Important habitats for lichens
The Celtic rain forest
The west of Scotland holds very wet and mossy, oak and hazel woods which are internationally important for their lichens, mosses and liverworts. Their higher plant flora is not particularly special. It is the epiphytes (plants which grow on other plants without being parasitic) clothing the trees that are exceptional. The woods are best developed along the sides of sheltered sea lochs such as Sunart, Shiel and Moidart. Some of these woods are so remote that they still have not been fully explored for lichens.
It is quite an experience to visit these Celtic rain forests. The trees are of only a moderate size and the first impression is of how mossy the habitat is - a deep blanket of moss covers the floor, climbs the tree trunks, festoons horizontal boughs, and even extends into the canopy. By absorbing rain and mist, then releasing this moisture slowly, the blanket of mosses and liverworts helps maintain the very high humidity. The moss forms the base on which the rare lichen communities are found, especially one called the 'Lobarion' after its most conspicuous family. There are four species of lungworts (Lobaria) that can grow to the size of dinner plates. Growing with these are lead-grey Pannaria, Parmeliellaand Degeliaspecies, dark Stictas and jelly lichens, Biatora sphaeroideswith pink fruits and Dimerella luteawith bright yellow ones. The trees often have a skirt of dog lichen (Peltigera) which has abundant shield-like, chestnut-coloured fruit bodies. The weight of the lichen cover eventually becomes so great that the moss peels off the trees, leaving heaps of these internationally rare lichens around their base.
You do not have to know the names of the lichens to realise that you are somewhere very special.
The best lichen sites are on sunny, south-facing slopes while the rare Atlantic mosses and liverworts, which need more moisture, favour northern aspects. The optimal sites for lichens are also well lit being around the edge of glades or along woodland margins.
While the whole area is a lichenologists’ Mecca, sites where burns and gorges cut through the woodland are particularly special. Here, in ravines full of tangled vegetation, there is a wider range of trees - oak together with ash, elm, cherry and willows. Ash and hazel have a more alkaline bark and are a home to 'jewels' such as the handsome Specklebellies (Pseudocyphellariaspecies) and Blackberries and Custard (Parmentaria chilense).
On higher ground the oak woods give way to grazed birch woodland which is a different, but equally distinctive, lichen-rich variant of Celtic rain forest. It is dominated by silvery-grey leafy species many of which grow directly on tree bark. This bark has a high acidity due to 'washing out' or leaching by the high rainfall.
These western woods are far from being undisturbed relics of the ancient forest. In times past, these woods were extensively coppiced, and the wood used for charcoal, tanning and bobbin-making. This activity ceased about 150 years ago, but it is thought there was always sufficient wildwood remaining for the entire suite of rare lichens to survive. Today the best examples of these woodlands are managed for nature conservation.
It is a myth that the Scottish Highlands were once covered by a vast pine wood called 'The Great Wood of Caledon'. Pollen analysis tells us that pine has always been patchily distributed. The native pinewoods are the surviving fragments of the ancient Caledonian pine forests. Today pinewoods cover an area of around only 10,000 hectares. Some of their strongholds are Glen Affric, the Black Wood of Rannoch, Rothiemurchus, and hill slopes beside Loch Maree. The woods are composed of Scots pine Pinus sylvaticasubspecies scotica. This is restricted to Scotland, has short needles and maintains a pyramidal form until late in life when it becomes round-headed.
These woods have so far yielded more than 430 epiphytic lichens of which 222 have been found on pine. The rest are on associated rowan, birch and juniper. Seventeen of these lichens are found nowhere else in the British Isles so this is another particularly Scottish community.
Lichen cover is most often best developed on ancient 'granny pines', in sheltered situations. Here the bark plates are welded together by thick, multi-coloured lichen crusts, scrambling leaf-like species and festoons of yellow-grey Usneaand horse-hair brown Bryoria. The rare Witches’ Hair (Alectoria sarmentosa) occurs in spectacular swathes on the lower branches of pine in valleys in the west.
A major difference from deciduous forests is that a high proportion of the lichens are found on stumps and standing dead trees. This habitat may be very sparse, less than one tree per hectare, but is of the utmost importance as slowly decaying pines that have lost their bark hold many of the pinewood specialities. These standing pillars of dead wood, and recently fallen ones, are picturesquely known as the 'bones of the forest'. Pinhead lichens looking exactly like their name, often form swards on sheltered parts of the 'bones'. Being small it is often easiest to locate them by manoeuvring so that a patch of bark is seen against the sky. If present, they can be seen in silhouette. Up to eight types of Pinhead lichen may be present and can be distinguished by their colours - green, yellow, rusty, grey, black or brown.
Hazel forms a lower storey in oak woods or is a hedgerow shrub growing throughout most of Britain. In the west of Scotland, however, it forms pure woods on steep slopes facing the sea. Hazelwoods have probably occupied these sites for 9,000 years, the extreme exposure preventing invasion and over-topping by other trees. Hazel enjoys the benefit of being able to persist indefinitely under grazing and has a partiality for soils developed over basalt that are widespread along the coast around Oban and on the islands of Mull, Eigg and Skye.
