Regulation of transpiration water age by plant root-rock fissure interactions in epikarst

Abstract. Under climate change, rock water becomes an important source of transpiration water for plants. However, in karst regions, rock fissure water and rooting depth are often not adequately considered in existing determination systems. This study conducted monthly field sampling over a year in a rock-dominated subtropical karst region, focusing on deep-rooted trees (Ailanthus altissima and Juglans regia) and shallow-rooted trees (Zanthoxylum bungeanum and Eriobotrya japonica). It integrated stable isotope tracing, the piecewise isotope balance method (quantifying the replenishment ratio of root-zone water), the piecewise linear mixing water age model (estimating the mean residence time of water utilized by plants), and geological drilling techniques. The results showed that rock fissure and root depth regulated the root zone water recharge rate of plants: deep-rooted trees (32 %) were lower than shallow-rooted trees (44.3 %) in the rainy season, while deep-rooted trees (10.4 %) were higher than shallow-rooted trees (3.8 %) in the dry season. Differences in water recharge to the root zone affected the age of plant transpiration: deep-rooted trees (46.4 d) were higher than shallow-rooted trees (35.1 d) during the rainy season, while the opposite was true during the dry season (deep-rooted trees: 139.6 d; shallow-rooted trees: 128.5 d). In addition, geological boreholes revealed that a large number of roots were distributed in rock fissures at a depth of 1.8–3.2 m below ground. The study showed that rock fissures are not only important channels for the formation of preferential flow in the rainy season, but also interact with the root system to regulate the water recharge in the root zone of plants and thus influence the change of transpiration water age, which provides a new perspective to understand the complex hydrological processes in karst areas.