For anyone used to looking at birds, butterflies or bryophytes, it is hard to perceive that lichens are not one, but two organisms. To put it simple, a lichen is a symbiosis between a fungus species – the mycobiont – and a species of either a green algae or cyanobacterium – the so-called photobiont. As the name suggests, the photobiont is able to photosynthesize and provide sugars (Baron, 1999) to the mycobiont. In return, the mycobiont provides the photobiont protection. Yet, this is only a simplified rendition of this story. The exact nature of the lichen symbiosis is at the forefront of current research and it has been suggested that more than two organisms may be involved, including bacteria. When we see a lichen on a tree or rock, however, we usually only see the fungal partner. This begs the question where the photobiont is found in the thallus and how its presence can be made visible without cutting open the lichen.

When first starting to look at lichens, this symbiosis startled me. For example, when looking at the fruticose Ramalina fraxinea draping from a rowan I was only seeing the exterior of the lichen. In most lichen species the photobiont is hidden within the thallus. In the case of fruticose or foliose lichens, the green algae or cyanobacterium usually sits between the outer layers – or cortex. In the case of crustose lichens, the photobiont is squashed between the fungal cortex (‘skin’) and the bark or rock the lichen is attached to. Only in so-called leprose lichens like Lepraria lobificans, there is no protective outer layer of fungus so that, with a hand lens, the algal cells can be seen scattered between fungal hyphae.

In most lichens, however, the photobiont is hidden behind the cortex. Yet, its presence becomes apparent when it rains or, for so-called chlorolichens that involve a symbiosis with a green algae, air humidity is high, and the lichen is able to absorb water. Lichen species like Physconia distorta, Parmelia sulcata and Ramalina fastigiata will turn distinctively green because the cortex becomes transparent to allow the green algae that sits below to photosynthesise.

In the lichen symbiosis, the fungus is always a unique species that lends the symbiosis its name, while the photobiont species can match with more than one fungal species. There are a number of commonly found photobiont species, some of them only found in the lichen symbiosis. Around 60 % of lichen species involve a symbiosis with a unicellular green algae like Trebouxia, while around 10 % of lichen symbioses involve a cyanobacterium like Nostoc. That leaves roughly 30 % of lichen symbioses to involve a green algae that consists of a chain of cells, like Trentepohlia (Dobson, 2018). Rather confusingly, the green algae Trentepohlia is actually bright orange. As Trentepohlia can also be found living outwith the lichen symbiosis, many people are familiar with it is often found growing on shaded and damp substrata like rock or bark as orange fluff. As Trentepohlia prefers shaded conditions, it is generally found as the photobiont of crustose lichens that grow in shaded conditions as well. As such, it can be found in Anisomeridium polypori, Graphis scripta, Gyalecta jenensis, Lecanactis abietina, Opegrapha atra, Porina aenea, Schismatomma decolorans and Thelotrema lepadinum (Dobson, 2018). One of the commoner lichens in North-East Scotland in which it is found is Arthonia radiata, a lichen that I often find growing in pioneer communities on bark and on rowan. Even though it is not mentioned in Dobson as a diagnostic feature, it can be useful to scratch the surface of some Arthonia radiata as it will reveal the orange or yellowish hue of Trentepohlia below the cortex.

That said, a few lichen species quite literally wear their heart on their sleeve. Responsible for photosynthesis and the production of sugars, the photobiont can be seen as the engine in the symbiosis. In most cases, the fungi in the symbiosis is only able to form a symbiosis with one particular photobiont species, and the lichen then has particular environmental conditions that suits it best. As we have seen in the case of Trentepohlia, lichens containing this photobiont prefer shaded conditions. In a few cases, however, the fungal partner lays its eggs in two baskets, and forms a symbiosis with both a green algae and cyanobacterium. One line of thinking is that this may help with nitrogen metabolism in lichens (Gilbert, 2000), another line of thinking is that it could be a risk management strategy as the different photobionts may function optimally under slightly different environmental conditions. Lichens containing two photobionts may develop wart-like cephalodia. In the genus Placopsis, for example, the thallus itself contains the green algae Trebouxia, while on top of the thallus there are flesh-coloured cephalodia that contain a blue-green cyanobacterium. Though rare, and in Scotland mainly restricted to the genera Lobaria, Placopsis, Solorina and Stereocaulon, the presence of cephalodia at the very least visualises the presence of the photobiont in the lichen symbiosis.

