United States 

Department of Fuel Consumption and Fire 

Agricutture Characteristics during Understory - 

Forest Service Burning in a Mixed ite PineHardwood Stand in the Southern 

Appalachians 

Southern 

Research Station 

Research Paper Barton D. Clinton, James M. Vose, Wayne T. Swank, 

SRS-12 Eric C. Berg, and David L. Loftis 
The Authors: 

Barton D. Clinton, Research Ecologist, James M. Vose, Research 

Ecologist, Wayne T. Swank, Project hder, USDA Forest 

Service, Southern Research Station, Coweeta Hydrologic 

Laboratory, Otto, NC 28763; and Erik C. Berg, Program Manager, 

and David L. Loftis, Project Leader, USDA Forest Service, 

Southern Research Station, Bent Creek Experimental Forest, 

Asheville, NC 28806. 

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

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

July 1998 

Southern Research Station 

P.Q. Box 2680 

Asheville, North Carolina 28802 
Fuel Consumption and Fire Characteristics 

during Understory Burning in a Mixed 

White Pine-Hardwood Stand in the 

Southern Appalachians 

Barton D. Clinton, James M. Vose, Wayne T. Swank, 

Eric C. Berg, and David L. Loftis 

Abstract 

We characterized fire behavior and fuel consumption resulting from an 

understory prescribed burn in a mixed eastern white pine-hardwood stand 

in the Southern Appalachians. Three stands were used for the treatment. 

Flame lengths, ranging from 0.3 to 1.5 meters (m) for backing fues and 

from 1.2 to 4.5 m for head fires, reached maximum heights where 

evergreen understory was found. Rates of spread ranged from 1.8 to 3.0 m 

per minute for head fires and 0.3 m per minute for backing fires. Fire 

intensity, measured with ceramic tiles painted with heat-sensitive paint, 

varied across stands. Mean peak flame temperature ranged from 129 to 

290 T. Pre-burn mass totals were similar among stands, except for stand I, 

which had substantially greater humus mass than the other stands. 

Consumption of litter and humus layers in the forest floor was positively 

correlated with flame temperature. Small wood (43 centimeters diameter) 

consumption was not correlated with temperature. Over all stands, 50 

percent of the mass in small wood and litter was lost during burning, and 

20 percent of the humus layer was consumed. The losses in the humus 

layer represented about 40 percent more humus mass consumption than 

would have occurred in a fell-and-burn treatment. The humus layer is an 

important nutrient reservoir for plant growth. Maintaining this layer 

through careful selection of burning conditions will minimize losses during 

burning and maintain long-term site productivity. 

Keywords: Blue Valley, eastern white pine, fuel consumption, prescribed 

fire, Southern Appalachians, understory burning. 

Introduction 

In the Southern Appalachians, fire is commonly used as a 

site preparation tool in yellow-pine (includes Pinus rigidu 

Mill., P. virginiana Mill., P. pungens Lamb., and P. 

echinata Mill.)-hardwood ecosystems (Vose and Swank 

1993) to stimulate oak advanced regeneration after harvest 

(DeSelrn and Clebsch 1990, Van Lear 1990) and to restore 

degraded pine-hardwood sites (Vose and Swank 1993). The 

role of fie in regenerating mixed white pine (P. strobus L.)- 

hardwood ecosystems is less understood. Moreover, 

information about the effects of fie on other basic 

ecosystem attributes is also lacking. 

Changes in mass and nutrient pools are key parameters used 

to assess the effects of fire on ecosystem processes. Fire 

intensity and severity (i.e., fire behavior) largely determine 

the amount of fuels consumed and site nutrients lost 

(Clinton and others 1996, Vose and Swank 1993). Fire 

behavior is dependent on fuel type, amount, and size 

distribution; moisture content; site macroclimate (Swift and 

others 1993); and burning techniques such as head fies vs. 

backing fires. Typically, the forest floor is the largest 

reservoir of available nutrients for plant growth (Vose and 

Swank 1993). Fires of high intensity but low severity result 

in minimal consumption of the forest floor and, hence, 

minimize nutrient losses from this reservoir (Clinton and 

others 1996, Vose and Swank 1993). 

