Abstract
We calculated water use efficiency (WUE) using measures of gross primary production (GPP) and evapotranspiration (ET) from five years of continuous eddy covariance measurements (2009–2013) obtained over a primary subtropical evergreen broadleaved forest in southwestern China. Annual mean WUE exhibited a decreasing trend from 2009 to 2013, varying from ~2.28 to 2.68 g C kg H2O−1. The multiyear average WUE was 2.48 ± 0.17 (mean ± standard deviation) g C kg H2O−1. WUE increased greatly in the driest year (2009), due to a larger decline in ET than in GPP. At the diurnal scale, WUE in the wet season reached 5.1 g C kg H2O−1 in the early morning and 4.6 g C kg H2O−1 in the evening. WUE in the dry season reached 3.1 g C kg H2O−1 in the early morning and 2.7 g C kg H2O−1 in the evening. During the leaf emergence stage, the variation of WUE could be suitably explained by water-related variables (relative humidity (RH), soil water content at 100 cm (SWC_100)), solar radiation and the green index (Sgreen). These results revealed large variation in WUE at different time scales, highlighting the importance of individual site characteristics.
Highlights
Ecosystem water use efficiency (WUE) indicates the coupling of carbon and water vapor flux exchanged between the atmosphere and an ecosystem and it quantifies how much water an ecosystem uses relative to carbon gained[1,2]
A widespread and severe drought occurred in southwestern China in 2009 and 2010, providing a unique opportunity to directly evaluate how WUE changes with drought stress in the primary subtropical forest
Unlike the diurnal dynamic of WUE, the underlying water use efficiency (UWUE) reached a peak in the evening and decreased sharply
Summary
Ecosystem WUE indicates the coupling of carbon and water vapor flux exchanged between the atmosphere and an ecosystem and it quantifies how much water an ecosystem uses relative to carbon gained[1,2]. Consistent and continuous observations for accurate evaluations of WUE can provide insight into how the ecosystems respond, or have responded, to climate fluctuations at different temporal scales, from hourly to multiannual[10,12]. Most long-term characteristics of WUE in forest ecosystems are measured using the tree-ring carbon isotope method[8,13]. Tree-ring isotopes have the advantage of recording long-term changes, but may not be reliable in quantifying the responses of WUE to local climate change. The Ailaoshan Nature Reserve in Yunnan province, southwestern China hosts about 5000 ha of primary subtropical evergreen mountain cloud forest. This area is exposed to monsoon precipitation (P) regimes from the southwest and from the southeast. The impact of vegetation phenology modification (the timing of leaf emergence, developmental, and senescence stages) on WUE is likely to be critical
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