Bark, pt.1

It has been cold outside for the last few weeks; and whenever we go outdoors, we tend to bundle up with warm coats, gloves, hats, boots, and long underwear.  Animals and plants have their own strategies for living through the winter, including trees.  Bark is an essential part of any tree.  Bark provides protection, insulation, and plays a role in getting food to the rest of the tree.  Bark can be used to identify tree species from the way it grows, its color, and its texture.  However, bark can look different from tree to tree, and from branch to branch on the same tree.

There are several layers to a tree.  The innermost layer is called wood and is made up of dead cells from previous year’s growth.  Wrapped around the wood is the vascular cambium where new cells are growing inward to become the xylem layer that transports water and nutrients from the roots upward, and new cells growing outward to become the phloem that transports sugar and nutrients from the leaves to all parts of the tree.  During winter, the need for food and water is much less as the tree enters a non-growing, dormant state, but circulation continues enough to keep the tree alive and hydrated. 

Bark consists of several layers starting with the phloem, also known as the inner bark.  The periderm makes up the outer layers of bark protecting the phloem and other inner layers from environmental elements.  The cork skin is the first layer of the periderm, containing cells to store food and waste products including tannins and resins.  The cork cambium is the second layer where new cells are growing to add to the outside and final layer, the cork.  Cork, mostly hollow, dead cells, prevents desiccation, provides insulation, and is a protective barrier from fungal and bacterial diseases. 

Bark’s appearance is caused by both internal and external factors including growth, gas exchange, and  the environment.  At what rate are new cells being added to expand the tree?  Does the bark stretch?  Are there cracks or breaks in the bark?  Bark thickness may vary from one-quarter inch on a mature beech to over an inch on a mature oak.  Are there visible openings in the bark?  Lenticels are specialized cells in the bark allowing an exchange of gases with the outside environment.  Is temperature speeding or slowing growth?  Has fire influenced the area?  What affect has the wind had?  Has the tree been attacked by bacteria, fungi, or animals?  Have other plants used or damaged the tree? 

Six bark appearance categories are commonly used to distinguish trees:

1. Smooth: Cork maintains a constant thickness throughout the life of the tree, and old cells slough off easily.  An example is the American beech, Fagus grandifolia.
2. Horizontal peel: Expansion causes the external layer to peel away in strips when it is still quite thin.  An example is river birch, Betula nigra.
3. Visible lenticel: Often a different color than the bark, lenticels can be oval, round, linear, or diamond-shaped.  Some retain their shape over time, while others stretch as the tree grows.  An example is pin oak, Quercus palustris.
4. Vertical cracks: Cracks often begin at lenticels, and they may be a different color depending on the color of the phloem layer that is exposed below.  An example is pignut hickory, Carya glabra.
5. Scales, plates, or vertical strips: Each of these structures are separated on one or more sides from one another, and appear as overlapping sections of a similar size.  Examples include black cherry, Prunus serotina, and American sycamore, Platanus occidentalis.
6. Ridges and furrows: As trees with hardened cork expand, the cork begins to crack.  Sections are held together by fibrous tissue, creating furrows between ridges.  Examples include Eastern cottonwood, Populus deltoides, and bur oak, Quercus macrocarpa

As you walk in the woods this season, consider taking along a tree guide to see how many trees you can identify by their bark.  In a future installment on ecology, we will take a closer look at the role bark plays in the relationship between trees and their environments.