Riparian forests throwback at the Eurasian beaver era: a woody vegetation assessment for Mediterranean regions

Several biotic and abiotic interactions will contribute to riparian ecosystem changes. The impact of Eurasian beaver ( Castor fiber ) on woody vegetation is still unknown for the Mediterranean biogeographical area. Through a replicable approach applied on a cluster of three rivers, we studied how the tree layer of Mediterranean riparian sites is impacted by the beaver's recent comeback. For each site, we collected data (e.g., stem diameter, species, distance from riverbank) for all standing trees and additional information only for gnawed trees at plot level. Data elaboration allowed to characterise impacts on riparian vegetation. Salix spp. and Populus spp. are the main gnawed species, but sporadically other species can be selected based on their size and spatial distribution (e.g., Alnus glutinosa ). Diameter means of gnawed trees are significantly lower than the not gnawed ones. Most of the selected trees have low diameter classes (< 12 cm), even if diameter preferences may vary on the basis of overall stand tree size range and distribution. Over 90% of the gnawed trees are entirely harvested, with stumps as the remaining standing element. Main changes on the overall forest stand occurred in the first ten metres from the riverbank, as beaver gnawing activity is significantly influenced by the interaction among tree distance from the river and diameter size. Our approach can be used as a model system to be implemented in other Mediterranean sites where beaver is expanding, with the aim of predicting mid-term riparian forests vegetation changes. Graphical abstract