Festive lichenometry

It’s Christmas, and I’ve been visiting my Mum in the village I grew up in. There’s a ‘new’ housing estate up the road – I say new, it was built over thirty years ago – with a wall at the entrance that’s got a really solid population of a crustose lichen called Ochrolechia parella (Fig.1). I’ve always quite liked this species, it’s fairly easy to recognise and a familiar sight when I’m out climbing on sea cliffs. While Christmas lunch was cooking, I thought I’d take the opportunity to try out a bit of lichenometry.

Fig.1. A wall, Hampshire

Lichenometry is the technique of using lichen growth rates as a way of measuring time. Lichens, as you may know, grow slowly. There’s a bunch of reasons for this (as well as a lot of exceptions) that I won’t go into here, but generally if you’re looking at a crustose lichen on a bit of rock you can assume it’s been there for a fairly long time. How long? The easiest way to get an idea of this is to go to a churchyard and look at gravestones. Older gravestones have more lichens on them. A few of those lichens might be pretty big and assuming that they’ve been there the longest we can get a good idea of their age by looking at the date on the stone, and from this you can work out their growth rate. This is the essence of lichenometry. It’s a technique that’s found a home in geography, particularly in dating the exposure of glacial moraine; if you have an estimated growth rate for a particular lichen that appears on the rocks of the moraine (geographers typically use Rhizocarpon geographicum, the map lichen) then you can have a stab at the date that those rocks were first revealed, giving you a picture of the dynamics of the glacier that deposited them. Neat.

Cool as this is, trying to work out dates from a single population of lichens is kind of impossible. In an uncomplicated world, a rock would be colonised by the first fungal spores on the day it was first exposed to the environment. All would germinate simultaneously and the lichens would grow at identical rates. No more lichens would establish themselves after that first cohort and nothing would interrupt their expansion. Every lichen would be a perfect circle and determining the age of that initial exposure would be straightforward.

This is of course wishful thinking (thankfully so – a world built on such simple rules would be missing all of the complexity that makes geography, ecology, and evolution such fascinating fields of study) and there are a bunch of things to confuse the situation. There was probably a lag prior to lichens getting established and while some individuals might have got off to an early start, others may have taken their time. Opportunities for colonisation don’t stop after the arrival of the first pioneers, and colonisation rates might increase once the first lichens reach maturity and start reproducing themselves locally. Once they’re established lichens might encounter environmental barriers, mosses, other lichens of both the same and different species, and the developing shapes of lichens are highly heterogeneous. With so much noise inherent in the technique, proper lichenometry studies use 1,000+ samples from multiple locations to calibrate measurements, and even then its been called into question how accurate they can actually be (Winchester, 2023).

But just because things aren’t perfect doesn’t mean that we shouldn’t see what we can find out, and I’ve always been a firm believer that useful insight can be obtained from even the crappiest of data. With that in mind, I went to measure some lichens. There’s not a lot in the way of moraine in Hampshire, but there is that wall at the edge of the housing estate.

My methods were simple:

  • Measure the diameter of O. parella thalli (plural of thallus, the word we use to refer to the body of a lichen) on the top of a waist-height brick wall in the south of England (Fig. 2).
  • When large thalli were clearly confluent (multiple lichens joined together), each part was measured as a separate thallus.
  • Only lichens that were easily identifiable as O. parella due to the presence of thick-rimmed apothecia (fruiting bodies) were counted.
  • Growth rates were calculated as the radius (i.e. half the diameter) in mm divided by the approximate length of time the wall had been standing (32 years) to give a growth rate in mm per year.
  • Eat Christmas lunch.
Fig. 2. Clockwise from top left: The wall again (TL), a small thallus (TR), a big thallus (BR), and a confluent thallus (BL).

After filling up on cheese and mince pies I imported the measurements for 84 thalli into R and did some basic analysis. Lets pretend for a second that we live in that fantasy world where everything landed on the wall at the same time. Based on the size of the thalli and a known elapsed time of 32 years, if everything that’s there now had started growing simultaneously then we’d be looking at growth rates of O. parella ranging from 0.6 mm per year for the smallest thalli to 2.3 mm per year for the largest thalli (Fig. 3).

Fig. 3. A weird violin plot with some pretty unlikely growth rates

There are a couple of things to note here. First of all, this would mean the biggest thalli were growing four times faster than the smallest thalli. Is it likely that there’s this much variation in growth rate between individuals of the same species, in the same habitat? I guess it’s possible but I’d be inclined to think that this is probably supporting colonisation as a continuous process, with the smaller lichens having started growing later. Using the predicted growth rate of the biggest (and supposedly oldest) lichens of 2.3 mm per year, we can work out that an O. parella thallus 40 mm across might have been growing for somewhere between 8.5 and 9 years. This leads onto my second point; although these rates are slow I don’t think they’re CRAZY slow. Some lichens like R. geographicum, have average growth rates around 0.6 mm per year and as slow as 0.1mm per year in montane environments (Bradwell and Armstrong, 2007), so these measurements suggest O. parella is relatively rapid grower in the crusty lichen world. This fits with its habitat – rather than the extremely nutrient poor and snow covered environment of the mountains, the temperate south of England is much more favourable to faster rates of growth.

