Coarse Woody Debris Following Fire and Logging in Wyoming Lodgepole Pine Forests

Coarse Woody Debris following Fire and Logging in Wyoming Lodgepole Pine Forests

Daniel B. Tinker* and Dennis H. Knight

Department of Botany, University of Wyoming, Laramie, Wyoming 82071, USA

ABSTRACT The accumulation and decomposition of coarse ing a single clear-cut. However, the total mass of woody debris (CWD) are processes that affect hab- downed CWD plus the mass of snags that will be- itat, soil structure and organic matter inputs, and come CWD was nearly twice as high in burned energy and nutrient flows in forest ecosystems. Nat- stands than in clear-cuts. In YNP, approximately ural disturbances such as fires typically produce 8% of the downed CWD was consumed by fire and large quantities of CWD as trees fall and break, an additional 8% was converted to charcoal, for an whereas human disturbances such as timber har- estimated loss of about 16%. In contrast, approxi- vesting remove much of the CWD. Our objective mately four times more wood (70%) was removed was to compare the amount of CWD removed and by clear-cutting. Considering all CWD more than left behind after clear-cutting to the amount con- 7.5 cm in diameter that was either still present in sumed and left behind after natural fires in Rocky the stand or removed by harvesting, slash treat- Mountain lodgepole pine. The masses of fallen logs, ment, or burning, clear-cut stands lost an average of dead-standing trees, stumps, and root crowns more 80 Mg haϪ1 whereas stands that burned gained an than 7.5 cm in diameter were estimated in clear-cut average of 95 Mg haϪ1. Some CWD remains as slash and intact lodgepole pine forests in Wyoming and and stumps left behind after harvesting, but stands compared to estimates made in burned and un- subjected to repeated harvesting will have forest burned stands in Yellowstone National Park (YNP), floor and surface soil characteristics that are beyond where no timber harvesting has occurred. Estimates the historic range of variability of naturally devel- of downed CWD consumed or converted to char- oping stands. coal during an intense crown fire were also made in YNP. No significant differences in biomass of Key words: coarse woody debris; lodgepole pine; downed CWD more than 7.5 cm in diameter were Pinus contorta; timber harvesting; fire; Yellowstone detected between burned stands and those follow- National Park; Wyoming; clear-cutting.

INTRODUCTION aquatic environments (Harvey 1982; Harmon and others 1986; Franklin and others 1987; Bull and Coarse woody debris (CWD) which is present as others 1997). Microbes and fungi, as well as logs and snags in most western coniferous forests, many insects and other invertebrates, facilitate plays an important ecological role within these decomposition and derive energy and nutrients ecosystems. For example, it provides a habitat for from rotting logs (Frankland and others 1982). many types of organisms in both terrestrial and Numerous species of vascular plants, including some tree seedlings, establish on CWD, and tree Received 16 November 1999; Accepted 31 May 2000. roots commonly grow into decomposing logs on *Current address for corresponding author: Department of Geosciences and Natural Resources Management, 207-A Stillwell, Western Carolina Uni- the forest ﬂoor (Grier and others 1981; Harvey versity, Cullowhee, North Carolina 28723, USA; e-mail: [email protected]. and Neuenschwander 1991; Little and others

