Temporal patterns in root water uptake and intrinsic water-use efficiency of overstory and understory tree species in a subtropical humid pine forest

Abstract

Root water uptake and leaf-level intrinsic water-use efficiency (WUEi) and their temporal variations are important determinates of plant water balance and carbon fixation, yet these processes in humid forest trees are much less explored. We investigated the monthly variations in xylem and soil water isotopes (δ¹⁸O, δ²H), leaf carbon isotope (δ¹³C), soil, xylem and leaf water contents as well as the seasonal variations in leaf oxygen isotope (δ¹⁸O) and N contents for the dominant overstory species (Pinus massoniana) and a neighboring common understory species (Camellia japonica) in a subtropical humid pine forest within April 2021-June 2022. Water stable isotopes revealed that the two species exhibited similar water uptake patterns over time and both shifted water uptake toward deeper and shallow soil layers during the wet and relatively dry seasons, respectively. Evident soil water partitioning only occurred during a spring drought when only C. japonica shifted water uptake toward deeper soil layers, indicating high interspecific competition for shallow water. For P. massoniana, the reliance on shallow water positively correlated with WUEi (leaf δ¹³C) through negatively affecting leaf relative (to xylem) water content and stomatal conductance (proxied by leaf ¹⁸O enrichment above source water). In contrast, C. japonica exhibited non-sensitive WUEi response to temporal changes in water uptake depth, and its low leaf N contents indicate severe N limitation on photosynthesis and WUEi. Our results highlight the tight coupling between water uptake depth and WUEi for the overstory species, but not for the understory species likely associated with the stronger water and particularly N limitation it was experiencing. Our analysis of temporal changes in water uptake depth and WUEi (leaf δ¹³C) provides a useful framework to evaluate belowground constraints on resource acquisition and leaf-level water use strategies for plants in humid forests.