Forest structure and species traits mediate projections of climate-driven recruitment declines in western US tree species

Background/Question/Methods

Seedling establishment is quite sensitive to climatic variation relative to growth and survival of adult life stages. This sensitivity makes the establishment phase important for understanding climate change impacts on forests. Climate change projections for the western US show widespread increases in temperature, and seasonably varying changes inprecipitation. These climate changes are expected to drive recruitment declines, range contractions, and shifts in species composition for tree species of the western US. However, most models that predict these changes do not explicitly consider the factors that influence recruitment in western forests or the role of stabilizing processes such as microclimatic buffering of climate by forest canopies. Forests have distinct below-canopy microclimates; forest structure and soil moisture buffer temperature, humidity, and radiation extremes at the forest floor where juvenile trees are found. This raises the prospect that potential climate-driven recruitment declines may be offset to some degree by the moderating effect of forest canopies.  In this study, we relate macroecological patterns of tree recruitment in the western US to climate, forest structure, and species traits to assess factors that may influence recruitment under projected climate change. We ask if juveniles of western US tree species show apparent differences in their climatic niche when compared to conspecific adults? We also develop recruitment models calibrated with FIA data and gridded estimates of climatic water balance and project changes in recruitment over the coming century. We ask if the sensitivity of the response of juvenile trees to climate is contingent on forest structure at a site? 

Results/Conclusions

A geographic comparison of the distribution of adult and juvenile trees of the western US reveals that, on average, juveniles occupy a mesic subset of the adult niche. These demographic niche differences vary by climatic context and by species traits. Projections from our models suggest recruitment declines for many western tree species. This is most evident for species at the extremes of the shade and drought tolerance spectrum. Lastly, our results show that the inclusion of forest structural variables into climatic niche models dampen climatic response functions compared to models solely fit with climate data. This in turn moderates projections of recruitment changes during the 21^st century.