Search

Using multi-scale trembling aspen tree-ring width data from Alberta, we show that scaling has a profound influence on dendroecological inferencing. At all scales of sampling, there is a significant climatological signal whose strength is nevertheless superseded by the pervasive effect of insect herbivory. At the smallest spatial scale, 20 km * 20 km, we demonstrate a quasi-periodic pattern of sharp growth reductions and the existence of negative spatial correlations among successive outbreaks. At the intermediate spatial scale of 20 km * 80 km, we show that the period 1930-1963 was marked by extremely low correlations in aspen ring widths, with much higher correlations occurring both before and after, despite high variance in precipitation that might have caused ring widths to correlate under moisture limitation. At the largest spatial scale of the entire province, aspen ring widths in the boreal forest became decorrelated abruptly in 1916, after cycling synchronously for the previous 80 years. This occurred despite a warming climate that heightened moisture limitation. Something in the environment prevents some outbreak cycles from rising to maximum intensity and from spreading to maximum extent. This appears to be the key to better predictability and management of forest insects and forests at short and long time-scales. Key words: forest tent caterpillar, insect dendroecology, outbreak periodicity, synchronization, trembling aspen, Introduction Resources, such as food and shelter, are distributed patchily in space, affecting the dynamics of both animal population growth and movement (Wiens 1976; Baker 1978; Hanski 1999). In the [...]

I outline the "definitional problem" in forest insect outbreak analytics and show how it is related to the "counting problem" in dendroentomology and the "forecasting problem" in forest insect population dynamics, through the ubiquitous presence of non-stationary complex periodicity. Using real-world examples from the spruce budworm (Choristoneura fumiferana Clem.) system, I show that regardless how outbreak patterning is characterized—whether by peak impact, cycle frequency, interval duration, or interval severity—the distribution in pattern attributes appears to be extremely variable, regardless how the data are processed through definitional filters. I show that this extreme variability is an unavoidable and key feature of the system's dynamics and argue that it needs to be viewed as an object of study, instead of a nuisance problem to be swept under the rug. The single biggest opportunity for rapid gains in spruce budworm predictive ecology is determining the environmental and ecological factors that separate high-intensity from low-intensity outbreak cycling. [ABSTRACT FROM AUTHOR]

I report on long-term patterns of outbreak cycling in four study systems across Canada and illustrate how forecasting in these systems is highly imprecise because of complexity in the cycling and a lack of spatial synchrony amongst sample locations. I describe how a range of bottom-up effects could be generating complexity in these otherwise periodic systems. (1) The spruce budworm in Québec exhibits aperiodic and asynchronous behavior at fast time-scales, and a slow modulation of cycle peak intensity that varies regionally. (2) The forest tent caterpillar across Canada exhibits eruptive spiking behavior that is aperiodic locally, and asynchronous amongst regions, yet aggregates to produce a pattern of periodic outbreaks. In Québec, forest tent caterpillar cycles differ in the aspen-dominated northwest versus the maple-dominated southeast, with opposing patterns of cycle intensity between the two regions. (3) In Alberta, forest tent caterpillar outbreak cycles resist synchronization across a forest landscape gradient, even at very fine spatial scales, resulting in a complex pattern of cycling that defies simple forecasting techniques. (4) In the Border Lakes region of Ontario and Minnesota, where the two insect species coexist in a mixedwood landscape of hardwood and conifers, outbreak cycle intensity in each species varies spatially and temporally in response to host forest landscape structure. Much attention has been given to the effect of top-down agents in driving synchronizable population cycles. However, foliage loss, tree death, and forest succession at stem, stand, and landscape scales affect larval and adult dispersal success, and may serve to override regulatory processes that cause otherwise top-down-driven periodic, synchronized, and predictable population oscillations to become aperiodic, asynchronous, and unpredictable. Incorporating bottom-up effects at multiple spatial and temporal scales may be the key to making significant improvements in forest insect outbreak forecasting. [ABSTRACT FROM AUTHOR]

