Abstract
Encroachment of riparian vegetation into regulated river channels exerts control over fluvial processes, channel morphology, and aquatic ecology. Reducing encroachment of terrestrial vegetation is an oft-cited objective of environmental flow recommendations, but there has been no systematic assessment of the evidence for and against the widely-accepted cause-and-effect mechanisms involved. We systematically reviewed the literature to test whether environmental flows can reduce the encroachment of terrestrial vegetation into river channels. We quantified the level of support for five explicit cause-effect hypotheses drawn from a conceptual model of the effects of flow on vegetation. We found that greater inundation, variously expressed as changes in the area, depth, duration, frequency, seasonality, and volume of surface water, generally reduces riparian vegetation abundance in channels, but most studies did not investigate the specific mechanisms causing these changes. Those that did show that increased inundation results in increased mortality, but also increased germination. The evidence was insufficient to determine whether increased inundation decreases reproduction. Our results contribute to hydro-ecological understanding by using the published literature to test for general cause-effect relationships between flow regime and terrestrial vegetation encroachment. Reviews of this nature provide robust support for flow management, and are more defensible than expert judgement-based approaches. Overall, we predict that restoration of more natural flow regimes will reduce encroachment of terrestrial vegetation into regulated river channels, partly through increased mortality. Conversely, infrequent deliveries of environmental flows may actually increase germination and subsequent encroachment. Electronic supplementary materialThe online version of this article (doi:10.1007/s00267-013-0147-0) contains supplementary material, which is available to authorized users.
Highlights
IntroductionEncroachment of Terrestrial Vegetation into Regulated River Channels
Encroachment of Terrestrial Vegetation into Regulated River ChannelsRegulation of rivers, and the resulting alteration of flow, threatens ecosystem functions and biodiversity globally (Nilsson and others 2005; Dudgeon and others 2006)
The evidence used in this study is available for re-use from the Eco Evidence database, and can be located by searching the ‘‘Question’’ field for ‘‘#Encroachment.’’ None of the standard terms that describe water regime in the current list of standard terms in the Eco Evidence database sufficiently captured the variation in inundation inherent in our hypotheses
Summary
Encroachment of Terrestrial Vegetation into Regulated River Channels. Regulation of rivers, and the resulting alteration of flow, threatens ecosystem functions and biodiversity globally (Nilsson and others 2005; Dudgeon and others 2006). River regulation can result in the encroachment of terrestrial vegetation into channels (Erskine and others 1999; Bejarano and others 2011; Bejarano and Sordo-Ward 2011). Deposited sediment provides suitable substrate for the Environmental Management (2013) 52:1202–1212 germination of terrestrial vegetation, which in turn stabilizes the sediment (Benn and Erskine 1994). Riparian terrestrial vegetation can be responsible for the largest amount of energy loss in fluvial corridors (Nepf and Vivoni 2000). The ‘‘clogging’’ of channels with terrestrial vegetation and subsequent energy losses change aquatic habitat availability and alter aquatic ecosystems
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