Lichens

Lichens at Dun Coillich

Lichens are unique living organisms formed by a mutually beneficial partnership between a fungus and a photosynthetic alga or cyanobacterium. The fungus provides structure, moisture retention, and a secure attachment to surfaces, while the alga produces food through photosynthesis to sustain both partners. Together they form a distinct body with properties neither could achieve alone, capable of growing on bare rock, bark, or other surfaces where no higher plant can survive. Over time, they contribute to the gradual breakdown of rock, releasing minerals and building the earliest layers of soil — a critical process in ecological succession that prepares the ground for mosses, ferns, and eventually flowering plants and trees.

At Dùn Coillich, a combination of clean air, varied geology, and mixed habitats supports a rich diversity of lichens. Along the Goulandie and Allt Mhòr burns, older birch trees provide smooth, slightly acidic bark that favours pale greenish-grey shield lichens (Parmelia species) and crustose map lichens, which form thin, patterned patches following the trunk’s surface. A single crab apple tree hosts a distinct community adapted to its harder, more neutral bark. Planted young oaks, part of habitat restoration work, are already being colonised by early-stage, adaptable lichen species, which will be replaced by slower-growing, specialised forms as the trees mature. Scots pine branches support pale, thread-like beard lichen (Usnea), which hangs in fine strands, and shrubby, pale green oakmoss (Evernia prunastri); both are sensitive to air pollution and their presence indicates very clean atmospheric conditions.

The geology of Dùn Coillich strongly influences the distribution and appearance of rock-dwelling lichens. The dominant rock type is schist — hard, fissured, and generally acidic — which supports orange sunburst lichen (Xanthoria), dark crustose species, and various grey-green leafy forms. Significant outcrops of Dalradian limestone occur in places; this calcium-rich rock supports a distinct community of pale and white crustose lichens, their light colouration likely reflecting the incorporation of calcium compounds from the substrate. Small areas of Schiehallion Boulder Bed, composed of harder, quartz-rich rock, host a different set of acid-tolerant species. Large glacial erratics, left behind by retreating ice, are scattered across the site; these boulders provide stable, long-term surfaces that support particularly diverse lichen communities, often hosting crustose, leafy, and shrubby forms growing together.

Different lichen forms serve different ecological roles: crustose species, tightly bonded to rock or bark, are highly resistant to drying and temperature change; foliose, leafy forms are more efficient at trapping moisture and dust; and fruticose, shrubby or thread-like types maximise surface area for nutrient uptake from the air. All contribute to nutrient cycling, particularly by fixing atmospheric nitrogen and accumulating minerals from dust and rain. They also provide food, shelter, and hunting grounds for a wide range of micro-invertebrates, including mites, springtails, and small beetles, which in turn support larger animals further up the food chain. As they have no true roots and absorb substances across their entire surface, lichens are highly sensitive to atmospheric pollutants such as sulphur dioxide and nitrogen compounds, making their diversity a useful indicator of good air quality. Their slow growth rate — often less than one millimetre per year for crustose species — means that many of the larger patches on rock surfaces are decades or centuries old, representing a long-term biological record of the site.