Barry J. Cooke

Potential for range expansion of mountain pine beetle into the boreal forest of North America.

Abstract The potential for mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae), to expand its historical range in North America from west of the continental divide into the eastern boreal forest was assessed on the basis of analyses of the effects of climate and weather on brood development and survival, and key aspects of the interaction of mountain pine beetle with its hosts and associated organisms. Variation in climate suitability and high host susceptibility in the boreal forest create a finite risk of establishment and local persistence of low-level mountain pine beetle populations outside their historical range. Eventually, these populations could become widespread and cause epidemic infestations, creating an ecological pathway eastward through the boreal forest. Such infestations would reduce the commercial value of forests and impose an additional disturbance on native ecological systems.

Mountain pine beetle host‐range expansion threatens the boreal forest

The current epidemic of the mountain pine beetle (MPB), an indigenous pest of western North American pine, has resulted in significant losses of lodgepole pine. The leading edge has reached Alberta where forest composition shifts from lodgepole to jack pine through a hybrid zone. The susceptibility of jack pine to MPB is a major concern, but there has been no evidence of host‐range expansion, in part due to the difficulty in distinguishing the parentals and their hybrids. We tested the utility of a panel of microsatellite loci optimized for both species to classify lodgepole pine, jack pine and their hybrids using simulated data. We were able to accurately classify simulated individuals, and hence applied these markers to identify the ancestry of attacked trees. Here we show for the first time successful MPB attack in natural jack pine stands at the leading edge of the epidemic. This once unsuitable habitat is now a novel environment for MPB to exploit, a potential risk which could be exacerbated by further climate change. The consequences of host‐range expansion for the vast boreal ecosystem could be significant.

15 – Insect Defoliators as Periodic Disturbances in Northern Forest Ecosystems

Recurrent outbreaks of forest insect populations have been discussed extensively in the ecological literature but rarely from the perspective of disturbance ecology. The reason lies, in part, in the traditional focus of quantitative animal ecologists on the species of interest contrasted with the traditional emphasis of plant ecologists on whole communities. This chapter presents the argument that herbivorous insects constitute a class of forest disturbance that is distinct from fire, windthrow, or flooding. Insect outbreaks tend to be spatially synchronized and temporally periodic (periods of high impact followed by periods of low impact over extensive areas), and insect outbreaks are predictably selective, and therefore result in different legacies than do abiotic disturbances. The chapter suggests that recognition of the distinct nature of disturbances caused by insect outbreaks may shed light on the analysis of other disturbances that are, by nature, aperiodic and difficult to forecast. It reviews the dynamics of several outbreak systems. To study these relationships, the chapter uses a comparative, process-oriented approach and illustrates the approach by comparing the ecologies of four species of insect defoliators.

Spatial analysis of large-scale patterns of forest tent caterpillar outbreaks.

Abstract A spatial analysis of forest tent caterpillar (Malacosoma disstria Hübner) outbreak data from Ontario indicates that previous studies examining the influence of forest heterogeneity and climate on outbreak vulnerability were pseudoreplicated. We adopted a spatially explicit modeling framework to re-examine, at several spatial scales, the influence of forest heterogeneity on outbreak duration, and also tested for effects of other plausible environmental factors including winter temperature, spring degree-day accumulation, and elevation. Of these, forest heterogeneity was the strongest predictor of the number years of defoliation recorded over three outbreak cycles. Nevertheless, correlations were inconsistent across districts and were often weak. We suggest that this is due either to the involvement of other factors or to neighbourhood effects associated with contagious population processes such as insect dispersal. The precise role of climatic perturbations in governing outbreak dynamics remains unclear but may be elucidated through studies at larger and smaller spatial scales.

The effect of winter temperature on forest tent caterpillar (Lepidoptera: Lasiocampidae) egg survival and population dynamics in northern climates

Abstract Overwintering mortality of forest tent caterpillar [Malacosoma disstria (Hübner)] eggs was estimated over a 360 km2 grid of 83 plots in north-central Alberta over the period 1992–1996 during a local outbreak. Egg mortality in the trembling aspen (Populus tremuloides Michaux) canopy was generally low; however, 20% of the eggs laid in the summer of 1995 failed to hatch in the spring of 1996. In the shrub layer, 70% failed to hatch. In both the canopy and shrub layers, the spatial pattern of mortality was density-independent, with high mortality occurring in low-lying areas. Daily temperature records suggested that the proximal cause of death was freezing during mid-winter. Caterpillar populations peaked in 1995, before perturbation, and collapsed during the summer of 1996, largely as a result of larval parasitism. The timing of this perturbation-assisted population collapse coincided loosely with the penetration of larvae down into the shrub layer. We illustrate how winter temperature, albeit a density-independent factor, probably acts in a partially density-dependent manner through interactions with density-dependent behavioral and physiological processes that influence spatial variation in vulnerability and susceptibility to winter cold. We argue that cold winter temperatures are an important factor influencing the long-term dynamics of forest tent caterpillar populations in northern climates.

Trembling aspen responses to drought and defoliation by forest tent caterpillar and reconstruction of recent outbreaks in Ontario

We investigated the long-term effects of drought and defoliation by forest tent caterpillars on trembling aspen radial growth in the province of Ontario using a dendroecological approach. Drought, as measured by Hoggs climate moisture index (CMI), was found to have no discernible impact on aspen radial increment in either northeastern or north- western Ontario during the study period 1930-2003. Forest tent caterpillar outbreaks were strongly decadal in periodicity in both regions and resulted in similar patterns of periodic variation in tree ring-width chronologies, indicating that, in hu- mid environments prone to spatially synchronized tent caterpillar outbreaks, herbivory is the main factor limiting aspen ra- dial growth. We show that the major decadal outbreak cycles of forest tent caterpillar can, by filtering with the computer program OUTBREAK, be reliably reconstructed from raw aspen ring-width chronologies. We determine the filtering pa- rameters that give the most reliable reconstruction fit to observed patterns of outbreaks in each region. We show that the periodic outbreak signal is present even in areas where aerial surveyors frequently failed to detect significant levels of de- foliation, and that the outbreak signal necessarily includes minor defoliation episodes that occur in between the major dec- adal outbreak cycles.

