Bird response to forest structure and composition and implications for sustainable mountain forest management

Assessing how bird richness across ecological guilds of mountain-breeding birds responds to changes in forest attributes may inform the development of sustainable management strategies. In this study, we surveyed forest structure and bird communities across 148 sampling units in France and Italy to identify threshold values for stand variables that influence bird species richness. The sampling covered an altitudinal range from 805 to 1555 m a.s.l. and was mainly dominated by mountain beech and Alpine coniferous forests. We modelled species richness of overall bird communities and individual ecological guilds in response to structural and compositional attributes and topographic variables using generalized modelling. Bird species richness was negatively associated with stand-level total tree density. However, higher silver fir density was associated with greater species richness across most bird guilds, which preferred mixed woods and heterogeneous stand structure. An increase in deadwood volume had a significant positive effect on all guilds, confirming the importance of old-growth forest features for forest birds. Threshold values indicating a shift in species richness of bird guilds were identified only for some stand structural attributes (i.e., total basal area and deadwood volume). Our results suggest that the conservation of biodiversity in forest bird communities may benefit from management strategies that maintain mixed, relatively open canopies and promote deadwood retention to ensure the availability of associated habitats. Our approach lays the groundwork for further research, applying this methodology to additional variables and biodiversity metrics.