472 Coarse Woody Debris in Lodgepole Pine Forests 473

1994; Vogt and others 1995; Jurgensen and oth- amount of ‘inherited‘ CWD is present on the forest ers 1997). floor before the initial harvest. This CWD is a legacy Snags represent a critical habitat for many species of the trees that were killed during the last stand- of cavity-nesting birds (Davis and others 1983; Har- replacing fire (Maser and others 1979; Wei and mon and others 1986). Amphibians, reptiles, and others 1997) and trees from the present stand that mammals rely on many forms of CWD as habitat for have died and fallen as a result of natural mortality cover, feeding, and reproduction (Harmon and oth- (Gore and Patterson 1986; Franklin and others ers 1986). CWD is an important source of organic 1987). This inherited CWD, and therefore total matter inputs to forest soils (Edmonds 1991), and CWD, would likely decrease with continued de- nitrogen, much of which will become available to composition and repeated harvesting. various organisms, can accumulate in the decom- The few recommendations that exist for the posing CWD of western coniferous forests (Harvey amount of CWD that should be left on a site fol- 1982; Fahey and Knight 1986; Hart 1999). In lowing timber harvesting vary widely, from less aquatic systems, CWD is recognized for its effect on than 10 Mg haϪ1 in drier Rocky Mountain sites the geomorphic structure within streams and lakes, containing grand fir (Abies grandis [Dougl. ex D. as well as serving as a habitat for fish and many Don] Lindl.) or ponderosa pine (Pinus ponderosa aquatic invertebrates (Harmon and others 1986). [Dougl. ex Laws.]) (Graham and others 1994) to The amount of CWD in a forest depends on pro- 125 Mg haϪ1 for mixed conifer forests of the north- cesses that affect its accumulation from tree mor- ern Rockies (Reinhardt and others 1991). The de- tality and breakage, as well as processes that affect sired amount of CWD left following harvesting de- its loss, such as decomposition, burning, and har- pends on habitat type, method of regeneration, and vesting. The presence of CWD on a harvested site preharvest levels of soil organic matter and CWD surely affects ecological processes associated with (Brown and See 1981; Jurgensen and others 1997). nutrient cycling. In Canadian lodgepole pine for- Given the increasing interest in the use of harvest- ests, Wei and others (1997) found that if posthar- ing to mimic natural disturbances (Hammond 1991; vest CWD is left in place, nutrient removal by har- Keenan and Kimmins 1993), the differences in vesting was within the range of nutrient removal by CWD and potential CWD biomass in burned and wildfires. In contrast, the complete removal of harvested stands should be better understood. CWD from a clear-cut area could result in signifi- Quantified CWD amounts following fire and log- cant nutrient losses and a potential reduction in ging are the net result of how much CWD is pro- long-term site productivity. In a study comparing duced and eliminated by each process. Estimating postharvest slash treatments in clear-cut Wyoming the amount of CWD removed and created by clear- lodgepole pine forests, Benson (1982) reported that cutting is relatively straightforward (for example, burning slash removed approximately 87% of the Brown 1974). On the other hand, comparable esti- residue present following clear-cutting. For this mates are more problematic in stands burned by study, we compared the proportion of CWD re- intense wildfires because they require an estimate moved by clear-cutting to the proportion consumed of the amount of downed CWD that is completely by natural fires in Rocky Mountain forests domi- consumed and the amount that is converted to nated by lodgepole pine (Pinus contorta ssp. latifolia charcoal. Fahnestock and Agee (1983) concluded [Engelm.ex Wats.] Critchfield). In addition to pro- that there is no reliable method for verifying the viding a comparison that is relevant to ecosystem quantity of woody fuels consumed by wildfire. In management, our method of estimating the fact, most studies on wood consumption have been amount of wood consumed by an intense wildfire conducted in prescribed burns or under laboratory may be applicable to other forest types. Natural conditions (Kauffman and Martin 1989; Reinhardt forest disturbances typically kill a large number of and others 1991; Albini and Reinhardt 1995; Call trees, which then augment the amount of CWD and Albini 1997) that exhibit different thermal dy- when the trees fall to the ground (Spies and others namics and behavior than natural fires. Previous 1988). Most of the live trees killed by intense, estimates of large woody fuels that are burned vary stand-replacing fires fall to the forest floor within a widely, ranging from 4% to 100% of prefire few decades (Mitchell and Preisler 1998). Con- amounts (Brown and others 1985, 1991; Kauffman versely, human disturbances, such as timber har- and Martin 1989; Reinhardt and others 1991). One vesting, and some forms of postharvest slash treat- of the primary objectives of our research was to ment, such as broadcast burning or pile-and-burn, develop a method that would produce reasonable remove much of the wood that would have become estimates of the amount of CWD consumed during CWD (Jurgensen and others 1997). Notably, a large natural fires. 474 D. B. Tinker and D. H. Knight

