Abstract: Aims The impoundment of the Three Gorges Reservoir (TGR) has led to the transformation of a large number of natural riparian zones into reservoir drawdown zones, characterized by reverse seasonal water level fluctuations. This transformation has fundamentally altered the local habitat, significantly impacting the differentiation of plant morphological traits. This study aims to reveal the impact of habitat transformation from "riverbank to drawdown zone" as a result of the TGR backwater disturbance on the functional trait differentiation of adapted herbaceous plants as well as their adaptation strategies. Methods This study used a typical small watershed (Baijiaxi) in TGR area as a case. Four herbaceous plant species (Cynodon dactylon, Bidens tripartita, Xanthium strumarium, and Persicaria hydropiper) suitable for the local environment were selected as objects. The spatial differentiation characteristics of 17 plant functional trait indicators were analyzed across the habitat transformation from the riparian zone to the drawdown zone. Pearson correlation analysis was employed to explore the co-evolutionary relationships among functional traits, while redundancy analysis (RDA) was conducted to assess the influence of habitat factors on the differentiation of these traits. Important findings (1) The transition from natural riparian habitats into the drawdown zone can result in significant variations in the morphological traits of the four plant species. These variations are characterized by increased plant height and aboveground growth, along with decreased belowground growth. These changes are primarily attributed to the restricted growth period imposed by imposed by the water-level-fluctuating and the relatively simplified plant communities’ structure with reduced competitive pressure. Concurrently, leaf thickness increases and leaf area expands in most plants, which was mainly driven by the intensified drought stress during summer in the drawdown zone. It highlighted that the backwater effects of the TGR significantly amplifies local-scale shape variations among species. (2) The chlorophyll content and net photosynthetic rate of leaves in four plant species were significantly higher in the drawdown zone compared to those in the natural riparian zone, indicating that plants may have developed an adaptation strategy to enhance photosynthetic efficiency and accelerate growth rates as a response to the habitat limitation in the drawdown zone. (3) Under the habitat screening effect of the backwater in the TGR, four plant species have developed a trade-off strategy between between “growth” and “drought tolerance”, showing convergent adaptation. Cynodon dactylon exhibits relatively stronger phenotypic variability, especially in its photosynthetic traits, indicating stronger adaptability to habitat changes. In contrast, Bidens tripartita, Xanthium strumarium, and Persicaria hydropiper have formed more conservative adaptation strategies. (4) RDA analysis shows that the variations in functional traits of the four plant species within the upstream riparian zone are primarily associated with soil moisture. While, in the midstream and downstream drawdown zone, these variations are predominantly linked to flooding depth, soil pH temperature, suggesting that differences in the flooding environment have driven the differentiation of plant functional traits.

Key words: Three Gorges reservoir area, Fluctuation zone, Dominant species, Functional traits, Adaptive strategy