Cader Idris: A Study of Certain Plant Communities in South-West Merionethshire

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Abstract
The arctic-alpine vegetation of Cader belongs chiefly to lower arctic-alpine associations. Floristically rich localities coincide with the outcrop of calciferous rocks (chiefly "pillow-lavas" of the lower and upper basic volcanic rocks of the Ordovician). Asplenium viride occurs along a narrow band of pillow-lava, in places only a few feet in width, for some miles along the scarp parallel to the strike. When the rock is difficult to determine in the field, the presence of this plant at once indicates its calciferous nature and is a rough guide to the course of the "pillow-lava" band. The chomophytic vegetation of the crags and corries consists of some "Highland" species, of alpine forms of maritime plants, and of calcicole species. Buried wood (Beteula sp. and Salix sp.) at about 1750-1800'' indicates the former upper limit of trees (present limit C. 1100''). The presence of Genista pilosa (recorded from only one other locality[long dash]Pembrokeshire[long dash]in the principality) and of Pertusaria monogona (the warted and the rarer, somewhat plane areolate form), up to the present the only British record, is noteworthy. Juncus acutiflorus communities cease at about 1800'' and high-level junceta are dominated by J. communis. Hybrid oaks [Quercus robur (pedunculata) X sessiliflora] are rather frequent in the oakwoods and the hybrid birch [Betula alba (verrucosa) X pubescens] dominates the birchwoods. The presence of pine in the submerged forest (presumably late Neolithic) may indicate the persistence of pine in this area.

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