Comparing ET-VPD hysteresis in three agroforestry ecosystems in a subtropical humid karst area

Understanding evapotranspiration-vapor pressure deficit (ET-VPD) hysteresis for different ecosystems can improve our knowledge of vegetation adaptability to a changing environment. However, no previous studies have compared the controlling factors of ET-VPD hysteresis for different agroforestry ecosystems in karst landscapes. Therefore, this study explored ET-VPD hysteresis patterns and the controlling factors for three agroforestry ecosystems, using field observations in a subtropical humid karst catchment from March 12, 2015 to May 26, 2016. The results showed both similarities and differences in ET-VPD hysteresis among the three ecosystems. For example, all three agroforestry ecosystems exhibited noticeable ET-VPD hysteresis, and the hysteric magnitude showed noticeable seasonality with higher values during the growing season. For all three ecosystems, the ET-VPD hysteretic magnitude had no significant relationship with the daily mean environmental factors (e.g. meteorological factors and soil moisture (θ)), but it was highly (R2 > 0.59) correlated to changing rates of meteorological factors (e.g. temperature (Ta), relative humidity (RH), and vapor pressure deficit (VPD)) in the morning. The ET-VPD hysteretic magnitude of the forest-grass mixed ecosystem (142.48 ± 74.16 min) was significantly higher than that of the grass ecosystem (104.37 ± 82.93 min). Hysteresis reflected the extent of imbalance between water demand and water supply for ecosystems during a day. Therefore, this study may be helpful in modeling ET and further understanding ecosystem responses to environmental change.