Many ways of governing the development of forest stands rest heavily on cuttings and other treatments that alter or modify the factors of the stand environment that regulate the growth of the vegetation. The reliance on ecological knowledge is all the firmer for resting on the virtues of the necessity and not simply on philosophical principle. The economic returns from forestry are not high enough to make it feasible to shield forests from all the vicissitudes of nature. Therefore, silviculture is usually far more the imitation of the natural processes of forest growth and development than a substitution for them. The necessity that nature should be understood and emulated does not mean that silviculture should slavishly follow either the reality of natural processes or abstract theories about them.
Most forests live longer than people.
It is, therefore, not easy to recognize that the natural disturbances that renew forests, often after intervals of centuries, are usually catastrophes such as fires, windstorms, and insect outbreaks. There are also forests that are slowly and continuously renewed by minor disturbances, but these are far from being any universal or typical form of nature. Paramount among the objectives of forestry in general and of silviculture in particular is the maintenance of the productivity of the living forest. The site is the total combination of factors, living and inanimate, of a place that determines this productivity. The site factors that are most subject to long-lasting harm are those of the soil, which is the most nearly nonrenewable of the resources in silviculture.