United States Department of Agriculture

Forest 

Service

Pacific Northwest 

Research Station

Research Note

PNW-RN-574

February

2017

Decomposition Rates for 

Hand-Piled Fuels

Clinton S. Wright, Alexander M. Evans, and Joseph C. Restaino1

Abstract

Hand-constructed piles in eastern Washington and north-central New Mexico were 

weighed periodically between October 2011 and June 2015 to develop decay-rate 

constants that are useful for estimating the rate of piled biomass loss over time. 

Decay-rate constants (k) were determined by fitting negative exponential curves to 

time series of pile weight for each site. Piles at the Washington site (k = 0.027/year) 

decomposed significantly more slowly than piles at the New Mexico site (k = 0.064/

year). Significant differences in k for each site may be a function of a between-site 

variation in pile composition (with or without large woody material), the environmental dissimilarities between sites (Mediterranean climate regime in Washington 

vs. monsoonal climate regime in New Mexico), or a combination of factors.

Keywords: hand piles, decay-rate constant, fractional mass loss, fuels management, Washington, New Mexico.

1 Clinton S. Wright is a research forester emeritus, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station, 400 North 34th Street, Suite 201, Seattle, 

WA 98103; Alexander M. Evans is the research director, Forest Stewards Guild, 2019 

Galisteo Street, Suite N7, Santa Fe, NM 87505; and Joseph C. Restaino is a researcher, 

University of Washington, School of Environmental and Forest Sciences, Box 352100, 

Seattle, WA 98195.
2

RESEARCH NOTE PNW-RN-574

Introduction

Many conifer-dominated forests with historically low-severity fire regimes in the 

Western United States are at risk of stand-replacing wildfires (e.g., Fulé et al. 2004, 

Noss et al. 2006). To reduce the likelihood of uncharacteristically severe wildfire 

in these forests, fuel reduction treatments, which typically consist of removal of 

small-diameter trees, are being implemented widely. These types of fuel reduction 

treatments produce large amounts of unmerchantable woody material and elevated 

surface fuel loadings (Agee et al. 2000, Finkral and Evans 2008, Fulé et al. 2002, 

Hjerpe et al. 2009). Currently, there are few commercial markets for this material, so it is commonly piled by hand or with heavy machinery and burned on site 

(Evans and Finkral 2009, Han et al. 2010).

In addition to biological and ecological effects, research on pile burning is 

expanding to address key topics such as smoke emissions and carbon dynamics. Because of concerns about smoke and air quality in particular, fire, fuel, 

and air-quality managers must understand and be able to quantify fuel loading 

accurately to best estimate and manage emissions when piles are burned (Weise 

and Wright 2014, Wright et al. 2010). Besides potentially affecting downwind 

air quality, piling and pile burning also affect terrestrial ecosystem carbon pools 

and fluxes (DeLuca and Aplet 2008, Finkral et al. 2012, Forbes et al. 2006, 

González-Pérez et al. 2004).

As with broadcast prescribed burning and wildland fires, smoke emissions 

from pile burning are affected by the amount and physical characteristics of the 

fuels involved, which change over time as existing fuel particles decompose. Little 

is known about how the physical characteristics, particularly the fuel loading, of 

hand-piled fuels change over time. As part of a larger experiment to document how 

time-since-piling and season-of-burning influence fire behavior and fire effects 

(Wright et al. 2015), we constructed and periodically weighed hand piles located at 

one site each in eastern Washington and north-central New Mexico to determine 

rates of decay. Because smoke emissions are a function of fuel loading and fuel 

consumption (Hardy et al. 2001), knowledge about the rate of decay can be used to 

more accurately estimate the biomass of piles as they age and the resulting emissions when they are burned.
3

Decomposition Rates for Hand-Piled Fuels

Methods

Study Areas

Two study sites, one each on the Naches Ranger District of the OkanoganWenatchee National Forest in eastern Washington and on the Santa Clara Pueblo in 

north-central New Mexico, were selected for study. Both sites were in dry conifer 

forests that had been recently thinned of small diameter (<20 cm diameter at breast 

height; d.b.h.) trees to reduce stand density. Thinning debris at both sites had been 

piled by hand during fuel management operations; piles were approximately 2.5 to 

3 m in diameter and 1.2 to 1.5 m tall (fig. 1). Piles at the Washington site included 

tops, branchwood, and 2-m-long pieces of bole wood up to approximately 20 cm 

in diameter. In contrast, piles at the New Mexico site were composed of <7.6-cmdiameter woody material only (branches and small tops), as the area had been made 

available for firewood collection prior to piling.

