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美国波克纳山区增连续发生卷叶蛾,为弄清其危害程度,在该地区设置了130块样地,并从1970—1979年进行了详细的观查,现将其林分变化情况概述如下:
In order to find out the extent of its damage, a total of 130 plots of plots were set up in the Pokner Mountains in the United States and detailed observations were made from 1970 to 1979. The change of their stands is summarized as follows: