Spruce Beetle Outbreak Research Articles

Post-fire forest dynamics and climate variability affect spatial and temporal properties of spruce beetle outbreaks on a Sky Island mountain range

The spruce beetle (Dendroctonus rufipennis) is known for extensive outbreaks resulting in high spruce mortality, but several recent outbreaks in the western United States have been among the largest and most severe in the documentary record. In the Pinaleño Mountains of southeast Arizona, U.S.A., an outbreak in the mid-1990s resulted in 85% mortality of Engelmann spruce >7cm diameter. To put this outbreak into historical perspective, we used dendrochronology to examine the effects of host species’ distribution and growth rates on spruce beetle outbreak initiation, frequency and size over three centuries. We used multiproxy records of understory spruce growth release, snag death dates, failed attack scars, and historical records to identify 12 distinct outbreak events over a 319-year period of reconstruction. Outbreaks were defined by spatial criteria, affecting 25% or more of sites, instead of mortality-based criteria that could not be assessed in older outbreak events. We also tested outbreak associations with summer temperature and spring drought as they relate to thresholds associated with larval development and host stress, respectively. In the decades following fire exclusion in adjacent mixed-conifer forest, the area occupied by Engelmann spruce and corkbark fir doubled in size, coinciding with the first wide-spread outbreaks in the reconstructed period and a doubling of outbreak duration. Outbreaks lagged spruce establishment by 40–90years depending on forest type and time since fire, and were correlated significantly with several years of antecedent warm summer temperatures followed by up to a decade of persistent spring drought. Mean annual growth increment, a factor associated with host susceptibility, was significantly higher in recently colonized mixed-conifer forest than in historically spruce- and fir-dominated sites. Increasing size and severity of outbreaks appears to have been influenced by host range expansion due to fire exclusion, coupled with increasing incidence of warm summer temperatures associated with persistent spring water stress. The combined effects of expansion of spruce into mixed-conifer forest and climatic trend toward warmer, drier conditions have the potential to promote extensive spruce beetle outbreaks across the host range of the western United States.

Are density reduction treatments effective at managing for resistance or resilience to spruce beetle disturbance in the southern Rocky Mountains?

While bark beetle disturbance is an inherent component of coniferous forest ecosystems throughout the northern hemisphere, associated tree mortality and ensuing changes in forest composition and structure may conflict with timber, wildlife, water and other resource management objectives. Therefore, host tree density reduction has been suggested as a management option to increase forest stand resistance to beetle infestation, protect remaining trees and maintain forest resources. However, little is known about the effectiveness of such treatments to mitigate spruce beetle (Dendroctonus rufipennis) infestation or their influence on the stand structural controls of beetle disturbance in subalpine spruce-fir forests in the Rocky Mountains. We addressed this research gap in a retrospective assessment of the impact of density reduction treatments on stand composition and structure and subsequent (ca. 5–20years later) spruce beetle infestation in southwestern Colorado. The study area was located at the fringe of an ongoing spruce beetle outbreak and at the time of sampling was affected by endemic to incipient beetle pressure. Stand structural attributes and beetle infestation were measured in treated and untreated control stands at four sites. Classification tree analyses revealed spruce diameter and its interaction with spruce basal area percentage as the most important drivers of tree-level beetle infestation. The number, basal area and proportions of beetle-infested spruce were lower in treated stands at sites where treatments significantly reduced the abundance of large spruce trees and where the abundance of large spruce was relatively high prior to tree removal. However, spruce density reduction did not result in a reduction of infestation rates in the remaining large (>25cm DBH) spruce during the ongoing beetle outbreak. While confirming previous assessments on the limited effectiveness of density reduction treatments for mitigating stand-level beetle infestation, this study provides further insights on the stand structural controls that mediate forest management effects on beetle disturbance dynamics. We conclude by suggesting that priority should be given to management practices that enhance resilience by increasing spruce advance regeneration in the understory as opposed to treatments aimed at achieving resistance to beetle disturbance.

Drought induces spruce beetle (Dendroctonus rufipennis) outbreaks across northwestern Colorado.

This study examines influences of climate variability on spruce beetle (Dendroctonus rufipennis) outbreak across northwestern Colorado during the period 1650 2011 CE. Periods of broad-scale outbreak reconstructed using documentary records and tree rings were dated to 1843-1860, 1882-1889, 1931-1957, and 2004-2010. Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index (PDSI), and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). Classification trees showed that outbreaks can be predicted most successfully from above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Notably, we find that spruce beetle outbreaks appear to be predicted best by interannual to multidecadal variability in drought, not by temperature alone. This finding may imply that spruce beetle outbreaks are triggered by decreases in host tree defenses, which are hypothesized to occur with drought stress. Given the persistence of the AMO, the shift to a positive AMO phase in the late 1990s is likely to promote continued spruce beetle disturbance.

