Carbon and water fluxes in an arid-zone Acacia savanna woodland: An analyses of seasonal patterns and responses to rainfall events

The study of landscape gas exchange in arid and semi-arid regions is less common than those of more mesic environments, despite their large geographical extent, their importance to regional climate, their socioeconomic values and the carbon and water balances of such regions. In this study we used eddy covariance measurements to examine net ecosystem exchange and water fluxes of a landscape dominated by a N-fixing tree (Acacia aneura; Mulga) as a function of soil moisture content, vapour pressure deficit, leaf area index and pulses of rain. Seasonal budgets of carbon and water, ecosystem-scale water-use-efficiency (the ratio of net ecosystem exchange to evapotranspiration) and inherent water-use-efficiency (ecosystem water-use-efficiency × vapour pressure deficit) were also examined. Across the 12 month study, the landscape was a net sink for carbon, despite prolonged periods of zero rain. Changes in both net ecosystem exchange and evapotranspiration were tightly coupled to changes in the moisture content of the upper (10 cm) soil profile, but not the deeper profile and both responded rapidly to changes in soil moisture content. As vapour pressure deficit increased over the course of several consecutive days in the wet season there was no significant response of ecosystem water-use-efficiency. In contrast, in the dry season, as vapour pressure deficit increased ecosystem water-use-efficiency declined curvilinearly. However, in both wet and dry seasons, ecosystem water-use-efficiency declined with increasing soil moisture content. Daily inherent water-use-efficiency increased gradually following each rainfall event. As daily mean vapour pressure deficit increased between rain events, inherent water-use-efficiency increased in both the wet and dry seasons but with a steeper slope in the wet season. However, inherent water-use-efficiency decreased with increasing soil moisture in both seasons, and the slope of a semi-log plot of inherent water-use-efficiency versus soil moisture content decreased faster in the dry season than in the wet season. Similarly, the marginal carbon cost of water was smaller (0.3) in the wet than dry season (0.6). Variations in ecosystem leaf area index were correlated with the under storey component, which was highest in the wet season and lowest in the dry season. We therefore conclude that changes in under storey leaf area index were significant drivers of seasonal changes in canopy gas exchange. Mulga, despite maintaining leaf area index through the dry season in a semi-arid environment, supports little dry season evapotranspiration and relies, to a very large extent, on soil moisture in the upper soil profile rather than deeper stores of water.