MIXED-SEVERITY FIRE REGIMES IN THE DRY FORESTS OF BRITISH COLUMBIA: HISTORICAL RECONSTRUCTIONS USING TREE-RING EVIDENCE 2012 REPORT

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1 MIXED-SEVERITY FIRE REGIMES IN THE DRY FORESTS OF BRITISH COLUMBIA: HISTORICAL RECONSTRUCTIONS USING TREE-RING EVIDENCE 2012 REPORT LORI D. DANIELS 1 AND ZE'EV GEDALOF 2 WITH CONTRIBUTIONS FROM JED COCHRANE 1,3, ERIC DA SILVA 2, HÉLÈNE MARCOUX 1, GREGORY GREENE 1, JOHN NESBITT 1, RICK KUBIAN 4, ROBERT GRAY 5 1 Tree-Ring Lab at UBC, Departments of Geography and Forest Sciences, University of British Columbia, Vancouver, BC 2 Climate and Ecosystem Dynamics Research Laboratory, Department of Geography, University of Guelph, Guelph ON 3 Parks Canada Agency, Calgary, AB 4 Parks Canada Agency, Kootenay, Yoho, Lake Louise National Parks, Radium Hot Springs, BC 5 RW Gray Consulting, Inc., Chilliwack, BC

2 MIXED-SEVERITY FIRE REGIMES IN THE DRY FORESTS OF BRITISH COLUMBIA: HISTORICAL RECONSTRUCTIONS USING TREE-RING EVIDENCE INTRODUCTION In the forests western North America, wildfire is a primary disturbance driving the diversity, age structure, and population dynamics of plant communities. In some forests, climatic warming combined with the exclusion of natural fires has created conditions conducive to catastrophic wildfires as vegetation has become increasingly drier and fuels have accumulated. Understanding the relationships between wildfire and environmental change is a fundamental concern, particularly in forests near the wildland-urban interface where human lives and property are at risk. This report summarizes research on fire history of dry forests in British Columbia conducted by Ze'ev Gedalof's research group in the Climate and Ecosystem Dynamics Research Laboratory at the University of Guelph and the Lori Daniels' research group in the Tree-Ring Lab at the University of British Columbia. Using fire-scarred trees as evidence of low-to-moderate-severity fires, we have developed crossdated, annually-resolved fire records to quantify fire return intervals at 92 sites. We have documented stand-replacing fires at another 41 sites based on age cohorts determined from stand age-structure analyses from high-quality increment cores. Combined, these fire records indicate mixedseverity fire regimes historically dominated the dry forests of British Columbia. All study areas indicate low-to-moderate severity fires were common historically but have been effectively eliminated during the 20 th century, with potentially important implications for fire and fuels management, sustainable forest management and timber supply, and conservation of biodiversity. RESEARCH APPROACH We have used two approaches for sampling fire history. In four case studies, the 22 sample sites were selected to address specific management questions or objectives. In four landscape studies that collectively include 111 sites or plots, study sites were selected using a stratified-random research design so that they are statistically representative of the landscape and environmental gradients within those landscapes. For case studies 1 and 2, mixed-conifer stands with old-growth forest structures were sampled (Daniels et al. 2007, 2011; Gray and Daniels 2007). Case study 3 included the Rocky Mountain Airport and McLeary Park in Cranbrook; two sites for which fuels mitigation and ecological restoration treatments have been implemented (Gray et al. 2009). At each site included these three case studies, we searched 20 to 50 hectares of forest to identify trees, snags and stumps with multiple, external basal scars. We collected nine to 30 partial or full cross-sections per site for a total of c.600 fire-scarred samples. Case study 4 was conducted in the Cariboo forest region (Daniels and Watson 2003). In this study, forests with old-growth structures were sampled but the search for fire scars was constrained to 1- hectare areas. We also used tree-rings to reconstruct forest dynamics at these sites. 2