Increasingly, researchers and managers are ex- others 1997). Crown fires are those that consumed ploring the idea of using the historic range of vari- the canopy foliage and most small branches and ability as a reference to guide the management of twigs, as well as most of the forest floor litter. In- human-influenced ecological systems (Swanson tense surface fires are those that did not actually and others 1994; Cissel and others 1999; Landres burn in the canopy but are nonetheless hot enough and others 1999). Because of the limited anthropo- to turn the tree foliage to a reddish-brown color. genic influence in Yellowstone National Park, one Unburned and uncut stands of similar age, as they of the primary areas for this study, we had an existed just before the fires, were located as close to opportunity to examine the variability in CWD, each burned stand as possible. In addition, a single which may be used as a reference system for lodge- stand that burned in 1996 near Pelican Creek in pole pine forest management. By estimating CWD YNP was also sampled for CWD consumption esti- amounts in stands of various successional stages in mates. All stands in YNP were located at approxi- Yellowstone, we were able to provide baseline in- mately 2300 m elevation on rhyolitic soils. formation on CWD biomass that may help define a In the MBNF, we used eight recent clear-cut range of variability by which to model managed stands (harvested in 1991–93; 5–15 ha) and six forest systems. uncut and unburned stands ranging in age from 98 We compared the mass of CWD after a single to 244 years (preharvest age for clear-cuts). Roller- clear-cut timber harvest and a natural fire in lodge- chopping, which leaves the CWD on site, was the pole pine forests to estimate the net loss or gain of postharvest treatment for all clear-cuts. The soils CWD following each type of each disturbance. We are Alfisols (Typic Cryoboralfs), which have devel- considered CWD to be all downed woody material oped from Precambrian gneiss and granite, with more than 7.5 cm in diameter, including stumps occasional Inceptisols (Typic Cryochrepts). Com- and woody lateral roots, as well as snags more than mon understory species in our study areas were 7.5 cm in diameter. Mitchell and Preisler (1998) buffalo berry (Shepherdia canadensis (L.) Nutt.), found that 90% of the dead lodgepole pine trees grouse whortleberry (Vaccinium scoparium Leiberg killed by mountain pine beetles in central Oregon ex Cov.), and common juniper (Juniperus communis fell within approximately 2 decades. Snags were L. var. depressa Pursh). therefore included as CWD because they fall and become part of the forest floor CWD within a short Estimates of CWD Biomass time. Twenty-five 15.2-m (50-ft) transects were used in each stand for estimating CWD biomass with the STUDY AREA AND METHODS planar intercept method (Brown 1974). This method only requires that logs be classified as Data were collected during 1995–97 from 31 sound or rotten, but instead we classified each log stands dominated by lodgepole pine in the Med- into one of five decay classes using the classification icine Bow National Forest (MBNF) in southeast- of Maser and others (1979). Decay class I logs are ern Wyoming (41°N, 106°W) and in Yellowstone sound and still have bark, branches, and twigs. At National Park (YNP) in northwestern Wyoming the other end of the decay gradient, highly-decayed (44°N, 110°W) (Table 1). Because the stands in class V logs are elliptical in cross section and are YNP were essentially undisturbed by human ac- partially buried in the forest floor. tivity, they provided a unique opportunity to in- In addition, two or three 20 ϫ 20 m plots were vestigate CWD dynamics under natural distur- established in each stand. All downed CWD more bance regimes. In contrast, much of the MBNF than 7.5 cm in diameter, root crowns more than 7.5 has been heavily influenced by timber harvesting cm in diameter, seedlings (less than 1 m tall), and for well over 40 years. Also, many stands that standing-live and-dead trees more than 7.5 cm in burn in the MBNF have been altered as well by diameter were mapped in each plot. From these salvage logging or firewood gathering, either be- maps, stand density, percent cover, and biomass of fore or after fires, making accurate estimates of standing trees (live and dead) and stumps were natural CWD production difficult. calculated using allometric equations developed by In YNP, stands burned by crown fires and intense Pearson and others (1984) in the Medicine Bow surface fires were sampled along with unburned National Forest. The equations provide estimates of stands (Table 1). Stands that burned in 1988 were the biomass of tree root crowns, boles, branches, selected based on a burn severity classification done and foliage using individual tree basal area. Where in 1989 as part of an ongoing postfire study of tree boles had been removed or separated from the vegetation and successional dynamics (Turner and stump, either through clear-cutting, fire, or natural Coarse Woody Debris in Lodgepole Pine Forests 475

Table 1. Characteristics of Stands Sampled in Yellowstone National Park (YNP) and the Medicine Bow National Forest (MBNF)

Stand Name Treatment Elevation (m) Trees/haa Est. Age (y)

YNP (17) Hllp0un Unburned 2204 950 27 Hllp1un Unburned 2292 725 61 CClp1un Unburned 2280 2725 103 CClp2un Unburned 2216 975 173 Fsunb Unburned 2377 1387 193 Lewis Falls Unburned 2438 1194 — Flsb Surface fire 2304 2087 106 Fssb Surface fire 2371 912 251 Hltsb Surface fire 2423 525 262 Mallard Surface fire 2286 933 185 Flcrown Crown fire 2310 587 150 Hltcrown Crown fire 2423 775 189 Fscrown Crown fire 2384 500 242 Lewis Canyon Crown fire 2387 683 208 Pelican Creek Crown fire 2420 983 — Arrow Double burn 2362 733 12 Cascade Double burn 2432 367 35 MBNF (14) 95-01 Uncut 2646 1233 108 96-01 Uncut 2758 667 244 96-02 Uncut 2734 1075 230 96-03 Uncut 2755 1683 233 96-04 Uncut 2621 2367 131 97-01 Uncut 2743 800 98 cc1 Clear-cut 2667 983 273 cc2 Clear-cut 2646 283 — cc3 Clear-cut 2737 1567 222 cc4 Clear-cut 2725 700 222 cc5 Clear-cut 2627 1667 112 cc6 Clear-cut 2649 850 240 cc7 Clear-cut 2743 825 238 cc8 Clear-cut 2743 1000 104 aDensity prior to either fire or clear-cutting.

