The first and definite answer to the question at the head of this blog is, no. Lichens (along with various other clinging organisms such as moss and algae) are not damaging your tree. They may look like a fungus, and have the appearance of something flakey and decaying, but they are not harming the trees on which they dwell.
But what are they?
The interesting and wonderful world of lichens begins when we ask exactly what is a lichen? A lichen is in fact the organic body of two or three types of organisms, living together. The body, structure and shape of lichen is made up of fungi - the decomposing kingdom of life that produces, among other phenomena, mushrooms. There can be multiple species of fungi in the formation of a lichen. Alongside and within the fungi is algae. Yes the stuff from the sea, but in this case they are land-loving algae. Fungi and Algae are sometimes joined by Cyanobacteria. No doubt you've heard the word SYMBIOSIS - well that word was coined 150 years ago when biologists first started probing into what lichens actually were. They found that these two distinct types (even kingdoms) of life were living together in a mutually beneficial way - which basically breaks down thus:
The fungi create a structure using the proteins they produce which is purpose-built to house algae and cyanobacteria. These in turn specialize in photosynthesis, and to a lesser degree, nitrogen fixing. If you didn't already know, photosynthesis is the ability to produce sugars from sunlight and carbon dioxide. They then share these sugars with the fungi, which enables them to grow and maintain their growth. Nitrogen fixing is a way in which a group of bacteria can absorb the nitrogen from the air and transfer it as a bio-available form into the ecosystem - in this case directly into the fungi.
One scientist described lichens as "Fungi that have discovered agriculture". That sums it up nicely I think.
Some quick facts about Lichen:
Some lichen are up to 10,000 years old.
They cover 7% (!) of the Earth's surface.
There are 18,000 species of them.
Certain extremely hardy lichens have survived being strapped to the outside of space ships and sent into orbit.
They can photosynthesize at temperatures as low as -20oC. So they are found in Antarctica, and all the other continents.
The variety of colours and forms of
lichen are what attracts me to them. They flake, peel, curl, engross, creep, encrust, bulge, burst while looking like leathery leaves or dry paint, or graffiti or horse hair or molten cheese. The colours are lurid and extravagant. They derive their palate from the algae which generate compounds such as chlorophyll, antioxidants and photoprotective stuffs most of which are unpronounceable. The structures or 'growth forms' of lichen are very much designed around enabling them to 1. attach and stay attached to things; 2. manifest surface area for the purpose of gathering water and sunlight and 3. Reproducing - mostly done A-sexually through spreading, creeping or breaking off and re-colonizing.
These unique and odd organisms can sometimes look as though they are somehow consuming the tree. This isn't the case. The reason they cling to trees is, basically, it's a great free space to get air flow, water and sunlight without being out-competed. The lichen we find on rocks are in fact very, very, very slowly breaking down the minerals in the rocks and digesting them. But the lichens on trees are just hitching a free ride up into some quality living conditions. Free maybe the wrong word...because lichen pay their way in the ecosystem by providing amazing materials for birds' nests, habitat for invertebrates as well as in death offering the rich minerals and chemicals they have consumed back into the soil in which the trees root.
There isn't time enough here to go that much further into the world of lichens - but here are a few links to resources that I've explored:
https://www.youtube.com/watch?v=Z2haTr0j4hk
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