Forests in a changing world: conifer leaf traits, community shifts, and climate change adaptation

Abstract

Climate change is increasing forest vulnerability worldwide. While forest management has been an integral part of human history, climate change poses a new challenge. Mitigating and adapting to the impacts of this novel challenge likely requires the use of novel methods alongside tried-and-true management strategies. But to properly implement these methods, we need a robust understanding of the patterns of functional biodiversity in current forests, the degree of changes they have experienced in response to climate change, and the synergy between more traditional and more novel management techniques. In this dissertation, I present work that addresses these knowledge gaps in the coniferous forests of the Pacific Northwest. In chapter 1, I examined the variation and covariation of leaf traits of conifer communities across a climatic gradient. I found that the contribution of species turnover to community-level trait values was tightly linked to climate while the contribution of intraspecific trait variation, despite being more important, was not. This suggests that species turnover will likely be the main mechanism by which these communities respond to climate change, as least as it pertains to leaf traits. In chapter 2, I compared historical and modern montane plant communities (both overstory and understory) to assess whether they had shifted towards more novelty and increased biotic similarity. I found that both overstory and understory communities had indeed shifted towards more novel species combinations over time, and had also become more differentiated from one another across space. This, in combination with our finding of potential disequilibrium between modern plant communities and climate, suggests that while organisms are responding to climate change, they are doing so individualistically resulting in increased potential for novelty in future communities. Finally, in chapter 3, I assessed whether thinning forest stands for old-growth restoration reduces landscape buffering capacity (in the form of climate change refugia) in a watershed-level experimental forest. I found that variation in remote sensing vegetation indices can be good proxies for climate change refugia and that strategic forest thinning does not have negative impacts on landscape buffering capacity. Overall, this body of work highlights the importance of examining community-level biodiversity from multiple perspectives and demonstrates the value of testing the efficacy of combining old and new methods to achieve multiple management goals.