Vegetation

Maritime Communities

The major stands of salt-influenced vegetation occur on Ruaival, and some parts of the Cambir, with smaller areas elsewhere along St Kilda’s shores. These swards tend to be poor in species, sea plantains forming dense mats, with short sea pink and the salt-tolerant fescue grass. A non-tussocky sea pink community extends 50m inland from the Dun Gap, and in places reaches almost to the summit of Ruaival because of the salt spray from prevailing storms. On the more sheltered east face of Ruaival the vegetation is more varied with some bog pimpernel, eyebrights (Euphrasia spp.), moss campion and tormentil. In gullies where sea foam can become particularly concentrated common bent grass thrives. The Cambir communities suffer less exposure to salt spray and have better plant cover. These areas accessible to sheep are heavily grazed in summer but the exposure prevents much dung enrichment. Nonetheless biotic species such as white clover and meadow buttercup establish. Where Atlantic puffins and northern fulmars are present fescue assumes a more tussocky growth. Glen Bay is an area much favoured by roosting gulls, and the short fescue sward has less plantain but more meadow grass and common bent grass.

The sea cliffs have a characteristic but species-poor vegetation where typical seaside plants like sea campion and sea pink grow together with luxuriant versions of familiar land plants such as common sorrel, spear thistle, devil’s bit scabious and the grass, Yorkshire fog. Ferns are a conspicuous feature in some places, e.g.
Polypodium, Athyrium, buckler fern and spleenwort, along with umbellifers such as lovage and angelica. Absence of sheep grazing and fertilisation by northern fulmars are important ecological factors; roseroot is often found growing out of fissures above these enriched areas. The ungrazed island of Dun shows the extreme form of bird-dominated vegetation with very rank sorrel and Yorkshire fog dominating the Atlantic puffin colonies, together with lesser celandine, pinks, campion and waist-high angelica and scentless mayweed.

The high salt content of the strong winds favours salt-tolerant or halophytic mosses, the high humidity throughout the year reducing the risk of their desiccation. Halophytes tend to be the only species present on the smaller, spray-washed islands and stacks, but the presence of two salt-tolerant mosses Schistidium maritimum and Ulota phyllantha on the summit of Conachair reflects the totality of the sea spray effect. The most striking feature of the bryophyte flora is the abundance of the extreme oceanic and salt-tolerant liverwort Frullania teneriffae all over the island. The absence of lime-rich rocks and woody vegetation excludes many bryophytes but, with a hill 430m high and two deep valleys, Hirta affords a considerable variety of bryophyte habitats for an island of its size; it boasts a total of 104 moss and 56 liverwort species.

Similarly, for a remote Scottish island, St Kilda is unusually rich in lichens (194 species). The altitude of its sea cliffs enables lichens that dislike salt to persist in sheltered places near the top. Indeed the upper reaches of the cliffs are striking in supporting so many large foliose or bushy lichens, which over most of their British range are epiphytic on other plants; here on St Kilda they can spread on to rock faces. Lichens achieve their maximum development on large, guano-splattered boulders in the seabird colonies such as Dun and Carn Mor, with up to 25 species on a single, well-manured boulder. Here they are less affected by shade, verticality and run-off that impoverishes the lichen flora of cliff nesting sites. The rare lichen Lecanora straminea has even colonised the aluminium body of the crashed Wellington bomber on Soay.

Grasslands

The inbye grasslands, within the Head Dyke around the Village area, lie on relatively fertile, well-drained acid soils. The most important grass species in order of abundance are common bent grass, Yorkshire fog and fescue, with lesser amounts of sweet vernal grass, velvet bent grass, meadow grass and rye grass. The most conspicuous herbs – again in order of abundance – are white clover, common mouse-ear, plantain and common dog violet. The most notable feature of these grasslands is their reduced species-richness.

