Water depletion of climax forests over humid karst terrain: Patterns, controlling factors and implications

Knowledge of vegetation water depletion and the underlying mechanisms are essential for understanding eco-hydrological processes and fragile ecological environment protection. Based on a 2-year field experiments, this study investigated the transpiration and its influencing factors for three karstic climax forest communities located in the depression (DeC), mid-slope (MsC) and summit (SuC) of a typical karst hill in southwestern China. The meteorological factors, sap-flow, eddy covariance and soil moisture were measured. Results showed that the annual transpiration for SuC, MsC and DeC, were 701.0 mm yr−1, 626.4 mm yr−1 and 663.1 mm yr−1, respectively. The daily transpiration was significantly correlated to the air temperature (r > 0.6, P < 0.05), solar radiation (r > 0.8, P < 0.05) and water vapor pressure deficit (r > 0.3, P < 0.05), but showed no strong relationship with the shallow layer water content (SLWC, 0–40 cm). Moreover, the transpiration of the three communities generally maintained high rate (e.g., > 4.0 mm d−1 in the growing seasons), even when the SLWC dropped to about 0.15 cm3 cm−3 in the SuC during dry spells. Overall, 77.6%, 78.3% and 90.8% of the transpired water in the SuC, MsC and DeC respectively, came from the shallow layer. These suggested that the plants in this region could uptake water from deep layers when the SLWC is limited, and implied that the climax forest communities were generally not limited by water, at least in normal years.