Are bark beetle outbreaks less synchronous than forest Lepidoptera outbreaks?

Comparisons of intraspecific spatial synchrony across multiple epidemic insect species can be useful for generating hypotheses about major determinants of population patterns at larger scales. The present study compares patterns of spatial synchrony in outbreaks of six epidemic bark beetle species in North America and Europe. Spatial synchrony among populations of the Eurasian spruce bark beetle Ips typographus was significantly higher than for the other bark beetle species. The spatial synchrony observed in epidemic bark beetles was also compared with previously published patterns of synchrony in outbreaks of defoliating forest Lepidoptera, revealing a marked difference between these two major insect groups. The bark beetles exhibited a generally lower degree of spatial synchrony than the Lepidoptera, possibly because bark beetles are synchronized by different weather variables that are acting on a smaller scale than those affecting the Lepidoptera, or because inherent differences in their dynamics leads to more cyclic oscillations and more synchronous spatial dynamics in the Lepidoptera.     

How does synchrony with host plant affect the performance of an outbreaking insect defoliator?

Phenological mismatch has been proposed as a key mechanism by which climate change can increase the severity of insect outbreaks. Spruce budworm (Choristoneura fumiferana) is a serious defoliator of North American conifers that feeds on buds in the early spring. Black spruce (Picea mariana) has traditionally been considered a poor-quality host plant since its buds open later than those of the preferred host, balsam fir (Abies balsamea). We hypothesize that advancing black spruce budbreak phenology under a warmer climate would improve its phenological synchrony with budworm and hence increase both its suitability as a host plant and resulting defoliation damage. We evaluated the relationship between tree phenology and both budworm performance and tree defoliation by placing seven cohorts of budworm larvae on black spruce and balsam fir branches at different lags with tree budburst. Our results show that on both host plants, spruce budworm survival and pupal mass decrease sharply when budbreak occurs prior to larval emergence. By contrast, emergence before budbreak decreases survival, but does not negatively impact growth or reproductive output. We also document phytochemical changes that occur as needles mature and define a window of opportunity for the budworm. Finally, larvae that emerged in synchrony with budbreak had the greatest defoliating effect on black spruce. Our results suggest that in the event of advanced black spruce phenology due to climate warming, this host species will support better budworm survival and suffer increased defoliation.     

Dendroentomological and forest management implications in the Interior Douglas-fir zone of British Columbia, Canada

Douglas-fir (Pseudotsuga menziesii var. glauca Mirb. Franco) forests in the Interior of British Columbia, Canada, show periodic defoliation due to western spruce budworm (WSB) (Choristoneura occidentalis Freeman) outbreaks. Tree defoliation causes a reduction in radial growth and is therefore visible in tree rings. In this paper, we identify WSB defoliation history, and critically examine the potential for using dendrochronological analysis by comparing tree-ring estimates with insect surveys. WSB defoliation history was investigated using cores from Douglas-fir growing in the Lac du Bois region of the Kamloops Forest District. Years with an abrupt decrease in radial growth were considered as negative pointer years that potentially reflected WSB outbreaks. The comparison with ponderosa pine (Pinus ponderosa Dougl., ex P. & C. Laws.) (a non-host species) permitted differentiation between growth reductions in Douglas-fir due to climatic effects and those due to defoliation by WSB. The dendrochronological data were matched with information reporting visible damage in Forest Insect Disease Survey (FIDS) and British Columbia Ministry of Forest records. Our objective-based method using ring-width measurements from host and non-host chronologies was compared with qualitative techniques based on the software program OUTBREAK. We were able to distinguish seven distinct outbreak events in 300 years of record.     

