Do trees ever stop growing?

Do trees stop growing? The answer is both yes and no. The trunks of trees keep getting wider, and trees add new rings year after year. But, for all practical purposes, trees do stop growing in height. You can notice the similar height of the trees when you see a stand of trees of the same species.

Just like people, the slowing in the growth of trees is related to their age. Trees grow more slowly as they age. At a certain age, they essentially stop gaining height.

Consider the mountain ash. As a young tree, it might grow two to three meters – or about seven to 10 feet – every year. But by the time the tree reaches 90 years of age, its growth has slowed to about half a meter – roughly a foot and a half – a year. By the time the tree is 150 years old, height growth has virtually stopped, even though the tree may live another 100 years.

Some scientists suggest that tree cells are like animal cells: that is, they have to stop growing after a certain number of divisions. If a tree’s cells stop dividing, then it stops getting taller. Experiments have not proven anything conclusive about this theory.

Another idea is that a tree’s height is limited by the way it transports water from its roots to its leaves. Inside a tree, water moves upward – pulled towards the leaves – where it evaporates into the air through tiny pores. The cells inside the leaves and stems are inflated with water, kind of like air in a bike tire. Some scientists hypothesize that once a tree reaches old age and is therefore tall, the downward pull of gravity makes it more and more difficult for the tree to pull the water all the way to its upper branches and leaves.

So do trees stop growing? They do, and they don’t. At some point, their height is more or less fixed at the same height as other trees of that species. But trees continue to add width to their trunks – to put out new branches and sprout leaves – throughout their lives.

March 27, 2011

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