We investigated the extent and severity of a recently reported decline in bigleaf maple (Acer macrophyllum), which was first reported in Washington State in 2010 and has also been reported in California, Oregon, and British Columbia. Symptoms of this decline include yellow flagging of large branches, small leaf size, partial or entire crown dieback, and mortality. An initial exploratory survey throughout the range of bigleaf maple in Washington State revealed spatially widespread decline, though sampling was limited to bigleaf maple along roadways. At these sites, foliar and xylem samples were collected and tested for the presence of several plant pathogens that are known to cause tree mortality. We conducted more comprehensive study by sampling both a subset the original survey sites and by randomly selected sites from within the range of bigleaf maple in western Washington to better quantify the prevalence, spatial extent, and biotic and abiotic variables associated with decline. We recorded basic forest measurements and the health of bigleaf maple at 106 sampling sites.
Results/Conclusions
Bigleaf maple decline was represented throughout the spatial extent of our study region, Washington State west of the Cascade divide. Forty-two percent of bigleaf maple trees within 0.5 km of a previously identified declining bigleaf maple, from the initial exploratory survey, showed symptoms of decline. Among randomly sampled sites, 18% of bigleaf maple showed symptoms of decline. Assays that were used to test for the presence of Ganoderma root disease, Armillaria root rot, Verticillium wilt, and Xylella fastidiosa did not reveal the presence of any of the pathogens in sufficient abundance, which suggests the causative agent could be a pathogen not yet identified or another biotic or abiotic cause. Proximity to major roads was significantly associated with the presence of bigleaf maple decline; this could indicate an anthropogenic cause of decline or be a proxy for some habitat characteristic. Additionally, increased drought in recent summers was significantly correlated with bigleaf maple decline. Future research will involve an elemental analysis of the leaf and soil samples to determine the relationship between plant and soil chemistry and decline, comparisons of archived weather records and decline, and a dendrochronological analysis on the tree cores to quantify a temporal record of decline.