Dendroentomological and forest management implications in the Interior Douglas-fir zone of British Columbia,Canada

Douglas-fir (Pseudotsuga menziesii var. glauca Mirb. Franco) forests in the Interior of British Columbia, Canada, show periodic defoliation due to western spruce budworm (WSB) (Choristoneura occidentalis Freeman) outbreaks. Tree defoliation causes a reduction in radial growth and is therefore visible in tree rings. In this paper, we identify WSB defoliation history, and critically examine the potential for using dendrochronological analysis by comparing tree-ring estimates with insect surveys. WSB defoliation history was investigated using cores from Douglas-fir growing in the Lac du Bois region of the Kamloops Forest District. Years with an abrupt decrease in radial growth were considered as negative pointer years that potentially reflected WSB outbreaks. The comparison with ponderosa pine (Pinus ponderosa Dougl., ex P. & C. Laws.) (a non-host species) permitted differentiation between growth reductions in Douglas-fir due to climatic effects and those due to defoliation by WSB. The dendrochronological data were matched with information reporting visible damage in Forest Insect Disease Survey (FIDS) and British Columbia Ministry of Forest records. Our objective-based method using ring-width measurements from host and non-host chronologies was compared with qualitative techniques based on the software program OUTBREAK. We were able to distinguish seven distinct outbreak events in 300 years of record.     

Landscape host abundance and configuration regulate periodic outbreak behavior in spruce budworm Choristoneura fumiferana

Landscape‐level forest management has long been hypothesized to affect forest insect outbreak dynamics, but empirical evidence remains elusive. We hypothesized that the combination of increased hardwood relative to host tree species, prevalence of younger forests, and fragmentation of those forests due to forest harvesting legacies would reduce outbreak intensity, increase outbreak frequency, and decrease spatial synchrony in spruce budworm Choristoneura fumiferana outbreaks. We investigated these hypotheses using tree ring samples collected across 51 sites pooled into 16 subareas distributed across a large ecoregion spanning the international border between Ontario (Canada), and Minnesota (USA). This ecoregion contains contrasting land management zones with clear differences in forest landscape structure (i.e. forest composition and spatial configuration) while minimizing the confounding influence of climate. Cluster analyses of the 76‐yr time‐series generally grouped by subareas found within the same land management zone. Spatial nonparametric covariance analysis indicated that the highest and lowest degree of spatial synchrony of spruce budworm outbreaks were found within unmanaged wilderness and lands managed at fine spatial scales in Minnesota, respectively. Using multivariate analysis, we also found that forest composition, configuration, and climate together accounted for a total of 40% of the variance in outbreak chronologies, with a high level of shared variance between composition and configuration (13%) and between composition and climate (9%). At the scale of our study, climate on its own did not explain any of the spatial variation in outbreaks. Outbreaks were of higher frequency, lower intensity, and less spatially synchronized in more fragmented, younger forests with a lower proportion of host species, with opposing outbreak characteristics observed in regions characterised by older forests with more concentrated host species. Our study is the first quantitative evaluation of the long‐standing ‘silvicultural hypothesis’ of spruce budworm management specifically conducted at a spatio‐temporal scale for which it was intended.     

Variation in wood anatomical structure of Douglas-fir defoliated by the western spruce budworm: a case study in the coastal-transitional zone of British Columbia,Canada

Key message

An outbreak of the western spruce budworm temporarily modifies cellular wood anatomy of stem wood in natural and mature Douglas-fir stands impacting wood quality properties.

Abstract

Western spruce budworm (Choristoneura occidentalis Freeman) is a widespread and destructive defoliator of commercially important coniferous forests in western North America. In British Columbia, Canada, Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] is the most important and widely distributed host. Permanent sample plots were established at a number of locations in southern interior at the beginning of a severe western spruce budworm outbreak in the 1970s. Two of the sites were sampled in 2012 to determine whether modifications had occurred in the anatomical characteristics of stem wood formed during outbreak years. We determined that rings formed during the western spruce budworm 1976–1980 outbreak had a significantly lower proportion of latewood, reduced mean cell wall thickness and smaller radial cell diameters. While the cellular characteristics of the earlywood remained fairly constant, significant reductions in lumen area occurred in 1978 and 1979 at each site. Our study shows that western spruce budworm outbreaks not only reduce annual radial growth, but also temporarily modify cellular characteristics in latewood cells, which has implications for wood density and quality in Douglas-fir.     

