Studies documenting organismal responses to climate have become increasingly important, given ongoing climate change that is projected to accelerate in the future. Although many studies document climate impacts on species physiology and distributions, we lack a generalizable understanding of how entire communities will respond to climate change. Responses could vary from the predictable (thermophilization – increase in warm-adapted at the expense of cold-adapted), to stasis (i.e. no change) to unpredictable reorganization (novel communities). These different pathways have differing implications for ecosystem functioning and forest management. To determine likely future responses of forest communities in the Pacific Northwest, we revisited vegetation plots at Mount Rainier National Park originally surveyed four decades earlier, offering a unique opportunity to assess how tree and the understory communities reshuffle following recent warming. Specifically, we quantified changes in species richness, community dissimilarity (presence-absence Bray-Curtis dissimilarity), and floristic temperatures over time, and compared these temporal changes in community metrics to spatial changes over the large spatial gradients in climate. Using these methods, we asked whether community turnover varies between understory and tree communities, and whether low elevation vs. high elevation communities are more strongly influenced by recent warming.
Results/Conclusions
In all, our results suggest complex responses of forest communities at Mt. Rainier National Park to climate change. Specifically, we found that species richness has decreased slightly for both tree and understory communities, with large site to site variability that was not related to elevation. We also found that the relationship between community turnover in space and climatic factors sensitive to climate change (e.g. snow, temperature) were different for tree and understory communities, implying different sensitivities to warming. Consistent with these patterns, we found that understory communities have shifted their composition more strongly than tree communities, implying that novel communities may arise with future warming. Finally, although community shifts in tree communities were in the direction expected (leading to warmer floristic temperatures), shifts were lagged relative to observed warming – implying that lagged responses are likely. In total, our results suggest that communities are partly responding to climate change as expected, but that we should expect community reorganization and lagged community shifts. Such complex responses to climate change may have implications for ecosystem functioning.