Ecosystem carbon and water budgets under elevated atmospheric carbon dioxide concentration in two California grasslands

Stomatal conductance and photosynthesis play central roles in regulating the fluxes of water (H₂O) and carbon (C) between the land surface and the atmosphere. Anthropogenically driven increases in atmospheric carbon dioxide (CO₂) concentration may have a significant impact on these fluxes by decreasing stomatal conductance and increasing photosynthesis. Here, using a combination of gas-exchange and modeling results, I describe the effects of elevated atmospheric CO₂ concentration (720 ppmv) on ecosystem water and carbon budgets in serpentine and sanstone annual grasslands. Averaged across grasslands, daily evapotranspiration (ET) decreased by an average of 0.53 mm H₂O day−1 under elevated CO₂ during the winter and spring months. Decreases in ET were accompanied by increases in soil moisture and deep drainage. During the summer, early season water savings resulted in an average ET increase of 0.12 mm H ₂O day−1 under elevated CO₂. On an annual basis, simulation results show that ET decreased by an average of 26.5 mm H₂O y−1. For the net ecosystem CO₂ exchange analyses, I used soil CO₂ flux measurements in conjunction with a soil CO₂ transport model to remove the bias introduced by chamber pressurization. In the sandstone grassland, net ecosystem production (NEP) increased from 55.6 g C m−2 y−1 under ambient CO₂ to 59.6 g C m−2 y −1 under elevated CO₂, a difference of 4.0 g C m −2. Annual canopy net photosynthesis (A_n) increased under elevated CO₂ by 35.8 g C m−2 y −1, but the bulk of this increase was returned to the atmosphere via root and heterotrophic respiration. Canopy A_n decreased under elevated CO₂ early in the growing season, an observation that may reflect nitrogen limitation. These results suggest that future increases in atmospheric CO₂ concentration will significantly reduce evapotranspiration but have only minimal effects on ecosystem C uptake.