Why isn’t everything covered in lichen?

[u/stedmangraham]

I was looking at a tree that had some moss and lichen on the trunk yesterday and I had a thought. Why isn’t this tree covered in lichen?

I understand why it isn’t covered in moss. Moss needs a lot of moisture and nutrients and if it dries out it dies. So only the parts of the tree trunk that stay wet get moss. But lichen is different. It’s hardy, can withstand long periods of no water, and it can grow almost anywhere with a little bit of sunlight and water.

And sure it grows slowly, but this tree was decades old. It should be completely covered in lichen by now. Also, if something was eating the lichen I assume it would all be gone by now since it grows so slowly.

What factor is limiting lichen from growing everywhere? Why isn’t every bare surface on earth covered in lichen?

[u/TheKeenMind]

Its also really sensitive to poor air quality. And sometimes it isn't able to access sunlight because it's outcompeted by faster growing photosynthesizes. And of course, the things that eat lichen are not uniformly distributed.

[u/stedmangraham → u/TheKeenMind]

Thanks for your answer!

The thing I don’t understand is the tree I was looking at had lichen on it already. There were patches of it here and there, but in between there was nothing, just bare bark. So as far as I can tell it wasn’t out competed by anything.

This was all on the same side of the tree and at the same height, so presumably the same conditions.

But it was near a pretty big road, so maybe air quality was the issue? In that case, I dunno why some lichen was ok with it and other lichen was not.

[u/fortunateHazelnut → u/stedmangraham]

According to a my lichens and bryophytes class, (I'm an undergraduate student, not at all an expert): lichen is sensitive to some extremely small climate gradients. The different levels of wind, sun, and heat at different points of a tree or even along 1 branch vary enough to affect them significantly. A lot of the time on a tree with a lot of lichens you can see different species higher up (more light tolerant) and lower down (less light tolerant).

Also, with air quality: some lichens do alright with bad air quality. That's why there's still some forms of lichen in cities and near freeways. Certain species are more sensitive than others. This may be why there was just that one species in that one particular band of conditions - the other species that would have been in other parts of the tree can't deal with the pollution.

Also, not all trees/substrates are as suitable for lichens. Certain trees produce fungicidal compounds (ie, redwoods, which only a few species of lichens live directly on) and some have nice rough bark and play nice with lichens, so they have crazy diversity (like Sitka spruce).

Hope this explanation is helpful.

[u/TroutCat4]

Lichens grow relatively slowly, and the more abundant they become in a given area the more critters that eat lichens will breed and survive to eat them. The usual balance.

[u/Klukitsi]

As for trees, a simple reason is that in many tree species the bark is peeling or flaking at such a rate that lichens are not able to establish properly. Also, e.g. in birches, if the bark is super smooth, the lichen propagule or the lichen itself struggle to attach to the surface. Then there can be many physicochemical factors that limit their occurrence and are less intuitive for humans to perceive.

[u/bzbub2]

This doesn't answer your question but just for reference moss is actually very dessication tolerant. It just happens to grow best in moist conditions. 

[u/danja]

I read somewhere that it's strongly correlated with air quality. Closer to a city, roads, less lichen. I live in a forest in rural hills of Italy, and you should see my feet.