Soil depth changes everything: How limitations of ecosystem water storage govern the ecology of trees in grassland

Background/Question/Methods . Woody encroachment into grasslands is thought to have largely common causes, remedies and, above an annual precipitation threshold, significant negative effects on water yield through increases in evapotranspiration.  However, recent studies have increasingly highlighted variation in the patterns and consequences of woody encroachment at the landscape level.  Building upon the insights gained from recent field studies conducted on the Edwards Plateau in central Texas, we develop a heuristic ecohydrological model to examine how soil depth might interact with precipitation patterns to modify tree demography and water use.  

Results/Conclusions .  Even though trees growing on shallow soils extend roots into fractured bedrock, such substrates appear to limit tree access to water, thus reduce plant-available storage capacity for water.  Thus, shallow soil conditions require a degree of drought tolerance that is met by some but not all woody encroachers.  For example, while honey mesquite can establish and survive on soils of about one meter depth on the Edwards Plateau, mature trees suffer near complete reproductive failure in drier than average years.  By contrast, Ashe juniper reproduces in most years, even on the shallowest soils, because it sets cones, fruits and seeds during winter when the demand for water storage capacity is minimal.  We suggest that soil depth limitations weaken the establishment bottleneck in woody encroachment, thereby permitting encroachment by species that, as seedlings, are relatively weak competitors against grasses.  Concurrently, the reduced water storage capacity of shallow soils intensifies reproduction bottleneck, excluding species with a high water demand for reproduction.  Further, because shallow soils select for drought tolerant tree species, impacts of woody encroachment on ecosystem ET, and negative impacts on recharge, are therefore reduced on shallow soils. Native tree species of the Edwards Plateau appear to contain both isohydric and anisohydric species, suggesting that both strategies are viable on shallow soils under past and current climate conditions.  However, changes predicted by climate models that include increases in precipitation variability may in the future select against anisohydric species, potentially reversing juniper encroachment.   The talk concludes by highlighting major research needs for savanna and woodland ecosystems whose water requirements are in part supplied by fractured bedrock.