山西北部雁北地区地广人稀,有大面积的宜林荒山荒地。五十至七十年代初,全区营造了大面积的小叶杨人工纯林,收到了较大的生态效益。但除了沿河及部分丘陵区下部长势较好外,约有60%的林分形成了“小老树”。这些低产林林龄20—30年,长势严重衰退,防护效益、经济价值逐年降低。因此,选择适宜的更新树种,提高更新速度和质量是生产中急待解决的问题。从1972年以来,在本区进行了樟子松引种试验工作。截止1984年春,已试栽樟子松3750亩,其中5年生以上的幼林占45%。经十多年的观测,无论在平川沙荒地或风积沙 North Shanxi Yan Ge area sparsely populated, a large area of ​​barren mountain wasteland. From the early 1950s to the early 1970s, a large area of ​​Artificial Pure Populus euphratica was created in the whole district and received great ecological benefits. However, apart from growing well along the lower reaches of the river and part of the hilly areas, about 60% of the stands formed “little old trees.” These low-yielding forests are 20-30 years old, with a serious recession, protection and economic value decreasing year by year. Therefore, the selection of appropriate updated species, improve the speed and quality of the update is an urgent problem in production. Since 1972, Pinus sylvestris var. Mongolica has been introduced in this area. As of the spring of 1984, 3750 mu of Scotch Pine had been planted, of which 45% were young people over 5 years of age. After more than 10 years of observation, no matter in Pingchuan sand wasteland or aeolian sand