University of Montana, Maureen and Mike Mansfield Library
Abstract
Whitebark pine (Pinus albicaulis), a keystone species of high-elevation ecosystems of western North America, is warranted for listing under the Endangered Species Act due to the combined threats of fire exclusion, white pine blister rust (Cronartium ribicola), and increased pressure from insects due to climate warming. Despite widespread knowledge of the potential causes of the tree’s decline, there is limited understanding of its ecology, including successional dynamics in forests disturbed by white pine blister rust and mountain pine beetle. Specifically, there is a need for more information on successional replacement by subalpine fir and whitebark seedling regeneration dynamics. I assessed changes in forest composition and structure and seedling regeneration dynamics over a 22-year period (1990-2012) at 19 sites in the Cascade Mountains of Washington State (USA). Over the two decades analyzed, 26% of sites experienced wildfire, 58% were disturbed by mountain pine beetle, and 68% were infected by white pine blister rust. Only one of 19 sites showed no evidence of disturbance by these agents. Tree community composition changed significantly during the study period, primarily due to a significant decline in mature (≥20-cm DBH) whitebark pine . Despite loss of mature whitebark pine, I found little evidence of successional replacement by other tree species. I also found insignificant correlation between whitebark seedling density and basal area of mature whitebark pine. Seedling density was positively correlated with herb and shrub cover. These observed regeneration dynamics appear to contradict the conventional view that seedling establishment is contingent on local seed production and microsites free of competing vegetation. Additional, long-term studies are needed to comprehensively quantify the effects of novel disturbances on successional trajectories in whitebark pine ecosystems