treefall, stumps were included as root crown bio- Estimates of CWD Conversion to Charcoal mass (which provides a conservative estimate of and Consumption total live biomass, since lateral woody roots and fine Our CWD consumption estimates were made dur- roots are not included). In burned stands, stumps ing 1997 in a single stand near Pelican Creek that were included in estimates of total live- and dead- standing biomass. Within each stand, 10 of the burned in YNP during the summer of 1996. CWD largest trees were cored using an increment borer, consumed by fire or converted to charcoal was to- and cores were subsequently mounted on boards taled from (a) the amount of CWD converted to and sanded. Annual rings were counted to estimate charcoal that is still present and measurable, and (b) the age of the stand. Two-tailed t tests were per- measurements of logs completely consumed by the formed on biomass data to test for significant dif- fire. We attempted to include estimates of the bio- ferences between mean values of biomass estimates mass of logs partially consumed by the fire, which, of clear-cut vs burned, unburned vs uncut, clear- when it occurs, typically creates cupped depressions cut vs uncut, and crown fires vs surface fires, with on the log surface. However, our methods for this an alpha level of 0.05. estimate proved inaccurate (Tinker 1999), so data 476 D. B. Tinker and D. H. Knight for paratially consumed CWD are not included in our results. This and other potential sources of error are discussed below. CWD converted to charcoal. Our approach to estimating the amount of CWD converted to charcoal is based on Newton’s formula for the volume of a tapered cylinder (Harmon and Sexton 1996):

ϭ ͑ ϩ ϩ ͒ V L Ab 4 Am At /6 (1) where V is the volume, L is log length, and Ab, Am, and At are the cross-sectional areas of the base, middle, and top of the log, respectively. The maximum diameters at the middle and at each end were measured using tree calipers on each of 50 downed logs that had fallen to the ground and had been charred or subsequently partially burned in 1996. This calculation provided diameters of the log, including the burned charcoal exterior. Each log was then cut at the three points of measurement using a chainsaw, and diameter measurements were repeated on the inner, unburned portion of the wood. These two sets of measurements provided the volumes of two tapered cones—one larger cone that included the charcoal and one smaller cone that was the unburned portion of the log. The difference between the two cone volumes is an estimate of charcoal volume, which was converted to mass us- Figure 1. Photographs taken immediately after the Pel- ing density values for the five different decay classes ican Creek fire in Yellowstone National Park in August of lodgepole pine from Harmon and Sexton (1996). and September 1996, showing log trenches (A) and log Estimates of this type are based on the assumption shadows (B). The size of the light-colored area (log that logs have been uniformly charred around their shadow) on the forest floor is indicative of the amount of circumference. There is variability in charcoal depth wood that was consumed. Similarly, the cupped depres- around some logs, but our observations in the field sion (log trench) shows where a decaying log once occu- corroborate the assumption of relatively even char- pied the forest floor before being consumed by the fire. ring. Still, some diameter reduction of charred logs Note the unburned woody lateral root from a lodgepole pine that had grown into the decaying log, suggesting could have occurred before the remaining charcoal that the log was a source of water and nutrients for the was formed, and our measurements must be con- growing root. sidered as underestimates. Logs completely consumed by fire. We used two types of evidence to estimate the volume of CWD of the unburned CWD using the ARC/INFO Geo- that is completely combusted: log shadows and log graphic Information System (ESRI Inc. 1995). In trenches. Log shadows are linear, tapered, light gray most cases, the overlays associated the shadows and patches on the otherwise blackened soil surface; trenches with unburned portions of logs of identi- they are apparently created by the high tempera- fiable size and decay classes. The associations were tures of glowing combustion on logs in close prox- used to estimate the probable dimensions and decay imity to the forest floor and the accompanying ox- stage of the logs prior to the fire. idation of surficial organic matter (Figure 1a). Log The dimensions of missing logs in log trenches trenches are elongated, concave depressions in the not associated with adjacent unburned logs were forest floor that were occupied by decayed logs estimated from regression models we developed (decay classes 3–5) prior to the fire, but where the during this study in unburned forests in YNP, which logs were completely consumed (Figure 1b). Log use trench width to predict log diameter (Table 2). shadows and trenches were mapped separately We determined that almost all logs that burned and from the unburned CWD within the 20 ϫ 20 m formed trenches were of decay classes IV and V, plots, and the maps were overlaid with digital maps which are elliptical rather than round in cross sec- Coarse Woody Debris in Lodgepole Pine Forests 477

Table 2. Regression Models Developed during this Study in Unburned Stands in Yellowstone National Park (YNP) for Predicting Log Diameters from Trench Widths

