Insect defoliators trading spaces in a hemiboreal forest: Implications for diversity-stability and ecosystem...

Periodic outbreaks of forest insect pests shape northern forest dynamics, yet interactions between multiple insect-host systems-and their role in ecosystem metastability and resilience-remain poorly understood. To elucidate these spatiotemporal interactions, we investigated the coupled dynamics of two dominant insect-host systems-spruce budworm (SBW) associated with late-successional fir and spruce, and forest tent caterpillar (FTC) associated with early-successional aspen. We assess whether asynchronous outbreak patterns shift over time and whether those shifts are consistent with forest compositional change. Using tree-ring data from mixedwood sites across a 20,000-km² hemiboreal landscape at the United States-Canada border, we reconstructed outbreak histories (1928-2005) and applied multi-scaled analyses guided by resilience theory. We analyzed outbreak dynamics across a continuous range of temporal resolutions, summarizing patterns into short (years), intermediate (decade), and long (multi-decade) windows to evaluate spatial persistence and cross-species correlations, and examined whether patterns over the longest windows align with contemporary forest composition. SBW and FTC cycled independently over short windows, but cross-correlations became increasingly negative as windows lengthened to multi-decadal scales, consistent with compositional feedbacks. Spatial persistence within species shifted from positive over short windows to strongly negative over long windows, where outbreak centers reciprocally inverted between early and contemporary periods-a "trading spaces" dynamic indicative of metastability. These patterns show that insect outbreaks are not passive responses but feedback-driven agents interacting with anthropogenic legacies that accelerate compositional change. Our findings highlight how cross-scale feedbacks involving slow and fast regulatory processes shape disturbance regimes, thus supporting resilience theory and the concept of dynamic stability. Forest homogenization through fire suppression and past logging may collapse the stability landscape into a biphasic regime dominated by SBW and FTC, whereas increasing compositional and structural diversity can expand it into multiphasic regimes that attenuate disturbance amplitudes and promote asynchronous dynamics-underscoring the role of diversity-stability and resilience principles in guiding forest management.