Forest Ecology and Management 359 (2016) 210–220
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Forest Ecology and Management
journal homepage: www.elsevier.com/locate/foreco
Quantification of Dryocoetes confusus-caused mortality in subalpine fir forests of southern British Columbia
Lorraine Maclauchlan
Ministry of Forests, Lands and Natural Resource Operations, 441 Columbia Street, Kamloops, B.C. V2C 2T3, Canada article info abstract
Article history: The western balsam bark beetle, Dryocoetes confusus Swaine (Coleoptera: Scolytinae), is considered the Received 19 May 2015 most destructive mortality agent in high elevation subalpine fir ecosystems, yet the actual impact of this Received in revised form 6 October 2015 beetle is not well quantified. Past estimates of in-stand losses from D. confusus in subalpine fir-dominated Accepted 8 October 2015 ecosystems have been unreliable due to historically limited and sporadic aerial survey coverage in British Columbia prior to the 1990’s. Tree level aerial assessment of mature subalpine fir in southern British Columbia in 1996–1997, and again in 2014, found mortality from this beetle to be as high as 70%, with Keywords: mean annual mortality rates ranging from less than one percent to 1.6% in some of the ecosystems sur- Western balsam bark beetle veyed. Over 10% of 101–120 year old stands surveyed in 1996–1997 had no visible mortality, and the High elevation ecosystems Engelmann Spruce-Subalpine Fir mean percent mortality of this youngest cohort was 11% compared to over 17% mortality in all older biogeoclimatic zone age classes. By the second survey time, all age classes saw mean mortality estimates more than double, Outbreak dynamics and all stands had some level of D. confusus attack. The highest increase in mortality was recorded in Tree level surveys stands over 250 years of age. Stands surveyed in the driest, coldest Engelmann Spruce-Subalpine Fir ecosystem in southern British Columbia sustained the highest levels of attack between survey times, averaging slightly less than 27% mortality, and by 2014 total subalpine fir mortality averaged more than 46%. The rate of mortality over the past two decades has increased. As extreme weather events become more common, particularly in wetter subalpine fir ecosystems, mortality from western balsam bark bee- tle will be amplified, causing both ecological and economic repercussions in these ecosystems. Results clearly show that subalpine fir stands over 100 years in all ecosystems sustain continuous attack from D. confusus. To minimize future losses to this bark beetle, subalpine fir stands should be managed for a rotation age less than 100 years and mixed species stands promoted where climatically feasible. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction 2002). The western balsam bark beetle has been recorded through- out the range of its primary host. Other reported hosts of D. con- Western balsam bark beetle, Dryocoetes confusus Swaine fusus include Abies amabilis (Douglas ex Louden) Douglas ex (Coleoptera: Scolytinae), is the dominant mortality agent of sub- Forbes and Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. alpine fir Abies lasiocarpa (Hook.) Nutt. in British Columbia (Bright, 1963). The beetle generally kills spatially discrete clumps (Garbutt, 1992). Although other factors such as root disease or single stems of subalpine fir at relatively low rates each year (Merler, 1997), defoliation, animal damage and climate events within infested stands, thus causing significant mortality in chron- (drought, lightning) play a role, western balsam bark beetle is ically infested stands over time (Stock, 1991; Unger and Stewart, the primary mortality factor (Maclauchlan, 2001; Maclauchlan 1992; Maclauchlan, 2001; Maclauchlan et al., 2003; Buxton et al., et al., 2003; Negrón and Popp, 2009; Buxton et al., 2012; Buxton 2012; Buxton and Maclauchlan, 2014). Although cumulative mor- and Maclauchlan, 2014). The range of subalpine fir extends tality may reach significant levels in many subalpine fir stands, throughout B.C., south to the Cascade and Olympic Mountains of western balsam bark beetle is less aggressive than other tree- Washington and Oregon, down to Arizona and New Mexico killing bark beetles such as the mountain pine beetle during epi- (Furniss and Carolin, 1977). It is found at moderate to high eleva- demics (Wood, 1982). When subalpine fir is attacked by western tions, preferring wetter sites to dry ones, and summer precipitation balsam bark beetle, tree foliage turns bright red the year following is a limiting factor for growth (Lloyd et al., 1990; Peterson et al., attack; however, D. confusus-killed trees retain their foliage and coloration longer than other tree species attacked and killed by their respective bark beetles (Stock, 1991), thus making the E-mail address: [email protected] http://dx.doi.org/10.1016/j.foreco.2015.10.013 0378-1127/Ó 2015 Elsevier B.V. All rights reserved. L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220 211 estimate of new attack and mortality more difficult over the life of 2. determine if stand age class influences attack rates of D. a stand. confusus; Western balsam bark beetle preferentially attacks the larger 3. elucidate potential differences in D. confusus attack rates among (Stock, 1991), older and slower growing subalpine fir trees in a biogeoclimatic zones in southern B.C.; and, stand, but among stands, there is a wide range of selection in terms 4. determine if D. confusus attack dynamics have changed over the of tree size (Maclauchlan, 2000, 2001; Bleiker et al., 2003; Buxton last two decades. et al., 2012). Maclauchlan (2000) showed the mean diameter of subalpine fir killed by other factors was significantly less than trees Effects of climate change on these high elevation forests, includ- killed by D. confusus and that these smaller, often suppressed ing additional stress on the host, may create more favorable attack understory trees were the first to die in a stand. This low-level conditions for the beetle, or conversely may be detrimental to the mortality creates gaps and may condition the stand to subsequent beetle’s survival. Therefore, it is important to determine a baseline attack by D. confusus. It has been hypothesized that this beetle may level of mortality in these ecosystems. be limited by the abundance and distribution of susceptible, weak- ened hosts (Bleiker et al., 2003). This in part explains the constant, scattered, yet low-level attack observed throughout the range of 2. Methods susceptible subalpine fir. Bark beetle population success, including that of western bal- Tree level aerial surveys using rotary wing aircraft were con- sam bark beetle, is influenced indirectly by the effects of climate ducted in 1996–1997 and again in 2014 to determine the relative on community associates, host vigour