Decay Classa Regression Model R2 ϭ ϭ ϩ III (n 30) D1 1.00(TW) 3.64 0.72 ϭ ϭ ϩ IV (n 30) D1 1.01(TW) 1.06 0.93 ϭ ϭ ϩ V(n 30) D1 0.87(TW) 2.96 0.89

D1, predicted log diameter; TW, trench width aBecause decay class I and II logs do not form trenches, no regression models are included for them.

tion. The estimated volumes of each elliptical log sumption by partial burning nor partial and bulk densities for each decay class were multi- consumption of snags or stumps was estimated, our plied by wood density values for the appropriate values represent a conservative estimate of the ac- decay class (Harmon and Sexton 1996) to calculate tual amount of CWD consumed by fire. the biomass of the logs that had occupied the trenches. Estimates of Net Loss or Gain of The application of this log shadow/log trench CWD Biomass method provides a conservative estimate, because CWD biomass measurements, together with esti- some of the consumed CWD more than 7.5 cm in mates of the amount of CWD removed by harvest- diameter does not leave any obvious trace of its ing and consumed or converted to charcoal by fire, existence. However, at the present time, there is no were used to calculate the losses and gains of CWD alternative for studies of natural fire. Notably, this during natural fires and clear-cut timber harvesting. method cannot be applied beyond 1 year after a fire In burned stands, live trees killed but not consumed because most of the log shadows disappear and the by fire were considered a potential source of CWD. log trenches are less conspicuous. In clear-cuts, live trees removed by harvesting rep- Logs partially consumed by fire. To account for the resented a loss of potential CWD. Estimates of pre- CWD lost through partial burning of logs, we at- harvest live-tree biomass in clear-cuts, which rep- tempted to estimate pre- and postfire log biomass resents the loss of potential CWD, were made from by applying a repeated measure design using the individual stump basal areas using the allometric planar intercept method twice along the same equations of Pearson and others (1984). Posthar- transects. Other studies have used this procedure vest slash more than 7.5 cm in diameter, along with for the same purpose in prescribed burns (Kauff- stumps left after bole removal, were considered man and Martin 1989; Brown and others 1991). CWD in clear-cuts. Postfire biomass was first estimated along transects using the planar intercept method. To estimate prefire biomass, each log was cut in half at the point RESULTS along the log where the postfire measurement was Uncut and unburned stands. Comparisons be- taken, and the maximum radius that was exposed tween uncut stands in the MBNF and unburned by the cross-sectional cut was recorded. The maxi- stands in YNP were performed to detect any re- mum radius was then doubled to provide a prefire gional differences that might affect subsequent diameter of each log, which was used to calculate comparisons. No significant differences for any size prefire biomass. The difference between pre- and classes of CWD were observed between the two postfire estimates would provide an estimate of the sites (P Ͼ 0.3) (Figure 2). CWD consumed by partial burning of logs. Clear-cut and burned stands. No significant differ- However, using this method, we found small and ences between any size class of downed CWD more nonsignificant differences between pre- and post- than 7.5 cm in diameter or total downed CWD fire biomass estimates. We therefore concluded that (CWD plus fine woody debris more than 7.5 cm in the planar intercept method is not appropriate for diameter) were found when comparing clear-cut to estimating the amount of biomass lost through par- burned stands; (P Ͼ 0.2) (Figure 3). However, as tial burning of logs. These estimates are not in- expected, there was significantly more CWD in the cluded in our analysis. Because neither CWD con- form of snags in burned stands. If the snags are 478 D. B. Tinker and D. H. Knight

