Annual variations of T/ET in a semi-arid region: Implications of plant water use strategies

Abstract Understanding the annual variation in the transpiration to evapotranspiration ratio (T/ET) remains a challenge and is essential for a thorough understanding of plant responses to the changing environment. We obtained the annual dynamics of T/ET in a semi-arid area of the southwestern United States based on the medians of monthly T/ET derived from two ET partitioning methods. The variation in monthly T/ET was analysed, and plant water use strategies were discussed based on the water use efficiency evaluated by the transpiration (WUE_T). The results show that physiological changes in plants are vital in the annual dynamics of T/ET. Switches in plant physiological status (growth and dormancy) at the start and end of growing seasons induce two dramatic changes in T/ET. Consequently, there is an annual bimodal dynamic of monthly T/ET, with a maximum of 0.84 in October and a minimum of 0.14 in December. Physiological/biochemical variations of plants indicated by solar-induced chlorophyll fluorescence (SIF) are linearly related to T/ET in growing seasons at a monthly scale (T/ET = 3.40 × SIF + 0.36, R2 = 0.987). Generally, a stable high monthly T/ET occurs under sufficient energy and water conditions and a highly variable monthly T/ET occurs under energy and water deficient conditions. In semi-arid regions, plants can flexibly adjust WUE_T following different water use strategies to survive or gain as much gross primary productivity (GPP) to compete. Saving water by greatly elevating WUE_T is the main strategy by which plants survive the non-growing season when WUE_T is linearly related to SIF (WUE_T = -114.93 × SIF + 3.25, R2 = 0.970). However, GPP and not WUE_T, becomes the goal of plants in growing seasons when they employ a stable and moderate WUE_T (around 2.1 gC kg−1 H2O) despite the abundant energy and precipitation. There are obvious reductions in WUE_T during the transition periods of the plants’ ‘growth-dormancy’ cycle. Our study highlights the importance of studying annual T/ET variations and water-use efficiency dynamics to better understand water use strategies in plants.