and abundance (Bentz frequency of live and dead subalpine fir in subalpine fir- et al., 2010). Temperature driven variation in scolytid development dominated ecosystems. During low, tree level aerial surveys, indi- is common and life cycles may be shortened or lengthened with vidual live and dead subalpine firs are easily distinguished from warmer or colder weather, respectively (Amman, 1973; Schmid other conifers due to their long, narrow crowns of stiff branches and Frye, 1977; Wermelinger and Seifert, 1999). Both western bal- (BCMFLNRO, 2014) and remain visible for many years following sam bark beetle and spruce beetle, Dendroctonus rufipennis, bark death (Fig. 1)(Maclauchlan, 2000, 2001; Maclauchlan et al., beetles common to high elevation ecosystems, have two-year life 2003). The B.C. Provincial Forest Cover Inventory was used as the cycles (Mathers, 1931; Schmid and Frye, 1977). If D. confusus could sampling base for this project because it was the most accurate complete its life cycle in one year (Bright, 1963), as does spruce delineation of the forest landscape, represented on maps by an beetle occasionally (Schmid and Frye, 1977), that coupled with assemblage of discrete stands. Each stand was identified by a stress caused by drought could greatly increase subalpine fir mor- unique mapsheet and stand number with a set of attributes that tality in the future. Drought, fire, insect and disease disturbance included but was not limited to: biogeoclimatic ecosystem classifi- and extreme weather events may occur over relatively short time cation; stand age class; tree species composition; and, area of the intervals and have important implications for forest management stand (hectares). Age class is defined as any interval into which (Williamson et al., 2009). Forest managers must recognize, under- the age range of trees, forests, stands, or forest types is divided stand, and adapt to the cumulative impacts of climate change. for classification (BCMFR, 2008b). The age classes used in this study Western balsam bark beetle is emerging as a dominating and included: age class 6 (101–120 years); age class 7 (121–140 years); enduring disturbance factor in the high elevation forests of south- age class 8 (141–250 years); and, age class 9 (251 plus years). The ern B.C. Across North America, the rise in temperatures is projected 1995 Version 1 Forest Inventory and Planning File was obtained to exceed global mean increases, particularly at high latitudes and (pers. comm. Bob MacDonald, B.C. Min. Forests, Inventory Branch) elevations, and more frequent extreme weather events are for the Kamloops Forest Region (now the Thompson Okanagan expected (IPCC, 2007). Such events may pre-condition hosts for Region) and stratified for sampling as follows: bark beetle attack, including D. confusus, creating the potential to trigger localized outbreaks. Biogeoclimatic Ecosystem Classification (BEC) zone, subzone Infestations of western balsam bark beetle in high elevation (Lloyd et al., 1990; Meidinger and Pojar, 1991; BCMFLNRO, spruce-fir forests have been reported since comprehensive forest 2014)(Appendix A); pest surveys began in British Columbia, Canada, in the 1930’s subalpine fir leading stands (>50% subalpine fir by basal area); (Garbutt, 1992). Until recently, aerial survey coverage of these for- age classes 6–9; and, ests has been limited and sporadic; therefore, reliable annual or stand area (hectares). cumulative mortality or volume loss has not been readily available. With uncertainty around British Columbia’s mid-term timber sup- Inventory data were sorted by BEC zones, subzones and age ply due to the large-scale mortality caused by the mountain pine class, and weighted according to the area (hectares) contained beetle Dendroctonus ponderosae Hopkins (Coleoptera: Scolytinae), within each grouping, called a sampling stratum. Biogeoclimatic harvesting and intensive forest management in B.C. are now focus- zone, subzone and stand age were used as the main selection cri- ing on high elevation mixed spruce-fir forests, and thus the impact teria, because BEC best describes the vegetation of mature forested of D. confusus is of concern. Harvest levels are set in B.C. based ecosystems and integrates the environmental factors affecting a upon a number of factors including inventory of mature stands, site (BCMFLNRO, 2014). Sampling was then divided according to projected growth rates and insect and disease losses to forests the relative proportional area of each stratum, and stands greater (BCMFR, 2008a). As salvage harvesting ends in pine forests than five hectares in size were selected for sampling. Stands less impacted by mountain pine beetle, there will be greater reliance than five hectares were not time or cost efficient to survey by air, on other conifer species to fill the harvest profile. In order to not and thus were not sampled. overestimate the timber supply, it is imperative to have reliable Subalpine fir were recorded as live or dead during the surveys, estimates of tree mortality in high elevation, subalpine fir- and placed into one of three categories: green (live, dominant sub- dominated ecosystems (BCMFLNRO, 2015). alpine fir with green foliage); red (dominant subalpine fir display- The objectives of this study were to: ing chlorotic to bright red foliage, indicating western balsam bark beetle attack in the previous calendar year); and, grey (dominant 1. quantify the mortality from D. confusus attack in mature sub- subalpine fir displaying dull red to no foliage, indicating the tree alpine fir-dominated forests over time; had been dead for more than two years). Knowledge of the 212 L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220
Fig. 1. Top left: observer view from a 206B helicopter. The red rectangle represents the line of sight followed by surveyors to tally red (1), grey (2) or live (3) subalpine fir. Spruce (4) or other species were not recorded. Top right: high level of subalpine fir mortality (greys and snags). Lower left: landscape view of a typical subalpine forest in southern B.C. timeline of foliage fade after attack by D. confusus allowed us to helicopter was a Bell 206B Jet Ranger, but occasionally a Bell 206 or quantify attack years depending upon the date of the survey. 