The influence of the Eurasian beaver’s gnawing activity on the structure of riparian forests in three Italian rivers

The Eurasian beaver (Castor fiber Linnaeus, 1758) can be considered a hydrological ecosystem engineer as it shapes the characteristics of the environment through its building activity and feeding behaviour. Even if several studies have so far reported beaver impact on multi-taxon biodiversity and forest regeneration, there is a lack of research on forest stand structure evolution following beaver direct activity on trees. This represents a pivotal topic for the implementation of restoration outcomes and to suggest sound silvicultural and management practices for maintaining specific forest conditions. Specifically, the study aims at investigating forest stand structure and tree species diversity changes considering river variability, distance from the riverbank and beaver’s gnawing activity intensity. The Eurasian beaver is only recently recolonising the three Mediterranean analysed rivers, but stand structure seems to be already significantly impacted by the species. The number of trees was reduced, increasing mean diameter at breast height at stand level, as most of the youngest and/or smaller trees are entirely cut down. Strongest structural variations can be detected in intensively impacted stands and in the forest portions closer to the riverbank. The absence of significant effect on most of the diversity indices is probably due to the initial homogeneous composition of the tree layer of each river stand and for the beaver not very varied diet within the sites. Future resprouting of secondary tree shoots, as well as beaver gnawing activity changes in intensity over time and space can further produce variations in structural parameters and woody species diversity in the medium- and long-term period. Therefore, it will be crucial to further monitor the long-term effects, as structural shifts can produce significant effects on riparian ecosystem functions.