Do tree and stand-level attributes determine susceptibility of spruce-fir forests to spruce beetle outbreaks in the early 21st century?

Stand-level hazard ratings are widely used to determine the susceptibility of forests to insect outbreak. Many of these risk systems were developed from observations and empirical studies of one to a few insect outbreaks and consequently may not reflect variability expected under different climatic conditions. Given the strong and nonlinear linkages between bark beetle outbreaks and climate, there is increased need for understanding the applicability of existing hazard and risk classification systems under current and future climate conditions. Here we examine the susceptibility of 100+ year-old Engelmann spruce to spruce beetle (Dendroctonus rufipennis) infestation during the 2002–2012 period of stand-mesoscale outbreak (i.e. patchy distribution of affected stands rather than a continuous outbreak across a large outbreak). In addition, we examine the effects of pre-1900 disturbance history on stand structural traits related to spruce beetle outbreak. We document the effect of a 1840s spruce beetle outbreak on stand structure 170years after the outbreak. In contrast, stand structure was not drastically different amongst stands with different last stand-replacing fire dates, which ranged from CE 1740 to 1879. Spruce beetle infestation in the early 2000s was influenced by tree-level characteristics (e.g. DBH), but not by topographic variables or stand structure; thus stand-level effects of pre-1800 disturbance did not discernibly affect the pattern of recent spruce beetle occurrence. Stand-level hazard ratings systems could be improved to consider warm and dry climate conditions, which may remove or relax stand constrains such as in the current outbreak.

Spruce Beetle Biology, Ecology and Management in the Rocky Mountains: An Addendum to Spruce Beetle in the Rockies

Spruce beetle outbreaks have been reported in the Rocky Mountains of western North America since the late 1800s. In their classic paper, Spruce Beetle in the Rockies, Schmid and Frye reviewed the literature that emerged from the extensive outbreaks in Colorado in the 1940s. A new wave of outbreaks has affected Rocky Mountain subalpine spruce-fir forests beginning in the mid-1980s and continuing to the present. These outbreaks have spurred another surge of basic and applied research in the biology, ecology and management of spruce and spruce beetle populations. This paper is a review of literature on spruce beetle focusing on work published since the late 1970s and is intended as an addendum to Spruce Beetle in the Rockies.

Organic, elemental, and geochemical contributions to lake sediment deposits during severe spruce beetle (Dendroctonus rufipennis) disturbances

Recent outbreaks of native bark beetles are unprecedented during the historical period. The aim of this manuscript is to develop a proxy-based methodology to infer past bark beetle outbreaks using lake sediments to provide long-term context for recent outbreaks. We test three hypotheses to determine how the ecological impacts of severe spruce beetle (Dendroctonus rufipennis Kirby) disturbances are recorded in lake sediment deposits. The resulting mortality and defoliation of Engelmann spruce is hypothesized to: (1) decrease the ratio of spruce to fir pollen; (2) reduce canopy interception of precipitation leading to an increase in soil erosion and/or enhanced mobilization of terrestrial carbon; and (3) leach foliar nitrogen and enhance algal productivity resulting in increased nitrogen values in lake sediments. To test these hypotheses, we analyzed sediment cores from six spruce beetle-affected basins in Utah for spruce/fir pollen ratio (hypothesis 1), loss-on-ignition, magnetic susceptibility of sediments (hypothesis 2), and δ13CBOM, δ15NTN, elemental C and N, and the C/N ratio of bulk organic material (hypotheses 2 and 3). The dataset was statistically tested using general linear mixed models (GLMMs) to determine if the response variables differed significantly between outbreak and non-outbreak period. The spruce/fir pollen ratio responded significantly to outbreaks at all sites suggesting that this metric may be the most suitable for identifying past spruce beetle outbreaks. For our second hypothesis we found little support for an influx of terrestrial C due to strongly individualist responses of the basins. For our third hypotheses we found little support for increased sedimentary nitrogen, likely due to alterations to nutrient cycling from human activities. Therefore the host/non-host pollen ratio provides the most promising metric for detecting past outbreaks.

