Role of plant hydraulics in influencing the spatial distribution of carbon flux across the sagebrush-steppe ecosystem—A quantitative analysis
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The primary objective of this research was quantifying the biophysical controls on spatial heterogeneity of soil respiration at the sagebrush-steppe ecosystem. The study scale of my research, focused at the process and tower level has been subdivided into five main chapters with the second, third and fourth chapters forming the core of this research. The focus of the research was the sagebrush-steppe ecosystem as it encompasses 11% of North America and is an important carbon sink. The primary focus of the second chapter was the development of a coupled, low dimensional parsimonious mechanistic model in order to capture the dynamics of carbon and water fluxes across the sagebrush ecosystem during the summer season. The unique aspect of this model was that it also examined the importance of incorporation of a multi-patch system against the conventional single patch system to model carbon and water flux within the footprint of an eddy covariance tower data. While plant hydraulics was used for the first time to model carbon and water flux using eddy covariance data, any limitations on the application of this theory remained inconclusive, unless this theory could be extensively examined across different biomes. However, the fact that the same model parameters for sagebrush was obtained, irrespective of whether the model was constrained with water or carbon reflects upon the robustness of the model structure. The primary focus of the third chapter was to have an improved understanding of the various physiological functioning of the sagebrush vegetation at a plot level at different times during the summer period. The empirical study, conducted in the summer of 2009 was aimed to understand the shortfalls of the mechanistic model in the second chapter so that it would help to provide the theoretical foundations for the construction of a more mechanistic ecohydrological model. Weak correlation between photosynthesis and stomatal conductance highlighted the fact that the parameters that influence photosynthesis could vary temporally, thereby having important implications for current coupled photosynthesis-stomatal conductance models. Total soil and heterotrophic respiration was found to be sensitive to both soil moisture and temperature. Contrary to expectations, isotope analysis and modeling studies did not find fine roots to be sinks for recent plant photosynthate, even though leaves were identified as one possible sink for recently assimilated carbon in August. The fourth chapter was in essence, convergence of the fundamental ideas of the second and third chapters with the primary objective to study the spatial structure of soil respiration. Presence of spatial structure would reflect underlying physical mechanisms, which had been ignored in soil respiration models. Pulse input was found to be the primary driver of soil respiration with both biotic and abiotic processes influencing the spatial autocorrelation of soil respiration. The fourth chapter provides a unique perspective on the mechanisms of soil respiration. The significance of this study lies in the fact that both biotic and abiotic factors can be incorporated to construct mechanistic soil respiration models, which could then be used to compare and explain respiration fluxes across different ecosystems as well as reduce uncertainty in the estimation of soil respiration at the landscape and ecosystem level.