Agricultural and Forest Meteorology
Volumes 162–163, 15 September 2012, Pages 127-135
High efficiency in water use and carbon gain in a wet year for a desert halophyte community
Abstract
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.
Highlights
► Precipitation variation alters the water use strategy of the desert plant community. ► Water use efficiency increases in wet years for this desert ecosystem. ► Carbon gain benefits from increased inter-annual variation in precipitation.
Introduction
Increasing atmospheric carbon dioxide (CO2) concentration is expected to increase the mean global surface temperature and modify the hydrological cycle, resulting in higher frequencies of extreme drought and flood events (Easterling et al., 2000, Houghton, 2001, Millennium Ecosystem Assessment, 2005). Shifts in the amount, frequency or seasonality of precipitation accompanying global climatic changes are likely to differ among regions. In addition, the impact of these changes on the structure and function of ecological systems will differ depending on the local environmental and biological conditions (Houghton et al., 1996, Mahlman, 1997, Giorgi et al., 1998, McAuliffe, 2003).
Arid and semi-arid regions comprise > 40% of the global terrestrial surface (Asner et al., 2003). Their distinct vegetative communities have low leaf-areas, above-ground biomass and net productivity (Schilesinger, 1991, Unland et al., 1996, Rotenberg and Yakir, 2010), and are highly dependent upon precipitation. In these regions, precipitation exerts overwhelming control over the local water and carbon balance. Slight changes to amount and timing of precipitation may trigger a complex interaction of physical, chemical and biological processes at the ecosystem level (Mielnick et al., 2005, Williams et al., 2006, Potts et al., 2006).
Potential responses to changes in precipitation regimes may include shifts in plant composition, distribution and abundance (Stephenson, 1990, Figueroa and Davy, 1991, Flanagan et al., 2002, Snyder and Tartowski, 2006, Scott et al., 2010). In turn, such responses may cause strong feedbacks on the water and carbon (C) balances (Mielnick et al., 2005, Scott et al., 2010). The seasonal timing of precipitation may be as important as its magnitude in controlling ecosystem water and C fluxes (Weltzin and Tissue, 2003, Schwinning et al., 2004a, Schwinning et al., 2004b). For example, low precipitation and high evaporative demand, coupled with limited plant canopy development during the pre- and early-growing season could constrain transpiration and CO2 assimilation.
Most soils in halophyte desert ecosystems are highly saline and alkaline (Sinha et al., 2002, Smedema and Shiati, 2002), because precipitation in these regions is significantly less than the potential evaporation rate. Groundwater discharge can be a major component of the water and salt balance (Fitzpatrick et al., 2001). 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, e.g., perennial shrubs) and herbaceous plant groups (which depend mostly on precipitation, e.g., annuals or ephemerals) (Xu and Li, 2006, Collins and Bras, 2007, Guswa, 2010). In addition to directly altering water supply, precipitation in these ecosystems also determines affects the water availability to plant indirectly by diluting or flushing down salts (Herczeg et al., 2001).
Arid and semi-arid halophyte desert ecosystems are heterogeneous with respect to root depth and vegetation structure. They 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 (Huxman et al., 2004, Troch et al., 2009). This, in turn, will allow more accurate predictions of possible effects of current and future climate change scenarios at local and regional scales (Jenerette et al., 2012).
The objectives of this paper are to (1) compare the differences in carbon flux in a dry and wet year; (2) compare the differences in water supply and demand (groundwater vs. precipitation) and water use efficiency; and (3) investigate mechanisms that result in differences in water use and C gain in dry and wet years.
To meet the above objectives we will evaluate two linked ecohydrological hypotheses and predictions. (1) Differences in water use and C gain may be due to adjustments of community structure to precipitation. Heavy precipitation may lead to more germination and growth of annuals or ephemeral plants in the wet year, which in turn use rain water to assimilate C and increase community productivity. (2) The capability that the composition of the desert community is adjustable to precipitation may provide an explanation to increase in ecosystem water use efficiency in the wet year.
Section snippets
Site description
The study site is located at the Fu-Kang Station of Desert Ecology, Chinese Academy of Sciences, in the hinterland of the Eurasian continent (44°17′N, 87°56′E and 475 m a.s.l.). The station is 8 km from the southern edge of the Gurbantonggut Desert and 72 km south of the highest peak of the eastern Tianshan Mountains (5445 m a.s.l.). The study area is fairly flat (slope < 1°), and the groundwater table fluctuates between 2 and 5 m depth. This shallow groundwater, which is recharged by snow-melt runoff
Meteorological conditions during two climatologically contrasting years
The seasonal patterns of average daily Rn and air temperature (Ta) were similar in 2006 and 2007 (Fig. 2a–d), with minimum values (−17 to −24 W m−2) in winter and maximum values (120–150 W m−2) in summer. Maximum and minimum Ta were between 34 and 38 °C and −29 to −32 °C, respectively. The annual means of Rn and Ta were 53 W m−2 and 6.7 °C. In contrast, the amount of precipitation at the study site varied significantly between the years. The annual total precipitation was 129 mm in 2006 and 231 mm in 2007
Discussion
Across dryland sites, functions of ecosystem carbon uptake vary largely from a net C sink to a source, and these variations depend on precipitation regimes (Emmerich, 2003, Mielnick et al., 2005, Nagy et al., 2007). Recently, eddy covariance measurements in desert shrub environments have provided valuable insights in ecosystem C exchange patterns (Kurc and Small, 2007, Scott et al., 2009), but many of the ecosystem processes remain poorly understood. The annual NEE in this study showed the
Acknowledgements
Financial support was provided by the National Key Basic Research Project (Grant No. 2009CB825104), a grant to YL from the Natural Science Foundation of China (No. 40725002) and a cooperative China-New Zealand research project (No. 2011DFA31070). Thanks to all the staff of the Fu-Kang Station of Desert Ecology for their help. The authors gratefully acknowledge the many useful comments provided by Drs. Yu-gang Wang and Cheng-hua Li.
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