What do trees die of when they die of old age?

Answer 1

Old age is not the problem for plants that it is for animals. Being modular, plants can grow new limbs when old ones die off. More crucial to the longevity of a tree is its size. A tree reaches a stage when it cannot get taller, owing mainly to the difficulties of bringing water up from the roots, and when its side branches cannot grow longer, because they are too expensive to support. So the number of leaves a tree holds becomes more or less fixed, and this means that the tree's ability to produce food--the sugar made in leaves by photosynthesis--also levels off.

Yet each year the tree adds a new layer of wood under the bark, and the amount of wood needed to coat the whole tree increases, just as, in a set of Russian dolls, each new doll on the outside has to be bigger. As the tree grows, the amount of food needed for running it rises. The tree resembles a bank account whose income (sugary food) is fixed but whose outgo (respiration and new wood) keeps mounting. The tree compensates for a time by producing narrower and narrower rings, but there comes a point when a ring cannot get any narrower. Something has to give, usually the water-deprived top most branches. The result is a stag-headed tree, so named for the antlerlike dead branches sticking out of the top. A downward spiral begins: the loss of branches means fewer leaves, and fewer leaves means less new wood.

But many trees can slow the process. Some have buds in the trunk that sprout new branches. These may hold enough leaves to make up for those lost higher up, so the tree can keep the leaf area constant while cutting out the expensive-to-maintain upper trunk and its big branches.

Although these new trunk branches are fairly short-lived (a hundred years in oak, sixty years in hornbeam and beech, and less in birch and willow), an oak with plentiful trunk buds can stave off death for centuries. As the old saying goes: "Oak takes 300 years to grow, 300 years it stays, 300 years it takes to decline." Perhaps we should think of a stag-headed oak as merely entering middle age and, like many humans, just going a little bald on top.

A tree has no fixed life span. To live long, it must stay small. One way to do this is to grow slowly. Bristlecone pines are the supreme example: they live on poor soil in a dry, cold environment with a short growing season. One bristlecone in the American Southwest has been documented at three feet tall, less than three inches in diameter, and 700 years old! The other way to stay small and live long is, paradoxically, to be cut down repeatedly. (This strategy, of course, will work only for trees capable of regrowing when cut.) The ash Fraxinus excelsior normally lives for 250 years, yet Suffolk, England, hosts a coppiced ash with a stump almost seventeen feet in diameter. It is at least a thousand years old.

A tree's bank balance is also influenced by savings in the form of food reserves. As a tree gets bigger, however, it has less food left over. At the same time, the larder--the sapwood--gets smaller. Eventually, infections penetrate inner structures, and storage capacity is lost behind a barrier zone, a layer of new cells produced in the inner bark to seal off infected wood. The living part of the tree is walled into a thinner and thinner space under the bark. Part of the tree dies. New branches on the trunk can still save its life, but a large old tree is not good at producing new shoots, perhaps because it is running out of stored buds or because they are trapped behind thick bark. New sprouts on weak trees often die just when people think the tree is going to live. This may be because the barrier zone is missing or because there are too few reserves left for the tree to grow a strip of tissue from the new branch down to the roots. Either way, disease easily overtakes the tree, and the branch withers away. At this point, the tired old tree bows out gracefully.