Soil fertility andTheobroma cacaogrowth and productivity under commonly intercropped shade-tree species in Sulawesi, Indonesia

Background and aims Trade-offs between ecological benefits and potential yield and growth reductions associated with the inclusion of shade trees in cocoa agroforests remain poorly understood. In this study we investigate interactions between shade and cocoa trees in cocoa agroforests in terms of soil fertility and cocoa productivity. Methods We quantified the effects of individual shade trees from 11 commonly intercropped species on cocoa growth (aboveground biomass) and yield and soil fertility indicators (total soil carbon, nitrogen, phosphorus contents and soil aggregation) at field sites in Southeast Sulawesi, Indonesia. Results Shade trees had a net positive effect on soil fertility in cocoa agroforests, with a 6% increase in soil carbon, a 4% in soil nitrogen and a 24% increase in mean weight diameter (used as an indicator for median soil aggregate size), under shade tree canopies compared to open areas. We found that shade trees had a net negative effect on cocoa tree growth and no net effect on cocoa yields. We were not able to link costs versus benefits with specific shade tree traits, but nevertheless observed significant differences between shade tree species.G. sepium(gliricidia) had significantly positive effects on yields, soil carbon and aggregation.N. lappaceum(rambutan) andD. zibethinus(durian) had significantly positive effects on soil carbon and nitrogen contents and on aggregation, but not on yields. Conclusions Our findings confirm the potential for soil improvements under shade trees and suggest that the inclusion of individual shade trees does not always constitute a direct trade-off for farmers in terms of yield losses.