References

Baron, G,. 1999. Understanding lichens. Richmond Publishing, Slough, England.

Dobson, F. S., 2018. Lichens. An illustrated guide to the British and Irish species. The British Lichen Society. Seventh revised edition.

Gilbert, O., 2000. Lichens. HarperCollinsPublishers, London.

Copyright text and images Petra Vergunst

I’ll admit it upfront, I’m a bit of a map addict. I have an obsession with landscape and habitats and can spend hours studying distribution map of birds and butterflies – and, of course, lichens. As I started out, I took those maps as given, a reflection of the real distribution of lichen species in North-East Scotland, but I have increasingly come to question the extent to which these distribution maps reflect the actual distribution of lichens in the field.

The British Lichen Society uses three colours to indicate the date of the most recent record of a species: green for records from between 1650 and 1959, blue for records found between 1960 and 1999, and red for species found since the year 2000. This map of the distribution of Cornicularia normoerica shows records found in each of these periods. Indeed, lichens are long-lived and so it could well be that a species found in 1960 is still present in 2022, but with changes in land use, changing pollution levels and climate changes that is not a given. And it is not always clear whether a hectad (a 10 x 10 km square on an OS map) is blue because it was between 1960 and 1999 that a species was last found there, or because nobody has returned to the area since 2000 to check whether the species is still there. Even more fraught with difficulty is the interpretation of the absence of a coloured square in an area. Does that mean the species genuinely does not occur in the area or that nobody has ever looked for it?

This is what I know has happened in Kincardineshire (VC91), Aberdeenshire, where I have done a significant amount of my recording to date. In the late 1960s and 1970s each of the hectads in this vice county were visited by the famous lichenologist Ursula Duncan and fellow lichenologists. In a systematic manner they visited one site in each hectad, with an apparent preference for the coastline, churchyards and dykes. Since then, the vice county has been visited occasionally by other lichenologists who seemed to have had a preference for the road over the Cairn o’ Mount and through Glen Dye. The only area that has been given more elaborate attention by lichenologists in recent decades was that around Edzell, where the famous gorge, the Rocks of Solitude and the neighbouring estate grounds of The Burn have been surveyed in detail. Marking the exact locations that have been surveyed on a map suggests that no lichenologist ever visited the conifer plantations that cover the northern half of the vice county. I have always felt attracted to these mature conifer plantations, not least because of the hidden stone circles, abandoned farmsteads and pockets of broadleaved woodland these plantations contain. Fifty years ago, Ursula Duncan and co-workers never visited these plantations, which may well be because they had just been planted and presented themselves as oppressive and impenetrable young spruce plantations. Had there been records of that time, my records of the last few years may well have shown increased lichen diversity, reflecting the maturing and subsequent diversification of those plantations. 

The accuracy of distribution maps is not only reliant on whether we visit an area, but also the approach we take to collect records. Perhaps not surprising for a map addict, I am also a keen walker. No, not that kind of walker that need the hills, but one that strolls and rambles local fields and wood several times a week, exploring or revisiting paths, looking at the habitats and trees along on the way, following the dykes that line fields. Some months my rambling searches out stone circles indicated on maps, other months I follow the river Dee to look at aquatic lichens. The lists of lichen species seen while out walking tend to be more diverse and longer than those that I collate when recording in one limited area close to a road. The act of walking often involves walking away from areas easily accessible by car and almost unequivocally allows for an encounter with a wider range of habitats and substrates and hence more lichen species. As I have been studying maps and walking the landscape ever since I moved to North-East Scotland seventeen years ago, I have developed an intimate knowledge of the landscape that is based on a detailed understanding of the habitats and substrates available to lichens. By revisiting hectads again and again, but looking at different habitats and substrata, I have developed a multi-layered picture of the lichens present.

