DO LICHENS HARM TREES?
Lichens which grow on trees and shrubs do not harm them. Although the lichens are attached to the bark or penetrate a short distance, they do not enter the inner bark where food is transported, and hence do not rob the tree of nourishment. Neither do lichens cause disease. In winter in the Willamette Valley, the oak and ash trees seem to be choked with lichens. Although we might expect these lichens to shade the leaves and thus harm the trees, this does not occur. In the spring when new leaves emerge, they grow only at the ends of twigs, where there are no lichens. Thus, the leaves grow on the outside of the tree and shade the lichens on the older twigs and branches, rather than the lichens shading the leaves. In addition, lichens grow in the winter, after the leaves have fallen from the trees and no longer block light from reaching the lichens. Thus, the leaves and lichens take turns: leaves use sunlight in summer, lichens use it in winter.