We use simulation models of the winter biology of two forest insects--emerald ash borer (Agrilus planipennis Fairmaire, 1888) and mountain pine beetle (Dendroctonus ponderosae Hopkins, 1902)--to illustrate the anticipated effect of the polar vortex of the winter of 2019 on overwintering survival. Cold spells in late January and early and mid-February were cool enough that relatively high levels of mortality of both species may be expected in many parts of northern Canada. However, the high mortality response is not ubiquitous, and neither is it particularly severe compared with winter weather patterns from earlier decades. Forest insect pest managers will be required to carry on their usual work of population assessment and response. As Earth's climate continues to warm, overwintering survival should, on average, increase in both species. However, the common occurrence of a split polar vortex associated with increasingly meridional jet stream flows should bring the odd winter of heavy mortality. Key words: insect ecophysiology, cold tolerance, winter mortality, predictive modeling. Nous avons utilise des modeles de simulation de la biologie hivernale de deux insectes forestiers--l'agrile du frene (Agrilus planipennis Fairmaire, 1888) et le dendroctone du pin ponderosa (Dendroctonus ponderosae Hopkins, 1902)--pour illustrer les effets anticipes du vortex polaire de 2019 sur la survie hiemale. Les vagues de froid qui sont survenues a la fin janvier ainsi qu'au debut et a la mi-fevrier ont ete suffisamment intenses pour qu'on puisse s'attendre a des taux de mortalite relativement eleves chez les deux especes dans plusieurs regions du nord du Canada. Cependant, la mortalite elevee n'est pas une reponse generalisee ni particulierement severe comparativement a l'allure du climat hivernal des decennies precedentes. Les gestionnaires des insectes forestiers nuisibles devront poursuivre leur travail habituel concernant l'evaluation et la reaction des populations. La survie hiemale devrait en moyenne augmenter chez les deux especes a mesure que le climat de la terre continue de se rechauffer. Cependant, l'occurrence frequente d'un vortex polaire divise associe a une circulation de plus en plus meridionale du courant jet devrait produire des hivers inhabituels accompagnes d'une forte mortalite. [Traduit par la Redaction] Mots-cles: ecophysiologie des insectes, tolerance au froid, mortalite hivernale, modelisation predictive., Introduction One of the most important factors that has historically limited the spread and impact of terrestrial invasive species in Canada has been the country's relatively cold climate. This climate [...]

Defoliators cause extensive damage in boreal and temperate forests of the world. Considerable effort has been invested to understand their individual population dynamics, and despite ample theorising, there is little empirical evidence on factors causing spatial synchrony of pest eruptions at landscape scales.We report on the landscape‐level effect of forest configuration and composition on the intensity of outbreaks of spruce budworm and forest tent caterpillar in a mixedwood boreal forest in northern Minnesota (USA) and adjacent Ontario (Canada), and how this is related to the degree of spatial synchrony in each species' outbreak cycling.Using a large spatiotemporal tree‐ring reconstruction of outbreak impacts across these two systems, we evaluate two contrasting theories governing defoliator outbreaks: harmonic oscillation (a.k.a. 'clockwork') and relaxation oscillation (a.k.a. 'catastrophe'), each with consequences linked to top‐down versus bottom‐up influences on outbreak behaviour in time and space.We find synchrony varies temporally, among outbreak cycles and in direct proportion to cycle peak intensity; however, cycle peak intensities are distributed bimodally in time, and so, therefore, are synchrony coefficients. Spatially, the area where each pest species currently cycles with the greatest peak intensity and synchrony is where their preferred host trees are currently found in greatest proportion.Despite overall synchrony in cycling, we found, in both systems, a persistent negative spatial correlation among successive eruptive pulses of defoliation. Many of these eruptions failed to spread spatially and to coalesce with other spot eruptions to form extensive area‐wide outbreaks. Eruptions often fail to spread at the hardwood‐conifer interface, resulting in outbreak pulses that systematically bounce back and forth between landscape types, particularly when systems were cycling at low amplitude. These over‐dispersed spatial patterns of pulse impact are consistent with a contagious theory of eruption and outbreak spread. They could be considered consistent with harmonic oscillation theory only for populations cycling at different frequencies, with cycling frequency determined by host forest landscape structure.Synthesis. We find that defoliator outbreak dynamics across systems include spatiotemporal signatures of each theoretical paradigm—suggesting a hybrid approach will better characterise outbreak behaviour. Host concentration influences which paradigm dominates the spatial dynamics in any given forest landscape context. Because of the synchronising effect of host concentration on forest insect spatial dynamics, mixedwoods appear to be less prone to intense, synchronised defoliator attack than forests of pure hardwood or pure conifer. [ABSTRACT FROM AUTHOR]