The Washington site is located on a small, flat bench in an area of gently sloping 

terrain (5- to 15-percent slope, easterly aspect) at 1140 m elevation above Rimrock 

Lake, approximately 50 km southeast of Mount Rainier in the eastern Cascade 

Range (N46° 39′ 25.9′′; W121° 09′ 26.6′′). The area is characterized by a Mediterranean climate regime with warm, dry summers (mean July–August maximum 

temperature of 23 °C; July–August precipitation of 3 cm) and cold, wet winters 

(mean December–February minimum temperature of -5 °C and precipitation of 

Figure 1—Typical hand piles approximately 6 years after piling at the study site in central Washington.

Clinton S. Wright
4

RESEARCH NOTE PNW-RN-574

43 cm), with most precipitation falling as snow (Prism Climate Group 2016). The 

site is in a mixed-conifer stand comprised of 15- to 20-cm d.b.h. ponderosa pine 

(Pinus ponderosa Lawson and C. Lawson), western larch (Larix occidentalis Nutt.), 

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and grand fir (Abies grandis

(Douglas ex D. Don) Lindl.), with scattered, remnant, old-growth (70- to 125-cm 

d.b.h.) ponderosa pine and western larch. The stand was thinned to a basal area of 

10.0 m2/ha in 2011; small trees of all species were cut during the thinning. Thinning 

debris was piled by hand at a density of 90 piles/ha. 

The New Mexico site is located on relatively flat terrain at 2400 m elevation 20 

km west of Española, in the Jemez Mountains (N36° 0′ 56.4′′; W106° 16′ 59.9′′). The 

area is characterized by hot summers (mean July–August maximum temperature 

of 27 °C) and cold winters (mean December–February minimum temperature of 

-9 °C), and receives 56 cm of precipitation annually, with approximately half of it 

coming in the summer and early fall during the monsoon period (Prism Climate 

Group 2016). The site is in a mature ponderosa pine stand (±29 cm d.b.h.) that 

was thinned to a basal area of 18.5 m2/ha in 2011; thinning debris (ponderosa pine 

branches and tops only) was piled by hand at a density of 50 piles/ha. 

Field Methods

We built five hand piles at each study site atop galvanized steel frames. The steel 

frames were constructed of 2 m × 2 m chainlink fence panels and reinforced with 

two galvanized steel cross braces joined to the panel edges with split-T braces (fig. 

2). Galvanized wire mesh (6-mm diameter hole size) was attached to each panel 

frame with steel wire to minimize material loss through fine branch breakage and 

needle excision as piles aged. The entire assembly was weighed (tare weight) before 

building the pile on top of it.

2-m x 2-m chain-link fence panel

covered with 6-mm diameter galvanized

steel hardware cloth wired to the frame

3.5-cm diameter galvanized steel fence

post attached to outer frame with split-T

fencing connectors

Pile diameter

Numbered steel tag wired to corner of

frame to allow long-term monitoring

Figure 2—Pile frame assembly.

USDA Forest Service
5

Decomposition Rates for Hand-Piled Fuels

Hand piles were paraboloid, 1.8 m in diameter, and 1.2 m high when built. 

Thinning debris was placed on top of the chain-link panel assemblies until the pile 

reached its 1.2-m build height. Piles at the Washington site were constructed based on 

contract piling specifications that are typical on the Okanogan-Wenatchee National 

Forest and closely matched those created during the fuel management operations that 

immediately preceded the start of this study. The Washington piles were composed 

of alternating layers of small (<7.6-cm diameter) and large (7.6- to 20-cm diameter) 

fuels of mixed species (ponderosa pine, western larch, Douglas-fir, and grand fir), and 

were covered with a layer of waxed craft paper. The waxed craft paper is intended to 

keep the pile center dry to facilitate ignition during the spring and fall when weather 

conditions are moist and when most pile burning occurs in Washington. Piles at 

the New Mexico site only included ponderosa pine branchwood and tops less than 

approximately 8 cm in diameter and were not covered. We attempted to build piles of 

comparable volume, weight, and size class composition within each site.

