Lichen

The accepted advice on lichen is that it doesn't do any harm to the host plants...fair enough. But the grey-green, flat lichen that grows on hard surfaces turns into a black, tarry blotch that actually eats into concrete, terracotta and stone. Is there a scientific basis for the assumption that the vegetable-dwelling sort is harmless? Asking for a friend

 

I'm not sure the term host is appropriate in relation to lichens that live on plants and my understanding is that they are more like epiphytes that simply grow on the plant surface rather than draw anything from the plant. Not done any scientific experiments but I have not seen any damage done by lichens to plants, concrete, terracotta or stone. As far as I was taught they are just made up of filaments of fungi that support algae in a symbiotic relationship and there was no suggestion that anything in the lichen was involved in feeding off anything outside the lichen.

 

@noisette47 Trees and shrubs basically provide a platform for the lichen to live on. They tend to form on slower growing parts of trees, so unhealthy branches may have more lichen on them as they are growing more slowly; the lichen is not the cause of the unhealthiness.
They do require a clean environment to thrive. On rocks, tombstones etc they do remove minerals from the area below them and the surface is etched and weakened.
I thought lichen were a symbiotic relationship between an algae and a fungus, then I read "Entangled Life" by Merlin Sheldrake and the latest research shows they are much more complicated being an ecosystem of algae, fungi and bacteria living together. Would recommend this book well written and readable.

 

Well what clear, concise and thoroughly useful replies! Thanks, @NigelJ and @Macraignil