Abstract: Stable isotope analysis can be used to investigate water sources of plants because of no isotopic fractionation during water uptake by terrestrial plants. Ecosystems in the karst region of southwest China are very fragile due to the thin soil layer and intensive infiltration capacity of rock fracture, which result in a very limited amount of water storage for plant uptake. Water retention in the soil zone and shallow fractured rock zone (subcutaneous) is a key factor for plant growth. However, the source of water for plant growth in karst region remains unclear. Distinction of water sources taken by karst plants is a challenging task for botanists and hydrologists, and has important implications for ecosystem management. In this study, we selected 5 dominant plant species grown in various micro-habitats in the Wangjiazhai catchment, a typical karst desertification area in Guizhou Province. The spatial heterogeneity of plant water sources at niche scale and the response of the heterogeneity to different karst rocky desertification degree were studied. Proportions of water sources for plant uptake were determined by the δD and δ¹⁸O values of plant stem water, and the water taken from different soil layers and the subcutaneous zone. The following conclusions were drawn from the analysis: (1) The δD and δ¹⁸O values of soil water were significantly different among various micro-habitats, stony surface had the most positive δD and δ¹⁸O values, stony crevice had the most negative δD and δ¹⁸O values, the others had the medium δD and δ¹⁸O values. The δD and δ¹⁸O values of soil water were significantly different in soil profile and the δD and δ¹⁸O values of soil water in the 0-10 cm layer were more positive than those in the 10-30 cm layer. (2) The plant species absorbed water from both soil layers and subcutaneous zone and these plant species extracted more water from soil layers than subcutaneous zone in rainy seasons in general. The proportion of plants water use from the subcutaneous zone decreased with increasing degree of karst rocky desertification. Pyracantha fortuneana, Rhamnus davurica and Viburnum utile in slight and non karst desertification grade extracted water from both soil layers and subcutaneous zone, while these plants only used water form soil layers in medium and severe grade, and Platycarya longipes and Carpinus pubescens in non karst desertification grade extracted water from both soil layers and subcutaneous zone, while these plants only used water form soil layers in slight grade, which is mainly due to various vegetation types, disturbance regimes, soil conditions and crack growth degree of rocks in different grades plots.

Key words: karst rocky desertification, micro-habitats, water sources, stable hydrogen and oxygen isotopes