Evapotranspiration and water use efficiency in a Chesapeake Bay wetland under carbon dioxide enrichment

Wetlands evapotranspire more water than other ecosystems, including agricultural, forest and grassland ecosystems. However, the effects of elevated atmospheric carbon dioxide (CO₂) concentration (C_a) on wetland evapotranspiration (ET) are largely unknown. Here, we present data on 12 years of measurements of ET, net ecosystem CO₂ exchange (NEE), and ecosystem water use efficiency (EWUE, i.e. NEE/ET) at 13:00–15:00 hours in July and August for a Scirpus olneyi (C3 sedge) community and a Spartina patens (C4 grass) community exposed to ambient and elevated (ambient+340 μmol mol^?1) C_a in a Chesapeake Bay wetland. Although a decrease in stomatal conductance at elevated C_a in the S. olneyi community was counteracted by an increase in leaf area index (LAI) to some extend, ET was still reduced by 19% on average over 12 years. In the S. patens community, LAI was not affected by elevated C_a and the reduction of ET was 34%, larger than in the S. olneyi community. For both communities, the relative reduction in ET by elevated C_a was directly proportional to precipitation due to a larger reduction in stomatal conductance in the control plants as precipitation decreased. NEE was stimulated about 36% at elevated C_a in the S. olneyi community but was not significantly affected by elevated C_a in S. patens community. A negative correlation between salinity and precipitation observed in the field indicated that precipitation affected ET through altered salinity and interacted with growth C_a. This proposed mechanism was supported by a greenhouse study that showed a greater C_a effect on ET in controlled low salinity conditions compared with high salinity. In spite of the differences between the two communities in their responses to elevated C_a, EWUE was increased about 83% by elevated C_a in both the S. olneyi and S. patens communities. These findings suggest that rising C_a could have significant impacts on the hydrologic cycles of coastal wetlands.