Biological factors influencing stem deterioration rates and salvage planning in balsam fir killed after defoliation by spruce budworm

Abstract

Balsam fir (Abiesbalsamea (L.) Mill.) trees dead between 0 and 5 years were sampled in three widely separated areas that had distinctly different population levels of the secondary, stem-attacking, balsam fir bark beetle (Pityokteinessparsus (Lec.)). The percentage of stem volume affected by sap rot increased significantly with the number of years since tree death. The wood volume lost in debarking tests, simulating pulp mill conditions, averaged 23% in trees dead 4–5 years in the two areas with the most sap rot, and in individual trees, debarking losses were significantly correlated with the extent of sap rot. Sap rot developed most quickly where stems had the most bark beetle activity, whereas where the bark beetle population was very low, sap rot development was significantly slower. These results are consistent with those of many earlier studies and observations in which a close relationship has been noted between the extent of sap rot and the intensity of bark beetle attack in stands of budworm-killed balsam fir. Evidence is presented indicating that stem deterioration rates and beetle activity frequently change with time in specific regions during any one budworm outbreak. It is postulated that inoculum levels of Polyporusabietinus (Dicks ex Fr.), the fungus responsible for the sap rot, and population levels of the balsam fir bark beetle are similarly and greatly affected by earlier occurrences of balsam fir mortality in a region. The average density of bark holes, caused by bark beetle tunneling in recently killed trees, can be used as a nondestructive and quick indicator of the average rate of development and probable extent of stem sap rot in budworm-damaged balsam fir stands. This information can be useful in assessments of the economic feasibility of proposed salvage operations.