每一篇SAT阅读素材均有其主要观点或中心主题。典型的围绕文章主要观点的问题大多是：在这篇文章中作者的主要目的是什么?这篇文章主要涉及什么问题?这篇文章主要建议是什么?这篇文章总体上想要回答什么问题等。读完每一篇SAT阅读素材，我们都要针对SAT素材想想这几个问题。
What Do They Do?
1、Read the patterns of wide and narrow rings in wood from different trees. They match patterns from different trees to identify the year that a tree produced a particular ring. This process of matching across specimens is "cross-dating."
2、Estimate archaeological ages. Andrew Ellicott Douglas was the first dendrochronologist. He became famous for his studies of Pueblo ruins, which he analyzed by cross-dating, the tool that he developed as his "key to prehistoric chronology."
3、Predict future drought, flood, and other conditions. A. E. Douglas proposed that tree rings would be allow us to measure past climate and, potentially, to predict drought and flood over a period of years, which would provide economic advantage.
What Causes Variations in Tree Rings?
When conditions encourage growth, a tree adds extra tissue and produces a thick ring. In a discouraging year, growth is slowed and the tree produces a thin ring. Much of the variation in tree rings is due to variations year-to-year in:
1、Higher springtime temperature. If spring starts early, the growing season is likely to be longer than usual, causing a tree to have a wider ring.
2、Lower springtime temperature. A late spring is likely to shorten the growing season, causing a tree to have a narrower tree ring.
3、Abundant rainfall increases growth, producing a wider ring.
4、Drought decreases growth, producing a narrower ring.
5、Species of tree do differ in their response to weather changes. One might respond strongly to changes in overall rainfall, another might be more sensitive to the amount of rain during the late summer, and another to a temperature change that alters the length of the growing season.
6、Crowding from neighboring trees. This causes a series of narrow rings. Crowding is suspected when the series of narrow rings is more than three, because droughts are usually only one to three years.
7、If the rings are narrow on one side of a tree with wide rings on the other, the tree was crowded on the side of the tree where the rings are narrow.
8、A series of many narrow rings followed immediately by wide rings probably means that an encroaching neighbor died, releasing the crowded tree into a growth spurt.
9、Fire scars suggest past forest fires. The number of annual rings between fire scars shows the period between fires.
10、Scars due to insect plagues indicate insect infestations.
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