The present and projected needs for increased food production in the world are well recognized. Less attention has been directed towards future demands for fiber crops, which may be in more critical shortage than food crops. Trees-the main fiber-producing species-also provide fuel, animal feed, human food, and chemical feedstocks, as well as aesthetic and nonfiber benefits. In the United States, as in most of the world, the productivity of forest lands is far below potential levels. This situation contrasts sharply with that of food crop lands, which in many areas appear to be approaching maximum productivity. The 200 million hectares or more of commercial forest land in the United States are likely to decrease in size in the future. To meet the demand for more forest products, these lands will receive much more intensive management than in the past (Clawson 1979). Intensive culture of tree crops-"agroforestry"has already begun in this country and others (Anon. 1978, Douglas and Hart 1976, MacDaniels and Lieberman 1979, Smith 1953). Two important components are implicit in the development of future agroforestry systems. First, the use of genetically improved tree varieties and hybrids has been slow, and the use of improved materials is limited. This situation must change if intensive tree culture is to be successful. Second, there must be a broad base of knowledge about and practical application of tree growth control by environmental factors. Although much progress has been made in the science of tree physiology, little is known about how to achieve maximum tree growth by manipulating the environment. The use of growth control techniques on both nursery seedlings and trees established in plantations will greatly enhance productivity. To illustrate some of the problems and poten-