditions are 

most vulnerable to repeated disturbance? 

Although the findings from most single-entry studies reported in this synthesis do not indicate any major soil concerns when using well-planned fuel 

reduction practices, little reassurance is afforded for addressing soil responses to 

multiple-entry, site-specific treatments. Resource managers can help safeguard 

against unwanted soil damage by continuing to document treatment intensity and 

extent, landscape pattern, and resulting soil quality of all repeated treatments until 

additional knowledge from studies like the FFS advances. More information on 

the FFS study can be found at http://frames.nbii.gov/portal/server.pt/community/

fire_and_fire_surrogates_study/363.
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Soil in a Changing Climate

Key points:

Ecological effects Management considerations

• Sequestering or “hoarding” soil carbon (C) reduces atmospheric carbon 

dioxide concentrations and helps 

mitigate climate change. 

• Current science to suggests that this 

process will play only a minor role in 

climate change mitigation.

• Most evidence shows that soil C 

content is relatively stable following 

fuel reduction treatments, and that 

attempts to increase C storage will be 

limited to previously degraded land. 

• Until a better appreciation of the 

balance between sequestering soil 

C versus using it to fuel essential 

ecological functions is met, soil C 

sequestration might be best thought 

of as a lesser goal of forest soil 

management.

• United States soils will adapt to 

changes in annual precipitation, temperature, snowpack, vegetation, and 

wildfire risk (they have no choice).

• How well they adapt to environmental pressures is obviously unknown, 

as is the role of fuel reduction 

practices in helping buffer our soils 

against environmental change.

Soil’s ability to function effectively in forest ecosystems wil be put to the test in 

the next century by our changing climate. Exactly how U.S. soil will be affected 

is uncertain; however, widely variable conditions and responses will likely be the 

case. Factors such as regional differences in climate change severity, latitudinal 

and elevational hotspots, soil vulnerability to climatic pressure, and complex 

interactions between soils and changing vegetation patterns add to the uncertainty. 

These unknowns also instill a sense of urgency in the natural resources community 

to identify and evaluate potential mitigative actions and adaptive strategies. For 

example, how does soil fit in the climate change equation? Does it offer any leverage, perhaps providing an untapped sink for long-term forest C capture (C sequestration), effectively lowering greenhouse gas concentrations? Or, are the benefits 

of relative insignificance compared to other socioenvironmental options. Here we 

briefly introduce soil C sequestration as an option for mitigating climate change 

and explore the projected impacts of climate change on soil quality.
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Fuel Reduction Practices and Their Effects on Soil Quality

Soil carbon sequestration and mitigating global climate change— 

Soil C sequestration is the process of removing CO2 from the atmosphere (via 

photosynthesis) and storing it as SOM. Usually a portion of incorporated plant 

residues are stored in long-term, decay-resistant humus compounds (Six et al. 

2002), which not only serve to sequester C and reduce greenhouse gases but also 

to improve soil quality. The remaining residues are more easily respired, returning 

CO2 to the atmosphere in a relatively short-term turnaround process. Thus, soil C 

is sequestered and atmospheric CO2 levels are lowered when the C input rate from 

residue incorporation is greater than the output rate from heterotrophic respiration. 

This is not as easy as it sounds, however, since C input and output rates are often 

comparable on a decadal timeframe (Janzen 2006). 

Soil has an amazing ability to store C. Approximately 80 percent of all terrestrial C, or about 1500 gigatons (Gt) of C, is found in soil (Batjes 1996). This 

explains, in part, why soil continues to receive considerable scientific attention 

for its potential role in climate change mitigation (Kimble et al. 2003, Powlson et 

al. 2011). Atmospheric CO2 levels increased from 315 parts per million (ppm) in 

1960 to 383 ppm by 2009 and are expected by some to approach 800 ppm by 2100 

(Solomon et al. 2007). By increasing soil C sequestration, more CO2 would be 

withdrawn from the atmosphere, resulting in a negative feedback that reduces the 

effects of climate change. On the other hand, C losses through forest disturbance 

(e.g., wildfire) would cause more CO2 to enter the atmosphere in a positive feedback 

that promotes climate change (Philander 2008). The germane questions become (1) 

how much more C can soils store to help offset rising greenhouse gas concentrations, and (2) can forest management practices assist in this process? 

Most evidence suggests that land conversion from abandoned agricultural fields 

to grasslands or forests is the most viable means for increasing soil C storage (Post 

and Kwon 2000, Richter et al. 1999). Historical tilling and erosion of these lands led 

to substantial soil C loss which, in turn, has been shown to slowly aggrade to nearoriginal levels once native vegetation is restored. Whether forest management practices such as thinning and prescribed burning modify soil C storage is in question, 

however (Hoover 2003). At present, most studies suggest that these practices do not 

substantially affect mineral soil C storage (Boerner et al. 2009, Johnson and Curtis 

2001), and thus would not be expected to either mitigate or exacerbate atmospheric 

CO2

 levels. No confirmation of this with long-term data exists, however. 

So the debate continues. Can soils store more C and play an important role in 

climate change mitigation? Lal (2004) suggests that global soils can sequester an 

additional 0.4 to 1.2 Gt of C each year, which is the equivalent of 5 to 15 percent of 

global fossil fuels emissions. Countering this optimistic view, Powlson et al. (2011) 
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argued there has been an overemphasis on the potential benefit of soil C sequestration and that it may detract from other more effective measures (e.g., slowing 

global deforestation). They reasoned that (1) soil does not accumulate C indefinitely 

(soil C storage is finite), and (2) the process of C sequestration is easily reversible 

unless the change in management practices that led to increased C is continued 

indefinitely. Instead, they discussed the value of a “no regrets” soil policy, one that 

first encourages management practices that increase soil C content for the benefit 

of soil functioning and, second, reaps the benefits of climate change mitigation if 

actualized. 

Janzen (2006) offers keen insight into the debate on soil C in asking “shall we 

hoard it or use it?” The dilemma, as he points out, is that the ecological value of soil 

carbon is derived primarily from its decay (releasing nutrients, fueling biological 

processes, providing glues for soil structure). Now we are asking systems to accrue 

C while simultaneously allowing them to decay. Is such a “win-win” proposition realistic or not? He suggests that we must find approaches to (1) increase C 

inputs, (2) optimize the timing of decay to match ecosystem needs, and (3) better 

understand the input and output flows of soil C, not just evaluate C stocks. Forest 

management approaches for dealing with soil C sequestration are clearly in an 

experimental phase.

