The Ecology of the Cairngorms: IV. The Juncus Zone: Juncus Trifidus Communities

Abstract

This zone extends roughly from 100 m upwards. Most of the ground is bare. The soil is loose granular gravel with granite boulders, lacking humus, of pH near 4. Juncus trifidus communities occur wherever there is bare gravel and snow-cover is not excessive: on south-west slopes because of exposure to wind; on north-east slopes (generally bearing a snow-tolerant grassy turf) where gravel is exposed by erosion, solifluction, floods or screes. The important species are tussock-formers: Juncus trifidus, Luzula arcuata, Deschampsia flexuosa and Festuca vivipara. Two facies are recognized (1) with Rhacomitrium lanuginosum (2) with Salix herbacea. (1) occurs on exposed sites where gravel is blown away: the Juncus tussocks are raised, convex, more or less eroded, and bear R. lanuginosum and hard encrusting lichens as epiphytes. (2) occurs where gravel (and probably snow) accumulate by drifting: the tussocks are submerged and funnel-shaped, the centers containing a pocket of humus with a hepatic crust and Salix herbacea. The Juncus colonizes by seed, probably at irregular intervals. This association is very different from those found at equivalent altitudes on base-rich soils in Britain. Compared with similar European associations, it is more open, and poor in species partly because some of the European associates of Juncus trifidus are confined to lower altitudes in the Cairngorms: a separate status - as Juncetum trifidi scoticum - is proposed.