3 The four landscape-scale studies were designed to be statistically representative of historic fire regimes of the montane forests in the East Kootenays (MSdk subzone, Cochrane 2007), Joseph and Gold Creek watersheds near Cranbrook (DaSilva 2009, Marcoux in progress), the Darkwoods property owned by the Nature Conservancy of Canada (Greene 2011) and montane forests in the West Kootenays near Nelson (ICH and ESSF zones, Nesbitt 2010). In these studies, forest vegetation inventory data and digital elevation models were analysed in a geographic information system (GIS) to stratify the study areas according to biophysical attributes including elevation, slope aspect, biogeoclimatic units, Historical Natural Fire Regime classes, and forest composition, structure and age. Potential study sites were selected using project-specific objective criteria and a random subset of sites were selected for sampling and the location of study plots was determined using the GIS to avoid biased location of plots in the field. In each of these studies, a 1-hectare plot was established at each site and systematically searched for fire-scarred trees, snags and stumps. We sampled up to 10 cross-sections or partial crosssections from each site. For sites that did not include fire scars, we extracted increment cores from large canopy-dominant trees to represent stand age structure. For even-aged stands, the age of the oldest tree estimated the time since last fire. METHODS In the field, fire scars were differentiated from scars caused by mountain pine beetle or other disturbance agents based on scar morphology, presence of charcoal and fungal stains in the wood (McBride 1983, Dietrich and Swetnam 1984). Basal scars that were triangular in shape, with all bark missing from the face of the scar were considered fire scars. Often these trees had multiple scars and charcoal was present. The wood samples were examined to determine if they were stained by fungi. Red stain generally indicates fire, whereas blue stain indicates disturbance by mountain pine beetle. In this study we report only scar dates caused by fire; however some trees had been disturbed or killed by mountain pine beetle as indicated by the blue stain in the wood closest to the bark. All samples were dried and prepared for analysis using a planer, belt sander and/or palm sander with successively finer sand paper to 600 grit (Stokes and Smiley 1968). Disks from live trees were visually crossdated by matching the negative marker rings from our site- and regional-scale speciesspecific chronologies. For all dead trees and c. 70% of living trees that had missing rings, the ring-width series were measured and statistically crossdated as follows. The rings along one radius of each disk were measured to the nearest 0.01 mm using a Velmex bench interfaced with a computer or using WinDendro digital imaging software. To ensure that calendar years were accurately assigned to each ring, the resulting ring-width series were statistically crossdated using the program COFECHA (Grissino Mayer 2001a). COFECHA compares each tree-ring series against all other series in each site to identify errors in tree-ring dates due to false and missing rings. In the dry forests of British Columbia, mid- to late-growing season drought may cause false rings and, during extremely dry conditions or during multiple-year droughts, rings may be incomplete or missing from the radius. The ring-width series of samples from dead trees and stumps were crossdated against the standard chronologies from live trees to determine the calendar year of the outer-most ring on the disc and estimate the year of death. Once accurate calendar years were assigned to individual rings, the date associated with each fire scar was determined. 3

4 We used the annually-resolved fire-scar dates to quantify the intervals between fires. At the site level, composite fire intervals were compiled using the computer program FHX2 (Grissino-Mayer 2001b). Fire intervals were analysed using all fire scars and were computed for scar-to-scar dates; the interval between stand initiation and the first fire scar and the interval between the last fire scar to the present were excluded. For each site, we analysed the full fire record and calculated the minimum, maximum and mean or Weibull median interval (WMI) from fire interval distributions. WMI is a measure of central tendency of the Weibull distribution in which half of the fire intervals in the modelled frequency distribution are longer than the median and half are shorter than the media. RESULTS Case Study 1 Old-Growth Forests in the East Kootenays The 100 fire scar samples from the 10 old growth study sites in the East Kootenays yielded 296 fire scars during 100 fire years between 1540 and 1973 (Table 1, Figure 1; Daniels et al. 2007, 2011). The period of analysis and number of scars varied among individual sites. Fire scar records were shortest for the three northern sites, Spring Creek, Bittern Lake and Jubilee Mountain. They ranged from 177 to 279 years and included 14 to 27 fire scars, which represented four to seven fire intervals at each site. At the other seven sites, the fire records ranged from 400 to 557 years and included 18 to 47 fire scars and eight to 23 fire intervals per site. The WMI ranged from 10.3 to 25.6 years at each site. Two to 123 years separated successive fires within sites. With the exception of the late 20th century fire-free period, the longest intervals between fires occurred early in each record. For the majority of fire years (79 of 100 years), scars were recorded at only one site and the majority of fires scarred only one tree at one site. Regional fires, fires that formed scars at 2 sites indicating synchronous fires in the landscape, burned in 21 years from 1652 to In eight of these regional fire years, multiple trees were scarred at multiple sites: 1665, 1701, 1720, 1808, 1831, and These dates represent the most severe, regional fire years, likely in years when climate was suitable for fire and ignition sources were active. The fire-free interval, measured as the number of years since the last fire (time since fire) ranged from 32 to 121 years at individual sites. At all sites, time since fire exceeded the median fire interval and it exceeded the maximum interval in the historic record at seven of 10 sites. 4

