Oxygen-rich Air and Lichen

The real plants-woman of the family, Mrs Irene Nairn, has long been saying ‘that lichen is choking the trees and killing them off’ ‘No, no, no’! says I, ‘Its all good…lichen is pretty and surely oxygen-rich air is what the world aches for?’ But when wandering around yesterday I saw this on several trees…deed branch choaked by lichen! Gardening friends – what your thoughts?
Oak tree and lichen