Fire history of a central Nevada pinyon juniper woodland

Transcription

1 1589 Fire history of a central Nevada pinyon juniper woodland John M. Bauer and Peter J. Weisberg Introduction Abstract: Our study reconstructed fire history ( ) from tree rings for a Great Basin single-needle pinyon pine (Pinus monophylla Torr & Frém.) Utah juniper (Juniperus osteosperma (Torr.) Little) woodland. Information from multiple lines of evidence, including dateable fire scars (n = 83), tree demography, and charred coarse woody debris, was used to quantify fire frequency, severity, and extent. Fire cycle models were developed using survivorship analysis of timesince-fire estimates. We investigated the spatial and temporal variation in historical fire regime, addressing the plausibility of postsettlement fire exclusion as an explanation for increased woodland area and density since the late 1800s. Historical fire regime was characterized by infrequent, small, high-severity fires. Estimated fire cycle ( ) was 427 years, with no evidence of postsettlement stand-replacing fires. Topographic analyses indicated that in this drought-prone landscape, more mesic conditions favor continuous fuels that lead to more frequent or extensive fire. Superposed epoch analysis showed increased fire occurrence during drought years but with no influence of antecedent climatic conditions. More frequent grassland and shrubland fires were recorded by fire scars near valley floors. Thus, anthropogenic fire exclusion in adjacent, shrub-dominated communities presents a plausible mechanism for woodland expansion in the study area. However, there is little ecological justification for reintroducing fire to areas of historic woodland, where effects of fire exclusion have been minimal. Résumé : Notre étude dendrochronologique a permis de reconstituer l historique des feux ( ) dans un boisé de pin faux-arolle (Pinus monophylla Torr & Frém.) et de genévrier (Juniperus osteosperma (Torr.) Little) situé dans le Great Basin. L information provenant de plusieurs sources, incluant les cicatrices de feu datables (n = 83), la démographie des arbres et les débris ligneux grossiers carbonisés, a été utilisée pour quantifier la fréquence, la sévérité et l étendue des feux. Des modèles du cycle de feu ont été développés en utilisant l analyse de survie des estimations du temps écoulé depuis un feu. Nous avons étudié les variations spatiales et temporelles du régime de feux passé en abordant le fait que l exclusion du feu après la colonisation pourrait expliquer l augmentation de la superficie et de la densité des boisés depuis la fin des années Le régime de feux passé était caractérisé par des feux de forte sévérité, petits et peu fréquents. La durée du cycle de feu ( ) a été estimée à427 ans et il n y avait pas d indices de feux causant le remplacement des peuplements après la colonisation. Des analyses topographiques indiquent que, dans ce paysage sujet à la sécheresse, des conditions plus mésiques favorisent la présence continue de combustibles associésàdes feux plus fréquents et plus étendus. Une analyse par superposition d époques a montré que l occurrence des feux a augmenté durant les années de sécheresse mais sans que les conditions climatiques antérieures exercent une influence. Des feux de prairie et de broussailles ont été enregistrés par les cicatrices de feu près du fond des vallées. L exclusion du feu par l homme dans les communautés adjacentes dominées par des arbustes constitue par conséquent un mécanisme plausible pour expliquer l expansion des boisés dans la zone d étude. Cependant, il y a peu de justification écologique pour réintroduire le feu dans les zones historiquement boisées, où l exclusion du feu a eu peu d effets. [Traduit par la Rédaction] Received 13 August Accepted 7 May Published on the NRC Research Press Web site at cjfr.nrc.ca on 15 August J.M. Bauer and P.J. Weisberg. 1 University of Nevada, Reno, Department of Natural Resources and Environmental Science, 1000 Valley Road, Mail Stop 186, Reno, NV 89557, USA. 1 Corresponding author ( pweisberg@cabnr.unr.edu). Characterization of historical fire regimes has emerged as an important component for ecological restoration of North American forests and woodlands (Landres et al. 1999; Bergeron et al. 2002). However, fire regimes are difficult to quantify for many vegetation types, and existing methodological approaches cannot be universally applied (Veblen 2003). Fire-history studies for low-severity fire regimes typically apply a chronology-based approach, determining fire return intervals at a location based on precisely dated fire scars (e.g., Brown and Swetnam 1994). Fire-history studies for high-severity fire regimes typically apply a chronosequence-based approach, where fire cycle is estimated statistically from reconstructed time-since-fire maps (e.g., Johnson and Gutsell 1994), or natural fire rotation is calculated empirically from maps of historical fire perimeters (e.g., Floyd et al. 2004). Methodological approaches for fire history are complicated by the prevalence of mixed-severity fire regimes across much of North America (Barrett et al. 1991; Fulé et al. 2003; Lentile et al. 2006). Mixed-severity fire regimes describe fire behavior resulting in a complex mixture of high-severity and low-severity burn patches. Because firehistory methodology differs for high- versus low-severity fire regimes, this can cause lack of consistency within and Can. J. For. Res. 39: (2009) doi: /x09-078

