High efficiency in water use and carbon gain in a wet year for a desert halophyte community

Eddy covariance measurements of water and carbon (C) fluxes were carried out in a desert halophyte community in western China, during two years differing greatly in precipitation (2006 and 2007). The first year was dry, with annual precipitation 22% below the long-term mean (163 mm) and the second year was wet (42% above the long-term mean). The main goal of this study was to develop an understanding of how ecological and hydrological processes and vegetation composition respond to precipitation variability in halophyte desert ecosystems. On an annual basis, the desert halophyte community was a weak sink or source in the dry year (−5 ± 12 g C m−2 year−1), but a strong sink in the wet year (−40 ± 12 g C m−2 year−1). Groundwater was a stable water source for evaporation and transpiration, supplying average of 14 mm in both the dry and the wet years for each part. However, water supply for plant transpiration from precipitation differed remarkably between the two years: 17 and 48 mm for the dry and wet years, respectively. Connecting water use and C gain, ecosystem water use efficiency was markedly different for the dry and wet years, with values of 0.03 and 0.15 g C per kg H2O, respectively; however, plant water use efficiency was differed only slightly (3.58 and 3.51 g C per kg H2O). Vegetation community surveys and root investigations revealed that more shallow-rooted herbaceous plants occurred in the wet year compared to the dry year. Thus the inter-annual variation of water and C fluxes may have resulted from adjustment of community structure to precipitation with more annuals or ephemeral plants in the wet year. These shallow-rooted plants use the extra water input in the wet year, and consequently the community productivity increases. Water use efficiency at the ecosystem level increased in the wet year at this desert, contrary to findings in more humid environments where water use efficiency increases in dry years.