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

Abstract

Increasing the proportion of unmanaged forests in multi-functional forest landscapes is a primary goal of international and national conservation strategies aiming at restoring natural properties in structurally simplified forests. However, the development of structural features and associated habitat suitability for forest species is largely unknown and even controversially discussed, as the development of newly established reserves is unidirectional and passes through dense maturation stages. This may negatively affect open forest species in the first phase after reserve designation. We evaluated the effects of management cessation on key habitat characteristics of four mountain forest bird species indicative of different structural components: Capercaillie (Tetrao urogallus), Hazel grouse (Bonasa bonasia), Three-toed woodpecker (Picoides tridactylus) and Pygmy owl (Glaucidium passerinum) across four mountain regions in Central Europe. Habitat suitability was modelled based on 300 forest sites selected independently of their management status, and predicted to an independent dataset of 42 strictly protected forest reserves in the same regions. We then compared forest reserves to managed forests with species presence or absence with regard to habitat suitability and key habitat structures and related both 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 species associated with open forest structures (Capercaillie, Hazel grouse, Pygmy owl) habitat suitability was significantly related to the “reserve age”: reserves in the first three decades after management cessation showed a significant decrease in suitability, which increased afterwards up to the maximally recorded time of 100 years. No such correlation was found for the Three-toed woodpecker associated with deadwood and barkbeetle infestations following temporally unpredictable disturbance events. Structural characteristics varied greatly in abundance and distribution, with open structures being related to the time since reserve designation. We therefore recommend focusing on mature, near-natural and structurally diverse forests when designating new strict forest reserves.