This paper addresses one part of a project designed to 

examine the effect of understory burning on hardwood and 

conifer regeneration.' The specific objectives of this part of 

the study were twofold: (1) to quantify forest floor and 

small wood mass consumption resulting from prescribed 

burning in mixed white pine-hardwood ecosystems; and (2) 

to relate mass loss to fire intensity. 

Methods 

Study Site-The Blue Valley Experimental ore st^ is 

located in the Blue Ridge physiographic province of the 

Southern Appalachians in Macon County, NC (lat. 35" N, 

long. 83"W). The 460-hectare (ha) experimental forest is 

approximately 5 kilometers south of Highlands, NC, and 

ranges in elevation from 700 meters (m) on Overflow Creek 

along its southern boundary to 950 m on Ostin Knob. 

Annual precipitation ranges from 200 to 250 centimeters 

(cm) per year. The area has a large component of naturally 

regenerated eastern white pine. Stands for the most part are 

' Berg, E.C. 1996. Evaluation of shelterwood cutting and understory 

burning on hardwood regeneration. 24 p. Unpublished study plan and 

establishment report for Blue Valley Experimental Forest. On file with: 

Southern Research Station, Bent Creek Experimental Forest, 1577 Brevard 

Road, Ashevitle, NC 28806. 

' Sluder, E.R. February 11, 1964. Establishment report for the Blue Valley 

Experimental Forest, Southeastern Forest Experiment Station. 5 p. 

Unpublished report. On file with: Highlands Ranger District, 2010 Flat 

Mountain Road, Highlands, NC 2874 1. 
mixed white pine-hardwood, although white pine stocking 

often exceeds 35 m2 of basal area per ha. Stand age is 

approximately 70 years. Eastern white pine accounts for 63 

percent of the basal area on these sites; oaks (Quercus alba 

L.= Q, pn'nus L.> Q. coccineu Muenchh> Q, velutim Lam.) 

make up 25 percent; and hickory (Carya spp.) accounts for 

6 percent. The remaining basal area is distributed among 

six species. The understory consists of scattered rosebay 

rhododendron (Rhododendron maximum L.), mountainlaurel (Kalmia htifolia L.), huckleberries (Gaylussacia 

spp.), and blueberries (Vuccinium spp.). Little or no eastern 

white pine advanced regeneration is present, but saplings of 

red maple (Acer rubrum L.) and several oak species are 

found. 

Treatment-Three stands were selected for understory 

burning. Fire lines were established and a combination 

backing firehead fire was set on each stand. Burning was 

completed on two stands on April 12, 1996, and on the third 

stand on April 13. 

Sampling 

Forest Floor-Three 16- by 63-m plots were located within 

each burn stand and in adjacent unburned control stands. A 

transect was established through the center of each plot 

along the long axis. Forest floor measurements were taken 

from five 0.09-m%ubplots that were systematically 

distributed at 10-m intervals along each transect, and offset 

on either side of the transect 5 m in a randomly determined 

direction. The forest floor (wood <8 cm diameter, litter, and 

humus) was removed from each subplot for mass 

determination by cutting and removing the litter (Oi) and 

humus (Oe + Oa) layers separately. Sample mass was 

determined by drying at 70°C to a constant weight and 

weighing to the nearest 0.0 1 gram. 

Weather and Fire Behavior-Understory air temperature 

("C), relative humidity (%), and wind speed (m per second) 

and direction were recorded every 30 minutes throughout 

the burning period. Cloud cover was also noted and 

expressed as a percent. Flame length (FL; m) and rate of 

spread (ROS; rn per minute) for head fires and backing fires 

were estimated for each stand. 

Flame Temperature--Hame temperature was measured 

using heat-sensitive indicator paint applied to ceramic tiles 

(Omega Engineering, Inc.) suspended 30 cm above the 

forest floor and collocated with each forest floor 

measurement subplot. Heat sensitivities ranged from 52 to 

804 "C, at approximately 15 "C intervals between 52 and 

430 "C, and la) "C intervals for > 500 "C. Wen threshold 

temperatures were reached during burning for each level of 

sensitivity, melted paint indicated that the temperature 

exceeded the threshold temperature but was lower than the 

next threshold temperature, 

Statistical Analysis-Relationships between mass 

consumption and mean peak flame temperature were 

examined using regression analysis (PROC GLM; SAS 

1987) and Pearson correlation analysis (PROC CORR; SAS 

1987). 