2 1590 Can. J. For. Res. Vol. 39, 2009 among studies. Not all forest types include tree species that consistently survive fire with dateable fire scars, creating difficulties for the chronology-based approach. On the other hand, where stand ages are used to reconstruct fire cycle from time-since-fire maps (Van Wagner 1978), slow successional responses make it challenging to reconstruct fire years from landscape-level age-class patterns. Finally, many vegetation types occur as patches or bands within heterogeneous landscape mosaics and, thus, experience a fire regime that is influenced by proximity to other vegetation types. As a result, fire-regime reconstructions have been limited for certain vegetation types that occupy extensive areas. This is the case for the pinyon juniper woodland communities, which are estimated to occupy at least ha across 10 western states. There is incomplete and fragmentary information regarding natural fire regimes of pinyon juniper woodlands (reviewed in Baker and Shinneman 2004): some researchers describe fire as having played an integral role in landscape dynamics, and others describe woodlands that seldom burn, with other disturbance agents (drought, insects) being of greater ecological importance (Eisenhart 2004; Floyd et al. 2004). Fire regimes in persistent pinyon juniper woodlands are generally characterized as infrequent and of high severity, but significant uncertainty remains (Baker and Shinneman 2004; Romme et al. 2008). Fire-regime characterization is particularly important for pinyon juniper woodlands in the context of widespread, dramatic changes in woodland extent and structure as well as recent efforts at landscape restoration. Increased tree density and expansion of pinyon juniper woodland to other vegetation types has been evident for over a century and is now a major source of concern (Leopold 1924; Miller and Wigand 1994; Weisberg et al. 2007). Management practices intended to reverse or mitigate these trends include use of prescribed fire (Rau et al. 2008). Ecological justification for such management to reduce tree cover on western rangelands is predicated on the hypothesis that postsettlement fire exclusion is at least a partial cause for recent pinyon juniper woodland expansion. Although many fire-history studies for diverse forest types in the western United States have documented a significant reduction in postsettlement fire frequency (e.g., Weisberg and Swanson 2003), recent studies on the Colorado Plateau suggest this may not be universally true for pinyon juniper woodlands (Eisenhart 2004; Floyd et al. 2004). Scientific investigation of this key hypothesis depends upon accurate fire history reconstruction, which is generally lacking for pinyon juniper woodlands in the Great Basin (Baker and Shinneman 2004). This study reconstructs the fire regime of a pinyon juniper woodland in central Nevada based on tree rings. Our study focused on the following questions: (i) what was the overall importance of fire in shaping the presettlement woodland and landscape structure; (ii) how has fire regime varied spatially, considering effects of topography, landforms and adjacent vegetation types; (iii) how has fire regime varied temporally, according to variability in climate and land use practices; and (iv) is fire exclusion a plausible explanation for woodland expansion and increased tree density? Information from multiple lines of evidence, including stand ages, stand demographics, fire scars, charred coarse woody debris, and landscape context, was integrated to quantify Fig. 1. Map of Barrett Canyon study area showing sample plot locations and 25 m elevation contours. ~, Intensive plots; *, extensive plots. Shaded areas indicate present-day woodland extent. historic fire severity, frequency, and extent. Climatic influences on fire regimes were investigated on short-term and multi-decadal scales using a regionally derived drought index. Spatial relationships of fire scars and stand ages were used to determine the influence of the surrounding landscape on the woodland fire regime. Methods Study area The 1880 ha Barrett Canyon watershed ( @N, @W) is located within the Shoshone Mountain Range in central Nevada (Fig. 1) and is oriented towards the southeast, with intermittent streams draining into the Reese River Valley. Lithology of the Shoshone Range consists mostly of welded and nonwelded tuff. Alluvial fan soils were classified as coarse loamy mixed frigid Typic Haploxerolls (Rau 2005). In the Shoshone Range, measured annual precipitation increased linearly with elevation from 230 mm at canyon bottoms to 500 mm at the top of the drainage (Rau 2005). Most precipitation falls as snow in winter or spring rains. Summers are characterized by seasonal drought with hot dry days and cool nights. Euro-American settlement in nearby the Reese River Valley commenced in the 1860s, with seasonal sheep and cattle grazing presumably occurring in the upland mountain ranges shortly after. Barrett Canyon s long distance from 19th-century silver mining operations resulted in it being bypassed from the large-scale logging operations for charcoal production (Young and Budy 1979), leaving a nearly intact woodland. Fire atlas records extend back to 1970, with no recorded fires in the Barrett Canyon. Elevation ranges from 2100 to 2900 m. The terrain is highly dissected, with narrow canyon bottoms, occasional basaltic outcrops, and a median slope in the woodland portion. Woodland cover is discontinuous, with woodlands

3 Bauer and Weisberg 1591 Fig. 2. Distribution of (a) fire event years as reconstructed from fire scar data and (b) char density observed on 90 fire history plots. Figures 2a and 2b share a common background illustrating stand age classes of woodland patches mapped from aerial photography and field vantage points. on southern to southwestern slopes alternating with mountain shrub communities on northern and eastern slopes. Valley floors are dominated by Wyoming big sagebrush (Artemisia tridentata var. wyomingensus (Beetle & A.L. Young) S.L. Welsh), mountain big sagebrush (Artemisia tridentata var. vaseyana (Rydb.) B. Boivin), Great Basin wildrye (Leymus cinereus (Schribn. & Merr.) A. Löve), and yellow rabbitbrush (Chrysothamnus viscidiflorus (Hook.) Nutt.). Woodlands are dominated by single-needle pinyon pine (Pinus monophylla Torr & Frém.), with Utah juniper (Juniperus osteosperma (Torr.) Little) forming a minor component (<2% stem