Review
Restoration of riparian vegetation: A global review of implementation and evaluation approaches in the international, peer-reviewed literature

https://doi.org/10.1016/j.jenvman.2015.04.033Get rights and content

Highlights

  • Hydrogeomorphic is the restoration approach most often assessed by academia (50%).

  • Planting and seeding is also popular (39%) but in combination with other techniques.

  • Assessing trajectories of change is a legitimate alternative to using reference sites.

  • Longer-term evaluations (>6 yr) at large spatial scales (>meander) are needed.

  • Reporting failure and assessing the multidimensional nature of rivers is necessary.

Abstract

We examined how restoration of riparian vegetation has been implemented and evaluated in the scientific literature during the past 25 years. A total of 169 papers were read systematically to extract information about the following: 1) restoration strategies applied, 2) scale of monitoring and use of reference sites, 3) metrics used for evaluation, and 4) drivers of success. Hydro-geomorphic approaches (e.g., dam operations, controlled floods, landform reconfiguration) were the most frequent, followed by active plant introduction, exotic species control, natural floodplain conversion and grazing and herbivory control. Our review revealed noteworthy limitations in the spatio-temporal approaches chosen for evaluation. Evaluations were mostly from one single project and frequently ignored the multi-dimensional nature of rivers: landscape spatial patterns were rarely assessed, and most projects were assessed locally (i.e., ≤meander scale). Monitoring rarely lasted for more than six years and the projects evaluated were usually not more than six years old. The impact of the restoration was most often (43%) assessed by tracking change over time rather than by comparing restored sites to unrestored and reference sites (12%), and few projects (30%) did both. Among the ways which restoration success was evaluated, vegetation structure (e.g., abundance, density, etc.) was assessed more often (152 papers) than vegetation processes (e.g., biomass accumulation, survival, etc.) (112 papers) and vegetation diversity (78 papers). Success was attributed to hydro-geomorphic factors in 63% of the projects. Future evaluations would benefit from incorporating emerging concepts in ecology such as functional traits to assess recovery of functionality, more rigorous experimental designs, enhanced comparisons among projects, longer term monitoring and reporting failure.

Introduction

Evaluation of success has been the main weakness of restoration ecology since this discipline emerged in the early 1980s (Walker et al., 2007). The assessment of restoration outcomes has been traditionally jeopardized by underfunding and the lack of systematic and objective evaluation criteria (Bernhardt et al., 2007, Kondolf et al., 2007), even though these criteria are necessary for adaptive management as well as the advancement of the field of restoration ecology as a whole (Kondolf et al., 2007, Walker et al., 2007, Shafroth et al., 2008, Suding, 2011). Because of this, numerous efforts have been made to establish standards and common criteria for evaluating restoration (Hobbs and Norton, 1996, Palmer et al., 2005, Dufour and Piégay, 2009, Suding, 2011). In this review we examine the extent to which recommendations for project monitoring of vegetation restoration are being followed across the globe for one ecosystem type, riparian vegetation, and where we might improve as a discipline.

The most widely cited metrics of restoration success are by the Society for Ecological Restoration International (SERI), which proposed a list of nine key attributes that a successfully restored ecosystem should have, such as being resilient to natural disturbances and self-sustainable (SERI, 2004). The SERI Primer also suggested that it is necessary to compare metrics from restoration sites with values from natural reference sites or sites that have not sustained the degradation that restoration is intended to redress (SERI, 2004). Others have recommended that project sites should be monitored before and after restoration implementation in combination with comparisons to control sites (e.g., Before-After Control-Impact design, Palmer et al., 2005, Bernhardt et al., 2007). It is still debatable whether restoration should target reproducing the species composition and physical structure of desirable final states represented by reference sites vs. focusing on changes over time of certain ecosystem components and recovery of functionality (Dufour and Piégay, 2009). Finally, one must consider spatial and temporal scales of monitoring, given that recovery of restored ecosystems usually takes years or even centuries, and responses of ecosystems to restoration activities may be often non-linear or stochastic and vary in space (Trowbridge, 2007). Because of this, some authors have proposed that integrating multi-scale information should be an essential part of restoration assessments, including considerations of impact beyond project boundaries (González del Tánago and García de Jalón, 2006, Aguiar et al., 2011).

It has been argued that few studies have the resources to follow these recommendations (Ruiz-Jaen and Aide, 2005); however, a comprehensive review of the international literature is necessary to investigate the actual extent to which this is true. Given that the most appropriate monitoring approach will also depend upon the ecosystem being restored, we will restrict our review to a specific ecosystem. We have selected restoration of riparian vegetation along river systems as our focus because studies dating back to the 1990s (e.g., Battaglia et al., 1995, Friedman et al., 1995, Roelle and Gladwin, 1999) exist to document these projects, and because degradation of river ecosystems is an international rather than local concern. A synoptic view of these restoration strategies, and especially an assessment of how to evaluate their success, is still lacking despite major investments annually worldwide for restoring riparian habitats. As pointed out by Weisberg et al. (2013), many of the first studies on the ecology of riparian vegetation focused on understanding the decline of Salicaceae gallery forests and the mechanisms underlying their regeneration (Johnson et al., 1976, Stromberg and Patten, 1991, Stromberg, 1993, Rood and Heinz-Milne, 1989, Rood and Mahoney, 1990, and Rood, 1998); the first restoration trials that received academic attention often equated riparian restoration with restoration of Salicaceae forests (e.g., Friedman et al., 1995, Roelle and Gladwin, 1999, Rood and Mahoney, 2000, Rood et al., 2003). With a significant and growing body of literature, restoration of riparian vegetation nowadays goes beyond restoration of Salicaceae forests. We believe that a comprehensive review of riparian vegetation restoration strategies and the empirical evaluations that have been completed up to this point could be a first step towards the development of standard protocols for monitoring river restoration projects.

In this study, we reviewed how restoration success of riparian vegetation has been evaluated in the scientific literature in order to answer the following questions: (1) Which restoration strategies were evaluated in different regions in the world and how have they been applied? (2) Which assessment approaches have been used (e.g., scale of monitoring and use of reference sites)? (3) Which evaluation metrics have been used as success criteria and how have they been computed? (4) Which factors explained success? Finally, by answering these four questions, our goal was to learn from previous experiences and suggest methods to improve the evaluation of restoration projects of riparian vegetation in the future.

Section snippets

Selection of articles

A literature search in ISI Web of Science was done in October 2014. The chain of keywords used in the topic category were “(riparian or floodplain or river or stream) near (vegetation or forest* or plant*) and (resto* or rehabilit* or recover* or remov* or reforest* or planting) and (success* or reference or degrad* or fail*)”. We also manually browsed all volumes of Ecological Restoration and Ecological Management and Restoration (SERI journals) between 1990 and 2012, given that these thematic

Results and discussion

Our review of 169 papers revealed strong convergence of approaches for both the restoration activity itself and also how and which vegetation was evaluated. The 169 papers were published in 58 different journals, but 41% were from just five, and only 25 articles (15%) were published in journals with an impact factor higher than 3.000 (Appendix S2). We found that the number of publications progressively increased since the first article in 1991, but more than half of them were published during

Conclusions

Three decades after the emergence of restoration ecology, articles evaluating success in the recovery of riparian vegetation have begun to appear frequently in the scientific literature. However, published evaluations have been mainly local. This is not necessarily bad and indeed we hope that more of those studies are published in the future. However, broadening the assessment approach (e.g., metrics and criteria used, factors of success explored, number of projects assessed, and larger spatial

Acknowledgments

This research was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme.

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