For my first writings of 2024, I thought I’d go for something slightly more unconventional. We’ve all seen them on a woodland walk, on dry stone walls, fallen twigs or even on roadside trees, but they tend to at a glance be quite unassuming commodities and ones to which many will just be passersby.

Well, for a time I may have been guilty of this. But that is certainly no longer the case.

Last year I was fortunate enough to have received a visit at work from The North East Lichen Society, a group of members from the British Lichen Society to survey areas of the site for lichen diversity, and my word did they know a thing or two about these multi-faceted, heterogenous organisms. I learnt a huge amount, although being able to confidently ID them to species level can certainly be problematic. Most importantly though, I have been hooked with intrigue ever since.

But what actually are lichens?

A lichen is not a single organism. It is the result of symbiosis and is a partnership between a fungus and algae (and/or cyanobacteria). It is thought to be more specifically a symbiosis of mutualism whereby both organisms benefit from the association. Here a lichen can be compared to the mycorrhizal partnership between tree roots and fungi in terms of its mutual gain. The fungus element of lichens provides the benefit of physical protection for the alga, whereas the latter will provide nutrients in return from sunshine, water and air as it is capable of photosynthesis.

So, are they plants or fungi? They are in fact both. Due to their unique biological structure, they are unlike any other organism on earth.

The visible appearance of lichens can appear to be incredibly varied but this can be broken down into three main groups, foliose, fruticose and crustose. However, there are also squamulose lichens which sit somewhere between crustose and foliose.

Foliose lichen are species that form a flattened, leaf-like thallus (body) with an upper and lower cortex (surface layer or skin). Fruticose lichen form coral-like and bushy structures that will anchor themselves to trees or rocks. Crustose lichen species form thin, crust-like coverings that will be tightly bound to a surface. For this reason, we can often quite easily witness three very different looking lichens as you can see below.

Many individual species of lichen can be classed as keystone species within the habitat that they reside due to their deep importance to everything around them. Some are without doubt the glue that holds it all together. They provide nesting materials for birds, food for deer and small mammals, protection for trees and rock habitats from extreme elements and are an important part of soil binding and production as their acidity can free mineral gains from rock faces over time. They can convert carbon dioxide into oxygen and can tolerate a vast scale of environmental conditions, allowing them to be found growing on mosses, trees, leaves, rocks and soil. Their fungi element provides a safe environment in which many algae species could not normally grow.

Lichens are also pioneer species as they are one of the first entities to grow in bare areas lacking in vegetation, even if there is nothing but rocks on offer. The majority will simply not grow in polluted areas so if you find yourself wandering through an old woodland full of mature oaks, hawthorn and birch and notice branches literally dripping with lichens, then this will tell you just how important an environment it is that you are standing in.

A very common lichen which is present and fairly easy to find is Xanthoria parietina, common orange lichen. It is used as shelter and as a food source for the tree snail Balea perversa.

It is a bright yellow to orange foliose lichen very tolerant of pollution and in particular high levels of nitrogen to which has no damaging effect. Areas with nutrient enrichment from bird droppings encourage it to grow, often directly beneath bird perches.

Wherever there are high levels of nitrogen in the atmosphere you will find this species. Which is why it is often found along roadsides. As more Xanthoria parietina indicates more nitrogen pollution, a lot of presence in a concentrated area is a bad indicator for nature. I have found that the further you move away from a roadside, the more diverse the lichens you will find on a tree.

There are over 1,800 species of lichen recorded in the UK and just on ash trees alone, there have been a colossal 536 species found! Sadly, as ash dieback disease (Hymenoscyphus fraxineus) ravages the UK countryside, the effect that it is having on our ash-dependant lichens is yet unknown but it is hard for it not to feel a little ominous. Only time will tell. Though, nature does seem to always find a way.

One thing that nature constantly teaches me is that if you take the time to look just beyond the peripheral, the rewarding value can be incalculable. Nothing could be more true than when dabbling in the world of lichens. I have found these mini, incredibly diverse, self contained eco-systems to be as captivating as they are purposeful.

They are the ultimate symbiotic alliance.