count). Limber pine (Pinus flexilis James) is found in small pockets at the highest ridgelines. Curlleaf mountain mahogany (Cercocarpus ledifolius Nutt.) codominates with pinyon at the higher elevations of the study area. North-facing slope cover is dominated by mountain big sagebrush, low sagebrush (Artemisia arbuscula Nutt.), and Idaho fescue (Festuca idahoensis Elmer). The invasive cheatgrass (Bromus tectorum L.) is found throughout the study area but forms only a minor cover component. Tree-ring data Using systematic nonaligned sampling, where one sampling point was randomly placed within each 300 m 300 m rectangular grid cell, 90 plots were located in the woodland portion of the watershed, occupying 840 ha of 1880 ha contained within Barrett Canyon. Sampling was restricted to woodland sites with canopy coverage >10%, determined through photointerpretation of digital orthophotography. The slope-corrected, 0.1 ha plots were randomly designated as intensive (n = 31) or extensive (n = 59). Both categories of plots recorded basic topographic attributes such as slope, aspect, and rock cover. Basal diameter and species of all trees, logs, and snags were recorded, along with presence of char. In the extensive plots, three to five of the oldest trees determined from basal diameter and other morphological attributes were increment cored for aging as close to root collar as possible, with height above root collar recorded. In the intensive plots, all coniferous trees were sampled for aging, using increment cores for larger trees or cross sections for trees less than 5 cm in diameter. Of 1609 cores and cross sections collected across all 90 plots, 36 were from Utah juniper, 3 were from limber pine, and 1570 were from singleneedle pinyon pine, reflecting the dominance of the latter species across the study area. A canopy coverage polygon map of the watershed was developed from an object-oriented classification of panchromatic digital orthophotographic quadrangles at 1 m 1 m resolution (Weisberg et al. 2007). Polygons encapsulated areas of similar tree canopy coverage. The polygons were subsequently surveyed in the field and classified with respect to tree age distributions, with young stands having 95% of the trees <125 years old, old stands having 15% of trees <125 years old, and mixed stands between the two percentages. Polygon boundaries were edited for improved accuracy in defining areas of homogeneous tree canopy coverage on the basis of the field surveys, and changes were incorporated into a geographic information system (GIS). Edited

4 1592 Can. J. For. Res. Vol. 39, 2009 polygon boundaries that have been aggregated according to the two stand age categories are shown in Fig. 2. All fire scars within plots were sampled, although only four scars were found within plots. Fire scars were opportunistically sought outside of plots, with focus given to ecotones and areas of significant canopy cover variability. A wedge from each fire-scarred tree was extracted with a chainsaw. Criteria for distinguishing fire scars were refined using field observations of scars in modern pinyon juniper burns (Bauer 2006). Scars were treated as high confidence or medium confidence : high-confidence scars had char present on nubs, undersides of branches, or on the catface, with the catface extending to the root collar. Scars with medium confidence lacked char, but a triangular-shaped catface morphology and presence of high-confidence scars from the same year and in the immediate vicinity suggested formation from a fire event. Death cohorts of snags killed in the same fire event were opportunistically sought and sampled for tree-ring dating. Death years of fire-killed trees may allow dating of fires not recorded elsewhere by fire scars or further quantify the spatial extent of past fires that were recorded by fire scars (Miller and Rose 1999). Using observations from modern burns (Bauer 2006), morphological characteristics of trees that died in a high-severity fire included charred branch nubs and limited char on bole. Extensive char on the bole would be more likely to indicate trees that had burned after outer bark had already sloughed off following tree death. Increment cores, fire scars, and death cohorts were processed using standard dendrochronological techniques (Stokes and Smiley 1968). Increment cores used for reconstructing age-class structure and plot origin years were generally not crossdated but were counted and cross-checked independently by at least two researchers. However, the oldest trees in extensive plots (n = 59) were crossdated to a pinyon reference chronology assembled from the Shoshone Range and two nearby mountain ranges (F. Biondi, Department of Geography, University of Nevada, Reno, Nevada, personal communication, 2005). We used overlaid concentric circles on cores that did not reach pith. Adjustments for age at coring height used correction factors derived from previous research (R.J. Tausch, USDA Forest Service, Rocky Mountain Research Station, Reno, Nevada, personal communication, 2005), according to the following regression relationship (R 2 = 0.82, p < 0.01): Age correction ¼ 24:60 þ 0:218 coring height ðcmþ Fire scars (n = 83) and death cohorts (n = 11) were visually crossdated to the same pinyon reference chronology. COFECHA software was used to verify crossdating. For death cohorts, the year of the outermost ring was recorded as a fire year, although this may have underestimated the year of the fire in some cases because of decay. Death cohort dates were never used as primary evidence for reconstructing a fire event but only as secondary evidence to expand the reconstructed spatial extent of a fire event with primary evidence from precisely dated fire scars. Death cohort dates were only used when they were within 5 years of the precisely dated fire scar year. Approximately 50% of sampled death cohort stems exhibited bark beetle galleries, which indicated that the last growth increment observed