For those that saw the stream on the second repotting of the ponderosa pine with lichen growing on the trunk, here is an interesting article on the importance of lichen as an indicator of healthy old growth forests. Keeping lichen on a tree would thus be an awesome indicator of age.
Here is the paper itself, i am not sure if I agree with the author in all their points but that’s not relevant.
https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/fee.2016