5 Table 1. Fire history summary statistics for 92 study sites reported in four case studies and four landscape studies conducted in the dry, mixed-conifer forests of the East and West Kootenays and Cariboo, British Columbia. WMI is the Weibull median interval; values in italics are means. TSLF is time since last fire in years, calculated relative to the year the samples were collected; bold values exceed WMI; bold values in italics exceed the maximum fire scar interval for the site. Site Recording Number of Fire Intervals Period Samples/Scars N Range WMI TSLF Case Study 1 Old-Growth Forests in the East Kootenays Spring Creek / Bittern Lake / Jubilee Mountain / Fenwick Creek / Jack Creek / Bootleg Creek / Dublin Creek / Palmer Bar / Etna Creek / Joseph Creek / Case Study 2 Joseph Creek West and East Joseph Creek West / Joseph Creek East / Case Study 3 McLeary Park and Rocky Mountain Airport, Cranbrook McLeary Park / Rocky Mtn Airport / Case Study 4 Mixed Douglas-fir and Lodgepole Pine Forests in the Cariboo Plot / Plot / Plot / Plot / Plot / Plot / Plot / Plot / Plot / Landscape Study 1: Montane Spruce Zone of the East Kootenays / / / / / / / / / NA / NA / / / / / NA / NA / /

6 Table 1 continued. Site Recording Number of Fire Intervals Period Samples/Scars N Range WMI TSLF Landscape Study 2: Joseph and Gold Creek Watersheds, East Kootenays NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA Landscape Study 3: The Darkwoods Property, Nature Conservancy of Canada / / / / / / / / / / 1 0 NA NA / / / / / / / NA / / / / / / / /

7 Figure 1. Historical fires from 1500 to 2005 at 10 old-growth forest study sites in the East Kootenays. The map indicates the location of the 10 sites (stars) relative to the distribution of the Montane Spruce biogeoclimatic zone. In the fire chart, sites are arranged according to location from north (top) to south (bottom) in the study area. Horizontal lines represent the composite fire chronology for each site. Triangles mark the year of fires that scarred 1 tree at each site. Combined, there were 100 fire years (all fires, open triangles); 28 were major fires that scarred >2 trees and >10% of recorder trees (all fires, solid triangles). Regional fires scarred 2 trees at 2 sites. Case Study 2 Joseph Creek West and East At the Joseph Creek West site, 13 fire-scarred trees were successfully crossdated, including five lodgepole pine and eight western larch (Table 1, Figure 2; Gray and Daniels 2007). The period for fire analysis was from 1449 through 2006 and included 18 fire years between 1540 and The WMI was 17.2 years; the minimum and maximum intervals between successive fire scars were 3 and 93 years. Fire years in which multiple trees scarred were 1677, 1701, 1721, 1831, 1869, 1905 and The last fire scar was formed in 1910; so it has been 97 years since the last fire. The 1910 fire scarred three of six trees that were living at that time At the Joseph Creek East site, 15 fire-scarred trees were successfully crossdated, including one lodgepole pine, four Douglas-fir, three western larch and seven ponderosa pine (Figure 2, Table 1; Gray and Daniels 2007). The period for fire analysis was 1302 through 2006 and included 27 fire years between 1323 and The WMI was 20 years; the minimum and maximum intervals were 3 and 69 years. Fire years in which multiple trees scarred were 1522, 1688, 1721, 1739, 1831, 1869, 1898, and 1910; in 1721, 1831, 1869 and 1910 fires also burned at the Joseph Creek West site. The last fire scar was formed in 1936, so that it had been 71 years since the last fire when we sampled the study site in