Figure 4. Biomass of clear-cut stands (MBNF, hatched Figure 2. Biomass of uncut stands (MBNF, cross- bars) and uncut stands (YNP, solid bars). Total DW ϭ total hatched bars; n ϭ 6) and unburned stands (YNP, solid downed wood of all sizes; Total CWD ϭ total downed bars; n ϭ 6). Total Downed Wood ϭ total downed wood wood of all sizes ϩ stumps ϩ snags. Stumps and snags are of all sizes; Total CWD (more than 7.5 cm in diameter) ϭ both more than 7.5 cm in diameter. Error bars are Ϯ 1 SE. total downed wood ϩ stumps ϩ snags (all more than 7.5 Asterisks indicate significant differences between clear- cm in diameter); Total CWD (all sizes) ϭ total CWD of all cut and uncut stands. sizes ϩ stumps ϩ snags; Total Biomass ϭ total CWD of all ϩ sizes live trees (including boles, branches and leaves). Ͼ Error bars are Ϯ 1 SE. clear-cut and uncut stands (P 0.05) (Figure 4). However, clear-cuts contained almost three times the amount of fine woody debris (less than 7.5 cm in diameter) and more than double the amount of stump biomass than uncut stands (P Ͻ 0.05). Crown fires and surface fires. No significant differences in any size classes of CWD were observed between stands that were subjected to crown fires and those that were burned by intense surface fires (P Ͼ 0.10 for all size classes). The biomass of snags was not statistically different among the two burn intensifies (P Ͼ 0.10). In fact, tree mortality was 100% in both types of fire. CWD converted to charcoal or completely consumed. The amount of CWD of all decay classes converted Figure 3. Biomass of clear-cut stands (MBNF, hatched to charcoal during the Pelican Creek fire in 1996 ϭ bars) and burned stands (YNP, solid bars). Total DW was estimated to be at least 6.4 Mg haϪ1 (around total downed wood of all sizes; Total CWD ϭ total 8% of CWD present in the stand). Charcoal thick- downed wood of all sizes ϩ stumps ϩ snags. Stumps and snags are both more than 7.5 cm in diameter. Error bars ness was relatively consistent for decay classes I–IV are Ϯ 1 SE. Asterisks indicate significant differences be- (Table 3). Conversion of decay class V CWD to tween clear-cut and burned stands. Stump biomass is charcoal could not be estimated because of the dif- higher in clear-cuts than in burned stands only because ficulty we encountered in locating and obtaining bolewood removal creates stumps during timber harvest. intact cross sections. Analysis of log shadow and log trench evidence indicated that downed CWD was reduced by at least 6.4 Mg haϪ1 (around 8%) dur- included, there is more than double the biomass of ing the 1996 Pelican Creek fire. Decay class V ac- CWD more than 7.5 cm in diameter in burned counted for only 10% of the biomass loss estimated stands than in clear-cut stands (P Ͻ 0.05) (Figure from log shadows but 44% of the biomass loss 3). In contrast, clear-cut stands had five times more estimated from log trenches. Three-fourths of the fine woody debris less than 7.5 cm in diameter than log trenches had been occupied by class V logs burned stands (P Ͻ 0.05; 20.6 and 4.5 Mg haϪ1, (Table 4). When both losses are added, 12.8 Mg respectively). haϪ1 of downed CWD were consumed or converted Clear-cut and uncut stands. There were no signif- to charcoal by the Pelican Creek fire (around 16% icant differences in CWD of any size class between of the total). Coarse Woody Debris in Lodgepole Pine Forests 479

Table 3. Coarse Woody Debris (CWD) Biomass (Mg haϪ1) Converted to Charcoal by Decay Class during the 1996 Pelican Creek Fire, Based on Data from 50 Logs

Outer Inner Decay Cone Cone Biomass Percent Class Biomass Biomass Loss Loss

I 0.35 0.32 0.02 7.1 II 4.3 4.0 0.3 7.7 III 2.5 2.3 0.2 7.7 IV 0.15 0.14 0.01 7.6 V no data no data no data no data All 7.3 6.7 0.5 7.6

Table 4. Coarse Woody Debris (CWD) Consumed during the 1996 Pelican Creek Fire, Estimated Using 100 Postﬁre Log Shadows and 27 Log Trenches

Log Shadows Log Trenches

Biomass Biomass Decay Class Number (%) (Mg haϪ1) (%) Number (%) (Mg haϪ1) (%)

I ———— II 15 (15) 0.6 (16) — — III 50 (50) 2.2 (56) 3 (12.5) 1.1 (44) IV 27 (27) 0.7 (18) 3 (12.5) 0.3 (12) V 8 (8) 0.4 (10) 21 (75) 1.1 (44) Total 100 (100) 3.9 (100) 27 (100) 2.5 (100)

Net loss or gain of CWD biomass. Clear-cut harvesting resulted in an average net wood loss of 83 Mg haϪ1 (137 Mg haϪ1 removed as live trees minus 54 Mg haϪ1 gained from downed CWD) (Figure 5). In contrast, assuming an approximate 16% reduction in CWD by consumption and conversion to charcoal, as calculated above, lodgepole pine stands that burn still would have an average net CWD increase of 99.2 Mg haϪ1 after all snags fall (112 Mg haϪ1 gained minus 12.8 Mg haϪ1 burned) (Figure 5). Clear-cutting removes almost 11 times as much wood biomass as a natural ﬁre (137 Mg haϪ1 removed by clear-cutting divided by 12.8 Mg haϪ1 removed by CWD consumption) (Figure 5).

