Substantial uncertainty exists on how terrestrial ecosystems will respond to climatic change. The diversity in forest structure, composition, and genetics contribute to this uncertainty. Revealing the range of climatic sensitivity (or tree-level correlations to climate) in a population is one way to infer vulnerability. An example: if most trees in one population grow better with more summer water availability but in a second population there is a greater range of climatic sensitivity (grow positively, neutrally, or negatively with greater water availability), more trees at the former site will be vulnerable to a severe, multiannual drought. Naturally, tree rings are a powerful tool to investigate how individual trees respond to decades of climatic variation. Given this framework, we present a case study using 18 Chamaecyparis thyoides populations along the species’ contiguous distribution. Examining prior research, we hypothesize that northern populations will be more vulnerable to cold winters while southern populations will be more vulnerable to dry summers. Upscaling how variability from trees to populations and regions could affect conservation in the context of climatic change and species vulnerability: If some trees survive climatic change, the population could persist. If some populations survive, the species could persist at the regional level.
Results/Conclusions
Supporting our first hypothesis, cool winter temperatures constrain the growth of northern Chamaecyparis thyoides populations. Contrary to our second hypothesis, Chamaecyparis thyoides populations near the center of our network have the strongest climatic sensitivity to growing season drought (positive correlations to precipitation and negative correlations to maximum temperatures). Additionally, their relatively narrower distributions of correlations to climate at the population level suggest they are the most climatically vulnerable with their sensitivity to growing season drought, a result that generally mirrors findings in Europe that indicate central populations of Fagus sylvatica are the most vulnerable to climatic change. While maximum temperatures have the strongest negative impact on central populations, the common response among all trees in this region to low precipitation during the summer is particularly unique. While the growth of central and northern populations is similarly constrained by cool winter temperatures, the narrower distributions of climatic sensitivity in the northernmost populations during spring make these populations the most vulnerable to cool spring temperatures. Our findings can aid management and conservation of the unique Chamaecyparis thyoides ecosystems and lead to research on the potential interactions between forest structure and genetic composition for ecosystem resiliency in the context of climatic change.