Thinning, the selective removal of some trees from a forest, is one way forest managers can reduce the probability that a forest will be susceptible to attack by bark beetles. Although this method has been shown to be effective, it is not clear whether the effect arises when pre-outbreak populations are small or during the epidemic phase when outbreaks are growing. We adopted a population dynamics approach to determine if the effect of limit or basal area thinning could be observed in the form of differential beetle recruitment using lodgepole pine (Pinus contorta Dougl. ex Loud.) and ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) mortality data from previously published studies as a proxy measure of mountain pine beetle (Dendroctonus ponderosae Hopkins) population size. We found that mountain pine beetle populations exhibit density-dependent population dynamics that are influenced by the silvicultural history of their host's stand. Thinning did not change the epidemic equilibrium but instead caused a shift in dynamics from linear to nonlinear. In a validation test, the models developed for thinned and unthinned stands predicted reproductive rates in independent locations. These data also suggest the epidemic dynamics of mountain pine beetle may be sensitive to perturbations and to systematic trends associated with climate variability and climate change. L'eclaircie, c'est-a-dire l'elimination selective de certains arbres dans une foret, est un des moyens qu'utilisent les amenagistes forestiers pour reduire la probabilite qu'une foret soit susceptible aux attaques des scolytes. Meme si cette methode s'est averee efficace, on ne sait pas si l'effet survient lorsque les populations qui precedent une epidemie sont faibles ou durant la phase epidemique lorsque les epidemies progressent. Nous avons adopte une approche de dynamique de population pour determiner si l'effet de l'eclaircie basee sur un diametre limite ou sur la surface terriere pouvait etre observe sous la forme d'une difference de recrutement de scolytes a l'aide de donnees de mortalite du pin tordu latifolie (Pinus contorta Dougl. ex Loud.) et du pin ponderosa (Pinus ponderosa Dougl. ex P. & C. Laws.). Ces donnees viennent d'etudes deja publiees et ont ete utilisees comme mesure indirecte de la taille des populations du dendroctone du pin ponderosa (Dendroctonus ponderosae Hopkins). Nous avons decouvert que les populations de dendroctone du pin ponderosa ont une dynamique de population dependante de la densite qui est influencee par le passe sylvicole de leurs peuplements hotes. L'eclaircie n'a pas modifie l'equilibre epidemique mais a plutot change la dynamique, de lineaire a non lineaire. Dans un test de validation, les modeles developpes pour les peuplements eclaircis et non eclaircis predisaient le taux de reproduction pour des endroits independants. Ces donnees indiquent egalement que la dynamique des epidemies du dendroctone du pin ponderosa pourrait etre sensible aux perturbations et aux tendances systematiques associees a la variabilite du climat et au changement climatique., Introduction Bark beetles (Coleoptera: Scolytinae) are one of the most significant natural disturbance agents of coniferous forests in North America. Since the mid-1990s, large areas of forest have been damaged [...]

Abstract: 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 [...]

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. [ABSTRACT FROM AUTHOR]

Management of spruce budworm, Choristoneura fumiferana (Clem.), outbreak spread requires understanding the demographic processes occurring in low, but rising populations. For the first time, detailed observations were made in the early stages of outbreak development. We sampled populations over a three-year period in both treated and untreated populations in the Lower St-Lawrence region of Quebec, Canada, and measured the density-dependence of survival and population growth rates, and the impact of natural enemies and insecticides. Insecticides tested were Bacillus thuringiensis (Berliner 1915) and tebufenozide. We recorded strong density-dependence of survival between early larval stages and adult emergence, explained largely by the variation of natural enemy impacts and overcrowding. We also observed inverse density-dependence of apparent fecundity: net immigration into lower-density populations and net emigration from the higher, linked to a threshold of ~25% defoliation. Because of high migration rates, none of the 2013 treatments reduced egg populations at the end of summer. However lower migration activity in 2014 allowed population growth to be reduced in treated plots. This evidence lends support to the conclusion that, for a budworm population to increase to outbreak density, it must be elevated via external perturbations, such as immigration, above a threshold density of ~4 larvae per branch tip (L4). Once a population has increased beyond this threshold, it can continue growing and itself become a source of further spread by moth migration. These findings imply that populations can be brought down by insecticide applications to a density where mortality from natural enemies can keep the reduced population in check, barring subsequent immigration. While we recognize that other factors may occasionally cause a population to exceed the Allee threshold and reach outbreak level, the preponderance of immigration implies that if all potential sources of significant numbers of moths are reduced on a regional scale by insecticide applications, a widespread outbreak can be prevented, stopped or slowed down by reducing the supply of migrating moths. [ABSTRACT FROM AUTHOR]