The following is a modified extract from the ‘Flora of Assynt’ by P.A. & I.M. Evans and G.P. Rothero 2002 and is reproduced here with the kind permission of G.P. Rothero
Bryophytes – The physical background
Assynt has a large total annual precipitation but more important for the bryophytes, particularly those described as ‘oceanic’, is the total number of wet days or conversely, the absence of any prolonged period of drought. Residents in the area will not be surprised that much of Assynt has in excess of 200 wet days per year (Ratcliffe 1968), with a ‘wet day’ defined as one with at least 1mm of rain falling. The ameliorating effect of the relatively warm sea means that Assynt as a whole has fewer than 60 frosty days and fewer than 20 days with snow lying (Page 1982), although these sea-level figures need to be adjusted for the higher ground in the east of the parish.
To give an expression to this type of climate, an index of ‘oceanicity’ (see Averis 1991 and Page 1982), can be derived from the number of wet days per annum divided by the annual temperature range (mean maximum July temperature minus the mean minimum February temperature). Plotting the areas with the highest index gives a narrow band of ‘wet mildness’ (or ‘cool wetness’) that extends down the coast of Highland Scotland, including the Hebrides, and the extreme west of Ireland, and which includes all of Assynt. This index has proved a useful indicator of the distribution of oceanic bryophyte species. This very wet climate has also had a marked effect on bryophytes in another way, through the formation of mire areas where mosses, particularly Sphagnum, are often dominant.
Bryophytes are mostly small plants and do not compete well with the larger flowering plants and ferns except where conditions are favourable. The landforms of Assynt with a preponderance of rocky slopes, crags, boulders and ravines provide a wonderful assortment of niches where bryophytes can become established and form long-lived communities. The effects of glaciation are everywhere apparent but particularly so on north and east-facing slopes where the resultant crags and screes provide an important habitat for the hepatic heath described below. The run-off from the melting of the glaciers with an increasingly mild and wet climate, over a landscape largely devoid of vegetation and with prodigious quantifies of rock debris gave rise of the, sometimes spectacular, ravines which seam the area. These now form very important sites for bryophytes. Another very recent landform with a distinctive bryophyte community is wind-blown shell-sand which has a patchy distribution from Achmelvich round to Oldany.
The effects of the underlying rocks are as apparent for the bryophytes as they are for the flowering plants, though the physiological mechanism through which the rock type operates to determine presence or absence is even less well-understood. The large expanse of limestone and the associated Fucoid Beds and the run off from the calcareous areas have a nationally important bryophyte flora with a number of species limited to this zone. The sandstone is largely rather acid and often has a limited flora although the gritty texture of the rock is ideal, particularly for the smaller liverworts.
As with the flowering plants, it is the complex mineralogy of the gneiss that poses most of the problems. Some facies of the gneiss are strongly base-rich and have a number of ‘calcicole’ species which others are absent (though frequent enough on the limestone). Schoenus nigricans flushes on the gneiss have a limited number of species, Scorpidium scorpioides, Drepanocladus revolvens, Aneura pinguis and Blindia acuta, but the expected Philonotis fontana, Dicranella palustris, Jungermannia exhertifolia ssp. cordifolia and Bryum pseudotriquetrum are rarely seen. Some large gneiss crags are almost completely devid of bryophytes except at the base and in some seepage lines. The autecological studies necessary to provide an explanation for these distribution patterns have not been carried out. However it would seem likely that it is the balance of a range of minerals including pyroxene, hornblende and plagioclase and their breakdown products that are critical, along with the depletion of several trace elements in the gneiss in general (Johnstone and Mykura 1989).
Assynt bryophytes in context
In order to put bryophyte flora of Assynt into context it seems sensible to discuss briefly the importance of the Scottish Highlands as a whole. Britain has an internationally important bryophyte flora; we have approximately 70% of the European bryophytes as compared to only 18% of the European flowering plants. In general, the diversity of bryophytes in Britain increases as you move north and west because of the more diverse geology, more and bigger hills, higher rainfall and less pollution. Within this diverse bryophyte flora, the most important of the bryophyte communities, in global terms, are those which are often described as ‘Atlantic’ or ‘oceanic’.
