Assessing spatial and temporal dynamics of a spruce budworm outbreak across the complex forested landscape of Maine, USA

Key message Simulated spruce budworm (SBW; Choristoneura fumiferana (Clem.)) defoliation generally becomes ubiquitous in 3 years after its initiation in agreement with historical observations despite varying environmental and stand conditions over large ranges. Current-year defoliation has almost no correlation with defoliation more than 1 year ago at the same location, which may be related to the role of SBW dispersal in sustaining defoliation across space and time. Mitigation practices like insecticide spraying may be more efficient if applied early to initial spots (epicenters) of defoliation, while management probably should focus on improving forests' resilience to withstand repeated defoliation by altering species composition. Context SBW defoliation during its periodic and extensive outbreaks greatly affects forest productivity at large spatial and temporal scales. A generalized modeling framework that simultaneously accounts for both highly variable spatial and temporal dynamics of SBW outbreaks has not been developed. Aims To develop a flexible parametric spatiotemporal model to explicitly predict defoliation in continuous space and time in order to evaluate the dynamics of SBW outbreaks across a complex forested landscape. Methods A novel model was developed on extensive defoliation data covering approximately 50,000 km(2) and 10 years of the last SBW outbreak during the 1970s and 1980s in Maine, USA. Simulations of various outbreak scenarios were performed using this model. Results The developed model provided a sufficient fit of the data (R-2 of 0.63 and mean bias of +0.3%) and was relatively consistent with expectations. Simulations show that defoliation generally becomes ubiquitous in 3 years despite varying environmental and stand conditions. Current-year defoliation has almost no correlation with defoliation more than 1 year ago at the same location, which may be related to the role of SBW dispersal in sustaining defoliation across space and time. Conclusion Mitigation practices like insecticide spraying may be more efficient if applied early to initial spots (epicenters) of defoliation, while management probably should focus on improving forests' resilience to withstand repeated defoliation by altering species composition. Our model provides quantitative information flexible in spatial and temporal scales yet directly usable in existing forest growth and yield modeling frameworks and management decision support systems. This generalized spatiotemporal model is readily extendable for evaluating spatial and temporal dynamics of other forms of defoliation across complex forest landscapes.