And intimate knowledge of the local landscape pays off. When I explored the branches of a few hazel trees on the slopes above a burn no more than a mile from home, I was surprised to find Hypotrachyna revoluta, a species that only had one or two other records in NE Scotland. On a stone circle and nearby dyke in the conifer plantations near Strachan, I found Mycoblastus sanguinarius, a species that had not been recorded previously in Kincardineshire. Likewise, a dreich Saturday afternoon stroll up the Barmekin of Echt, a hillfort on a low hill surrounded by farmland, revealed Cornicularia normoerica, a species usually found only further west in the hills of Deeside.

When walking my local landscape I often think of the mammoth effort by Pete Martin in Cumbria who, during lockdown, made a detailed record of the lichens in each of the hundred 1 x 1 km squares in the hectad where he lives. And in a quest to update and extend the lichen distribution maps for Cumbria, his fellow members of of the Cumbria Lichen and Bryophyte Group Caz Walker and Chris Cant visit the fells and limestone outcrops in an attempt to refind historic records of upland species. The lichens you find in a landscape are dependent on the substrate available and environmental conditions. What we share is an understanding that changes in land use, pollution levels and changes induced by climate change may bring about changes in the lichens we find around us. Each blue square we turn red indicates continuity, but squares that remains blue, and squares that turns newly red pose questions may trigger questions as to the health of our environment. It is rainy today, a good time to unfold that map.

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If you are interested in lichen recording and would like help with lichen identification, there are a number of useful Facebook groups where you can get help with identification, including Scottish Lichens, Cumbria Lichens and Lichen Spotters UK. In addition to this, the websites Scottish Lichens and Dorset Nature provide photos and species descriptions for selected species. The standard guide used for lichen recording is the 2018 edition of Frank Dobson’s Lichens – An Illustrated Guide to the British and Irish Species. For instructions on where and how to submit lichen records, please see the website of The British Lichen Society.

Copyright text and images Petra Vergunst

To celebrate the Cairngorms Nature BIG Weekend from 13 to 15 May 2022, I was commissioned by the Cairngorms National Park Authority to write a self-guided walk to explore the lichens of the birch and pinewoods at Muir of Dinnet NNR (managed by NatureScot). The walk follows the Parkin’s Moss Trail (3 miles) which passes through birch and pinewoods, dykes and ruined farmsteads, and follows a boardwalk through Parkin’s Moss, famous for its dragon and damselflies in summer.

A species gallery of lichen species found in upland birch and pinewoods, with links to species descriptions, can be found here.

In May, the coastal cliffs of the RSPB reserve at Fowlsheugh are like an overcrowded apartment block that rises from a choppy pavement all the way up to a buzzing sky. Binoculars in hand, I spent quite some time distracted by the squabbles and rumours squashes that unfold beneath me. The volunteer warden looks slightly bemused when I explain that I haven’t come to look at the birds, but have an interest in lichens.

True, it would have been more efficient had I planned this trip outwith the breeding season, but spring is a good time to look for lichens on the clifftop. The vegetation is waist-high, the red campion flowers abundantly and the thrift turns the upper ledges pink. Yet, the buzz of bees and hoverflies, butterflies and moth is completely sounded out by the calls of kittiwakes and fulmars, guillemots and razorbills that seems amplified by the steep rising cliffs. The smell of the excrements that have turned the cliffs white irritates and early on I decide to eat my lunch somewhere else.