The lichen interest of these pure hazelwoods only started to be recognised in the late 1970s when Brian Coppins, lichenologist at the Royal Botanic Garden, Edinburgh, realised that they contained many distinctive species, some new to Britain, others new to science. An example is the White Script lichen (Graphis alboscripta) which has so far not been found outside this habitat and is regarded as a Scottish endemic.
On entering a hazelwood the 'brush' of thin stems that make up a high proportion of each individual can be seen to be covered with a dense, pale, mosaic of small lichens which give the bark a silvery hue. These can be distinguished by subtle shades of colour and the small dots, scribbles and tiny 'volcanic craters' through which their spores are ejected.
High-quality, old-growth hazelwoods contain trunks of all sizes including very large individuals with rough moss-covered bark which are around 70-100 years old. These support a very different range of lichens to those found on the smooth thin stems. Here large leafy species of the 'Lobarion' are dominant. Despite the woods having the appearance of a past history of coppice management this is not the case. On Eigg, Cleadale Woods were last exploited during the 1940s when a wartime coal shortage led the crofters to cut the larger stems for firewood.
The best season to explore this unique habitat is late spring, before the interior becomes shaded by the expanding leaves. At this time the woods are bright with primroses, violets and bluebells, sunlight filters through the branches, and birdsong is at its height. To investigate a lichen flora of international importance against this background is magical and a particularly Scottish experience.
Only a tiny fraction of Scotland is over 914 metres high (or 3,000 feet if you are a Munroist) but this small area, above the upper limit of heather, is home to a vegetation dominated by lichen, moss and sedge heath. This is the nearest we have to tundra in Britain. The Scottish montane lichen flora comprises about 400 species and includes nearly a third (52) of the British Red List lichens so it is very special (see p34). It is possible to walk all day in the hills and encounter only common upland species. The rarities have particular habitat requirements that need to be appreciated and some of these are outlined.
The Cairngorm Plateau is the place to study lichens associated with late snow patches. These species are left-overs from the last Ice Age. The largest patches are almost permanent, melting on average once every 25 years. The lichens are concentrically-zoned around them depending on their tolerance to snow lie; the inner-most ones can survive years of burial. Ice-cold melt-water streams issuing from the toe of the snow patches are also good lichen-hunting grounds. Lichens form a skin over the soft ground or occur on the moss cushions covering small boulders. The handsome, green Lecanora leptacinais absolutely restricted to these sites. Recently moss cushions were found supporting an undescribed lichen with tiny reddish fruits and the discoverer, Alan Fryday named it in honour of his young daughter, Amelia.
A curious factor, which restricts the abundance of lichens on the granite boulders that litter the summit plateau, is abrasion from windblown gravel and needle-like ice crystals which blast and erode during winter blizzards. Several of the rarest lichens of this high 'tundra' have not been seen for over 20 years and are suspected of being casualties of global warming.
In contrast, Ben Nevis in the damper, milder west, supports a somewhat different alpine lichen flora. A species more common here than elsewhere is Catolechia wahlenbergii. This elegant lichen forms vivid yellow rosettes in dark crevices. It is protected under Schedule 8 of the Wildlife and Countryside Act 1981, as amended, and therefore has been given an English name. After some discussion it was named 'Goblin lights'.
As Ben Nevis is the highest point in Britain it was hoped that a recent survey of the summit area would produce at least one special lichen. We were not disappointed and in 1990 the first UK record of the suitably named Staurothele arctica was made a few yards from the summit cairn.
A feature of these western hills that sets them apart from those in the east is that lichens which grow on the ground are rare. Instead, carpets of the Woolly Hair Moss (Racomitrium lanuginosum) form an undulating mattress over the ground. The distinctive lichens of the western Highlands mostly grow on rock.
The mountains considered so far have all been acid, which is the norm for the Highlands. But the foremost areas for arctic-alpines, flowering plants, mosses, and lichens alike, are those where calcareous rocks occur. The outstanding locality for this feature is the Breadalbane range in Perthshire. Here extensive deposits of soft, calcareous schist outcrop over a distance of 11.2 kilometres reaching a height of 1219 metres at one point. For two centuries botanists have been making pilgrimages to this area and, though the higher plants are now well-known, the lichens, which are twice as abundant, are still being investigated.
Calcareous outcrops on these mountains are restricted and often stumbled on by chance. But once such a site has been located it pays to spend several hours with hand lens and kneeling mat examining every 'square inch' of its surface. The rewards are great, ledges are crowded with rare and beautiful species that scramble over rock, soil, dead vegetation and moss. The communities are reminiscent of a minature rock garden and they have probably been growing undisturbed since the end of the Ice Age.
