Response of bark beetles and their natural enemies to fire and fire surrogate treatments in mixed-conifer forests in western Montana

Four treatments (control, burn-only, thin-only, and thin-and-burn) were evaluated for their effects on bark beetle-caused mortality in both the short-term (one to four years) and the long-term (seven years) in mixed-conifer forests in western Montana, USA. In addition to assessing bark beetle responses to these treatments, we also measured natural enemy landing rates and resin flow of ponderosa pine (Pinus ponderosa) the season fire treatments were implemented. All bark beetles were present at low population levels (non-outbreak) for the duration of the study. Post-treatment mortality of trees due to bark beetles was lowest in the thin-only and control units and highest in the units receiving burns. Three tree-killing bark beetle species responded positively to fire treatments: Douglas-fir beetle (Dendroctonus pseudotsugae), pine engraver (Ips pini), and western pine beetle (Dendroctonus brevicomis). Red turpentine beetle (Dendroctonus valens) responded positively to fire treatments, but never caused mortality. Three fire damage variables tested (height of crown scorch, percent circumference of the tree bole scorched, or degree of ground char) were significant factors in predicting beetle attack on trees. Douglas-fir beetle and pine engraver responded rapidly to increased availability of resources (fire-damaged trees); however, successful attacks dropped rapidly once these resources were depleted. Movement to green trees by pine engraver was not observed in plots receiving fire treatments, or in thinned plots where slash supported substantial reproduction by this beetle. The fourth tree-killing beetle present at the site, the mountain pine beetle, did not exhibit responses to any treatment. Natural enemies generally arrived at trees the same time as host bark beetles. However, the landing rates of only one, Medetera spp., was affected by treatment. This predator responded positively to thinning treatments. This insect was present in very high numbers indicating a regulatory effect on beetles, at least in the short-term, in thinned stands. Resin flow decreased from June to August. However, resin flow was significantly higher in trees in August than in June in fire treatments. Increased flow in burned trees later in the season did not affect beetle attack success. Overall, responses by beetles to treatments were short-term and limited to fire-damaged trees. Expansions into green trees did not occur. This lack of spread was likely due to a combination of high tree vigor in residual stands and low background populations of bark beetles.     

Effects of prescribed fire and season of burn on direct and indirect levels of tree mortality in Ponderosa and Jeffrey Pine Forests in California,USA

Many forests that historically experienced frequent low-intensity wildfires have undergone extensive alterations during the past century. Prescribed fire is now commonly used to restore these fire-adapted forest ecosystems. In this study, we examined the influence of prescribed burn season on levels of tree mortality attributed to prescribed fire effects (direct mortality) and bark beetles (Coleoptera: Curculionidae, Scolytinae) (indirect mortality) in ponderosa pine, Pinusponderosa Dougl. ex Laws., and Jeffrey pine, Pinusjeffreyi Grev. and Balf., forests in California, USA. A total of 816 trees (9.9% of all trees) died during this 3-yr study. Significantly higher levels of tree mortality (all sources) occurred following early and late season burns compared to the untreated control, but no significant difference was observed between burn treatments. The majority (461 trees) of tree deaths were attributed to direct mortality from prescribed burns and was strongly concentrated (391 trees) in the smallest diameter class (<20.2 cm diameter at breast height, dbh). For the largest trees (>50.7 cm dbh), significantly higher levels of tree mortality occurred on early season burns than the untreated control, most of which resulted from indirect mortality attributed to bark beetle attacks, specifically western pine beetle, Dendroctonus brevicomis LeConte, and mountain pine beetle, D. ponderosae Hopkins. Red turpentine beetle, D. valens LeConte, was the most common bark beetle species found colonizing trees, but tree mortality was not attributed to this species. A total of 355 trees (4.3% of all trees) were killed by bark beetles. Dendroctonus brevicomis (67 trees, 18.9%) and D. ponderosae (56 trees, 15.8%), were found colonizing P. ponderosa; and Jeffrey pine beetle, D. jeffreyi Hopkins, was found colonizing P. jeffreyi (seven trees, 2.0%). We also found pine engraver, Ips pini (Say) (137 trees, 38.6%), and, to a much lesser extent, Orthotomicus (=Ips) latidens (LeConte) (85 trees, 23.9%) and emarginate ips, I. emarginatus (LeConte) (3 trees, 0.8%) colonizing P. ponderosa and P. jeffreyi. Few meaningful differences in levels of indirect tree mortality attributed to bark beetle attack were observed between early and late season burns. The incidence of root and root collar pathogens (Leptographium and Sporothrix spp.), including species known to be vectored by bark beetles, was low (18% of trees sampled). The implications of these and other results to management of P. ponderosa and P. jeffreyi forests are discussed in detail.     