DISCUSSION Ϫ1 The range of our CWD biomass estimates is broad Figure 5. Average CWD biomass (Mg ha ) gained (gray (29–284 Mg haϪ1), but it is consistent with CWD bars) and lost (black bars), and the net gain or loss (cross- hatched bars) for all clear-cut stands (MBNF) and burned estimates from other studies in Rocky Mountain stands (YNP). In clear-cuts, gain is from postharvest slash and Pacific Northwest coniferous forests (Table 5). and loss is from removal of live tree boles. In burned Although both clear-cutting and natural fires create stands, gain is from fire-killed trees and loss is from wood CWD, our data show that clear-cutting removes consumption and conversion to charcoal. Error bars Ϫ1 47–116 Mg ha (mean, 83) more potential CWD are Ϯ 1 SE. There is no error bar for the loss category for (live trees) than is added by downed CWD left on burns because this estimate is from a single stand. 480 D. B. Tinker and D. H. Knight

Table 5. Comparison of Coarse Woody Debris (CWD) (more than 7.5 cm in Diameter) Biomass Estimates in Western Coniferous Forests

CWD Biomass Study Location Forest Type (Mg haϪ1) Reference

Cascade Mountains, Abies amabilis 147–809 Grier and others 1981 Washington, USA Olympic National Park, Picea sitchensis/Tsuga 120–161 Graham and Cromack 1982 Washington, USA heterophylla MBNF, Wyoming, USA Pinus contorta 123–180 Pearson and others 1984 Sequoia National Park, Various mixed 29–400 Harmon and others 1987 California, USA conifers British Columbia, Canada Pinus contorta 156–392 Comeau and Kimmins 1989 Rocky Mountain National Picea engelmannii/Abies 123 Arthur and Fahey 1990 Park, Colorado, USA lasiocarpa British Columbia, Canada Pinus contorta 118 Blackwell and others 1992 British Columbia, Canada Pinus contorta 52–141 Wei and others 1997 MBNF and YNP, Pinus contorta 29–121 Present study Wyoming, USA (undisturbed) YNP, Wyoming, USA Pinus contorta (burned 41–284 Present study in 1988) MBNF, Wyoming, USA Pinus contorta (clear- 52–123 Present study cut in 1991–93)

