The importance of being natural in a human-altered riverscape: Role of wetland type in supporting habitat heterogeneity and the functional diversity of vegetation

Human activities have dramatically affected the status of river ecosystems, mainly by completely altering their natural dynamics. One of the main questions in this regard is: 'How do the origin (natural or artificial) and hydrology (lentic or lotic) of a riverine wetland influence its heterogeneity and the functional diversity of its vegetation?' To answer this question, data from 60 wetlands were collected along the Oglio River (northern Italy), a ~700 km2 ecosystem that is a typically over-exploited plain. In order to perform a quantitative assessment of habitat heterogeneity and vegetation diversity, a hierarchical approach was applied to sample sites divided into four functional zones (FZs) according to distinct seasonal inundation patterns (persistently aquatic, riparian, seasonally emergent and lateral zones). Plant communities were randomly surveyed in each FZ by using five replicates of 4 m2 plots for aquatic and herbaceous plant communities and five replicates of 64 m2 plots for woody and shrub communities. Data on species cover were estimated by standard methods (geometric-centered scale) The highest values of heterogeneity (FZs per site) and vegetation diversity (plant communities per site) were found in natural lentic sites, whereas natural lotic sites exhibited the lowest values. A clear dependence of plant community assets on wetland origin was detected, thereby confirming the close relationship between heterogeneity and vegetation diversity. Present results highlight the fundamental role played by natural sites in maintaining aquatic and wetland vegetation diversity in human-altered riverscapes. The pivotal role of water level fluctuations in promoting the diversity and distribution of aquatic and hygrophilous vegetation also clearly emerges.