Lichen: Complex, Confusing Conundrums Wrapped in An Enigma?  

Lichen Are Everywhere, Even In Our Gardens. But What Are Lichen & Are They good For Gardens? Lichen Appear On Trees, Walls & The Soil & Are Actually Good News For Gardeners.

In Devon we are fortunate in having many lichen species growing locally, especially in the Sid Valley where I live. Lichen are a bioindicator. Their growth indicates low levels of atmospheric pollution. Fortunately we have loads of lichen on our fruit trees. In London it’s a different story as discussed later in this article. Lichen are a strange life form, They are a symbiotic relationship between algae or cyanobacter and fungi. So not a single species but a symbiotic relationship between different life forms. The algae or cyanobacteria species grow amongst the fungal filaments and become a composite organism that has properties different from either of their constituent parts. We know the relationship is normally mutualistic but it’s not always clear if its a 50:50 relationship or if one species gains more than the other.  Interestingly, though lichen are composite organisms that include fungi, some lichen also have fungi growing on them in addition to the fungi that is part of the lichen,  There are two types of lichen. Micro-lichen, as the name suggests, are small. But by small we don’t mean in total size but to the life form they exhibit. So they hug the surface and look small on the substrate they grow. The other type of lichen are the macro-lichen. They look bigger as they may rise above the substrate and form coral type structures above the substrate. Lichens grow in many places including on rock, on soil, on plants including trees, and are found across the globe. Though the lichen anchor themselves to particular substrates such as rocks, they do not have roots. They don’t need roots as they produce their own nutrition via photosynthesis similar to plants. But they aren’t plants. 

Lichen Confusion (Often Leads to Asking What Are Lichen?)

If all this confuses you, you are not alone. Lichen are unlike other lifeforms. We do however try to shoehorn them into our understanding and use words to describe them that aren’t always accurate. For example we say that reindeer eat reindeer moss. But the reality is that they eat a lichen we call a moss. So though lichen are formed by microscopic partners, they feed creatures as large as a 180kg reindeer.  There are over 18,000 described species of lichen worldwide with perhaps several times that number likely to exist in total. In the UK we have around 1800 lichen species. I say species but the word is really inadequate to describe a life from that consists of several species. Lichen are not found in deep shade. Some deserts at low elevation don’t see lichen. But some are found in seawater. They love humidity and light because they can photosynthesise better.  Lichen aren’t parasitic on plants. Though found on trees they live independently and only use the tree as somewhere to live. 

Does That Explain The Question, What Are Lichen?

Unsurprisingly, lichen have been around for millennia, some lichen experts claim they first appeared in the Permian extinction period, some 250 million years ago. What is more surprising to many people is the age of individual lichen. One has been dated as being 9600 years old and that would make it the oldest life form on earth. The only problem with ascribing an age to lichen is whether we know it’s the same organisms that survive year after year or are there new generations of fungi and bacteria or algae? With lichen nothing can be certain as the normal measures and words don’t seem to apply.  

What Are Lichen? .. Colour, Shape and Form

Most lichen are easy to identify in that they have specific shapes, colours and forms. And their names are often based on this. And their uses are also sometimes based on these features. For example their colour often lead to the production of dyes.  Lichen reproduce by spores, often produced during wet times. The spores spread by drifting on the air currents. Once they land they germinate, produce short lived hyphae and need to “lichenise” with algae or Cyanobacteria. It’s a precarious way to reproduce and it’s only the fungi that spores. Hence it’s hard to ascribe the word species to the lichen, in that the lichen doesn’t reproduce, only part of it does. That’s weird!  Once the two parts come together they form a “thallus”. The function of the thallus is to to protect the algae or cyanobacter. They need to be protected from drying out. Plus they need protection from ultraviolet light. The colouration does this in a similar way to how we use sunscreen! 

Look on a gravestone in the Sid Valley or elsewhere and you’ll often see a lichen battle taking place. One lichen will often overgrow another. In this case the one above can steal the algae from the one below. And in some cases a new lichen can result.  Lichen don’t have it easy though. Not only reindeer eat them. So do mites. Lichens may not be hugely nutritious but mites will eat them. Even humans can eat lichen (in moderation). But the secondary metabolites can cause problems. The lack of nutrition and possible metabolic problems are perhaps the reason we don’t see them on restaurant menus whereas seaweeds and other “unusual” organisms are eaten and are nutritious.

Lichen in London

Sadly this promises to be a short paragraph.  According to Kew there was only one pollution tolerant lichen in London until recently. That was Lecanora dispersa.  London was a lichen désert. Today polution tolerant Xanthoria parietina is edging back in. But whereas the south west has a wealth of lichen, London remains a desert. The Kew lichen paper can be found by following this link.