Key structural factors and their thresholds for promoting bird diversity in spruce-dominated production forests of central Europe

Many of native forests in Europe have been transformed into even-aged production forests of commercially attractive conifers. In recent decades, a moderate shift back to a more native tree species composition accompanied by the maturation of some close natural stands has been evident in the Czech Republic. We aimed to investigate the effects of increasing age and contribution of native tree species on bird species and to identify the potential critical thresholds of these factors in central European forests. For this purpose, bird monitoring and forest structure measurements were carried out at 120 plots in production forests and 20 forest reserves located on 20 study sites throughout the Czech Republic. These plots covered gradients of native tree species contribution and stand age. Birds were counted during the 2018, 2019 and 2020 breeding seasons using passive acoustic monitoring, followed by subsequent computer analysis of the recordings. We assessed relationships between differences in bird species composition and structural and environmental factors. We also used generalised additive models (GAMs) to investigate the effects of individual structural and environmental factors on birds, taking into account their occurrence frequencies and habitat preferences. Our results convincingly documented, that dissimilarities in bird species composition, especially species turnover, strongly coincided with differences in the share of conifer basal area and stand age. The effect of tree species composition on the bird species turnover reflected the habitat preferences of individual species. In addition, we found that some cavitynesting species were strictly associated with stands with a low contribution of conifers and high stand age. The presence of forest older than at least 125, but sometimes up to 280 years is a critical factor for rare and oldgrowth bird species. Conversely, the high contribution of conifers (more than app. 60 % of basal area) inhibited the occurrence of the species-rich communities, especially the birds associated with close natural stands. Therefore, increasing the area of mature close-natural stands would be beneficial for bird diversity, especially for rare species. In central Europe, however, the rotation length of forest stands is usually less than 120 years in order to maximise timber production. Forest management practices should therefore support the maturation of forest stands and the transformation of tree species composition towards more native broadleaved forest stands. These measures are essential for the protection of species-rich bird communities, especially old-growthassociated birds, in central Europe.