Adapting to global climate change— 

Regional changes in the frequency and severity of precipitation events, heat waves, 

drought, floods, and hurricanes remain a key uncertainty in future climate change. 

Intensified storm events will affect soil erosion rates. Higher moisture and temperature will result in deeper, more leached soil profiles with clay eluviation to 

lower horizons: however, these effects are slow and will be overshadowed by erosion events and changes in management practices (Paul and Kimble 2000). Heat 

waves and droughts will heat soils, increase soil evaporation rates, and reduce soil 

moisture content. Drier soils will stress trees, increasing tree dieoffs from insect 

infestation. Dead trees and longer fire seasons will result in an expansion of areas 

subject to wildfires. Table 11 provides a qualitative, first-approximation of projected 

impacts of climate change on U.S. soils and the environment.
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Fuel Reduction Practices and Their Effects on Soil Quality

Table 11—Projected soil impacts based on predicted U.S. regional temperature, precipitation, snowpack, 

wildfire risk through 2099a

Projected environmental change

Region

Temperature 

increase

Annual 

precipitation

SnowpackWildfire

risk Projected soil impacts

°C Centimeters - - - Percent - - -

Southern 

California

3 to 4 12 to 25 ↓ ↓ 40 ↑ Warmer temperatures, lower precipitation, 

reduced snowpack, and earlier snowmelts will 

result in reduced soil moisture and increased soil 

temperatures. Drought periods will be extended 

and interspersed with El Nino-driven intense rain 

events. Soil erosion will increase. Heavy winter 

rains will increase fine fuels, leading to greater 

fire hazard.

Northern 

California

4 to 6 12 to 25 ↓ ↓ 55 ↑ Warmer temperatures will reduce available soil 

moisture, especially from mid to late summer, 

and increase the length of wildfire season in some 

middle and upper elevation forests. Fires will be 

fueled by increased tree mortality and summer 

flammability. Some project a 30 to 50 percent decrease in forest productivity; however, increased 

atmospheric carbon dioxide (CO2) concentrations 

may result in a 10 to 20 percent increase in forest 

growth and an increase in soil and forest floor 

organic matter.

Sierra 

Nevada, 

California

↑ ↓ 90 ↓ ↑ Reduced snowpack and soil moisture along with 

hotter air temperatures will increase the number 

and severity of wildfires, affecting soil heating 

and erosion. An increase in rain-on-snow events 

will also lead to greater streambank erosion and 

runoff. 

Oregon/ 

Washington

2 to 5 ↓ 80 ↓ ↑ Declining snowpack will lead to reduced summer 

streamflows. Higher summer temperatures and 

earlier snowmelt will increase potential for wildfires and insect outbreaks and alter forest species 

composition.
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Alaska 3 to 7 5 to 25 ↑ ↓ ↑ Although precipitation is projected to increase, 

longer summers and higher temperatures will lead 

to decreased soil moisture and greater thawing of 

permafrost. Earlier warming of soil will increase 

the growing season, but will also increase wildfires and insect outbreaks. 

Arizona/New 

Mexico

4 to 6 10 to 30 ↓ 75↓ ↑ Soil moisture will decrease as drought periods are 

projected to increase. The severity of monsoon 

storm events is expected to increase owing to 

warming land and water temperatures. Current 

drought (1999–2010) has resulted in substantial 

die-off of pinyon pines. A decline in riverflow 

will alter riparian vegetation and increase the loss 

of wetlands.

Rocky 

Mountains

2 to 4 ↑ ↓ ↑ Less snow and more winter rain will lead to 

earlier snowmelt and lower summer streamflows. 

Changes will result in longer summer drought, 

reduced soil moisture, more wildfires, and greater 

insect infestations. Forest carbon storage will 

decrease during prolonged drought seasons. 

Great Basin, 

Nevada/Utah

2 to 5 ↑ ↓ ↓ ↑ Projections for precipitation will vary across 

the Great Basin on account of large differences 

in topography. Higher levels of CO2 may favor 

cheatgrass and other forb and shrub invaders. 

Increased temperatures, winter precipitation, and 

fine fuels (cheatgrass) will increase the wildfire 

season and potentially lead to greater soil erosion 

and loss of biological crusts and soil nutrients. 

Texas/ 

Oklahoma

3 to 6 ↑ NA ↑ Projected increases in precipitation are based on 

localized intense rain events and are unlikely to 

offset decreasing soil moisture and water availability because of rising temperatures, drier 

spring and summers, lower humidity, and aquifer 

depletion. Changes in precipitation patterns will 

alter wetland ecosystems. Increases in temperature and drought severity will multiply the risk 

of grass and shrub fires. An increase in wildfires 

could deplete soil organic matter (SOM) reservoirs overtime.

Table 11—Projected soil impacts based on predicted U.S. regional temperature, precipitation, snowpack, 

wildfire risk through 2099 (continued)

Projected environmental change

Region

Temperature 

increase

Annual 

precipitation

SnowpackWildfire

risk Projected soil impacts

°C Centimeters - - - Percent - - -
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Fuel Reduction Practices and Their Effects on Soil Quality

Great Plains 3 to 6 13 ↑ ↓ ↑ Northern areas will experience the greatest 

change in temperature. Projected increased precipitation will be highly variable over the region. 

Increased evaporation from rising air temperatures will likely overwhelm the extra moisture 

from precipitation, causing soil moisture to 

decline throughout much of the region. Areas of 

increased soil moisture may show gains in SOM 

and C sequestration. Increased severity of storm 

events will lead to greater flooding and erosion. 

Great Lakes 4 to 8 10 to 25 ↑ ↓ ↑ Longer warm seasons will increase wildfire risk. 

Soil moisture is projected to increase up to 80 

percent during winter in some areas, but decrease 

by up to 30 percent in summer and fall. This will 

result in some wetland ecosystems drying up 

entirely during summers. Increased soil moisture 

will result in increased C loading and sequestration.

