Replenishment and mean residence time of root-zone water for woody plants growing on rocky outcrops in a subtropical karst critical zone

Abstract

Quantifying root-zone water replenishment (β) is essential to understanding how root-zone water is partitioned and released, and the associated residence time of water in Earth’s critical zone. However, dynamics of such water replenishments and mean residence time in karst environments remain poorly understood. In this study, stable isotope compositions of precipitation and plant xylem water were investigated for eight woody plant species growing on two rocky outcrops in a subtropical karst critical zone. A new method was proposed for estimating mean residence time of root-zone water with a consideration of precipitation replenishment. The results show that all these species had seasonal water sources during the growing seasons, reflecting by seasonal variations in isotopic compositions of stem xylem water and β. Rapid increases in β were generally found for most species during the early wet season. The high values of β (>80%) appeared frequently during the main growing seasons accompanied with frequent precipitation events, indicating that precipitation frequency is particularly important for these plants growing on the outcrops. Meanwhile, precipitation was likely to bypass the root zone (β = 0) after several high replenishment events, which suggests that not all precipitation events evenly contribute to root-zone water. In addition, mean residence time of root-zone water varied seasonally and differed among species, with a range from 2 to 80 days for most species. The differences of mean residence time of root-zone water among these species during the main growing seasons may suggest that less competition exists among these species in such rocky outcrops. This study provides insights into understanding vegetation response to future changes in precipitation patterns, and ecohydrological dynamics of karst ecosystems.