406 Long Ranger was used. Subalpine fir attacked by D. confusus begins to show foliar discol- The navigator located sample stands and directed the helicopter oration starting in mid-July (July 15) the year following attack to a pre-determined survey line making any necessary changes to and by late summer the foliage is brilliant red. Using this knowl- the flight path during the survey on the air photograph. Once the edge of foliage fade, the number of years in which western balsam helicopter was positioned at the start of the survey line, the navi- bark beetle attack could have occurred between survey times was gator would say ‘‘start” or ‘‘stop” to inform the surveyors of when calculated based upon dates flown in the first and second surveys. to start and stop data collection. The helicopter maintained an ele- The 1996, 1997 and 2014 surveys began in April of each year and vation of approximately 30 m above the tree canopy and an aver- continued through September. Conifer species such as Engelmann age ground speed of 40 kilometres per hour, with variations to spruce (Picea engelmannii Parry ex Engelm.), lodgepole pine (Pinus account for wind and local terrain conditions. contorta var. latifolia Engelm.) and other species encountered in Surveyors looked down at the stand using a narrow line of sight surveys were not tallied. just outside the landing skid of the helicopter as a reference (Fig. 1) and tallied all mature, dominant canopy subalpine fir within this sight-line, by the categories described above. The number of stands 2.1. 1996–1997 survey methodology surveyed each day varied depending on the number of stands per mapsheet, length of survey line in each stand, proximity of stands Stands from the selection criteria outlined above were located to one another, fuel limits and flying conditions. The same survey- and highlighted on 1:20,000 Forest Cover Maps (BCMOF, 1995) ors were used in all survey years. and then outlined on aerial photographs, labelled with stand num- bers and arranged into photo-mosaics. As a general navigational aid, the 1:20,000 Forest Cover map grid and the general location 2.2. Survey methodology of the candidate stands were drawn on 1:100,000 NTS (National Topographic System) series topographical maps. The flight path The 2014 survey methodology was identical to that of 1996– of all survey lines were marked on the color aerial photographs 1997 except for streamlining in map construction and navigating. at the time of the survey. ArcMap version 10.1 (Environmental Systems Research Institute, Surveys were conducted using helicopters, and originated from Inc.) was used for all spatial data preparation and map production the helicopter company based nearest to sampling areas. Survey in 2014. Map bases consisted of 0.5 m resolution ortho-rectified areas were flown in a sequential pattern optimizing the number color aerial photographs, overlaid with recent forest harvesting of stands surveyed and flight time on each survey day. Most aerial activity (provided by the BCMFLNRO Forest Analysis and Inventory surveys were conducted using three people: one person in the Branch). Survey-specific information added to the maps included front left seat navigating to survey stands; and, two observers in the outlines of the forest cover stands surveyed in 1996–1997, the back on opposite sides of the helicopter recording survey data location of the original 1996–1997 aerial survey lines (which were using three handheld tally meters each. The most commonly used digitized from the original 1996–1997 survey aerial photographs), L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220 213 and the 1:20,000 British Columbia Geographic System (BCGS) map- based on the day, month and year flown in each survey time. If sur- sheet grid (Fig. 2). A single map file was output for each mapsheet veys were done prior to July 15th in a given year then only attack to be surveyed. Output consisted of a series of full-color geo- from two or more years prior would be visible. Surveys after July referenced .pdf files, clipped to each 1:20,000 mapsheet grid. 15th would capture the previous year’s attack (new foliar fade) The geo-referenced .pdf files were transferred to a Samsung plus older attacks. The number of potential attack years between Note 3 Android smartphone. Using the combination of paper maps surveys was then calculated and used to estimate the mean annual and smartphone, which was loaded with the PDF Maps application attack rate. (Avenza Systems Inc. 2014 https://play.google.com/store/apps/de- The number of red and grey trees tallied in each stand was tails?id=com.Avenza), the navigator knew the helicopter’s exact summed to determine total dead subalpine fir per stand. All three position at all times and could direct the pilot precisely along the categories were summed to get total trees per stand and the num- 1996–1997 survey lines. Printed paper versions of the maps were ber red attack, or number dead, was divided by total trees to get used for general navigation and survey route planning. percent red attack or percent dead, respectively. Red and grey cat- egories were kept separate while doing the surveys to determine 2.3. Data analysis recent activity of D. confusus within surveyed stands and to com- pare between survey times. If attack rates remained the same over Not all of the 1996–97 survey lines were re-surveyed in 2014. time, there should be no difference in red attack levels in survey Some areas had been logged or burned in wild fires, while in other time one and survey time two. No statistical analyses were per- cases, inclement weather or time constraints prevented the survey formed using the red category alone, as this number is not as pre- of some stands. Summaries were done for the complete 1996–1997 cise as the combined red plus grey category, due to variations in data set and then comparative analyses were conducted on only the timing of foliar fade (Stock, 1991; Garbutt, 1992; Harder, those stands surveyed in both survey times. The 1996–1997 data 1998; personal observations). were merged with the 2014 data, matching mapsheets and stands Analyses comparing the difference in percent mortality flown in both time periods. The number of potential attack years between the first and second survey times used only data from between the 1996, 1997 and 2014 assessments was calculated stands that were flown in both surveys. For ease of reference, the
