Functional responses of aquatic and riparian vegetation to hydrogeomorphic restoration of channelized lowland streams and their valleys

1. Streams and riparian zones are highly heterogeneous ecosystems. Their high biodiversity is promoted by variable flow velocities and water depths, strong hydrological gradients and disturbance regimes. However, human interventions like damming and channelization have degraded these ecosystems worldwide. And, although restoration efforts have increased in the past decades, ecological improvement is lagging. 2. We assessed vegetation development in channelized lowland stream valleys in the Netherlands, combining innovative restoration measures to the stream and stream valleys. This ‘stream valley restoration’ entailed construction of narrower and shallower channels to increase flow velocities during base discharges, meandering of the watercourse to increase flow and depth heterogeneity and excavation of banks to create wide v-shaped stream valleys. We evaluated the effects on functional aspects of the developing in-stream and riparian vegetation by comparing restored stream reaches to nearby unrestored reaches. 3. The reduced channel dimensions led to higher flow velocities, which, through interaction with meandering, triggered a higher variability in flow and depth. Combined with enlargement of the floodplain, this promoted flooding in stream valleys and created wider environmental gradients. Plant diversity strongly increased in the floodplain area, the land-water interface and the shallow water habitat at the channel margins, but decreased in the central parts of stream channels. There, higher flow velocities led to more typically lotic (running water) in-stream plant communities, indicated by a sharp decrease in floating-leaved species and an increase in trailing species. Riparian vegetation showed a higher beta-diversity across the wider valley slopes of restored reaches, with more wetland species in areas with water tables between 0.0 and -0.6 m, and more upland species as well. 4. Synthesis and applications. This study demonstrates that the combination of strongly reduced channel dimensions, remeandering and widening of riparian zones, is effective in restoring in-stream and riparian habitat heterogeneity. The restoration efforts lead to distinct immediate increases in total and beta-diversity of many typical stream and riparian plant species. Overall, this stresses the importance of applying restoration measures to both streams and stream valleys simultaneously, considering them as a single landscape unit. 27-Nov-2018

1. 溪流与河岸带（riparian zones）属于高度异质的生态系统。多变的流速、水深、显著的水文梯度与干扰制度（disturbance regimes）共同维持了其较高的生物多样性。然而，筑坝、渠化（channelization）等人类活动已在全球范围内对这类生态系统造成退化。尽管近几十年来生态修复工作不断增加，但生态改善的进度仍相对滞后。
2. 本研究针对荷兰渠化平原河谷展开植被发育状况评估，同时将针对溪流与河谷的创新性修复措施纳入研究范畴。本次“河谷修复”措施包括：修建更窄更浅的河道以提升枯水期的基流（base discharges）流速；对河道进行蜿蜒化改造以增加水流与水深的异质性；开挖河岸以形成宽V型河谷。研究通过将修复河段与附近未修复河段进行对比，评估了修复措施对正在发育的溪流内植被与河岸带植被功能特征的影响。
3. 河道尺寸的缩减提升了流速，结合蜿蜒化改造的作用，进一步加剧了水流与水深的变异性。加之漫滩（floodplain）的拓宽，该措施促进了河谷内的洪水漫溢，并形成了更宽泛的环境梯度。漫滩区域、水陆交错带（land-water interface）以及河道边缘的浅水生境中，植物多样性显著提升，但河道中部的植物多样性却有所下降。在河道中部，更高的流速促使群落向更典型的流水生境植物群落（lotic plant communities）转变，具体表现为浮叶植物物种数量骤减，而匍匐植物物种数量增加。河岸带植被在修复河段更宽阔的河谷坡面上呈现出更高的β多样性（beta-diversity）：在地下水位介于0.0至-0.6米的区域，湿地物种占比更高；同时旱地物种的占比也有所提升。
4. 总结与应用启示。本研究表明，同时大幅缩减河道尺寸、对河道进行蜿蜒化改造以及拓宽河岸带的综合修复措施，能够有效恢复溪流内与河岸带生境的异质性。该修复措施可快速提升多数典型溪流与河岸带植物物种的总多样性与β多样性。总体而言，本研究强调了将溪流与河谷视为统一的景观单元，同时对二者开展修复措施的重要性。2018年11月27日