Scotland also has an extremely rich arctic-alpine bryophyte flora, often occurring in close proximity to the oceanic species, giving plant communities that are unique in Europe. Two special habitats are particularly worthy of mention:
- the hills with outcrops of calcareous rocks, and · the high hills where snow lies late into the summer.
A high proportion of Scotland’s Red Data Book species come from these two habitats.
Scotland has more lime-rich rock on its higher hills than anywhere else in the British Isles and many arctic and alpine species occur only in our mountains. Ben Lawers alone has over 50 nationally rare species, and other important sites include the other Breadalbane hills, Caenlochan Glen, the Ben Alder area, the Ben Nevis massif and, further north, Beinn Dearg and Ben Hope. Ledges and crevices on broken, lime-rich crags are probably the most productive habitat and the ‘good ground’ is usually heralded by relatively common calcicoles like Frizzled crisp-moss Tortella tortuosa and more restricted arctic-alpine mosses such as Red Ieskea Orthothecium rufescens. Winter frosts and the occasional rockfall keep this habitat open so that a variety of species persist here and are not overwhelmed by flowering plants. Special plants include Scottish beard-moss Bryoerythrophyllum caledonicum a Scottish endemic, and Alpine Comb-moss Ctendium procerrimum which, in Britain, is found only on Ben Lawers and in Glen Feshie.
The calcareous scree below the crags is also important and mosses and liverworts are usually abundant, both in the sheltered crevices and on the open surfaces of the rocks. Himalayan fringe-moss Racomitrium himalayanum, which has its only European sites in Scotland, prefers the sun-warmed rocks on south-facing slopes. Rarities like Dimorphous tamarisk-moss Heterocladium dimorphum and Spinose thyme-moss Mnium spinosum prefer the shaded cavities. Another plant of these shaded crevices is Hair silk-moss Plagiothecium piliferum, found on Ben Lawers and in Caenlochan Glen but not seen for more than 60 years. The flushes below the crags and scree are also important habitats, being both attractive and having some rare and delightful species. Green spur-moss Oncophorus virens prefers the wet grassland at the edge of the flushes but Broad-nerved hump-moss Meesia uliginosa is a plant of mossy hummocks within stony springs. Even though these calcareous hills have been the target of bryologists for years, their richness is such that they are still the most likely sites for new finds.