In my low fields along the streams I notice bark lichens growing on my oak, maple, ash and walnut trees, so I thought that I’d take a closer look and attempt to identify them. Easier said than done.
Richard Orr of the Howard County Bird Club has prepared an excellent taxonomic identification key of Howard County lichens, which is attached as PDF file. The document more than adequately covers much of central Maryland, and the more common species in the document have a range throughout the mid- Atlantic states.
Lichen found on many trees in the forest. It looks like the tree has splotches of whitish gray when dry, and grayish green when wet. They are found more frequently in younger growth disturbed stands than in more mature undisturbed stands and may be useful in determining the disturbance level of a forest.
Rough Speckled Shield Lichen (Punctelia rudecta) is usually found on bark and less commonly on shaded rock. It is common in the County and is tolerant of pollution, often found in urban areas.
Common Greenshield Lichen (Flavoparmelia caperata) is almost always found on bark in either sun or shade. It is very common, and may be the most conspicuous and widespread lichen in the County.
Sea-Storm Lichen (Cetrelia chicitate/olivetorum) is widespread throughout the County and the uplifted edges of the lichen reminds one of foam on ocean waves. Separation of the two species can only be done by spraying liquid bleach on the lichen. C. chicitate will turn red, while C. olivetorum does not react to bleach.
These four common species are very similar in appearance, all in the Parmelia family and are likely being observed in the photographs.
Another lichen, in the Physcia family, is called Star Rosette Lichen (Physcia stellaris) and is widespread/common on bark in various habitats and is a small-lobed pale to dark gray foliose (leaflike) lichen. Several species occur and all look very similar to each other. This lichen is on these trees too, but outdone by the larger, more obvious Parmelia’s.