Northeast 2 to 3 ↑ ↓ ↓ ↑ Precipitation is projected to increase by 14 

percent during the winter, but decrease by 2 to 3 

percent during the summer. Extreme storm events 

will increase resulting in greater flooding and erosion. Warmer coastal areas may allow hurricanes 

to survive longer and gain more strength.

Southeast/

Central

2 to 6 ↑ ↓ NA ↑ Warming waters will increase in the intensity of 

hurricanes. Fall precipitation will increase and 

summer precipitation will decrease. Declines 

in forest growth will result from heat stress and 

reduced soil moisture.

Florida 1 to 3 Little change NA ↑ Warmer air temperatures will affect the biogeochemical cycles of C, nitrogen, phosphorus, 

sulfur, and redox cations in wetland soils and 

sediments. Higher air temperatures will lead to 

increased evapotranspiration and reduced soil 

moisture content.

NA = not applicable.

a

 Climate ranges are based on Grassland Fire Danger Index A2 and B1 scenarios (low-to-high temperature scenarios) and are adapted from Adams et 

al. 2009, Cayan et al. 2008, Chambers and Pellant 2008, Field et al. 1999, Hayhoe et al. 2006, Kling 2003, Romanovsky et al. 2007, Running 2009, van 

Mantgem et al. 2009, Westerling and Bryant 2008.

Table 11—Projected soil impacts based on predicted U.S. regional temperature, precipitation, snowpack, 

wildfire risk through 2099a (continued)

Projected environmental change

Region

Temperature 

increase

Annual 

precipitation

SnowpackWildfire

risk Projected soil impacts

°C Centimeters - - - Percent - - -
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Conclusion

Most fuel reduction projects are driven by the desire to reduce or rearrange hazardous fuels. Although obvious, in practice this means that fuel priorities may take 

center stage while other affected resources fade into the background to absorb the 

impacts. Soils can be profoundly affected by mechanical operations, prescribed 

burns, and combination fuel treatments. However, with thoughtful planning and 

careful implementation, reducing fuels and proactively managing soil resources can 

be complementary objectives. This report provided a synthesis of soil chemical, biological, and physical responses to fuel treatment practices, as well as tools to help 

predict these effects. This information can be used to help prevent detrimental soil 

impacts. For example, compaction can be minimized by operating heavy equipment 

when soils are dry, and soil heating can be minimized by burning when soils are 

wet. But beyond preventive measures, we hope this information is also used to create desired soil conditions through fuel treatment practices. For example, prescribed 

fires can be used to create coarse woody debris (CWD), or mechanical thinning can 

create soil microclimate conditions that promote faster nutrient cycling, if those are 

important ecological objectives locally.

Soil ecosystems are dynamic, and we can anticipate both short- and long-term 

effects from treatment operations. For example, the rate at which fuels accumulate 

and decay will influence not only how long a fuel treatment is effective (Keane 

2008), but also the depth and composition of the forest floor. Without continued 

treatment or wildfire, litterfall will accumulate in the form of falling leaves, cones, 

branches, and snags for decades (Keifer et al. 2006). Snags are often created by prescribed burning treatments, which serve as recruitment material for downed woody 

materials over several years to decades (Stephens and Moghaddas 2005). Snag 

retention and creation during fuel treatments will contribute to meeting longer term 

woody debris objectives. Whether N is removed during burning or thinning, forest 

managers might find it beneficial to monitor the response of N-fixing plants. These 

species can play a crucial role in restoring site N and maintaining site productivity. Knowing if a treatment will enhance or diminish N fixers can help managers 

determine how long it will take to replenish the N lost from a site. This may be an 

important consideration when planning followup treatments.

Under historical fire regimes, wildfires burned until available fuels were 

consumed or moisture conditions extinguished them. In the dry West, these fires 

typically occurred during late summer when ground and surface fuels were at their 

driest. Many fires resulted in patchy fuel and highly variable forest floor conditions, 
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Fuel Reduction Practices and Their Effects on Soil Quality

likely ranging from exposed mineral soil to areas that did not burn. In many ecosystems, extended fire exclusion has led to increased fuel accumulations, prompting 

the need for fuel reduction treatments. In designing these treatments, consideration 

should be given to the historical fuel conditions that likely occurred in the treatment 

area—in the variability of load, depth, and distribution across space and time.

This synthesis highlighted soil effects and concerns applicable to a broad 

variety of fuel treatment scenarios, soil types, and vegetation communities. The 

take-home messages that can be applied across most landscapes include:

1. Low- to moderate-severity burning results in nominal damage from soil 

heating, N loss, exposed mineral soil and erosion potential, or root and soil 

microbial mortality. In particular, burn prescriptions that encourage incomplete duff consumption generally produce benign effects on soil quality 

and functioning. Alternatively, mixed-severity burning that results in forest 

structural and compositional heterogeneity, variable forest floor consumption, and some changes in soil properties may be highly appropriate for 

meeting desired fuel conditions and safeguarding soil quality.

2. Severe burning and harvesting based compaction are high risk factors for 

detrimental soil damage if applied across a large percentage of the land. 

Resulting soil erosion, nutrient loss, and soil quality decline in these cases 

may be acute. 

3. Burn piles that contain a high percentage of large-diameter wood generate 

extreme soil temperatures of lasting duration. This is of limited concern on 

a site or landscape basis if the piles occupy only a small percentage of the 

land surface. In comparison, burning of slash piles containing a mix of fuel 

sizes (e.g., precommercial thinning slash) generally does not produce excessive soil temperatures or changes in soil functioning. 

4. Whole tree harvesting removes a relatively small percentage of a site’s 

nutrient capital, and, in most cases, postharvest nutrient reserves remain 

adequate to meet ecosystem needs. Exceptions to this rule include harvesting in extremely nutrient-poor forests or in northern U.S. aspen stands. 

Simple apriori calculations of nutrient export are encouraged to gain perspective on anticipated site nutrient loss.

5. Soils are easily compacted by mechanical harvesting. Whether this translates to a loss in soil quality or reduced site productivity is uncertain, however, based on results from recent compaction studies. Therefore, a cautious 

approach when planning fuel reduction thinning operations is to first identify high-risk soil conditions (e.g., seasonally wet; clayey texture; low rock 
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content; low organic matter content; minimal forest floor development), 

then select best management practices based on site-specific restrictions 

(e.g., avoid harvesting when soil moisture content is near field capacity).

