三峡水库坝下江心洲植物群落及其性状对不同水淹强度的响应

三峡水库的运行，改变了其坝下游的水沙情势，使坝下游沙洲生境呈现出明显的水淹强度梯度变化。阐明这一梯度变化下沙洲植被组成、分布和性状结构特征，是理解植被与沙洲稳定关系的基础，更是阐明三峡工程对长江中下游地区生态环境影响的核心内容之一。选取上荆江河段第一个江心洲-太平口心滩作为研究样地，通过植被组成和分布特征的调查，对不同水淹强度下群落物种组成、多样性和功能特征进行了深入分析。结果表明：太平口心滩植被组成以草本植物为主，稀布小型灌木川三蕊柳。调查共记录物种21科33属39种，以禾本科和菊科植物为主要优势种。轻微（20-40 d）和极强水淹强度（100+d）条件下的生境物种组成同其他水淹强度生境具有显著性差异，轻微水淹强度下牛鞭草和节节草为主要优势种，极强水淹强度下虉草为主要优势种。不同水淹强度下物种多样性指数差异显著，功能多样性指数和生物多样性指数趋势基本一致。随着水淹时间的延长，植被更倾向于表现出花果期位于出露期、植株高度更加低矮、须根系、进行营养繁殖的功能性状。江心洲植被的群落结构和功能性状特征都在水淹梯度下呈现出明显的梯度变化特征。这些研究结果表明水淹强度的梯度变化是沙洲植物群落变化的重要驱动因子，为进一步研究沙洲植物群落动态变化以及沙洲植被协同演替机制，明晰大坝影响下的生态环境变化提供重要依据。

Responses of plant communities and traits to flooding durations in a middle-channel bar downstream from the Three Gorges Dam

The composition and distribution characteristics of vegetation play a vital role in the development and stability of the sandbars. Operation of the Three Gorges Reservoir has changed the hydrological and sediment transport regimes in the downstream of the dam, causing a significant gradient on the sandbar habitats along the flooding durations. To clarify the composition, distribution, and structural characteristics of vegetation in the sandbar is fundamental for understanding the roles of vegetation in stabilizing the sandbar. In addition, they provide key information to clarify the relationship between the Three Gorges Project and the ecological and environmental problems in the middle and lower reaches. In this study, we selected the first sandbar, the Taipingkou Middle bar in the upper Jingjiang river section, and used the equidistant systematic sampling plots to investigate the vegetation composition and distribution characteristics. Based on the elevation information of plots and the water level data over the years, we set the plots at 5 flooding gradients levels by the flooding days in a flooding cycle:20-40 d, 40-60 d, 60-80 d, 80-100 d, and over 100 d. Statistical methods, such as importance value, species diversity index, functional diversity index and functional traits analysis, were used to analyze the species composition, diversity and functional traits of communities under different flooding duration days. The results showed that the vegetation composition was dominated by herbaceous plants, and the small shrubs Salix triandroides were sparsely distributed. A total of 39 species, belonging to 21 families and 39 genera, were recorded and the main dominant families were Poaceae and Compositae. The species composition of habitats under intermediate level (20-40 d) and maximum flooding intensity (100+ d) condition is significantly different with that found in other flooded gradients. Under the intermediate flooding intensity, Hemarthria sibirica and Equisetum ramosissimum are the main dominant species, and Phalaris arundinacea is the main dominant species under the maximum flooding intensity. Along the flooding gradient, species diversity is significantly varied. Species richness is low in both intermediate and maximum flooding intensity habitats, but the highest species diversity in the intermediate flooding condition habitats and the lowest in the maximum strong flooding intensity habitats. Basically, the trend of functional diversity index keeps same with biodiversity index, while the functional diversity index under maximum flooding conditions is not representative. As increasing of intensity of flooding stress, the vegetation tends to show the functional traits with flowering and fruit stage under the outcrop stage, while plant height reduction, fibrous root system and vegetative reproduction also show difference. The community structure and the functional traits of the sandbar plants showed obvious gradient change with the flooding gradient. The results show that the gradient change of flooding duration is an important driver shaping sandbar plant community, and provides an important basis for further research on the dynamic succession of sandbar plant community and vegetation succession mechanism of sandbar, and clarifying the ecological environment changes under the influence of dams.