They can also be incidentally fertilised by bird and insect dung. Lichens will and do grow on just about everything, natural or manmade. Different species of lichens prefer, or only grow on different substrates. Thus some species will be found on smooth barked trees, some on rough barked and some on only one species of tree. Also some lichens grow on basic rocks while others only grow on acidic rocks and some have particular mineral requirements, thus Acarospora sinopica only grows on rocks with a high iron content.
Lichens grow differently at different times in their lives.
When young and very small they grow slowly, then once they are reasonably well established they grow much more quickly, obviously when they are dying, for what ever reason they grow more slowly again, or not at all. Lichens grow by extending their thallus outwards, from either its tips or edges. They grow very slowly, some species more slowly than others. Rate of growth can vary from 0.5mm per year to 500mm per year. Their slow growth rate equates with their long life.