The Vulnerability of Balsam Fir to Spruce Budworm Attack in Northwestern Ontario, With Special Reference to the Physiological Age of the Tree

The annual defoliation by spruce budworm and the progressive mortality of balsam fir trees were recorded in nine study plots in northwestern Ontario over a period of 11 years. In addition to general observations on the relationship of tree mortality to defoliation, some information was obtained on the relative vulnerability of the trees with respect to size, physiological age (flowering condition), and vigor (site quality).