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Tuesday, February 08, 2005

How Trees Grow (Hint: not like us)

Trees don't grow like us.

Trees have many appealing features. One of them is that they seem to reach for the sky. Another is the texture of the bark. Both can be explained (at least on one level) in terms of the way trees grow.

We, and "higher" animals generally, grow by expansion. We start out small, and end up larger, by expanding each part. Our feet get larger, our liver gets larger, etc. Not exactly all in synchrony, but basically all parts get larger. In other words, we grow more or less like a balloon.

Not so trees. Trees add on to what is already there. If there's a forty-year old tree in our yard, the outside layer of the wood, the inside layer of the bark, and the tips of branches, are all new. The newest are 2004 vintage. (Assuming no growth yet this year.) Then, next to them, 2003's production, then 2002, etc.

One consequence of this pattern is that the oldest bark is on the outside. Since the oldest bark isn't as large as newer bark, it gets stretched, as the wood, and newer bark, push it out. That causes the cracks in bark, and causes it to come flaking off. The cracks, and the flaking off, differ considerably from species to species, but all the ways that they do it are wonderful, and give different species their character.

Meanwhile, back in the trunk, the oldest wood gets further and further from the outside, as new layers of wood and bark are built up outside it. In most trees, eventually it gets plugged up with various chemicals, naturally produced by the tree, and eventually is no longer useful for water transportation. This inner, older wood is usually darker than the outer, younger wood, and is called heartwood. It still is useful for support. Sometimes it's more useful for support than the outer wood. I remember some unhappy experiences trying to cut the heartwood of some types of trees with an axe. The outer wood, the sapwood, carries water and minerals up to the leaves, and also supports them, and the branches above.

As the inner wood isn't absolutely essential, in some trees, it is eaten away by various processes, such as fire or disease, and you get a live tree which is partly hollow.

Since trees don't grow by expansion, human markings on trees have an interesting history. Carving your initials into bark at, say, four feet off the ground, doesn't lead to the initials moving gradually up the tree as it grows. No, the initials will remain at the same height. But they won't stay as they were. The bark will expand, and eventually the initials, on the outermost bark, will fall off, leaving no trace of them, unless they are re-carved every few years.

There are lots of photos of trees out there. Also, for many of you, there are trees you can see by looking out the window, or by taking a few steps. You know what trees look like. Nonetheless, I recommend Bonnie's recent photo gallery, featuring collections on "Winter," "Fall," and "Leafless Trees."

I also recommend looking at real trees.

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