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Monday, April 9 • 2:30pm - 2:45pm
INSECT & DISEASE OUTBREAKS: Fragmentation of Forest Host Disrupts Cycling Behavior of Defoliator Outbreaks: Evidence from Spruce Budworm and Forest Tent Caterpillar in a Heterogeneous Mixedwood Landscape

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AUTHORS: Barry J. Cooke*, Canadian Forestry Service; Brian R. Sturtevant, USDA Forest Service; Louis-Etienne Robert, University of Montreal; Daniel Kneeshaw, University of Quebec at Montreal

ABSTRACT: The spruce budworm (Choristoneura fumiferana Clem.) and forest tent caterpillar (Malacosoma disstria Hbn.) are early-season defoliators of spruce/fir trees and aspen/maple trees, respectively. Both exhibit periodic outbreaks – albeit at different time scales – so they are often perceived as stereotypical “cyclic” forest defoliator species. We contrasted the outbreak dynamics of these two defoliators as they related to forest landscape structure via tree-ring studies within a common landscape. The Border Lakes landscape is a large (20,000 km2) ecoregion containing contrasting land management zones with clear differences in forest landscape structure (i.e., concentration and spatial configuration of host species for each defoliator) while minimizing the confounding influence of climate. We found that outbreaks spruce budworm were more strongly periodic, more synchronous, and more severe in regions with higher concentrations of its host trees, with analogous results for forest tent caterpillar related more strongly to forest fragmentation metrics. However, we were surprised to find each species exhibited complex patterns of spatio-temporal autocovariance that led to a significant departure from purely cyclic, synchronous behaviour. Temporally, cycle peaks were distributed tri-modally, not uni-modally, as predator-prey theory would predict. Spatially, successive outbreak cycles tended to occur in disparate parts of the study area, which is not consistent with Moran’s theorem of cycle synchronization. The net emergent effect was a breakdown in cycle synchrony in those parts of the landscape where host trees were sparse. Spruce budworm tended cycle synchronously where forest tent caterpillar did not, and vice versa. Taken together, this suggests that forest landscape structure modulates cycle amplitude and synchrony, regardless of the herbivore-plant species association. It further suggests a homeostatic mechanism whereby severe outbreaks associated with high abundances of host-tree species tend to be followed by less severe outbreaks occurring in more diverse residual forests.

Monday April 9, 2018 2:30pm - 2:45pm CDT
LaSalle 2 (7th Floor)

Attendees (5)