8 Figure 2. Chronology of fire dates by sample and for the entire sample site for Joseph Creek West (top) and Joseph Creek East (bottom). 8

9 Case Study 3 McLeary Park and Rocky Mountain Airport, Cranbrook Thirty cross-sections were sampled from dead trees containing fire scars within McLeary Park (Gray et al. 2009). Twenty nine of these disks (97%) were successfully crossdated, including 21 western larch and eight ponderosa pine. They contained 123 fire scars and their outermost ring dates ranged between 1809 and 1938 (Table 1, Figure 3). The period for fire analysis was 1533 through 1938 and included 33 fire years between 1621 and The WMI was 7.7 years. The minimum interval between fires was three years: 1626 to 1629, 1634 to 1637, and 1645 to The maximum interval between fires was 18 years from 1881 to While other fires may have burned without scarring trees, the last fire recorded by samples in McLeary Park was in Of 18 crossdated disks with outer-ring dates following the 1899 fire, five contained external charcoal. Eighteen trees with fire scars were sampled from the Rocky Mountain Airport site (Gray et al. 2009). Fourteen of these cross-sectional disks (78%) were successfully crossdated, including two western larch, eight ponderosa pine, and four Douglas-fir. They contained 46 fire scars (Table 1, Figure 4). Outermost ring dates of sampled trees ranged from 1889 to The period for fire analysis was 1519 through 2006 and included 27 fire years between 1534 and The WMI was 12.1 years. The minimum interval between fires was two years from 1857 to The maximum interval between fires was 35 years from 1722 to The last fire recorded by samples from Rocky Mountain Airport was in Of 12 crossdated disks with outer-ring dates following the 1923 fire, only two contained external charcoal. Figure 3. Fire history of McLeary Park, Cranbrook. Each horizontal line represents one tree. The bottom line is the composite for all samples combined. 9

10 Figure 4. Fire history for the forest surrounding Rocky Mountain Airport near Cranbrook. Each horizontal line represents one tree. The bottom line is the composite for all samples combined. Case Study 4 Mixed Douglas-fir and Lodgepole Pine Forests in the Cariboo The 139 fire scar samples from nine study plots in the mixed-conifer forests of the Cariboo region yielded 289 fire scars (Table 1, Figure 5; Daniels and Watson 2003). Between 1701 and 1995, local, major fires (fires that scarred 2 trees) burned in 28 years and regional fires (major fires that scarred trees at 2 plots) burned in 11 years. Fire intervals were similar among the study plots with 2 to 78 years separated successive fires in each plot. WMI ranged from 9.9 to 19.1 years among plots. At plot 7, two intervals averaged 35.7 years between fires. Time since fire ranged from 25 to 81 years. At all plots, it exceeded the median fire interval and it exceeded the maximum interval in the historic record at seven of nine plots. 10

11 Figure 5. The nine study plots (triangles) are in the Cariboo Region of central British Columbia (left). Fire occurrence from 1700 to 1995 at plots 1 to 9 (right). Horizontal lines represent the lifespan of recorder trees in each plot. Triangles mark local, major fires that scarred 2 trees in each plot. Combined there were 28 fire years, 11 of which were regional fires that scarred trees at 2 plots (vertical dashed lines). Landscape Study 1: Montane Spruce Zone of the East Kootenays External fire scars were present on trees at 18 of the 20 randomly selected study sites (Cochrane 2007). The forest at two sites was structurally complex and old, but there was no direct evidence of fire. For the remaining 18 study sites, the 149 fire scar samples yielded 272 fire scars between 1509 and 2003 (Table 1, Figure 6). At the plot scale, the period of analysis and number of scars varied among individual sites. Fire records ranged from 229 to 620 years and included five to 10 fire scarred trees and five to 33 fire scars per plot. The WMI between fires ranged from 25.3 to 77.5 years; it did not differ significantly between plots on south- versus north-facing slopes. WMI could not be calculated for four plots in which only one or two fires had resulted in scars. Five to 137 years separated successive fires at each site. For the majority of fire years (60 of 76 years), scars were recorded at only one site. Notable years in the fire record occurred in years when fires scarred trees at more than one site. The highest scar frequencies were in 1706, 1718, 1831, 1847, 1886, 1888 and 1889, when three or more sites burned. Fire-free intervals ranged from 3 to 147 years and exceeded the maximum historical interval at 9 of 18 sites. 11