Drought-driven disturbance history characterizes a southern Rocky Mountain subalpine forest

The view that subalpine forest vegetation dynamics in western North America are “driven” by a particular disturbance type (i.e., fire) has shaped our understanding of their disturbance regimes. In the wake of a recent (1990s) landscape-extent spruce beetle ( Dendroctonus rufipennis Kirby) outbreak in the southern Rocky Mountains, we re-examined the temporal continuity in disturbance types and interactions and the possible role of drought on their occurrence by reconstructing antecedent disturbances for 11 sites across the Markagunt Plateau, southern Utah, USA. Multiple consistent lines of evidence suggested that historic fires were the primary antecedent disturbance, while relatively minor, stand-specific spruce beetle activity occurred later in stand development but prior to the recent outbreak. Unlike the recent outbreak, antecedent fires were spatially and temporally asynchronous over the period examined (~1600–2000). Reconstructed fire events primarily occurred during periods of prolonged drought. Similarly, historic spruce beetle activity, indicated by species-specific tree-ring release, and timing of Engelmann spruce ( Picea engelmannii Parry ex Engelm.) death dates from the recent outbreak were related to drought conditions. Vegetation dynamics on this landscape were strongly driven by historic fires and the recent spruce beetle outbreak, and drought conditions likely influenced the occurrence of both disturbance types.

Pollen accumulation in lake sediments during historic spruce beetle disturbances in subalpine forests of southern Utah, USA

Paleoecological reconstructions using lake sediments provide important information about ecological dynamics and forest disturbance processes that occurred prior to the historic period of scientific observation. In high-altitude and high-latitude ecosystems where landscape-scale disturbances recur at time intervals exceeding observation in many regions, e.g. western North America, reconstructed environmental data are essential in providing context for land managers. During the most recent two decades eruptive populations of bark beetles ( Dendroctonus spp.) have rapidly and profoundly altered subalpine forest ecosystems across western North America. Outbreaks of these insects are unprecedented in scale and severity, at least historically. Currently, little information exists about these destructive outbreaks and in general, the information that exists, does not extend beyond the most recent few centuries. The research presented here examines sedimentary pollen records from six subalpine basins affected by severe spruce beetle ( D. rufipennis) epidemics during the 20th century in the high-elevation plateaus and mountain ranges of south-central Utah. Reciprocal exchanges in dominance between pollen abundance of host spruce ( Picea engelmannii) and non-host subalpine fir ( Abies lasiocarpa) associated with historic outbreaks are conspicuous. Calculating simple ratios of host and non-host pollen accumulations offers a useful metric to visually identify spruce beetle outbreaks using sedimentary records. However, supporting lines of evidence may be required to identify these disturbances with greater certainty over the Holocene. Our data and findings provide a platform with which to begin exploration of other paleoecological proxy methods for the ultimate purpose of generating more temporally extensive reconstructions of bark beetle disturbances using sedimentary records.

Factors Influencing the Spatial and Temporal Dynamics of Engelmann Spruce Mortality during a Spruce Beetle Outbreak on the Markagunt Plateau, Utah

Host conditions are known to influence spruce beetle population levels, but whether they influence the spatial and temporal patterns of beetle-caused mortality during an outbreak is unknown. Using dendro- chronological techniques, we quantified the spatiotemporal dynamics of a modern (late 1980s through the early 2000s) spruce beetle outbreak in Engelmann spruce on the Markagunt Plateau, Utah. Erupting beetle populations occurred in multiple areas across the Plateau over the course of 8 years. Early in the outbreak, the timing of spruce mortality was strongly and predictably related to latitude, slope, aspect, stand structure, host species composition, and site productivity potential. Exogenous forcing from landscape-scale drivers such as winter minimum temperatures, summer maximum temperatures, and landscape-wide host suitability probably contrib- uted to spruce beetle population success and heightened spruce mortality. As the outbreak progressed, the timing of spruce mortality became less well correlated to stand and environmental variables. The outbreak subsided when suitable host material was depleted, including smaller spruce in remote stands at the fringe of the spruce-fir zone. Tests for spatial and temporal autocorrelation of beetle-cased spruce mortality did not provide strong evidence for the common perception that spruce beetle outbreaks originate in a specific location (epicenter) and then spread across the landscape; rather, support for positive spatial synchrony of multiple, building popula- tions during development of the outbreak was found. Based on these results, silvicultural intervention before the onset of an outbreak might mitigate Engelmann spruce losses; however, the same is almost certainly not true later in an outbreak of this scale and magnitude. FOR .S CI. 58(1):1-14.

