Abstract. Shade trees reduce the stress of coffee (Coffea spp.) and cacao (Theobroma cacao) by ameliorating adverse climatic conditions and nutritional imbalances, but they may also compete for growth resources. For example, shade trees buffer high and low temperature extremes by as much as 5 oc and can produce up to 14 Mg ha-1 yr-1 of litterfall and pruning residues, containing up to 340 kg N ha-1 yr-1 • However, N 2 fixation by leguminous shade trees grown at a density of 100 to 300 trees ha-1 may not exceed 60 kg N ha-1 yr-1 • Shade tree selection and management are potentially important tools for integrated pest management because increased shade may increase the incidence of some commercially important pests and diseases (such as Phythphora palmivora and Mycena citricolor) and decrease the incidence of others (such as Colletotrichum gloeosporioides and Cercospora coffeicola). In Central America, merchantable timber production from commercially important shade tree species, such as Cordia alliodora, is in the range of 4-6 m 3 ha-1 yr-1 _ The relative importance and overall effect of the different interactions between shade trees and coffee/cacao are dependent upon site conditions (soil/climate), component selection (species/varieties/provenances), belowground and aboveground characteristics of the trees and crops, and management practices. On optimal sites, coffee can be grown without shade using high agrochemical inputs. However, economic evaluations, which include off-site impacts such as ground water contamination, are needed to judge the desirability of this approach. Moreover, standard silvicultural practices for closed plantations need to be adapted for open-grown trees within coffee/cacao plantations.

A review is made of the ecological interactions that occur between shade trees and the perennial crops: coffee (Coffea spp. L.L cacao (Theobroma cacao L.) and tea (Camellia sinensis L. Kuntze). These interactions are classified firstly as advantages or disadvantages, and secondly as: effects on crop management; effects on the hydrological cycle; effects on pathogens, insects and climatic conditions; and effects on soils. References are given for the 20 advantageous and 16 disadvantageous consequences of using shade trees, emphasizing publications that provide original data and useful methodologies. Finally a check list of desirable characteristics for perennial crop shade trees is presented.Resumen. Se hace revisidn de las interacciones ecoldgicas que ocurren entre grboles de sombra y los cultivos perennes: caf~ (Coffea spp. L.), cacao (Theobroma cacao L.) y t6 (Camellia sinensis L. Kuntze). Estas interacciones fueron clasificadas en primer nivel como ventajas o desventajas, yen segundo nivel como: efectos sobre manejo de los cultivos; efectos sobre el ciclo hidrol6gico; efectos sobre pat6genos, insectos y condiciones climfiticas; y afectos sobre los suelos. Se dan referencias para 20 consecuencias ventajosas y 16 consecuencias desventajosas al utilizar firboles de sombra, dando 6nfasis a publicaciones que proveen datos originales y metodologias fitiles. Finalmente se presenta una lista de las caracteristicas deseables para firboles de sombra para cultivos perennes.

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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 60kg.N/ha/a. However, these trees contribute 5,000-10,000kg. 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.

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