Ecosystem and Large-Scale Climate Impacts of Plant Leaf Dynamics

Thesis (Ph.D.)--University of Washington, 2019Vegetation modifies Earth's climate by controlling the fluxes of energy, carbon, and water. Of critical importance is a better understanding of how vegetation responses to climate change will feedback on climate. Observations show that plant leaf traits respond to elevated carbon dioxide concentrations. These leaf trait responses have the potential to modify plant functioning and competitive dynamics, and could therefore alter carbon cycling and surface energy fluxes with implications for regional and global climate. Yet the climate impacts of changes in leaf structural traits − such as increases in leaf mass per area and leaf carbon to nitrogen ratio − in response to elevated carbon dioxide are not included in most climate projections and remain to be tested and quantified. Here we show that one leaf trait response to elevated carbon dioxide − a one-third increase in leaf mass per area − significantly impacts climate and carbon cycling in Earth system model simulations. Higher leaf mass per area enhances warming in response to elevated carbon dioxide by reducing the increase in leaf area, which lowers carbon uptake and evapotranspirative cooling by plants and leads to enhanced solar radiation absorbed at the Earth's surface. Our results suggest that leaf trait responses to carbon dioxide should be considered in climate projections and provide additional motivation for ecological and physiological experiments that improve our mechanistic understanding of plant responses to environment. Tropical forests exert extensive control over global energy, carbon, and water fluxes and thus play a critical role in determining future climate. Using an ensemble of demographic vegetation model simulations we quantify the influence of two leaf trait responses to elevated carbon dioxide − increases in leaf mass per area and leaf carbon to nitrogen ratio − on tropical forest functioning and competitive dynamics. We find that consideration of these leaf trait responses reduces projected carbon uptake and evapotranspirative cooling when plant type abundance is held invariant with time. However, given that more competitively advantageous leaf trait responses also maintain higher levels of plant productivity and evapotranspiration, including changes in plant type abundance may mitigate these decreases in ecosystem functioning. Models that explicitly represent competition between plants and leaf responses to elevated carbon dioxide are needed to capture these influences on tropical forest functioning and large-scale climate. Lastly, we improve the simulation of present-day tropical forest functioning and structure in a demographic vegetation model by including a gradient of leaf mass per area with canopy depth, following observations. By benchmarking the modified model's performance against observations at a tropical forest test site across nearly 300 plausible plant trait parameterizations, we identify high-performing parameter sets and areas for further model development