was, in fact, Table 1. Guidelines used to classify fire severity for ninety 0.1 ha plots. Fire severity classification High severity Old growth, no fire Expansion Low severity Attributes Char present on logs within 10 m of plot; plot origin date can be attributed to nearby fire scar(s) or adjacent plots with similar origin date; if plot is bifurcated from a past disturbance, the boundary between old and young age cohorts is distinct. At least three trees >200 years old; high density (>100 logs/ha) of coarse woody debris; no char present within 10 m of plot; cannot attribute plot origin date with a nearby fire scar or with other nearby plot origin dates. Fewer than three trees >200 years old; minimal density of coarse woody debris (<20 logs/ha); no char present within 10 m of plot; no fire evidence of any kind present within 10 m of plot. Large temporal gap or gaps of greater than 50 years in the diameter age scatterplots, suggesting multiple age cohorts; abundant char on trees and coarse woody debris (>100 charred logs/ha or trees/ha); two or more fire dates from a single fire-scarred tree in or near plot. the precise death year. Fire scars recorded during dormancy were assumed to have occurred during the year of the last crossdated annual ring. This assumption was based on a comparison of sampled increment cores from 2005 with the temporal distribution of modern lightning-initiated wildfires in Nevada pinyon juniper woodlands, showing that fires generally occur late in the season after ring growth is complete (Bauer 2006). Analysis Guidelines were created for classifying fire severity according to the following plot-level characteristics: abundance of coarse woody debris, presence of char within 10 m of the plot, synchrony of plot origin date with nearby fire scars or adjacent plot origin dates, and presence of temporal gaps in diameter and age distributions (Table 1). Plot-origin dates were considered synchronous with nearby fire scar dates when the fire scar date predated the plot origin date by <35 years, the fire scar fell within the same or similar canopy-coverage polygon as the sample plot, and there were no topographic barriers that would be expected to act as fire breaks separating the fire scar and sample plot. Fire severity was classified for each plot as high, low, or no fire. Low-severity fires would not have resulted in extensive tree mortality and, thus, would be characterized by an abundance of char or fire scars on living trees or, in the case of limited but patchy tree mortality, by multiple age cohorts within the plot. Plots designated as no fire were further classified as expansion plots showing no evidence of woodland cover prior to Euro-American settlement or as old-growth pinyon

5 Bauer and Weisberg 1593 woodlands with maximum tree age >200 years old and no evidence of past fire. Each plot was assigned a time-since-fire value according to the ages of the oldest trees in the plot. Although it is conceivable that time-since-fire values may have been biased by whether a plot was sampled extensively or intensively, mean time-since-fire values did not differ according to sampling methodology (t 88 = 1.41, p = 0.16). Three or more trees were required to establish evidence for a postfire cohort; fewer were regarded as remnants from an earlier burn. A 35 year adjustment to the oldest tree age was incorporated to account for the lagged regeneration following fire that is typical for single-needle pinyon. Researchers have reported periods from 5 to 55 years for initial establishment, with slow initial return rate, followed by more rapid establishment (reviewed in Bauer 2006). Large variation in postfire establishment rate may be due to site-specific characteristics, corvid and rodent dispersal mechanisms, and dependence of seed source and seedling establishment on periodicity of pinyon masting and favorable climatic conditions. For plots whose evidence suggested a high-severity regime, fire-cycle estimates were developed from mathematical models using survival analysis (Allison 1995; Grenier et al. 2005). These methods handle censored data points, specify confidence intervals, and quantify effects of covariates. Plots that were not accurately aged because of the presence of heart rot were treated as right censored (i.e., stand ages were considered to be minimum ages). In survival analysis, right-censored observations are assumed to have eventually failed at some time after the final recording point. The effect of right censoring is an increase in the accuracy of the estimate at the cost of precision. Ages from plots classified as expansion and old growth were used in survival analysis, with the assumption that their initiation was from a fire-related event in the past that is no longer detectable from the fire-scar record. Analyses of fire cycle using stand ages commonly use time-since-fire maps to construct area burned for each time interval (Johnson and Gutsell 1994). For our semiarid woodland, such a map was not feasible because of very slow successional trajectories, heterogeneous within-stand structures, highly variable topography, and interspersion of woodland with other vegetation types. Burns occurring in the past 150 years are often, but not always, distinguished in canopy coverage analysis and field surveys. However, older woodlands lose distinct boundary features, with tree age height relationships obscured (Tausch et al. 1981). Thus, a random sampling design was employed, wherein the ages of plots were considered to be a representative sample of the landscape s disturbance history (Larsen 1997; Reed 1997). Graphical analysis of cumulative percentage of stand survivorship was first done to examine for temporal heterogeneity and to suggest temporal breakpoints for further analysis. Based on previous research (Miller and Tausch 2001), we suspected that a temporal change in fire frequency occurred with Euro-American settlement but had no other information that suggested