Several years ago I found myself passing time at a sheep fank at the foot of Shiehallion. It was early July, the morning skies were blue, and the car park at the foot of the munro had already filled up. Walkers would stop at the fank and ask, “I am looking for lichens, look at the colour of this little one”, and I would continue exploring the dykes until the next family would stop. A young meadow pipit seemed unconcerned about my slow progress along the dykes. This was my happy place – one so very different from the granite dykes I was used to exploring in Deeside. This sheep fank was a treasure trove with many lichen species I had not – or rarely – seen before.
The sides of the dykes were covered in crustose lichens like Acarospora cervina, Protoblastenia rupestris, Verrucaria nigrescens, Aspicilia contorta, Pertusaria corallina and Tremolicia atrata. Among the mosses on top of the dykes Cladonia species like Cladonia furcata had made themselves at home. The rocks exposed on the slopes of Shiehallion are a mosaic of limestone, quartzite and schist and the lichen species I was finding suggest that each of these rock types was used to construct the fank. An unimaginable long time ago, these rocks were mollusc shells, sand and mud at the bottom on a seabed, and since then, they have become metamorphosed, folded and upturned to arise right here, at what we know as the slopes of Shiehallion.
In Scotland, limestone exposures are a bit of a specialty, and many of them occur in the Breadalbane Mountains that Shiehallion is part of. Limestone areas are not only important for the unique flora associated with it, many of the lichens species found on limestone are rare as well. Acaraspora cervina, Protoblastenia rupestris and Aspicilia contorta are clear indicators of the calcareous conditions created by the shells from which limestone is formed.
In fact, the limestone, quartzite and schist used to build the sheep fank each have their own chemical composition, texture and water holding capacity. Species like Lecanora polytropa, Lecanora intricata, Fuscidea cyathoides and Tremolecia atrata suggest the presence of acidic and metal-rich rock such as schist. One of the first questions someone studying lichens will ask is whether the rock the lichen attaches to is basic or acidic. Lacking roots and a vascular system to absorb and transport water and nutrients within their bodies, lichens absorb water across their entire thallus and any nutrients they need will have to be dissolved in that water. Though the crustose lichens on the side of the boulders are attached to the rock with their entire lower body, lichens are only able to absorb water and any nutrients dissolved in it from runoff that flows across the upper side of their body. Likewise, lichens that grow on the flattish top of boulders derive their nutrients from the thin film of water that will gather on flattish rock surfaces.
When geological processes expose rock at the surface of the earth, lichens are among the first species groups to colonise their surface, paving the way for mosses and eventually vascular plants. Even for lichens rock is a harsh environment to live on as it fully exposes them to scorching sun and drying winds with no protection whatsoever. And soon after the lichens arrive, the invertebrates that graze lichens will do too. For some lichen species this herbivory by slugs and snails may be so heavy that they regenerate endolithically, immersed in the rock.
Among the first colonists of recently exposed rock, lichens immediately set to work to break down the surface they are living on. On top of dykes you will often find thin sheaths of parched soil that flake off. Look carefully and you will find that these consist of organic matter mixed with tiny rock particles. Though there are other causes of rock weathering too, lichens play an important role in the breakdown of rock. While some lichens grow entirely immersed in rock, the hyphae of crustose lichens that grow on top of rock may still penetrate the surface several millimetres. The hyphae that thread between rock particles will swell and shrink in response to humidity and temperature, eventually contributing to the physical breakdown of the rock surface and soil formation.
If not interfered with, the soil layer on the sheep fank will build up to allow heath and grasses, and eventually trees to take root. The crustose lichens that are now covering the sides of the sheep fank will become smothered by vegetation. Where soil enters streams, water will flush the rock particles downstream, eventually disposing of them on the seabed where sediments, over time, will start a new phase in the cycle of rock formation. It is hard to believe that lichens, small as they are, play a significant role in processes that take place across geological time scales.
If you are interested in learning more about the lichens found near Shiehallion, I’ll be giving a guided walk at the adjacent Dun Coillich for the Highland Perthshire Communities Land Trust on Friday 11 August from 10 am to 1 pm. For more information, and to sign up, please click here.
Copyright text and images Petra Vergunst
scottishlichens.co.uk 