Larval mortality from phenological mismatch can affect outbreak frequency and severity of a boreal forest defoliator

Climate change affects species’ phenology. This, in turn, may affect the dynamics of interacting species, such as consumer-resource systems, when the species react differently to a change in temperature regime. We study one particular mechanism of climate change impact on the spruce budworm - balsam fir system in Canada's boreal forest. More precisely, we investigate how changes in larval survival due to phenology mismatch can affect frequency and severity of outbreaks. The spruce budworm is a defoliator whose recurring outbreaks are considered the most severe biotic disturbance in the North American boreal forest. Its main tree host is balsam fir. We use a stochastic, dual-timescale stand-level model that integrates individual phenology with community dynamics of the spruce budworm - balsam fir system to explore the potential long-term effects of differentially changing phenologies of the two species. In our model, we use realistic temperature time series to estimate budburst phenology as well as spring emergence of the insect. The potential mismatch between these dates affects larval survival in any given year and thereby long-term outbreak dynamics. Our simulations indicate that a succession of three years of strong mismatch (i.e., low survival) could end a local outbreak prematurely while leaving the forest susceptible to subsequent outbreaks. Similarly, strong mismatch could delay the beginning of a local outbreak. Simulations across the geographic distribution of spruce budworm show that phenological mismatch is expected to increase with climate change, especially in southern regions, with the consequence that outbreaks could become more frequent, but potentially less severe. Our study indicates that including phenology effects of climate change is important to predict future outbreak dynamics of boreal consumer resource systems.