Bark beetle-caused mortality in a drought-affected ponderosa pine landscape in Arizona,USA

Extensive ponderosa pine (Pinus ponderosa Dougl. ex Laws.) mortality associated with a widespread severe drought and increased bark beetle (Coleoptera: Curculionidae, Scolytinae) populations occurred in Arizona from 2001 to 2004. A complex of Ips beetles including: the Arizona fivespined ips, Ips lecontei Swaine, the pine engraver beetle, Ips pini (Say), Ips calligraphus (Germar), Ips latidens (LeConte), Ips knausi Swaine and Ips integer (Eichhoff) were the primary bark beetle species associated with ponderosa pine mortality. In this study we examine stand conditions and physiographic factors associated with bark beetle-caused tree mortality in ponderosa pine forests across five National Forests in Arizona. A total of 633 fixed-radius plots were established across five National Forests in Arizona: Apache-Sitgreaves, Coconino, Kaibab, Prescott, and Tonto. Prior to the bark beetle outbreak, plots with mortality had higher tree and stocking compared with plots without pine mortality. Logistic regression modeling found that probability of ponderosa pine mortality caused by bark beetles was positively correlated with tree density and inversely related with elevation and tree diameter. Given the large geographical extent of this study resulting logistic models to estimate the likelihood of bark beetle attack should have wide applicability across similar ponderosa pine forests across the Southwest. This is particularly true of a model driven by tree density and elevation constructed by combining all forests. Tree mortality resulted in significant reductions in basal area, tree density, stand density index, and mean tree diameter for ponderosa pine and for all species combined in these forests. Most of the observed pine mortality was in the 10–35 cm diameter class, which comprise much of the increase in tree density over the past century as a result of fire suppression and grazing practices. Ecological implications of tree mortality are discussed.     

Bark Beetle Demography in a Jeffrey Pine Stand as Influenced by Mechanized Thinning and Prescribed Fire

Forest thinnings implemented with cut-to-length and whole-tree harvesting systems followed by underburning were evaluated for their effects on bark beetle prevalence in pure, uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) interspersed with isolated California white fir (Abies concolor var. lowiana [Gord.] Lemm.). Based on pitch tube counts in a stand with a moderate bark beetle population in its pine component, the Jeffrey pine beetle (Dendroctonus jeffreyi Hopkins) generally preferred larger trees before treatment implementation, but after exhibiting mixed pretreatment tendencies concerning stand density demonstrated a posttreatment proclivity toward higher density. Cut-to-length thinning followed by underburning increased the pine beetle population while whole-tree thinning unaccompanied by burning reduced it. Tree mortality was induced by the bark beetle infestation but was not its sole cause. Pitch tube abundance on white fir far exceeded that on Jeffrey pine, and the greatest influence on the fir engraver (Scolytus ventralis LeConte) population was the prevalence of its host tree. The responses presented herein to these thinning and burning practices, which are being increasingly utilized in forest restoration efforts in the western USA, provide natural resource managers insight into potential forest health outcomes when implemented in Jeffrey pine and similar dry site forest types.     

Risk rating for mountain pine beetle infestation of lodgepole pine forests over large areas with ordinal regression modelling

The mountain pine beetle Dendroctonus ponderosae Hopkins is endemic to lodgepole pine, Pinus contorta var. latifolia Engelmann, forests in western Canada. However, the current beetle epidemic in this area highlights the challenges faced by forest managers tasked with prioritizing stands for mitigation activities such as salvage harvesting and direct control methods. In western Canada, the operational risk rating system for mountain pine beetle is based on biological knowledge gained from a rich legacy of stand-scale field studies. Due to the large spatial (millions of hectares affected) and temporal (over 10 years) extents of the current epidemic, new research into large-area mountain pine beetle processes has revealed further insights into the landscape-scale characteristics of beetle infested forests. In this paper, we evaluated the potential for this new knowledge to augment an established system for rating the short-term risk of tree mortality in a stand due to mountain pine beetle. New variables explored for utility in risk rating include direct shortwave radiation, site index, diameter at breast height, the temporal trends in local beetle populations, Biogeoclimatic Ecosystem Classification and beetle–host interaction variables. Proportional odds ordinal regression was used to develop a model for the Vanderhoof Forest District in west-central British Columbia. Prediction on independent data was assessed with the area under the receiver operator curve (AUC), indicating good discriminatory power (AUC = 0.84) for predicting levels of mountain pine beetle-caused pine mortality.     

