The bark is the protection of the tree. The tree also takes care of the bark itself, so you don't really have to worry about it.
Seal damage with tree wax
If the bark is injured, the tree can become ill from pathogens, bacteria and pests. For this reason, damage should be closed with tree wax if it is very large. This at least ensures that no pathogens can penetrate the tree bark. As a rule, however, the tree or the bark regenerates itself, so hardly anything needs to be done here.
Lichens and mosses on the bark
But it often happens that trees on the bark, especially on the side that faces the weather is, has lichens and mosses. How about that? Are these growths harmful, possibly even taking away nutrients or even oxygen from the tree - and how can you treat them?
Growths are not harmful
First things first: These growths are in no way harmful. They may not always be pretty to look at, but they are not harmful. They even protect the tree or the bark additionally, which is why they should not be removed. Many gardeners fiddle around with brushes here, but this can lead to the bark becoming damaged and cracks forming - and the tree is again susceptible to pathogens. So: doing nothing is the best option here.