Harvest Retention Survivorship of Endangered Whitebark Pine Trees

Abstract

Whitebark pine (Pinus albicaulis Engelm.) is a widely distributed high-elevation species in western North America that is threatened primarily by an introduced disease and other disturbances. In British Columbia, this tree is a component of harvested forests, yet knowledge of post-harvest survivorship and factors that promote successful retention is lacking. Our objectives are to describe the temporal attrition of retained mature whitebark pine trees and to identify factors that likely influence survivorship during the critical initial post-harvest period. We assessed five separate harvest units in southeastern British Columbia. Dendrochronological investigation revealed that retained trees experienced high annual mortality rates (3–16%) across harvest sites during the initial five-year post-harvest period. By eight years post-harvest, retention survivorship ranged from 17–80%. After eight years post-harvest, mortality rates drastically declined. The preponderance of fallen stems oriented towards the northeast suggests that storm system events arriving from the Pacific Ocean are the most significant drivers of blowdown. We estimate that survivorship is positively associated with shorter tree heights and longer crown lengths, a lack of disease cankers, a greater presence of rodent wounding, and higher numbers of surrounding retained trees. We found little effect based on slope and aspect. As these trees are an endangered species, harvest operations should be practiced cautiously in associated forests. We recommend carefully selecting retention trees, ensuring an adequate number of neighbor trees, and orienting retention patches to avoid predominant storm wind directions.