The effects of mechanical fuel reduction treatments on the activity of bark beetles (Coleoptera: Scolytidae) infesting ponderosa pine

Selective logging, fire suppression, forest succession and climatic changes have resulted in high fire hazards over large areas of the western USA. Federal and state hazardous fuel reduction programs have increased accordingly to reduce the risk, extent and severity of these events, particularly in the wildland–urban interface. In this study, we examined the effects of mechanical fuel reduction treatments on the activity of bark beetles in ponderosa pine, Pinus ponderosa Dougl ex. Laws., forests located in Arizona and California, USA. Treatments were applied in both late spring (April–May) and late summer (August–September) and included: (1) thinned biomass chipped and randomly dispersed within each 0.4 ha plot; (2) thinned biomass chipped, randomly dispersed within each plot and raked 2 m from the base of residual trees; (3) thinned biomass lopped-and-scattered (thinned trees cut into 1–2 m lengths) within each plot; (4) an untreated control. The mean percentage of residual trees attacked by bark beetles ranged from 2.0% (untreated control) to 30.2% (plots thinned in spring with all biomass chipped). A three-fold increase in the percentage of trees attacked by bark beetles was observed in chipped versus lopped-and-scattered plots. Bark beetle colonization of residual trees was higher during spring treatments, which corresponded with peak adult beetle flight periods as measured by funnel trap captures. Raking chips away from the base of residual trees did not significantly affect attack rates. Several bark beetle species were present including the roundheaded pine beetle, Dendroctonus adjunctus Blandford (AZ), western pine beetle, D. brevicomis LeConte (AZ and CA), mountain pine beetle, D. ponderosae Hopkins (CA), red turpentine beetle, D. valens LeConte (AZ and CA), Arizona fivespined ips, Ips lecontei Swaine (AZ), California fivespined ips, I. paraconfusus Lanier (CA) and pine engraver, I. pini (Say) (AZ). Dendroctonus valens was the most common bark beetle infesting residual trees. A significant correlation was found between the number of trees chipped per plot and the percentage of residual trees with D. valens attacks. A significantly higher percentage of residual trees was attacked by D. brevicomis in plots that were chipped in spring compared to the untreated control. In lopped-and-scattered treatments, engraver beetles produced substantial broods in logging debris, but few attacks were observed on standing trees. At present, no significant difference in tree mortality exists among treatments. A few trees appeared to have died solely from D. valens attacks, as no other scolytids were observed in the upper bole. In a laboratory study conducted to provide an explanation for the bark beetle responses observed in this study, monoterpene elution rates from chip piles declined sharply over time, but were relatively constant in lopped-and-piled treatments. The quantities of β-pinene, 3-carene, -pinene and myrcene eluting from chips exceeded those from lopped-and-piled slash during each of 15 sample periods. These laboratory results may, in part, explain the bark beetle response observed in chipping treatments. The implications of these results to sustainable forest management are discussed.     

Multi-scale nest-site selection by black-backed woodpeckers in outbreaks of mountain pine beetles

