Life Cycle of a Tree


We have so much to do with human nature in February, in connection with the celebration of the birthdays of great men, it suggested for this chapter some comparison between the lives of trees and the lives of men; among other things, about how trees write their autobiographies, as great men have so often done, one way or another. Some of these tree autobiographies are quite sad, as in the case of the tree with a broken heart, and the one with a chapter in it about the dreadful fire and the year of the plague. On the other hand, in reading these life records, we shall find that trees have curious traits that are so much like those of human beings they make us smile. For example, we shall learn that at a certain average age for their families, they stop growing taller, and then, usually after they get to middle age, begin getting larger around the waist -- just as men are so apt to do if they don't keep up their gym work. We shall not only see that this is so but why it is so; and why trees lose the keen appetite of youth with advancing years. I'm sorry to say I shall also have to tell that all trees have a tendency to grow intolerant -- that's the forester's own word, "intolerant" -- as they grow in age, while others are just naturally born that way.

I used to think that intolerance was confined to mankind!


But don't you think it will make interesting reading? Moreover, as I just finished saying in the January chapter, everything about the nature of trees is a part of the science and art of forestry, and you can't get along with trees, any more than you can with people, unless you know something about their dispositions and their ways; and the more you know the better.

Take this matter of intolerance. The word doesn't mean quite the same thing, to be sure, when applied to trees as it does when we are speaking of human beings, but there is a striking similarity in the nature of "intolerance" and its bad results among trees as among men. Among human beings an intolerant person is one who doesn't want to share the light of truth with other people. Always, to his way of thinking, he is not only entitled to all the light there is on a given subject, but he believes that he has it already; that wisdom shall die with him, as poor Job said to his opinionated friends, Bildad the Shuhite, and the rest. In the world of the forest an intolerant tree is one that will not willingly share the sunlight with its neighbors; or, if it is compelled to do so, gets grouchy about it and so doesn't amount to much. Such trees also remind me of the folks that always want to have the front seat and to be seen of men; particularly when I think of it in the language of forestry:

The ability of a tree to endure shade is called tolerance.


Trees, like people, differ widely in the amount of shade they can bear. For example, the Aspen, the Pitch-Pine, and the Western Yellow Pine must have all the sunlight there is or they "won't play"; while the Beech, the Hard Maple, and the Western Red Cedar -- this last of the very same family, mind you, as the two other Pines I named -- are perfectly contented in quite heavy shade, and get along beautifully with their neighbors. Of the "Christmas Tree family," the cone-bearers, the Larches, Lodge-Pole Pines, Western Yellow, Pitch, and Norway Pines are classed among the intolerants; while the Balsam and White Firs, the Hemlocks and the Spruces are tolerants, and the Red and Douglas Firs and the Loblolly, White, and Scrub Pines take a rather middle-of-the-road attitude on the sunlight question. Among the hardwood trees the Hard and Red Maples, the Beech, Black-Gum, and White Elm are classed as tolerants; the Black Locust, Tulip, Gray Birch, Black Cherry, and Hickory as intolerants; while the Chestnut, Red Oak, Butternut, Ash, and Black Walnut rank in between.

Why does a Pine forest occur in a given region, instead of a hardwood forest, or vice versa? Of course, soil is a factor, but the largest factor is light. Take the White-Pine region of New England, where formerly there were the finest Pine forests in the world. After these forests were cut the hardwoods began to take the land. Why? Because they responded more vigorously to the light than the Pines. Now, with the hardwoods established, certain species of Pine seedlings are coming in underneath, where shade conditions are more favorable to their growth.


As we all know from experience, it is a great advantage, as well as much pleasanter, to be able to get along with other people. The same is true of trees. Tolerant trees, owing to their ability to stand shade, will remain and occupy large areas after the intolerants have disappeared.

Tolerance, as applied to trees, means that the ones that cannot only do with the least sunlight, but with the least food and moisture, will hold their own, while other trees that have always been used to a good living and feel that they just can't get along without it, will pass out of the picture. So, from a certain standpoint, we may say that of two of the great men whose birthdays we celebrate in February, Washington and Lincoln, the life of Washington is more remarkable than that of Lincoln; for Washington, born to a life of wealth and ease, voluntarily chose the hard path because it was the path of duty, while Lincoln's equally strong and unselfish character was developed in the severe school of necessity in which he was born.


Trees that have a tendency toward intolerance get more intolerant as they grow older. For instance, the seedlings of the White Pine, which is not so intolerant as some of its relatives, will do very well for a few years in the shade of older trees, but after a while you see a difference. Their leaves, no longer that deep lustrous green which marks the healthy tree, turn pale yellow, and the buds begin to look small and discouraged.

So, using the word "intolerance" now in the same sense it is applied to trees, we know that people as they get on in years are more sensitive to their surroundings, less able to put up with things, than they were in their younger days.

"My growing love of easy shoes, My growing hate of crowds and noise, My growing fear of taking cold, All whisper, in the plainest voice, I'm growing old!"

Yet, taking the meaning of "intolerance" as it is usually used when speaking of human beings, youth is often more intolerant than age, more sure that its opinions are absolutely right and all opposing opinions absolutely wrong; while those who keep their minds open and "incline the ear to instruction" grow more mellow, broad, and tolerant with the advancing years.


Trees, like people, have less vigorous appetites as they leave the days of youth behind. Often, when new groves have been started on sandy soil, the trees have done well up to the age of forty or so; then, for no apparent reason, growth rate has begun to decline, and after that, frequently, they became the easy prey of insect and fungus enemies; for trees are like us in that respect also--the better their general state of health the less liable they are to be attacked by specific ailments of any kind.

There is a similar difference between the older and the younger leaves on the very same tree. In younger leaves the power of digestion in proportion to the chlorophyll!, the green substance that does the work, is greater than in the older leaves.


In John G. Saxe's humorous lament on growing old, from which I quoted the lines about his fear of taking cold, there are two more that, with a little modification, we can apply to the trees:

"I see it in my thinning hair, I see it in my growing waist."

Like mankind, trees do most of their growing in height in their earlier years. Then, having reached the average height of the people in their families, they "begin to put on flesh" as we would say of a human being, and particularly "around the waist," that is to say, around their trunks. The reason trees attend, first of all, to growth in height is that this enables them to help themselves to more of the sunlight. The crowns of all young trees grow faster at the tops than at the sides, for there is unlimited room above, while on every side, in a well-populated tree community, are the competing branches of other trees.

If it is true, as I have been told, that boys do most of their growing when they are asleep, that is to say at night, then we have here another curious parallel between ourselves and our tall brothers of the forest, for trees do most of their growing at night also!

Growth in trees, however, goes on until extreme old age, if they are kept sound and well, but growth in height gradually decreases and finally ceases altogether, while growth in diameter goes right on. The chief reason trees stop growing in height is that they are not able to keep the upper parts of their crowns properly supplied with food and water above a certain distance from the ground. This distance varies with different species of trees. It depends, also, as you would naturally suppose, on the health and vigor of the tree.1 (And I wonder if it doesn't make a difference with trees, as it does with the rest of us, whether they take sufficient exercise. See what you think about this when, in the March chapter, we visit the gymnasium of the winds.)

It is from this weakening of circulation, which comes with age, that trees have a tendency to grow "bald-headed". The topmost branches of the crown die and the leaves fall off, producing what are generally called "stag heads", because of their resemblance to a stag's horns. These bare branches show conspicuously above the mass of foliage and are favorite outlooks for crows. Stag heads are most common as the result of the removal of undergrowth which helps to keep the soil moist and to enrich it by holding the annual leaf fall from blowing away.

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