Potential Role of Spruce Budworm in Range-Wide Decline of Canada Warbler

ABSTRACT The Canada warbler (Wilsonia canadensis) is one of many common neotropical migrants whose populations are in decline across their range. Influences of habitat loss and degradation on breeding or wintering grounds have been postulated as possible causes, but few empirical data exist to support a specific cause. Based on previous studies linking abundances of Canada warbler and spruce budworm (Choristoneura fumiferana), we hypothesized that the Canada warbler may be influenced by a persistent decline in spruce budworm throughout the bird's breeding range, a hypothesis that has received little attention. This hypothesis makes 5 predictions: 1) budworm outbreaks and warbler detections should be spatially and temporally coincident; 2) the relationship between Canada warbler and spruce budworm outbreaks should be similar to relationships for other warblers known to be spruce budworm associates; 3) the relationship should be stronger than for warblers lacking an association with spruce budworm; 4) because temporal trends of both spruce budworm and Canada warblers have varied throughout Canadian provinces, declines in Canada warblers should be seen only in provinces where spruce budworm also declined; and 5) variation in Canada warbler abundance should reflect variation in supply of preferred habitat for the spruce budworm if habitat rather than budworm abundance is the key. Our analyses supported predictions 1–4, suggesting that Canada warbler may be even more closely associated with spruce budworm than are known associated species, a phenomenon noted in the literature but previously unexplained. Prediction 5 was not supported, because budworm habitat (area of mature and older balsam fir [Abies balsamea] and white spruce [Picea glauca]) remained constant in Ontario while warbler abundance declined. Although the correlative nature of these results precludes inference of a causal relationship between the declines of the Canada warbler and spruce budworm, we postulate that potential links may exist directly, where spruce budworm outbreaks provide elevated levels of insect prey items for breeding Canada warblers, or indirectly through changes in forest structure and composition following outbreaks. These results have implications when considering long-term trends in Canada warbler populations, because it may be impossible to alter population trends for species linked to the timing and magnitude of spruce budworm outbreaks.     

Modelling Spatial Interactions Among Fire,Spruce Budworm,and Logging in the Boreal Forest

In the boreal forest, fire, insects, and logging all affect spatial patterns in forest age and species composition. In turn, spatial legacies in age and composition can facilitate or constrain further disturbances and have important consequences for forest spatial structure and sustainability. However, the complex three-way interactions among fire, insects, and logging and their combined effects on forest spatial structure have seldom been investigated. We used a spatially explicit landscape simulation model to examine these interactions. Specifically, we investigated how the amount and the spatial scale of logging (cutblock size) in combination with succession, fire, and spruce budworm outbreaks affect area burned and area defoliated. Simulations included 30 replicates of 300 years for each of 19 different disturbance scenarios. More disturbances increased both the fragmentation and the proportion of coniferous species and imposed additional constraints on the extent of each disturbance. We also found that harvesting legacies affect fire and budworm differently due to differences in forest types consumed by each disturbance. Contrary to expectation, budworm defoliation did not affect area burned at the temporal scales studied and neither amount of logging nor cutblock size influenced defoliation extent. Logging increased fire size through conversion of more of the landscape to early seral, highly flammable forest types. Although logging increased the amount of budworm host species, spruce budworm caused mortality was reduced due to reductions in forest age. In general, we found that spatial legacies do not influence all disturbances equally and the duration of a spatial legacy is limited when multiple disturbances are present. Further information on post-disturbance succession is still needed to refine our understanding of long-term disturbance interactions.     

