Changes in vegetation structure in pulse-driven, water-limited systems can have important and non-linear affects on ecosystem function and biogeochemical cycling. Conversion of grasslands to woodlands in these systems through woody plant encroachment also results in greater patch heterogeneity, and can promote the presence of a more diverse plant community. Along a semi-arid riparian corridor in S.E. Arizona, annual and ephemeral herbaceous plants are present primarily in areas with significant woody plant establishment. The herbaceous component of this ecosystem is active during late summer in response to seasonal monsoon precipitation, and is productive until the first frost. These herbs can fill in 75% of the understory with plants 1m in height and an average of 2.5 meters of leaf per meter of ground. Given this increase in aboveground photosynthetic biomass, at their peak seasonal activity, annual and ephemeral plants should have a significant contribution to ecosystem carbon and water cycles, and ecosystem water-use efficiency. We tested these effects through herbaceous plant removals at two sites concurrently monitoring ecosystem-level carbon and water fluxes in the San Pedro Riparian National Conservation Area: a shrubland with 50% woody shrub cover, and a woodland with a mature overstory of 75% woody cover.

Results/Conclusions

We found that the presence of the herbaceous understory contributed significantly to both positive carbon fixation and aboveground respiration. These plants had little effect on soil respiration, and appear to allocate resources primarily to the support of photosynthesis and reproduction, with little investment in belowground structures. Over the duration of the growing season at the woodland site, we found that the herbaceous understory accounted for a significant proportion (~ 1/3) of the seasonal increase in total ecosystem carbon fixation during the period of peak ecosystem activity. The influx of ephemeral photosynthetic biomass also affected ecosystem water use by significantly drying surface soils and increasing rates of evapotranspiration up to 65%. The combination of increased carbon fixation and water use demonstrates that these plants have important effects on ecosystem water-use efficiency and carbon storage. Study of the herbaceous annual and ephemeral understory is important to our understanding of the ecology of these systems, how they affect biotic and abiotic controls of carbon and water cycling, and to improve our estimates of the components of ecosystem carbon fluxes given shifts in physiognomy and climate.