Fig. 2. Example of map used in aerial surveys showing the outline of stands surveyed in 1997, the flight path and areas that have been harvested since the first survey time. 214 L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220 original age class was retained in both the results tables and in the harvesting in 2014. Of the remaining stands, over one quarter of text; however it is acknowledged that by 2014, all stands had aged them had less than half their total area harvested. ±18 years. To test for observer bias, the data collected by the two 9.3–50% mortality in first assessment to 17.4–44.8% in final observers in each sampling year was converted to percent mortal- assessment. ity and compared using a paired two sample for means t-test and In 1996–1997 ground checks of 132 aerially surveyed stands Pearson Correlation. (unpublished data) saw mortality from D. confusus range from zero Data analysis for comparing percent mortality at the two survey to 100%. Buxton and Maclauchlan (2014) monitored the attack times (proc GLM ANOVA done as a repeated measures analysis) dynamics of D. confusus in permanent sample plots and deter- and the difference between survey times (proc GLM ANOVA) was mined the mortality per plot attributable to D. confusus ranged generated using SAS/STAT software, Version 9.1.3 (SAS, 2004). To from 25% to 53% by 2013. D. confusus is clearly the primary mortal- determine if the rate of D. confusus attack changed in the time ity agent of subalpine fir and these studies support levels of mor- between surveys, a full ANOVA model of the change in percent tality recorded in aerial surveys. Other mortality agents included dead trees per stand between the two survey times was fitted root disease, secondary bark beetles, animal feeding, and a combi- using SAS proc GLM. The model incorporated BEC (Biogeoclimatic nation of D. confusus with other factors. zones and subzones) and age class as fixed factors and an interac- The mean percent mortality of subalpine fir nearly doubled in tion term: Change in percent dead trees per stand (% dead second the second survey time (2014), at 31.3 (0.6) (Standard Error of survey time – % dead first survey time) = BEC AgeClass BECxAge- the Mean = SEM) compared to that recorded in the first survey Class. The interaction term means were assessed using the time (1996–1997), at 16.7 (0.3) (Table 2) suggesting continuous LSmeans option in proc GLM. Means separations were performed and substantial western balsam bark beetle attack in subalpine for BEC and age class main effects, using Tukey’s Studentized Range fir stands over the time interval between surveys. Subalpine fir Test or Scheffé’s post hoc test. mortality, which included both red and grey attack in a stand, ran- ged from 0% to over 55% in the first survey, to a maximum of 70% mortality in the second survey (Table 2). 3. Results Red attack was observed throughout the survey area during both time periods, across all biogeoclimatic zones and age classes There was no significant difference in the percent mortality surveyed (Fig. 4). Of the stands surveyed in 1996–1997, 23% had recorded per stand by any two surveyors (t-test; P P 0.05) in no visible red attack compared to less than 3% having no red attack 1996, 1997 or 2014. Therefore, these data were combined as one in 2014. In both survey periods, the highest levels of red attack observation for each stand. The number of potential attack years were recorded in age class 7 stands (121 to 140 years) averaging between the two survey times ranged from 16 to 19 years. The 2.4 (0.4) percent and 7.0 (1.4) percent in 1996–1997 and 2014, majority of stands surveyed (90%) had potentially 17–18 years of respectively. Conversely, the lowest red attack levels were additional, visible mortality in the time interval between survey recorded in age class 6 stands in both survey times, with mean per- dates. cent red attack 0.9 (0.2) in 1996–1997 and 4.8 (0.9) in 2014. If The 1995 forest cover inventory (BCMOF, 1995) identified just there was no change in relative attack rates by D. confusus over over 372,000 hectares of leading subalpine fir stands 100 years or time, the mean red attack recorded in 2014 in age class 6 stands older, age class 6–9, within the Thompson Okanagan Region. This should be comparable to that recorded in age class 7 stands in represents the total sampling pool from which stands were 1996–1997. However, as noted above the mean red attack was selected, stratified by age class and BEC. Both age class and BEC 2% higher by 2014. The level of red attack was noticeably higher were sampled proportionally to their representation in the total in the second survey in all age classes (Fig. 4), suggesting both sample pool; with age class 8 (141–250 years) stands representing on-going presence and augmented activity of D. confusus in these 73% of the sample pool. In the first survey time (1996–1997), a stands. The percent red attack tallied in stands ranged from 0% to total of 807 stands were surveyed over 36 days, covering approxi- over 17% in the first survey time and from 0% to over 37% in the mately 68,448 hectares of subalpine fir forest, or 18% of the poten- second survey (Fig. 4). The presence of red attack can be used as tial sample pool (Table 1 and Fig. 3). In 2014, 503 stands were a gauge for both the vigour of the insect population and the suscep- surveyed covering just over 51,000 hectares, or 13% of the potential tibility of the host. sample pool (Table 1 and Fig. 3). Only 17 stands in the first survey had no visible mortality and Using the PDF Maps application in 2014 greatly assisted in the no stands in 2014 were without some level of mortality (Table 2). prompt and accurate location of survey lines. The mean number of Six stands surveyed had no visible red attack in either the first or stands surveyed per day increased from a mean of 22.4 stands per second survey times, although all 6 had some level of grey attack. day in 1996–1997 to almost 28 stands per day in 2014 (Table 1). Of the stands surveyed in both time periods, 55 of the 503 stands The area harvested between the two survey times accounted for (11%) saw no change in the percent of dead trees over this time less than 9% (5875 ha) of the total area surveyed in 1996–1997. interval. Four of these stands contained no visible red attack. Red Of the total 807 stands surveyed in 1996–1997, 571 had no attack levels in the other 51 stands ranged from under 1–11% in 2014, highlighting the fact that even these stands sustained contin- uous low levels of mortality between 1996 and 2014, and that D. Table 1 confusus remained active in most stands. Despite the static nature Summary of the number of days, mapsheets and stands flown in 1996, 1997 and 2014 of red attack levels in these stands, very low mortality was still and hectares surveyed. occurring. With dead trees falling in the between-survey interval, Survey N (no. Mapsheets Stands Hectares there would be little change in standing dead. This ongoing, in- year days) surveyed Total Mean per Total Mean per stand mortality caused by D. confusus reinforces past observations no.a day no. day of low level, discrete patches of subalpine fir mortality in southern 1996 25 74 3.0 498 19.9 44,280 B.C. (Garbutt, 1992). 1997 11 63 5.7 309 28.1 24,168 The ANOVA model showed that age class and BEC were both Total 36 134 3.8 807 22.4 68,448 significant at P < 0.05 and the BECxAgeClass interaction term was 2014 18 114 6.3 503 27.9 51,346 marginally significant (P = 0.054). The model showed percent mor- a Three maps were partially flown in both years. tality among age classes significant in the first survey (df =3; L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220 215
Fig. 3. Map of the Thompson Okanagan Region depicting mapsheets that were surveyed in both 1996–1997 and 2014 (shaded squares) and those surveyed only in 1996– 1997 (no shading).