6. The effects of repeated fuel reduction treatments on soil quality are poorly 

studied. Although findings from most single-entry studies indicate few soil 

concerns when best management practices are adhered, any translation to 

multiple-entry, site-specific treatments is premature until more results from 

long-term studies become available. As a precaution, soil scientists can 

develop simple, qualitative accounting systems that track local nutrient loss 

(or gain) or changes in soil physical properties for consecutive entries of 

fire or thinning. Then, adjustments to the frequency of burning and harvesting can be suggested as appropriate. 

7. Soil properties can vary widely across all spatial scales, from broad landscapes to individual microsites. Not only is heterogeneity the norm, but it 

is a desirable attribute that can be augmented by fuel reduction practices. 

For example, using burning techniques to create an uneven mosaic of forest 

floor consumption is sufficient to limit erosional loss, even in the presence 

of fire-induced soil water repellency. Similarly, acknowledging the natural 

variability in CWD content at the landscape scale helps avoid the need for 

strict managerial control on a treatment-area basis, which may offer little 

benefit to soil productivity anyway. 

8. Several soils topics such as the long-term consequences of masticated fuel 

beds or fuel treatments in riparian areas remain largely unstudied, making 

rule-of-thumb assessments unverifiable. 

Bear in mind that soil properties and functions are inextricably linked to the 

vegetation, fuels, land use, disturbance history, and climate conditions in which 

they are found. In practice, the most important knowledge first comes from a local 

understanding of how these factors vary and interact across a landscape. When 

coupled with local knowledge, the information provided here will allow for a 

more powerful interpretation and assessment of fuel treatment effects on our soil 

resources.
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Fuel Reduction Practices and Their Effects on Soil Quality

English Equivalents

When you know: Multiply by: To get:

Centimeters (cm) 0.394 Inches

Meters (m) 3.28 Feet

Hectares (ha) 2.47 Acres

Kilograms (kg) 2.205 Pounds

Tonnes per hectare (Mg/ha) 0.446 Tons per acre

Kilograms per hectare 0.893 Pounds per acre

Tons 907 Kilograms

Gigatons (Gt) 1×109 Tons

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit

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Appendix: A Soil Quality Primer 

This section introduces soil quality as a conceptual tool for managing soil and 

provides examples of soil quality indexes, visual disturbance classes, and risk 

ratings used by land managers. Our intent is to provide supplemental information 

on soil quality principles, which themselves help define the limit of disturbance for 

long-term changes in soil function (Page-Dumroese et al. 2006a), in support of the 

12 fuel reduction topics covered in this synthesis report. 

Soil quality represents the ability of a soil to function as part of a healthy 

ecosystem (Karlen et al. 1997, Schoenholtz et al. 2000, Seybold et al. 1999). It is 

often used synonymously with the term soil productivity, although subtle differences exist between the two (Burger et al. 2010). Basic attributes of soil quality or 

soil productivity include (1) providing for plant growth, water retention, and carbon 

storage; (2) maintaining and sustaining biological activity, diversity, and productivity; (3) governing and partitioning water and solute flow; (4) filtering, buffering, 

decomposing, immobilizing, and detoxifying organic and inorganic materials; and 

(5) storing and cycling of nutrients and other elements within the Earth’s biosphere 

(Burger and Kelting 1999, Karlen et al. 1997). 

The present day concept of soil quality with an emphasis on holistic and sustainable soil management began to evolve in the early 1990s with substantial input 

from the Soil Science Society of America (Doran and Parkin 1994, Karlen et al. 

2003). The development of guidelines to assess forest soil quality at the national and 

international levels then began in earnest as part of the United Nations Conference 

on Environment and Development (O’Neill et al. 2005). This led to the Santiago 

Declaration of 1995 and produced the Montreal Process, consisting of seven criteria 

and 67 indicators applicable to the conservation and sustainable management of 

temperate and boreal forests. Criterion 4 addresses the conservation and maintenance of soils, with indicators focused on best management practices for limiting 

soil degradation. 

The origins for managing lands using the soil quality concept were already 

well established before the 1990s. For example, the U.S. Forest Service (USFS) 

developed soil guidelines in the early 1970s and identified threshold levels which, 

if exceeded, were considered detrimental to soil productivity (Page-Dumroese et al. 

2000). Similar guidelines and thresholds were adopted shortly thereafter by other 

public land agencies. The National Forest Management Act (NEPA) of 1976 gave 

legal credence to these efforts by requiring the development of resource management plans that ensured the productivity of forested lands be sustained in perpetuity. In support, the Forest Inventory and Analysis and Forest Health Monitoring 
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programs of the Forest Service now provide systematic monitoring of soil properties across all forested regions of the United States and help develop benchmarks 

for regional, national, and international reporting on sustainable forest management 

and soil quality (O’Neill et al 2005). 

Present day objectives of the USFS are to “maintain or restore soil quality” 

and manage soils “to sustain ecological processes and function so that desired 

ecosystem services are provided in perpetuity” (USDA FS 2010). Agency guidance 

encourages both qualitative and quantitative methods to evaluate soil quality, yet 

specifies that qualitative measures are generally sufficient to meet soil management 

objectives. Forest Service regions and local management units supplement this 

guidance with specific standards, guidelines, and threshold values as needed. In 

practice, the Forest Service advocates three elements for managing its soils (USDA 

FS 2010): 

(i) Soil quality assessments. Local assessments incorporate past and ongoing 

management activities and are used to establish benchmark (or pretreatment) soil conditions. 

(ii) Soil quality analyses. Soil analyses use a combination of local monitoring results, local knowledge, and scientific literature to predict future soil 

conditions. The purpose is to assist managers when writing environmental documents (such as NEPA analyses) and proposing treatment options. 

(iii) Soil quality monitoring. Monitoring is required to evaluate changes in 

soil properties with time. Repeated measurements provide an understanding of the extent, intensity, and duration of management impacts on soil. 