When built, and approximately every 6 months thereafter for a period of almost 

4 years, each pile was weighed intact by lifting the frame assembly a short distance 

off the ground using a hand-operated chain hoist with an attached 500-kg-capacity 

hanging scale (Intercomp CS750). The frame assembly was suspended from a 

4.2-m-tall aluminum utility tripod (Spanco ATA-02-1107)2 with 1000-kg-capacity 

centered over each pile by nylon slings attached to each corner of the frame (fig. 3).

Moisture-content subsamples were collected during pile construction and at the 

time of each remeasurement and were used to adjust all field weights to a common 

oven-dry basis. Three to five subsamples each from the surface and the interior of 

adjacent sacrificial piles (i.e., piles scattered throughout the stand that were created 

during piling operations immediately preceding the beginning of this study) were 

collected in heavy-gauge, resealable plastic bags and weighed shortly after collection. 

Subsamples were returned to the laboratory, oven-dried at 100 °C to a constant weight 

(at least 48 hours) and reweighed to determine the ratio of dry weight to wet weight for 

use in correcting field weights to a dry-weight basis. In addition to collecting material 

from the surface and the interior of sacrificial piles, at the Washington site, moisture 

content subsamples were further stratified by size class (<7.6 cm in diameter and >7.6 

cm in diameter). A volume-weighted (and size-weighted for the Washington site) moisture ratio ( ̄m) was calculated and multiplied by the field weights to adjust to an ovendry basis (eq. 1). We assumed that moisture content subsamples collected from the 

surface of adjacent sacrificial piles represented an outer 0.5-m-thick shell for each pile. 

This outer shell accounted for 35 percent of the pile’s volume, with the remainder of 

the pile’s volume (65 percent) represented by the interior moisture content subsamples.

2 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.
6

RESEARCH NOTE PNW-RN-574

(1)

Where

m ̄ = the volume- and size-weighted net-dry:net-wet weight moisture ratio,

w = the starting oven-dry weight of the pile,

ws = the starting oven-dry weight of fuel particles <7.6 cm in diameter in the pile,

ve = the proportion of the pile’s volume included in a 0.5-m-thick shell on the 

surface (i.e., 0.35),

vi = 1 – ve (i.e., 0.65),

mse = the ratio of net-dry:net-wet weight of moisture subsample particles <7.6 cm in 

diameter from the exterior, or surface, of the pile,

msi = the ratio of net-dry:net-wet weight of moisture subsample particles <7.6 cm in 

diameter from the interior of the pile,

mle = the ratio of net-dry:net-wet weight of moisture subsample particles 7.6 to 20 

cm in diameter from the exterior, or surface, of the pile, and 

mli = the ratio of net-dry:net-wet weight of moisture subsample particles 7.6 to 20 

cm in diameter from the interior of the pile.

Figure 3—Pile suspended from tripod during periodic remeasurement.

Robert E. Vihnanek
7

Decomposition Rates for Hand-Piled Fuels

Analysis

The rate of pile mass loss was modeled by fitting a negative exponential curve to 

the mean fractional mass loss for the periodic remeasurements from each site (eq. 2) 

as in Olson (1963).

Xt /X0 = e-kt (2)

Where

Xt = the oven-dry weight at time t (in years),

X0 = the starting oven-dry weight, 

k = the fractional mass loss or decay-rate constant, and

t = the time since piling (in years).

Decay rate constants were derived for individual piles (i.e., replicates) and 

also for the mean of all decay rate measurements for each of the New Mexico and 

Washington sites. Mean decay-rate constants for the New Mexico and Washington 

sites were compared statistically with the Mann-Whitney U test (Zar 1984), the nonparametric equivalent of the two-sample t-test.

