Response of Mesquite (Prosopis velutina Woot.) Carbon-Water Relations to Variations in Soil Texture and Precipitation

by Fravolini, Alessandra.

Variability in climate and rainfall are commonly cited by aridland ecologists to explain shifts in plant community structure and land-cover change in waterlimited environments. While pulses of precipitation undoubtedly play an important role in driving vegetation dynamics of arid and semiarid lands, the relationship between precipitation and plant-water acquisition is not straightforward. The amount of soil moisture that is effectively available to plants is ultimately controlled by a complex combination of biotic and abiotic factors such as physical soil attributes and plant developmental stage. This indirect relationship between precipitation and plant water uptake can lead to profound differences in plant distribution and ecosystem function across arid and semiarid landscapes, greatly contributing to mesoscale physiognomic heterogeneity. In this doctoral research I investigated the interaction between soil texture and summer precipitation and its implications on the carbon-water acquisition of a widespread woody legume: velvet mesquite (Prosopis velutina Wooten). Specifically, the goal was to answer the following questions: how are precipitation pulses translated into plant-available water across different soil types? Do mesquite responses to different sized pulses vary across soils with different soil textures? Does the response of mesquite to pulses of precipitation vary with ontogeny? 13 This research revealed that variations in soil texture across the landscape led to differential utilization of seasonal precipitation by mesquite trees beginning in the earliest stages of mesquite development. Specifically, we found that on coarse-textured soils both adult and one year old mesquites are highly coupled to and dependent on the occurrence of both large and small summer precipitation pulses. In contrast, on fine textured soils, the response of mesquite to both large and small summer precipitation pulses is limited by the presence in the soil of antecedent winter water. Substantial differences in seasonal water balance between mesquite occurring on different soil textures were also detected with mesquite on fine-textured soils experiencing a smaller pre-monsoon water deficit than mesquite on coarse-textured soils. Differential utilization of seasonal water by mesquite across the landscape coupled with predicted changes in seasonal precipitation may have important consequences in future patterns of plant community structure and ecosystem water balance of this semi-arid lands. 14