Tree Anatomy and Growth

The fact that grain exists in forest products is not the only characteristic of significance. We also must concern ourselves with the fact that wood fibers can be very different and the patterns of these differences can work in our favor, or against us if we are not careful. As a tree grows from a sapling into a mature tree we can start to see differences in structure in the cross-section of the tree.

The oldest part of the tree, at it’s center, is called the pith. The pith is useless to woodworkers as it tends to crack very easily. Adjacent to the pith is the heartwood - the region of the interior of the tree that is dead. Chemicals called extractives build up in these dead wood fibers and they serve to protect them from fungal decay and insect damage. It is the heartwood that woodworkers build with. Next is a lighter colored region known as the sapwood. The sapwood is alive and is resistant to decay and insect as long as the tree lives, but once the tree is cut down or dies, it loses this ability, which is why you should avoid choosing lumber with sapwood. The next layer is very thin and lies between the sapwood and the bark. It is called the cambium and is the fastest growing part of the tree. Every year a new growth ring develops at the cambium, and every year the oldest ring of sapwood dies and turns to heartwood. The year’s growth is evident in the form of growth rings.

Each ring is composed of two distinct parts; the earlywood and the latewood. Earlywood, the inner portion, is formed in Spring and is typified by cells with larger cavities and very thin walls which grew very quickly as the tree came alive in anticipation of the summer ahead. Latewood is comprised of cells with small internal cavities and thick walls. Called Summer wood, they tend to appear darker due to the higher concentration of cellulose. In fall, the tree retreats into dormancy to repeat the process again next spring.

This seasonal pattern of growth produces the distinctive growth rings we see in the cross-section of the tree. Growth rings are important as they provide visual indication of the grain of the wood. We cannot see the wood fibers themselves, but the variation in the appearance of the wood caused by growth rings allows us to quickly see the direction in which the grain runs when looking at the face of a board. When looking at the end grain, they provide us with even more information. From the end we can tell from what part of the trunk the board was cut and use that information to anticipate how dimensionally stable, or unstable, that piece of lumber might be.