Lisa Dionne, age 13, of Coventry, R.I., for her question:
WHAT DO THE RINGS IN A TREE TRUNK MEAN?
By the rings in a giant sequoia tree found in California, we know that these trees have lived 2,000 years. Scientists know that the trees are continuing to thrive and cannot guess how many more years they will survive. X
Trees, of course, continue to grow taller and bigger each year. And as they grow bigger, they also add a layer around the trunk, just beneath the bark. The wood added in the earlier part of the year is different from the wood added during the summer, so when the tree is cut down, we can distinguish between one year's growth .and the next.
When the tree is cut across, it shows a number of rings, one inside the other. It is easy to read the history of the tree this way.
But we can only use this method of giving a tree's age after the tree has been cut down. While it is still standing, we can only make a guess at its age from its height, size and species.
we can also measure the age of very old pieces of wood by a method called radiocarbon dating. All plants, as long as they are alive, absorb small amounts of radioactive carbon along with ordinary carbon from the atmosphere.
Radiocarbon decays that is, it loses its radioactivity at a definite rate. But while the tree is living, it constantly is absorbing more of the radiocarbon to take the place of that which is decaying.
After a tree dies, it cannot continue to absorb carbon. But the decay of radioactive carbon continues.
At the moment of death there are 15.3 disintegrations per minute for every gram of carbon in the tree. After 5,600 years there will be just half that many, or 7.65 per minute for every gram of carbon, and 5,600 years later still the number will be cut in half again to 3.825.
Scientists can burn a sample of wood to be tested and capture the carbon dioxide that is formed in the burning.
They measure the carbon and then apply a special kind of Geiger counter. They count the impulses produced and thus figure the amount of radioactivity per gram per minute and from that the age of the wood. X
As a tree grows older, it must continue to bear the leaves by which it breathes and manufacturers its food. The tree's branches grow in such a way that the leaves are spread to the sunlight. The branches grow fairly equally on every side, and in growing sideways all around they are able to expose more leaves to the sun's rays.