Thinnings of forest stands invariably increase the diameter of trees, but not their heights. The question of why an augmented supply of light and water promotes the division of lateral cambial cells but not of the apical cells has so far failed to received a satisfactory answer from plants physiologists. On the other hand, heavily thinned stands supported by weed—free soils attain an average height equal to that of adjacent fully stocked stands in spite of a decrease of soil organic matter. This study of plantations of red pine, Pinus resinosa Ait., explains the cybernetic—like growth equalization of thinned and fully stocked stands on the basis of differential water availability and corresponding compensatory effects of organic matter and nutrients. A part in the restoration of growth of thinned stands may be played by grafted roots of stumps which stubbornly resist the invasion of wood—destroying fungi but preserve their water—conducting mycelia of mycotrophic symbionts. See full-text article at JSTOR