The climatological and geomorphological features already described for Assynt are common to much of western Scotland. The combination of equable temperatures and consistently high humidity occurs only in a very few areas of the globe, principally on temperate oceanic margins and in high montane zones closer to the equator. In Europe, this zone is limited to the extreme western margins of the continent, where very small areas occur on the costs of France and Spain and again in south-west Ireland to north-west Scotland.
In addition to the climatic factors, buffering from changes in humidity can also be enhanced by a reasonably continuous tree canopy, deeply incised river valleys and by very rocky terrain. The latter two conditions are common in the west of Scotland as a result of glacial and fluvio-glacial processes; the burns tend to be steep and have ravine sections and there are rock-falls and scree slopes, not least those associated with the extensive raised-beaches around the coast. Broadleaf tree cover was probably fairly continuous at one time from Kintyre to Torridon and extensive in favoured spots north of this, probably including much of Assynt.
These various factors have given rise to distinctive and, in global terms, very rare, moss and liverwort communities. Several of the species involved have their only European sites in the British Isles, while many other species that are reasonably common here are extremely rare elsewhere. Perhaps more important is that, though there may be isolated records for most of the species from elsewhere in Europe, it is principally in the British Isles that there are extensive and distinct communities containing these species.
The west coast of Scotland has the largest area of this kind of habitat in Britain and has the largest number of typical oceanic species as well as many of the most important populations. The available habitat in Assynt, as in much of Scotland, has been much reduced in the last few millennia as most of the species, as well as requiring an oceanic climate, require the further buffering from changes in temperature and humidity provided by a broadleaf tree cover. As the area under permanent broadleaf tree cover had decreased, so the populations of the oceanic species have become increasingly fragmented. This means that any semi-natural woodland with populations of oceanic bryophyte species is scientifically important, both nationally and internationally. A much more detailed analysis of the processes giving rise to this rich woodland flora is given by Averis (1991) and Hodgetts (1993) has a good account of the species involved.
These woodland and ravine communities of bryophytes have a better representation further south than Assynt, with woodlands or larger size, a greater diversity of species and more rarities. However, some sites in Assynt are extremely rich in oceanic species and are of international importance. Outside of the woodlands, there is one oceanic community that has some of its best Scottish sites in Assynt and in similar areas in West Ross just to the south. This is the liverwort community under ericaceous shrubs which has been variously described as ‘oceanic’ heath, ‘oceanic-montane’ heath, liverwort-rich heath or the ‘mixed Northern Atlantic hepatic mat’ (Ratcliffe 1968). This community is described more fully below but, essentially, it is a community of rocky slopes with a north or north-easterly aspect, consisting of Calluna vulgaris or, at higher altitudes, Vaccinium myrtillus, and with a bryophyte layer below which includes a number of large, leafy liverworts with an extremely restricted and disjunct global distribution. The best sites for this community, in the bigger hills in the east of Assynt, are, again, of international importance.
Averis, A.B.G., 1991. A survey of the bryophytes of 448 woodlands in the Scottish Highlands. Unpublished report for the Nature Conservancy Council, Edinburgh.
Hodgetts, N.G.,1993. Atlantic bryophytes of the western seaboard. British Wildlife 4, 287-295.
Johnstone, G.S. and Mykura, W., 1989. British regional geology: the Northern Highlands of Scotland. 4th edition. H.M.S.O.: British Geological Survey.
Page, C.N., 1982. The ferns of Britain and Ireland. Cambridge: C.U.P.
Ratcliffe, D.A., 1968. An ecological account of Atlantic bryophytes in the British Isles. New Phytologist 67, 365-439.