Responses of Chihuahuan Desert herbaceous annuals to rainfall augmentation

Densities and biomass of annual herbaceous plants were compared on plots receiving supplemental water of 25 mm at 4-week intervals or 6·3 mm per week, and plots receiving only natural rainfall. During the first summer, there were no significant differences in total biomass between treatments, but the density and biomass of two species, Astragalus tephrodes and Euphorbia micromera, were greater on the watered plots. Several spring annuals responded to supplemental water and by late spring (April) there were decreases in biomass on the watered plots, with the greatest decreases on the 6·3 mm per week plots. Few species of summer annuals responded to supplemental water during the second year, possibly as a result of smaller rain events and/or limitation of available nitrogen. Root shoot biomass ratios were between 0·15 and 0·25 for winter–spring annuals and 0·04 and 0·03 for summer annuals.