MEASURING ANNUAL GROWTH Cut down a tree, measure the diameter of its stump and count the rings in the outside inch of wood—the first inch inside the bark. Multiply the diameter by the number of rings to this inch. Divide 40o by the product obtained by this multiplication. The quotient is the percentage of yearly increase of the tree.
This seems like an arbitrary formula, and it is not accurate to a hair. But it is a practical method for estimating the yearly accretion that a tree makes. It is the method used by the Bureau of Forestry in estimating the annual growth of woodlots, and it is so simple that anyone can use it. Farmers can tell how much interest they are getting by letting their trees grow, and when they are cutting into their wood principal in harvesting the crop. It replaces guesswork by knowledge.
One tree does not make a forest, nor a woodlot. But one
tree is a key to the rest. Take each kind of tree by itself. Cut sixteen or twenty white oaks of different sizes and grown under varying conditions at different places on the woodlot. Their average will fairly represent the individuals of this species. Take the tulip trees in the same way, and get the increase of the average tree. When the different species have been considered separately, they may be averaged to get the general per cent. of growth for all. Then the owner knows about what amount of wood cut in a winter will be replaced by the growth of the following summer. The secret of success in the best-kept forests of Germany and France is the management that does not cut more than the annual increase will restore. It explains the perennial vigour and productiveness of these secular forests.