Climate variability and spruce beetle (Dendroctonus rufipennis) outbreaks in south-central and southwest Alaska

We used tree ring data (AD 1601-2007) to examine the occurrence of and climatic influences on spruce beetle (Dendroctonus rufipennis) outbreaks in south-central and southwest Alaska and found evidence of regional-scale outbreaks dating from the mid-1700s, related to climate variability at multiple temporal scales. Over interannual time scales (approximately 1-3 years), El Niño years, combined with severe late-summer drought, appeared to contribute significantly to spruce beetle outbreaks in the study area. Over multidecadal time scales (up to approximately 40 years), cool-phase Pacific Decadal Oscillation (PDO) conditions tended to precede beetle outbreaks, regardless of the phase of El Niño-Southern Oscillation (ENSO). All sites showed low-severity disturbances attributed to spruce beetle damage, most notably during the 1810s. During other major periods of disturbance (i.e., 1870s, 1910s, 1970s), the effects of spruce beetle outbreaks were of moderate or higher severity. The highly synchronized timing of spruce beetle outbreaks at interannual to multidecadal scales, and particularly the association between cool-phase PDO conditions and beetle disturbance, suggests that climate (i.e., temperature, precipitation) is a primary driver of outbreaks in the study area. Our disturbance chronologies (mid-1700s to present) suggest that recent irruptions (1990s to present) in south-central and southwest Alaska are within the historical geographic range, but that outbreaks since the 1990s show greater spatiotemporal synchrony (i.e., more sites record high-severity infestations) than at any other time in the past approximatly 250 years.

Combining dendrochronological data and the disturbance index to assess Engelmann spruce mortality caused by a spruce beetle outbreak in southern Utah, USA

Forest disturbances such as bark beetle outbreaks are increasing in severity and extent across western North America. Classification of remote sensing imagery is a powerful way to analyze and detect large-scale disturbances. We used a temporal sequence of four Landsat TM images (1991, 1995, 1999, and 2003) to detect the spatiotemporal change in spectral response of Engelmann spruce ( Picea engelmannii Parry ex. Engelm.) killed by an unprecedented spruce beetle outbreak in southern Utah, USA. After co-registration and masking out non-vegetation the Disturbance Index (DI) was calculated for each image. DI values associated with Engelmann spruce mortality, determined by comparing each image to a no outbreak baseline image, were then used to classify the images. Dendrochronologically determined dates of spruce death collected from across the outbreak area were used to assess the ability of the DI to accurately differentiate stands of dead spruce from live conifer forest. The overall classification accuracy of the DI varied from 80 to 82% while the accuracy to detect spruce beetle-killed spruce varied from 59 to 71%. Both user's and producer's accuracy to classify beetle infested stands increased over the temporal sequence of image dates. However, confusion matrix-derived statistics varied by image date. Consistent with previous studies, the spruce beetle outbreak began building in multiple, seemingly independent locations across the study area. Over time, areas attacked earlier in the outbreak enlarged and coalesced on the landscape.

Recent infestation of forest stands by spruce beetles does not predict habitat use by little brown bats ( Myotis lucifugus) in southwestern Yukon, Canada

Insect outbreaks affect forest structure which may have significant effects on the habitat of other animals. Forest-dwelling insectivorous bats are likely affected by associated changes in the abundance of roost trees and insect prey, altered foraging and flying efficiency, and predation risk. We examined the short-term effects (3-13 years post-infestation) of an outbreak of spruce beetles ( Dendroctonus rufipennis) on the habitat use of little brown bats ( Myotis lucifugus) in the boreal forest of the southwestern Yukon, Canada. We measured bat activity, using Anabat II bat detectors, in 90 forested stands that had experienced from 0 to 90% tree mortality due to spruce beetles. We used generalized linear models to assess whether bat activity varied with tree mortality, season, tree density, canopy closure, or distance to the nearest lake or town. Bat activity did not vary significantly with tree mortality, season, or canopy closure, but decreased with increasing tree density. Bat activity was significantly greater in areas close to both the nearest lake and nearest town, and was low in areas that were far from either. Our results indicate that in the short-term, habitat use by little brown bats was not related to the severity of spruce beetle infestation, but suggest that in the long-term, bats may be positively affected by decreased tree density as beetle-killed trees fall down.