Results and Discussion 

Weather and Fire Behavior 

Weather conditions were markedly different between the 

two burn days (table 1). The first day was clear, warm, and 

dry, while the second day was overcast, cooler, and more 

humid. On day 2, temperatures were 6 to 8 "C cooler and 

relative humidity was 34 to 40 percent higher than on day 1. 

Winds were steady out of the west with occasional gusts on 

both days. 

Fire behavior also varied between the 2 days. The fist day 

produced longer FL and faster ROS for head fires, where 

FL was 1.2 to 4.5 m and ROS was 1.8 to 3.0 m per minute. 

For baclung fires, FL was 0.3 to 1.5 m and ROS was 0.3 m 

per minute across both burning days. Where evergreen 

understory was found, FL was commonly 3 to 4.5 m. 

Burning on both days was aided by a dry litter layer and 

steady winds. 

Pre-burn Mass 

Pre-burn mass totals were similar among stands, except for 

stand 1, which had substantially greater humus mass than 

the other stands (fig. 1). For all stands, the largest pre-burn 

mass pool was in the humus layer. Within and across 

stands, equal amounts of mass were found in the wood and 

litter layers. Other studies in yellow pine-hardwood stands 

in the Southern Appalachians have also shown comparable 

amounts of litter and smatl wood mass, ranging from 5 to 13 

and 5 to 16 megagrams (Mg) per ha, respectively ('Jose and 

Swad 1993, Vose and others, in press). 
Tabie 1-Mean, maxirnunn, and rninimum flame temperature (OC) by stand (N = 15) and mean air temperature, 

relative huddity, and wind speed (meters per second) during burning 

Flame temperature 

Air Percent Wind 

Standa Mean Max. Min. temp. RH speedb 

a Stand 3 was burned on day 2. 

Wind direction varied between W and SW on both days. 

Values in parentheses are standard errors. 

I 

- Wood 

Litter 

Humus 1 

Stands 

All stands 

Figure 1-Pre-burn mass (Mg per ha) for three fuel types by stand and for all stands combined. Error bars represent 1 

standard error (N = 15). 
Fire intensity is defined as the upward heat pulse produced 

by the fire (Ryan and Noste 1985), and flame temperatures 

measured by heat tiles provide a direct measure of fire 

intensity. Fire intensity varied across stands (table 1). 

Stand 2 had the greatest fire intensity, followed by stands 1 

and 3. Swift and others ( 1993) reported mean peak flame 

temperatures of 630 to 8 12 "C for a site preparation burn of 

chainsaw-felled slash in the Southern Appalachians. In 

their study, heavy fuel loadings (>I35 Mg per ha), higher 

surface wind speed (1.6 to 4.5 m per second), head fires, 

and steep topography resulted in greater fie intensity. In 

the present study, fuel loadings were moderate (fig. l), wind 

speed was low (table I), and the topography was gently 

sloping. Yet, even under moderate conditions of fuel 

loading, high intensity or high severity fies can result in 

bole damage that may ultimately lead to mortality. 

However, in the first year after burning, mortality directly 

attributable to the fire was negligible. 

Fuel consumption, which indicates fire severity (Van kr 

and Waldrop 1988, Wells and others 1979), followed a 

slightly different pattern (stand 2 > stand 3 > stand 1) (fig. 

2). In addition to fie behavior, fuel size and flammability 

are important determinants of mass consumption, and 

variation in these factors may explain the consumption 

patterns. We measured wood <8 cm but did not further 

quantify fuel size nor flammability. Across all stands, 

consumption of the humus and litter layers was positively 

correlated with flame temperature (2 = 0.72 for humus and 

0.30 for litter), but wood was not (fig. 3). The poor 

correlation for wood may result from the variability in size, 

amount (fig. 2), and flammability of woody fuels across 

stands that may not be reflected in measures of fie 

intensity. Because small wood is more completely 

consumed at lower temperatures than larger wood, plots 

high in wood mass in small size classes would lose more 

mass than plots with similar mass in larger size classes. 

1 2 3 All stands 

Stands 

Figure 2-Reduction (%} in mass from understory burning by fuel type and stand and for all stands combined. Error bars 

represent 1 standard error (N = 15). 
Litter 7 

rv 0 

r Wood (n.s.) 

