hanges in vegetation structure in pulse-driven, water-limited systems can have important and non-linear affects on ecohydrological and biogeochemical cycles. We have studied the impacts of seasonally present annual and ephemeral herbs on ecosystem processes in two systems within a semiarid region of SE Arizona: a riparian system with woody plant encroachment and a true Sonoran Desert upland. In both systems the herbaceous plants are present and active only during periods of optimal resource availability.  Although their standing biomass is small compared to the dominant perennial plants, ephemeral herbs represent seasonal bursts of increased activity and primary production disproportional to their standing stock.

Results/Conclusions

In the encroached riparian woodland, ephemeral herbs can fill in 70% of the understory with plants 1m in height and an average leaf area index of 2.5 in response to summer monsoon precipitation. We found that given this increase in aboveground photosynthetic biomass, annual and ephemeral herbs at their peak seasonal activity have a significant contribution to this ecosystem’s carbon and water exchange. In the Sonoran Desert upland, we found that the presence of winter annual plants can significantly affect a long-lived perennial shrub’s access to shallow soil moisture, and constrain the shrub’s photosynthetic gas exchange. In both a riparian setting and a desert upland, annual and ephemeral herbs affected coupled carbon and water exchanges between soils, vegetation and the atmosphere.  Study of the ephemeral components of ecosystems is important to our understanding of the ecology of these systems, how they affect and interact with biotic and abiotic controls of carbon and water cycling, and to improve our estimates of the components of ecohydrological fluxes given shifts in physiognomy and climate.