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

Abstract

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.

Tree diversity drives associational resistance to herbivory at both forest edge and interior

“…Our two studied sites belong to the same biogeographical area, but vary in their forest cover (18.5% vs. 9.2%). The amount of habitat and distances between habitat patches are known to influence metapopulation processes (Gilpin & Hanski, 1991) and hence the colonization probability of host trees by forest insect herbivores (Robert et al, 2018). Forest insect herbivory can be thus driven by a complex interplay between local tree diversity and stand isolation in the landscape (Castagneyrol, Giffard, Valdés-Correcher, & Hampe, 2019).…”

Incorporating Insect and Wind Disturbances in a Natural Disturbance-Based Management Framework for the Boreal Forest

“…Outbreaks are known to occur periodically, at about 30 year intervals on a supra-regional scale, at least during the 20th century [18,[83][84][85][86] (Table 1). In eastern North America, three major outbreaks occurred during the 20th century: the first in 1909-1925, the second from 1947 to 1957 and the last one between 1966 and 1992 [87,88].…”

“…In eastern North America, three major outbreaks occurred during the 20th century: the first in 1909-1925, the second from 1947 to 1957 and the last one between 1966 and 1992 [87,88]. However, the temporal cycles of outbreak and their amplitude (e.g., severity) varied among landscapes being influenced by stand composition and their spatial arrangement [86]. In the beginning of the 21th century, budworm populations were on the rise again and signs of defoliation were observed in the Quebec North-Shore region in 2006.…”

“…As SBW is host-species dependent, the range and distribution of the host species can either limit or allow spatial extent of outbreaks [17,86]. The spatial extent of an outbreak will also be influenced by the dynamics of insect populations (e.g., whether the outbreak starts at an epicenter and spreads, or starts synchronously at several locations in a region as a result of climatic and/or biotic influences) [17,83,93].…”

“…The spatial extent of an outbreak will also be influenced by the dynamics of insect populations (e.g., whether the outbreak starts at an epicenter and spreads, or starts synchronously at several locations in a region as a result of climatic and/or biotic influences) [17,83,93]. However, recent findings also suggest a feedback between forest structure and outbreak spatial dynamics [86]. Such controls of outbreak dynamics will result in large-scale spatial variability in outbreak severity from one outbreak to the next.…”