Phenological synchrony between eastern spruce budworm and its host trees increases with warmer temperatures in the boreal forest

Climate change is predicted to alter relationships between trophic levels by changing the phenology of interacting species. We tested whether synchrony between two critical phenological events, budburst of host species and larval emergence from diapause of eastern spruce budworm, increased at warmer temperatures in the boreal forest in northeastern Canada. Budburst was up to 4.6 ± 0.7 days earlier in balsam fir and up to 2.8 ± 0.8 days earlier in black spruce per degree increase in temperature, in naturally occurring microclimates. Larval emergence from diapause did not exhibit a similar response. Instead, larvae emerged once average ambient temperatures reached 10°C, regardless of differences in microclimate. Phenological synchrony increased with warmer microclimates, tightening the relationship between spruce budworm and its host species. Synchrony increased by up to 4.5 ± 0.7 days for balsam fir and up to 2.8 ± 0.8 days for black spruce per degree increase in temperature. Under a warmer climate, defoliation could potentially begin earlier in the season, in which case, damage on the primary host, balsam fir may increase. Black spruce, which escapes severe herbivory because of a 2‐week delay in budburst, would become more suitable as a resource for the spruce budworm. The northern boreal forest could become more vulnerable to outbreaks in the future.     

A dendroecological reconstruction of western spruce budworm outbreaks (Choristoneura occidentalis) in the Front Range,Colorado, from 1720 to 1986

Radial increment cores from Douglas-fir (Pseudotsuga menziesii) and blue spruce (Picea pungens), defoliated by western spruce budworm (Choristoneura occidentalis), were analyzed by means of dendrochronological methods and compared with cores from undefoliated ponderosa pine (Pinus ponderosa) and lodgepole pine (Pinus contorta) growing on the same sites in the Front Range, Colorado. Extensive deforestation during the gold and silver booms in the second part of the nineteenth century led to dense and almost pure stands of shadetolerant budworm host species. By using the skeleton plot method, the number of trees with clear growth reductions is obtained, thus representing an exact record of forest insect attacks. The analysis of abrupt growth reductions revealed at least nine outbreaks of western spruce budworm between 1720 and 1986, the majority occurring in the nineteenth century. The outbreaks were graphically compared with periods of attack in New Mexico and Colorado which were detected by other scientists employing tree-ring measurement techniques. No increase in the frequency of severe outbreaks during the twentieth century was observed, yet there is some evidence that the most recent outbreak might be the most severe ever recorded. Open Douglas-fir stands on higher sites were more susceptible to heavy budworm attack than dense stands on lower sites. Blue spruce was less frequently and less severely attacked than Douglas-fir. The spatial pattern of historical outbreaks generally was very patchy.     

Warming counteracts defoliation‐induced mismatch by increasing herbivore‐plant phenological synchrony

Climate change is altering phenology; however, the magnitude of this change varies among taxa. Compared with phenological mismatch between plants and herbivores, synchronization due to climate has been less explored, despite its potential implications for trophic interactions. The earlier budburst induced by defoliation is a phenological strategy for plants against herbivores. Here, we tested whether warming can counteract defoliation‐induced mismatch by increasing herbivore‐plant phenological synchrony. We compared the larval phenology of spruce budworm and budburst in balsam fir, black spruce, and white spruce saplings subjected to defoliation in a controlled environment at temperatures of 12, 17, and 22°C. Budburst in defoliated saplings occurred 6–24 days earlier than in the controls, thus mismatching needle development from larval feeding. This mismatch decreased to only 3–7 days, however, when temperatures warmed by 5 and 10°C, leading to a resynchronization of the host with spruce budworm larvae. The increasing synchrony under warming counteracts the defoliation‐induced mismatch, disrupting trophic interactions and energy flow between forest ecosystem and insect populations. Our results suggest that the predicted warming may improve food quality and provide better growth conditions for larval development, thus promoting longer or more intense insect outbreaks in the future.     

