Tree bark

This time of year in a deciduous forest in the northern hemisphere you get to see a great deal of tree bark - gray, brown, flaky, smooth, furrowed and fissured, sometimes with lichen or moss growing on it.

This is the bark on a dogwood tree growing near the Apex Reservoir in Cary, North Carolina. If you click on the image to see it full-sized, you can easily count layers along the descending sides of the fissures in the outer bark or cork. I have not found any confirmation that these layers represent annual growth increments of the outer bark, but if they do, I can count as many as 22 layers in this image.

This outer bark of a tree is a protective layer of dead cells meant to be partially shed as they shield the tree from physical impacts of the weather as well as the grazing of animals from deer, beavers, and birds to insects, and the biological attacks of bacteria, fungi, and viruses. In some trees, this outer bark layer contains materials that resist fire, allowing the tree to survive all but the most intense, canopy fires. A tree can also excrete waste products into the cells of the outer bark.

At or just below the base of the fissures seen in this photograph lies the cork cambium, a layer of actively dividing cells that produce the largely dead outer bark layers. Just beneath the cork cambium lies a layer of living cells called the phelloderm that can serve a variety of roles including photosynthesis, active disease defense, and storage.

Below the phelloderm lies the phloem, the inner bark layer filled with the vascular sieve tubes that carry the sugars produced during photosynthesis in the leaves down to the rest of the tree and its underground roots.

Dig just beneath the phloem and you hit the cambium, the layer of actively dividing cells that produces the thickening or radial growth of the tree trunk. This is the inner end of the bark of the tree, and also the outer beginning of the inside structure of the trunk.

The actively dividing cells of the cambium layer produce not only the bark of the tree, but also the entire inner trunk of the tree. This inner wood, called the xylem, includes a variety of tubes that carry water and dissolved minerals up from the roots to the stems and leaves. The xylem also contains stiff vertical tubes called fibers that support the heavy aboveground weight of the tree.

For an overview of bark found on trees around the world, take a look at ArtSylva's post on the biology of barks. This beautiful site created by photographer Cédric Pollet talks succinctly about the variety of barks, their function for the tree, and their uses for people. And the collection of pictures of bark of all colors and bark found on many different kinds of trees is amazing. If you have not seen a baobab tree, visit this site and find one in Cédric's "Photo Reports" link!