Part 1: Beautiful Bark
By Mary Free, Certified Extension Master Gardener
Colorful flowers and foliage that dazzle the eyes, fragrant to pungent odors that tickle the nose, textures and temperatures that warm the hands and the heart—these are the joys of the growing seasons. So where, beyond snow-covered landscapes, does one search in winter for wonders of the natural world? If one looks carefully at that which winter has laid bare, then the beauty of bark emerges.
With no leaves to obscure the view, one can truly appreciate the varying colors, textures, and patterns of deciduous tree barks. Bark functions as the first line of defense for every tree, but its physical aspects show considerable variability among species. Why is this? The appearance and composition of bark is the product of each species’ response to the internal pressures from growth, and to the external pressures from the environment.
Outer bark—primarily dead cells—includes the external epidermis (found in early growth) and an underlying periderm (composed of three layers of tissue). It protects the tree against the weather elements (rain, snow, hot and cold temperatures) and foreign bodies (pests, fungi, and bacteria). The inner bark is made of living tissue called phloem, which carries sap (water mixed with the sugars made through photosynthesis by the leaves) throughout the tree. Next to the phloem is growing tissue called vascular cambium, which makes new phloem cells, and new xylem (wood), the layer on the inner side of the vascular cambium. Active xylem (sapwood) carries water and nutrients from the roots up to the leaves; inactive xylem becomes the heartwood of a tree.
As new xylem is being created, it pushes the vascular cambium and phloem outward and each species’ response to that pressure produces that species’ distinct bark characteristics. In beech trees, “continuous internal periderms [are] formed immediately underneath the initial periderm,” resulting in a smooth bark (Hirons 2018, 43). In other species, the inside pressure of growing wood splits the initial periderm, and successive layers of periderm form in different patterns and textures depending on the species. For example, in some birch trees, proof of the internal pressures are seen in the lateral peeling of their bark. In Acer rubrum (red maple), the bark starts off light and smooth but as the tree ages the bark darkens and cracks, developing vertical, scaly plates. Commonly, the initial periderm replaces the epidermis over a period of time—typically from less than 10 years in most temperate trees to over 50 years for trees like Betula (birch) and Prunus (cherry) (Hirons 2018, 43). How species deal with environmental concerns also affects bark: thick outer bark indicates a more passive defense, greatly dependent on the composition of the bark itself, while thin outer bark, more easily penetrable, indicates a greater reliance on chemical defenses to combat external threats. (Morris and Jansen 2017, 58)
Bark is an important identifying feature of trees. Among our native trees, one looks for the mottling of Platanus occidentalis (American sycamore), the diamond-patterning of ash, the square-plating of dogwood, the shaggy strips of Ostrya virginiana (Eastern hop-hornbeam), the shredding of Juniperus virginiana (Eastern redcedar), the corky, wing-like projections of Liquidambar styraciflua (sweetgum), and the plating of Quercus alba (white oak). Some descriptions evoke vivid images of the “burnt potato chip” bark of Prunus serotina (black cherry) (Hayek 2015, 25) and the smooth, sinewy bark of Carpus caroliniana (musclewood or American hornbeam). Then there is the image evoked by one word—birch—a lean trunk with white, paper-like bark and horizontal lines.
Not all birches are white or have bark that peels off in papery strips. However, birches do have distinctive horizontal markings called lenticels, which are pores that allow the direct exchange of gases between the inner tissues and the atmosphere. It takes decades for some trunks to lose this identifying characteristic as a more furrowed or platy periderm emerges and obscures the lenticels. The mid-Atlantic region is home to a number of native birch species, although not all are at home everywhere in the mid-Atlantic. Five of those birch tree species can be seen in “The Beauty of Birch” video (below) filmed at the Connecticut College Arboretum in New London, Connecticut. Learn more about each of these species in Are You Lichen the Bark? Part 2. Five Birch Trees of the Mid-Atlantic Region.
When looking closely at bark, one may notice the presence of organisms like lichens and mosses. Some gardeners worry that they might pose a threat to the tree or are indicative of disease, but neither is parasitic. A lichen is a complex life form that is a symbiotic union of two separate life forms, a fungus and an alga. Moss, on the other hand, is a non-vascular plant. Both lichens and mosses produce their own food and for the most part, the filaments that anchor them to their substrates are non-penetrating. Lichens and mosses appear on healthy trees, as well as those in decline, and generally will not harm the trees to which they attach themselves.
There are about 17,000 species of lichens worldwide (including 3,600 known species of lichens in North America) and about 15,000 species of mosses from the Arctic to Antarctica (Monthey and Dudzik 2010, 2). Their intricate forms and subtle to bold color palettes are beautiful to behold, and as winter provides the best opportunity to view bark, it also provides the best time to search for and observe lichens and mosses. Learn more about these fascinating organisms in Are You Lichen the Bark? Part 3. Lichens and Mosses.
- Hayek, Jay C. 2015. “Identifying Trees by Bark and Buds.” University of Illinois at Urbana-Champaign, Department of Natural Resources and Environmental Sciences. https://www.extension.iastate.edu/forestry/tri_state/tristate_2015/Talks/PDFs/hayektree%20ID.pdf
- Hirons, Andrew and Peter A. Thomas. 2018. “Applied Tree Biology.” Hoboken, NJ: John Wiley & Sons Ltd., 43.
- Master Gardeners of Northern Virginia. 2015. “Tried and True Native Plant Selections for the Mid-Atlantic.” https://mgnv.org/plants/trees/
- Monthey, Roger and Kenneth R. Dudzik. 2010. “Learning About Mosses in Our Northeastern Woodlots: an Introduction.” Newtown Square, PA: U.S. Department of Agriculture, Northeastern Area State and Private Forestry Forest Service. https://www.fs.usda.gov/naspf/sites/default/files/naspf/pdf/moss_article_web_access.pdf
- Morris, Hugh and Steven Jansen. 2017. “Bark: Its Anatomy, Function and Diversity.” Kington, UK: International Dendrology Society. https://www.researchgate.net/publication/318457183_Bark_its_anatomy_function_and_diversity
- Steele, Mike. “How a Tree Grows.” Virginia Department of Forestry. http://www.dof.virginia.gov/infopubs/_forest-facts/FF-How-A-Tree-Grows_pub.pdf
- U.S. Department of Agriculture, Forest Service Rangeland Management & Vegetation Ecology—Botany Program. “Lichens.” https://www.fs.fed.us/wildflowers/beauty/lichens/
Are You Lichen the Bark? is a three part series:
- Part 1: Beautiful Bark (January 30, 2019)
- Part 2: Five Birch Trees of the Mid-Atlantic Region (February 6, 2019)
- Part 3: Lichens and Mosses (February 13, 2019).