Table 2 Results comparing subalpine fir mortality in the first (1996–1997) and second (2014) survey times.
Survey time N Percent subalpine fir mortality No. stands with no attack No. stands with no red attack Mean (SEM)a df Max. Min. First survey 807 16.7 (0.3)a 1289 55.7 0.0 17 187 Second survey 503 31.3 (0.6)b 70.0 0.3 0 6
a Means within columns followed by a different letter are significantly different (t-test P < 0.001).
F = 4.39; P = 0.005) and second survey time (df =3; F = 4.39; P = 0.046). The mean percent mortality caused by D. confusus within the four age classes highlighted a significant difference between age class 6 and the two oldest age categories (141 to over 251 years) in the first survey time (Table 3). Age class 6 stands had 11.3% mortality in the first survey, while age class 7–9 stands had over 17% mortality, resulting in a mean difference of approxi- mately 6% (Table 3). There was no significant difference in percent mortality between age classes 6 and 7, which could reflect the onset of stand susceptibility in these two younger age categories. Similarly, there was no significant difference in percent mortality between age classes 7 through 9 where trees are at their highest susceptibility to western balsam bark beetle. In 1996–1997, 25% of age class 6 stands had less than 5% mortality, compared to between 7% and 15% of stands in age classes 7–9 with less than 5% mortality, respectively. Almost all stands over the age of 120 years (age classes 7–9) had some level of western balsam bark beetle activity in the first survey time, with only 1.3% of age class 8 stands having no attack, the most abundant age class in the study. At the second survey time, almost a third of the subalpine fir in all age classes was dead with some of the highest mortality in age class 9 (Table 3). The data elucidate the early stages of western balsam bark bee- Fig. 4. Box plot showing percent red attack within all subalpine fir stands surveyed, tle attack in the youngest stands, with higher mortality observed as grouped by age class, in the first (1996–1997) and second (2014) survey times. stands age. This is clearly seen in the 1996–1997 data when com- Codes for age class are (as of 1995): 6 = 101–120 years; 7 = 121–140 years; 8 = 141– paring mean mortality among the age classes, and in the 2014 data, 250 years; and, 9 = 250 years and older. 216 L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220
Table 3 Summary of percent mortality caused by D. confusus, by age class, in all stands surveyed and mean annual percent mortality between 1996 and 1997 and 2014.
Age class (stand age in N (both survey First survey mean Second survey mean Mortality between surveys mean Mean annual percent mortality 1995) times) (SEM)a (SEM)a (SEM)a (SEM) 6 (101–120 years) 33 11.3 (1.7)a 29.4 (3.6)ab 18.2 (3.0)ab 1.0 (0.2) 7 (121–140 years) 31 17.1 (1.8)ab 31.6 (2.4)ab 14.6 (2.4)ab 0.8 (0.1) 8 (141–250 years) 359 17.4 (0.5)b 30.6 (0.7)a 13.2 (0.7)a 0.8 (0.04) 9 (251 + years) 80 17.4 (1.0)b 35.0 (1.3)b 17.6 (1.5)b 1.0 (0.1) df 333 F value 4.39 2.69 3.45 P 0.005 0.046 0.017
a Means within columns followed by a common letter are not significantly different (Tukey’s Studentized Range Test P > 0.05).
Table 4 Percent subalpine fir mortality in five biogeoclimatic zones (BEC). BEC zone codes: AT = Alpine Tundra; ESSF = Engelmann Spruce-Subalpine Fir; ICH = Interior Cedar-Hemlock; MS = Montane Spruce; and, SBS = Sub-Boreal Spruce.
BEC N (2014) 1996–1997% mortality 2014% mortality Mean annual percent mortality (SEM) Mean (SEM)a Max. Min. Mean (SEM)a Max. Min. AT 22 9.9 (1.5)a 26.5 0.0 26.8 (2.3)a 56.4 0.3 1.0 (0.1) ESSF 469 17.4 (0.4)b 55.7 0.0 31.5 (0.6)a 70.0 0.4 0.8 (0.04) ICH 4 11.2 (4.6)ab 30.7 0.0 20.9 (8.2)a 40.4 3.0 0.5 (0.4) MS 6 15.5 (3.9)ab 47.4 1.9 38.8 (5.3)a 54.9 25.5 1.4 (0.2) SBS 2 10.7 (0.5)ab 13.9 0.0 28.5 (15.0)a 43.6 13.5 1.1 (0.9)
a Means within columns followed by different letter are significantly different (Scheffé test P < 0.01).
