Understanding the response of semi-arid systems to changes in woody plant cover and global change factors has been an area of active research for some time. When coupled with climate change, shifts in vegetation structure or function in these water-limited systems can have important and non-linear affects on ecosystem function and biogeochemical cycling. By focusing on the end-members of grasses and woody plants, we may be missing crucial information about the encroachment process or the function of the resulting ecosystem. We have found that short-lived, small, annual plants are important components to Sonoran Desert ecosystems, facilitating the formation of fertile shrub islands and affecting co-occurring perennial shrub water availability and function. The presence of these annuals also significantly affects soil respiration, water infiltration, decomposition rates and biogeochemical cycling. Yet the effects of annuals and herbaceous understory plants in semi-arid systems remain understudied. Our goal was to extend our research from the desert upland to a riparian corridor, utilizing the San Pedro National Conservation Area in Southeastern Arizona to better understand the importance of annual and herbaceous perennial understory plants to ecosystem function in the semi-arid Southwest.

Results/Conclusions

We utilized understory removal plots at two sites along a space-for-time gradient of woody encroachment into grassland: a mature, closed-canopy mesquite woodland, with the native Sacaton bunchgrass limited to a patchy distribution in the understory; and a grass-mesquite shrubland with approximately 50% woody plant cover. During the summer monsoon growing season, herbaceous understory plants can fill in the understory open space with vegetation that regularly reaches 1m in height. We compared plots with herbaceous plant removal to paired control plots nearby (<3m distant), using chamber measurements of plot and leaf-level gas exchange, soil CO₂ flux, soil water content, and soil C to monitor carbon and water dynamics in this system. During periods of peak photosynthesis, we found that plots with an intact herbaceous understory could show rates of CO₂ uptake of 6 μmol m^-2s^-1 compared to the release of -5 μmol CO₂ m^-2s^-1 in the paired removal plots. Rates of evapotranspiration were 60% higher on average in the vegetated plots than the removal plots. Soil respiration rates were also higher in the vegetated plots. Our results suggest that herbaceous understory plants can have important effects on carbon and water cycling during periods of peak activity in semi-arid ecosystems.