Pollen Evidence of Historical Forest Disturbance on the Wasatch Plateau, Utah

Environmental indicators from lake sediments provide excellent opportunities to improve understand- ing of forest disturbance processes and corresponding changes in forest composition. Our research provides a methodol- ogy for assessing recent, historic, and prehistoric disturbances using lacustrine sediment records. We collected sediment cores from Blue Lake, a small subalpine lake on the Wasatch Plateau in central Utah. These cores record environmental changes caused by both spruce beetle (Dendroctonus rufipennis Kirby) and human (logging and livestock grazing) modi- fication. We observed deteriorated insect remains in the lake sediments. These remains correspond temporally with a historic spruce beetle outbreak, though alkaline conditions in the lake water may have inhibited preservation of bark beetle remains. Pollen data reveal that despite the unprecedented level of mortality among Engelmann spruce (Picea engelmannii Parry ex Engelm.) resulting from the spruce beetle epidemic, logging activities subsequent to Euro-Ameri- can settlement appear to be the most severe disturbance to the Blue Lake watershed over the last 750 years.

The pollen record of a 20th century spruce beetle ( Dendroctonus rufipennis) outbreak in a Colorado subalpine forest, USA

The frequency and intensity of ecosystem disturbance, including outbreaks of forest insects and forest fires, is expected to increase in the future as a result of higher temperatures and prolonged drought. While many studies have concentrated on the future climatic impacts on fire, little is known about the impact of future climate on insect infestation. Paleoecological techniques are important in this regard in identifying the potential relationships between climate and insect outbreaks in the past, as a predictive tool for the future. We examine a high-resolution 20th century record of spruce beetle ( Dendroctonus rufipennis) infestation from a small, subalpine lake, comparing the paleoecological record to the historical and tree-ring record of the event. An extensive spruce beetle outbreak occurred in northwestern Colorado during the 1940s and 1950s, causing widespread mortality of mature Picea engelmannii. Pollen analysis of this period documents the decline of Picea and its replacement locally by Abies lasiocarpa, paralleling age and composition studies of modern forest stands in the region. This study is a proof of concept that, when applied to longer sedimentary records, could produce a detailed record of infestation for the Late Holocene or older time periods. This information will be useful to forest managers in efforts to plan for the effects of D. rufipennis infestations, and subsequent succession within high elevation conifer forests.

Wildfire and spruce beetle outbreak: Simulation of interacting disturbances in the central Rocky Mountains

Infrequent large-scale natural disturbance regimes are an integral component of Engelmann spruce (Picea engelmannii) forests of the central Rocky Mountains. Wildfires, bark beetle outbreaks, winds, and avalanches cause relatively drastic changes in community structure, composition, and function. These disturbances may occur independently or interact where the incidence of one may change the potential for another. We assessed potential wildfire behaviour change in the wake of a catastrophic, landscape-wide spruce beetle (Dendroctonus rufipennis) outbreak in southern Utah, USA. Using data collected in spruce forests affected by the outbreak, the Forest Vegetation Simulator and Fire and Fuels Extension were used to simulate long-term (100 y) stand dynamics and potential fire behaviour under 3 reconstructed scenarios: no spruce beetle outbreak (low-severity), 50% spruce beetle-caused mortality (mid-severity), and 95% spruce beetle-caused mortality (high-severity). Simulations suggested a likely reduction in probability of active crown fire for 1 or 2 decades on near-pure Engelmann spruce sites after high-severity mortality. This counterintuitive result suggested extreme fire behaviour is not an inevitable consequence of spruce beetle outbreaks. No change in potential fire behaviour was predicted in stands with the least reduction in spruce basal area (low- or mid-severity). In one stand with a history of surface fire, stand structure and potential fire behaviour from low- and high-severity simulations were influenced by surface fire ˜100 y ago. These results are indicative of complex disturbance interactions that were influenced by the host-specific spruce beetle, resultant stand structures and fuel profiles, and in one case antecedent disturbance.

Large Woody Debris and Salmonid Habitat in the Anchor River Basin, Alaska, Following an Extensive Spruce Beetle (Dendroctonus rufipennis) Outbreak

Abstract A widespread and intense spruce beetle outbreak during the 1990s has killed most of the mature white spruce (Picea glauca) trees across many watersheds in south-central Alaska. To investigate the potential habitat impacts in a salmon stream, we characterized the current abundance and species composition of large woody debris (LWD), examined the linkages between LWD and salmonid habitat, and estimated changes in LWD abundance and associated pool habitat over time. LWD abundance was relatively low (97 pieces/km overall) and varied widely according to riparian vegetation typology, ranging from 15 pieces/km at sites with non-forested riparian zones to 170 pieces/km at sites adjacent to cottonwood forest. LWD provided significant fish cover in pools, especially in cottonwood forest stream reaches. LWD-formed pools were relatively rare (15% of total), but LWD abundance explained much of the variation in pool frequency (r2 = 0.86 in spruce forest reaches) and in the proportion of pool habitats (r2 = 0.8...