Table 5 Mean percent mortality and annual mortality mean in six Engelmann Spruce-Subalpine Fir (ESSF) subzones.
ESSF subzones N Percent mortality mean (SEM)a Mean annual percent mortality (SEM)a First survey (1996–1997) Second survey (2014) Between survey times Dry, cold (dc) 47 16.6 (1.3)a 32.9 (2.1)a 16.3 (2.0)a 1.0 (0.1)a Dry, very cold (dv) 22 15.5 (2.4)a 33.7 (2.5)a 18.2 (2.8)ab 1.1 (0.2)ab Moist, warm (mw) 73 17.3 (1.2)a 26.7 (1.5)a 9.1 (1.6)a 0.7 (0.1)a Very wet, cold (vc) 29 16.9 (1.5)a 29.4 (1.8)a 12.6 (1.5)a 0.7 (0.1)a Wet, cold (wc) 258 17.3 (0.6)a 30.3 (0.6)a 13.0 (0.8)a 0.8 (0.04)a Very dry, cold (xc) 51 19.9 (1.3)a 46.6 (1.2)b 26.7 (1.8)b 1.6 (0.1)b
a Means within columns followed by a common letter are not significantly different (Scheffé test P > 0.05). by the overall increase seen during the time interval between sur- rates mid-way between the annual mortality rates calculated for veys. The data confirm that by 100 years of age, subalpine fir is sus- other zones. The sample size of zones other than the ESSF, such ceptible to western balsam bark beetle, and beyond this age, attack as the Interior Cedar Hemlock, Montane Spruce and Sub-Boreal is persistent (Table 3), with other factors such as tree size and vig- Spruce, was very small because very few stands in these ecosys- our presumably playing more important roles in individual tree tems met the survey criteria of >50% subalpine fir by basal area. susceptibility (Bleiker et al., 2005; Buxton and Maclauchlan, 2014). In southern B.C., subalpine fir is predominately found in the The mean annual mortality rates were very consistent among ESSF (Lloyd et al., 1990; Michael Ryan, personal communication). age class, averaging 1% or less for all age classes (Table 3) with Ninety-three percent of stands surveyed during both survey times the maximum annual mortality reaching 3.2%. Despite fewer age were located in ESSF subzones. In a Repeated Measures ANOVA, the class 6 and 7 stands available for surveying, Table 3 illustrates sus- factor ESSF subzone is significant (df =5;F = 9.07; P = 0.0001). Also, tained attack across all age categories in both survey times. survey time (df =1;F = 409.27; P = 0.0001), and the interaction sur- The 503 stands surveyed in both survey times were grouped by vey time x ESSF subzone is significant (df =5;F = 11.6; P = 0.0001). biogeoclimatic zone to examine potential differences in percent There was no significant difference among ESSF subzones in the mortality in the first or second survey (Table 4). All ecosystems first survey time (Table 5), but in the second survey time the sampled suffered mortality from D. confusus, and overall subalpine ESSFxc displayed significantly higher mortality, (almost 47%) than fir mortality increased from the first survey time to the second sur- any other subzone. Proc GLM looking at difference in mortality vey time in all biogeoclimatic zones (Table 4). In a Repeated Mea- between survey times showed a difference in mortality between sures ANOVA there was a significant difference in mean percent subzones (df =5; F = 11.69; P = 0.0001). The ESSFxc was signifi- mortality between the first and second survey time (df =1; F cantly different (Scheffé test P < 0.05) (Table 5) from all other sub- value = 41.3; P = 0.001) but no interaction effect of time and BEC zones except the ESSFdv. zone. The only significant difference among BEC zones was seen in the first survey (1996–1997) between the Alpine Tundra and Engelmann Spruce Subalpine Fir zones (df = 498; F = 2.390; 4. Discussion P = 0.05). There was no significant difference in mortality among zones in 2014 (Table 4). The maximum level of mortality was The data confirm that subalpine fir stands over 100 years of age recorded in ESSF stands in both survey times with annual mortality in southern B.C. sustain continuous and substantial attack by L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220 217
Appendix A Detailed descriptions of Biogeoclimatic Ecosystem Classification (BEC) zones and subzones used in this study. Adapted from Lloyd et al., 1990; Meidinger and Pojar, 1991; BCMFLNRO, 2014; and, Michael Ryan (personal communication, August 12, 2015).
Alpine Tundra (AT) Elevation (m) 2347 Location in Thompson Okanagan Region On high mountains chiefly along the Coast and Cascades mountains in the west and Columbia Mountains in the east Mean annual precipitation (mm) 756 Mean annual temperature (°C) À1.8 General description Generally treeless but at lower elevations have stunted subalpine fir, Engelmann spruce, white spruce, mountain hemlock, and whitebark pine. Alpine vegetation is dominated by herbs, bryophytes, and lichens.
