Snow beds – Scotland’s Arctic
Walking across the Cairngorm plateau on a fine, windy day in the depths of winter, the snakes of spindrift and the occasional maelstrom, reducing visibility to zero, reminds you that snow does not stay put once it has fallen. Most snow that falls is blown onto lee slopes and into gullies where it can form drifts of prodigious depth. These ‘snow-beds’, on north and north-east facing slopes, can persist right into the summer months and, in the deepest coires and gullies, snow can often last throughout the year. This pattern of snow accumulation and melting means that snow patches usually build up in the same places each year. But it also means that the growing season, for plants underneath them, is short and erratic. Bryophytes and lichens, which can persist and even grow under the snow, have an advantage over flowering plants in these harsh conditions and they dominate in these ‘snow-bed communities.’
Where meltwater from the snow patch percolates through the gravel soil, the predominant colour is green from mosses like Ludwig’s thread-moss Pohlia ludwigii, Northern haircap Polytrichum sexangulare and Starke’s fork-moss Kiaeria starkei. Large patches of the pale, blue-green of Mountain thread-moss Pohlila wahlenbergii var. glacialis are visible from several hundred metres away. In the best snow-beds, these damp gravels can also have large patches of the liverworts Snow threadwort Pleurocladula albescens and Alpine ruffwort Moerckia blyttii, both restricted to areas of late snow-lie. On the drier slopes a liverwort crust forms, often much contorted by frost-heave, and the colours here are dark reds and blacks, with the most common plants being species of Rustwort Marsupella. This liverwort crust can be very species-rich with a two centimetre square containing ten or more species of moss and liverwort, as well as a number of lichens. On the surface of the rocks, species of Andreaea, the ‘granite-mosses’, are often abundant and include rare species such as Snow rock-moss Andreaea nivalis and Icy rock-moss Andreaea frigida, on irrigated rocks in burns fed by snow patches, and Blytt’s rock-moss Andreaea blytti on flat rocks in areas of very late snow-lie.
There are a number of nationally rare species restricted to this habitat, which is limited in the UK to the higher mountains in Scotland, and even here it probably covers, at most, 1,000 hectares – our own small piece of the Arctic. The largest areas of snow-bed vegetation are in the Cairngorms, which has by far the greatest amount of ground over 1,000 metres, but there are important sites elsewhere, including the Ben Nevis area, the Ben Alder plateau, Creag Meagaidh and, further north, Ben Dearg. The snow-beds have a particular attraction because of their beauty and remoteness, and days spent studying these patches, with snow buntings catching insects on the remaining snow and dotterel nearby, provide some of the most memorable times in the hills.