Drought impacts on forest responses to precipitation and biological drivers of the surface energy imbalance
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The eddy covariance (EC) technique is used across the world to study the interaction between the biosphere and the atmosphere. This thesis consists of two research chapters, one devoted to analyzing historical EC data to gain new insights into how drought shapes forest responses to precipitation, and another study aimed at advancing scientific understanding of the surface energy imbalance (SEI). For both research chapters, historical ecosystem gas and heat flux data from the Missouri Ozark AmeriFlux site (MOFLUX) were analyzed. The first study focused on drought impacts on forest responses to precipitation. By analyzing predawn leaf water potentials ([psi]pd) to quantify drought and focusing on short-term responses (timescales [less than] 1 week) of latent and sensible heat fluxes (LE and H, respectively), as well as net ecosystem carbon dioxide (CO2) exchange (NEE), this study determined that the forest was most responsive to precipitation while under moderate antecedent drought stress (-1 [less than] [psi] pd [less than] -0.5 MPa). The second study focuses on the SEI, an issue apparent at EC sites globally whereby we cannot account for all energy within an ecosystem using EC measurements, introducing biases into the data and how we interpret it. Drought impacts forest heat fluxes and also ecosystem level photosynthesis, therefore affecting transient energy storages associated with photosynthetic energy. These energy storages are often neglected in surface energy budget studies and may be responsible for a portion of the SEI. Using Random Forest regression and Shapley Additive Explanations, both machine learning tools, NEE was identified as a primary driver of SEI, pointing towards the possible importance of biochemical and photochemical energy storages in surface energy closure. Taken together, this thesis advanced scientific understanding of forest drought responses and the SEI problem that has plagued the EC community for decades.
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M.S.