Structural complexity in managed and strictly protected mountain forests: effects on the habitat suitability for indicator bird species

Abstract

Increasing the proportion of unmanaged forest in multi-functional forest landscapes is a central goal of international and national conservation strategies. However, the structural development in newly created forest reserves and its impact on forest species remain are controversially discussed, especially with regard to potential negative effects on light-demanding species in the first phase after reserve designation. We evaluated the effect of management cessation on habitat characteristics of four bird species indicative of different seral stages and structural components: Capercaillie (Tetrao urogallus), Hazel grouse (Bonasa bonasia), Three-toed woodpecker (Picoides tridactylus) and Pygmy owl (Glaucidium passerinum). We analysed the forest structure and composition in 42 forest reserves and 300 managed forest sites in four mountain regions in Southwestern Germany and Switzerland. We first modelled habitat selection independent of forest management status and then compared habitat characteristics and suitability of forest reserves to managed forest with species presence or absence. Further we evaluated habitat suitability in relation to the time since reserve designation. For all model species, except Pygmy owl, habitat suitability in forest reserves was significantly higher than in managed forests with species’ absence, but not different from managed forests with species presence. For the three species associated with open forest structures habitat suitability decreased significantly in the first three decades after reserve designation and increased afterwards up to the maximally recorded time of 100 years. No such correlation was found for the three-toed woodpecker, probably because this species is mostly associated with temporally unpredictable bark-beetle infestations. While on average forest reserves can provide suitable structures for different indicator bird species, structural characteristics vary greatly in abundance and distribution, with variance being only partly linked to the reserve age. Especially open structures can be limiting for light demanding species in the first decades after designation, when relatively young forests of about 80-120 years grow dense. We therefore recommend focusing on old, near natural or recently disturbed and structurally diverse forests when new reserves are designated.