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Riparian vegetation is an important inhabitant of numerous forest ecosystems, although it is vulnerable and affected by the changes in climate or microclimate. Due to the fluctuation in rainfall and surface water availability, riparian plants usually exposed to two different seasons throughout the year: a dry season and a rainy season. However, water use patterns among riparian plants are still poorly understood in the tropical monsoon region. In this study, we used stable isotope techniques (delta D and delta O-18) coupled with MixSIAR model to identify the water sources for riparian plants in a tropical forest of SW China. The results showed an apparent seasonal variation in water use patterns for most trees, and the spatial variations were less pronounced between the terrace and floodplain sites. Among most riparian trees, deep soil (below 50cm) was the main water source in the dry season, and shallow soil (above 15cm) was the primary water source during the rainy season. However, there was no change in water source for herbs and other shallow-rooted species with the fluctuation in water availability. This result indicates that shallow-rooted plants (i.e., herbs and seedlings) struggle for their survivability in a riparian ecosystem because of the water scarcity in shallow soil during the dry season. Thus, lower rainfall and/or extended dry period may affect the productivity and sustainability of riparian vegetation under a future climate change scenario.