Marie Echenique, age 11, of Costa Mesa, Calif., for her question:
HOW DO TREE RINGS TELL THE YEARS?
The average tree keeps a diary throughout its entire life. It records the years, plus a number of other interesting details. It notes down the rainy and dry seasons and points where branches sprouted from this side and that. This remarkable diary can be read in the circles of rings around a sliceof'the tree's trunk.
The story begins with the young sapling and ends when the tree is cut down or dies of old age. The young tree grows by adding layers of woody cells around its trunk and branches and by extending the tips of its twigs. These new cells are constructed in a special layer just below the bark. Each layer of new growth forms a circle around the outside of the trunk.
The average tree has to cope with changing seasons, winds and sometimes forest fires. Growth requires moisture, warmth and sunshine. During the summer season, the tree adds a thick layer of large woody cells around its trunk. In winter, growth slows down. When new cells are added, they are small and the ring is thin. The summer growth appears lighter, the winter growth is darker.
When the tree is young, all the woody cells in its trunk transport food in the form of liquid sap. Later these duties are taken over by the outer rings, and the core of the trunk dies. It forms a sturdy column of tough heartwood, which helps to support the growing tree. The paler, softer rings around the heartwood are called sapwood because they still carry the life giving sap.
The annual rings are seen when the tree is cut down and sliced across the trunk. Each ring is a double layer of lighter cells built during the summer and small darker cells built during the winter. The counting begins in the center of the heartwood and moves outward, ring by ring. Each circle of light and dark wood records one year in the life of the tree.
A pine tree drops its lower branches as it reaches for the sky. These wounds are healed with gobs of hard, dark resin and covered over with rings of new growth. They form the hard knots in knotty pine and tell how old the tree was when its branches fell.
Sometimes a fierce gale bends a tree to one side. Later, the tree adds wider rings with extra.weight on the leaning side of the trunk. A buried scar may mean fire damage. A very narrow growth ring means that the tree lived through a season of miserable weather, when new cell production was reduced to a minimum.