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Schütz, M.; Krüsi, B.O.; Edwards, .PJ. (eds): Natl.park-Forsch. Schweiz 89, 89 - 105

Development of species richness in mono-dominant colonies of tor grass (Brachypodium pinnatum) — an indicator of the impact of grazing upon subalpine grassland?

Entwicklung der Artenvielfalt in monodominanten Kolonien der Fiederzwenke (Brachypodium pinnatum) — ein Indikator für den Einfluss der Beweidung in subalpinem Grünland?

Andrea Bärlocher1, Martin Schütz1, Bertil O. Krüsi1, Helena Grämiger1, Jakob J. Schneller2

1 Swiss Federal Research Institute WSL, CH-8903 Birmensdorf, Switzerland
2 Institute of Systematic Botany, University of Zürich, CH-8008 Zürich, Switzerland
On subalpine pastures, mono-dominant colonies of tor grass (Brachypodium pinnatum) offer an excellent opportunity to study how the exclusion of grazing animals affects the number of vascular plant species. First, both cattle and red deer avoid tor grass. Second, some of the Brachypodium-colonies are very old, up to 250 years, and we have accurate data about their radial growth during the past 50 years. In colonies of tor grass the number of vascular plant species per unit area depends on both the extent (per cent cover) and the duration of tor grass dominance. Between colonies, differences in species richness were mainly related to the abundance of tor grass, i.e. the vitality of the colony. Vitality and cover of tor grass most likely depend primarily on the availability of nutrients and the age of the colony. In young colonies on nutrient-rich sites tor grass is more abundant and the number of plant species is lower than in old colonies on nutrient-poor sites. Within a given colony, in contrast, differences in species richness depend primarily on the duration of dominance by tor grass: it decreases sharply from the edge to the centre of the colony. On average, ten years of dominance by tor grass lead to the loss of one plant species. The reversal process after the death of a colony proceeds five times faster; thus, every second year a new plant species can establish itself in an area once dominated by tor grass. The observed development of the number of vascular plant species in colonies of tor grass supports the «competitive exclusion hypothesis». Consequently, the spatial pattern of species richness in mono-dominant colonies of tor grass give us reasonable indication of the long-term development of the number of vascular plant species in the absence of grazing.

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