The influence of soil texture and vegetation on soil moisture under rainout shelters in a semi-desert grassland

Climate change and invasions by non-native organisms are two factors of global change likely to alter the structure and function of arid and semi-arid ecosystems. We conducted a large-scale field experiment to determine how changes in amount of summer precipitation and invasions by an African grass (Eragrostis lehmanniana) may interact with soil texture to affect community and ecosystem processes in temperate grasslands and savannas of southern Arizona, USA. In particular, we investigated the response of soil moisture at several soil depths within seventy-two 2.7 m2 plots under six 9 m×18 m rainout shelters to a 3-fold difference in irrigation between February and November 2002, as well as to a single large pulse of irrigation water in June 2002. In addition, we documented the effects of the rainout shelters on microclimates under the shelters. Results indicate that relatively more water is available to plants at greater depths on sand-rich soils, whereas on clay-rich soils the greatest water availability is at the surface. Typically, soil moisture under E. lehmanniana was lower than under plots with the native grass Heteropogon contortus, although soil texture and depth modified this pattern. The precipitation shelters had minimal impacts on windspeed, soil temperature, and relative humidity; small differences in microclimatic effects between the soil types were attributed to effects of surrounding vegetation on wind velocity. Differences in soil texture, grass cover and species identity, and amount of irrigation exert interactive controls over soil moisture in this semi-arid environment, but responses are modified by time of year and depth in the soil profile.