Shifting disturbances in a warming environment : the western spruce budworm and Douglas-fir beetle in British Columbia

Abstract

Outbreaks of western spruce budworm (Choristoneura occidentalis Freeman), a native defoliator of Douglas-fir, appear to have been occurring further north in British Columbia (BC) compared to the early 1900s potentially as a result of climate change, but there is a lack of quantitative evidence to support this. I analysed the distribution of western spruce budworm infestation centers in forest health survey data for BC from 1967 to 2011 using a geographic information system (GIS) and linear regression. There was a significant (p<0.05) range shift towards higher latitudes and elevations. I also tested the prediction that this range shift was a result of a climate change-induced northward shift in optimal synchrony between springtime budworm larval emergence and Douglas-fir budburst, since the budworm must feed on newly flushed, highly nutritious needles for the best chance of survival. I simulated the dates of first larval emergence and budburst annually from 1901 to 2011 using phenology models and long-term climate data, then analysed them with GIS and linear regression. The mean difference in dates at higher latitudes and elevations has indeed significantly (p<0.05) converged towards optimal synchrony over the last century, while lower latitudes show divergence from optimal synchrony. Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins), a native bark beetle that infests Douglas-fir in BC, has been associated with stands recently defoliated by western spruce budworm, leading to increased mortality of large-diameter trees. I predicted that moderate defoliation intensity, which reduces tree defenses while leaving thick phloem for optimal Douglas-fir beetle breeding material, will be most closely associated with subsequent Douglas-fir beetle infestation. Evaluation of generalized additive models with long-term insect and biophysical variables showed that moderate cumulative severity of a defoliation event greatly improved model fit for predicting the presence of subsequent Douglas-fir beetle infestation. I concluded that interactions between biotic disturbance agents is an important consideration in future research and forest management because of additive or synergistic effects and how changes in the outbreak dynamics of one species under climate change will affect the other.