There is a misconception that big trees are old and small trees are young, but in reality tree size at a given age is more related to the species and growing conditions. It is not uncommon to have two trees side by side, one two foot through, the other the size of a fence post, and both be the same age.
Different species of trees grow at different rates. Paulownia, a non-native tree that has naturalized and is now common, can grow 2 inches of stem diameter per year. Yellow poplar is pretty fast as well, and under the right conditions can put on ½ inch of growth per year. Oak, a valued timber and wildlife species, takes its time with cell division and probably averages maybe ¼ inch of diameter growth per year. Foresters generally gauge tree growth in 10-year increments.
The main reason trees grow at different rates is the conditions they are growing in. Sunlight amount, soil depth and moisture, which compass direction the hill faces, location on the hill (ridge top or hollow?), all affect growth rate. The tree growing potential of a certain area is called “site index” in forester-speech, and we can actually give it a numerical score for each species.
Site index is determined by measuring the height of the tree and comparing that to its age. We use a device called an increment borer to pull a small core of wood out of the tree and count the growth rings. It’s sort of like drawing blood and does not harm the tree. We use a neat tool called a clinometer to determine tree height.
The age and growth rate of trees are used to make important forest management decisions. If the trees are really old and mature the landowner may want to harvest them before they begin to degrade with age. If the trees are young but growing slowly, it could be the forest is too crowded and needs thinning out to give more sunlight to the better trees.
For more information on trees, their age, and their care, contact your local state forestry office.