Influence of Grassland Heterogeneity on Land-Atmosphere Coupling

Abstract

Land surface processes and interactions with the atmosphere have been identified as a weak point in our understanding of the Earth's climate system and contribute to uncertainty in projections of future climate. This weakness is due, in part, to the inherent complexity of land-atmosphere (LA) interactions and the highly heterogeneous nature of land cover across a variety of spatial and temporal scales. The research included in this dissertation looks at the significance of these issues over a central US grassland. Influence of spatiotemporal variability is investigated through comparison between two proximate grassland sites with differing land cover. High-frequency observations from eddy covariance towers over a study period covering a range of environmental forcings, including two strong droughts and woody encroachment at one site provides a unique opportunity for study. First, changes in the water, energy, and carbon budgets are studied, with focus on the influence of woody encroachment on carbon sequestration, water-use efficiency, and drought response. How these changes manifest in the nature of turbulent fluxes, including at which spatial and temporal scales, are invested through deviations from similarity theory, quadrant analysis, and wavelet decomposition. Second, the nature of coupling between the land surface and atmosphere is studied by utilizing a variety of LA feedback metrics and analysis tools that allow for investigation of several steps in the LA feedback chain. This research includes the first use of some of these tools (self-organizing maps and mixing diagrams) in an study of this nature. Results indicate that woody encroachment increases resilience to drought due to changes in canopy structure and root access to soil moisture, and highlight the need to carefully consider scale and objective when selecting a metric of LA coupling.