Areas of mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks in the Black Hills can provide habitat for black-backed woodpeckers (Picoides arcticus), a U.S. Forest Service, Region 2 Sensitive Species. These outbreaks are managed through removal of trees infested with mountain pine beetles to control mountain pine beetle populations and salvage timber resources. To minimize impacts to black-backed woodpeckers while meeting management objectives, there is a need to identify characteristics of these areas that support black-backed woodpeckers. We examined the habitat associations of this species nesting in areas of beetle outbreaks in the Black Hills, South Dakota in 2004 and 2005. We used an information theoretic approach and discrete choice models to evaluate nest-site selection of 42 woodpecker nests at 3 spatial scales—territory, nest area, and nest tree. At the territory scale (250 m around nest), availability and distribution of food best explained black-backed woodpecker selection of beetle outbreaks versus the surrounding forest. Selection at the territory scale was positively associated with densities of trees currently infested by mountain pine beetles and indices of wood borer (Cerambycidae and Buprestidae) abundance, and was greatest at distances of 50–100 m from the nearest patch of infestation. At the nest-area scale (12.5 m radius around the nest), densities of snags positively influenced nest-area selection. Finally, at the nest-tree scale, aspen (Populus tremuloides) and 3–5-year-old ponderosa pine (Pinus ponderosa) snags were important resources. The association between abundant wood-boring insects and black-backed woodpeckers creates a difficult challenge for forest managers. In the absence of fire, areas of beetle outbreak might serve as the only substantial source of habitat in the Black Hills. Regulating insect populations via salvage logging will reduce key food resources to black-backed woodpeckers during nesting. Therefore, given the relatively infrequent occurrence of large-scale fire in the Black Hills, management should recognize the importance of beetle-killed forests to the long-term viability of the black-backed woodpecker population in the Black Hills.     

Susceptibility of ponderosa pine, Pinus ponderosa (Dougl. ex Laws.), to mountain pine beetle, Dendroctonus ponderosae Hopkins,attack in uneven-aged stands in the Black Hills of South Dakota and Wyoming USA

Mountain pine beetle, Dendroctonus ponderosae Hopkins can cause extensive tree mortality in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests in the Black Hills of South Dakota and Wyoming. Most studies that have examined stand susceptibility to mountain pine beetle have been conducted in even-aged stands. Land managers increasingly practice uneven-aged management. We established 84 clusters of four plots, one where bark beetle-caused mortality was present and three uninfested plots. For all plot trees we recorded species, tree diameter, and crown position and for ponderosa pine whether they were killed or infested by mountain pine beetle. Elevation, slope, and aspect were also recorded. We used classification trees to model the likelihood of bark beetle attack based on plot and site variables. The probability of individual tree attack within the infested plots was estimated using logistic regression. Basal area of ponderosa pine in trees ≥25.4 cm in diameter at breast height (dbh) and ponderosa pine stand density index were correlated with mountain pine beetle attack. Regression trees and linear regression indicated that the amount of observed tree mortality was associated with initial ponderosa pine basal area and ponderosa pine stand density index. Infested stands had higher total and ponderosa pine basal area, total and ponderosa pine stand density index, and ponderosa pine basal area in trees ≥25.4 cm dbh. The probability of individual tree attack within infested plots was positively correlated with tree diameter with ponderosa pine stand density index modifying the relationship. A tree of a given size was more likely to be attacked in a denser stand. We conclude that stands with higher ponderosa pine basal area in trees >25.4 cm and ponderosa pine stand density index are correlated with an increased likelihood of mountain pine beetle bark beetle attack. Information form this study will help forest managers in the identification of uneven-aged stands with a higher likelihood of bark beetle attack and expected levels of tree mortality.     

Nitrogen cycling following mountain pine beetle disturbance in lodgepole pine forests of Greater Yellowstone

Widespread bark beetle outbreaks are currently affecting multiple conifer forest types throughout western North America, yet many ecosystem-level consequences of this disturbance are poorly understood. We quantified the effect of mountain pine beetle (Dendroctonus ponderosae) outbreak on nitrogen (N) cycling through litter, soil, and vegetation in lodgepole pine (Pinus contorta var. latifolia) forests of the Greater Yellowstone Ecosystem (WY, USA) across a 0-30 year chronosequence of time-since-beetle disturbance. Recent (1-4 years) bark beetle disturbance increased total litter depth and N concentration in needle litter relative to undisturbed stands, and soils in recently disturbed stands were cooler with greater rates of net N mineralization and nitrification than undisturbed sites. Thirty years after beetle outbreak, needle litter N concentration remained elevated; however total litter N concentration, total litter mass, and soil N pools and fluxes were not different from undisturbed stands. Canopy N pool size declined 58% in recent outbreaks, and remained 48% lower than undisturbed in 30-year old outbreaks. Foliar N concentrations in unattacked lodgepole pine trees and an understory sedge were positively correlated with net N mineralization in soils across the chronosequence. Bark beetle disturbance altered N cycling through the litter, soil, and vegetation of lodgepole pine forests, but changes in soil N cycling were less severe than those observed following stand replacing fire. Several lines of evidence suggest the potential for N leaching is low following bark beetle disturbance in lodgepole pine.     