Results and Discussion

The experimental hand piles monitored for this study showed a trend of decreasing 

weight over time (tables 1 and 2). On average, piles lost 10 percent and 17 percent 

of their starting weight after 45 months for the Washington and New Mexico sites, 

respectively. Although a variety of model forms have been proposed to represent 

biomass decomposition (see Harmon et al. 1986 for a thorough discussion on this 

Table 1—Actual changes in hand-pile weight over 

time at the Washington state site

Date Mean pile weighta Sample size

Kilograms ± SE n

October 2011b 160.6 ± 3.8 5

June 2012 153.4 ± 4.0 5

October 2012c 179.6 ± 5.9 5

May 2013 153.4 ± 3.6 5

October 2013 154.5 ± 2.9 5

July 2014 148.2 ± 4.1 4

June 2015 143.5 ± 4.8 4

a Oven-dry weight.

b Starting weight.

cRainy day; likely that moisture content subsamples did not 

adequately account for surface moisture and properly adjust for dry 

weight; data point not used in model development.

SE = standard error.

Table 2—Actual changes in hand-pile weight over 

time at in New Mexico site

Date Mean pile weighta Sample size

Kilograms ± SE n

October 2011b 74.9 ± 5.3 5

April 2012 70.8 ± 4.3 5

November 2012 68.5 ± 4.1 5

March 2013 65.8 ± 4.6 5

October 2013 62.9 ± 4.8 5

February 2014 63.0 ± 4.0 5

June 2015 61.7 ± 4.4 5

a

 Oven-dry weight.

b

 Starting weight.

SE = standard error.
8

RESEARCH NOTE PNW-RN-574

topic), the decay rate of hand-piled fuels at our study sites was well-represented 

with site-specific, single, negative exponential models (fig. 4). Site-specific models 

explained more than 80 percent of the variance in the data (r2 = 0.82 for Washington 

and 0.83 for New Mexico), at least for the relatively early stages of decomposition 

(first 4 years) monitored in this study. The decay rate constant, k, was 0.027/year 

and 0.064/year for the Washington and New Mexico sites, respectively. The shape 

and slope of the curve, which is determined by the product of the exponents in 

equation 2, was significantly steeper for the New Mexico site than the Washington 

site (Mann-Whitney U ν(5, 5) = 25, p = 0.01), indicating a more rapid rate of mass loss 

over time in New Mexico.

Harmon et al. (1986) suggested that larger logs decay more slowly than smaller 

logs, branches, and twigs, but acknowledge that this size/decay rate relationship 

has not been found for all forested systems or studies. Piles at the Washington site 

were 43 ± 6 percent (mean ± standard deviation) bole wood >7.6 cm in diameter; 

none of the piled material in New Mexico was >7.6 cm in diameter. Based on our 

empirically derived values of the decay-rate constant, it appears that piles with 

a large woody material component (i.e., the Washington site) decay more slowly 

than piles with only small-diameter branchwood, tops, and needles (i.e., the New 

Mexico site).

Figure 4—Mass loss of hand-piled biomass as a function of time, in years, for sites in Washington 

and New Mexico, with modeled mass loss projected for 10 years. The decay constant (k) and the 

coefficient of determination (r2) are indicated for each site-specific negative exponential model.
9

Decomposition Rates for Hand-Piled Fuels

Leaf litter and woody material decomposition have been widely studied in wet 

and moist conifer forests in the western United States, especially in the western 

Cascade Range (e.g., Edmonds 1987, Harmon et al. 1986, Spies et al. 1988). Information about litter and woody material decomposition is limited for dry conifer 

forests, however, which are widespread across the intermountain West where thinning, piling, and burning is a common forest and fuel management practice. Based 

on the limited information available in forest types comparable to our study sites, 

it appears that the decay-rate constants for whole hand piles observed in this study 

are intermediate between the faster rates observed for coniferous leaf litter 

(k = 0.08-0.20/year) (Hart et al. 1992, Stohlgren 1988) and the slower rates 

observed for solid wood in different size classes (k = 0.010/year) (Erickson et al. 

1985), as we would expect of a pile that includes both types of material. 

Litter and wood of different species have been shown to decay at different rates 

(Daubenmire and Prusso 1963, Edmonds 1987, Erickson et al. 1985). The piles 

in New Mexico were constructed entirely of ponderosa pine and it was a major 

component of the piles at the Washington site as well; however, the Washington site 

also included logs, branches, twigs, and needles of Douglas-fir, western larch, and 

grand fir. Species composition was not a factor in the design for this study, so we 

cannot say whether, or to what degree, it affected our observed differences in decay 

rate between sites.

