Implementation and evaluation of riparian vegetation restoration in the world
Evaluation of success has been the main weakness of restoration ecology since this discipline emerged in the early 1980s. The assessment of restoration outcomes has been traditionally jeopardized by underfunding and the lack of systematic evaluation criteria, despite the critical importance of such criteria for adaptive management as well as the advancement of the field of restoration ecology as a whole. A synoptic view of restoration strategies for riparian vegetation in particular, and an assessment of how to evaluate their success, is still lacking despite major investments annually worldwide for restoring riparian habitats. With a significant body of literature growing in recent years, we believe that a comprehensive review of riparian vegetation restoration strategies and the empirical evaluations that have been done up to date could foster the development of standard protocols for monitoring river restoration projects. We systematically examined 169 papers published in the peer reviewed, international literature dating from 1991 to 2015, where vegetation of floodplains and riverbanks was restored and extracted information about: 1) restoration strategies applied, 2) use of reference sites and scale of monitoring, 3) evaluation metrics and 4) drivers of success.
North America was by far the most studied region (61% of the papers) followed by Europe (15%) and Asia (14%). Hydro-geomorphic approaches were the most frequent (50%), followed by active plant introduction (39%), exotic species control (31%), conversion to a natural floodplain (29%) and grazing control (21%). The impact of the restoration was most often (43%) assessed by tracking change over time rather than by comparing restored sites to control and reference sites (12%), and few projects (30%) did both. 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 at the meander scale. Monitoring rarely lasted for more than six years. Among the ways which restoration success was evaluated, vegetation structure was assessed more often (152 papers) than vegetation processes (112) and vegetation diversity (78). Success was attributed to hydro-geomorphic factors in 63% of the projects, but biotic factors such as biological interactions, grazing and seed dispersal were also important (52%) in explaining restoration outcomes, especially when restoration approaches were not hydrogeomorphic. Future evaluations would benefit from more rigorous experimental designs, enhancing comparisons among projects, longer term monitoring and reporting failure.