Impact of prairie management on turbulent transport of carbon and water fluxes during drought

Recent severe droughts in the Central U.S. and predictions of increased drought frequency in the future raise concern for regional ecosystem resilience. Understanding how changes in species composition have altered regional drought response is vital for understanding future carbon and water cycling issues in response to climate change. Two eddy covariance towers located on neighboring native prairie sites with differing burn regimes provide a unique opportunity to investigate the impacts of prairie management and woody encroachment on carbon and water cycling during drought. Daytime turbulent fluxes are analyzed throughout the growing season during two wet years and one drought year. The site with less frequent burning (K4B) is experiencing woody encroachment. Compared to the annually burned site, K4B had greater carbon uptake during the drought, along with greater water loss, likely due to increased root access to deeper sources of moisture. Changes in the nature of the turbulent fluxes are investigated using 1. Monin-Obukhov Similarity Theory (MOST), 2. Conventional quadrant analysis of sweeps and ejections, and 3. Wavelet decomposition. It is well known that heterogeneity weakens applicability of MOST. K4B exhibits greater deviations from MOST flux-variance predictions due to the increased patchiness of species composition with woody encroachment. Ejection and sweep eddy motions are responsible for much of the land-surface evaporation, sensible heat, and momentum transfer between the land surface and atmospheric surface layer. The sites typically display a similar pattern of sweeps and ejections, though they differ the most during drought conditions. Wavelet decomposition is used to determine the dominant scales of these motions, and the size of eddies involved in net exchange and anomalous events. Weakened correlation of carbon dioxide and water vapor flux (from -1 during peak growth) at these dominant scales of transport occurs at the end of the growing season with senescence. This change is seen earlier in the drought year at the annually burned site only. Eddy size of maximum transport decreases over the growing season, and is always smaller at K4B, likely corresponding to increased canopy height and roughness. However, the smallest dominant eddy sizes at K4B are observed during the drought year. Overall, results are indicative of dampened response to fluctuations in precipitation and increased resilience to drought with woody encroachment.