Tree regeneration and future stand development after bark beetle infestation and harvesting in Colorado lodgepole pine stands

In the southern Rocky Mountains, current mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks and associated harvesting have set millions of hectares of lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) forest onto new stand development trajectories. Information about immediate, post-disturbance tree regeneration will provide insight on dynamics of future stand composition and structure. We compared tree regeneration in eight paired harvested and untreated lodgepole pine stands in the Fraser Experimental Forest that experienced more than 70% overstory mortality due to beetles. New seedlings colonized both harvested and untreated stands in the first years after the beetle outbreak. In harvested areas the density of new seedlings, predominantly lodgepole pine and aspen, was four times higher than in untreated stands. Annual height growth of pine and fir advance regeneration (e.g., trees established prior to the onset of the outbreak) has doubled following overstory mortality in untreated stands. Growth simulations based on our regeneration data suggest that stand basal area and stem density will return to pre-beetle levels in untreated and harvested stands within 80-105 years. Furthermore, lodgepole pine will remain the dominant species in harvested stands over the next century, but subalpine fir will become the most abundant species in untreated areas. Owing to terrain, economic and administrative limitations, active management will treat a small fraction (<15%) of the forests killed by pine beetle. Our findings suggest that the long-term consequences of the outbreak will be most dramatic in untreated forests where the shift in tree species composition will influence timber and water production, wildfire behavior, wildlife habitat and other forest attributes.     

Management strategies for a large-scale mountain pine beetle outbreak: Modelling impacts on American martens

We assessed the effectiveness of alternative forest management strategies for maintaining American martens (Martes americana) in a sub-boreal landscape subject to an extensive mountain pine beetle (Dendroctonus ponderosae) outbreak, associated salvage logging, and climate change in north-central British Columbia, Canada. We structured the analysis in a Bayesian network (BN) meta-model that incorporated the results of spatially explicit modelling of landscape conditions (natural and logging disturbance, habitat quality, number of potential territories, and connectedness of territories) with analytical population modelling. The BN meta-model was then used to examine habitat and population size responses (adult females only are presented) to management scenarios, in the context of uncertainty of model parameters, management objectives, and climate change. Status quo management is dominantly clear-cutting with 3–20% of each harvest unit retained as mature patches, with reforestation by planting in the remainder. Management options we examined were: (1) the status quo, (2) varying the total annual timber harvest on the landscape (100%, 80% or 50% of current long-term sustained yield estimates), (3) the protection of understory trees during logging, and (4) 30–70% retention of overstory (partial cutting in distinct patches <1 ha in size) in each harvest unit, for 33% or 50% of the annual timber harvest. We found that marten habitat and population size declined substantively with the beetle outbreak and associated salvage cutting. The choice of management strategy then had a long-term effect on the potential for marten recovery after the beetle outbreak. Partial cutting scenarios had the greatest average long-term marten population levels, followed in order by reduced harvest rates, understory protection, and the status quo. Management scenarios with the best chance of meeting conservation goals without over-protecting habitat (and thus unnecessarily constraining timber management) varied with the population objective chosen. The choice of management strategy will depend on the weighting of marten outcomes against the economic desirability of timber harvest strategies, willingness to gamble on climate change, and the time-frame of interest.     

Nutritional and pathogenic fungi associated with the pine engraver beetle trigger comparable defenses in Scots pine

Conifer bark beetles are often associated with fungal complexes whose components have different ecological roles. Some associated species are nutritionally obligate fungi, serving as nourishment to the larvae, whereas others are pathogenic blue-stain fungi known to be involved in the interaction with host defenses. In this study we characterized the local and systemic defense responses of Scots pine (Pinus sylvestris L.) against Ophiostoma brunneo-ciliatum Math. (a blue-stain pathogen) and Hyalorhinocladiella macrospora (Franke-Grosm.) Harr. (a nutritional fungus). These fungi are the principal associates of the pine engraver beetle, Ips acuminatus (Gyll.). Host responses were studied following inoculation with the fungi, singly and as a fungal complex, and by identifying and quantifying terpenoids, phenolic compounds and lignin. Although the length of the necrotic lesions differed between control (wound) and fungal treatments, only two compounds (pinosylvin monomethyl ether and (+)-α-pinene) were significantly affected by the presence of the fungi, indicating that Scots pine has a generic, rather than specific, induced response. The fact that both nutritional and blue-stain fungi triggered comparable induced defense responses suggests that even a non-pathogenic fungus may participate in exhausting host plant defenses, indirectly assisting in the beetle establishment process. Our findings contribute to the further development of current theory on the role of associated fungal complexes in bark beetle ecology.     

