Hello, the lichens that are growing on the bark are not known to have any detrimental effects, the are harmless epiphytes (plants growing on other plants).  If your trees are dying, that is likely from some other factor.  It can be difficult to really pin down the cause of tree death, but we have been seeing a lot of tree mortality we believe is related to drought and heat.  We have been recommending watering of trees in late summer and early fall to help them get thru the summer dry period.  If you don't think drought is the problem, we recommend having a certified arborist examine the tree.