INTERACTIONS BETWEEN WINTER AND SUMMER ANNUALS IN THE CHIHUAHUAN DESERT

Using 18 years of census data from permanent quadrats, we examined the interactions between spatially coexisting but temporally segregated w inter and summer ephemeral plant communities in the Chihuahuan Desert. The ability of winter and summer annuals to achieve nearly complete t emporal segregation by partitioning the bimodal annual rainfall permit s the coexistence of a diverse flora of annual (and perennial) plants in this unproductive arid environment. Despite the differences in thei r biogeographical affinities and temporal segregation, long-term data indicated that at the scales of both the entire 20-ha study site and s mall 0.25-m(2) sample quadrats, abundances of plants were never high i n two successive growing seasons, suggesting a negative interaction be tween winter and summer annuals. We evaluate alternative hypotheses fo r this phenomenon.