Engelmann Spruce-Subalpine Fir (ESSF) Biogeoclimatic units dc dv mw vc wc xc BEC unit label Dry Cold Dry Very Cold Moist Warm Very Wet Cold Wet Cold Very Dry Cold Elevation (m) 1520–1950 1500–1900 1275–1675 1250–1800 1400–1800 1650–2060 Location in Thompson Okanagan Upper slopes of the West of the Fraser Western boundary of West side of Most extensive unit West side of the Region Okanagan River on the upper region, on lee side of Monashee in the region and Fraser River, Highlands and slopes of the Cascade Mountains Mountains occupies the upper mountain tops North Thompson Yalakom, Hurley, from upper slopes of the across River Stein and Bridge Seymour River Monashee Mountains Thompson rivers to south of and Shuswap Plateau and on Revelstoke Highlands the Graystokes Plateau east of Kelowna Mean annual precipitation (mm) 856 986 1424 1640 1307 713 Mean annual temperature (°C) 1.8 0.7 2.1 0.6 0.6 1.3 General description Wetter than the Moderately dry Wet and warm ESSF Wettest ESSF Widespread interior Driest ESSF unit. ESSFxc but drier forests dominated by forests in coast unit in the ESSF unit that is wet Stands than other ESSF a mix of subalpine fir transitional areas south central and cold. Drier than predominantly a subzones with and Engelmann dominated by mixed BC. Stands the ESSFvc and mix of subalpine subalpine fir, spruce. Lodgepole stands of subalpine composed of ESSFmw but wetter fir, Engelmann spruce forming pine and whitebark and amabilis firs and subalpine fir, than all other units. spruce and climax stands and pine common and Engelmann spruce. Engelmann Open stands lodgepole pine. lodgepolepine in sometimes abundant. One of the wettest spruce and dominated by Stand destroying successional Wetter than the ESSF units and mountain subalpine fir and fires common. stands. ESSFxc, similar to the warmer than all other hemlock. Engelmann spruce. ESSFdc, and drier units. than all other ESSF units. Stand destroying fires sometimes occur. Interior Cedar-Hemlock (ICH) Elevation (m) 314–1085 Location in Thompson Okanagan Region On north- and west-facing slopes northeast and east of Okanagan Lake; low to mid- elevations south of Shuswap Lake; and mid-slopes of North Thompson drainage Mean annual precipitation (mm) 897 Mean annual temperature (°C) 3.2 General description Has the highest diversity of tree species of any zone with the presence of cedar and generally hemlock and occasionally subalpine fir Montane Spruce (MS) Elevation (m) 1275–1530 Location in Thompson Okanagan Region West of the Fraser River; on the lee side of the Cascade Mountains from Merritt to the U.S. A. border; and the east side of the Thompson Plateau Mean annual precipitation (mm) 797 Mean annual temperature (°C) 2.2 General description Dry cold montane forests distinguished by the presence of subalpine fir and spruce often with lodgepole pine stands on seral sites and on dry sites. Colder than the IDF but warmer and more productive than the ESSF. Sub-Boreal Spruce (SBS) – very limited in the Thompson Okanagan Region Elevation (m) 1100–1300 Location in Thompson Okanagan Region Limited to the Clearwater area in the northern part of the Region Mean annual precipitation (mm) 122 Mean annual temperature (°C) 0.8 General description Transitional areas to the boreal forests of central and northern BC. Stands dominated by subalpine fir and hybrid white spruce. Lodgepole pine is common in seral stands and on dry sites. Winters are cold and summers are cool with moderate amounts of precipitation.
D. confusus until the majority of dominant canopy subalpine fir is that attack dynamics of D. confusus is changing, and levels of attack dead. Age is likely a factor in the susceptibility of trees to D. con- are increasing with our more vulnerable forest condition. In the fusus, however, the beetle was already established across all age first survey, the youngest cohort of stands, between 101 and classes at the first survey time, and by the second survey time, 120 years old, had significantly lower levels of attack than the in-stand mortality in both the youngest and oldest cohorts had older age classes, suggesting that D. confusus had not been active at least doubled. One key observation is that stands 121–140 years for long in these stands. Other studies in British Columbia show old in 1996–1997 recorded substantially less mortality than stands D. confusus attacking subalpine fir at ages younger than 100 years, approximately the same age in 2014. This supports the hypothesis with 118 years being the mean age of attacked trees (range 218 L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220
57–240 years) (L. Maclauchlan unpublished results). In the first (wet, cold) is the most predominant ESSF subzone in southern B. survey, over 30% of the youngest stands had little or no D. confusus C., and displayed intermediate levels of attack, averaging 30% sub- activity. Once western balsam bark beetle becomes active in a alpine fir mortality. The lowest level of mortality (27%) occurred in stand, beetle populations are maintained over many years and the ESSFmw (moist, warm), while the dry Engelmann Spruce- newly colonized trees are evident, until the overstory canopy is Subalpine Fir subzones, ESSFdc and ESSFdv, had intermediate depleted or understory canopies reach susceptible size (personal levels of mortality, similar to the ESSFwc. This study highlights that observations; Maclauchlan, 2000; McMillin et al., 2003). The highest drier stands are more susceptible to D. confusus and as the climate level of attack in 2014 was seen in stands over 250 years of age. changes in B.C. and summer droughts become more common, the These stands are senescent and as such, contain the oldest and ESSFwc could experience significant increases in D. confusus attack. often the largest or most stressed trees, putting them at high risk Trees growing in the driest, coldest ESSF subzones already suf- of attack to D. confusus. Compared to other conifers such as spruce, fer longer duration and more frequent climatic extremes than subalpine fir is a shorter lived species, generally dying by age those growing in the moist or wet Engelmann Spruce-Subalpine 300 years (Aplet et al., 1988). Aplet et al. (1988) also showed dead Fir subzones (Lloyd et al., 1990; Meidinger and Pojar, 1991; subalpine fir basal area peaking by age 275 and live subalpine fir Michael Ryan, personal communication). Extreme climate events basal area decreasing by almost 50% from age 175 to age 375, in southern B.C., including low winter snow packs and summer meaning most of the oldest, largest trees were dead, thus support- drought, could