Bigleaf maple, Acer macrophyllum Pursh, decline in western Washington, USA

Acer macrophyllum is a prominent component of the western Washington landscape where it performs ecological, economic, and cultural functions. Reports of its decline and increased mortality in the Pacific Northwest were documented beginning in 2011. Symptoms of this decline include a systemic loss of vigor, loss of transpiration, and reduced photosynthesis due to leaf loss. We conducted a preliminary study of A. macrophyllum decline across western Washington in 2014-2015 and observed decline symptoms across the region, but we did not detect any specific biotic causative agents. We subsequently conducted a multi-approach study in 2017 to quantify the spatial and temporal patterns of A. macrophyllum decline in western Washington, and to examine biotic and abiotic associations with its decline. We sampled in urban and suburban areas, and in wildland forests, and collected site-specific data to test for associations with decline. We also measured elemental concentrations in foliar and soil samples to determine their association with decline. Lastly, we conducted a dendrochronological analysis to ascertain the spatial and temporal patterns of decline. We report that A. macrophyllum decline is a recent phenomenon, particularly since 2011, that was positively associated with sites closer to roads and with increased development, and with increases in summer temperatures. Site conditions, especially hotter urban sites, are predisposing A. macrophyllum to mortality. We did not detect a consistent biotic agent that could be implicated in A. macrophyllum decline. We contend that abiotic factors are either causing direct mortality to A. macrophyllum, or making then vulnerable to opportunistic biotic agents. The results of this study inform mitigating management strategies for A. macrophyllum in the forest of the Pacific Northwest.