That’s not all we can learn from the frequency/size relationships of these lichens. Most thalli I measured were of a middling size; there were few very big lichens but also few small lichens. This is a bit different from most of the more thorough lichenometry studies that I’ve looked at, which tend to show a mean thallus diameter towards the smaller end of the spectrum. Have a look at these from lichens on bridges in Glen Coe (Fig. 4).

Fig. 4. A figure from Innes (1983) showing size frequency distributions of lichens on bridges in Glen Coe (N.B. this is a forty year old paper, I wonder what these lichens look like now?).

See how the graphs tend to lean to the left (counterintuitively we call this ‘right skewed’ data because most of the tail of the distribution is on the right)? I think this makes sense; you’re not going to end up with high numbers of very big individual lichens – there’s just not enough space – whereas you can pack quite a lot of small lichens into the same area. At early stages of colonisation the ‘elders’ of the lichen pack have little in the way of competition and are free to get big. Further down the line though, as more and more lichens get established and competition for space grows, individuals become restricted in how big they can get and the graph becomes biased towards higher frequencies of small thalli. So where are all the small thalli in my study? And why are there no lichens less than 30mm across (Fig. 5)?

Fig. 5. Size frequency distributions of lichens on the wall

One possibility is that the whole lichen community on the wall isn’t yet fully developed and that in time more smaller thalli will appear. This fits with the theory but still doesn’t explain the total absence of very small thalli, and I reckon there are some demographic factors that I haven’t taken into account. If you remember, I restricted my sampling to thalli with obvious fruiting bodies, ignoring anything that was infertile. There were lots of smaller white crusts and I have a suspicion that they are young O. parella thalli and it’s these that are missing from my distribution, which leads to one more piece of insight. Based on our calculations above, and assuming that our hypothesis is correct and the smallest thalli are infertile, we can work out that on this little wall O. parella is taking about 6.5 years to reach maturity. This is all quite speculative of course but not bad for twenty minutes staring at a wall on Christmas Day.

  • Bradwell T, Armstrong RA. 2007. Growth rates of Rhizocarpon geographicum lichens: a review with new data from Iceland. Journal of Quaternary Science 22: 311–320.
  • Innes JL. 1983. Size Frequency Distributions as A Lichenometric Technique: An Assessment. Arctic and Alpine Research 15: 285–294.
  • Winchester V. 2023. Lichenometric Dating and Its Limitations and Problems: A Guide for Practitioners. Land 12: 611.

Climbing to lichen discoveries at Pentire Point

Since I started ‘doing lichens’ in 2020, the potential for combining lichenology and climbing has been at the front of my mind. The places that ropes let you access are seldom visited by lichenologists and I’ve always speculated that hidden gems might be revealed by a more ‘adventurous’ approach to lichen hunting. Earlier this summer, I was able to realise this potential on the basalt cliffs of Pentire Point.

Pentire is a cliff that’s on the radar of a lot of climbers in the South West and further afield, mainly due to the appeal of the Great Wall and its most famous routes: ‘Eroica’, ‘Black Magic’, and ‘Darkinbad the Brightdayler’. Andy Moles wrote about Darkinbad on UKC and as folks familiar with the work of James Joyce will know, anything that invokes Ulysses (or worse still Finnegan’s Wake) should be approached with considerable caution.

The Great Wall

My aspirations for the day were a bit more modest and I suggested to my pal Dexter that we climb a VS called ‘Our Stars, Our Sky’, a wandering a traverse above the sea. It was scorching hot and the north-facing sea level crags of Pentire offered a welcome escape from the sun. Dex led the zig-zagging first pitch which was covered in a slick of the ‘tar lichen’ Hydropunctaria maura (more on this in another blog) while I took the second, a fun traverse back above the rocks below.

Dex on Our Stars, Our Sky P1, some cool tension gashes in the rock between the ropes.
Me on Our Stars, Our Sky P2

As we reached the top and scrambled back to our bags something caught my eye on a shady outcrop. I like to think I’m developing a pretty good eye for lichens and they tend to jump out at me from the background, especially if they’re something unusual. The form of this particular lichen was familiar but it seemed out of place. I was sure I was looking at Pectenia atlantica, a species I know pretty well from oceanic woodland in the northwest of Scotland.

