On The Trails: The outer bark of trees

The outer bark of spruce trees offers a variety of things for various users...hiding places for food, substrate for growing, habitat for living. (Photo by Mary F. Willson Willson)

By Mary F. Willson

The outer bark of trees evolved chiefly to protect the trees from pathogenic microbes, fungi, and some insects, and it may provide some incidental protection from damaging forces such as ice or falling branches. It is structurally and chemically resistant to damage and invasion, a set of adaptations that take different forms in different species. We observe some of the external differences along our trails — the ridged bark of hemlock, the scaly bark of spruces, and so on.

Depending on the characteristics of a tree’s out bark, other organisms can make use of it. For example, chickadees, nuthatches, jays, and so on may store seeds of prey in bark with crevices. Very few animals eat outer bark: most consumers aim for the more easily digestible and nutrient-dense inner bark — the phloem tissues that carry nutrients from the leaves and the cambium tissues that are actively growing. But some herbivores chew through the outer bark to get to the inner bark and are able to digest the tough outer bark too; porcupines and beavers are two examples, as discussed in a previous essay. The outer bark is also a substrate where various lichens and mosses can establish themselves. Some reportedly prefer tree bark (e.g., Usnea, the beard lichens).

Quite recently, researchers have learned that bark is a habitat for numerous microbes that actively oxidize air pollutants such as methane (much of it produced by the trees themselves), as well as aerial carbon monoxide and hydrogen. The oxidation forms compounds less damaging to the air. For example, hydrogen exacerbates the effects of methane, and the microbes oxidize hydrogen and less hydrogen means less trouble from methane. Some of these beneficial microbes also fix nitrogen into biologically useful forms. The emphasis of these studies was on the chemistry and the relationship to climate quality, and much remains to be learned about the ecology and evolution of these fascinating systems. These microbes seem to form a community quite different from the microbial communities of soil and water. Does the tree gain anything from the microbial community in its outer bark? Do different kinds of trees have differing microbial communities in their bark?  

Addendum: The world of microbes is more widespread than we thought and more is just being discovered. Recent work notes the existence of the so-called "aeonophiles" — microbes that live for "eons." They exist in energy-poor, deep subsurface sediments and rocks of the Earth’s crust. Their growth is very slow: it seems that their metabolisms focus more on repair than on cell division, conserving energy in places that provide little sustenance. Do these microbes interact with each other? 

There are more worlds of life than we ever imagined, but they are being discovered by curious researchers that keep looking and thinking. Some of these "new worlds" are found in places more difficult of access by ordinary folks, such as the Earth’s crust, but some of them occur in ordinary places such as tree bark, which we see every day. Even such common-place things as tree bark provide lots of things about which to wonder.  

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com