一些林业先进国家在实践中证明:疏伐只能提高林木生长量的4—6%,集约经营也只能提高林木生长量的10—20%,采用良种一般可使林木生产量提高30%,有时可达50—60%,甚至成倍地增加。说明了良种是增产的有效措施,是保证造林速生丰产的重要条件。在我省,北京杨是六十年代引进,七十年代得以大力推广,青杨在东部农业区 Some advanced forestry countries have proved in practice that thinning can only increase 4-6% of the growth of trees and intensive management can only increase 10-20% of the growth of trees. By using improved varieties, the yield of trees can generally be increased by 30% Sometimes up to 50-60%, or even multiplied. It shows that the elite is an effective measure to increase yield and is an important condition to ensure fast growing and yielding afforestation. In our province, Beijing Yang was introduced in the 1960s and was vigorously promoted in the 1970s. In the eastern agricultural area