Abstract: To clarify the variations of water isotopic composition and mechanism underlying their responses to environmental factors during water transport and conversion in subtropical forest ecosystems in heavy rain periods, we measured the hydrogen and oxygen stable isotopes of typical species in the subtropical evergreen coniferous forest across the source area of Xin’an River during the East Asian rainy season in 2020. Combined with the environmental factors monitored by the eddy covariance measurements of Huangshan Hydrological Station, we analyzed the diurnal variation of water isotopic compositions (δ¹⁸O and δ²H) in different parts (roots, bark, xylem and leaves) of Cunninghamia lanceolata and the correlation between δ¹⁸O and δ²H in leaf water of other dominant species in 2nd-4th July. The main environmental controlling factors of leaf water δ¹⁸O and δ²H in different plants were investigated. The results showed that on the diurnal scale, water isotopic compositions of roots, bark, and xylem were similar and changed gently, while the isotopes in leaf water were most enriched and changed dramatically. Influenced by the random heavy rainfall, there was no significant consistency in the diurnal variations in δ¹⁸O and δ²H from different sources, showing single peak, single valley and fluctuation in each day of 2nd-4th July respectively. By linear regression of leaf water δ¹⁸O-δ²H, the slopes of the transpiration line of five dominant species from high to low were as follows: Woodwardia japonica, Camellia sinensis, Glycine max, Cunninghamia, lanceolata, Phyllostachys heterocycla, indicating that water isotope fractionation effect was the strongest in Phyllostachys heterocycla and the weakest in Woodwardia japonica. The environmental controlling factors of δ¹⁸O and δ²H were different among the five species. Soil moisture and soil temperature were the dominant factors affecting δ¹⁸O and δ²H in leaf water of Cunninghamia lanceolata and Woodwardia japonica, while air temperature and net radiation were the main factors affecting δ¹⁸O and δ²H in leaf water of Camellia sinensis and Glycine max. The δ¹⁸O and δ²H in leaf water of Phyllostachys heterocycla were strongly correlated with air temperature, relative humidity, soil temperature, soil moisture, and wind speed. The results are helpful to clarify the eco-hydrological process in humid region and provide data support for further establishing isotope hydrological model.

Key words: forest ecosystem, stable isotope, leaf water, environmental factor.