other temporal breakpoints. The SAS LIFE- REG model procedure (SAS Institute Inc. 1999) estimates parameters for survival analysis. We used the exponential parametric hazard model to derive estimates of fire cycle and goodness of fit. Univariate effects of abiotic variables (see Table 2) were evaluated using the log-rank test in SAS Table 2. Univariate c 2 tests for fire cycle as predicted by topographic variables using the log-rank test (Allison 1995). Covariate c 2 p > c 2 Effect Aspect (transformed) Slope Rock cover Elevation Distance to valley floor Relative distance Slope position Solar radiation Note: Analysis period was AD with 44 fire event years (i.e., failures in survival analysis) and 43 censored points. LIFETEST procedure (Allison 1995). The log-rank test statistic compares estimates of the hazard function of two groups (here, a control group and a group with one explanatory variable) at each observed event time. Explanatory variables included the plot s slope position (position relative to valley bottom and ridgeline), cosine-transformed aspect, distance to valley floor, and solar radiation index (Kumar et al. 1997). Fire-event years determined by fire scars were compiled into a single fire chronology for the entire study area, for the purposes of superposed epoch analysis (SEA) of the temporal relationship between climate variability and fire occurrence (Stephens et al. 2003). The reconstructed Palmer drought severity index (PDSI) for 408N, W was used as an annually precise, synoptic record of drought (Cook et al. 2004). The SEA was used to estimate the strength and influence of PDSI on recorded fire occurrence. Bootstrapped simulations (n = 1000) for obtaining a comparison distribution of PDSI within a randomly chosen 5 year window were compared to the 5 year superposed window of actual PDSI during the fire event years, including two antecedent years. Confidence limits indicate the strength of the difference between the control (randomly chosen) window and the superposed window corresponding to observed fire event years. Fire years obtained from fire-scarred trees and death cohorts were spatially referenced to stand ages and canopy coverage maps (Fig. 2) to reconstruct extent of past woodland burns. The 35 year fixed postfire recovery period assumption used in the survival analysis was broadened to a year postfire recovery window. Thus, sample plots with oldest trees dating to within years of a reconstructed fire were assumed to have been burned in that fire. Fires may have extended to or originated from surrounding shrub- and grass-lands, but in general, the extent of such burns was not possible to determine. When two or more fire scars recorded the same year of fire, stand ages and landscape context were examined for possibility of a continuous fire between the two points. Death cohorts and mediumquality fire scars were not used as primary evidence for fire occurrence but expanded the estimates of spatial extent for individual fires. Fires were classified as small (<0.1 ha); medium (0.1 1 ha), and large (>1 ha). Results Most sample plots showed evidence of high-severity fires

6 1594 Can. J. For. Res. Vol. 39, 2009 Table 3. Estimates of fire cycle parameters for exponential model of stand survivorship. Temporal division Fire cycle (years) 508±64 349±44 427±64 187±43 95% CI Lagrange p < Note: Fire cycle values are means ± SE. Covariates are not included in this analysis. The Lagrange multiplier test checks whether the hazard function was constant over time, with low values of p suggesting that the hazard was not constant for the time interval. (73 of 90 plots) and contemporary tree age-class structures reflect long-term succession following such fires. The stand-origin dates of 38 of the 71 plots originating from a high-severity event could be attributed to a fire event recorded by a nearby fire scarred tree. Six plots were classified as pinyon old growth, with no evidence for fire in or near the plots. Three plots were classified as mountain mahogany old growth, where ancient mountain mahogany trees had long been established, but there had been recent expansion of pinyon into these plots. Mountain mahogany oldgrowth plots were not considered in further analysis. Six plots were classified as expansion woodlands. There was no evidence for low-severity fires within the expansion woodlands. The time-since-fire cumulative distribution (Fig. 3) revealed significant temporal heterogeneity. No plots originated in the 20th century. Three temporal divisions were suggested by the graphical analysis ( , , and ), which were subsequently used in the survival analysis. Estimates of fire cycle ranged from 187 to 508 years, depending on the time periods considered and models used (Table 3). A noticeable decrease in fire cycle (i.e., period of more frequent fire) occurred in the mid-16th century. The exponential model, as indicated by the Lagrange p value, did not adequately model the overall time period of nor did it adequately model the time period (p 0.05 for both). These results supported the graphical analysis suggesting strong temporal heterogeneity that would be inappropriate to model with a single distribution and suggesting different exponential models were needed to provide fire cycle estimates for different time periods. Presettlement fire cycle was 187 years (95% CI years) for the period, and 427 years (95% CI years) for the period. The period was further analyzed for physiographic influences on fire cycle using an exponential model for the cumulative time-since-fire distribution. Several covariates had significant effects on fire cycle (Table 2). The combined effects suggest less frequent fire on southwestern, steeper slopes with high rock cover. Eighty-three fire scars and 11 death cohorts were successfully crossdated from 134 collected samples, indicating 44 unique fire-event years. All successfully crossdated fire scars were from singleneedle pinyon pine. Scarred trees appeared