Abstract: Mixed conifer-broadleaf forest is a transitional type from coniferous forest to zonal evergreen broadleaved forest in subtropical area of southern China. Research on water use efficiency (WUE) of forest ecosystem is helpful in predicting the impacts of environmental change on water and carbon processes in lower subtropical China. In this study, the changes of WUE and the effects of main environmental factors in the mixed coniferbroadleaf forest ecosystem during 2005 to 2010 were analyzed based on the eddy covariance system installed at the Dinghushan ForestEcosystem Research Station. The results showed that, at the annual scale, mean annual WUE of the mixed coniferbroadleaf forest was (2.85±0.22) g C·kg^-1 H₂O. The seasonal trend of WUE presented a typical single-peak curve, with the highest values occurring in spring and winter, followed by autumn, and the lowest values in summer. The highest monthly WUE was 4.75 g C·kg^-1 H₂O in January 2006, while the lowest was 1.45 g C·kg^-1 H₂O in July 2007. WUE showed a decreasing trend during the study period (2005-2010). The annual WUE was significantly correlated with air temperature and vapor pressure deficit (VPD) during the dry season (November-January of the next year), while WUE was significantly correlated with photosynthetically active radiation (PAR) in the wet season (from June to September) (P<0.001). These results are helpful to clarify carbonwater coupling in forest ecosystems in lower subtropical China and provide scientific basis for ecosystem management in the context of climate change.

Key words: subtropical forest, eddy covariance, water use efficiency, driving factor.