Environmental Filtering by Hydrological Stress Shapes Stress-Tolerant Plant Ecological Strategies in the Drawdown Zone of the Three Gorges Reservoir, China

The anti-seasonal drawdown zone of the Three Gorges Reservoir (TGR) presents a unique ecological filter characterized by prolonged winter flooding and summer drought. This study investigated plant functional traits (PFTs) and CSR (Competitor–Stress tolerator–Ruderal) ecological strategies across an elevation gradient (145–175 m) in the TGR drawdown zone and the adjacent non-inundated reservoir shore slope (>175 m), to examine plant adaptation to the hydrological gradient. We measured 15 PFTs from 32 dominant species across three sites and calculated CSR strategies using three leaf traits (leaf area, specific leaf area, and leaf dry matter content). The results showed that elevation zone had significant effects on PH, SDMC, LL/LW, SLW, LDMC, SPAD, and SRA. LL/LW, LDMC, RL and FRTD were identified as hub traits in the trait correlation network. Stem traits were significantly related to slope, flooding and TP. Overall mean CSR values for the plant community were 47.90% for C, 37.01% for S, and 15.10% for R. However, the data revealed a clear elevation-dependent shift in the C–S balance: S declined from 45.78% in Zone I to 21.46% in Zone IV, while C increased from 41.86% to 55.61% and R increased to 22.93% in Zone IV. Key stress-tolerant species, such as Cynodon dactylon and Cyperus rotundus exhibited high CS-strategy values across all elevations. CSR strategies were correlated with SWC, flooding, elevation, TC and TN. Correlation heatmaps revealed distinct trait–strategy syndromes: the C strategy was negatively correlated with SDMC and LL/LW but positively correlated with SPAD and LN, whereas the S strategy showed the exact opposite pattern. Water-related factors mainly drove the C and S strategies, with SWC negatively correlated with C and flooding duration negatively correlated with S. These are correlative associations rather than causal pathways. These observed associations are consistent with the hypothesis that intense hydrological stress acts as an environmental filter shaping plant ecological strategies, although the historical filtering process itself is not directly observed in this cross-sectional study. Within this framework, niche differentiation is realized through subtle trait variations and mixed strategies.