| Abstract: | Savannas have undergone extensive amount of woody plant encroachment (WPE) over the past two centuries. The process of WPE can substantially alter the hydrology of a landscape. However, our understanding of ecohydrology of woody encroached landscapes derives mostly from arid ecosystems. Subhumid savannas which have also seen large increases in woody cover remain understudied. The overarching goal of this study is to improve our understanding of ecohydrological shifts associated with WPE in subhumid savannas. In this dissertation, these changes were investigated at the three junctures as recognized by Wilcox et al. (2017): a) the surface changes in lateral redistribution(runoff-runon) and infiltration, b) the rooting zone changes in transpiration fluxes, and c) the vadose zone changes in deep drainage. Two subhumid savannas were considered in this study, a) a Prosopis scrubland savanna landscape in South Texas (mean annual precipitation = 660 mm) and b) a woodland pasture in Post Oak savannas in East Central Texas (mean annual precipitation = 930 mm). In the Prosopis scrublands in South Texas in contrast to the arid woodlands where runoff-runon process is associated with intercanopy-canopy connectedness, no evidence was found for such process in the current state of the landscape. The absence of connectivity was explained by a substantial increase in infiltration since the grazed conditions of these landscapes which can be further explained by the greater productivity supported in subhumid woodlands relative to arid woodlands. The collapse in ecohydrological connectivity has implications for soil moisture patterns and transpiration rates across the landscape. Soils seemed to be consistently dry across the woodlands with very low moisture content even at depths beyond 120 cm. Further, sapflux measurements indicate that the dense woodlands in the drainage areas are transpiring at much lower rates than the uplands. In the Post Oak savannas, similar trends with regards to drying up of deep soils were noticed in the thicketized portions of the landscape. The effect of thicketization was more starkly captured through chloride mass balance which indicated very high deep drainage rates in the pastures and almost no deep drainage under the thicketized oak woodlands. Together, these studies demonstrate the different ways woody plant cover modify a landscape's hydrology in a subhumid climate. Measurements at the surface showed that the higher productivity in a subhumid climate can bring about recovery from the historical grazing leading to collapse in ecohydrological connectedness of landscape. Transpiration and soil moisture measurements showed that density of woody plants can not only cause deep soils to dry-up, but also cause its own transpiration rates to decline over time. Lastly, the deep drainage measurements showed that dense thickets of woody cover can cause complete decline in deep water fluxes and soil water storage even in a subhumid climate. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/198028 |
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