When do trees stop growing? The question may be asked by woodlot owners looking to harvest their timber or curious minds who gaze into the tree tops and wonder. In truth, trees never actually stop growing. As they reach maturity, their growth slows dramatically, and most of the growth volume is put on in width while height remains relatively constant. There are a great variety of factors that contribute how exactly trees grow and what ultimately limits their growth, but before you dig in, it’s important to understand the fundamentals of how trees grow.
Growth Slows Dramatically as Trees Mature
When trees first germinate, it can take years before they accumulate any noticeable growth. For a small seedling, an entire season can yield only a new twig. Those twigs add up as the years go by, and eventually the plant will have enough photosynthetic material (leaves) to ramp up growth. As the tree ages, its growth accelerates rapidly until it reaches maturity, at which point growth slows to a crawl. The process is visualized in the graph below.
If you look at tree rings, you can see this same pattern. As the tree gets older, the width of the tree rings shrinks. Keep in mind, however, that just because tree rings are narrowing does not necessarily mean growth is slowing down. Because the circumference grows larger as the tree ages, narrower rings can still equal greater volume than wider rings at younger ages. Even so, rings will eventually become sufficiently narrow so that total volume growth slows. You can see the progressive narrowing well in the rings of this nearly 100-year-old black spruce below.
Tree rings only tell the story of diameter growth, however. It is likely the height growth on this spruce tapered off long before the rings narrowed.
Why Does Tree Height Stop Growing Before Diameter Growth?
Tree growth is ultimately an evolutionary function. Trees grow primarily for one reason: sunlight. Trees must grow tall to to reach the canopy and into the sunlight, otherwise they will be outcompeted and shaded by neighboring trees, possibly resulting in the tree’s death. When a tree is young and short, its first priority is to grow tall to avoid being shaded out.
Once a tree is successful in its sunlight-seeking endeavors, it still has a problem: the tree is thin and spindly. It is supported by the trees growing around it for now, but if the trees around it were to die or fall down (as they eventually will), it would only take a good gust of wind or snow storm to knock the skinny tree down. The organism now has to deal with this existential threat by growing outward and gaining diameter. It will sacrifice height to put more energy into producing horizontal mass to stabilize itself.
These, of course, are not conscious decisions of the tree, but rather outcomes and threat responses genetically programmed into the organisms based on growing conditions. Interestingly, if a tree is grown out in the open from germination, it may never put on much height relative to similarly-aged trees in a forest. It will instead put its growth heavily into diameter, and the trunk will take on a tapered shape to move its center of gravity downward to further protect against blowdown. Survival of the fittest is a law of nature.
Will Factors Other Than Age Stop a Tree’s Growth?
Absolutely! Tree growth can be limited by a number of factors, including bedrock and water table depth. Tree roots are an unseen but vital part of a tree’s growth. When a tree seems to stop growing at a young age, and it is getting plenty of sunlight, it is likely that the site does not provide much room for roots to grow. A good example of this is the forests of small trees found toward the tops of mountains. These forests, known as Krummholz, are grown on thin, sandy, and nutrient-deficient soils and are subjected to the harshest weather conditions. The growth is subsequently stunted, and despite growing for decades, they seldom reach more than a few feet in height.
As mentioned earlier, trees can also stop growing when they are shaded out by neighboring trees. This, like the trees of a Krummholz, is a response to limited resources. If overtopping trees are harvested and sunlight introduced into the environment, these suppressed trees will respond and recommence growth. This effect can be seen in the rings of the spruce below. The tree was overtopped or crowded out for years, but a shelterwood harvest in 1999 create room to grow and increased available sunlight. The tree responded well and growth accelerated massively for the next 20 years.
Tree growth is a fascinating and complicated subject, as trees tend to alter their growth patterns to respond to their external environments. Understanding these patterns is crucial for any forester or woodlot owner to be able to qualitatively assess a forest’s health and make good management decisions. Next time you pick up a log from your property, look at the rings and see what you can learn.