The abundance and distribution of guilds of riparian woody plants change in response to land use and flow regulation.
Many riparian ecosystems in Mediterranean Europe are affected by land use and flow alteration by dams. We focused on understanding how these stressors and their components affect riparian forests in the region. We asked the following questions: (1) Are there well-defined, responsive riparian guilds? (2) Do dam-induced streamflows determine abundance and distribution of riparian guilds? (3) What are the main drivers governing composition and cover of riparian guilds in regulated rivers? We inventoried the cover of riparian woody species in free-flowing rivers and downstream of dams. We performed a cluster analysis and ordination to derive riparian guilds, using abundance data from 66 riparian woody species and 26 functional plant traits. We used a reduced set of principal components for the environment, land use and hydrology, and general linear modelling to explore the effect of these factors (separately and combined) on riparian guilds. We found that: (1) four dominant guilds are responsive to disturbance in southwestern European streams, namely the obligate riparian, water-stress tolerant, deciduous competitive and Mediterranean evergreen guilds; (2) a set of land use and hydrological variables differentially affect the diverse co-occurring riparian guilds; (3) frequency and duration of high flow pulses and the low-flow conditions were major drivers of change in landscapes dominated by intensive agriculture and forestry; (4) storage reservoirs reduced the cover of obligate riparian and Mediterranean evergreen guilds, and increased the abundance of water-stress tolerant and deciduous competitive guilds, while run-of-river dams, having limited water storage, reduced both obligate and deciduous competitive guilds. Synthesis and applications. Future research in southwestern Europe should address the resilience of riparian guilds and the effects of interacting landscape factors and stressors on guild distribution. Streamflow regulations downstream of reservoirs should focus on specific flow components, namely the magnitude of flows, and frequency and duration of extreme flow events. For successful mitigation of the dam-induced effects on riparian vegetation, river management plans must incorporate the environmental and land use site-specific contexts.