It's a dry heat: Quantifying effects of increasing atmospheric moisture demand on native Oklahoma trees
Wilson, Luke Alan
Hammond, William M.
Adams, Henry D.
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Anthropogenic climate change is predicted to alter precipitation frequency and intensity across Oklahoma in the coming decades, leading to an increase in the frequency, intensity, and duration of extreme events such as soil drought. Concurrently, temperature is predicted to continue rising, causing an ever-increasing atmospheric demand from plants. While the effect of soil droughts has been extensively studied in recent years, the impact of ever-increasing atmospheric droughts on trees is less characterized. Trees regulate photosynthesis though the interactive effects of availability of soil water (supply) and atmospheric demand for water (Vapor Pressure Deficit, VPD). Using recent innovations, and a novel experimental design, we set out to test gas exchange response for three native Oklahoma tree species to varying levels of VPD, with the hypothesis that drought adapted species would be less sensitive to increasing VPD. Two of the species, Quercus stellata and Quercus marilandica, often occur on unfavorable dry sites, while Cercis canadensis is found in riparian areas and wet forest interiors. We exposed six trees of each species to a range of VPDs, between 1kPa and 3kPa, at a constant temperature under well-watered conditions. We measured rates of carbon assimilation and stomatal conductance at five intervals across our VPD measurement range using a LI-COR LI-6800 infrared gas analyzer. Relative rates of carbon assimilation and stomatal conductance decreased as VPD increased across taxa. However, C. canadensis decreased carbon assimilation much quicker than the Quercus species as VPD increased in support of our hypothesis. Our results provide a preliminary understanding of photosynthetic response across a range of VPDs for deciduous forest trees in Oklahoma. Additionally, our methods provide a clear and repeatable way forward, as we aim to disentangle the effects of soil and atmospheric drought on photosynthetic rates in future experiments.