AgroJbrestry Systems 11: 175-197, 1990. © 1990 Kluwer Academic Publishers. Printed in the Netherlands. 175 Iterative increase of economic tree species in managed swidden-fallows of the Amazon JON D. UNRUH The Woods Hole Research Center, P.O. Box 296, Woods Hole, MA 02543, U.S.A. Key words: Agroforestry, fallow utilization, spontaneous community enrichment, fruit trees, frugivores, Amazon Abstract. Fallow utilization schemes are becoming increasingly popular in agroforestry designs. However little attention has been given to the fate of valuable fallow plants after the end of the initial fallow cycle, and over successive fallow cycles on a regional or inter-fallow level. Evidence is presented here for the spontaneous long-term community enrichment of an area with valuable plants in a cyclic or iterative fashion, in the context of an existing fallow utilization scheme in the Amazon. Review of the ecological processes which operate at microsite, field and community levels indicate that: a. Due to valuable plants left uncut in the swidden, seeds from valuable species are better able to survive the burn, and may experience a favored germination and establishment because of reduced competition, enhanced fertility of the young swidden, nutrient input from litterfall, and reduced soil temperatures and soil moisture evaporation. b. The much greater frequency of suitable germination and establishment sites in and around canopy gaps created by management practices during the fallow cycle interacts with a spatial and temporal distribution of fruit trees which may encourage a specific optimal foraging strategy among large numbers of frugivores. The net effect of this interaction would be to decrease competition for, and increase efficiency of, seed dispersal into the large number of available establishment sites. c. A greater opportunity for frugivore generated seed shadows exist in managed fallows due to the larger number of trails in and around these sites, which are used as flyways. d. A presence of the more efficient 'specialist' fruit tree seed dispersal strategy in managed fallows - as opposed to the 'opportunist' strategy - which produces patterns of seed dispersal more beneficial to the plant, and may have a longer more evenly spread fruiting season; which, along with relatively large numbers of trees would encourage frugivores to 'camp out' on the resource, depositing seeds nearby. The existence of 'relict' plant species that reflect pre-historic forest management which has led to long term and persistent effects on forest tree species composition. e. Introduction Presently much attention is being placed on investigation and improvement of traditional subsistence farming systems which are able to provide sus- tained yields in tropical countries [Horowitz 1986; Rambok 1980]. This interest has arisen in the face of alternative (imported) systems failing to