12 Figure 6. Fire-scar record from 1500 to 2003 for 18 plots in the montane spruce forests of the East Kooetnays. Horizontal lines show the time span of each plot-level composite fire chronology and triangles indicate a fire scar (white = one tree scarred and black = 2 trees scarred). Vertical dashed lines indicate fire years when 3 plots burned. 12

13 Landscape Study 2: Joseph and Gold Creek Watersheds, East Kootenays In the East Kootenays, Da Silva (2010) conducted a stratified-random sample of 33 sites in the Joseph and Gold Creek watersheds. Sites were stratified along an elevational gradient that spanned the Interior Douglas-fir, Montane Spruce and Englemann Spruce Subalpine fir biogeoclimatic zones and two Historical Natural Fire Regime classes. Fire scars were observed at 16 of 33 sites, which ranged in elevation from 1149 to 1877 m.a.s.l., with most below 1550 m.a.s.l. (Table 1). The fire record for all sites combined included 96 fire scars and 34 fire years between 1484 and 1953 (Figures 8 and 9). Widespread fires scarred trees at many sites in 11 years, including The "Great Burn" of 1910 burned >1.2 million ha in Idaho, Montana and BC and was extensive in the study area, affecting the forests at all elevations, scarring 60 of 70 recorder trees, and killing even fire-resistant tree species. Using a combination of fire scars (Da Silva 2010) and stand history reconstructions from tree rings, Marcoux (in progress) classified 11 of 20 sites in the study area as having a mixed-severity fire regime ( m.a.s.l.) and nine sites as high-severity ( m.a.s.l.). Fire return intervals and time since last fire varied between mixed- and high-severity sites. Across all mixed-severity sites, sitelevel mean fire intervals were 7 to 139 years, with most intervals <56 years. Time since last fire was 56 to 104 years, which exceeded mean intervals at all sites and the historic maximum interval at half of the sites. The 1910 fire was the last to burn 6 of the 11 sites. At the high-severity sites, fire intervals could not be calculated due to the absence of fire scars. However, time since last fire ranged from 99 to 369 years, with 5 of 9 sites exceeding 150 years. Figure 7. East Kootenay study area southeast of Cranbrook, BC, stratified by fire frequency according to the Historic Natural Fire Regime (HNFR) classification. 13

14 Figure 8. Fire record ( ) for individual fire-scarred trees in the East Kootenay study area. Horizontal lines represent individual trees, with fire scars marked by bold vertical dashes. Dotted/solid segments are years before/after the initial fire scar; after the initial scar trees are more susceptible and likely to record subsequent fires. Vertical/slanted lines on the left end of each horizontal line are pith/inner ring dates. Vertical/slanted lines on the right end of each horizontal line are bark/outer ring dates. The bottom row is the composite fire record in which vertical lines shows years in which >10% of recording samples were scarred and scars appeared in more than one plot. 14

15 Figure 9. Composite fire records ( ) for 16 sites with fire scars in the East Kootenay study area. Horizontal lines represent individual sites, with fire scars marked by white (single trees) and black ( 2 trees) triangles. The bottom row is the composite fire record in which red triangles and grey vertical lines show years in which >10% of recording samples were scarred and scars appeared in more than one plot. Landscape Study 3: The Darkwoods Property, Nature Conservancy of Canada In the southeast portion of the Darkwoods, near Creston, Greene (2010) established 40 sample sites in a semi-systematic grid that ranged from 590 to 1690 m.a.s.l. (Figure 10). Sites were located in the Interior Cedar Hemlock and Englemann Spruce Subalpine Fir biogeoclimatic zones. Low- and moderate-severity fires resulted in fire scars at 25 sites. The 114 fire scar samples yielded 277 fire scars between 1439 and 1966 (Figure 11, Table 1). Site-level fire records ranged from 123 to 561 years and included one to seven fire-scarred trees and one to 18 fire scars per plot. Sample sizes were most reliable after 1690; therefore, fire regime statistics were calculated for the period from 1690 to The WMI was calculated for nine sites with 3 fire intervals and ranged from 18.7 to 32.8 years; averages for the other sites ranged from 18 to 67 years. Medium sized, low- to moderate-severity fires (>10% scarred) occurred in four separate years: 1703, 1795, 1866 and Large fires (>25% scarred), occurred in five separate years: 1718, 1768, 1823, 1831 and The largest, most widespread fires (>50% scarred) occurred in 1739, 1869 and Fire free intervals ranged from 44 to 141 years and exceeded the maximum historical interval at 19 of the 25 sites. At 11 of the 15 plots that lacked fire scars, trees established in an even-aged cohorts between the early-1870s and mid-1880s. Compared with dates from adjacent fire-scar samples, this implies the last stand-replacing, high-severity fire to reach the higher elevations was likely the widespread 1869 fire. Of the remaining four plots, located in the far northwest portion of the study area, one established in the 1520s, two in the 1640s and one in the mid-1930s. 15

