Forest structure and species traits mediate projected recruitment declines in westernUStree species

Abstract

Aim Determine if differences in the climatic niche between conspecific adult and juvenile trees of the western Unites States vary by species traits and to assess if forest canopies moderate the sensitivity of juvenile trees to climatic variation. Location The western Unites States. Methods Using data from the USDA Forest Inventory and Analysis 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.

Competition alters tree growth responses to climate at individual and stand scales

“…Our results also suggest that the effects of climate change on the relative abundances of tree species within forest communities may differ based on competitive environment (also see Dobrowski et al 2015). First, changes in community structure might be most rapid in low-competition environments because growth and the sensitivity of growth to climate are highest in these locations, potentially allowing thermophilic species to increase in abundance more quickly .…”

“…To calculate these variables, we used a Thornthwaite-type water balance model following Lutz et al (2010), an approach considered most appropriate when temperature and precipitation data are available but reliable measures of humidity and wind speed are not (Dingman 2002), as was the case in this study.…”

Global change and terrestrial plant community dynamics

“…In another study in California, declines in large trees and increased density of small trees with dominance of drought-tolerant oaks correlate with increases in climatic water deficit (47). In the water-limited forests throughout the western United States, lack of seedling regeneration at warmer range margins is more prevalent than expansion along cool margins (48), but forest cover buffers the negative effect of climate warming and drying on recruitment (49).…”

Shading and litter mediate the effects of soil fertility on the performance of an understorey herb

“…For example, we found that Trientalis occurrence is highly related to overstorey cover rather than topographic microclimate, suggesting that small shifts in climate may have little effect on distribution if overstorey cover does not significantly change, whereas loss of overstorey due to increasingly severe or frequent fires could have much more drastic consequences. Facilitative effects of canopy cover may also explain patterns such as persistence of species with 'cool-adapted' traits through long-term drying trends (Valiente-Banuet et al, 2006) and the positive effect of forest density on tree regeneration niche (Dobrowski et al, 2015). These results also imply that changes in biotic conditions independent of climate could lead to shifts in distribution and abundance that were not predicted by climate-based models.…”