Evaluating Soil Quality

No universal standards exist for measuring soil quality or specific ratings against 

which all soils can be compared (Karlen et al. 2003). Soil properties vary greatly 

at national, regional, and landscape scales owing to differences in parent material, 

climate, vegetation type, topography, age, and land use history (box 4). With these 

differences come a variety of responses by soils to disturbance (e.g., tolerance, 

resilience, or detrimental damage). Therefore, establishing soil quality indexes 

that would be a standard throughout the U.S. is futile (Burger and Kelting 1999, 

Burger et al. 2010, Page-Dumroese et al. 2000, Schoenholtz et al. 2000). Instead, 

soil scientists must select appropriate soil quality indicators for given locations and 

treatments. As an example, soil quality issues during harvesting differ between a 

well-drained, infertile soil where nutrient depletion is a primary concern versus a 

fertile clay soil that is susceptible to compaction, rutting, and displacement (Burger 
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and Kelting 1999). Fuel reduction treatments present their own unique concerns 

related to soil quality. Mechanical thinning can result in detrimental soil compaction, whereas severe burning can lead to unwanted soil heating and nutrient loss. 

Therefore, the goal of monitoring soil quality is to develop an indexing procedure 

that can be adjusted for different soils and be used to enumerate ratings that are site 

specific for a variety of ecosystems (Karlen et al. 2001). 

Box 4

Soil-Forming Factors

1. Parent material is the primary material from which soil is formed. Examples 

include bedrock, organic material, a stream deposit (alluvium), wind deposit (eolian 

material), glacial deposit (till), volcanic ash, or material moved down slope by gravity 

(colluvium). The composition and fragmentation of parent material affects the rate 

of soil formation and the chemical and mineralogical properties of soil. For example, 

rocks with small-grained minerals tend to undergo faster chemical weathering than 

rocks composed of larger grains. 

2. Climate forces such as heat, rain, ice, snow, and wind facilitate the breakdown of 

parent material and determine how fast or slow soil formation proceeds. For example, 

wet climates of the Eastern United States produce highly weathered soils by promoting rapid chemical and biological weathering, whereas weathering processes and soil 

development are slower in the dry Southwestern United States. 

3. Living organisms include all plants, animals, and microbes living in or on the 

soil. Vegetation type and density influences the soil by providing cover, organic 

matter, and belowground biomass. Organisms affect decomposition and transport of 

soil materials (e.g., preferential flow through root pores and bioturbation). The dead 

remains of plants and animals are recycled by micro-organisms into organic compounds that enrich the soil. This is evident when comparing the nutrient-rich soils 

of wet, northwestern Washington with the low-nutrient-status soils in southwestern 

pinyon/juniper forests.

4. Topography is the shape of the land surface in which a soil occurs. For example, 

soils tend to be shallow on steep slopes and exhibit little development of soil horizons 

because of the constant movement of soil downhill. Generally, soil temperatures are 

warmer on south-facing slopes than on north-facing slopes; soil moisture content is 

greater on north slopes and less moist on south slopes (moisture accelerates chemical 

and biological weathering); and vegetation is denser on north slopes as compared to 

south-facing slopes.

5. Time plays a role in all of the above factors. This role can assert itself over hundreds or thousands of years. With time, physical, chemical, and biological weathering 

processes result in the development of soil horizons that differ in color, texture, and 

chemical makeup. 

Adapted from Jenny (1941).
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For decades, the USFS has used soil properties that are easy to quantify and of 

known sensitivity to management practices as indicators of soil quality, including 

soil compaction, rutting, displacement, erosion, mass movement, groundcover, 

and burn severity. These properties are measured either visually or with rapid and 

inexpensive methods, and their cumulative change is assessed within an activity 

area using either a random or systematic sampling procedure (Page-Dumroese et 

al. 2009). Threshold values for each soil quality indicator have been established 

to predict when conditions become detrimental to soil quality. For example, a 

10-percent reduction in soil porosity has been used as a threshold for detrimental 

compaction (USDA FS 1995). Additionally, a threshold for the cumulative extent of 

detrimental disturbance within a treatment area is set as a management trigger to 

determine if mitigation activities are warranted. Traditionally, a value of 15 percent 

of an activity area was used as a disturbance threshold as this was thought to be the 

lowest value that will manifest as observable changes in soil productivity (fig. 41) 

(Powers et al. 1998). 

Traditional soil quality indexes—

Soil compaction—Soils become compacted when they are compressed to a 

smaller volume by surface traffic such as harvesting equipment, livestock, wildlife, or humans. This causes a rearrangement of soil particles with a concomitant 

reduction in the number and continuity of large pores (Greacen and Sands 1980). 

Common effects of compaction on soil quality include:

Figure 41—Thresholds for detrimental soil disturbance. Detrimental soil conditions exist when indicator values are 

above the threshold level (horizontal dashed line). An activity area is considered detrimentally impacted when the 

extent of detrimental soil conditions exceed the area threshold (vertical dashed line), gray area of chart. 
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Fuel Reduction Practices and Their Effects on Soil Quality

Figure 42—Soil compaction following a thinning operation.

• Decreased soil porosity, water infiltration, hydraulic conductivity, and air 

exchange. 

• Increased erosion by surface runoff.

• Restricted rooting area and inhibited root penetration (fig. 42).

• Reduced rates of organic matter decomposition and nutrient cycling.

• Reduced water storage and nutrient availability in clayey soils.

• Improved water storage in sandy soils. 

• Greater potential for anaerobic, waterlogged conditions.

Absolute changes in these properties vary greatly depending on specific site 

conditions such as (1) soil moisture—moist soils compact easier than dry soils, (2) 

soil temperature—frozen soils resist compaction, (3) soil organic matter (SOM) 

content—organic matter is more difficult to compact than soil minerals, (4) rock 

fragments—rocky soils are resistant to compaction, (5) snow or forest floor cover—

surface cover helps diffuse compaction forces, (6) number of passes by machinery—substantial compaction may occur after only a few passes by high ground 

pressure equipment, and (7) extent of ground coverage—designated skid trails 

produce less aerial disturbance than random movement. Because of this complexity, 

the task of assessing compaction severity is challenging. Several procedures are 

currently used, yet no method is ideal (inexpensive, quick, reliable, reproducible, 

sensitive to regional variability). Both ocular and tile-spade estimations are used 

frequently since they are rapid and inexpensive, although their ultimate usefulness 

depends on an operator’s experience and interpretational skills. Bulk density and 

cone penetrometer measurements are recommended when quantitative measures 

are required. 
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Rutting and puddling—Rutting occurs when heavy equipment is driven on wet 

soil (fig. 43). Evidence of rutting can remain for many years, altering soil hydraulic 

flow and decreasing site productivity (Han et al. 2006). Deep, rutted areas can keep 

soil temperatures cool, inhibit root growth, and slow organic matter decomposition. 

