A guild-based approach to assessing the influence of beech forest structure on bird communities

The advantages of using birds as indicators of biodiversity and/or habitat quality in forest habitats rely principally (1) on their relative ease of detection and (2) on their strong association with many forest features. Here we investigate how forest structure can shape a bird community at the stand level (ca. 30. hectares), using an expeditious sampling protocol and a simple but reasoned guild approach. We focused on three beech (. Fagus sylvatica) forests in Italy with different structures resulting from different management histories: Cansiglio, Chiarano-Sparvera and Marchesale. We hypothesise that the abundance of the selected guilds varies in relation to forest structure as a result of forest management; to test this hypothesis we modelled guild abundance along a latitudinal transect in a similar habitat (. i.e. beech forest) in order to offset the geographical and environmental sources of variability. Birds were surveyed through replicated aural/visual point counts. Thus we identified four guilds: generalist cavity nesters (TIT), generalist canopy nesters (WAR), insectivorous cavity nesters (INS) and granivorous canopy nesters (FIN). Forest structure and deadwood were estimated for each of the 27 sampling points. Guild abundance was estimated using the N-mixture models approach, which allows the mean abundance at each sampling location and the detectability to be estimated. Both the mean abundance and detectability were constrained to forest structure and time variables. Guild-estimated abundance was tested for differences among sites using ANOVA. Mean guild abundance proved different only for two guilds. INS was more abundant in Cansiglio than in Chiarano and Marchesale while between Chiarano and Marchesale there was no significant difference. Also WAR abundance was greater in Chiarano, followed by Marchesale and Cansiglio. Shelterwood management may result in a higher abundance of specialist guilds (. i.e. insectivorous cavity nesters), compared to other management options. Our results highlight this pattern in the Cansiglio forest, where the age and the applied treatments resulted in a lower tree density and in a larger mean diameter. Such features are often correlated with a larger amount of dead wood which, in turn, promotes the presence of cavity nesters, otherwise rare-to-absent in the bird community. Forest structure is largely influenced by forest management that can be guided to benefit specific bird assemblages by applying specific treatments focused at increasing structural diversity and at multi-functionality.