Spruce budworm (Choristoneura spp.) biotype reactions to forest and climate characteristics

The spruce budworm (Choristoneura fumiferana) is the most destructive insect defoliator of forests in North America. Climatic influences on this species' life history are considered a major factor in restricting the extent and intensity of outbreaks. We examine the life history traits of the spruce budworm and related Choristoneura populations with respect to forecasting the conifer‐feeding responses of these insects in changing environments. Analysis of the evolutionary relationships between Choristoneura entities, including their hybridization, genetic distances, and their degree of sympatry leads us to distinguish 15 possible Choristoneura‘biotypes’. Population trend has been associated with recruitment to the feeding stage, and two indicators of recruitment, egg weights and phenological development, are both ‘biotype’ and climate dependent. Among Abietoid feeding ‘biotypes’ and among spruce budworm populations, those from locations with extreme winters tend to have heavier eggs than those from the more benign environments. In spruce budworm, this genetically based adaptation allows populations to increase their potential recruitment substantially where winters are mild. All biotypes feed on the newly developed shoots of their host trees in spring, and are thus vulnerable to the uncertain timing of budbreak. Genetic control of spring emergence is weak so larvae from a single family typically exit from hibernacula over a prolonged period. This guarantees some synchronization with budburst. However, hybrid populations have high heritabilities. This allows rapid adaptation to new conditions (e.g. mixed host‐species stands). Geographic variation in phenological development after establishing feeding sites is largely genetically controlled. The importance of variation in these traits is examined with respect to competing population dynamics theories to evaluate their utility in forecasting future trends in defoliation. We finish with a plea for jointly using alternative approaches in forecasting spatiotemporal patterns of defoliation.     

Three years of aerial field experiments with Bacillus thuringiensis plus chitinase formulation against the spruce budworm

From 1971 to 1973 several Bacillus thuringiensis formulations were tested in the field against larvae of the spruce budworm under various conditions of population and tree defoliation. The results showed B. thuringiensis treatments can be a weapon in the control of spruce budworm outbreaks and the beneficial effect of B. thuringiensis treatments appear to be prolonged over 1 or 2 years. A new compact formulation was developed making B. thuringiensis treatments more economical and competitive with chemical insecticides.     

Interguild interactions between a non-native phloem feeder and a native outbreaking defoliator

1. Competitive and synergistic interactions directly or indirectly drive community dynamics of herbivorous insects. Novel interactions between non-native and native insects are unpredictable and not fully understood. 2. We used manipulative experiments on mature red spruce trees to test interactions between a non-native phloem feeding insect, the brown spruce longhorn beetle (BSLB), and an outbreaking native defoliator, the spruce budworm. We subjected treatment trees to defoliation by three densities of spruce budworm larvae. Treatment trees were: stressed by (i) girdling (to mimic beetle feeding) or (ii) girdling + BSLB before spruce budworm larvae were introduced on branches in sleeve cages. Budworm larvae then fed on foliage and developed to pupation. We assessed all branches for budworm performance, defoliation, shoot production and shoot growth. 3. Shoot length did not differ in response to stress from girdling or BSLB infestation. Neither stress from girdling, nor interactions with BSLB feeding affected spruce budworm performance or defoliation. Intraspecific impacts on performance and defoliation in relation to budworm density were stronger than the effects of tree stress. 4. Prior infestation of red spruce by BSLB in our experimental set-up did not influence spruce budworm performance. BSLB is a successful invader that has blended into its novel ecological niche because of ecological and phylogenetic similarities with a native congener, Tetropium cinnamopterum. 5. Outbreaks by BSLB will not likely impede or facilitate spruce budworm outbreaks if they co-occur. It would be useful to evaluate the reverse scenario of BSLB success after defoliation stress by spruce budworm.     

Landscape level analysis of mountain pine beetle in British Columbia, Canada: spatiotemporal development and spatial synchrony within the present outbreak

