Estimation of sources of water used by plant established in rocky karst habitats, subtropical China

Plant communities in continental ecosystems usually access to at least two pools of water that can be differentiated based on their turnover characteristics: a plant preferred pool with rapid recharge and depletion (dynamic pool), and a more consistent pool with less frequent recharge and slower loss rates (reserve pool). Identifying the use of different pools by community members is the key to estimating their ecological and hydrological functions. In regions with rocky, thin soils in which plant roots also take up water from rock fissures and crevices, it is usually very difficult to locate plant available water pools and quantify their water status or use by plants. Fortunately, we expect dynamic water pools to frequently change isotopic ratios due to rapid recharge and depletion, while reserve pools of water is expected to have distinct isotope ratios and maintain much less variability. Thus, we can use this fact to derive limited quantitative conclusions about the species differences in water use. In order to reveal sources of water used by one karst endemic tree species (Platycarya longipes) established in two typical karst habitats (cliff face and nearby loose rocky soils), stem samples for the tree and one coexistence shallow rooted shrub species (Tirpitzia ovoidea, which was proved to relied on shallow water sources) were collected for 9 times throughout a growing season. Linear relationships (regression slopes were closed to 1) were found between stem water isotope ratios of the two species in each habitat, indicating that the target tree species also relied on water in the dynamic pool. We further discussed the probable water movement mechanism based on the responses of stem water isotope ratios to rainfall.