Monitoring the impacts of mountain pine beetle mitigation

Since 1999, the mountain pine beetle (Dendroctonus ponderosae Hopk. [Coleoptera: Scolytidae]) has impacted over 13 million hectares of pine forests in western Canada. Successful mitigation of the beetle depends on the accurate and timely identification of currently infested trees and on sustained control activities over several successive years. We monitored the success of mitigation activities in reducing damage caused by mountain pine beetle at two sites (A and B) on the leading edge of the current beetle epidemic in western Canada. Using three years of digital high spatial resolution aerial imagery (2006–2008) and one season of field measurements (2008), we estimated retrospective ratios of trees attacked by beetle in the current year (green attack) to trees attacked in the previous year (red attack), hereafter referred to as G:R. Our results indicate that mitigation activities slowed the rate of population growth, with G:R found to be decreasing or stable over sites A and B while mitigation was ongoing in 2005 and 2006 (site A 1.06:1; site B 0.32:1). When mitigation was discontinued over site A in 2007, the G:R increased markedly (1.94:1), while continued mitigation at site B in 2007 further reduced the G:R (0.22:1). Despite the cost associated with mitigation, its efficacy is rarely assessed and even more rarely documented. The approach presented herein enables a sample-based appraisal of mitigation efforts.     

The influence of mountain pine beetle outbreaks and drought on severe wildfires in northwestern Colorado and southern Wyoming: A look at the past century

Outbreaks of bark beetles and drought both lead to concerns about increased fire risk, but the relative importance of these two factors is the subject of much debate. We examined how mountain pine beetle (MPB) outbreaks and drought have contributed to the fire regime of lodgepole pine forests in northwestern Colorado and adjacent areas of southern Wyoming over the past century. We used dendroecological methods to reconstruct the pre-fire history of MPB outbreaks in twenty lodgepole pine stands that had burned between 1939 and 2006 and in 20 nearby lodgepole pine stands that were otherwise similar but that had not burned. Our data represent c. 80% of all large fires that had occurred in lodgepole pine forests in this study area over the past century. We also compared Palmer Drought Severity Index (PDSI) and actual evapotranspiration (AET) values between fire years and non-fire years. Burned stands were no more likely to have been affected by outbreak prior to fires than were nearby unburned stands. However, PDSI and AET values were both lower during fire years than during non-fire years. This work indicates that climate has been more important than outbreaks to the fire regime of lodgepole pine forests in this region over the past century. Indeed, we found no detectable increase in the occurrence of high-severity fires following MPB outbreaks. Dry conditions, rather than changes in fuels associated with outbreaks, appear to be most limiting to the occurrence of severe fires in these forests.     

Determining the vulnerability of Mexican pine forests to bark beetles of the genus Dendroctonus Erichson (Coleoptera: Curculionidae: Scolytinae)

Bark beetles of the genus Dendroctonus are natural inhabitants of forests; under particular conditions some species of this genus can cause large-scale tree mortality. However, only in recent decades has priority been given to the comprehensive study of these insects in México. México possesses high ecological diversity in Dendroctonus–Pinus associations. The geographic coexistence of 12 Dendroctonus species suggests greater vulnerability or threat of tree mortality relative to other areas. We use a biogeographic strategy to identify and rank the areas most vulnerable to tree mortality caused by bark beetles in México. We aim to define the areas that might experience high impact by these insects and also to provide a geographic database useful to forest resource management and conservation policies in México. Using collection records of bark beetles and pines, we develop a quantitative estimate of the threat of beetle infestation of forest areas based on factors including pine and beetle species density, host preference and level of mortality caused by beetle species. A quantitative estimate of forest area vulnerability, the Bark Beetle Threat Index (BBTI) was calculated. Despite the vast area of geographic coincidence of Pinus and Dendroctonus in México, the regions of highest bark beetle pressure are restricted to small zones within some mountain systems. The region that has been most affected by this insect group during the past hundred years is the Transverse Volcanic Belt, followed by the Sierra Madre Occidental and Sierra Madre del Sur. Pine diversity is the major determining factor of BBTI at the regional level, while disturbances from extensive logging and ecosystem change are the key factors behind high BBTIs at the local level.     

