Long-term effects of jack pine budworm outbreaks on the growth of jack pine trees in Michigan

Growth of jack pine (Pinus banksiana Lamb.) trees from the Raco Plains area in the Upper Peninsula of Michigan was examined over an 18-year period (1978-1995) that included two jack pine budworm (Choristoneura pinus pinus Freeman) outbreaks. Specific volume increments were calculated for 84 trees grouped into three classes based on their status in 1996; 36 trees were undamaged, 24 trees had been recently top-killed, and 24 trees had been recently killed. Average growth was converted to proportion of previous years' growth for three periods: before the 1983-1985 outbreak, between the 1983-1985 and 1991-1993 outbreak, and after the onset of the 1991-1993 outbreak. Differences in growth over these periods among undamaged, recently top-killed, and recently killed trees were evaluated. Growth did not differ among the three groups before the 1983-1985 outbreak. From 1983-1990, undamaged and recently top-killed trees grew significantly more than recently killed trees. There was no difference in average growth from 1983 to 1990 between undamaged and recently top-killed trees. Growth of undamaged trees was significantly greater than growth of recently top-killed trees following the onset of defoliation from the 1991-1993 outbreak. Patterns of growth loss suggest that a history of defoliation stress from multiple budworm outbreaks was an important determinant of tree mortality.