Basically arise by growth rings that trees in summer and winter at different rates grow. In the spring, the tree produces large cells (earlywood), by the amount of wood grows. During the year, the size of these cells reduced more and more (late wood), be formed until no more new cells and the tree no longer grows. The next spring produces large wood cells, which in addition to the smaller cells again are from last year's fall. At this point, the growth ring boundary is visible as a sharp transition. Therefore, one can determine the age of a tree based on the annual rings. In addition to the annual rings can realize how much a tree is grown in a year. But also reveal something about the specific environmental conditions under which a tree is grown. Wide rings show eg on strong growth, narrow rings point to bad years with high pest infestation or maybe a little rain. Oval rings with a different strong growth indicate that the tree has stood on a slope or strong wind from one side was exposed.