Effect of forest stand and edge characteristics on the vulnerability of jack pine stands to jack pine budworm (Choristoneura pinus pinus) damage

We determined how factors originating within individual jack pine (Pinus banksiana Lamb.) forest stands (local effects) and factors originating in neighbouring forest stands (adjacency effects) affected and modified severity and consequences of defoliation during a jack pine budworm (Choristoneura a pinus pinus Freeman) (Lepidoptera, Tortricidae) outbreak in northern Michigan from 1992 to 1994. Defoliation, top kill, and tree mortality were monitored in 104 stands annually and forest inventory data were collected for each stand. Defoliation was widespread in 1992 and then declined in 1993 and 1994. The oldest stands were most susceptible to defoliation, top kill, and mortality. Contrary to expectations, mortality was highest in relatively good sites. However, characteristics of adjacent stands also affected the susceptibility of a focal stand: stands adjacent to younger trees sustained more defoliation than would be expected based on their age. We hypothesize that pollen cones, which contribute to survival of early instar larvae, may have been more abundant in trees exposed to full light, a situation likely to occur along the edges of stands growing next to openings, roads, or young trees. Thus, both the stand age and boundary to a younger and shorter forest stand may contribute to budworm outbreak at the local level.