In addition to material type (i.e., leaf litter vs. woody material, hardwood vs. 

conifer, etc.), moisture and temperature conditions influence how quickly vegetative matter decomposes (Erickson et al. 1985, Harmon et al. 1986, Hart et al. 1992, 

Jenny et al. 1949, Prescott et al. 2004, Zell et al. 2009). In general, warm, wet conditions favor and accelerate decomposition. While both study sites are in dry conifer 

forests dominated by ponderosa pine, they experience different climate regimes. 

The New Mexico site is characterized by a monsoonal climate pattern in which 

most of the precipitation falls as rain during the warm summer months, whereas the 

Washington site is characterized by a Mediterranean climate and receives most of 

its precipitation in the cold winter months in the form of snow. The coincidence of 

warm temperatures and summer monsoon moisture in New Mexico may provide 

for microclimates that are relatively more conducive to decomposition activity and 

a faster rate of decay than those found in Washington, where warm temperatures 

are temporally separated from moist conditions owing to the Mediterranean climate 

pattern (Hart et al. 1992).

It is a common practice in the Pacific Northwest to cover piles in an attempt 

to keep a portion of the pile dry and thereby facilitate ignition during wetter times 
10

RESEARCH NOTE PNW-RN-574

of the year when fuel managers prefer to conduct pile burning. Maintaining drier 

conditions within the pile by covering would theoretically slow decomposition. 

This supports the pattern we observed wherein the covered piles in Washington 

decomposed more slowly than the uncovered piles in New Mexico; however, our 

measurements of pile moisture content periodically throughout the study (table 3), 

show that fuels are not consistently significantly drier in the interior than on the 

surface of piles in Washington (paired two-sample t-test for difference between 

pile interior and pile surface) for large (t ν(7) = 0.03, p = 0.97) and small size classes 

(t ν(7) = 0.96, p = 0.37) of fuel particles. Thus, although the practice of covering 

piles may maintain a sufficient quantity of fuel in a dry, ignitable condition to 

justify the practice for operational purposes, based on our observations it appears 

as though the waxed craft paper used in this study did not maintain a consistently 

drier pile interior over the course of the study—a condition that would theoretically 

slow decomposition. Further work to document the fuel moisture dynamics of 

covered (and uncovered) piles is necessary to understand how within-pile moisture 

conditions affect decay rate.

Stand conditions (e.g., clearcut vs. mature forest, young forest vs. old growth, 

etc.) have also been shown to affect decomposition rate (Klemmedson et al. 1985, 

Prescott et al. 2004). Whether differences in decay rate are a function of the chemical qualities of the biomass inputs (e.g., from mature trees vs. young trees, from 

hardwoods vs. conifers, etc.) or the microclimate (e.g., with or without canopy 

coverage, northern vs. southern aspects, on or off the ground, etc.) is an open 

discussion in the literature worthy of additional study. As both of our sites were 

in established, forested stands, however, we were not able to assess the effects of 

Table 3—Mean moisture content by size class for samples collected 

from the interior and the surface of piles in Washington

Percentage of moisture content

Date

<7.6 cm diameter ≥7.6 cm diameter

Interior Surface Interior Surface

October 2011 21.6 31.0 59.3 70.0

June 2012 24.1 25.9 81.7 53.3

October 2012a 28.3 57.5 28.4 39.0

May 2013 31.2 37.8 40.3 42.0

October 2013 21.1 36.2 24.8 31.3

May 2014 36.0 22.7 36.0 36.3

July 2014 12.8 10.0 17.0 17.3

June 2015 24.3 15.4 34.7 32.0

a

 Rainy day; likely that moisture content subsamples included moisture from particle surfaces.
11

Decomposition Rates for Hand-Piled Fuels

overstory structure on decomposition of hand-piled fuels. Given the widespread use 

of piling as a forest management practice throughout the Western United States, 

further research is warranted to better understand the manner in which stand- and 

species-level variables affect decay rates; for instance, whether piles in clearcuts 

decay at a different rate than piles in intact forest or in thinned stands.

