| Summary: | The Arctic is warming at rapid, unprecedented rates, causing cascading ecological and environmental changes that threaten to destabilize the vast amounts of carbon stored in the vegetation and soils of the tundra. Foliar gas exchange, which is responsible for the initial fixation of carbon, is likely to respond to warming and associated environmental change in tundra plants, though the direction and degree of these responses are not well studied. This dissertation aims to quantify multiple cellular and leaf-level processes underlying carbon cycling in tundra plants, and to address the responses of these processes to abiotic and biotic effects of warming in the Arctic. To assess the impact of environmental change on foliar gas exchange physiology of tundra plants, a series of empirical studies were conducted on common and abundant plant species located near Toolik Lake, on the North Slope of Alaska. Long-term manipulated treatment plots that simulate the effects of climate change in this region, including elevated growth temperature and increased soil nutrient availability, served as the research setting for multiple experiments that addressed the response of variables such as foliar photosynthesis, respiration, photorespiration, mitochondria and chloroplast size and density, and physical leaf traits. Due to the extreme photoperiod experienced by arctic vegetation, respiration in the light (estimated using the Kok method) was quantified in addition to dark respiration for a more accurate depiction of plant carbon fluxes. Individual studies, presented as dissertation chapters, examine the responses of the aforementioned variables to a gradient of soil nitrogen and phosphorus availability; decades-long warming and fertilization; seasonal timing and short-term intra-season temperature fluctuations; and canopy position within a shrub community. Collectively, the results of these studies find respiration to be more sensitive to long- and short-term environmental variation than photosynthesis, indicating a decoupling of the processes controlling foliar carbon cycling. Across all species and environmental conditions, respiration is inhibited by light, emphasizing the need for the estimation of this physiological phenomena and its inclusion in regional terrestrial ecosystem carbon models. Also, foliar carbon fluxes in woody shrub species are significantly higher than non-shrub species across experiments, a finding that demands attention given the general trend of increasing shrub cover associated with warming in the Arctic tundra. The results presented in this study on the environmental controls on leaf-level gas exchange allow for a more thorough understanding of the current carbon balance of this region and provides new data the can inform predictions and models of its future status.
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