Monday, January 9, 2012


Finally, we're past "stick season" and snow makes it feel like true winter.The long slant of afternoon sunlight illuminatesintricate patterns of tree bark.Instead of a haze of green, trunks stand out as individuals.Some say that a tree's bark is as unique as your fingerprint.While I have yet to find a study that proves it, I have noticed that bark, even within the same species of tree, is incredibly variable.The age of a tree, its health, its growth rate, and its habitat all have an effect on the pattern of its bark.

While skiing or hiking through the forests around here,I tend to entertain myself by identifying plants while zooming down hills.Bark patterns among different species are fairly distinct once you know what to look for.In general, the fissure pattern in bark is a result of the genetics of the tree species, and the way trees grow.

Here's a riddle:If a pirate buries treasure underneath a tree and marks the tree with an X five feet above the ground, and the tree grows one inch taller per year, how high will the X be in 100 years?

The answer, of course, is about five feet, depending on local soil erosion.Trees only grow taller from the tips of twigs.Trees do get bigger around, but only by adding new layers of cells just under the outer bark.And there lies the key to bark patterns.

The center of a tree is dead; in fact, most of a tree is dead!In the trunk there are several layers of different cells.Xylem and phloem are two types of transport tissue. Xylem, which is dead at maturity, carries water and minerals up from the soil. Old xylem becomes what we think of as wood, and makes up the bulk of a tree's mass. The living phloem carries sugars down from the leaves. They have a layer between them called the vascular cambium, which creates the new xylem and phloem cells. Cambium is made of undifferentiated cells that can become anything - like stem cells. These three layers are considered the "inner bark."

Outside the phloem is the cork cambium.Just like the vascular cambium, the cork cambium produces a layer of cells inside it (living phelloderm cells), and outside it (dead, air-filled cork cells).These three layers make up what we call "outer bark."

As the cambium layers create new cells, the tree expands just under the cork layer.Since the cork is dead, it can't stretch or grow to accommodate the larger girth.The bark cracks instead, and eventually sloughs off like dead skin.I think that paper birch has such smooth bark because it peels off instead of cracking.

Why do trees need bark anyway? Outer bark is the armor of a tree, and it protects the inner bark.There is something very tasty hiding in the inner bark.Think ahead to maple sugaring season and you'll know what I mean.Insects, fugus, bacteria, yellow-bellied sapsuckers, deer, moose, snowshoe hares, porcupines, humans and many other things want the sugar created by photosynthesis and transported in the phloem.

Outer bark, like our skin, can help keep these invaders out, or scar over an injury when they break in.Also like skin, bark slows water loss, allows certain gasses to be exchanged, and protects from intense sunlight. The pigments in colorful bark, like on red osier dogwood, act as sunscreens; while greenish bark, like on aspens, can photosynthesize. Bark can also protect trees from extreme cold andheat, including fire.

One of the "active ingredients" in bark is a chemical called tannin.You may have encountered tannins in the way your mouth feels dry and puckery after drinking wine or eating unripe fruit.Tea also contains this bitter brown chemical, and the tea-colored water in bogs is the result of plants steeping in water and releasing the soluble tannins.

Tannins provide a unique defense mechanism against herbivores.Condensed tannins bind with proteins and inhibit digestion.That's one reason that animals tend to eat the newest twigs and softest bark - they have fewer tannins.The ability to bind with proteins made tannin useful to humans, too.We used it for many years (and some primitive skills buffs still use it) in the tanning of hides to produce leather.The tanning process makes leather more flexible and resistant to bacteria. Hemlock bark was once a source of tannin for the leather industry.

Bark from various species also supplies us with spices (cinnamon), medicines (quinine), resin, latex (rubber), poisons, landscaping mulch, and cork (for that wine!). We can also fashion it into cloth, rope, canoes, roofing,dyes, and baskets.

So when the sun's rays sweep through the afternoon forest, take a moment to enjoy the wonderful colors, textures, and patterns of the beautiful, protective, and very useful bark.

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