Grasslands in many semi-arid regions of the world have seen an expansion of woody vegetation over the past century and many now exist largely as woodlands or shrublands. This “woody encroachment” results in numerous changes to ecosystem function, including alteration of element and water cycles. Vegetation removal strives to reduce competition and therefore increase herbaceous vegetation by reducing woody vegetation thru a variety of methods. Two ways this activity has the potential to alter the water cycle are by reducing transpiration and changing rooting depths. The interactions of soil texture, soil water movement, and vegetation removal techniques are poorly understood for the mesquite woodland ecosystem of south Texas. We report here the results of a large manipulative experiment designed to assess the effects of woody vegetation removal on soil moisture movement in the critical zone. In this study, woody vegetation has been removed using two common methods over 3 soil textures that represent the range of soils typical in this area. The study location was selected for its potential to act as a recharge zone for the underlying Carrizo-Wilcox Aquifer. Fifty-four soil cores (2 m each) were taken across all treatments and soil types to determine pre-treatment rooting depth. Neutron probe measurements were taken from July thru April to determine volumetric soil water content. Soil and stem water isotopes were collected during summer to help understand depth of water acquisition for the dominant woody species Prosopis glandulosa.
Results/Conclusions
The data indicate no change in soil water volume for any treatment or soil texture below 1 m at any point in the study. Isotope analysis shows evaporative enrichment of δ018 and δH2 values at soil surfaces with a declining effect at depth. Comparisons of soil water to stem water do not indicate a water source with a different isotopic composition from the soil profile to 2 m. As expected, these results show under low rainfall conditions there is no potential for improved potential recharge to the underlying aquifer by woody vegetation removal. A significant interaction between soil texture and treatment suggested that the effects of brush removal on soil water content are soil dependent. This study improves our understanding of the relationships between below ground vegetation, soil water movement and an overall understanding of ecosystem processes. This study also provides valuable information to land owners and managers considering woody vegetation removal.