bark [ bahrk ] noun: the tough outer layer of a woody root or stem
periderm [ PER-i-durm ] noun: a corky protective outer layer of plant tissue that covers the roots and stems of woody plants in place of an epidermis
phelloderm [FEL-uh-durm ] noun: a layer of living cells called parenchyma produced by the inner surface of the cork cambium in woody plants
phloem [ FLOH-em ] noun: one of two specialized types of tissue in the vascular systems of highly developed plants that deliver and store synthesized foodstuffs throughout all parts of the plant. Phloem consists mainly of sieve tubes and elongated parenchyma cells. [See also xylem.]
Bark is the skin of the tree, specifically the tissue outside the vascular cambium that includes an inner layer of secondary phloem and an outer defensive barrier, periderm, comprised of three layers: phelloderm, cork cambium, and cork (primarily dead cells). The phloem carries sap (water mixed mainly with sugars made through photosynthesis by the leaves) throughout the tree.
Bark is a large factor in tree identification for many of us, along with form, leaf shape, and other characteristics. That’s a value to us as humans. To the tree, bark is crucial for its primary role in protecting it from adverse weather, harmful insects, bacteria and fungi, browsing animals, and, importantly, forest fires, where thickness and moisture content are important factors.
Bark varies from species to species and by the age of the plant. It is often thinner and smoother when young, thickening and cracking with age as the tree grows and expands. Surrounding ecological conditions also account for bark differences, and bark can vary on different parts of the same plant. Other variables in bark include moisture, density, and chemical makeup, which includes cellulose and lignin (a polymer that provides rigidity, is hard to metabolize, and stores carbon, contributing to the trees’ value as carbon sinks).
In some species, like Acer rubrum (top left, younger tree and top center, mature tree), bark can change as the tree ages. In some species, like Prunus serotina (top right), bark can vary on different parts of the same tree. Each species has a characteristic pattern and texture (e.g., smooth, papery, scaly, platy, furrowed, or fibrous) produced as the inside pressure of growing wood splits the initial and successive layers of periderm. Bottom left to right: Ilex opaca, Betula nigra, Quercus alba, Pinus rigida, Liriodendron tulipifera, and Juniperus virginiana.
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In addition to its protective role, bark is a pollution indicator. While rain can wash pollutants off leaves, the porosity and cracks and crannies of bark tend to retain contaminating acids, metals, and other elements longer. Many trees live long lives, and bark can contribute indicators to each tree’s history for hundreds or even thousands of years.
Bark has a history of ethnobotanic uses as food, medicine, and fiber for textiles and structures. Today it is still used in fiber products and as a source of alternative energy fuels, in particle board and other building uses, as mulch in agriculture and horticulture, and as litter for pets and domestic animals. Many chemicals found in bark are extracted for medicinal uses. A specialized bark use is cork production, for bottle stoppers, bulletin boards, and other items. As our society focuses on better use of natural resources, scientists continue to search for other ways to effectively use this by-product of lumbering, often treated as waste.
References
How a Tree Grows. University of New Hampshire Extension. (accessed April 11, 2023).
Pásztory Z, Mohácsiné IR, Gorbacheva G, Börcsök Z. 2016. The utilization of tree bark. BioRes. 11(3): 7859-7888.
Saunders MR, Mann DP, Stanis S, Wiedenbeck JK, Dey DC, Schuler TM. 2023. Prescribed Fire Causes Wounding and Minor Tree Quality Degradation in Oak Forests. Forests. 14(2): 227-. Doi: 10.3390/f14020227.
Starker TJ. Fire Resistance in the Forest. 1934. United States Department of Agriculture.
Zandonella C. 2017. Tree-bark thickness indicates fire-resistance in a hotter future. Princeton University.
MGNV Posts About Bark
Free M. January 30, 2019. Are You Lichen the Bark? Beautiful Bark
Free M. March 11, 2019 (updated April 12, 2022). Healthy Bark, Healthy Tree Part 1: Wildlife and Fungi Damage
Free M. March 13, 2019. Healthy Bark, Healthy Tree Part 2: Mechanical and Weather Damage
Mills E. January 5, 2020. Identifying Trees by Their Bark