Lichen

Is lichen harmful on my trees?

Not at all. Lichen, a symbiotic combination of an alga & a fungus, are epiphytes. They roost on the trees without deriving any significant nourishment from them. They are considered a beauty mark of old age on a bonsai.
Iris

I have seen cases, particularly on Larch where Lichen has taken over and covers over buds on old branches. In this case I have recomended brushing it off.

Wow, I forget parts of UK are practically a rainforest. Growers in the Pacific Northwest might encounter that problem, but I doubt if you would have it in the middle of Ohio. Here in Central NY lichen are few & far between. I occasionally get spots of it on my oldest trees.
Iris

Here in Central NY lichen are few & far between. I occasionally get spots of it on my oldest trees.

Most Lichen are EXTREMELY sensitive to air pollution. If you have lichen, breathe easy.

Lichen is a good sign it means the air is clean. The lichen which grows around the root base is slightly different but its presence is also good sign as it means your trees are not too wet.
Craig

YEAH!!! its sounds like i got something good goin on for once!!!