Forest structure and species traits mediate projected recruitment declines in western US tree species

Abstract

Aim Determine if differences in the climatic niche between conspecific adult and juvenile trees of the western U nites S tates vary by species traits and to assess if forest canopies moderate the sensitivity of juvenile trees to climatic variation. Location The western U nites S tates. Methods Using data from the USDA F orest I nventory and A nalysis programme, we compare the distribution of conspecific adult and juvenile trees for 62 western US tree species. We relate demographic niche differences to species traits including shade and drought tolerance. We model recruitment under projected climate change using generalized linear mixed models, probabilistic uncertainty accounting, forest structural data and projected changes in the climatic water balance. Results On average juveniles of western US tree species occupy a climatic subset of their conspecific adults. Demographic niche differences increase as species shade and drought tolerance increase and are greatest at climatic range margins, indicating the potential for range contractions. Models calibrated solely with climate data project recruitment declines for 2080 that are 47% larger on average than models that also account for forest structure. Main conclusions Climate change‐driven declines in recruitment in western US tree species may be partly offset by the moderating effect of forest canopies. The importance of this stabilizing process will depend on whether a given site is disturbed and the traits of resident species, including their ability to utilize sites that have buffered microclimates. Conversely, our results suggest that broad‐scale disturbances which result in the loss of forest canopy will amplify the effects of climate change on tree recruitment.

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