Since water availability is often the crucial limiting factor for plant recruitment and growth in arid ecosystems, we have altered the amount and timing of summer and winter precipitation by 25% to investigate how overall increases in seasonal precipitation may affect plant growth and species composition in a sotol-grassland community in 2002-2006 in comparison with natural precipitation. For aboveground annual net primary productivity, we focused on three dominant perennial plant species: Dasylirion leiophyllum, Bouteloua curtipendula, and Opuntia phaeacantha. Species richness and plant density values were used to investigate precipitation effects on species composition (e.g. shrubs, grasses, herbs, and succulents). None of the species showed a positive ANPP response to increasing annual precipitation. Only Bouteloua and Opuntia ANPP showed a significant treatment response: Bouteloua had greater ANPP in the winter treatment for 2002 and Opuntia ANPP was greater for the summer/winter treatment in 2003. Both years received above average annual precipitation. The following years were drier (2005-2006) and showed no treatment response. Canonical correspondence analyses (CCA) suggest that soil exchangeable nitrogen may have a greater impact on species ANPP during wet years (2002-2004) than annual precipitation and soil moisture. Over the last five years there has been no significant change in species richness due to natural precipitation or supplemental treatments. For plant density, only grasses and herbs showed a response to seasonal precipitation (spring and summer), but not to supplemental treatments. There were few changes in shrub or succulent plant densities.
