The Role of Abiotic and Biotic Factors in Douglas-fir Decline in the Western Cascades, Washington

Abstract

Douglas-fir (Pseudotsuga menziesii) dominated forests of the Pacific Northwest are an integral part of the region and provide numerous ecosystem services. They are particularly important in the Cedar River Municipal Watershed (CRMW), Washington, where they provide erosion control and ecosystem buffering for the reservoir, which supplies drinking water to >1.4 million people in the greater Seattle area. Recent records from the CRMW suggest that annual mortality of Douglas-fir has increased in recent years. The Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins) and fungal pathogens are key agents of mortality in mature or physiologically stressed Douglas-fir trees, but their contributions to the current rates of mortality in the CRMW is unknown. In this study, I measured relationships between temperature and Douglas-fir beetle seasonality through development of a phenology model. I then used this model to show how future climate change conditions will be conducive to an expansion of seasonality. Second, I quantified the individual and interacting roles of Douglas-fir beetle; the fungal pathogen, Armillaria spp.; and other abiotic and biotic variables as drivers of Douglas-fir mortality in the CRMW. The identified combination of variables could be used to inform managers and help develop adaptive management techniques for at-risk forests.