Bark, pt.3

Bark provides protection for the tree from organisms trying to get under it to lay eggs and let larvae grow, to live out of the elements, or to feed.  Whether bark is thick or thin, there are areas of good protection and areas of weak protection.  Weak areas may include lenticels, cracks, branch junctions, wounds, broken branches, or open spots created by fire, frost, or other environmental actions.  Many trees are weakened by drought and poor nutrition increasing the likelihood that the bark will not provide protection over the entire exterior surface.

A mature tree can provide food and shelter for dozens of species, but not all visitors are harmless.  Dutch Elm disease is caused by a fungal bacteria found on non-native beetles introduced to the U.S. through the landscaping trade in the early 1930s.  Native elm trees had no defenses, and native elm beetles also picked up the fungus and spread it further. 

Insects that lay eggs under bark cause very little damage, but their larvae burrow through the cork layer, sometimes creating extensive tunnels that can damage the tree’s circulatory system.  Where tunnels break through the surface, decay sets in.  Carpenter ants feed on the decaying wood, opening the wounds further.  Woodpeckers, especially the pileated woodpecker, feed on the carpenter ants, excavating wood to get to more ants.  Tree sap, leaking from the damaged circulatory system, draws even more insects, and the cycle of damage continually worsens.

Many animals feed on sap and inner bark in late fall and winter when food resources are scarce.  This is also the time of year when the tree is least able to defend itself.  Red squirrels and porcupines bite through bark on beech and maple to get to the sap.  Beavers break through the outer bark of willow, aspen, poplar, and cottonwood to eat the inner bark.  Native Americans and settlers harvested the inner bark of hemlock and pine to make flour.  Deer, moose, and black bears strip bark to mark their territories.

Many organisms use significant parts of a tree without harm.  Peeling bark is used by birds for nest building.  Smooth patch disease, easily observed on white oak, is caused by a fungus eating the exterior bark, but not penetrating to the inner bark.  Insects move along bark cleaning up moss, lichen, and other material that may be clogging lenticels.  Their presence attracts ants, spiders, and other predators.  When it rains, water flowing down the bark carries minerals, nutrients, and organic matter from plants such as lichen and moss, plus fecal matter from bark dwelling insects, and washes it all into the root area providing additional resources to feed the tree.

Trees have their own chemical and mechanical defenses to repel, isolate, and kill various organisms.  Resin seals wounds to prevent desiccation and infection.  The inner bark of black cherry contains hydrocyanic acid to repel bacteria, a chemical long used in cough drops.  Tannin refers to a variety of compounds found in many plant species that is used to deter epiphytes, insects, and fungi.  Trees may grow inner bark tissue to form a walled off area that isolates an organism that has breached the exterior bark.  This area, covered by callous tissue sealing off the wound, is known as a canker, and results in a visible patch on the outer bark.  Burls forms from abnormal wood growth around these areas. 

Bark is an important and integral part of all trees, as well as a complex subject with much information still to be learned. There are several good resources available for further study, including Bark, A Field Guide to Trees of the Northeast by Michael Wojtech.  As you are walking the woods this year, take some time to think about the role of bark not only in tree  identification, but as a resource for the woodland habitat.