Bigleaf Maple Decline in Western Washington

Abstract

Bigleaf maple (Acer macrophyllum Pursh) is a prominent component of the urban and suburban landscape in Western Washington, which lies at the heart of the native range of A. macrophyllum. Acer macrophyllum performs many important ecological, economic, and cultural functions, and its decline in the region could have cascading impacts. In 2011, increases in A. macrophyllum mortality were documented throughout the distributional range of the species. Symptoms of this decline included a systemic loss of vigor, loss of transpiration, and a reduction in photosynthetic potential, but did not display any signs or symptoms indicative of a specific causative agent. No pathogenic microbes, insects, or other biotic agents were initially implicated in causing or predisposing A. macrophyllum to decline. In my thesis research, I quantified the spatial extent and severity of A. macrophyllum decline in the urban, suburban, and wildland forests of western Washington, identified potential abiotic and biotic disturbance agents that are contributing to the decline, and conducted a dendrochronological analysis to ascertain the timing of the decline. I surveyed 22 sites that were previously reported as containing declining A. macrophyllum, and sampled 156 individual A. macrophyllum, of which 65 (41.7%) were observed to symptoms of declining health. In addition, I sampled 59 sites that were randomly selected from within the range of A. macrophyllum in western Washington, and sampled 132 individual A. macrophyllum, of which 29 (22.0%) were observed to symptoms of declining health. By comparing qualitative and quantitative measurements of A. macrophyllum decline severity, and dendrochronological patterns of tree growth and decline, elemental concentrations in soil and foliage, and geographic records of weather, landcover classes, and soil characteristics, I identified several environmental variables that are associated with mortality and loss of vigor in A. macrophyllum. Higher temperatures, vapor pressure deficits, decreased precipitation, high levels of developed land, low levels of forested or herbaceous land, and proximity to paved roads were all positively associated with increased prevalence of decline in A. macrophyllum. Ring widths of A. macrophyllum were negatively correlated with summertime temperatures, vapor pressure deficits, and drought, signifying reduced growth and vigor in the hotter, drier summers of the Pacific Northwest. Dendrochronologically-derived spatio-temporal estimations of A. macrophyllum decline did not indicate a spatially-correlated spread of symptoms characteristic of pathogen or disease introductions. Increased human habitation, activity, and development, as well as predicted increases in summer temperatures and drought, in the western coast of North America will likely continue, which could lead to increased mortality in A. macrophyllum in the future. The results of this study could be used to assist managers in their efforts to develop mitigating management strategies and adapt to changing conditions in the forest of the Pacific Northwest.