Agroforestry Systems

Concepts used by various authors are analyzed in order to define agroforestry and discuss, in detail, some ambiguous terms often encountered (biological and economic interactions, time sequences, etc.). Only those concepts which differentiate agroforestry from other non-agroforestry forms of land use are retained as part of the definition, which is: agroforestry is a form of multiple cropping under which three fundamental conditions are met: 1) there e exist at least two plant species that interact biologically; 2) at least one of the plant species is a woody perennial; and 3) at least one of the plant species is managed for forage, annual or perennial crop production.

The relative importance of N fixation, organic material inputs and nutrient inputs in litterfall, as justifications for including shade trees in plantations of coffee or cacao, is discussed. According to existing data, N fixation by leguminous shade trees does not exceed 60 kg.N/ha/a. However, these trees contribute 5,000–10,000 kg. organic material/ha/a. Comparisons are made between the leguminous shade tree Erythrina poeppigiana and the non-leguminous timber tree Cordia alliodora. The former, when pruned 2 or 3 times/a., can return to the litter layer the same amount of nutrients that are applied to coffee plantations via inorganic fertilizers, even at the highest recommended rates for Costa Rica of 270 kg.N, 60 kg.P, 150 kg.K/ha/a. The annual nutrient return in this litterfall represents 90–100 percent of the nutrient store in above-ground biomass of E. poeppigiana, and hence the consequences of competition with the crop should not be a serious limitation. In the case of C. alliodora, which is not pruned, nutrient storage in the tree stems, especially of K, is a potential limiting factor to both crop and tree productivity. It is concluded that, in fertilized plantations of cacao and coffee, litter productivity is a more important shade tree characteristic than N fixation.