The seasonal uneven distributions of precipitation results in significant difference of soil water content in subtropical forests of southern China. This potentially induces water stress in the dry season, which in turn may affect plant transpiration. In the present study, we intended to explore effects of environmental variables and plant hydraulic conductance on the whole tree transpiration (E_T) of Schima superba, a dominant tree species in subtropical forests, and paid a special attention to the mechanisms of hydraulic compensation for transpiration under water stress condition. E_T was determined from simultaneously measured stem sap flow using Granier's thermal dissipation probes, and soil-leaf water potential difference (Ψ_L-S) was measured with the aid of a PMS pressure chamber on sunny days in dry (November 3 to 5) and wet seasons (August 21 and 22) of 2009. In addition, atmospheric evaporative demands and soil moisture conditions were monitored. The results showed that the rainfall seasonality brought about distinct soil moisture conditions between wet and dry seasons, but had no significant effect on the E_T of S. superba. It turned out that E_T of S. superba was significantly correlated to photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) in both dry and wet seasons. However, in dry season E_T was co-regulated by both stomatal and hydraulic conductance, whereas E_T was mainly regulated by stomatal conductance in wet season possibly due to sufficient supply of water and radiation. Regression analyses of E_T and soil-leaf water potential difference (Ψ_L-S) allowed the prediction of E_T in both dry and wet seasons. A distinct discrepancy between predicted and measured E_T (calculated from sap flow measurements) was found, i.e. the measured E_T was 91.74% of the predicted E_T in dry season, while in wet season measured E_T was 1.33 times higher than the predicted one. This revealed a hydraulic compensation for E_T that usually occurred in the afternoon of the dry season (13:00-17:00), when VPD reached as high as 2.132 MPa, and the compensation value was calculated to be 0.08 g/s. The whole tree hydraulic conductance (k) decreased within the daytime in both dry and wet seasons; while it was generally lower in dry than wet season, especially in the morning (7:00-9:00) and afternoon (13:00-17:00), indicating higher hydraulic resistance during that time in dry season. Taken together, the present results suggested that the lower and continuously decreasing k in dry season especially in the afternoon when VPD was high, strengthened the water stress on leaves and increased the sensitivity of stomata to drought, which thus balanced the water loss and heat absorption and promoted plant survival under drought conditions. The dynamic variation of k and subsequent hydraulic compensation may represent one of the reasons why subtropical broadleaf trees have potentially similar E_T in dry and wet seasons. The quantification of hydraulic compensation may be useful for a better understanding the interactive regulations of stomatal and hydraulic conductance on E_T. Our results would also be helpful to evaluate the ecological functions of subtropical forests in southern China, in terms of water conservation under the background of seasonal uneven precipitations.