to occur at the boundaries of historical fires, where otherwise high-severity fires were near the point of extinguishment. No fire-scarred tree recorded more than one fire event. Secondary scars were not unusual; however, these could not be confidently characterized as fire scars because the requisite catface char or embedded char within the scar tissue was Fig. 3. Distribution of time since fire with a logarithmic axis for cumulative plot area that has survived to time t, expressed as a calendar year (n = 87 plots). Open circles represent ages of plots that were not accurately dated due to heart rot and treated as right-censored data in survival analysis. lacking. Time since death for the 11 crossdated pinyon logs (i.e., death cohorts) ranged from 58 to 220 years. Char on trees was found only on fire-scarred trees. Char on logs was present in 81% of the plots. Most fire event years, as recorded by fire scars, occurred in the eighteenth and nineteenth centuries (Fig. 4), although many woodland stands appeared to originate from more extensive fire events occurring in earlier centuries (Figs. 2 and 3). A significant decrease in the frequency of recorded fires occurred in the 17th century. Fires were not completely absent from the study area following settlement but were strongly reduced in frequency compared with the immediate presettlement period. The composite fire chronology used for fire climate analyses was based on 36 fire years from 1690 to 2004 as recorded with annual precision by fire scars. The SEA indicated a significant relationship between PDSI and the occurrence of a recorded fire in a given year, with no support for an effect of antecedent moist or dry years (Fig. 5). PDSI was more negative during fire years, which indicated drought conditions. Fire-extent reconstruction revealed 55 unique fire events occurring over 44 fire years from 1300 to Most recorded fires (34 of 55, 62%) appeared to have originated in canyon bottoms and along drainages, penetrating to varying degrees past the sagebrush woodland ecotone. An additional nine fires (16%) appeared to have originated in mountain

7 Bauer and Weisberg 1595 Fig. 4. Fire-year temporal distribution as recorded by fire-scarred trees and death cohorts of fire-killed trees (n = 94 samples and 44 fire event years). shrub communities at higher elevations, burning down into the pinyon juniper woodlands. The largest reconstructed fire was approximately 35 ha, with most fires <10 ha (38 of 55, with six fires unresolved). Discussion Ample evidence in the form of fire scars, charred coarse woody debris, stand age distributions, and canopy distribution patterns suggests that fire had a significant role in shaping the Barrett Canyon landscape. Evidence of past fires was found in most of the woodland portion of the study area (Fig. 2). Several lines of evidence support an interpretation of historical, infrequent high severity fires within the pinyon juniper woodland, with a significant 20th century reduction in fire events. Fire regime characterization Romme et al. (2008) differentiated three fundamentally distinct categories of pinyon juniper woodlands: persistent pinyon juniper woodlands, where trees are the dominant life form in the absence of disturbance (e.g., most Barrett Canyon woodlands); pinyon juniper savannas with a continuous grass understory; and wooded shrublands, where tree density fluctuates in response to climatic variability or other causes, as is likely the case for valley bottoms in Barrett Canyon. As inferred from fire scars in combination with landscape-level tree age structure, fires in Barrett Canyon were historically infrequent, with fire rotation in the multiple hundreds of years. This is consistent with other dendroecological firehistory studies of persistent pinyon juniper woodlands (Eisenhart 2004; Floyd et al. 2004, 2008; Huffman et al. 2008), Fires in Barrett Canyon were typically small (<35 ha) in comparison with other studies (Floyd et al. 2004, 2008). This may be due to the dissected terrain of the Shoshone Mountain range. Woodland patches are separated by frequent rocky outcrops and ridgelines and by valley bottoms with deeper soils that historically were sagebrush and bunchgrass dominated. Fire-cycle analysis indicated that topography significantly influenced the overall presettlement fire cycle. Statistical effects of topographic variables (Table 2) suggested that more Fig. 5. Superposed epoch analysis, comparing annual fire occurrence with reconstructed Palmer drought severity index (PDSI) values. Analysis was constrained to AD (n = 37 fire years). The 95% and 99% confidence intervals from the bootstrapped simulation are shown as broken lines. Shaded bars show the difference between mean PDSI from 1000 randomly chosen 5 year window intervals and mean PDSI centered on the fire-event years. mesic conditions (e.g., more northeastern aspects and valley bottom environments) reduced the fire cycle. Sites with more mesic conditions generally produce a higher fuel load, which can favor an increase in frequency or extent of fires on semiarid landscapes (Brooks and Minnich 2006). Floyd et al. (2004) observed a decreased fire rotation with elevation and attributed the increased chance of fire to greater soil depth in the northern, higher elevation portion of the cuesta that encourages more biomass production. The concept of fire rotation is problematic in several ways (Fox 1989; Reed 2006). Estimates of fire cycle from any fire-rotation model should be interpreted in a broad, quasiquantitative context. For our study, evidence of past fires (char or fire scars) was not found in all plots. The absence of evidence may be due to decay of the entire death cohort. It is also possible that some of the plots did not originate from a fire event, instead originating from some other disturbance, such as wind, drought, or disease. In the mountainous landscape of Barrett Canyon, fire regime of the pinyon juniper woodland appeared