MBNF, Medecine Bow National Forest; YNP, Yellowstone National Park

the site (Figure 5). Repeated timber harvesting that leaves amounts of CWD similar to those we measured will result in an overall decline in CWD over time. If a stand is subjected to regular, repeated harvesting, the inherited CWD that decomposes will not be replaced, since most of the bolewood is removed during each harvest event (Spies and others 1988; Harmon and others 1990, 1996; Wei and others 1997). The effects of this decline on biolog- ical diversity, nutrient cycling, and soil development should be considered when making decisions about sustainable forest management. Many questions arise that are difficult to answer. For example, considering that lodgepole pine often grow on nutrient-poor sites in the Rocky Moun- Figure 6. Biomass of total CWD (stumps including root tains, and considering that their roots often grow crowns, total downed wood less than 7.5 cm and more into decomposing logs, will tree growth decline as than 7.5 cm in diameter, and snags) in undisturbed the amount of CWD declines? Also, does the pres- stands, clear-cut stands, and burned stands (all burn se- ence of decomposing CWD on the soil surface have verities) of lodgepole pine forests in Wyoming. Values to a significant effect on the structure and chemistry of the right of each category are the percentage of total the underlying mineral soil? CWD. Unburned stands in YNP and unharvested stands in the MBNF had similar amounts of CWD (Figure 2). However, burned stands contained more than Wei and others (1997) found three to five times twice the amount of CWD more than 7.5 cm in more CWD in lodgepole pine stands that burned diameter than clear-cut stands (Figures 2 and 6), than in those that were clear-cut, although their largely due to the removal of live trees in harvested measurements did not account for consumption of stands and the presence of snags in burned stands. CWD by fire. The snags killed by crown or intense Coarse Woody Debris in Lodgepole Pine Forests 481 surface fires are an important source of CWD dur- from three previous studies of fuel consumption, ing early stand development (Figures 5, 6) (Harmon they found that large woody fuels were reduced by and others 1986) unless the stand should reburn 25%–40% when fuel moisture was near 30%, but during early development, when additional CWD they were reduced by 80%–100% when fuel mois- could be consumed. Spies and others (1988) found ture was only 10%. The variability between esti- that CWD accumulation was highest in very young mates of wood consumption suggests that addi- and very old Douglas fir stands, suggesting that fires tional research is required to gain a better that burn such stands could result in a greater loss understanding of the reduction of woody fuels by of CWD than those that burn mid-successional natural fires. stands with less CWD. Complete estimates of CWD consumption require The similarity in CWD between stands burned by measurements of all woody components of the crown fires and stands burned by intense surface standing-dead trees and forest floor. Therefore, our fires was not surprising. Intense surface fires create data provide underestimates of the wood burned sufficient heat to kill most or all of the trees in during a fire because no measurements were made lodgepole pine stands, as happens during crown of the losses from partially burned logs, or of com- fires. Notably, both surface fires and timber harvest- plete or partial consumption of stumps and prefire ing with roller-chopping consume less fine fuels dead-standing trees. Our field observations suggest than do crown fires. that these additional losses would be small com- Estimating the amount of CWD completely con- pared to downed CWD. Data from unburned stands sumed or converted to charcoal by natural fire was indicated that there is around 50% less wood in the challenging. Examinations of cross sections of form of snags and stumps than in downed CWD. burned logs during the initial stages of our study Therefore, even if a proportion similar to that of revealed a thin charcoal shell surrounding an oth- downed CWD was consumed from snags and erwise unburned log, suggesting that far less CWD stumps, the additional amount consumed would be was consumed during natural fires than we had around 8%, for a total of around 24% wood con- previously thought. Our estimate of 16% of CWD sumption. Most of the partial burning of snags and consumed or converted to charcoal is less than that downed CWD occurs when two pieces of wood are of Reinhardt and others (1991), who found that in contact—for example, when a log is lying at the 30%–39% of all CWD more than 7.5 cm in diam- base of a snag. We consistently noticed an apparent eter was consumed by prescribed fire in mixed co- loss of CWD in areas where the close proximity of nifer stands in northern Idaho. Brown and others two logs created an environment conducive to (1991) estimated that woody fuel consumption in glowing combustion, a condition that surely in- prescribed burns, also in Idaho, ranged from 12% to creases with CWD biomass. Albini and Reinhardt 65% (mean, 33%), which encompasses our esti- (1995) suggested that large woody material nor- mate. mally does not burn if the only source of heat is its Using data from prescribed fires and subjective own combustion, a condition that is common for estimates, Fahnestock and Agee (1983) estimated downed CWD in natural fires. that the amount of snag and CWD biomass con- Although our calculations of CWD consumption sumed by fire was 20% during fires of moderate are underestimates, we have attempted to account intensity and 30% during high-intensity fires. for both additions and removals from burning and However, it seems unlikely that the spatial and clear-cut harvesting. As noted by Fahnestock and thermal dynamics of a prescribed burn would be Agee (1983), additional work is needed to develop comparable to an intense forest fire, which burns methods for estimating the amount of CWD con- through a stand more rapidly. Hall (1991) found sumed during natural fires. For now, measure- that high-intensity fires consumed 23% less large ments of CWD from natural stand conditions, both woody fuels than moderate-intensity fires under burned and unburned, provide the best data avail- otherwise similar conditions, apparently due to the able for estimating the historic range of variability shorter duration of high-intensity fires, which typ- for CWD, which may be used as a management ically do not reduce fuel moisture to the same de- guideline for the amount of CWD that should be gree as moderate-intensity fires. present on a given site. Our CWD data from YNP Of course, CWD consumption by fire will vary provide important baseline information that can be seasonally and annually, depending on tempera- used as such a guide. In addition, our information ture, precipitation, and fuel moisture content, and on the amount of CWD left after clear-cutting pro- our estimates do not account for this variability. vides managers with a look at how closely this When Brown and others (1985) summarized data practice does (or does not) resemble naturally de- 482 D. B. Tinker and D. H. Knight veloping stands with respect to CWD amounts. Per- Brown JK, See TE. 1981. Downed dead woody fuel and biomass haps if more CWD was left on clear-cut sites, de- in the northern Rocky Mountains. USDA Forest Service gen- veloping stands would more closely resemble eral technical report INT-117. 19 p. conditions that are created by natural fires. How- Bull EL, Parks CG, Torgersen TR. 1997. Trees and logs important to wildlife in the interior Columbia River Basin. USDA Forest ever, simulation modeling is required to estimate Service PNW-GTR-391. Pacific Northwest Research Station, the long-term effects of different fire and harvest Portland, OR. regimes on CWD amount, distribution, and Call PT, Albini FA. 1997. Aerial and surface fuel consumption in dynamics. crown fires. Int J Wildland Fire 7(3):259–64. Cissel JH, Swanson FJ, Weisberg PJ. 1999. Landscape manage- ACKNOWLEDGMENTS ment using historical fire regimes: Blue River, Oregon. Ecol Appl 9:1217–31. This work was supported by grants from the Uni- Comeau PG, Kimmins JP. 1989. Above- and below-ground bio- versity of Wyoming/National Park Service Research mass and production of lodgepole pine on sites with differing Center and the US Department of Agriculture (NRI soil moisture regimes. Can J For Res 19:447–54. 96-35101-3244). David Melkonian, Kristofer M. Davis JW, Goodwin GA, Ockenfels RA. 1983. Snag habitat man- Johnson, Sharon Stewart, Donna Ehle, and Sally agement. USDA Forest Service general technical report RM- Tinker assisted in the field and in the laboratory. 99. 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