Litter(r2=0.30) 

O Humus (r2 = 0.72) 1 

Temperature (OC) 

Figure 3-Reduction (96) in mass for three fuel types as a function of mean peak flame temperature ('C) at 0.3 m above 

the forest floor. Regression lines represent a least squares best fit regression. 

Overall, more than 50 percent of the mass in litter and small 

wood was lost during burning and about 20 percent of the 

humus layer was lost (fig. 2). Vose and Swank (1993) 

found similar consumption rates for litter and estimated that 

only 10 percent of the mass in the humus layer was 

consumed during a fell-and-bum treatment. The humus 

layer was the largest pool of nitrogen (N) in the forest floor, 

and they concluded that high intensity (fig. 4), low severity 

fires were optimum for &ntaining this plant nutrient 

resource. In our study, fire intensity was lower than in the 

fell-and-bum prescription but apparently severity was 

slightly higher. We attribute this higher severity to a more 

slowly moving fire with a long residence time (figs. 5 and 

6). If the humus on our study sites had N pool sizes similar 

to those described by Vose and Swank (1993), 

approximately 10 kilograms (kg) per ha more N was lost 

from the humus in this study (36 kg N per ha) than in their 

study of the fell-and-burn prescription (26 kg N per ha). 

Conclusions 

In this study, both fire intensity and severity were moderate. 

Burning conditions that produce a more intense and less 

severe fire would conserve more of the humus layer and 

associated nutrients. To consume less of the humus layer, 

head fires, which produce faster ROS, hence, shorter 

residence times, may be necessary. However, head fires 

produce the greater FL, associated with more intense fires, 

which could potentially result in higher overstory mortality 
as well as hinder the success of white pine reproduction. 

To maintain long-term site productivity, understory burning 

conditions must be carefully monitored to achieve the 

desired result while minimizing loss of site nutrients by the 

consumption of deeper forest floor layers. Forest managers 

must recognize the importance of this pool of site nutrients 

when burning, and design prescriptions that minimize 

consumption of site nutrients and maintain long-term site 

productivity. 

Acknowledgments 

We thank Patsy Clinton, Katherine Elliott, Bob McCollum, 

and Chris Sobek for help with sample collection, and Pat 

Brose for collecting and sharing weather and fire behavior 

data. We also thank District Ranger Erin Bronk and the 

staff of the Highlands Ranger District of the Nantahala 

National Forest for administering the burning treatments 

and for cooperating with our research efforts. This project 

was a cooperative effort among the Coweeta Hydrologic 

Laboratory, the Bent Creek Experimental Forest, and the 

National Forests in North Carolina. 

Figure &Extremely intense fires, as seen in this fell-and-burn treatment, typically result in heavy consumption of aboveground fuels but 

minimal forest floor consumption because severity is low. (U.S. Department of Agriculture, Forest Service, Coweeta Hydrologic Laboratory 

file photo.) 
Figure 5-Head fires are fast moving and typically have a wide burning front. Fire intensity diminishes toward the backside of the 

burning front. (Photo by Pat Brose.) 

Figure &Backing fires typically have a narrow burning front but are slow to move, resulting in increased consumption of 

belowground fuels because fire severity increases as fire residence time increases. (Photo by Pat Brose.) 
Literature Cited 

Ctinton, B.D.; Vose, J.M.; Swank, W.T. 1996. Shifts in abvegound 

and forest floor carbon and nitrogen pools following prescribed fire in 

the Southern Appalachians. Forest Science. 42(4): 43 1-44!. 

DeSelrn, H.R.; Clebsch, E.E.C. 1990. Response types to prescribed fire 

in oak forest understory. In: Proceedings, Fie and the environment: 

ecological and cultural perspectives; 1990 March 20-24; Knoxville, 

TN. Gen. Tech. Rep. SE-69. Asheville, NC: U.S. Department of 

Agriculture, Forest Service, Southeastern Forest Experiment Station: 22- 

33. 

Ryan, K.C.; Noste, N.V. 1985. Evaluating prescribed fires. In: 

Proceedings of a symposium workshop on wilderness fire; 1983 

November 15-1 8; Missoula, MT. Gen. Tech. Rep. INT-182. Ogden, 

UT: U.S. Department of Agriculture, Forest Service, Intermountain 

Research Station: 230-238. 