Our results document a difference in the rate of decomposition for hand-piled 

fuels in Washington and New Mexico, although the nature of the experimental 

design does not allow us to definitively attribute a cause to the difference. Piles 

with a large woody material component in Washington decayed more slowly than 

those composed solely of small-diameter branchwood in New Mexico. This is to be 

expected given that the New Mexico piles were weighted more heavily, in a relative sense, to faster decomposing small woody material and needle litter. Similarly, 

assuming that the monsoonal weather pattern in New Mexico is relatively more conducive to decomposition than the Mediterranean climate of eastern Washington, the 

larger decay-rate constant we measured for the New Mexico site is to be expected. 

The observed differences in decay rate between sites may be related to differences 

in pile composition (both size class and species), differences in environmental 

conditions attributed to climate regimes, different within-pile microclimates related 

to covering or stand structure, or a combination of these and potentially other factors not measured in this study. Future research should be conducted to isolate and 

examine these and other factors.

Management Implications

It is quite common for piles to age on the landscape for a period of time before they 

can be burned. As piles age they both compress and gradually lose mass—changes 

that affect the quantity and quality of smoke emissions. The site-specific decay 

constants (k) reported in this study can be used to adjust pile weights as piles age 

so that fuel characteristics (i.e., loading and bulk density), fuel consumption, smoke 

emissions, and charcoal production can be estimated based on pile weights at the 

time of burning rather than the time at construction (box 1). It should be noted, 

however, that the decay constants reported herein are based on hand piles composed 

of variable-diameter, dry mixed-conifer species in Washington and of smalldiameter ponderosa pine in New Mexico. Further study to clarify which factors 

are driving decomposition in piled fuels (i.e., climate regime, species composition, 

piece size distribution, overall pile size, covering, etc.) is necessary before these 

values can be generalized beyond the cases reported in this study.

The total amount of carbon emissions (i.e., carbon lost to the atmosphere through 

decomposition processes plus carbon emitted to the atmosphere during combustion) 
12

RESEARCH NOTE PNW-RN-574

Box 1: Steps to Estimate the Change in Pile Weight Over Time

Step 1—Determine the weight of your pile when built. The Piled Biomass and 

Emissions Calculator (http://depts.washington.edu/nwfire/piles/) will allow 

you to estimate the starting pile weight based on simple measurements of pile 

dimensions. See Wright et al. (2010) for additional details about estimating 

hand pile weight.

Step 2—Select the decay constant (k) that suits your situation. The decay constant for Washington (0.027/year) is probably most appropriate for piles with 

a large woody material component, and the decay constant for New Mexico 

(0.064/year) is probably most appropriate for piles comprised only of smaller 

branches.

Step 3—Determine the amount of time (t) that has passed (in years and fractions of a year) since the pile was created. This assumes that the pile was created from fresh logging or thinning slash (within approximately 3 months of 

harvesting).

Step 4—Calculate the fractional mass loss by using the k and t values from 

steps 2 and 3 (above) in equation 2. Below is the generic form of eq. 2 for use 

in a spreadsheet.

Fraction of pile remaining = EXP(-k * t)

Example—To calculate the dry weight of a 40-month-old, 2-m diameter, halfsphere-shaped, coniferous hand pile in Washington that includes large woody 

material:

Starting dry weight = 156.6 kg

k = 0.027/year

t = 40 months ÷ 12 months/year = 3.33 years

Fraction of pile remaining = EXP(-0.027 * 3.33) = 0.9125

Dry weight remaining = 0.9125 * 156.6 kg = 142.9 kg
13

Decomposition Rates for Hand-Piled Fuels

will be comparable for new piles and old piles. The effects of pile burning on air quality in general and on downwind receptors in particular, however, could potentially 

be reduced to some degree by allowing biomass loss and reductions in fuel loading 

through decomposition to occur for a period of time before burning (box 2). Forest 

and fuel management decisions that undertake piled slash disposal through decomposition, however, must also consider the risk involved with delaying pile burning or 

not burning piles at all. While choosing not to burn will potentially eliminate a large, 

short-duration pulse of smoke and carbon emissions, this only occurs if the piled 

landscapes do not burn in wildland fires before the piles have fully decomposed. This 

kind of a strategy could be viable in mesic and wet forest types with very long firereturn intervals and low probability of wildfire occurrence. In dry forest types that 

are characterized by relatively high-frequency fire regimes, however, such as those 

documented in this study, wildfires are inevitable and piles create concentrations of 

fuel that burn with elevated intensity, which could limit suppression options, affect 

firefighting success (Evans and Wright, in press), injure residual trees (Safford et al. 

