| The encroachment of deeply rooted woody plants into grasslands throughout the world has the potential to alter local, regional, and global water balance. The consequence of encroachment by woody plants on ecosystem water balance is, in part, related to the sensitivity of these plants to summer and winter precipitation pulses. This dissertation addresses the primary question: does pulse sensitivity of a dominant warm-desert woody plant, velvet mesquite (Prosopis velutina Woot.) vary across soil texture and water availability gradients? To address this question, sap flow and xylem anatomy and function were evaluated in mature velvet mesquite trees at two upland sites varying in soil texture at the Santa Rita Experimental Range (SRER), and one floodplain site along the San Pedro River National Conservation Area (SPRNCA). Experimental irrigation was used to assess the sensitivity of mesquite plants to small and large precipitation pulses. There was a moderate response to both small (10 mm) and large (35 mm) irrigation inputs by trees on sandy-loam soil, while trees on loamy-clay soil were only responsive to the large pulse. The differential response between sites was associated with differences in infiltration of the experimental pulses between the two soil types. Model predictions of the critical transpiration rate (Ecrit )---above which hydraulic conductivity through the soil-plant continuum falls to zero---showed that trees at the sandy-loam site operated well below their maximum transpiration rate before the onset of the monsoon. Conversely, plants on loamy-clay soils likely operate closer to their maximum permissible transpiration rates throughout the growing season. Hydraulic redistribution was observed and rates were tightly coupled to growing season and dormant season precipitation inputs. Hydraulic redistribution could enhance pulse sensitivity by transferring soil water to regions of the root zone that are otherwise dry, thereby allowing a greater proportion of the root system to participate in the extraction of pulse water during transpiration. Results from this research suggest that patterns of mesquite water relations are strongly mediated by soil texture. Nevertheless, once established, mesquite plants substantially modify ecosystem water balance, due to their responsiveness to growing season precipitation pulses, and their ability to withstand severe water deficits between precipitation pulses. |
