A systematic review of published evidence linking wetland plants to water regime components

We used the Eco Evidence method for systematic review to analyze the literature for evidence of causal relationships between components of wetland water regimes (waterlogging, inundation, depth, duration, frequency, timing) and their effects on plant establishment, growth, reproduction, assemblage composition and diversity. Using systematic keyword searches of Thompson ISI's Web of Science, we located 102 studies. Some studies examined multiple water regime components and/or plant characteristics. Overall, the papers yielded 281 pieces of evidence relevant to 38 cause–effect hypotheses. Of these papers, 49% studied plant growth responses. Inundation and water depth were the most-commonly studied water regime components (55% of papers each). Very few studies considered flood frequency (4 studies) or timing (8 studies), and in many cases there was insufficient evidence to test the effect of these components on wetland plants. Overall, our results show that increases in the magnitude of water regime components are related to negative effects on plant characteristics. Within this, the strongest set of results related inundation depth to negative impacts on nearly all plant characteristics (we found a positive relation for shoot length). Waterlogging (as opposed to complete inundation) was generally related to improvements in plant characteristics, but there was insufficient evidence to test several hypotheses. A large number of hypotheses (17) showed inconsistent effects of water regime components, reflecting the diversity of responses of different plant taxa, wetland types, and study methods. We were unable to find sufficient evidence to test 10 of the hypotheses, and are confident that these represent true knowledge gaps that can only be filled by future targeted research. Evidence of causal links improves our understanding of the water requirements of wetland plants. Such evidence can inform the development of environmental flow programs to maintain and restore wetland plant communities on regulated rivers.

► We systematically reviewed effects of water regime on wetland vegetation. ► We used the Eco Evidence method to synthesize evidence from the literature. ► Most research examined growth responses to changes in inundation and depth. ► There was little research on the effects of frequency or timing of inundation. ► Systematic review can inform evidence-based environmental flow recommendations.