Eruptive herbivores can exert profound landscape level influences. For example, the ongoing mountain pine beetle outbreak in British Columbia, Canada, has resulted in mortality of mature lodgepole pine over >7 million ha. Analysis of the spatio‐temporal pattern of spread can lend insights into the processes initiating and/or sustaining such phenomena. We present a landscape level analysis of the development of the current outbreak. Aerial survey assessments of tree mortality, projected onto discrete 12×12 km cells, were used as a proxy for insect population density. We examined whether the outbreak potentially originated from an epicenter and spread, or whether multiple localized populations erupted simultaneously at spatially disjunct locations. An aspatial cluster analysis of time series from 1990 to 2003 revealed four distinct time series patterns. Each time series demonstrated a general progression of increasing mountain pine beetle populations. Plotting the geographical locations of each temporal pattern revealed that the outbreak occurred first in an area of west‐central British Columbia, and then in an area to the east. The plot further revealed many localized infestations erupted in geographically disjunct areas, especially in the southern portion of the province. Autologistic regression analyses indicated a significant, positive association between areas where the outbreak first occurred and conservation lands. For example, the delineated area of west‐central British Columbia is comprised of three conservation parks and adjacent working forest. We further examined how population synchrony declines with distance at different population levels. Examination of the spatial dependence of temporal synchrony in population fluctuations during early, incipient years (i.e. 1990–1996) suggested that outbreaking mountain pine beetle populations are largely independent at scales >200 km during non‐epidemic periods. However, during epidemic years (i.e. 1999–2003), populations were clearly synchronous across the entire province, even at distances of up to 900 km. The epicentral pattern of population development can be used to identify and prioritize adjacent landscape units for both reactive and proactive management strategies intended to minimize mountain pine beetle impacts.     

Movement of outbreak populations of mountain pine beetle: influences of spatiotemporal patterns and climate

Insect outbreaks exert landscape-level influences, yet quantifying the relative contributions of various exogenous and endogenous factors that contribute to their pattern and spread remains elusive. We examine an outbreak of mountain pine beetle covering an 800 thousand ha area on the Chilcotin Plateau of British Columbia, Canada, during the 1970s and early 1980s. We present a model that incorporates the spatial and temporal arrangements of outbreaking insect populations, as well as various climatic factors that influence insect development. Onsets of eruptions of mountain pine beetle demonstrated landscape-level synchrony. On average, the presence of outbreaking populations was highly correlated with outbreaking populations within the nearest 18  km the same year and local populations within 6 km in the previous two years. After incorporating these spatial and temporal dependencies, we found that increasing temperatures contributed to explaining outbreak probabilities during this 15  yr outbreak. During collapse years, landscape-level synchrony declined while local synchrony values remained high, suggesting that in some areas host depletion was contributing to population decline. Model forecasts of outbreak propensity one year in advance at a 12 by 12  km scale provided 80% accuracy over the landscape, and never underestimated the occurrence of locally outbreaking populations. This model provides a flexible approach for linking temperature and insect population dynamics to spatial spread, and complements existing decision support tools for resource managers.     

The phenological window for western spruce budworm: seasonal decline in resource quality

• 1 Western spruce budworm Choristoneura occidentalis Free. (Lepidoptera: Tortricidae) emerge in the spring before budburst and then face a rapidly deteriorating host quality each season.
• 2 Measures of fitness, survival and fecundity, were made on cohorts of final‐instar spruce budworms deployed on host trees at several times during the season in four field locations in coastal and interior British Columbia, Canada.
• 3 Survival and fecundity were strongly correlated throughout the season and varied as much as four‐fold from maxima at mid‐season to minima at the end of the season.
• 4 Fitness values overall were greatest in the coastal compared with interior locations. Among interior locations, fitness was greatest at the highest elevation and least at the lowest elevation. Both cohort and sample‐based estimates of survival of wild, final‐instar budworms were relatively high in these outbreak populations.
• 5 The influence of the phenological window and degree of synchrony with the host plant on herbivore abundance often depends on other processes affecting population rates of change.

