Environmental Flows Can Reduce the Encroachment of

Authors: Kimberly A Miller J Angus Webb Siobhan C de Little Michael J Stewardson

Publish Date: 2013/08/17

Volume: 52, Issue: 5, Pages: 1202-1212

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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 oftcited objective of environmental flow recommendations but there has been no systematic assessment of the evidence for and against the widelyaccepted causeandeffect 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 causeeffect 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 hydroecological understanding by using the published literature to test for general causeeffect relationships between flow regime and terrestrial vegetation encroachment Reviews of this nature provide robust support for flow management and are more defensible than expert judgementbased 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 encroachmentRegulation of rivers and the resulting alteration of flow threatens ecosystem functions and biodiversity globally Nilsson and others 2005 Dudgeon and others 2006 Among many other effects river regulation can result in the encroachment of terrestrial vegetation into channels Erskine and others 1999 Bejarano and others 2011 Bejarano and SordoWard 2011 Moreover the extent of encroachment can increase with greater reductions in flow Poff and Zimmerman 2010Vegetation encroachment results from a predictable set of conditions and has welldefined consequences for riverine environments Frequent high flow events result in regular deposition and removal of sediments from channels Deposited sediment provides suitable substrate for the germination of terrestrial vegetation which in turn stabilizes the sediment Benn and Erskine 1994 When flow is reduced over many years the vegetation may establish in the base of the channel The development of largestatured terrestrial vegetation in channels results in a reciprocal relationship with hydrogeomorphic processes Corenblit and others 2007 with encroachment exerting control over fluvial processes and in turn channel morphology and aquatic ecology Hickin 1984 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 ecosystemsRiparian and aquatic species are welladapted to survive and exploit the natural flow regime Lytle and Poff 2004 Therefore river restoration often relies on environmental flows designed to reinstate a more natural flow regime eg Rood and others 2005 Konrad and others 2012 Environmental flows are deliberate releases of water to benefit the environment Poff and others 1997 Environmental flow recommendations often include flows expected to remove and/or prevent the encroachment of terrestrial vegetation in channels eg VEWH and others 2011 Konrad and others 2012 While the ecological relationships that underpin recommendations for terrestrial vegetation encroachment are accepted as fact by many in the research and management community the evidence for and against them often has not been rigorously tested Sutherland and others 2004In this study we aimed to rigorously test the assumptions underlying environmental flow recommendations by systematically reviewing the effects of streamflow on riparian vegetation Our results demonstrate that increased base flows and flooding events of longer duration can prevent the encroachment of terrestrial vegetation into regulated river channels However we also show that infrequent inundation may actually increase the germination of terrestrial species potentially exacerbating encroachmentExperiencebased models of environmental management such as those described above have sometimes been proven false after systematic reviews For example there is little evidence that instream structures improve the production of salmonid fishes Stewart and others 2006 despite the millions of dollars spent annually for just this purposeEffective management and restoration relies on understanding the causeandeffect relationships that determine how environmental stressors influence ecological responses However demonstrating causality in ecology is difficult because of natural variability lack of replication the presence of confounding influences and limits to experimental manipulation When faced with similar issues in studying the causes of disease epidemiologists developed “causal criteria” in 1960s Causal criteria analysis is a method for assessing causeeffect hypotheses in the face of weak experimental evidence and is widely used in medical research Weed 1997 Tugwell and Haynes 2006 The approach commonly uses a systematic review to test causeeffect hypotheses This contrasts to most reviews in ecology which use a “narrative” approach to survey the current state of knowledge Systematic reviews of the literature can play a key role in the move toward evidencebased environmental policy and management Pullin and others 2009 Conceptual models underpin the recommendations to use environmental flows to reduce terrestrial vegetation encroachment Testing such models against the available scientific evidence strengthens their credibility may provide new recommendations and informs the development of statistical models to test the effects of environmental flows

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