2、Do you know something about tree rings（年轮）? Do you know they can tell us what the weather was like, sometimes even hundreds of years ago?
A tree will grow well in a climate with lots of sunshine and rainfall（雨量）. And little sunshine or rainfall will limit（限制）the growth of a tree. We can see the change of climate by studying the tree rings. For example, to find out the weather of ten years ago, count the rings of a tree from the outside to the inside. If the tenth ring is far from the eleventh ring, then we're sure that it was sunny and rainy most of that year. If it is near to the eleventh ring, then the climate that year was bad.
Tree rings are important not only for studying the history of weather but also for studying the history of man. Many centuries ago there live a lot of people at a place in New Mexico. But now you can find only sand there － no trees and no people. What happened?
A scientist studied the rings of dead trees there. He found that the people had to leave because they had cut down all the trees to make fires and buildings. As all the trees had gone, the people there had to move.