Tree demographic and neighbourhood responses to regional environmental gradients of the northwestern United States

1. Studies of drivers shaping forest communities frequently include abiotic or biotic factors, while their interactive effects remain understudied. Here, we combined data on two prominent abiotic gradients, climatic moisture deficit and wildfire probability, along with tree functional neighbourhoods (i.e. trait differences of close neighbours) to assess variation in survival and growth of 56 tree species in the northwestern US.2. We asked two questions: (1) How does functional neighbourhood dissimilarity vary with environmental gradients? and (2) How do demographic rates of tree species in the northwestern United States vary with the interactive effects of environmental gradients and functional neighbourhood? We expected functional neighbourhoods to become more similar as environmental stress increased, due to a convergence of species towards an optimum stress tolerance strategy. We also predicted the interactive effects of abiotic and biotic factors on tree demography and high variation in species-specific responses to these interactive effects due to divergent species life history strategies.3. Functional neighbourhoods defined by dissimilarities in stem conductivity, litter decomposition, resprouting ability and specific leaf area changed with climate, shifting to more diverse neighbourhoods as climatic moisture deficit and wildfire probability increased. Results supported interactive effects of the functional neighbourhood and climatic moisture deficit or wildfire probability on tree demography, but only when the identity of dominant species was considered. Species-specific responses were highly variable in their direction and magnitude and often demonstrated opposing effects of climate and the functional neighbourhood and climatic moisture deficit on tree demography.4. Synthesis. Our findings show that climate and tree neighbourhood functional dissimilarity jointly impact tree demography; however, the effects are species-specific. Results of this study highlight the need to consider the interactive effects of abiotic and biotic contexts and individual species responses to their environment to adequately understand tree persistence under current and future climate conditions.