Rock-hugging marvels

In late November the Atlantic oakwoods of the west have taken on a stoical, contemplative mood. They survive by keeping their heads down, tucked in behind bare forelands, or at the back of mountains. Now the gales have whipped away their canopies, the sun can finger the full length of mossy trunks and search the shadows for forgotten fungi: little orbs and chalices corroding among the fallen leaves.

In this setting, well on the way towards the minimal and melancholy, the rude good health of lichens is suddenly apparent.

Wreaths of Lobaria pulmonaria, tree lungwort, perk up to a full, leafy greenness in the extra wash of rain (the name comes from the wrinkled diagram of this lichen's surface; appropriately, it flourishes only in the purest of air). There's an extra spring and curl to the long, grey-green beards of Us- nea, a new shine to the crusts and rosettes of other lichens that share its unearthly but beautiful colour.

The acidic bark of oak and birch, well drenched in showers, makes a prime habitat for lichens. In Shannawoneen Wood, north of Spiddal in Connemara, no fewer than 56 kinds clothe the old trees, right to the smallest twigs.

But lichens are everywhere that unpolluted air will let them grow. From the black, tar-like Verrucaria that covers our spraywet coastal rocks, to crusted mosaics on mountain summits, Ireland provides the conditions for upwards of 900 species, a total hugely boosted by the fieldwork of recent decades. About half of these can be found around the Connemara National Park alone. And just one big boulder in the Burren, well-fertilised with birddroppings, may have 50 kinds to show, among them the brilliant yellow-orange Xanthoria, pinned to the rock like some imperial decoration.

The big botanical discovery about lichens, historically, was their duality - the plant is really a fungus living with a microscopic alga. Now it is known that the relationship can sometimes involve three or even more partners, creating a miniature ecosystem all its own.

The main substance is the fungus, which cannot manufacture its own food, or live alone. Carbohydrate is supplied by a photosynthetic partner, a green alga. And nitrogen can be fixed from the air by a cyanobacterium (the bluegreen alga). The algal partners benefit from a stable and durable home, filtered of harmful radiation. Most lichens grow slowly, some by less than a millimetre a year, and they can endure for centuries on a tree or rock, wall or gravestone.

A lichen has no roots like those of flowering plants, nor a protective cuticle to help it store water. Its metabolism switches on within minutes when moistened by rain or dew, and off again when the tissues are dry and crisp. This "skinless" state is what makes it so vulnerable to aerial chemical pollution.

A decade ago, Ireland was the scene of important studies relating the amount of sodium dioxide (SO2) in the air to the distribution of lichens. In Dublin, a team headed by Eanna Ni Lamhna and helped by senior schoolchildren surveyed the lichens of more than 2,000 trees. Compared with European studies, Irish lichens seemed vulnerable to lower levels of SO2 - perhaps because of extra rainfall which boosts their activity.

On the mountains, in the purest of air, the incessant nibbling of sheep is the most obvious threat, and one that now confines many ground-hugging mountain lichens to the last rocky ledges. Ireland also has the famous reindeer lichen, Cladonia rangiferina, and 10,000 years or so ago there were reindeer to eat it. Today it survives as rarity on coastal turf in the north and west.

Cladonia lichens belong mostly to the drier parts of bogs. One big group, the cladinas, have no flat lobes or base but grow vigorously in all directions as dense "forests", rather like the tangle of sphagnum mosses they compete with. They also carve out their space by producing a chemical that stops heather seedlings from germinating among them. The bushy Cladonia portentosa, in particular, can form spectacular carpets.

The second group of Cladonia do not spread in thickets but push up cups and fingers so bright and intriguing as to warrant common names for their species. These clusters of fruiting bodies are often a vivid scarlet, suggesting "devil's matches" or "cock's comb lichen". The green "pixie cups" of Cladonia fimbriata, like little golf tees, seem designed to catch rain-drops as a way of spreading spores in the splashes.

An Irish lichenologist, Howard Fox of Athy in Co Kildare, has spent years working out the habitat preferences and strategies of Cladonia species on the bogs. But when he brings the British Lichen Society to Connemara and Joyce country for their spring meeting next April, he will also be hoping to excite them with "acres upon acres of lichen-covered bedrock . . . amazing saxicolous Lobarion . . . countless drystone walls, each with subtle variations", not to mention the oceanic woods and the lichens that infiltrate the dense, mysterious shrubberies of lake islands.

They are almost bound to find something new for the Irish lichen catalogue. But visiting British botanists can sometimes be a bit snooty about our range of plants. "Brandon, the second highest mountain, is the richest site for montane lichens," ran a recent paper, "but in the context of the British Isles it is of only minor importance . . . considerably less rich than Snowdon and considerably exceeded in interest by numerous Scottish hills."

Where we try harder, as it were, is in the sheer abundance and exuberance of the species we do have, in habitats that elsewhere are eroded by pollution, development, farming pressures. But this is far less true than it used to be: bogs, hills, wayside walls, are all degrading or diminishing: a whole fabric of diversity falling into shreds. Lichens may often seem the least of plants - mere smudges upon rock, at times - but where they vanish, a whole swathe of life goes with them.