Alnus viridis is a shrub species that has been increasing in many European mountains over the past decades, leading to a loss of agricultural areas and several negative environmental impacts. Recently, targeted livestock management systems have been investigated to reduce its encroachment. This study aims to provide an exhaustive assessment of A. viridis leaf composition and its temporal variation across the grazing season. A. viridis leaves were collected throughout the summers of 2020 and 2021 in four encroached sites across the Swiss and Italian Alps, characterized by different pedo-climatic conditions. Based on the data collected by meteorological stations, the growing degree days (GDD) were calculated for each site and year. The leaf functional traits (i.e., Leaf Dry Matter Content – LDMC – and Specific Leaf Area – SLA), chemical composition (macro- and microelements and fibre content), phenolic content, in vitro organic matter digestibility (IVOMD), and related gas production (CO2 and CH4) were assessed. The LDMC significantly increased throughout the season, whereas the SLA decreased. All macro- and micro-elements significantly varied during the season, with leaf nitrogen (N) and phosphorus (P) decreasing. In contrast, leaf fibre contents significantly increased as the season advanced. There was a significant increase in condensed tannin content during the summer season. Finally, adding A. viridis leaves (20% of diet on a dry matter basis) to cattle diets significantly reduced IVOMD, methane produced per digested organic matter (CH4/dOM) and CO2/dOM, compared to a 100% hay diet. Moreover, CH4/dOM and CH4/CO2 ratio increased during the summer season. These results highlight the potential of A. viridis leaves as a valuable forage resource, especially at the beginning of the summer. Such information could be used to optimize grazing of robust livestock in A. viridis-encroached alpine pastures in order to reduce its invasion and minimize greenhouse gases production at the same time.
