根据不完全统计,美国用材林地面积约为两亿公顷,其中杂灌木占优势及存在所谓“森林杂草”问题的面积不少于六千八百万公顷。这就是说,在近三分之一的用材林地上生长着各种低价值的杂灌木林。在美国北部及南部地区,杂灌木林的比重高达40～45%。造成这些次生林的原因很多,诸如不 According to incomplete statistics, the total area of ​​timber forest in the United States is about 200 million hectares, of which the area of ​​shrubs is dominant and the so-called “forest weeds” are not less than 68 million hectares. This means that a variety of low-value shrubs grow on nearly a third of the timber land. In the northern and southern parts of the United States, the proportion of miscellaneous shrubs is as high as 40-45%. There are many reasons for these secondary forests, such as no