Bark beetle (Coleoptera: Scolytidae) diversity in spaced and unmanaged mature lodgepole pine (Pinaceae) in southeastern British Columbia

Variation in the number and diversity of bark beetles in spaced mature lodgepole pine stands in the East Kootenay region of British Columbia was analyzed in relation to location (site), spacing treatment and years following treatment. We analyzed the number of bark beetles and the number of bark beetle species that emerged from stumps or were captured in flight traps in the first five years following spacing. We also investigated the incidence of bark beetle attacks on the remaining trees and the mean dates of emergence from stumps and of capture in flight traps for the common species. Observations were made on three sites, each having three treatments: 4 m × 4 m spacing, 5 m × 5 m spacing, and an untreated control. The mean density of bark beetles emerged from stumps was different among sites and years but not between spacing treatments. There was no statistically significant variation in the number of bark beetle species captured in flight traps by site, spacing treatment, years, or spacing treatment and years. Significantly more bark beetles were captured in the 4 m × 4 m spacing treatment than in the control. The number of bark beetles captured was the highest in the first 2 years following treatment. Up to 26 species of bark beetles, excluding ambrosia beetles, were captured in flight barrier traps. There was no difference in species diversity by site or treatment indicating that species diversity in mature lodgepole pine is relatively stable over large areas. Of the 213 trees that sustained at least 10 attacks by bark beetles on the lower 2 m of the bole, 59.1% occurred in the spaced plots but only 18.2% of those were successful, versus 74.7% success in the infested trees in the control plots. The majority of infested trees contained Ips sp., Dendroctonus valens and D. murrayanae. Of the seven trees attacked by mountain pine beetle (D. ponderosae) only one tree was located in a spaced plot.     

Stand characteristics and downed woody debris accumulations associated with a mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak in Colorado

Lodgepole pine (Pinus contorta Dougl. ex Loud.)-dominated ecosystems in north-central Colorado are undergoing rapid and drastic changes associated with overstory tree mortality from a current mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak. To characterize stand characteristics and downed woody debris loads during the first 7 years of the outbreak, 221 plots (0.02 ha) were randomly established in infested and uninfested stands distributed across the Arapaho National Forest, Colorado. Mountain pine beetle initially attacked stands with higher lodgepole pine basal area, and lower density and basal area of Engelmann spruce (Picea engelmannii [Parry]), and subalpine fir (Abies lasiocarpa (Hook.) Nutt. var. lasiocarpa) compared to uninfested plots. Mountain pine beetle-affected stands had reduced total and lodgepole pine stocking and quadratic mean diameter. The density and basal area of live overstory lodgepole declined by 62% and 71% in infested plots, respectively. The mean diameter of live lodgepole pine was 53% lower than pre-outbreak in infested plots. Downed woody debris loads did not differ between uninfested plots and plots currently infested at the time of sampling to 3 or 4–7 years after initial infestation, but the projected downed coarse wood accumulations when 80% of the mountain pine beetle-killed trees fall indicated a fourfold increase. Depth of the litter layer and maximum height of grass and herbaceous vegetation were greater 4–7 years after initial infestation compared to uninfested plots, though understory plant percent cover was not different. Seedling and sapling density of all species combined was higher in uninfested plots but there was no difference between infested and uninfested plots for lodgepole pine alone. For trees ≥2.5 cm in diameter at breast height, the density of live lodgepole pine trees in mountain pine beetle-affected stands was higher than Engelmann spruce, subalpine fir, and aspen, (Populus tremuloides Michx.), in diameter classes comprised of trees from 2.5 cm to 30 cm in diameter, suggesting that lodgepole pine will remain as a dominant overstory tree after the bark beetle outbreak.     