to be strongly influenced by fire regimes of adjacent vegetation types. More frequent fires were inferred for valley bottom communities. Fire spread along the valley floor would be expected to increase fire frequency (i.e., reduce fire cycle) for pinyon juniper communities on the slopes above. This finding differs from other fire-history studies in more mesic portions of the western United States, where fire frequency and severity are generally lower for riparian areas, but may be comparable with that of uplands in some forest types (reviewed in Dwire and Kauffman 2003). However, for dryland ecosystems it may be a useful generalization that lower order drainages act as fire conduits by bringing more frequent fire to upland habitats (Pettit and Naiman 2007). The topographic heterogeneity and rockiness of Barrett Canyon may have partially counteracted the influence of valley bottom fires by limiting fire spread and hence keeping fires small. Climate fire relationships The SEA (Fig. 5) indicated that fire events were more

8 1596 Can. J. For. Res. Vol. 39, 2009 Fig. 6. Averaged Palmer Drought Severity Index (PDSI), using 50 year moving window mean, for point 58, 40.08N, W (Cook et al. 2004), generalized late Holocene climatic divisions (Tausch 1999), and fire cycle (FC) estimates for Barrett Canyon watershed (Table 3). The conditions in the periods are as follows: (A) generally dry and cool; (B) Little Ice Age, with cooler and wetter conditions; (C) warmer and coincident with Euro-American settlement (starting around 1850). likely to occur during drought years than could be attributed to chance. The influence of a single drought year on probability of fire occurrence, lacking any association with antecedent climatic conditions, agrees with studies from the Colorado pinyon juniper system (Floyd et al. 2004) but differs from other studies (Miller and Rose 1999), where antecedent moist years are hypothesized to have allowed for accumulation of fine fuel loads that increase the probability and extent of fire during the subsequent drought year. Fuel loads in Barrett Canyon may have been more shrub dominated and not as influenced by precipitation patterns as are more monsoonal systems with dominant grass cover. Standreplacing fires in pinyon juniper woodlands of the Great Basin are likely rare events that are climate driven and are not dependent on a fuel buildup model (Veblen 2003), either from one or more antecedent moist years building up fine fuel loads, or from increased fuel loading as a stand ages. Broad century-scale changes in Great Basin climate are well described in the literature (Miller and Wigand 1994; Tausch 1999). Tausch (1999) described three late-holocene divisions that roughly coincide with the temporal divisions identified by fire cycle analysis as well as reconstructed PDSI (Fig. 6). The three late-holocene periods (Tausch 1999), the reconstructed PDSI (Cook et al. 2004), and the temporal divisions identified from this study s fire cycle analysis are in rough agreement. The PDSI time series (Fig. 6) suggests that the 16th century shift toward less area burned may have been influenced by the climatic shift to moister but cooler conditions starting in the late 15th century. The lower fire cycle estimate prior to 1575 may have been influenced by factors other than climate. If the oldest trees in a stand succumb to nonfire-related mortality, the measurement of the time-since-fire is reduced, causing a downward bias in fire-cycle estimate. Fox (1989) emphasized the need to publish tree mortality rate estimates concurrently with fire-cycle estimates, as mortality from nonfire agents can randomly and unknowingly censor the distribution of tree ages. This point is not commonly considered in fire-history studies (but see Sibold et al. 2006). Many tree species conform to a U-shaped mortality rate curve, with high seedling mortality, followed by low rates of background mortality, concluding with an increased mortality rate as a population reaches a maximum age (Harcombe 1987). We currently lack information on age-dependent background mortality rate for pinyon pine. However, even a low background mortality rate may significantly bias fire-cycle estimates. Using a constant background mortality rate estimate of 0.14%/year for pinyon, as determined by Shaw et al. (2005), a tree has a 50% chance of surviving to 495 years. In stands that experienced the last stand-replacing fire event >500 years ago, the initial cohort may disappear in the ageclass sample, decreasing the time-since-fire datum for that plot and, thus, decreasing the fire-cycle estimate. Fire-atlas records for central Nevada are limited to 1970 and later and confirmed a recent absence of fire in the study area (Brown et al. 2002). The most recent fire-cycle division (i.e., a near absence of fire) may have been driven in part by climate and by land-use history. Other studies attribute the dramatic reduction in postsettlement fire frequency to landuse history but, in general, cannot decouple climate effects from possible vegetation changes. Settlement of the nearby Reese River Valley commenced in the 1870s, with presumed late-season grazing in the higher elevations. Although effective fire suppression did not occur in remote Nevada woodlands until the mid-20th century, fire exclusion from overgrazing, resulting in reductions of fine fuel mass and continuity (Savage and Swetnam 1990) is a plausible explanation for the observed reduction in fire frequency since approximately A reduction in Native American burning practices may have occurred with Euro-American settlement, although such activity in the Great Basin is poorly understood in general (Griffin 2002). Plausibility of fire exclusion as a mechanism for woodland expansion and infill Expansion and infill of pinyon juniper woodlands are commonly attributed to fire exclusion, although alternative mechanisms involving climate change, grazing effects, and increased atmospheric CO 2 concentrations have been proposed (Miller and Wigand 1994; Knapp et al. 2001). However, the fire-exclusion hypothesis has seldom been tested empirically (Baker and Shinneman 2004). Expansion of woodland into neighboring vegetation types (e.g., sagebrush grassland) due to fire exclusion would depend upon a reduction in fire frequency of those neighboring types. Data limitations did not allow us to quantify changes in fire regime of