Standardized approaches in effectiveness monitoring programs and regional relevance: lessons from the Bowron River Watershed Riparian Evaluation Project

Streams and riparian areas in the Bowron River watershed were assessed using the riparian management routine effectiveness evaluation, a protocol developed for postharvest operational monitoring in British Columbia. Located in the British Columbia interior, the study watershed has had several decades to recover since a period of intensive salvage logging from 1977 to 1987 in response to a spruce beetle ( Dendroctonus rufipennis ) outbreak. Correlations were found with site-specific watershed characteristics and attributes that are used to answer the protocol indicator questions. Channel width was the strongest covariate, exhibiting the most significant relationships with the stream and riparian attributes. Suggestions on how to adjust standard protocols for stream size and landscape heterogeneity are discussed and include the use of appropriate reference sites for baseline data. We used correlations with watershed characteristics and multiple regressions based on regional reference sites to predict upper and lower threshold values by which attribute measurements could be compared. Sites that were assessed above or below threshold values for an attribute were considered to be outside the range of natural variation and were given a failing score. Attributes with the most failures 20–30 years postharvest were disturbed ground, bare soil, undercut banks, and shade.

White spruce regeneration following a major spruce beetle outbreak in forests on the Kenai Peninsula, Alaska

Between 1987 and 2000, a spruce beetle ( Dendroctonus rufipennis) outbreak infested 1.19 million ha of spruce ( Picea spp.) forests in Alaska, killing most of the large diameter trees. We evaluated whether these forests would recover to their pre-outbreak density, and determined the site conditions on which spruce germinated and survived following the spruce beetle outbreak in forests of the Anchor River watershed, Kenai Peninsula, Alaska. White spruce ( Picea glauca) and Lutz's spruce ( Picea × lutzii), a hybrid between white and Sitka spruce ( Picea sitchensis), dominate the study area. We measured the pre- and post-outbreak density of spruce in 108 3 m × 80 m plots across the study area by recording all live trees and all dead trees >1.5 m tall in each plot. To determine the fine scale site conditions on which spruce germinated and survived, we measured ground surface and substrate characteristics within 20 cm circular plots around a subset of post-outbreak spruce seedlings. The density of post-outbreak spruce (855/ha) was adequate to restock the stands to their pre-outbreak densities (643/ha) for trees >1.5 m tall. We could not accurately estimate recovery for pre-outbreak spruce seedlings because dead seedlings may have decayed in the 5–18 years since the beetle outbreak occurred. At the fine scale, spruce that germinated post-outbreak grew on a wide variety of substrates including downed log, stump, mesic organic mat, peat, hummocks and mineral soil. They exhibited a strong preference for downed logs (53%) and stumps (4%), and most (91%) of the downed logs and stumps that spruce rooted on were heavily decayed. This preference for heavily decayed logs and stumps was especially evident given that their combined mean cover was only 2% in the 3 m × 80 m plots. Within the 3 m × 80 m plots, spruce seedling survival was negatively correlated with bluejoint ( Calamagrostis canadensis) litter cover.

Climate Factors Associated with Historic Spruce Beetle (Coleoptera: Curculionidae) Outbreaks in Utah and Colorado

This study investigated relationships between climate and historic spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), outbreaks in northern and southeastern Utah and western Colorado between 1905 and 1996. A chronology of outbreak years was constructed from historic records, research papers, newspapers, and other sources of information. Historic climate data for the region included annual and mean monthly temperature and precipitation, in addition to Palmer drought severity index (PDSI) values estimated from tree rings. Classification and regression tree analysis (CART) was used to identify those climate factors most important for predicting historic spruce beetle outbreaks. The factors identified by the best CART model included mean December temperature, mean September temperature 1 yr before outbreak years, the mean estimated PDSI value of the 5-yr period before outbreak years, and mean October precipitation. The resulting model correctly classified nonoutbreak and outbreak years 67 and 70% of the time, respectively.

Climate Factors Associated with Historic Spruce Beetle (Coleoptera: Curculionidae) Outbreaks in Utah and Colorado

Climate Factors Associated with Historic Spruce Beetle (Coleoptera: Curculionidae) Outbreaks in Utah and Colorado