Comparing evapotranspiration characteristics and environmental controls for three agroforestry ecosystems in a subtropical humid karst area

Karst landforms account for 10% of the Earths land surface, and it is well-known that water is a limiting factor for karst ecosystems because numerous underground fractures and conduits induce rapid drainage. Therefore, an understanding of ecosystem water consumptions (evapotranspiration) and environmental controls is particularly important for land management on karst landforms. This study compared the actual evapotranspiration (AET) characteristics and analyzed the controls of three agroforestry ecosystems by field observations using a refitted ventilated-chamber, thermal dissipation probes and micro-lysimeters in a subtropical humid karst catchment from 12th March 2015 to 26th May 2016. Results show that: 1) the diurnal transpiration curves of trees plateau at noon with low fluctuations at a high level, while evapotranspiration curves of grass and crop ecosystems present single peaks; 2) AET of the forest-grass mixed ecosystem (7.64 ± 5.75 mm·day−1) was significantly higher than crop (4.24 ± 3.35 mm·day−1) and grass ecosystems (5.78 ± 3.53 mm·day−1); 3) AET of grass and crop ecosystems were most sensitive to temperature, while the forest-grass mixed ecosystem was more sensitive to leaf area index (LAI) than other factors; 4) AET of the forest-grass mixed ecosystem was more sensitive to deep soil moisture (>30 cm), while grass and crop ecosystems were more sensitive to shallow soil moisture (10 cm and 20 cm). This study suggests that soil water content was not key limit factors to AET in humid karst landscapes. However, LAI, which can be controlled by land management, heavily influenced AET rates and E/AET (E, evaporation) ratio through changing water demand.