While a few traits characterize the bark of all trees, these protective coverings are remarkably diverse. As with other tree components, such as leaves, branches and roots, every bark is adapted to suit its environment and the life history of the species.
Although bark hardly seems as exciting or fascinating as so many other aspects of trees, this complex tissue, comprised of many layers of both living and dead cells, tells an intriguing tale, to any curious enough to listen.
Catch-All Definition
Bark is actually an informal term that refers to several of the outer layers of trees and other woody plants. Specifically, it includes all layers occurring outside of the cambium – namely, the phloem, phelloderm, cork cambium and cork. This outer layer of dead cork cells comprises most of what people think of as “bark.”
Whereas the cambium is a lateral meristem (area of cell division and growth) that produces the xylem (wood) and phloem, and it is largely responsible for the tree’s increase in girth, the cork cambium is a secondary lateral meristem, which produces the cork and phelloderm.
The inner layers of bark – specifically the phloem – are responsible for transporting chemicals and calories through the tree. Accordingly, damage to this layer can cause great stress for a tree. This is one of the reasons trees are easily killed by “girdling” – a technique used for deliberately killing trees in which a wide swath of bark is removed around the trunk’s circumference.
Slipping into a Stronger Skin
Bark production is resource intensive, meaning that there is a very good reason trees produce it. After all, plenty of green plants thrive in the modern landscape, bereft of bark.
One key distinction is that trees are perennial plants whose lifecycle depends on lasting for many years – sometimes many hundreds of years. By contrast, many herbaceous plants, grasses and shrubs are annuals that die off each year – producing such a robust outer covering is a frivolous use of precious resources for these ephemeral species.
Bark protects the interior and vulnerable portions of the tree, just as skin or scales protect animals. It reduces water loss from the wood and deters some predators and pests. Some species have even developed spines and thorns that confer additional protection.
Bark Reflects Habitat History and Evolutionary Patterns
Many trees have bark that clearly reflects the specie’s survival strategy. For example, the deeply fissured bark of black oaks (Quercus velutina), tuliptrees (Liriodendron tulipifera) and other species that inhabit areas with cold winters is an adaptation that provides protection from freezing temperatures.
However, the bark of some trees reflects the evolutionary history of the species more than it serves a current need. For example, beech trees (Fagus grandifolia) arose from within a largely tropical lineage, native to habitats with saturated atmospheres. Epiphytic plants abound in such locations, where they have grown on the trunks of trees for millennia. To prevent the plants from colonizing the trunk, the ancestors of beech trees developed smooth, thin bark, making it hard for the plants to obtain a secure “grip.”
Animal Assaults
Bark provides a number of important resources for various animals. Porcupines, beavers and many other mammals munch on the delicate inner bark and cambium of aspens (Populus tremuloides), beech and basswood (Tilia sp.) trees. Countless insects and other arthropods take shelter under the bark of trees, using it to not only shield themselves from predators, but to provide them with a thermally appropriate microclimate.
Humans also derive resources from tree bark. Aspirin is derived from chemicals present in willow (Salix sp.) bark, while quina trees (Cinchona sp.) gave humanity quinine – an important malarial medication.
Incredible Examples
A number of species produce truly unique bark. Check out the following links to learn about some rather spectacular species and the bark that helps them survive.