It is not just the outcrops that hold arctic-alpine lichens. The lichens extend into burns, lochans, summit grasslands and bare, gravelly areas. There is a lifetime of investigation to be undertaken. It is every lichenologist's dream to discover a new 'Ben Lawers'. Just one such area has come to light in recent decades - 40 kilometres away in the remote Ben Alder range.
Several features combine to make the Scottish coast of special interest to lichenologists. The bird islands of St Kilda, North Rona and the Flannans; the machair; and the vast, east coast sand dunes deserve special mention.
The lichens produce distinct colour bands on coastal rocks:
- • A black zone at sea level, above that,
- • an orange zone heavily influenced by salt spray, and then,
- • an upper, grey zone which is richest in lichens.
Sea bird cities
Beyond the Outer Hebrides lies an arc of small remote islands known as the Furthest Hebrides. The islands are lapped by the waters of the Arctic Ocean, so sea temperatures remain low even in summer. These islands are home to millions of seabirds, their nests line the cliffs, the air is full of their cries, and they perch on every rock covering them with their droppings. It is these guano-spattered boulders that are special for the lichens. Each can support up to 25 species including such rarities as the golden-coloured Caloplaca scopularis, the brown Amandinea coniopsand the sulphur-tinted Lecanora straminea. This is the main British habitat for these three species.
Around these remote, storm-lashed islands the coloured bands are uplifted so the black zone on the west coast of North Rona extends vertically to 16 metres above sea level, compared with a sheltered shore on the mainland, where it might be only one metre deep.
A typical lichen of the grey zone is Sea Ivory Ramalina siliquosawhich forms dense shaggy swards in places exposed to the wind. On St Kilda, the agile Soay sheep supplement their diet by grazing on these swards during winter.
Machair is a habitat unique to the west coasts of Scotland and Ireland. It develops where calcareous shell-sand blows inland converting the wet, acid, peaty landscape into a well-drained, flowery pasture. It is a feature of the west coast of the Inner and Outer Hebrides with islands such as Coll, Tiree, South Uist and Harris supporting the finest examples.
The short, species-rich turf is sufficiently open to support a lichen community that has many species in common with chalk grassland. The richest sites are associated with low, partially sand-covered outcrops where crevice-loving species such as the socket lichens (Solorina saccata, S.spongiosa) and the apple-green Thrushwort (Peltigera leucophlebia)can find a foothold. Early summer is the time to explore the machair. The flowers are out, blue butterflies are on the wing, white surf lines the shore and the lichens - well, who knows what you will find?
East coast dunes
Sandy beaches backed by dunes are a characteristic feature of Scotland's maritime scenery. The largest of these are on the east coast at Culbin, Forvie and Tentsmuir. These vast, remote dunes are exciting places where the lack of grazing, trampling and fire has resulted in the lichen swards developing an unusual luxuriance. Reindeer lichens, for example, form a deep, extensive white carpet spreading into the conifers planted many years ago, and suppressing all other growth.
Over 30 species of the lichen genus Cladonia have been recorded at Culbin, including a northern element better represented here than anywhere else in Britain.
A number of the ground-dwelling lichens, favoured by disturbance, are found along the edge of forestry tracks.
At Cuthill Links, on the north side of the Dornoch Firth, the lichen interest is concentrated on sandy valley floors separated by gorse-covered dune ridges. Here is the best place to go to see the curious phenomenon whereby normally alpine species descend to sea-level in the far north of Scotland. Lichens typical of mountain summits, such as Thamnolia vermicularis, which looks like a cluster of white worms, are found here. They grow luxuriantly on the sand within sight and sound of the sea. These east coast dune lichen communities have links with those on the Danish coast.
Most level ground in the west of Scotland is naturally covered with a deep layer of peat, known as blanket bog. The surface vegetation, a mixture of Bog Moss (Sphagnum), various types of heather and cotton-grass, is very wet - too wet for most lichens. Lichens find an opening, however, where there has been disturbance perhaps associated with peat cutting, drainage or fire. Drier faces, ridges and baulks of peat are rapidly colonised by a range of attractive species. Particularly noticeable are a suite of red-fruited Cladoniaspecies with common names such as Scarlet cups, British soldiers or Bengal matches. Others have brown fruits and are branched like deer's antlers, coral, or have fantastic 'Disney-world' shapes. Another series, the cup-lichens, again Cladoniaspecies, have fruit bodies resembling trumpets or goblets.
These moorland lichens have attracted the attention of poets. Wordsworth wrote of them:
'Ah me what lovely tints are these,
Of olive, green, and scarlet bright!
In spikes and branches and in stars,
Green, red, and partly white.'
Though Cladonia species are dominant, and the first to attract attention, others are equally attractive. Very wet peat is the habitat of Pycnothelia papillaria, the thallus (plant body) of which has been compared to babies' molar teeth, and it is always a thrill to come across the large coral-pink fruits of the Heath Lichen (Icmadophila ericetorum).