16 Figure 10. Fire history of the southeast part of the Darkwoods Property near Creston BC was assessed using a systematically placed grid and sampling 40 plots. Figure 11. Fire history for 25 sample plots with fire scars in southeast part of the Darkwoods Property. Each horizontal line represents a plot and vertical dashes represent fire years. Broken lines indicate the period when trees were present but had not yet been scarred by fire; solid lines indicate the recording period after the first scar formed. Plots are arranged by elevation, from lowest (bottom) to highest (top). The bottom row is the composite showing all fire years in the record as vertical lines. 16

17 Landscape Study 4: West Kootenay Study Area near Nelson, BC In the West Kootenay study area, fire scars were observed at 11 sites, which ranged in elevation from 610 to 1350 m.a.s.l. (Nesbitt 2010, Figure 12, Table 1). The composite fire record for all sites combined included 67 fire scars and 19 fire years between 1679 and Seven sites were classified as low-to-moderate severity (fire scars present but no distinct cohorts) and four as mixed severity (fire scars and cohorts present). In individual stands, low-to-moderate severity fires burned at intervals ranging from four to 46 years with medians of 25 to 38 years. Five sites were classified as high severity with cohorts that established between 1850 and 1893 and included no fire scars. At two sites, the oldest trees were 212 and 338 years old and there was no evidence of past fire (no fire scars or cohorts). Time since last fire at the other sites ranged from 77 to 159 years. Figure 12. West Kootenay study area surrounding Nelson, BC, stratified by fire frequency according to the Historic Natural Fire Regime (HNFR) classification (top). Fire scar record for nine sites and a composite record for all sites in the West Kootenay study area (right). 17

18 Table 2. Summary of historical fires documented at 18 sites in the West Kootenay study area. Site Shore of lake Elevation (m.a.s.l.) Severity of historic fires Low-to-moderate severity fires (years) High severity fires (years) Time since last high-severity fire (years) Time since last fire (years) 1-1 North 611 Mixed North 737 Low-moderate 1679, 1706, 1720, , 1806, 1832, 1869, North 807 Low-moderate South 910 Low-moderate North 1061 Low-moderate South 1107 Mixed Present North 955 Mixed North 1061 Low-moderate 1834, South 1136 Mixed 1862, 1923, 1927, North 1161 Low-moderate Present South 1347 Low-moderate South 1494 High South 1572 No evidence North 1573 High North 1601 High South 1666 High South 1692 High North 1725 No evidence Scars present but not sampled due to advanced decay or wildlife danger tree status. 2 Oldest tree age indicates minimum time since last high-severity fire in stands with no scars or cohorts. Climate Variation and Historic Fires in the East Kootenays Variation in regional climate significant influenced historic fire occurrence in the East Kootenay study areas (Da Silva 2010; Daniels et al. 2007, 2011). Historic fires showed a strong statistical relationship with drought during the year of the fire, but not with preceding years. Fires burned more often than would be expected by chance during strongly positive (warm phase) years of the Pacific Decadal Oscillation (PDO). Although Daniels et al. (2007, 2011) found that interactions with El Niño- Southern Oscillation (ENSO) and Atlantic Multidecadal Oscillation (AMO) were significant drivers of drought and fire, Da Silva (2010) showed no significant relationship between historic fire activity and ENSO, AMO, or any combination of the climate indices. These contrasting results highlight the need for improved knowledge of climate-fire interactions and the variation in these relations among regions in British Columbia. 18