9 Bauer and Weisberg 1597 sagebrush-dominated communities that were adjacent to woodland. The use of fire scars from proxy ecotonal trees to reconstruct historical sagebrush fire frequencies is fraught with methodological bias and uncertainty (Baker 2006). However, fires in Barrett Canyon were apparently more frequent in the sagebrush- and grassland-dominated canyon bottom. Many fire-scarred trees in the study area were adjacent to the valley floor (n = 45 of 95 total scars) or occurred along side drainages that extended upwards in elevation from the valley floor (n = 20 scars). Age-class data suggested that the younger trees below the fire-scarred trees had established after a fire event had burned through the valley floor up to the point of the fire scar. Few fires that burned woodland were reconstructed as originating in the woodland, suggesting that historical fire frequencies of adjacent vegetation types were greater than the rather low fire frequencies observed for the woodland. Although our study does not provide a critical test for the fire-exclusion hypothesis, anthropogenic fire exclusion in shrub-dominated communities at lower and possibly higher altitudinal belts presents a plausible mechanism for pinyon juniper expansion in Barrett Canyon and in other areas of the central Great Basin (Weisberg et al. 2007). Infill of pinyon juniper woodlands refers to often dramatic increases in tree density over recent decades, as has been reported for the study area (Bauer 2006) and nearby mountain ranges (Tausch et al. 1981; Weisberg et al. 2007). It is plausible that infilling processes could result from fire exclusion if historical fires were either very patchy or of low severity. However, most of the sampled stands originated following high-severity fire events, with no evidence of subsequent fires. The observed pattern of fire-scarred trees near the edges of intense burns is supported by systematic surveys of fire evidence for three recent burns in pinyon juniper woodlands (Bauer 2006), where all observed fire scars occurred at burn boundaries. At Barrett Canyon, no stands had evidence suggesting a frequent, low-severity fire regime, which is consistent with other studies showing a lack of low-severity surface fires in pinyon juniper woodlands (reviewed in Baker and Shinneman 2004; Huffman et al. 2008). Although Barrett Canyon and the Shoshone Mountain Range have experienced dramatic increases in tree density since the 1880s (Bauer 2006; Miller et al. 2008), it is unlikely that the infilling process is related to fire exclusion because there is little evidence for low-severity or extremely patchy wildfires prior to fire exclusion. Management implications Although additional fire-history studies are needed to assess the generalizability of our results to other Great Basin watersheds, the Barrett Canyon study has important management implications. Firstly, not all young pinyon juniper woodlands are expansion woodlands representing novel tree establishment in areas that were formerly dominated by other vegetation types. Several of the sampled woodland plots were relatively young stands (<120 years) that established following high-severity fire, although diligent searching was required to uncover the evidence for such previous fire events. This is an important distinction for ecological restoration activities. Secondly, as pointed out by others (Baker and Shinneman 2004; Romme et al. 2008), pinyon juniper woodlands burn infrequently, with active crown fires occurring only under extreme conditions. Some old-growth woodland plots showed no evidence of having burned at any time over the past centuries. There is little ecological justification for reintroducing fire to areas of persistent (i.e., historic) woodland, at least within the context of restoring natural disturbance regimes, because these have not experienced the effects of fire exclusion. Furthermore, because historical low-severity fires were not reconstructed and have likely played an unimportant role in these woodlands, there is little justification for introducing low-severity underburns or for emulating low-severity fire through thinning treatments. These conclusions are similar to those of researchers who have conducted fire-history studies of woodlands dominated by twoneedle pinyon pine (Pinus edulis Engelm.) of the Colorado Plateau region (Eisenhart 2004; Floyd et al. 2004), although fire appears to have played a greater role in our Great Basin study area than is generally reported for Colorado Plateau woodlands, where any direct evidence of historical fire can be difficult to find. Shrub-dominated communities adjacent to pinyon juniper woodlands, including many areas now dominated by trees, are likely to have burned more frequently in the past. Reintroduction of fire to such communities may be warranted as an ecological restoration activity. However, our investigation was not able to develop robust estimates of fire frequency for sagebrush-dominated areas. Rigorous fire-history studies are urgently needed for sagebrush communities to determine if fire exclusion has been an influential cause for tree invasion of former shrublands (Baker 2006). It is critical that management plans for restoring woodlands consider the variability in presettlement woodland structure and the extent of old-growth woodlands (Miller et al. 2008). This requires an understanding of landscape-scale influences on tree establishment, productivity gradients, and historical fire regime. 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