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

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 measured by Hogg's climate moisture index (CMI), was found to have no discernible impact on aspen radial increment in either northeastern or northwestern 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 humid environments prone to spatially synchronized tent caterpillar outbreaks, herbivory is the main factor limiting aspen radial 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 parameters 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 defoliation, and that the outbreak signal necessarily includes minor defoliation episodes that occur in between the major decadal outbreak cycles.Résumé : Nous avons étudié les effets à long terme de la sécheresse et de la défoliation causée par la livrée des forêts sur la croissance radiale du peuplier faux-tremble dans la province d'Ontario à l'aide d'une approche dendroécologique. Nous avons trouvé que la sécheresse, mesurée par l'indice d'humidité du climat de Hogg, n'a pas eu d'impact discernable sur l'accroissement radial du peuplier faux-tremble, ni dans le nord-est, ni dans le nord-ouest de l'Ontario au cours de la péri-ode considérée dans l'étude, soit de 1930 à 2003. Les épidémies de livrée des forêts ont eu une périodicité fortement dé-cennale dans les deux régions et ont produit des patrons semblables de variations périodiques dans les chronologies de largeur de cernes annuels, indiquant que dans les milieux humides sujets à des épidémies de livrée des forêts synchronisées dans l'espace, l'herbivorisme est le principal facteur qui limite la croissance radiale du peuplier faux-tremble. Nous montrons que les principaux cycles décennaux d'épidémie de livrée des forêts peuvent, en filtrant avec le programme informatique « OUTBREAK », être reconstitués de façon fiable à partir des chronologies brutes de largeur de cernes de peuplier faux-tremble. Nous avons déterminé les paramètres de filtration qui permettent d'ajuster de façon la plus fiable la reconstitution avec le patron des épidémies observé dans chaque région. Nous montrons que le signal des épidémies périodiques est présent même dans les régions où les responsables de l'inventaire aérien ont souvent omis de détecter des niveaux significatifs de défoliation et que le signal d'une épidémie inclut nécessairement des épisodes mineurs de défoliation qui surviennent entre les cy...

Early 20<sup>th</sup> Century Climate-Driven Shift in the Dynamics of Forest Tent Caterpillar Outbreaks

“…Detailed methods of sample preparation, cross-dating, and data processing are described and discussed elsewhere [10].…”

“…is the principal host-tree-species consumed by forest tent caterpillar larvae [6] and tent caterpillar defoliation is the major factor accounting for large-scale temporal variation in aspen ring-widths, outweighing, by far, any effect of moisture limitations caused by drought [9]. Outbreaks of forest tent caterpillar can thus be inferred from careful measurements of aspen ring-widths [10].…”

Impacts of insect outbreaks on tree mortality, productivity, and stand development

“…Cooke and Roland (2007) used dendro-ecological analysis to investigate trembling aspen responses to drought and defoliation by forest tent caterpillar in Ontario and S144 Can. Entomol.…”

Repeated insect outbreaks promote multi-cohort aspen mixedwood forests in northern Minnesota, USA

“…Published historical records and maps were used to identify the agents potentially responsible for the canopy disturbances determined by methods outlined above. A GIS of digitized disturbances aerially surveyed from 1995 to 2009, as well as non-digitized aerial surveys spanning 1969-1994, provided by MNDNR, yielded qualitative information pertaining to the extent and severity of wind damage and defoliation by eastern spruce budworm (SBW: Choristoneura fumiferana) and forest tent caterpillar (FTC: Malacosoma disstria) -two insects known to affect aspen mixedwood systems (Sturtevant et al, 2004;Cooke and Roland, 2007). Other defoliating insects are known to affect aspen mixedwoods, such as the large aspen tortrix (Choristoneura conflictana) which defoliates P. tremuloides; however, we focus on SBW and FTC, both of which are held in the regional literature as the dominant defoliating insects in these forests, acknowledging that other sources of insect defoliation exist.…”