Environmental key factors and their thresholds for the avifauna of temperate montane forests

Abstract

Mixed montane forests cover large tracts of the low mountain ranges that dominate Central Europe and also contain much of the area that is important to forest related nature conservation. However, beyond general patterns little is known about ecologically effective driving factors in this habitat. This results in a lack of precise values that can be used to formulate guidelines for nature conservation oriented management strategies. To improve this situation, we used birds as indicators for forest habitat qualities. The avifauna of the Bavarian Forest National Park was investigated on 293 plots of 1 ha along four transects ranging from 655 to 1420 m a.s.l. The investigated forest includes a wide structural gradient ranging from very open, grass dominated areas of the high montane zone, where mature spruce trees have died after bark beetle infestation, to mature mixed montane forests at lower elevations. Altitude is the main factor driving composition of bird assemblages in the montane forests. We analysed occurrence of species using a subdivided dataset, in which the altitude could be eliminated as an important influencing factor, by implementation of General Linear Models under consideration of spatial effects and utilising maximally selected rank statistics. Our analysis identified canopy cover as the most influential factor in montane and high montane forests. Forest age is the second most important gradient in montane forests. The tree species composition has an influence on a limited number of species. In high montane forests, bark beetle infestations have led to a complete change in the avifauna composition. The distinct, species-poor group that is found here includes some species that are generally decreasing in abundance in Central Europe. By calculating threshold values for the environmental variables which were identified as important, we were able to formulate the following concrete management recommendations: (1) To create conditions suitable for the whole range of species associated with openings in the canopy, the full spectrum of canopy-openness ranging from 5% to 70% should be realized on the stand scale throughout a whole forest landscape. (2) To sustain the whole range of cavity-breeding species and species of mature forests, the age of Central European mixed montane forests needs to surpass 200–220 years. (3) To support the deciduous forest species in mixed montane stands a minimum of 60% cover of deciduous trees at least in single stands that are maximally few kilometres apart is required. Among the threshold values, we identified cascade-like threshold sequences that represent the ecological niches of the individual species with respect to the environmental variables, which contrasts with species groups where the collective threshold value is a universal threshold that applies to the whole group.