Large trees as a key factor for bird diversity in spruce-dominated production forests: Implications for conservation management

Abstract

Large trees (also termed veteran trees or habitat trees) are keystone structures for biodiversity worldwide. Retention forestry aims to keep large trees in production stands to support biodiversity. Nevertheless, there is insufficient information about the effect of large trees on biodiversity and how many large trees should be left in different types of stands, including spruce-dominated production forest. We aimed to investigate the influence of large trees on birds (included generalist and specialist species) in spruce-dominated production forest throughout the Czech Republic while taking into consideration effects of stand age, tree species composition, and distance to a clearing. At 20 study sites (each 600 ha), all trees with diameter at breast height (DBH) ≥ 70 cm were exhaustively searched. We then localized sampling plots representing a gradient of large tree numbers from zero to maximum on each plot. Birds were sampled using point count method in breeding seasons 2018 and 2019. Bird assemblages were analysed using generalized linear mixed models with Poisson error distribution. The total number of birds, number of generalist species, and number of specialist species increased significantly with the number of broadleaved trees ≥ 70 cm DBH. Furthermore, tree species diversity positively affected generalist species and negatively influenced specialist species. Our results suggest that 5 broadleaved trees ≥ 70 cm DBH/ha greatly improve bird diversity in spruce-dominated production forests and forest management should be adapted to reach ≥ 5 native, large, broadleaved trees to improve structural diversity of spruce-dominated production forests.