SAS Institute, h. 1987. SAS user's guide: statistics. SAS Institute, Inc. 

Cary, NC. 956 p. 

Swift, LW.; Elliott, KJ.; Ottmar, RD.; Vinanek, R.E. 1993. Site 

preparation burning to improve Southern Appalachian pine-hardwood 

stands: fire characteristics and soil erosion, moisture, and temperature. 

Canadian Journal of Forest Research. 23: 2242-2254. 

Van Lear, D.H.; Wddrop, TA. 1988. Effects of fire on natural 

regeneration in the Agpalachian Mountains. In: Smith, H. Clay; Perkey, 

kW.; Kidd, W.E, Jr., eds, Guidelines for regenerating Agpalachian 

hardwood stands: conference proceedings; 1988 May 24-26; 

Morgantown, W. SAF Publ. 88-03. Morgantown, W: West Virginia 

University Books: 56-70. 

Vose, J.M.; Swank, W.T. 1993. Site preparation burning to improve 

Southern Appalachian pine-hardwood stands: aboveground biomass, 

forest floor mass, and nitrogen and carbon pools. Canadian Journal of 

Forest Research. 23: 2255-2262. 

Vose, J.M.; Swank, W.T.; Clinton, B.D. [and others]. 1997. Using fire 

to restore pinehardwood ecosystems in the Southern Appalachians of 

North Carolina. In: Greenlee, J.M., ed. Proceedings of the conference on 

wildland fire effects on threatened and endangered species and habitats; 

1995 November 13- 16; Coeur d' Alene, ID: 149- 154. Available from: 

International Association of Wildland Fire, P.O. Box 328, Fairfield, WA 

990 1 2 ($40.00). 

Wells, C.G.; Campbell, R.E.; DeBano, LF. 1979. Effects of fire on soil. 

Gen. Tech. Rep. WO-7. Washington, DC: U.S. Department of 

Agriculture, Forest Service. 34 p. 

Van Lear, D.H. 1990. Fire and oak regeneration in the Southern 

Appalachians. In: Proceedings, Fire and the environment: ecological and 

cultural perspectives; 1990 March 20-24; Knoxville, TN. Gen. Tech. 

Rep. SE-69. Asheville, NC: U.S. Department of Agriculture, Forest 

Service, Southeastern Forest Experiment Station: 15-2 1. 
Clinton, Barton D.; Vose, James M.; Swank, Wayne T.; Berg, Eric C.; and Leftis, David L. 1998. 

Fuel consumption and fire characteristics during understory burning in a mixed white pine-hardwood 

stand in the Southern Appalachians. Research Paper SRS-12. Asheville, NC: U.S. Department of 

Agriculture, Forest Service, Southern Research Station. 8 p. 

We characterized fire behavior and fuel consumption resulting from an understory prescribed burn in a 

mixed eastern white pine-hardwood stand in the Southern Appalachians. Three stands were used for the 

treatment. Flame lengths, ranging from 0.3 to 1.5 meters (m) for backing fires and from 1.2 to 4.5 m for 

head fires, reached maximum heights where evergreen understory was found. Rates of spread ranged from 

1.8 to 3.0 m per minute for head fires and 0.3 m per minute for backing fires. Fie intensity, measured 

with ceramic tiles painted with heat-sensitive paint, varied across stands. Mean peak flame temperature 

ranged from 129 to 290 "C. Pre-burn mass totals were similar among stands, except for stand 1, which had 

substantially greater humus mass than the other stands. Consumption of litter and humus layers in the 

forest floor was positively correlated with flame temperature. Small wood (c8 cm diameter) consumption 

was not correlated with temperature. Over all stands, 50 percent of the mass in small wood and litter was 

lost during burning, and 20 percent of the humus layer was consumed. The losses in the humus layer 

represent about 40 percent more humus mass consumption than would have occurred in a fell-and-burn 

treatment. The humus layer is an important nutrient reservoir for plant growth. Maintaining this layer 

through careful selection of burning conditions will minimize losses during burning and maintain longterm site productivity. 

Keywords: Blue Valley, eastern white pine, fuel consumption, prescribed fire, Southern Appalachians, 

understory burning. 
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