Background/Question/Methods Species distributions are dependent on abiotic conditions and biotic interactions which vary spatially and temporally. Climate change is projected to cause shifts in tree species distributions, but the ability of trees to track current changes in climate is uncertain. Lags at the trailing end of species ranges may occur due to microclimate buffering by canopy trees and expansion at the leading edge is dependent on not only the dispersal ability of the species, but also the availability of sufficient resources (e.g., light, water, and nutrients) and substrates, which overstory trees can limit. Disturbances may provide a catalyst for tree species range shifts by altering the microenvironment so that it aligns with these requirements, which will vary by species. In this study, we use US Forest Inventory and Analysis data from across the interior western US to identify how tree species distributions are shifting in response to recent warming, and how disturbances alter these shifts. We compared adult vs. seedling climatic niches of twelve widespread tree species under different recent disturbances using multivariate analyses. We identified if shifts in climatic niche had occurred for each species, and whether the magnitude of shift was altered by disturbance history or species traits. Results/Conclusions The results of this study provide early evidence of tree species range shifts in the intermountain west and suggest that disturbances will have varying impacts on these shifts across species. Most species shifted in climate space away from sites with high climatic moisture deficit and hot temperatures, which is in alignment with the hypothesis that climate warming will drive species distributions upward in elevation and latitude, though less evidence was found for significant expansion into cooler sites. No disturbance type was found to be universal in promoting niche shifts, rather the largest shifts in climate niche were associated with different disturbances for each species. Shade intolerant species had larger magnitude shifts than shade tolerant species, especially in burned areas, while drought tolerance and seed mass did not predict range shift magnitude. This study demonstrates that the distributions of dominant tree species in the intermountain west are responding to climate warming as seen by a lack of regeneration in hotter climate niches and that disturbances are accelerating this response for some species. The increasing frequency and extent of disturbances across the western US may therefore accelerate range shifts, particularly for shade intolerant tree species at the trailing edge of their range.