Sea pinkThe sward is tussocky with the taller tussocks made up by common bent grass and Yorkshire fog, while the close-cropped gaps are dominated by clover, creeping common bent grass, meadow grass, fescue, sweet vernal grass and bryophytes. Trampling and grazing favours some species such as Yorkshire fog, meadow grass, mouse-ear, blinks, meadow buttercup, sorrel and clover but in the most intensively grazed situations only common bent grass and fescue tend to persist. A similar but less tussocky vegetation occurs around the enclosures in An Lag and on the sheltered eastern slopes of the Cambir.

The most extreme form of this grassland has been called ‘lair flora’, behind walls and around the entrance of cleitean, wherever sheep persistently seek shelter and leave copious dung and urine. A dense and vigorous growth of Yorkshire fog and meadow grass results, with the herbs mouse-ear, celandine and clover. Where northern fulmar guano is added, such as on Oiseval, on the north-west face of the Cambir and the north-west face of Ard Uachdarachd, sorrel becomes particularly abundant.

A rich ground lichen assemblage occurs all over this heavily grazed maritime grassland, especially between the 30 and 40m contours. The mass of dead vegetation decaying slowly in the salty conditions together with a long history of sheep grazing ensure an open, close-cropped sward that may contain up to 12 lichen species per square metre; with Solenopsora vulturiensis at its northernmost limit in Britain. Many of these species favour calcareous soils so must owe their presence on St Kilda to the salt spray imparting a soil pH from 5.8 to 6.2.

Common bent grass/fescue grasslands cover much of the interior of Hirta with the poorest assortment of species in the centre of the island where mat-grass or fescue becomes important. Another association has more moor-grass and there are a few patches that are more species-rich. A final type where woodrush thrives is to be found on and near the summits of Conachair, Mullach Bi and Oiseval. Sphagnum mosses occur widely in the wetter grasslands.

Flatter ground tends to be marshy and characterised by Sphagnum and moor grass with pondweeds (Potamageton spp.) and cotton grass, deer grass and rushes (Juncus spp.). The principal flowers are bog asphodel, bog pimpernel and the insectivorous sundew, with a variety of mosses. One such area between the summits of Mullach Mor and Conachair is a mix of peat hags and cotton grass where great skuas nest.

Heathlands

The lower, drier slopes of the hills are dominated by short, tussocky heathland of ling heather and occasional patches of crowberry, but bell heather tends to be sparse. The grasses are low, clump-forming species such as sheep’s fescue and tufted hair-grass or low, unpalatable grasses such as mat-grass, with a variety of typical flowers and mosses. Mat-grass becomes even more dominant over the clifftops and south-facing slopes of Hirta. The barren, wind-swept summits of the island have a low sward containing several interesting species including St Kilda’s only ‘tree’ – the dwarf willow, only centimetres high. The vegetation is distinctly patchy with clumps of stiff sedge, woodrush and mosses, and with extensive areas of bare, stony ground holding some interesting lichens.

In recent decades there has been a notable spread of ling heather and the decrease of mat-grass on moorland areas of St Kilda. The islanders’ blackface sheep would not have been allowed within the Village area in summer but the unmanaged Soay sheep have freedom to roam throughout the year. This will have reduced the pressure on the hill ground to favour ling heather, which being kept short and woody by the wind will not be particularly palatable anyway. Reduced grazing may be encouraging common bent grass and fescue to recover at the expense of mat-grass. It is possible that bracken may also be spreading, or at least becoming more obvious, around the Village.

The major plant community on Boreray is similar to the Yorkshire fog/common bent grass grasslands of the Village area on Hirta. Its vegetation is a lot more uniform than that of Hirta perhaps due to its simpler topography and geology, while the remarkable high densities of both sheep and seabirds seem to have affected the whole island; its two opposite faces are surprisingly similar. Although Soay is also grazed it is more varied with more grasses but less ling heather and crowberry than equivalent communities on Hirta. Its southeastern slope in particular is much influenced by burrowing Atlantic puffins, having a fringe of plantain sward near the sea.