If forest dynamics in Canada's west are driven mainly by competition, why did they change? Half-century evidence says: Climate change.

In a recent paper (1), Zhang et al. present analyses of “forest dynamics” inferred from measurements collected during 1958–2009 at permanent sample plots (PSP) distributed across Canada’s western forests. Their results are almost unanimous in showing widespread increases in mortality, and declines in relative growth and recruitment (figure 2 in ref. 1). Zhang et al. conclude these trends are explained primarily by changes in stand-scale competition, and that recent changes in climate are of secondary importance. Surprisingly, Zhang et al. do not explain the temporal changes in competition they detected. We accept that stand dynamics depend upon competition for light, nutrients, and water, but argue that climate affects the supply of these resources. We find some major problems with the report by Zhang et al., including misinterpretation of results and a critical lack of clarity on key model assumptions, which cast serious doubt on their conclusions.

Loblolly pine seed dormancy: constraints to germination.

Stratification by moist chilling is often used to break seed dormancy in loblolly pine (Pinus taeda L.). The role that moist chilling plays at the cellular level in preparing the embryo for germination and early seedling growth was investigated. Moist chilling did not affect the embryos ability to mobilize seed storage proteins, which is a biochemical marker of early seedling growth. Analysis of in vivo protein synthesis profiles indicated that moist chilling has only a subtle effect on gene expression in the embryo or germinant. The means by which moist chilling affects interactions between the seed coat and the living tissues of the seed also was studied. Although the seed coat is mainly a mechanical barrier to germination, seed coat replacement studies showed that moist chilling alleviates some factor(s) in the seed coat that significantly inhibit germination.

Model-specification uncertainty in future forest pest outbreak.

Climate change will modify forest pest outbreak characteristics, although there are disagreements regarding the specifics of these changes. A large part of this variability may be attributed to model specifications. As a case study, we developed a consensus model predicting spruce budworm (SBW, Choristoneura fumiferana [Clem.]) outbreak duration using two different predictor data sets and six different correlative methods. The model was used to project outbreak duration and the uncertainty associated with using different data sets and correlative methods (=model-specification uncertainty) for 2011-2040, 2041-2070 and 2071-2100, according to three forcing scenarios (RCP 2.6, RCP 4.5 and RCP 8.5). The consensus model showed very high explanatory power and low bias. The model projected a more important northward shift and decrease in outbreak duration under the RCP 8.5 scenario. However, variation in single-model projections increases with time, making future projections highly uncertain. Notably, the magnitude of the shifts in northward expansion, overall outbreak duration and the patterns of outbreaks duration at the southern edge were highly variable according to the predictor data set and correlative method used. We also demonstrated that variation in forcing scenarios contributed only slightly to the uncertainty of model projections compared with the two sources of model-specification uncertainty. Our approach helped to quantify model-specification uncertainty in future forest pest outbreak characteristics. It may contribute to sounder decision-making by acknowledging the limits of the projections and help to identify areas where model-specification uncertainty is high. As such, we further stress that this uncertainty should be strongly considered when making forest management plans, notably by adopting adaptive management strategies so as to reduce future risks.

Influence of water deficit on the molecular responses of Pinus contorta × Pinus banksiana mature trees to infection by the mountain pine beetle fungal associate, Grosmannia clavigera.

Conifers exhibit a number of constitutive and induced mechanisms to defend against attack by pests and pathogens such as mountain pine beetle (Dendroctonus ponderosae Hopkins) and their fungal associates. Ecological studies have demonstrated that stressed trees are more susceptible to attack by mountain pine beetle than their healthy counterparts. In this study, we tested the hypothesis that water deficit affects constitutive and induced responses of mature lodgepole pine × jack pine hybrids (Pinus contorta Dougl. ex Loud. var. latifolia Engelm. ex S. Wats. × Pinus banksiana Lamb.) to inoculation with the mountain pine beetle fungal associate Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield. The degree of stress induced by the imposed water-deficit treatment was sufficient to reduce photosynthesis. Grosmannia clavigera-induced lesions exhibited significantly reduced dimensions in water-deficit trees relative to well-watered trees at 5 weeks after inoculation. Treatment-associated cellular-level changes in secondary phloem were also observed. Quantitative RT-PCR was used to analyze transcript abundance profiles of 18 genes belonging to four families classically associated with biotic and abiotic stress responses: aquaporins (AQPs), dehydration-responsive element binding (DREB), terpene synthases (TPSs) and chitinases (CHIs). Transcript abundance profiles of a TIP2 AQP and a TINY-like DREB decreased significantly in fungus-inoculated trees, but not in response to water deficit. One TPS, Pcb(+)-3-carene synthase, and the Class II CHIs PcbCHI2.1 and PcbCHI2.2 showed increased expression under water-deficit conditions in the absence of fungal inoculation, while another TPS, Pcb(E)-β-farnesene synthase-like, and two CHIs, PcbCHI1.1 and PcbCHI4.1, showed attenuated expression under water-deficit conditions in the presence of fungal inoculation. The effects were observed both locally and systemically. These results demonstrate that both constitutive and induced carbon- and nitrogen-based defenses are affected by water deficit, suggesting potential consequences for mountain pine beetle dynamics, particularly in novel environments.