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Riparian vegetation: degradation, alien plant invasions, and restoration prospects

Authors

  • David M. Richardson,

    Corresponding author
    1. Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa,
      *Correspondence: David M. Richardson, Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa. Tel.: +27 21 8083711; Fax: +27 21 8082995; E-mail: rich@sun.ac.za
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  • Patricia M. Holmes,

    1. Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa,
    2. Cape Ecological Services, 23 Dreyersdal Road, Bergvliet 7945, South Africa,
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  • Karen J. Esler,

    1. Centre for Invasion Biology and Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa,
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  • Susan M. Galatowitsch,

    1. Department of Horticultural Science, University of Minnesota, 305 Alderman Hall, St. Paul, MN 55108, USA,
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  • Juliet C. Stromberg,

    1. School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ 85287, USA,
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  • Steven P. Kirkman,

    1. Avian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch 7701, South Africa,
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  • Petr Pyšek,

    1. Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 PrUhonice, Czech Republic, and Department of Ecology, Charles University Prague, CZ-128 01 Praha 2, Czech Republic and
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  • Richard J. Hobbs

    1. School of Environmental Science, Murdoch University, Murdoch, WA 6150, Australia
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*Correspondence: David M. Richardson, Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa. Tel.: +27 21 8083711; Fax: +27 21 8082995; E-mail: rich@sun.ac.za

ABSTRACT

Rivers are conduits for materials and energy; this, the frequent and intense disturbances that these systems experience, and their narrow, linear nature, create problems for conservation of biodiversity and ecosystem functioning in the face of increasing human influence. In most parts of the world, riparian zones are highly modified. Changes caused by alien plants — or environmental changes that facilitate shifts in dominance creating novel ecosystems — are often important agents of perturbation in these systems. Many restoration projects are underway. Objective frameworks based on an understanding of biogeographical processes at different spatial scales (reach, segment, catchment), the specific relationships between invasive plants and resilience and ecosystem functioning, and realistic endpoints are needed to guide sustainable restoration initiatives. This paper examines the biogeography and the determinants of composition and structure of riparian vegetation in temperate and subtropical regions and conceptualizes the components of resilience in these systems. We consider changes to structure and functioning caused by, or associated with, alien plant invasions, in particular those that lead to breached abiotic- or biotic thresholds. These pose challenges when formulating restoration programmes. Pervasive and escalating human-mediated changes to multiple factors and at a range of scales in riparian environments demand innovative and pragmatic approaches to restoration. The application of a new framework accommodating such complexity is demonstrated with reference to a hypothetical riparian ecosystem under three scenarios: (1) system unaffected by invasive plants; (2) system initially uninvaded, but with flood-generated incursion of alien plants and escalating invasion-driven alteration; and (3) system affected by both invasions and engineering interventions. The scheme has been used to derive a decision-making framework for restoring riparian zones in South Africa and could guide similar initiatives in other parts of the world.

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