Stand-level prediction of balsam fir mortality in relation to spruce budworm defoliation

Abstract

Stand production and sustained yield calculations are largely affected by tree mortality, which can be caused by many factors such as competition, insect damage, or climatic events. In the eastern Canadian boreal forest, spruce budworm (Choristoneura fumiferana (Clem.)) defoliation can produce varying levels of mortality in balsam fir (Abies balsamea (L.) Mill.) stands. This mortality was estimated for the entire range of balsam fir in Quebec, Canada, using historical records of insect defoliation and permanent sample plot (PSP) inventories for the 1970–2003 period, which includes the last insect outbreak. A two-step approach was used to model balsam fir mortality at the stand level. The first step predicts the probability that all balsam fir trees within a PSP will survive during a given time interval. The second step quantifies the amount of mortality for PSP observation periods during which mortality actually occurred. The whole model shows that spruce budworm defoliation may account for between 6% and 100% of the merchantable volume lost due to mortality, depending on outbreak severity. A model evaluation made with an independent data set indicates that the model is unbiased, although the prediction error is relatively large at the stand level but decreases with increasing prediction horizon.