Responses of plant sap flow rate to epikarst water availability under different karst water-bearing media

The water supply capacities of epikarst zones play an essential role in the restoration of vegetation and sustainable utilization of groundwater resources in karst areas. The permeability and water storage capacity of karst water-bearing media directly determines prospective water acquisition for plants. However, the degree to which karst water-bearing systems influence plant transpiration remains unclear. Consequently, this study selected three common karst water-bearing structures (pore and fissure (L1), fissure and conduit (L2), dissolution seam and pore (L3), to examine the sap flow rate (SF), infiltration rate, meteorological factors, soil water, and groundwater depth (GD) associated with the growth of Toona sinensis. The results revealed that: (1) Marked differences in the SF based on long term monitoring (2016/1-2022/6) were observed in three sample plots with the order of L2 > L1 > L3. Similar trends in the rock permeability coefficient were also found. (2) Groundwater depth and soil water are the major factors that determine the SF. (3) The threshold of GD (2.54 m for L1, 0.96 m for L2, and 3.56 m for L3) was identified, wherein the sensitivity of SF to GD (lambda(n)) was weakened with greater GD. (4) The fissure and conduit sample plot with a shallower GD and higher rock permeability provided more plant-available water, resulting in a larger SF. In contrast, the dissolution seam and pore sample plot with a more stable GD and media rock permeability hindered the absorption of water by plants, which translated to a lower SF. These results highlighted that the structures of karst water-bearing media determined the overall SF. This study provides useful information toward guiding the restoration of vegetation in subtropical karst areas with abundant rainfall and shallow groundwater reservoirs.