Red oaks are among the most common of the great deciduous trees of New England, renowned for their beauty, lumber and the mast they provide. While squirrels, deer and bear visit these trees for food, a host of lichens utilize their bark and branches as permanent habitat. Learning to identify a few of these obscure, symbiotic organisms can add a new dimension to your enjoyment of many forest habitats.

Lichens are among the most unique and interesting life forms on the planet. They are symbiotic organisms consisting of a fungal partner and an algal partner or cyanobacterium. The fungal threads, or hyphae, surround and even grow into the algal cells, while providing most of the lichen’s bulk and shape. Lichens absorb water, carbon dioxide and minerals from rainfall and atmospheric moisture. These substances and oxygen are absorbed by fungal hyphae and passed to algal cells. The algal cells, through photosynthesis, manufacture the carbohydrates necessary for the nutritional health of both the fungal and algal partners.

Lichens form a beautiful tapestry on our forest trees in New England, but look for them also on downed logs and branches, rocks, soil and other habitats. They grow on a wide variety of substrates in a variety of growth forms including foliose, fruticose, and crustose [see Massachusetts Wildlife, #3, 1997]. Foliose forms are leafy and spread horizontally over substrates. They anchor themselves with root-like threads called rhizines. Fruticose forms are shrubby or beard-like. They may be erect or pendulous and can easily be removed by hand. Crustose forms are encrusting and spread over and into the surface of the substrate on which they grow. They cannot be removed by hand without crumbling.

Many woodlot owners in New England have northern red oak, Quercus rubra, growing on their properties, and certainly anyone who hikes or hunts in the forests of Massachusetts should know these familiar and important mast producing trees. Most folks tend to take them for granted, noticing only their lobed leaves, large size and prolific acorns, but stop and examine one even briefly and all sorts of those strangely colored and mysteriously shaped vegetative growths known as lichens can be discerned on their bark and branches. What are these lichens doing there and what purpose do they serve?

Irwin Brodo, Sylvia Sharnoff and Stephen Sharnoff published Lichens of North America in 2001. Among the many interesting things the authors reveal about our native lichens in this book is the fact that these seemingly insignificant life forms are an important component of forest ecosystem function and diversity. Lichens are colonizers of tree bark, as well as other wood, dead vegetation, mosses, rock, soil, leaves and manmade structures. Those growing on trees absorb nutrients from rainwater, influence humidity levels by absorbing and then releasing rainwater and dew, and some species with specialized cyanobacteria contribute fixed nitrogen to ecosystems. Lichens also serve as food for many animals, as nesting material for many birds and mammals, and as camouflage and shelter for small, mostly invertebrate animals that live in lichen-covered habitats.

There are thousands of species of lichens worldwide, and few people have the inclination or determination to learn how to identify even a few hundred of them. That many species lack common names and must therefore be remembered by their scientific Latin names further compounds the public relations problem that lichens face. In the interest of creating a little more passion for these symbiotic miracles, we thought it might prove useful to profile a baker’s dozen of readily identifiable lichen species that can commonly be found growing on a specific habitat: the bark and limbs of red oak trees.

• Produce carbohydrates and accumulate biomass.
• Contribute to forest nutrient cycling through leaching by rainfall, and by decomposition when they fall to the forest floor.
• Provide organic material and increase the moisture-holding capacity of forest soils by litterfall.
• Capture rainfall/dew and retain this moisture for a time which helps to ameliorate soil erosion and to increase humidity which aids growth of forest vegetation.
• Provide food for wildlife such as flying squirrels and red-backed voles, and nestbuilding materials and camouflage for birds and small mammals.
• Provide food and/or habitat for a large assortment of invertebrates.
• Convert or “fix” atmospheric nitrogen into forms more readily usable by plants.
• Excellent indicators of the quality of our air, as they are sensitive to sulfur dioxide and other gases and are efficient accumulators of heavy metals.
• Potential for medicinal qualities and antibiotics.
• Dyeing of wool on a small scale in cottage industries (some lichens were formerly used commercially for dyeing of wool).
• Used in perfume industry in Europe.
• Dating substrates of unknown age such as archaeological remains and others.

Our co-author, Dr. Constance Stubbs, surveyed the lichens growing on red oak in the University of Maine Experimental Forest during a study of the associations between small invertebrates such as water bears (microscopic animals that have claws and a lumbering gait reminiscent of our mammalian bears; see box), nematodes and mites. She listed 15 species of foliose (leafy growth form) and fruticose (shrubby or hair like growth form) lichens that typically grow on red oak in the Northeast. She also listed 17 species of crustose (crustlike growth form) lichens.

The lichens appearing on the following pages are not intended to represent the complete list of the lichens that grow on red oak, but rather portray a representative suite of the foliose and fruticose species that anyone can expect to encounter while examining red oaks in the Northeast. Please keep in mind that the species listed are not restricted to growing only on red oak; most can also be found on other substrates. We have provded the Latin names as taken either from the 1994 publication, The Lichens of British Colombia by Goward, McCune and Del Meidinger, or the recent book by Brodo et al. mentioned above. We also borrowed liberally from Mason Hale’s book, How to Know the Lichens, (1979) to come up with the descriptions. We hope that by providing this partial list of the lichens that grow on red oaks, interested people can more readily identify — and thus more fully appreciate — these fascinating organisms.