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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Response of bird communities to the extensive collapse of tree stands in mountainous spruce forests of the Inner Bavarian Forest
Auteurs :	Wolfgang Scherzinger, Auteur
Type de document :	Article
Dans :	Vogelwelt (Die) (127(4) 2006)
Article en page(s) :	209-263
Résumé :	Due to heavy wind blows and unusual high temperatures during summer 1983/84, an outbreak of a bark beetle infestation stressed the mountain ridge of the Inner Bavarian Forest and killed most of the old spruce stands in an extensive scale. This study describes the reactions of the avifauna to this natural disturbance. A 75 ha study plot at 1050-1300 m a.s.l. was chosen, at the transition zone of mixed forest with beech and spruce (maple) at its upper limit to pure spruce stands, which are typical for the highland. Since 1970, when the Bavarian Forest National Park was founded, the forest in this area was free of any logging. Between 1989 and 2000 the bird census included a total of 301 visits (25 per year in average) and was based on a grid system squares of I ha), combined with a mapping of all individuals of birds within the squares. The 12 years study period covers a development from vital stands to a nearly open landscape, with extraordinary quantities of decaying wood.In total, 71 bird species were recorded. Within the study period the species set increased by 22 species, but lost another I I species. In contrast to the constancy of about 40 species during the breeding seasons, the number of species declined from 24 to 8 in winter. This effect was caused by a rising dichotomy in the development of habitat, resulting in favourable quality during summer and poor conditions in winter. Summarising the maximum densities of each species for all the years of the period 1989-2000 the total population came up to 1,498 individuals. As typical for mountainous forests, Chaffinch, Coal Tit and Wren were the most common species. The annual population size fluctuated between 18 1 individuals at the lowest level in 1995, and a maximum of 1, 159 individuals in 1996, a year with exceptional seed-mast. Comparing the frequencies of single species during the breeding seasons of 1989 and 1999 (by summarising all occupied squares from mid March to late June) ten species can be classified as loser and nine species as winner, while another eight species showed no distinct reactions to the forest dye back. At first, woodpeckers belong to the winners, increasing their number of species from two to six. During the phases of collapse and decay of heavily disturbed forest areas, the avifauna indicated a high level of habitat quality, equivalent to the attractiveness of uninterrupted old,growth of natural forest. But this level of quality lasted only as long as habitat conditions were suitable for species of the forest interior and for species of clearings and semi-open habitats as well.With respect to the history of similar catastrophic events in the Bohemian Forest, wind blows and successional bark beetle infestations should be accepted as manifestation of a natural development. Natural dynamics of ecosystems permanently trigger cycles of disturbance, regeneration, growing and establishment, which develop the preconditions for a rich diversity of habitats over a long time.

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