Along the wet, mild western edge of Britain, in the sea-facing valleys of Devon and Cornwall, the sessile oakwoods of Wales, the wooded gorges of the Lake District and the Atlantic glens of western Scotland, there survives a kind of woodland most people would never think to call a rainforest. Yet rainforest is the right word, and moss is the making of it.
What makes a wood a rainforest
A rainforest is defined less by heat than by the constancy of its moisture. Where the climate is oceanic, wrung wet by the prevailing winds off the Atlantic, mild in winter, cool in summer and humid the year round, woodland takes on a character quite unlike a dry lowland copse. Rain falls on more days than not, mist settles in the valleys, and the air rarely dries out enough to check the plants that drink straight from it. Foresters and ecologists call this Atlantic woodland or, increasingly, temperate rainforest; in Scotland it goes by the name of Celtic rainforest. Its signature is not a towering canopy but the sheer depth of life growing on every surface within it.
Why the bryophytes run riot
Constant humidity is exactly what mosses and liverworts, plants with no roots and no waterproof skin, have waited their whole evolutionary history for. In these woods they no longer huddle in the few damp refuges they hold elsewhere; they take charge. Trunks are sleeved to head height and beyond, boughs sag under green cushions, fallen logs disappear beneath them, and the great granite or schist boulders on the floor become mossy hummocks you could sink a hand into. Ferns join the crowd, the little translucent filmy ferns rooting in wet moss on the rock and polypody perched along the branches. The whole three-dimensional surface of the wood is clothed and alive, and that is what lends a rainforest its dripping, upholstered feel.
The Atlantic specialists
Much of what grows here grows almost nowhere else. Britain and Ireland, jutting out into a mild wet ocean, carry a large share of Europe's Atlantic woodland and with it a suite of "oceanic" mosses and liverworts whose world range is narrow and whose thirst for unbroken humidity is absolute. Robust leafy liverworts such as Bazzania and the scented Plagiochila mat the boulders and trunks beside mosses like Isothecium, Thuidium and the glossy Hylocomium of the ground layer, while lichens drape the twigs in grey-green beards. For a plant so often overlooked, moss here becomes the defining vegetation of the wood, and the desiccation-tolerant tricks that let it endure on dry stone are, in this soft climate, barely called upon. The liverworts, moss's quieter cousins introduced in liverworts, reach their British headquarters in exactly these woods.
Where the fragments remain
Once these woods ran in a broad band down the whole Atlantic fringe; today they cling on in fragments. The dwarf oaks of Wistman's Wood on Dartmoor, wind-stunted and boulder-strewn, are the picture-book example, though there is finer and larger rainforest in the oakwoods of Meirionnydd in Wales, in Borrowdale in the Lake District, and above all along the western Scottish seaboard from Argyll to Sunart, with a last great stronghold across the water in the woods of Killarney. Taken together they amount to a small and scattered acreage, which makes each surviving patch matter out of all proportion to its size.
Under pressure
Green and lush as they look, these woods are fragile in ways easy to miss. Overgrazing by sheep and deer strips out the seedlings, so the canopy ages without ever replacing itself. Rhododendron, escaped from Victorian gardens, throws so dense a shade that the bryophytes beneath it die away entirely. Nitrogen carried on the wind from farming and traffic slowly enriches ground these specialists need to stay poor, tipping the balance towards coarse grasses and a handful of weedy mosses. Add the drying and warming of the climate, and the steady humidity on which the whole system leans begins to waver. The encouraging news is a growing recognition of what these woods actually are, with restoration, fencing and rhododendron clearance now under way at many of the surviving sites.
Walking in one
If you can reach a fragment, go quietly and look closely. Kneel by a boulder and the moss resolves into a dozen distinct plants, each with its own texture and habit; the world going on within that green is the same teeming one described in the hidden world in a moss cushion, only vastly richer. Keep to the paths and off the cushions, since a mossy boulder recovers slowly from boots, and take nothing away, for these are among the last places where our commonest plant becomes something genuinely rare. The epiphytic sleeve you admire on the trunks, harmless to the trees that carry it, is the same lodger examined in moss on trees, here grown to a splendour it reaches nowhere else in these islands.