Water use efficiency in a primary subtropical evergreen forest in Southwest China

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.