With binoculars I scan the side of the cliffs for lichens. Only those rocks that are not topped by a ledge seem to have some yellow cover of what appear to be Xanthoria and Caloplaca species and the texture of Ramalinas. Along the clifftop path there are only few rocky ledges, and the vegetation needs to be bent back to reveal some of the Anaptychia runcinata, Diploicia canescens, Lecanora campestris, Ochrolechia parella and Tephromela atra on them. I even found some of the rarer Lecania aipospila, a species that is associated with perching birds. By far the most prolifically covered with lichens were the many fenceposts. The yellow Xanthoria parietina and Candelariella vitellina were abundant alongside the pale Physcia caesia and Physcia adscendens. In Scotland, these species are found by the dozen. More surprising was the find of large and conspicuous patches of the usually saxicolous Caloplaca verruculifera on fenceposts and the much more subtle Rinodina oleae on the plastic pipes that are meant to protect from them.

At first sight, most of these finds do not rock the world, but going beyond the species list and looking at their relative abundance reveals a picture of cliffs covered in lichens that cope well with eutrophication caused by seabirds. Species like Candelariella vitellina and Physcia ceasia are ubiquitous in nutrient-rich environments on the coast and inland, but Lecania aipospila and Caloplaca verruculifera are specifically associated with the eutrophication caused by perching birds, that is, species one would expect to thrive near seabird colonies.  

The management guidelines for lichens in seashore environments drawn up by the British Lichen Society acknowledges that ‘encouraging birds to nest or roost will certainly change the lichen communities’ and it discusses this in terms of competition and conflict. Standing on the clifftop at Fowlsheugh I can see where these comments come from. The excrements of nesting kittiwakes, fulmars, guillemots and razorbills have created a toxic environment on the sides of the cliffs and disturbed this environment so much by trampling that lichens have little chance to gain foothold. The eutrophicated environment that their excrements created also means that the herbaceous vegetation on top of the cliffs grows so abundantly that the few rocky ledges present are rather shaded and vegetation needs to be bent back to see if any lichens persist beneath. But I would probably not use the terms competition and conflict to describe this. The Aberdeenshire coast is known for its seabird colonies and these seabird colonies were enabled by a combination of a geology that involves steep cliffs with plenty of rocky ledges for breeding and plenty of fish in the sea below to feed the young. From the perspective of a single species (or species group) the presence of those bird colonies hinder the lichen assemblages to reach their full potential. As an ecologist I would prefer to say that the geology and availability of fish have created the conditions for thriving seabird colonies, which, in turn, have created a hyper-eutrophicated environment that has given rise to a lichen assemblage that comprises common eutrophication indicators and species characteristic for bird-perching along the coast. If, this spring, birds would not return to those cliffs to nest, this lichen assemblage would eventually disappear with species with a more limited distribution, such as Lecania aipospila and Caloplaca verruculifera, losing important habitat. In his article collection Claxton, nature writer Mark Cocker questions our obsession with rarity. Though many of the lichen species on the seacliffs at Fowlsheugh may be common, we should perhaps celebrate their presence and abundance as a unique reflection of a finely balanced ecosystem in which geology, fish and seabirds play a principal role.  

Click here for a picture gallery of coastal lichens.

Sources

Cocker, M., 2015. Claxton. Field Notes from a Small Planet. Vintage, London.

Dobson, F.S., 2014. A Field Key to Coastal and Seashore Lichens.

Ellis, J. C., 2005. Marine birds on land: a review of plant biomass, species richness and community composition in seabird colonies. Plant Ecology 181: 227-241.

Fletcher, A., 1973. The ecology of maritime (supralittoral) lichens on some rocky shores of Anglesey. The Lichenologist 5: 401-422.

Smith, C. W. et al, 2009. The Lichens of Great Britain and Ireland. The British Lichen Society. Second edition.

Wolseley, P. A., P. W. James, B. J. Coppins, O. W. Purvis, 1996. Lichens of Skomer Island, West Wales. The Lichenologist 28: 543-570.

Copyright text and images Petra Vergunst