In this paper an explanation for the spatial variation in the structure of a Sumatran rain forest is suggested. The forest area consisted of a series of topographical units (terraces mountain slopes) of increasing age and decreasing pH and fertility. With increasing soil age the production of leaf litter and fruit decreased whereas the proportion of mature-phase forest increased and gap cover decreased. There is variation in the growth strategies of trees depending on soil fertility; this results in higher tree longevity on poor soils and hence in forest with fewer gaps and with more large trees than on rich soils. Thus Janzen\s (1974) hypothesis of the effect of soil fertility on the prevalence of anti-herbivore defenses and fruiting periodicity is extended to include aspects of forest structure.