Spatial synchrony of spruce budworm outbreaks in eastern North America

We investigated the spatial synchrony of outbreaks of the spruce budworm, C horistoneura fumiferana, over much of its outbreak range in eastern North A merica during the period 1945-1988. Spatial synchrony decreased with distan ce between local populations and approached zero near 2000 km. Investigatio n of the synchrony of local population time series with cluster analysis re vealed a pattern of geographically distinct blocks of clusters oriented alo ng an east-west axis. Spatial synchrony also was identified in monthly temp erature and precipitation time series at 18 weather stations over the same time period and geographical range as the spruce budworm outbreaks. Cross c orrelations decreased linearly with distance between stations and approache d zero near 3000 km and 1800 km, respectively. We developed a spatially exp licit lattice model for a single species occupying multiple patches. Within patches, the model had first order logistic dynamics, and patches were lin ked by dispersal that depended upon their separation distances. Both local and regional stochasticity (i.e., a Moran effect) were present. The modeled lattice had the same spatial configuration as the outbreak region to facil itate investigating the relative effects of a Moran effect and dispersal on spatial synchrony. Simulations with and without a simple region-wide Moran effect and three levels of dispersal did not produce the decrease in spati al synchrony with distance observed with spruce budworm time series. Howeve r, when run at the highest dispersal rate, those simulations produced clust er maps similar to that observed for spruce budworm defoliation. Simulation s with a spatially autocorrelated disturbance that had either zero or high local variability and three levels of dispersal produced decreases in spati al synchrony with distance similar to that observed in the historical data. When run at the highest dispersal rate, simulations yielded cluster maps s imilar to the cluster map for defoliation. We discuss the potential signifi cance of the spatially autocorrelated disturbance factor in understanding r egional insect outbreaks. We also consider the plausibility of dispersal ra tes used in our simulations. We suggest in conclusion that spruce budworm o utbreaks were synchronized by a combination of a spatially autocorrelated M oran effect and a high dispersal rate.