Predicting mortality for five California conifers following wildfire

Fire injury was characterized and survival monitored for 5677 trees >25 cm DBH from five wildfires in California that occurred between 2000 and 2004. Logistic regression models for predicting the probability of mortality 5-years after fire were developed for incense cedar (Calocedrus decurrens (Torr.) Florin), white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), sugar pine (Pinus lambertiana Douglas), Jeffrey pine (P. jeffreyi Balf.), and ponderosa pine (P. ponderosa C. Lawson). Differences in crown injury variables were also compared for Jeffrey and ponderosa pine. Most mortality (70–88% depending on species) occurred within 2 years post-wildfire and had stabilized by year 3. Crown length and crown volume injury variables predicted tree mortality equally well; however, the variables were not interchangeable. Crown injury and cambium kill rating was significant in predicting mortality in all models. DBH was only a significant predictor of mortality for white fir and the combined ponderosa and Jeffrey pine models developed from the McNally Fire; these models all predicted increasing mortality with increasing tree size. Red turpentine beetle (Dendroctonus valens) was a significant predictor variable for sugar pine, ponderosa pine, and Jeffrey pine; ambrosia beetle (Trypodendron and Gnathotrichus spp.) was a significant predictor variable for white fir. The mortality models and post-fire tree survival characteristics provide improved prediction of 5-year post-wildfire tree mortality for several California conifers. The models confirm the overall importance of crown injury in predicting post-fire mortality compared to other injury variables for all species. Additional variables such as cambium kill, bark beetles, and tree size improved model accuracies, but likely not enough to justify the added expense of data collection.     

Influence of fire and mountain pine beetle on the dynamics of lodgepole pine stands in British Columbia,Canada

An outbreak of the mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB), currently affecting over 10.1 million hectares of lodgepole pine forests (Pinus contorta Dougl.) in British Columbia, Canada, is the largest in recorded history. We examined the dynamics of even-aged lodgepole pine forests in southern British Columbia, which were undergoing this MPB outbreak. Using dendroecology and forest measurements we reconstructed the stand processes of stand initiation, stand disturbances, tree mortality, and regeneration, and explained the current stand structure and the potential MPB impacts in selected stands. Our results indicate that stand-replacing fires initiated even-aged seral lodgepole pine stands in this region. In the absence of fire in the 20th century, multiple MPB disturbances, which each resulted in partial canopy removal, modified the simple one-layer structure of the fire-origin stands by the initiation of post-MPB disturbance regeneration layers, transforming the stands into complex, multi-aged stands. Despite high overstory mortality due to the current MPB outbreak, regeneration layers, which are likely to survive the current outbreak, will provide important ecological legacies and will contribute to mid-term timber supply.     

Prescribed fire effects on bark beetle activity and tree mortality in southwestern ponderosa pine forests

Prescribed fire is an important tool in the management of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests, yet effects on bark beetle (Coleoptera: Curculionidae, Scolytinae) activity and tree mortality are poorly understood in the southwestern U.S. We compared bark beetle attacks and tree mortality between paired prescribed-burned and unburned stands at each of four sites in Arizona and New Mexico for three growing seasons after burning (2004–2006). Prescribed burns increased bark beetle attacks on ponderosa pine over the first three post-fire years from 1.5 to 13% of all trees, increased successful, lethal attacks on ponderosa pine from 0.4 to 7.6%, increased mortality of ponderosa pine from all causes from 0.6 to 8.4%, and increased mortality of all tree species with diameter at breast height >13 cm from 0.6 to 9.6%. On a per year basis, prescribed burns increased ponderosa pine mortality from 0.2% per year in unburned stands to 2.8% per year in burned stands. Mortality of ponderosa pine 3 years after burning was best described by a logistic regression model with total crown damage (crown scorch + crown consumption) and bark beetle attack rating (no, partial, or mass attack by bark beetles) as independent variables. Attacks by Dendroctonus spp. did not differ significantly over bole heights, whereas attacks by Ips spp. were greater on the upper bole compared with the lower bole. Three previously published logistic regression models of tree mortality, developed from fires in 1995–1996 in northern Arizona, were moderately successful in predicting broad patterns of tree mortality in our data. The influence of bark beetle attack rating on tree mortality was stronger for our data than for data from the 1995–1996 fires. Our results highlight canopy damage from fire as a strong and consistent predictor of post-fire mortality of ponderosa pine, and bark beetle attacks and bole char rating as less consistent predictors because of temporal variability in their relationship to mortality. The small increase in tree mortality and bark beetle attacks caused by prescribed burning should be acceptable to many forest managers and the public given the resulting reduction in surface fuel and risk of severe wildfire.