Documenting how vegetation varies across large environmental gradients is foundational to understanding long-term persistence of forest systems under future climate conditions and disturbance regimes. Studies of forest communities frequently include only abiotic factors, failing to account for plant-plant biotic interactions that can have significant impacts on demographic rates. We combined data on two prominent environmental gradients, climatic moisture deficit and fire probability, with modelled tree functional neighborhoods, which determine the effects that close neighbors have on a focal tree, to assess variation in survival and growth of 56 of the most abundant tree species in the northwestern US. We asked two main questions: 1) How does functional neighborhood dissimilarity vary with environmental gradients? and 2) How does tree demography vary with the combined effects of environmental gradients and functional neighborhood?We expected functional neighborhoods to become more diverse as environmental stress increased, and to observe increased survival and growth in moist areas with lower fire probability and in more diverse functional neighborhoods. Overall, less diverse functional neighborhoods might exacerbate the effects of environmental stress, potentially through reduced strategies for trees to cope with novel conditions and increased competition for resources.
Results/Conclusions
We found that functional neighborhoods changed along the environmental gradients, with a shift from less diverse to more diverse neighborhoods as stress increased. For many species, we observed opposing effects of moisture deficit and fire probability on their demographic responses. Our results suggest substantial species variation in survival and growth responses to the combined effects of environmental variability and functional neighborhoods. In sum, our findings support that environmental variation and biotic interactions jointly determine tree demography. Results of this study offer a different perspective of tree species responses to variable environmental conditions necessary to better anticipate responses to future climate and disturbance changes.