2009), and remove the control over timing and dispersion of emissions from combustion that accompanies prescribed pile burning (Hardy et al. 2001). 

Box 2—Potential Emissions From Burning Fresh and Aged 

Hand Piles

Piles lose mass over time, affecting the amount of fuel available for consumption and potential smoke emissions during burning. 

Example—What is the potential difference in particulate matter (PM) and 

carbon dioxide (CO2) emissions for fresh (aged 0 years) and 4-year-old piles 

for a 15-ha burn unit with 65 piles/ha in New Mexico, where piles are mostly 

branches and other small-diameter woody debris with a 100-kg starting dry 

weight, and where 90 percent of the piled biomass is consumed during burning?

The following calculations assume that the emissions produced per unit 

mass of fuel consumed for the various components of smoke (i.e., the emissions factors) are the same for fresh and aged piles, even though combustion 

efficiency, proportion of combustion by combustion phase (i.e., flaming 

and smoldering combustion), and the resulting emissions factors are likely 

affected by increases in pile bulk density and other physical and chemical 

changes that occur as piles age. To simplify this example the below-noted 

comparison only takes into account changes in pile weight over time.
14

RESEARCH NOTE PNW-RN-574

This example also assumes that 70 percent of the fuel consumption occurs 

during the flaming phase of combustion and 30 percent during the smoldering 

phase of combustion. Fire-average PM and CO2 emission factors are 1663.7 kg/

Mg of fuel consumed and 10.95 kg/Mg of fuel consumed, respectively (based on 

the aforementioned combustion phase weighting and combustion phase-specific 

emissions factors in Wright et al. 2010). See Hardy (1996) and Hardy et al. (2001) 

for a more thorough discussion of combustion and emission factors.

Fresh piles—

Total unit biomass = 15 ha × 65 piles/ha × 100 kg/pile = 97.50 Mg

Consumed fuel = 97.50 Mg × 0.9 consumption rate = 87.75 Mg

PM emissions = 87.75 Mg × 10.95 kg/Mg = 961 kg

CO2 emissions = 87.75 Mg × 1663.7 kg/Mg = 145 990 kg

Aged piles—

k = 0.064/yr; t = 4 years

Fraction of piles remaining = EXP(-0.064 * 4) = 0.7741

Dry weight remaining = 0.7741 * 97.50 Mg = 75.47 Mg

Consumed fuel = 75.47 Mg × 0.9 consumption rate = 67.92 Mg

PM emissions = 67.92 Mg × 10.95 kg/Mg = 744 kg

CO2 emissions = 67.92 Mg × 1663.7 kg/Mg = 112 999 kg

Difference—

PM = 961 kg – 744 kg = 217 kg less PM for 4-year-old piles

CO2 = 145 990 kg – 112 999 kg = 32 991 kg less CO2 for 4-year-old piles

Approximately 23 percent lower emissions when burning 4-year-old piles
15

Decomposition Rates for Hand-Piled Fuels

Acknowledgments

Karen Haubensak, Alex Finkral, Mike Battaglia, and J. Morgan Varner provided 

helpful feedback and comments that improved the quality of the manuscript. We 

acknowledge funding from the Joint Fire Science Program under Project JFSP 

11-1-8-4.

English Equivalents

When you know: Multiply by: To find:

Millimeters (mm) 0.0394 Inches

Centimeters (cm) .394 Inches

Meters (m) 3.28 Feet

Kilometers (km) .621 Miles

Hectares (ha) 2.47 Acres

Square meters per hectare (m2/ha) 4.37 Square feet per acre

Kilograms (kg) 2.205 Pounds

Megagrams (Mg) 1.102 Tons

Degrees Celsius (°C) 1.8 °C + 32 Degrees Fahrenheit

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