Flooding stress drives plant community assembly and seed rain dynamics in reservoir water-level fluctuation zone

The construction of large reservoirs worldwide has led to the degradation of riparian vegetation. Since the completion of the Three Gorges project, the water-level fluctuation zone (WLFZ) formed by the high-water-level operation has experienced vegetation loss and ecological degradation. Despite extensive research on vegetation degradation in the WLFZ, the role of seed rain in revegetation and its response to flooding stress gradients remains unresolved. This study selected the Pengxi River of the Three Gorges Reservoir as the study area to (1) investigate the main factor influencing aboveground vegetation and seed rain formation, and (2) reveal the contribution of seed rain to revegetation in the WLFZ. The emergence period in May marks a critical stage for vegetation growth in the WLFZ. During this time, vegetation allocates more resources to vegetation growth, whereas in June and July, it shifts toward reproductive growth, with fruiting rates increasing over time—a key phase for seed rain formation. Annual plants under low flooding stress invest more resources in reproduction to sustain their populations, while those under high or moderate flooding stress, with shorter emergence periods, prioritize vegetative growth. Flooding stress was the dominant factor, explaining up to 51.3 % of the variation in plant community diversity and seed rain formation. Annual herbs under low stress prioritized reproduction, while high-stress conditions favored accelerated life cycles. These findings demonstrate how phenological adaptations (e.g., accelerated reproduction under high stress) enhance ecosystem resilience and highlight the need for flood-adaptive species in WLFZ restoration. Thus, long-term flooding stress at varying intensities is the primary driver shaping aboveground plant community and seed rain dynamics. Our findings offer a novel, seed-based restoration strategy tailored to flood gradients, advancing both theoretical and practical approaches for WLFZ sustainability.