High Efficiency in Water Use and Carbon Gain in a Wet Year for a Desert Halophyte Community

Arid and semi-arid regions comprise > 40% of the global terrestrial surface and are highly dependent upon precipitation. In these regions, precipitation exerts overwhelming control over the local water and carbon balance. Potential responses to changes in precipitation may cause strong feedbacks on the water and carbon (C) balances.

Usually, a typical desert halophytes community can be divided into phreatophyte (which obtain a large fraction of water from the near surface groundwater or the capillary fringe) and herbaceous plant groups (which depend mostly on precipitation). In addition to directly altering water supply, precipitation in halophytes desert ecosystems also determines the water availability to plant indirectly by diluting or flushing down salts.

Arid and semi-arid halophyte desert ecosystems offer a unique opportunity to investigate the sensitivity of water and carbon exchanges between the ecosystem and atmosphere in response to changes in precipitation regimes. Enhanced knowledge of these systems will aid in understanding the basic mechanisms linking hydrological and ecological dynamics across climatic gradients.

To develop an understanding of how ecological and hydrological processes and vegetation composition respond to precipitation variability in halophyte desert ecosystems, eddy covariance measurements of water and 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 with annual precipitation 42% above the long-term mean.

The result showed that, 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 mm 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.

The result has been published on Agricultural and Forest Meteorology, 2012, 162-163: 127-135. The paper can be downloaded from http://www.sciencedirect.com/science/article/pii/S0168192312001451.