Why is mesquite excluded from the eastern Edwards Plateau of central Texas?

Large portions of west to central Texas are currently encroached by deep-rooted honey mesquite (Prosopis glandulosa). A notable exception is the eastern portion of the Edwards Plateau, where mesquite is almost entirely excluded and Ashe juniper (Juniperus asheii) is the dominant woody encroacher. Soils in this region are often thin, and underlain by impenetrable limestone layers. We hypothesized that the limited soil depth poses a problem for mesquite but not for juniper water relations, especially during drought. We tested this at a site on the eastern Edwards Plateau where with a soil depth of 1 m both juniper and mesquite are encroaching a grassland. During the summer drought period, predawn water potentials were similar for all medium and large sized trees, but were much lower in the smallest size class in both species.  This suggests that medium and small trees of both species had reached maximal rooting depth, but that trees in the smallest size class had not. Under wet conditions in spring, net photosynthesis (A_net) was proportional to tree size class in both species. Within a size class, mesquite A_net often exceeded juniper A_net. However, this species difference reversed as summer drought conditions set in. These data suggest that mesquite’s carbon gain is more strongly inhibited by drought, indicating a perhaps greater importance of a deep root system for mesquite than for juniper. We speculate that sites with more shallow soils may exceed mesquite’s drought tolerance and so exclude this species from the eastern side of the Edwards Plateau.