Even shallow rutting can cause degradation in site quality by altering the flow of 

soil water and gasses. Rutting of clayey soils may lead to soil puddling, which further changes soil hydrologic function and adversely affects site productivity. 

Reliability of rutting as a visual indicator of soil disturbance is considered high 

because it is easily detected and measured. However, applying the same standards 

for rutting across regions may be misleading. For example, shallow rutting in arid 

environments may persist for many years, whereas the effects of deep rutting in 

moist southeast forests can be short lived when ameliorative practices such as bedding and providing drainage channels are used (Kelting 1999). 

Soil displacement—Soil displacement is the movement of surface soil horizon 

(mineral or organic) from one place to another by erosive or mechanical forces (fig. 

44). Displacement can also include soil mixing which occurs when the topsoil and 

subsoil are mixed during logging operations, or by ripping, tilling, or windthrow. 

Soil displacement degrades site quality by exposing the nutrient-poor subsoil and 

altering slope hydrology. Lateral displacement of soil may decrease productivity 

by disruption of water distribution, damage to root systems or, in extreme cases, 

uprooting and toppling of trees (Clayton et al. 1987). On marginal, low-nutrientstatus sites, the loss of the organic layer alone can be detrimental. On steep slopes, 

displacement of soil can change subsurface and surface hydrological flow resulting 

in soil slips and mass failures. 

Figure 43—Severe rutting from harvesting on top of moist soil. 

Ken Hubbert
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Figure 44—Soil displacement.

Carolyn Napper

Because of its qualitative and subjective nature (ocular assessment and dependence on the observer’s field experience), the reliability of this indicator may 

be considered moderate. In forest soils, for example, it is sometimes difficult to 

determine the boundary between the forest floor and the mineral soil. In addition, 

the depth of the surface horizon can vary widely across the landscape. 

Erosion—Soil erosion is the detachment and movement of soil by gravity, water, 

wind, or ice. It is a natural process that, in the absence of human intervention, often 

matches the rate of soil formation in a geological timeframe (about 1 to 4 Mg ha-1

yr

-1). Detrimental soil erosion occurs when soils are exposed (e.g., wildfire, road 

building, thinning operations, severe prescribed burning) and the rate of erosion 

exceeds the rate of soil formation. 

The primary types of erosion in wildland systems are dry erosion and water 

erosion, which result when plant cover or forest floor cover is removed, leaving 

hillslopes bare. Dry erosion, or ravel, is the unconsolidated flow of soil owing gravity (Rice 1974) (fig. 45). Any disturbance can initiate downslope movement of ravel 

when slopes are greater than 55 percent. Below 55 percent, wind acts as the catalyst 

to initiate movement of material downslope. Ravel is typically redistributed on the 

hillslope and eventually delivered to the drainage channel. Water erosion is caused 

by surface water runoff when either (1) rainfall intensity exceeds the soil infiltration rate (Horton 1945), or (2) the water storage capacity of the soil is exceeded by 

rainfall (saturation overland flow). Once overland flow begins, the extent of erosion 

is governed by numerous site factors including the intensity and duration of the rain 

event, slope steepness and length, surface cover, and soil texture, structure, and 

antecedent moisture content (Moffett et al. 2007). 
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Figure 45—Dry ravel erosion.

Ken Hubbert

Rills, gullies, channels, pedestals, and landslides are easily observed and 

symptomatic of excessive soil and nutrient loss from a site. Rills are the result of 

the cutting of numerous small water channels by concentrated surface runoff. As 

concentrated flows continue to cut into the hillslope, rills become larger, forming gullies or large channels. Pedestals are remnants of former surfaces that are 

indicative of interrill erosion (or sheet erosion). Landslides occur typically in steep, 

unstable landscapes when clayey soils become saturated. The weight load on the 

slope increases as previously empty pore spaces and fractures fill, which leads to 

increased pore pressure, reduced shear strength, and, ultimately, slope failure and 

landslides.

Unlike visual assessments of these erosional processes, quantitative measurements of soil loss are exceedingly difficult to implement on an operational scale. 

Soil erosion models offer an alternative approach for semi-quantitative assessments 

of erosion. The “water erosion prediction project” model (WEPP; http://www.ars.

usda.gov/Research/docs.htm?docid=10621) was developed to predict wet erosion 

within small watersheds and hillslopes, and its predictive accuracy has been further 

improved with a spatial erosion modeling tool, GeoWEPP, which uses geo-referenced, site-specific input parameters and defaults. 

Ground cover—Ground cover, including plants and forest floor residues, serves 

many soil quality functions. These include reducing the risk of surface soil erosion, 

regulating soil temperature and moisture, and storing carbon (C) and nutrients. In 

addition, forest floor residues function as a fuel source for fire and as a restrictive 

layer for plant germination and survival if excessively deep. Despite these diverse 

functions, soil quality guidelines are often primarily concerned with the role of 
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Fuel Reduction Practices and Their Effects on Soil Quality

ground cover in protecting against soil erosion (fig. 46). Loss of cover exposes bare 

soil to raindrop impact, causing the deterioration of soil structure and the onset of 

sheet erosion. Generally, the degree of patchiness of forest floor cover determines 

its effectiveness in intercepting rainfall and preventing surface runoff and erosion. 

Robichaud et al. (2000) noted that erosion is effectively controlled at 60 percent 

plant cover, even during high-intensity rain events. 

Burn severity—Soil quality is affected when burning is severe. Critical issues 

include soil erosion from loss of forest floor cover, change in soil pH and nutrient 

status, and loss of SOM. Severe heating within the mineral soil profile may occur 

beneath concentrated fuels such as large slash piles, downed logs, and thick masticated fuel beds (fig. 47). For example, Ulery et al. (1993) observed a 1- to 8-cmthick reddened layer beneath large burned out logs that they attributed to heatinduced iron (Fe) oxide transformations. In comparison, low- to moderate-severity 

burning results in blackened soil as a result of charring of organic matter, which 

may temporarily enhance soil quality resulting from an increase in base cations and 

available nitrogen (N). 