19 DISCUSSION In the mixed-conifer forests of the Kootenays and Cariboo, low- to moderate-severity fires were an integral component of historic fire regimes. At all study sites included in the four case studies (n = 22) and 70 of 111 sites in the landscape studies fire-scarred trees were documented, indicating fires had burned but did not kill all trees. Median or mean fire return intervals at the site level ranged from 7.7 to 77.5 years, with most between 20 and 45 years. Many fire-scar records exceeded 300-years long, providing strong evidence of multiple centuries of low-severity fires. The landscape studies consistently showed that fire regimes varied significantly with steep elevational gradients, with low-severity fires commonly burning at mid-elevations on hillslopes. For example, fire scars were common in Montane Spruce and Interior Cedar Hemlock forests up to 1350 in the West Kootenays, 1550 m.a.s.l. in the East Kootenays, and 1690 m.a.s.l. in the Darkwoods Property. As well, these low-severity fires were frequent in the past, with site-level mean or median return intervals of 25 to 50 years. These long records with short fire return intervals show a systematic error in the current natural disturbance type classification system, which incorrectly assumes severe standreplacing fires dominate at mid-elevations in mountain forests. This new knowledge of fire regimes provides an ecological precedent for uneven-aged silviculture, alternative harvesting, and regeneration practices that maintain diverse forest age and size structures. The effects of fire exclusion in the past half century are evident in all fire history reconstructions in all study areas. The lack of fire scars during the latter 20th century, despite the fire records having the highest number of samples and potential to record fires, demonstrates the strong human effects of fire exclusion and suppression. Historically, the low-severity fires would have killed the smaller thin-barked trees and consumed fine fuels such as needles and branches on the ground. In absence of low-severity fires, dense forests have been perpetuated and ground fuels have accumulated. Paradoxically, by protecting the forests from low-severity fires, we have likely increased the risk of severe wildfires in the low- and mid-elevation forests. We are concerned that these changes may have shifted the landscape towards a higher-severity fire regime, increasing the threat of wildfire to the surrounding communities and reducing resilience of the forest to the effects of climate change. IMPLICATIONS FOR FOREST MANAGEMENT Our research results have important implications for (a) sustainable forest management, (b) implementation of fire suppression, fuels mitigation and ecological restoration, and (c) anticipating the effects of global warming on fire regimes. Specifically, current forest classification systems consistently underestimate the abundance of low-severity fire and overestimate high-severity fires in low-elevation forests. This systematic error is of serious concern since this misconception underlies policies that encourage even-aged forest management and fire suppression. Over time, the net effects of such policies and practices are to simplify and homogenize the forest. The new knowledge on fire regimes provided by our research provides an ecological precedent for uneven-aged silviculture, alternative harvesting, and regeneration practices that maintain diverse forest age and size structures in managed forests. 19