stress subalpine fir, thus making it increasingly sus- ing our findings. ceptible to D. confusus attack, coupled with the fact that most of the Our data describes a slow, continuous successional pattern of forests surveyed were very old and reaching senescence. Drought mortality in subalpine fir-dominated ecosystems, with western is well documented as triggering outbreaks of bark beetles and balsam bark beetle as the primary driver. Antos and Parish other insects, and climate change is likely to accelerate drought- (2002) found that the abundance of both spruce and subalpine fir induced susceptibility in many tree species (Vega and Hofstetter, over the age of 150 years declined rapidly in stands they studied 2015). in southern B.C., which would support our findings. Subalpine fir One of the pivotal factors affecting tree and stand health is often remains in the understory for long periods, up to one hun- moisture availability (Mattson and Haack, 1987; Guarín and dred years, before responding to newly created openings in the Taylor, 2005), which directly affects the fitness and survivorship canopy, typically caused by dying, mature spruce or fir (Antos of bark beetles and other insect herbivores (Price, 1997). Suscepti- et al., 2000). This and other studies (Stock, 1991; Maclauchlan bility of subalpine fir to western balsam bark beetle attack is also et al., 2003; Buxton et al., 2011; Buxton and Maclauchlan, 2014) closely associated with tree vigour (Bleiker et al., 2003, 2005) show that D. confusus is the primary disturbance event creating and slower growing, stressed trees are preferentially attacked. these canopy gaps, which allow subalpine fir to respond rapidly Resin production is a main line of defence for most conifers under and assume dominance in the canopy until years later, new stress attack by bark beetles. The induced defence response of trees to factors cause reductions in annual growth, and increased suscepti- bark beetles is compromised under stress conditions bility to attack by western balsam bark beetle. Few stands sur- (Christiansen et al., 1987; Wallin and Raffa, 2001; Bleiker et al., veyed in this study were devoid of live trees but often the 2003; Wallin et al., 2003). Therefore, subalpine fir growing in dominant canopy was severely impacted (Fig. 1) and scattered moister, milder ecosystems may be better able to produce resin sub-dominant trees were beginning to be attacked. than those growing in harsher environments (Mattson and All stands surveyed contained over 50% subalpine fir by basal Haack, 1987; Waring, 1987). Winter precipitation and late summer area, however other than stands within the Engelmann Spruce- temperatures play a role in determining radial growth of subalpine Subalpine Fir zone, the sample size was too small in other ecosys- fir (Peterson et al., 2002) and as climate fluctuates, growth may be tems to infer much from the mortality rates observed. In general impeded, favouring successful reproduction and colonization by D. ecosystems that experience drier, harsher climatic extremes in confusus. terms of moisture regimes and summer droughts had higher levels Windthrow was evident in the majority of stands surveyed, of subalpine fir mortality. Ecosystems with much moister and although it was not quantified in this study. In other studies moderated climates, often typified by slow-melting snow packs (Huggard, 1999; Buxton and Maclauchlan, 2014), the quantity that help keep moisture levels high during the summer and rate of fall down of dead subalpine fir was significant. (Meidinger and Pojar, 1991), suffer less drought-related stress Buxton and Maclauchlan (2014) monitored the attack dynamics and subsequently less D. confusus-caused mortality. The milder of D. confusus in permanent sample plots and determined the aver- and moister biogeoclimatic zones and subzones containing a age annual fall down rates for dead subalpine fir at 0.5% in the higher component of other conifer species, such as spruce, did ESSFwc and ESSFxc, with a range of 0.1–1.5% (also Buxton et al., not sustain as high levels of attack and mortality from D. confusus. 2012). The total subalpine fir mortality attributed to western bal- McMillin et al. (2003) also found a positive linear relationship sam bark beetle in these plots ranged from 25% to over 50%, com- between the mortality caused by western balsam bark beetle in parable to levels determined in this study (Buxton and north-central Wyoming and the percentage of subalpine fir trees, Maclauchlan, 2014). Huggard (1999) showed that fall down rates subalpine fir basal area and subalpine fir stand density index. of subalpine fir were related to the condition of the snag. Newer The majority of subalpine fir in southern B.C. grows in Engel- snags with few decay symptoms had a fall down rate of 0.2% annu- mann Spruce-Subalpine Fir ecosystems and western balsam bark ally, while older, more decayed snags had fall down rates upwards beetle activity was most evident and predominant in these forests. of 6% per year. Therefore, in addition to the standing trees killed by The two driest and coldest of the Engelmann Spruce-Subalpine Fir D. confusus quantified in this study, an additional number of dead subzones sustained the highest levels of attack between survey would have fallen in the time period between surveys, thus times. The harshest of these, the ESSFxc (very dry, cold) had the increasing the estimate of mortality caused by this beetle espe- highest overall mortality in 2014 (>46%) and the highest annual cially in older stands by as much as 1% annually. mortality rate (1.6%). An annual rate does not reflect the actual In conclusion, this study highlights the significant and on-going attack dynamics of the bark beetle, which is both spatially and impact caused by western balsam bark beetle in subalpine-fir temporally discrete; however, it is a very useful estimate for forest dominated forests of southern British Columbia. As subalpine fir planners when calculating stand losses for long-term production dominated stands reach 100 years, D. confusus starts to build and and allowable annual cut estimates (BCMFR, 2008a). The ESSFwc by 121 years, tree susceptibility is high, allowing populations to L. Maclauchlan / Forest Ecology and Management 359 (2016) 210–220 219 prosper and expand more rapidly. The wet, cold Engelmann British Columbia Ministry of Forests, Lands and Natural Resource Operations, 2014. Spruce-Subalpine Fir ecosystem is the most abundant subalpine Research Branch, Biogeoclimatic Ecosystem Classification Program: 2014.
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