Bark

Now that most leaves have dropped from the deciduous trees -- the oaks, beeches, basswoods, birches, maples, hickories, and all the understory shrubs -- you can see deep into the forest and you can see tree trunks clearly. The various textures and colors of bark stand out, especially when lit by a late afternoon sun.

Look around at the various tree trunks. Better yet, get up close and feel the textures, although avoid hugging the ones oozing sticky sap. You will notice that tree trunks are more diverse than you thought. Bark textures include smooth, rough, scaly, shaggy, flaky, ridged, furrowed, or plated, among others. Some trees have thin bark others have thick bark.

Why so much diversity in tree bark? And did you know that trees carry on photosynthesis in their bark?

Michael Wojtech writes about bark in an article titled Getting to Know Bark in the latest issue of Northern Woodlands, a terrific magazine for anyone interested in plants, wildlife, woods, land stewardship, forestry, and so many other related topics. Michael points out the wonderful complexity of tree bark - or as he calls it "the multi-layered shell of a tree that can be detached from the wood." This layered shell includes a cork, cork cambium and cork skin which together are called the periderm, the outer part of the shell. The active phloem is next, the layer that transports food to all parts of the tree. The last layer before the wood is the vascular cambium, where cells divide and grow and produce the next ring of wood.

So back to photosynthesis. Michael notes that it occurs in the thin green cork skin, especially on new growth where sunlight can more easily penetrate the thin outer bark. Once the outer bark or cork layer gets too thick, photosynthesis ceases, except on new shoots and of course in the leaves. Scrape the outer bark of a sapling and you'll see the green cork skin.

Trees have different bark for specific reasons. The American beech has smooth, gray bark (unless damaged by beech bark disease), an adaptation that evolved in the tropics where beech originated. The smooth bark prevents epiphytic plants (lichens and mosses that grow on trees) which are common in the tropics from getting a foothold. Epiphytes in mass can weigh down and break or topple a tree.

Here in the north where winter temperatures fluctuate between day and night, freezing and thawing can damage cells in the tree. A tree with dark bark would absorb the intense winter sun, warming the cells which are then chilled at night. This would cause the equivalent of frost heaves on our road, except in trees the cells would burst and cause long term damage. Two of the most northern hardwoods - paper birch and quaking aspen - have light bark. This allows these trees to radiate, rather than absorb, the intense winter sunlight, preventing the freeze-thaw cycle. Other hardwoods that live in the north such as oaks have darker bark with ridges and furrows that help radiate heat soaked up by the winter sun, much like the fins of a radiator.

Softwoods of the north such as balsam fir and red spruce have dark bark, but they retain their needles year round which shades the bark. The resin in some tree bark helps repel insects. There are many more adaptations for trees to have different types of bark. Go out and get to know the tree bark in your neighborhood and think about why it is thick or thin, rough or smooth.