How Tree Crowns Differ in the Forest vs at the Edge: Competition and Growth
libre
Every one of us has a mental picture of what a forest is, yet if you suddenly asked someone (not a forester, of course) that question, they would probably answer: "Well, it's basically trees." Everyone has been in a forest and seen different trees there, gathered mushrooms and berries, listened to birdsong — but not everyone has noticed how different the crown of a tree in the forest is compared with a tree standing at the forest edge.
Why do trees at the forest edge and inside the forest look so different?
A tree growing at the forest edge and a tree growing deep inside the forest differ in both crown and trunk because of the space and light available to each. The edge tree has a denser, fuller crown shaped like a ball that reaches almost to the ground. The tree in the thicket carries its crown high above the ground, and its trunk is completely bare of branches or bears only their dried-out remnants.
Even if you come across a tree with such a high-set crown standing entirely alone, you can confidently say that this tree once grew inside a forest. The two trees also differ in their trunks: the edge tree's trunk approaches the shape of a cone, while the forest tree's trunk approaches the shape of a cylinder.
The reason for this difference in appearance is easy to work out. At the forest edge a tree has no competitors of its own kind. It does not need to fight for either light or nutrients in the soil, and there is no need to gain height, because nothing prevents it from using the available light.
What are dominant and suppressed trees?
Dominant and suppressed trees are the two groups foresters divide a stand into based on growth rate, and the division arises from the constant competition inside the forest. Inside the forest the situation is entirely different from the open edge: neighbours surround a tree on every side, and if it wants to stay in the light, it must stretch upward. So the trees reach toward the light with all their strength.
The trouble is that not every tree has the same strength, and soil conditions vary as well. Because of this, some grow faster and others more slowly, taking on a stunted look, their crowns squeezed from the sides and sickly — some even acquire the characteristic shape of a flag. Other trees outpace them in growth and rise a whole head taller, with fuller crowns and thicker trunks. It is important to note that both kinds were planted or seeded at the very same time. This forced foresters to split them into two groups — trees of good, rapid growth and trees of retarded growth, that is, dominant and suppressed trees.
What awaits the suppressed trees, what is their fate? They face starvation of both light and soil. Roots need nourishment from light too, in a sense: while they feed the tree, they themselves require food, and poorly developed crowns feed the root system badly.
Meanwhile the neighbours, which at that moment enjoy a more favourable position for light, confidently seize the living space of the tree that has fallen behind — both above ground and in the soil. The doomed tree, holding on for a while between life and death, finally dies. Such is the harsh law of the forest.
How fierce is the struggle for existence among trees?
The struggle for existence among forest trees is severe enough that more than 95 percent of them die before reaching maturity. Suppose 300,000 trees are planted. How many remain by the age of 100 to 200 years? Very few. A terrible mortality reigns among trees: of 100,000–200,000 seedlings, only about 500 trees per hectare may survive. It has also been observed that the most brutal competitive struggle occurs at an early age and gradually subsides toward old age.
Eventually the situation stabilises and balance sets in. There is enough light and soil for everyone. The strongest have survived — those which inherited a greater individual force of growth. But good heredity is not everything; external conditions are also needed so that this heredity can express itself.
Why is mortality among trees so high? The answer is not hard to give. At the start of their growth, trees occupy little space, which is quite enough for them. As they age and grow larger, as the crowns of the trees in the forest close together and the roots spread, the young trees need more and more living space and nourishment.
So they are forced to fight their neighbour, taking away its light and its food. The smallest advantages are used in this fight, because this is a genuine struggle for existence, the outcome of which is victory for some and defeat for others.
This law is universal for any forest. To survive and withstand the struggle, a tree must be not only strong but also fortunate — that is, it must land in favourable conditions.
It was noticed long ago that trees of good growth bear the most fruit, while the suppressed ones are condemned to barrenness. In this way natural selection takes place.
