Earlier Phenology Associated With Spring Warming Will Help Offset Some of the Negative Impacts of Climate Change on Temperate Tree Seedling Recruitment

Abstract

The warming temperatures and increased drought predicted to occur over the course of the next century have the potential to profoundly impact the composition and structure of global plant communities. Because of the relevance of forest ecosystems in storing a large amount of the planet’s carbon and thus in regulating the earth’s climate, there is a major effort to forecast forest composition, structure, and functioning. Accurate predictions will require the application of studies that identify how climate drivers (and interactions between multiple drivers) affect physiological processes that underlie patterns of demography and assembly. In forest systems, community composition is strongly shaped by bottleneck effects that occur during recruitment at small size classes, size classes that are highly vulnerable to climate change. In this dissertation, I investigated how climate change will affect the seedling demography of two temperate tree species that commonly co-occur across eastern North America: Acer saccharum (sugar maple) and Quercus rubra (northern red oak). In chapter 2 I investigated how potential climate-driven shifts in seedling foliar phenology (in relation to shifts in canopy phenology) could affect the ability of seedlings to maintain positive net carbon assimilation over the growing season, a dynamic that is commonly referred to as phenological escape. I also modeled how environmental conditions drive photosynthetic rates and used that information to estimate the relative proportion of carbon that is assimilated in different seasons. In my third chapter I used the same photosynthesis models to estimate annual carbon assimilation for individual tree seedlings and then modeled the relationship between carbon assimilation and demographic performance (growth and survival). I used results from both chapters to project how climate change in my study region could affect seedling demography directly (e.g., via changes in respiration rates associated with higher temperatures) and indirectly (e.g., via changes in access to light caused by different phenology shifts between seedlings and the canopy). In my last chapter I used a greenhouse study to investigate how seedlings of these two species respond to drought, specifically looking for differences in stomatal regulation of leaf water potential, reductions in photosynthetic capacity, reduction of non-structural carbohydrates, and loss of hydraulic conductivity.

My results suggest that climate change will primarily affect seedling recruitment via changes in annual carbon assimilation. Although I found evidence that seedlings are likely to gain access to light with warming spring temperatures (thereby increasing net carbon assimilation in spring), elevated leaf respiration rates in hotter and drier summers would outweigh these gains and lead to net reductions in annual assimilation. In turn, these reductions would reduce seedling demographic performance and lead to less growth and higher mortality rates. Access to water could affect plant performance via reductions in photosynthetic rates, but seedlings of both species are also highly vulnerable to hydraulic failure during severe drought events. In sum, my results indicate that seedlings of both species may experience steep reductions in performance under extreme climate change, but that phenological escape dynamics may be enough to compensate for these reductions under more conservative climate change scenarios.