There are many reasons why fir trees will die and lichen can move in. First the lichen does not kill or harm the trees. It is simply an opportunist growing where it has the right conditions. Native trees die from summer drought but the damage may not show up until the following summer. Compacted soil from machinery or building can also slowly kill trees and also smothering the roots with too much soil such as soil moved during construction and piled on top or roots. Root damage from moles, voles and ground squirrels can also damage trees. If the trees are along your driveway than perhaps the compacted soil from the concrete or weight of cars has damaged the trees. Keep growing, Marianne Binetti