Disentangling the independent and interacting impacts of biophysical factors on the transpiration of a black locust (Robinia pseudoacacia) plantation in the semiarid Loess Plateau, China

Transpiration (Tr) is the largest continental water flux. Tr is jointly affected by biophysical factors, including atmospheric demand, soil water supply, and plant physiological features, represented by potential evapotranspiration (PET), soil water content (SWC), and leaf area index (LAI) respectively. Disentangling the independent and interacting impacts of the biophysical factors on Tr provides a better understanding on the mechanism of ecohydrological controls on forest water use. In this study, taking the black locust plantation of the Loess Plateau as an example, independent contributions of the three factors and their interactions on Tr were quantified based on field observation, model development, and factorial analysis. First, sapflow density, meteorological variables, SWC, and LAI were observed during five growing seasons. Second, a coupled Tr model (R² = 0.85, RMSE = 5.92) was established by the long-term observed data of Tr, PET, SWC, and LAI. Finally, factorial experiment analysis showed that contributions of LAI, PET, and SWC and their interactions (PET×LAI, LAI × SWC, and PET×SWC) on Tr accounted for 50.82%, 9.77%, 9.30%, 13.88%, 13.75%, and 2.47%, respectively. The results indicated that LAI and PET×LAI had the largest independent and interacting impact on Tr of black locust. This study suggested that regulation of canopy leaf area through forest cultivation practices could reduce forest water use, which was beneficial for soil desiccation mitigation and meeting the soil water capacity for vegetation in this region. This study also provided a methodological reference for disentangling the impacts of biophysical factors on forest transpiration.