The trees in Michigan grow during certain seasons (spring and summer) and are dormant during others (fall and winter). During the seasons in which it is growing a tree produces wood (also called xylem). The cells grown during the early parts of this growing period is different from the wood cells grown during the latter parts. It is the difference between the wood produced during the early and late parts of the growth season that cause the visible rings found in many tree trunks.
In the early part of the growth season the cells produced (called early growth) are larger than the late growth. Gradually, as the growth season winds on, the cells produce become smaller and more closely packed as the tree moves from producing early growth to late growth. Eventually the growing season ends and the tree enters a dormant stage. The change between early and late growth in a particular season is gradual and the change isnít easily noticeable. However, when the next growth season comes around the tree again produces the large cells of the early growth of the next beings there is a very sudden change between the two types of cells. This sharp change is what we see when we look at the rings in tree trunks.