Pectenia atlantica on a tree near Inverkirkaig, Assynt

I knew that this species could be found in the South West and I wrote off its rocky habit here as an interesting anomaly. Basalt’s an interesting rock – not as acidic as the granite that makes up so much of the climbable terrain in Cornwall – and where it’s found it often hosts unusual lichens, so seeing P. atlantica here wasn’t a completely ridiculous proposition. Something did feel different about this specimen but I couldn’t put a finger on it, again attributing its not-quite-right appearance to its growth on a not-quite-right substrate. I was pleased to go home with what was an unusual, if not groundbreaking, record.

Pectenia ligulata on coastal basalt at Pentire Head

Later that evening, I was reading through the Pannariaceae chapter of the Lichens of Great Britain and Ireland (a lichenologist’s bedtime reading is edgy stuff) and something jumped out at me: the description for Pectenia ligulata. Superficially similar to P. atlantica, P. ligulata is often found on coastal rocks, just like those that I’d found my specimen on. The main difference between the two is in some special structures that some lichens have; whereas P. atlantica has dense isidia, P. ligulata has spoon-shaped schizidia (sorry for the technical terms – lichenology has a lot of them and I’ll do some explainers in future blogs). I looked back at some photographs I’d taken, and my specimen did indeed appear to have spoon-shaped structures. Nice.

Get in close and you might be able to pick out little spoon-shaped scales all over the middle of the lichen.

However, before I settled on my new identification I had another problem to resolve; whereas there were several records for P. atlantica in the region, there were none for P. ligulata. In fact, the only record for the species in all of England was made nearly 25 years previously by Peter James on the Isles of Scilly.

Known UK distribution of Pectenia ligulata https://britishlichensociety.org.uk/resources/species-accounts/pectenia-ligulata

Faced with the prospect of such a rarity, I reached out to fellow lichenologists who might be familiar with the species. The agreement was unanimous – I did indeed have P. ligulata and its first record for mainland England. What a find! If there ever was justification for combining climbing and lichens this was it. I can’t wait to get out and explore more cliffs with a lichenologist’s eye; who knows what else might be out there.

Many thanks to Maxine Putnam, Heather Paul and Brian Coppins for their help with identifying the lichen, and to Dexter for continuing to climb with me despite my inclination to stare at rocks when I could be climbing.

A version of this post was published in the Winter 2023 edition of the British Lichen Society Bulletin

Welcome to The Vagrant Lichenologist

Hello! This is a blog about lichens. It’s also about climbing, geology, woodlands, mountains, ecology, sea cliffs, hillwalking, deep time, moors, running, mosses, rivers, boulders, insects… Welcome.

I wanted to start a blog for a few reasons:

  1. I’ve been feeling that I’d like to write longer articles that don’t really work on Instagram or other short-form social media. I enjoy writing and want to practice it more.
  2. I want to document some of my scientific research in a way that’s accessible, engaging, and useful to people outside of the academic bubble.
  3. I’d like to build a permanent repository of ‘interesting stuff’ to act as a resource for me and for other people.

I’ve decided to call this blog The Vagrant Lichenologist. The Wikipedia page for vagrant lichen says this:

vagrant lichen is a lichen that is either not attached to a substrate, or can become unattached then blow around, yet continue to grow and flourish. . . . Vagrant lichens generally occur in open and windswept habitats, all over the world


This is a pretty appropriate name because a) I can be kind of hard to pin down, b) I’m often found in open and windswept habitats, and c) I really, really like lichens.

There will be a lot about lichens here, but that’s not going to be the end of it. I’m also interested in how all things (including us) interact with the environment and each other. How communities, ecosystems, and landscapes grow and change over millennia. There are endless stories to be told about the planet and the things that make and inhabit it. Every aspect of our world is dependent on something else at every possible scale – the shape and form of a lichen adapted to an imperceptible microclimate or the structural and chemical properties of a rock that were determined half a billion years ago – and I find zooming in and out of these concepts super cool.

I hope this is fun. That’s the bottom line of why I’m writing this; the fact that I find learning about the natural world fun. I don’t like calling myself an expert (even in the things that I’m an expert in1) because I don’t think you need to be an expert to appreciate this stuff. I think it’s all really exciting, how everything works and fits together, and talking about exciting things is fun. The fact that I get to explore all of this when I’m out in the hills and climbing is the icing on the cake.

Finally, I would love this to become an interactive thing. Ask questions. Send me emails. Tell me about stuff I don’t know about. Tell me about your favourite rock. Ask me for a belay. Whatever.

See you out there x

  1. I don’t have and formal geology training and I’ve only been working on lichens since 2020. Sure, I know a lot about how to assemble the genome of a cyanobacterium, or analyse communities of fungi based upon fragments of their DNA, but even in these fields I still have everything to learn. ↩︎