Soil moisture shapes the environmental control mechanism on canopy conductance in a natural oak forest

Canopy conductance (g_c) is an important biophysical parameter closely related to ecosystem energy partitioning and carbon sequestration, which can be used to judge drought effect on forest ecosystems. It is very important to explore how soil moisture change affects the environmental control mechanism of g_c, especially in natural oak forests in Central China where frequent extreme precipitation (P) and drought will occur in a context of climate change. In this study, variations of g_c and its environmental control mechanisms in a warm-temperate forest over three consecutive years under different hydroclimatic conditions were examined by using eddy-covariance technique. Results showed that the averaged g_c in the three growing seasons were 11.2, 11.3 and 7.8 mms^-1, respectively, with a CV of 19.7 %. The lowest g_c occurred in the year with the lowest P. Using three years of data, we found that vapor pressure deficit (VPD) exhibited the dominate effect on g_c, both diffuse photosynthetically active radiation (PAR_dif) and air temperature (T_a) were positively correlated with g_c. When relative extractable water content (REW) was larger than 0.4, however, inhibiting effect of high VPD on g_c disappeared and the effect of direct photosynthetically active radiation (PAR_dir) on g_c was larger compared to PAR_dif. When REW was <0.1, the positive relationship between T_a and g_c became negative. Our results indicated that soil moisture ultimately shapes the environmental control mechanism of g_c in a natural oak forest.