Impact of woody encroachment on carbon and water flux dynamics and drought response in native tallgrass prairie

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 closely located eddy covariance towers on 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 (2008, 2009) and one drought year (2011). The site with the less frequent, 4-year burn regime (K4B) is experiencing woody encroachment with fractional coverage of dogwood now over 50%. Compared to the annually burned site, K4B had greater carbon uptake each year. K4B also exhibits greater seasonal water use efficiency at all times except during drought conditions, when the site shows little change in carbon fluxes but has greater evapotranspiration (ET), likely due to increased root access to deeper sources of moisture. Evidence for water loss from deeper soil sources at K4B is seen through higher measured cumulative water vapor flux to the atmosphere than cumulative precipitation during drought conditions in the growing season of 2011.

Along with these changes in net fluxes, changes in the nature of the turbulent fluxes are investigated using conventional quadrant analysis of sweeps and ejections and wavelet decomposition. Ejection and sweep eddy motions are responsible for most of the land-surface evaporation, sensible heat, and momentum fluxes in the atmospheric surface layer. Analysis is carried out to determine whether sweeps or ejections dominate momentum and scalar transport. The sites typically display a similar temporal pattern of atmospheric motions, though differ the most during drought conditions. Wavelet decomposition is used to determine dominant scales of these motions, and the size of eddies involved in net exchange and in anomalous events. Weakened correlation of carbon dioxide and water vapor flux (from -1 during peak growth) at the dominant scales of transport occurs at the end of the growing season with senescence. This change is seen earlier during the drought year at the annually burned site only. Eddy size of maximum transport decreases over the course of the growing season with increasing leaf area index, 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.