Professor Jay O’Laughlin, retired director of the College of Natural Resource’s Policy Analysis Group at the University of Idaho, was noted for his many informative and entertaining presentations. His quick wit, along with a mastery of his subject matter and computer skills, kept the audience anxiously awaiting his next zinger. In one such presentation, combining carbon sequestration and solar energy, he pointed out that Mother Nature had long ago created the perfect solution to address both diverse topics. The professor described a structure consisting of a tower with thousands of solar panels attached to it. He explained that the tower stores carbon and the panels produce energy from the sun. Moreover, this structure also cleanses the air. After a slight hesitation, Jay clicked on to the next slide illustrating a picture of, you guessed it, a tree. His only comment, “we call them trees.” Point made!
Well, let’s use this cross-sectional diagram to briefly describe the wooden tower portion of Mother Nature’s marvel. Foresters refer to this as the trunk, stem or bole of a tree.
Bark provides the protective outer layer of the trunk. Usually corky in texture, it provides insulation from extreme temperatures. It is also the tree’s first line of defense against insect and disease attacks.
Phloem (pronounced flow-um) transports plant food, produced in the leaves during photosynthesis to the rest of the tree. Consisting of sucrose and fructose sugars in a water-based solution, we also know this plant food as sap. The phloem, a form of inner bark, is vulnerable to being girdled by bark beetles, which will kill the tree by blocking the flow of nutrients.
Cambium is the active growing layer of the tree. While it may be only one or two cells thick, the all-important cambium produces new phloem on the outside and new xylem on the inside.
Xylem (pronounced zi – lem), also called sapwood, is the waterworks of the tree as it transports water and dissolved minerals from the roots to the branches and leaves. Enlongated cells in the xylem act as pipes that create pressure within the tree, which seem to defy gravity and force water upward.
Although not depicted in the diagram, the new growth each year by the cambium forms annual rings in the xylem. As we all know, these annual rings reveal the age and growth pattern of the tree. Annual rings contain two bands: light colored springwood produced early in the growing season and dark colored summerwood formed during the latter part of the growing season. These two bands represent one year of radial growth in a tree.
Heartwood comprises the central column of the bole which provides structural support for the tree. It consists of dead xylem wood that no longer pumps water. The outer layers of the bark, phloem and xylem protect this dead wood from decay and insect damage. Over time, the color of heartwood darkens in some species. For example, the heartwood in Douglas-fir darkens to a reddish tint, hence the common name of red fir. But on the other hand, the heartwood in grand fir remains a very light color giving it a common name of white fir.
In conclusion, now you know that a tree trunk is far more complex that a wooden tower wrapped in bark. We will explore the solar panels that are attached to these towers in a future issue of Tree Talk.