一、天然林的集约经营目前美国的天然林多数都没有很好经营。在天然林中同时研究几种经营措施的也很少,而这一点对将来综合研究设计是很重要的。例如长针松的早期间伐对密度大的林分经营有利,这一点现已普遍应用于生产。但是在美国西部,只采取间伐措施往往还不够。如在东俄勒冈的干旱地区,对花旗松幼林来说,只有采取抑制其林下植物的方法才能得到最大的效果。威廉姆逊等最近研究花旗松和小干松林时指出,只有在林下植物稀少或土壤水分不太缺少的情况下,才能只用间伐措施来加快林木生长。近年在美国一些林区中进行各种施肥试验。多数试验结果证明,氮素是影响生长的主要养分。据称,阔叶林施肥的增产率平均约为80%。对北部针叶林的初步施肥试验说明,林 I. Intensive Management of Natural Forests At present, most natural forests in the United States do not operate well. There are few studies on several management measures in natural forests at the same time, and this point is very important for comprehensive research and design in the future. For example, early thinning of Pinus sylvestris favors the management of dense stands, which is now commonly used in production. However, in the western United States, it is not always enough to take only the thinning measures. For example, in arid regions of Eastern Oregon, for young Douglas fir trees, the best effect can only be achieved by adopting a method of restraining their understory plants. When studying Douglas fir and Pinus thunbergii recently, Williamson et al. Pointed out that thinning can only be used to speed up the growth of trees if the understory plants are scarce or soil moisture is not too scarce. In recent years, various forest experiments in the United States. Most test results show that nitrogen is the main nutrient that affects growth. Allegedly, broad-leaved forest fertilization yield an average of about 80%. Preliminary test of fertilization in the northern coniferous forest shows that Lin