Water Age Dynamics in Plant Transpiration: The Effects of Climate Patterns and Rooting Depth

Water ages in plant transpiration reveal important processes of water dynamics in soil-plant-atmosphere continuum, which is regulated by water variability and availability in both precipitation inputs and root zone. However, the impacts of climate patterns and rooting depth on water age dynamics in transpiration are inadequately investigated. In this study, variations in root-zone water replenishment are estimated based on stable isotope compositions of precipitation and plant xylem water, and a piecewise linear mixing water age model is employed to quantify water ages in plant transpiration under five different climates. The results show that water ages in transpiration are very dynamic in all climates with a range between 1 and 229 days. Water replenishments in the root-zone reveal whether and how much precipitation enters the root zone. Climate patterns control median (mean) water age in plant transpiration but modified by rooting depth. In wet climates, plants mainly rely on water source that originates from precipitation in the current month (accounting for about 60% on average of their water sources), while plants in dry climates largely rely on water source that originates from precipitation fallen on previous months/seasons. These indicate that climate has profound impacts on plant water source by regulating precipitation inputs, root zone water storage and residence time, and evapotranspiration. Despite the limitations of this study, our study provides an alternative way to reveal the precipitation partitioning in root-zone and seasonal changes in plant water use of precipitation, which highlights different vegetation responses to water variability between climate types.