20 Our research strongly supports initiatives by municipal and provincial government agencies to address fire hazards in the wildland-urban interface in the Kootenays and Cariboo regions. Our fire-scar evidence, combined with forest dynamics reconstructions (Marcoux, in progress) and recent fuels surveys (pers. comm. B.A. Blackwell), indicate that fire suppression has negative consequences in some forests. Our results support ongoing programs to mitigate fuels and restore fire through thinning and prescribed burns in many low- and mid-elevation forests. This approach is particularly prudent in the wildland-urban interface where damage by wildfire could be tremendously costly, as seen in Kelowna and Barrier, British Columbia in 2003 and Slave Lake, Alberta in We strongly recommend that treated areas be monitored to ensure that forest resistance to severe fire and resilience to climate change improves following mitigation treatments. Finally, understanding the range and effects of past climate variation helps us anticipate the potential effects of future climate change. Preliminary research in the East Kootenays indicates that historic fires were most likely to burn during single-year droughts in the warm, dry phase of the Pacific Decadal Oscillation. These results suggest that subtle changes in climate that increase drought may increase fire frequency. As well, in low-elevation forests, there may also be a shift towards higherseverity fires given current forest composition and densities that are perpetuated by fire suppression. CONCLUSIONS AND NEXT STEPS In the dry forests of BC, climate change is superimposed on a century of landscape-scale human impacts that have simplified and homogenized forests, reducing their resilience. The network of firescar records from the dry forests of British Columbia described in this report is a unique dataset that we will use to conduct inter-regional comparisons. The high resolution of the crossdated fire scars permits analyses at temporal scales of years to decades to centuries. In the next phase of this research, our research teams will collaborate to (a) identify years of widespread fire among regions, determine their historic frequency and the climatic conditions driving them, (b) differentiating climatic variation versus human impacts on historic fire regimes at stand-to-landscape spatial scales, and (c) set criteria for identifying altered landscapes and prioritizing management to increase forest resistance to severe fire and improve resilience to climate change. Based on our comparative analyses among regions, we will rank the forests in terms of their vulnerability to severe fire. Our analyses will also provide improved knowledge of the past occurrence of widespread fire years and the climatic conditions in which they occurred. Placed in context of modeled climate forecasts, we can estimate probabilities of future severe or widespread fires and prepare management plans for a range of scenarios. This would reduce uncertainty and the surprise of future extreme events. ACKNOWLEDGEMENTS We thank the British Columbia Ministry of Natural Resource Operations for providing funding for this compilation of research on historical fires in British Columbia. 20

21 LITERATURE CITED Cochrane, J.D Characteristics of Historical Forest Fires in Complex Mixed-Conifer Forests of Southeastern British Columbia. M.Sc. Thesis, Department of Geography, University of British Columbia, Vancouver, BC, Canada. Da Silva. E Wildfire History and Its Relationship with Top-down and Bottom-up Controls in the Joseph and Gold Creek Watersheds, Kootenay Mountains, British Columbia. M.Sc. Thesis, Department of Geography, University of Guelph, Guelph, ON, Canada. Daniels, L.D., J.D. Cochrane and R.W. Gray Mixed-severity Fire Regimes: Regional Analysis of the Impacts of Climate on Fire Frequency in the Rocky Mountain Forest District. Report to Tembec Inc., BC Division, Canadian Forest Products Ltd., Radium Hot Springs, and the Forest Investment Account of British Columbia. March p. Daniels, L.D., T.B. Maertens, A.B. Stan, S.P.J. McCloskey, J.D. Cochrane and R.W. Gray Direct and indirect impacts of climate change on forests: three case studies from British Columbia. Canadian Journal of Plant Pathology 33: Daniels, L.D. and E. Watson Climate-Fire-Vegetation Interactions in Cariboo Forests: a Dendroclimatic Analysis. Report to Forest Innovation and Investment, Forest Research Program, Vancouver, BC. April Dietrich, J.H. and T.W. Swetnam Dendrochronology of a fire scarred ponderosa pine. Forest Science 30: Gray, R.W and L.D. Daniels An Investigation of Fire History in the Lower Gold/Joseph Creek Watershed. Report to the City of Cranbrook, British Columbia. August p. Gray, R.W., J. Nesbitt and L.D. Daniels An Investigation of Fire History and Forest Dynamics and Their Impact on Wildfire Hazard in McLeary Park, Cranbrook. Report to the City of Cranbrook, British Columbia. August Greene, G.A Historical Fire Regime of the Darkwoods:Quantifying the Past to Plan for the Future M.Sc. Thesis, Department of Geography, University of British Columbia, Vancouver, BC, Canada. Grissino-Mayer, H. D. (2001a). Evaluating crossdating accuracy: A manual and tutorial for the computer program COFECHA. Tree-Ring Research, 57(2): Grissino-Mayer, H.D. (2001b). FHX2 Software for analyzing temporal and spatial patterns in fire regimes from tree rings. Tree-Ring Research, 57: McBride, J.R Analysis of tree rings and fire scars to establish fire history. Tree-Ring Bulletin 43: Nesbitt, J Quantifying Forest Fire Variability Using Tree Rings Nelson, British Columbia 1700 Present. M.Sc. Thesis, Department of Geography, University of British Columbia, Vancouver, BC, Canada. Stokes, M.A. and T.L. Smiley An Introduction to Tree-Ring Dating. University of Chicago Press, Chicago, IL. 21