Visual observations can identify color changes in the mineral soil and loss 

of surface cover, but they do not serve as effective surrogates for changes in soil 

chemical and biological properties. For example, the color of the mineral soil may 

naturally be red owing to the presence of hematite in weathered soils. In this case, 

color cannot be used as an indicator of severity class. 

Figure 46—Insufficient ground cover leading to surface wind 

erosion. 

Ken Hubbert
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Visual soil quality disturbance classes— 

Traditional guidelines and thresholds have been used effectively to evaluate soil 

changes associated with management practices. However, they can be cumbersome and costly to implement. Further, the lack of a standard, nationwide reporting 

system prohibits the development of rigorous statistics to help explain the effects 

of treatments such as fuel reduction activities on soils. These drawbacks led PageDumroese et al. (2009) to propose a monitoring protocol based on a visual disturbance rating system that is both practical to implement and can provide standardized information over a wide range of ecosystems and soils. Four soil disturbance 

classes and their identifying features developed by Page-Dumroese et al. (2009) are 

shown in table 12. The disturbance classes essentially account for the same set of 

soil quality indices as the traditional methods (compaction, rutting, displacement, 

erosion, ground cover, burn severity), using visual observations taken at numerous gridpoints within a treatment area to develop an aggregate score between 0 (no 

disturbance) and 3 (highly disturbed) for an entire unit. This assessment is particularly suitable for fuel reduction practices with its emphasis on tractor-derived 

compaction, forest floor integrity, and burn severity. Further, the expectation is that 

all disturbance scores will be entered into a nationwide database (in development), 

allowing resource managers to better understand trends in soil responses across 

landscapes, soil types, and treatments prescriptions. 

Figure 47—Visual scar left from high-severity pile burning.

Terry Craig
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Table 12—Soil disturbance classes based on Page-Dumroese et al. (2009)a

Severity class Identifying features

0 • Forest floor undisturbed 

• No evidence of past equipment operation

• No soil char or evidence of litter and duff burning

• No mineral soil displacement or erosion

• Water repellency at background levels

1 • Forest floor intact

• Faint wheel tracks and ruts

• No mineral soil displacement and minimal mixing with forest floor

• Slight increase in soil compaction

• Light burning may be evident

• Repellency similar to preburn levels

2 • Forest floor partially intact or missing

• Wheel tracks (5 to 10 cm deep) created by one or more passes

• Compacted, platy structure to a soil depth <30 cm 

• Moderate erosion

• Burning moderate, depth of char <5 cm 

• Increase in surface water repellency

3 • Forest floor missing

• Wheel tracks >10 cm deep

• Majority of soil surface is displaced

• Compaction extends beyond 30-cm depth

• Severe burning—duff fully consumed; soil char depth >5 cm

• Increase in surface water repellency

a Severity of impacts increases from class 0 to class 3. Severity class is determined at multiple gridpoints, 

systematically located within a treatment unit to determine a mean severity class rating.

Visual disturbance classes offer simple and standardized soil quality assessments. Their drawbacks are that they are highly qualitative in nature (Napper et al. 

2009) and the observations are dependent on the expertise of the observer and may 

be difficult to interpret. For these reasons, it may be necessary to supplement visual 

observations with quantitative measures of soil quality on a subset of treatment 

units that are of particular concern or are representative of benchmark soils and site 

conditions.

Soil Risk Ratings 

Risk ratings offer managers a decision-support tool for planning purposes that 

identifies vulnerability to disturbances such as compaction, erosion, and severe 

burning (Curran et al. 2005). Table 13 provides a simple example of a risk rating 

developed by USFS Pacific Southwest Region (California) soil scientists to identify 
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at-risk soils. This rating system provides a first-approximation of compaction risk 

based on soil texture and rock content, and is similar to a compaction rating system 

used in British Columbia (Curran et al. 2007). It essentially provides a rough assessment of site conditions where caution may be required and, conversely, where soil 

conditions are likely tolerant to disturbance (e.g., compaction in rocky soils of any 

texture). In turn, the success of this or any risk rating system relies on quality data 

provided by extensive field observations and research findings, along with a commitment to update the rating system as new information becomes available. 

An added benefit of risk ratings is that they offer an implicit decision tree 

for selecting best management practices for sustaining soil quality. For example, 

manageable options for harvesting on top of clay soil with low rock content (highhazard rating in table 13) might emphasize selection of the appropriate harvest 

timing to avoid wet soil, use of low-impact equipment, or the presence of thick slash 

and forest floor layers. 

While soil risk ratings offer a valuable “triage” assessment for project planning, 

they may be overly simplistic for some uses. For example, an assessment of compaction risk based only on texture and rock content fails to acknowledge countless 

other site factors such as slope, soil water content, forest type and assumed effects 

on site productivity, and cumulative effects from previous entries. Recognition of 

this limitation has led to recent advances in soil risk ratings through the use of soil 

and geology databases linked visually through geographic information systems 

(Kimsey et al. 2011, Reeves et al. 2012). This evolving technique holds promise as 

a user-friendly, cost-effective tool for planning purposes that can (1) identify sites 

with vulnerable soil physical, chemical properties, and (2) help select best management practices to protect the resource.

Table 13—Compaction risk ratings based on texture class and coarse fragments 

Compaction hazard Texture class Coarse fragments > 2 mm

Low Sandy

Any texture

Any amount

Greater than 70 percent

Moderate Loamy texture

Clayey

Any amount

Any amount

High Silty Less than 35 percent

Source: Adapted from USDA FS 2006.
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Quantitative Indicators of Soil Quality

Quantitative assessments of soil chemical and biological properties provide definitive evidence of changes in soil quality. Thus, they can be used to validate the 

findings of adverse soil conditions identified by qualitative (ocular) methods or to 

provide benchmark data for environmentally sensitive areas (e.g., riparian zones) 

or for analyses of cumulative treatment effects. Although far from a comprehensive 

list, we identify below several options for chemical and biological indicators of soil 

quality. Most soil chemical indicators can be analyzed at commercial laboratories 

for moderate cost, whereas there are few commercially available or affordable 

biological measures of soil quality. These, instead, fall in the category of researchbased indices.