     

The fine-scale population dynamics of spruce budworm: survival of early instars related to forest condition

Abstract.  1. A lagged, density-dependent relationship between survival of early instars and host-tree condition is revealed during outbreaks of spruce budworm, Choristoneura fumiferana Clem. Persistent damage to hosts leads to deterioration of the stand.
2. Resource limitation affects survival during early-instar dispersal of spruce budworm. Impediments to distinguishing these events with estimates of survival were overcome with a simple model that describes the dispersal and survival processes. The model was used to analyse a recent 15-year population series from Black Sturgeon Lake and two historical datasets from Green River, in Canada.
3. Defoliation-induced damage to the trees resulted in increased losses of spring-emerging larvae that are dispersing in search of feeding sites. Losses were further exacerbated by biotic factors such as maternal fecundity, rates of infection by the pathogen, Nosema fumiferanae , and by weather-related effects on the foraging period.
4. Survival of early-stage budworm larvae in persistent outbreaks declined and the likelihood of other density-related factors such as rate of mortality from natural enemies increased. These results may reconcile outstanding differences in interpretation of the role of the forest resource in spruce budworm population dynamics and point to a common process linking the dynamics of other well-known budworm species.     

Cross‐scale effects of spruce budworm outbreaks on boreal warblers in eastern Canada

Insect outbreaks are major natural disturbance events that affect communities of forest birds, either directly by affecting the food supply or indirectly by changing the vegetation composition of forest canopies. An examination of correlations between measures of bird and insect abundance across different spatial scales and over varying time lag effects may provide insight into underlying mechanisms. We developed a hierarchical Bayesian model to assess correlations between counts of eight warbler species from the Breeding Bird Survey in eastern Canada, 1966 to 2009, with the presence of spruce budworm (Choristoneura fumiferana Clem.) at immediate local scales and time‐lagged regional scales, as measured by extent of defoliation on host tree species. Budworm‐associated species Cape May warbler (Setophaga tigrina), bay‐breasted warbler (Setophaga castanea), and Tennessee warbler (Oreothlypis peregrina) responded strongly and positively to both local and regional effects. In contrast, non‐budworm‐associated species, Blackburnian warbler (Setophaga fusca), magnolia warbler (Setophaga magnolia), Canada warbler (Cardellina canadensis), black‐throated blue warbler (Setophaga caerulescens), and black‐throated green warbler (Setophaga virens), only responded to regional effects in a manner that varied across eastern Canada. The complex responses by forest birds to insect outbreaks involve both increased numerical responses to food supply and to longer term responses to changes in forest structure and composition. These effects can vary across spatial scales and be captured in hierarchical population models, which can serve to disentangle common trends from data when examining drivers of population dynamics like forest management or climate change.     

A hazards approach towards modelling pandora moth risk

Aim Pandora moth (Coloradia pandora Blake) is a phytophagous insect that produces a distinctive tree‐ring pattern in ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) during outbreak cycles. This paper describes the spatial characteristics of the outbreak regions, determines whether the size of the 1989 outbreak was within the historical range of variability, and constructs a hazard map identifying the forests in Oregon that are susceptible to future pandora moth outbreaks. Location South‐central Oregon along the eastern flank of the Cascade mountain range in the High Lava Plains and Basin and Range Provinces. Methods We used dendrochronological records of 17 pandora moth outbreaks on 14 sites over 31,200 km² area spanning 433 years. Using the site locations, we calculated minimum bounding polygons of adjacent recording sites to determine the relative size of each outbreak. Published literature on past pandora moth outbreaks and the environmental conditions of locally known outbreaks were used to create an outbreak hazard map using a geographical information system (GIS) model. Vegetation, climate, and soil layers were used to determine the potential susceptibility of Oregon forests to pandora moth. Results We found the area affected by past pandora moth outbreaks ranged in size from 12.4 to 3,391.5 km². The 1989 outbreak covered 807.9 km², which was well within the historical range of variability. The vegetation and soil layers greatly restricted the area susceptible to pandora moth while the climate layer seemed to have little effect in restricting the susceptible area. Main conclusions Pandora moth outbreaks did not increase in size over the last century as we have seen with spruce budworm outbreaks in this same region. Analysis of the environmental variables that are known to affect pandora moth outbreaks enabled us to produce a hazard map that predicts the suitable habitat for pandora moth. Temperature at the landscape scale did not restrain the range of pandora moth. The GIS model enabled us to propose areas susceptible to future pandora moth outbreaks providing a predictive model that can now be tested and refined with further sampling.