Chihuahuan Desert Annuals: Importance of Water and Nitrogen.

We examined the effects of water supplementation and nitrogen amendment on biomass, cover, and density of annual plants on a termite-free and a termite-present area in the Chihuahuan Desert. Soil moisture was higher in the termite than in the termite-free plots, and in the watered than in the unwatered plots during the spring and summer. There were no differences in soil moisture among plots during the winter. Soil nitrogen was higher in the termite-free than in the termite plots. There were no differences in total plant biomass produced in termite and termite-free areas. There were significant differences in relative abundances of species among treatments. natural rainfall was sufficient for maximum spring-annual biomass development on all plots except for the termite-free unfertilized, unwatered plots. These were the driest plots but had high soil nitrogen. Most of the herbaceous species responded to the water amendments by lengthening growing seasons, increasing density, or increasing biomass. When there was sufficient water for most of the spring annuals, high soil nitrogen levels favored increased densities and biomasses of Descurainia pinnata and Lepidium lasiocarpum. The absence of C4 summer annuals in the high-nitrogen plots suggests that relatively high soil nitrogen adversely affected the summer annuals. Termite-free watered plots had higher soil moisture than the termite-unwatered plots, but summer annuals were relatively abundant on the latter. Water amendments had a greater effect on the species abundances in the termite-free area than in the one with termites. In the area with termites, nitrogen amendments had a greater effect on species abundances. Species diversity and richness were affected by fertility as was species composition. This study demonstrates that we must understand patterns of soil nitrogen availability and processes affecting nitrogen availability in addition to water availability, in order to understand productivity and species composition of Chihuahuan Desert annual plants. © 1987 by the Ecological Society of America.