Chemical indicators— 

All fuel reduction practices affect soil chemical properties to a certain degree. 

Thinning removes nutrients from a site, alters plant competition and demand for 

available soil nutrients, and can alter soil hydrological properties that indirectly 

affect soil nutrient availability. Prescribed burning also affects soil nutrient availability and organic matter content—either positively or negatively—depending on 

fire severity. Useful chemical indicators include SOM, total and available N, soil 

pH, cation exchange capacity, total or available phosphorus, and heavy metals. For 

a more complete description of these and other potential indicators, see review articles by Doran and Jones (1996), Pankhurst et al. (1997), and Weinhold et al (2004). 

Soil organic matter. Soil organic matter is critical to the biological, physical, and 

chemical health of all soils and is considered a cornerstone measure of soil quality. 

Long-term monitoring of SOM may be warranted in some cases since as burning 

and thinning can reduce SOM levels if sufficiently severe or aggressive. 

Total nitrogen. Nitrogen is the second most limiting soil constituent (following 

water) that constrains plant growth and net productivity in most ecosystems. Like 

SOM, soil N can be altered by fuel reduction practices, justifying a long-term monitoring plan of forest floor and mineral soil N when cumulative treatment effects are 

anticipated.

Soil pH. Soil pH is a key indicator of soil quality. It controls the availability of 

many plant nutrients and toxic compounds, and regulates most microbial processes. 

Calcium, magnesium, and oxyanions such as sulfates and phosphates become more 

available as pH increases to alkaline levels. Conversely, many micronutrients and 

heavy metals become more available as pH decreases. Prescribed fire generally 

raises pH depending on the base cation content of ash, although the increase may 

be minimal or short lived (Murphy et al. 2006). 
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Other chemical indicators. Other options for chemical indicators include cation 

exchange capacity (useful for fire effects monitoring); available N (ammonium, 

nitrate; ephemeral measurement that can vary considerably with season of year); 

phosphorus (total P or available P); additional nutrients and heavy metals. These are 

generally high-cost analyses and may be most appropriate for research-level monitoring of fuel reduction practices.

Biological indicators— 

Soil biological activity is an important attribute of a healthy soil. Soil organisms 

regulate nutrient cycling, decompose SOM, protect against pathogens, and mediate heavy metal toxicity. They also help improve soil structure through the binding 

properties of fungal mycelium, which, in turn, improves soil porosity, water infiltration, and plant-available water content.

Biological indicators include individual species, functional groups, and their 

ecological processes (McGeoch 1998). They have proved to be useful tools for 

detecting changes in the environment because of their rapid response to disturbance. Drawbacks exist, however, such as their inordinate numbers, taxonomic 

challenges, general unfamiliarity, and ineffectual methods of detection (Andersen 

et al. 2002). Criteria for potential indicator organisms or processes include that 

they (1) play a key role in the functioning of the soil ecosystem, (2) occupy a wide 

range of ecosystems enabling comparisons between systems, (3) are abundant and 

easily recognized, (4) are easy to sample and analyze, and (5) are not too sensitive 

to environmental stresses that they become extinct (Edwards et al. 1996). Table 14 

lists common soil organism groups that function as indicators of soil quality. 
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Table 14—Soil organisms as bioindicators for determining soil ecosystem health

Organism group Use as bioindicator Soil function 

Macrofauna:

Earthworms General indicators of soil quality (presence is typically limited to nonacidic soils, 

hardwood forests, and moist climates 

• Mixing and transport of surface organic matter 

deep into the mineral soil 

• Improve soil structure, porosity, and water 

movement

• Improve soil fertility

Termites Limited primarily to subtropical and 

tropical climates

• Mixing and transport of surface organic matter 

into the mineral soil 

• Improve soil structure, porosity, water movement

• Digestion of soil wood

Ants Strong indicators of habitat disturbance • Transport organic matter and minerals from lower 

horizons to the soil surface

• Increase soil porosity and lower soil bulk density

• Govern soil hydrological processes

• Break down of soil wood

Millipedes Indicators of soil physical and chemical 

perturbation in moist areas 

• Assist in the breakdown of surface organic materials such as leaves, twigs, and roots

Carabid beetles General indicators of habitat alteration. 

(populations are often reduced following 

fire or mechanical treatments)

• Chewing and mixing of forest floor organics 

Mollusks (snails) Indicators of heavy metal pollution 

Mesofauna:

Springtails

Mites

Key indicators of soil food web health • Predators of bacteria, fungi, and algae 

• Help decompose surface organics

Enchytraeids

(potworms)

Indicators of chemical stress from pesticides, heavy metals, and fertilizers • Decompose and transport of surface organic matter into the mineral soil

Microfauna:

Nematodes Indicators of soil nitrogen (N) cycling 

and assorted soil chemical and physical 

processes 

• Predators of bacteria

• Plant pathogens

Protozoa (ciliates) Indicators of soil food web health • Enhance nutrient cycling processes

• Predators of bacteria

• Facilitate organic matter decomposition
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Micro-organisms:

Fungi, bacteria,

algae, viruses

Traditional indicators of soil quality and 

disturbance response in all ecosystems

• Nutrient cycling

• Organic matter decomposition

• Improve soil structure (fungi)

• Improve plant nutrient and water uptake

(mycorrhizal fungi)

• Sequester carbon

• Release greenhouse gases

• Symbiotic relationships with plants

• Degrade toxic chemicals

• Suppress plant pathogens

• Pathogens

Source: Adapted from Chen et al. 2006, Deka et al. 1983, Didden and Rombke 2001, Esquilin et al. 2007, Foissner 1999, Joschko et al 1989, Kobziar and 

Stephens 2006, Li et al. 2004, Luo and Zhou 2006, McCarthy and Brown 2006, Neher 2001, Rainio and Niemela 2003, van Straalen and Verhoef 1997.

Organism group Use as bioindicator Soil function 

Table 14—Soil organisms as bioindicators for determining soil ecosystem health (continued)
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