Forest ecosystems, whether natural or artificial, although greatly influenced by man’s activities, continue to be composed predominantly of wild species that have never been subjected to the processes of artificial selection and breeding. Therefore, forest ecologists and geneticists have been more concerned than agronomists and agricultural crop breeders with the inner mechanisms of natural and artificial ecosystems, of which many transitory forms exist. Only by knowing these mechanisms intimately could they hope to improve the economics of forestry ; not only through control of stand structure and species composition, maintenance of soil fertility, and choice of varieties, but also through reduction of ecological risks. The latter is regarded as being particulary important as the yields of forestry per unit area are likely to remain substantially below those of agriculture and horticulture. Direct control of diseases and parasites, which is commonly practiced and required in field and garden crops, amounts to an interference into the balance of tree species with these organisms, and this must be kept to a minimum.