Floodplain wetlands in Dongting Lake are characterized by pronounced seasonal water-level fluctuations, which create marked spatial heterogeneity and strongly influence plant community structure and ecosystem functioning. Fine roots play a central role in resource acquisition in wetland plants, yet variation in fine-root functional traits along hydrological and geomorphological gradients and their relationships with belowground productivity remain insufficiently understood. In this study, fine-root samples were collected using the ingrowth core method from five floodplain sites distributed along the flow direction of the Yuan River in West Dongting Lake. Analysis of variance and regression analyses were conducted to evaluate the distribution patterns and relationships of specific root length (SRL), specific root area (SRA), root tissue density (RTD), and fine-root productivity across gradients of site position, distance to water, and soil depth, as well as their associations with plant species diversity. The results showed clear spatial differentiation in both fine-root functional traits and productivity. SRL and SRA were generally higher in surface soils and at positions closer to the water body, whereas community-level fine-root productivity was mainly concentrated in the upper soil layer and declined with increasing soil depth. SRL was highly correlated with SRA, and both traits showed nonlinear relationships with productivity, characterized by an overall decline followed by a gradual leveling off at higher trait values. In contrast, RTD exhibited a unimodal relationship with productivity. No significant relationship was found between plant community diversity and fine-root productivity. Overall, the results indicate that fine-root traits and community belowground productivity do not respond in the same way under different hydrological and geomorphological conditions. Compared with species diversity, variation in fine-root traits provides stronger explanatory power for the spatial variation in community belowground productivity.