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Bark Ages

February 3, 2011

The stark winter forest has me looking closely at tree bark. Identifying trees by bark texture takes some practice. Like all of us, bark ages.

But first, a bark primer and reminder. Underneath that outer bark I have been looking at is, well, the inner bark. Technically called the phloem, this acts like a pipeline for leaves to pass food to the rest of the tree. Between the bark and the actual wood of the tree is a thin layer, really only one cell thick, called the cambium. Auxins are hormones that are produced each spring in leaf buds. These hormones travel to the cambium layer via the phloem and tell the cells to produce a little bit of new bark on one side and often a lot of wood on the other, the annual rings.

Outer bark has many functions. It helps to prevent water loss through evaporation, protects against attacks by insects and disease, insulates the tree from temperature swings, and in some species protects it from fast moving fires.

Each tree species outer bark has patterns that are often distinct and are a great aid for tree identification. The outer bark cells are dead and hardened and as trees grow and expand, the outer bark stretches and tightens. Eventually the pressure causes it to split, crack or flake. Add a little damage from rain, ice, wind and snow and you can find a lot of variability in bark. Understanding how bark ages and changes can help you better identify trees.

Take our iconic tree of New England, the Sugar Maple. Young trunks have bark that is relatively smooth and gray colored. As the tree ages the bark begins to fissure in long irregular flakes. When it is really old it becomes thick, dark and deeply furrowed. Each bark age gives the appearance that you are looking at a completely different tree species.

Black Cherry bark changes dramatically as it ages. Young trees have thin bark with narrow lenticels, corky pores that allow direct gas exchange. As the tree grows the bark remains smooth like a young birch tree. But as the tree grows old the bark becomes dark charcoal colored and very flaky.

Leaning trees or sides of trunks exposed to more severe weather conditions can be abnormally smooth even when a tree is old. The fissures and furrows in the bark catch and hold rain, snow and ice. The bark foliates from the action of freeze-thaw cycles creating a smoother appearance. Wind blown snow can also be abrasive and create smoother than normal bark on older trees.

Severe cold spells followed by rapid warming can cause trees to split. These are called frost cracks. Apparently, late growth stimulated by warm fall temperatures, high humidity, and high nitrogen levels can cause a tree to be more susceptible to frost cracks. The crack usually begins internally, but may expand with freeze-thaw cycles eventually splitting the bark. Such cracks are an opening for disease and insects to attack, sometimes killing the tree.

During the winter the sun is low on the horizon in the north woods. The air temperature during the afternoon may only read 10 degrees Fahrenheit on your thermometer. But on a clear winter day, the sun’s rays strike more directly on the trunks of trees. Light colored bark reflects the sun and keeps the tree from acting like a solar panel and heating the interior trunk well above freezing. The rapid expansion and contraction caused by extreme temperature changes can cause the tree to crack.

But smooth bark on some tree species may be an adaptation for protection too. A load of dark-colored epiphytes on a light colored trunk can cause the tree to become even more overheated. Smooth bark is harder for epiphytes, like lichens and moss, to cling and grow. Some species, like Paper Birch, not only have smooth, white bark, but they also peel, which constantly sheds the epiphytes and other debris keeping the trunk clean and white.

Bark can tell us a lot about tree identification and ecology. On your next walk in the woods, check out the bark ages.

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