Vulnerability of Fir-Spruce Stands During Uncontrolled Spruce Budworm Outbreaks: A Review and Discussion

The concept of vulnerability to spruce budworm refers to the probability of tree mortality resulting from a given level of budworm attack. This paper reviews and analyses available information from the literature on stand vulnerability and timing of mortality during several budworm outbreaks. Timing of mortality during outbreaks appears to be fairly straightforward when considered in a general sense, with trees usually starting to die after four or five years of severe defoliation. Prediction of the total expected mortality in a given stand is more difficult, however, and attempts to relate mortality to stand characteristics have not shown strong relationships. Nevertheless, the data analysed suggest that for approximation purposes in a management sense, we can probably treat mortality during uncontrolled budworm outbreaks as a host-density dependent process. This is particularly so for fir in mature stands, where mortality approaches 100%, while fir in immature stands and spruce show lower, more variable mortality rates.