M33 - Pohlia Wahlenbergii Var. Glacialis Spring Pohlietum Glacialis Mcvean & Ratcliffe 1962

Summary

Synonymy

Pohlia ‘glacialis’ spring Ratcliffe 1964; Pohlia albicans vax. glacialis spring Ratcliffe 1977.

Constant species

Deschampsia cespitosa, Saxifraga stellaris, Pohlia ludwigii, P. wahlenbergii var. glacialis.

Rare species

Alopecurus alpinus, Cerastium cerastoides, Epilobium alsinifolium, Phleum alpinum, Pohlia ludwigii, P. wahlenbergii var. glacialis.

Physiognomy

Pohlia wahlenbergii var. glacialis can be found as an infrequent and low-cover associate in a variety of vegetation types of wet ground through the uplands of north-western Britain but, in the Pohlietum glacialis, it dominates in spongy carpets, often of quite small extent, but exceptionally covering up to 200 m2, of a bright apple-green colour that makes the stands readily recognisable from a distance. Few other bryophytes occur with any frequency and none is consistently abundant. There is commonly a little P. ludwigii and Philonotis fontana sometimes attains a measure of prominence, though typically it is of nothing like such high cover as in the Philonoto-Saxifragetum. Other bryophytes recorded occasionally are Hygrohypnum luridum, Bryum weigelii, Calliergon stramineum, Scapania undulata, S. uliginosa, Dicranella palustris and Marchanda alpestris.

In this carpet, vascular plants are few in number and typically of low cover. Only Deschampsia cespitosa (presumably ssp. alpina at the high altitudes characterised by this community) and Saxifraga stellaris are constant, but the rare Arctic-Alpine Cerastium cerastoides is quite often found and there can also be some Stellaria alsine, Chrysosplenium oppositifolium, Epilobium anagallidifolium, Veronica serpyllifolia var. humifusa and Rumex acetosa. Other rare plants which find an occasional locus here are Epilobium alsinifolium, Alopecurus alpinus and Phleum alpinum.

Habitat

The Pohlietum is strictly confined to spring-heads associated with the late snow-beds of the higher reaches of the Scottish Highlands, where there is vigorous irrigation by cold, oligotrophic waters.

Although P. wahlenbergii var. glacialis occurs in small amounts over quite a wide range of altitudes through the uplands of north Wales, Cumbria and Scotland, it is found with the kind of dominance characteristic here only within the high-montane zone, at altitudes generally above 850 m, where mean annual maximum temperatures do not exceed 21 °C (Conolly & Dahl 1970).