Bark Beetle Activity and Delayed Tree Mortality in the Greater Yellowstone Area Following the 1988 Fires

After the 1988 Greater Yellowstone Area fires two studies were begun to monitor beetle activity in burned and unburned conifers and to evaluate the susceptibility of fire injured trees to bark beetle attack. An intensive survey was conducted annually from 1989 through 1992 on 24 permanent plots located in or near stands burned by surface fire. Stands were located adjacent to areas of extensive crowning and torching. By August 1992, 79% of the 125 Douglas-fir had been infested by bark beetles (primarily by the Douglas-fir beetle) and wood borers; 62% of the 151 lodgepole pine were infested (primarily by the pine engraver); 94% of the 17 Engelmann spruce were infested (primarily by the spruce beetle); and 71% of the 17 subalpine fir were infested (primarily by wood borers). Fire injury combined with subsequent insect attack killed 77% of the Douglas-fir, 61% of the lodgepole pine, 94% of the Engelmann spruce, and all of the subalpine fir. An extensive survey was conducted in 1991 and 1992 on 519 randomly located plots throughout the area. Plots were located in unburned and surface fire-burned areas. Insects killed 13% of the 1,012 Douglas-fir, 18% of the 4,758 lodgepole pine, 7% of the 439 Engelmann spruce, 8% of the 134 subalpine fire, and 3% of the 144 whitebark pine. Foe all species, insect infestation increased with the percent of the basal circumference killed by fire, except for Engelmann spruce where infestation was greatest with 40 to 80% of the basal circumference girdled. Infestation in Douglas-fir, lodgepole pine, and Engelmann spruce increased with time. The age level of infestation suggests that insect populations increased in fire-injured trees and the spread to uninjured trees. Increases form 1991 to 1992 suggest that additional tree mortality will occur in 1993, and that a major outbreak could occur in Engelmann spruce. Delayed tree mortality attributed to fire injury accounted for more mortality than insects. Both types of mortality greatly altered the original mosaics of green trees and dead trees that were apparent immediately after the 1988 fires