Distribution of coarse and fine roots of Theobroma cacao and shade tree Inga edulis in a cocoa plantation

Cocoa (Theobroma cacao L.) is often cultivated below legume shade trees, but root interactions between the species are not well known. The first step in understanding these interactions is the description of spatial root system distribution of cocoa and shade trees. We studied the root distributions of cocoa and Inga edulis Mart. (inga) in an agroforestry plantation in the humid tropics of Costa Rica. Fine roots were sampled by soil coring, and a fractal root architecture model parameterised with data from partial root excavations was used for determining coarse root distribution. Fine root length in the 0-50 cm soil layer was 26,762 and 22,026 km ha(-1) for cocoa and inga, respectively, with 24 % and 23 % of fine root length of cocoa and inga, respectively, in the 0-2 cm layer. Horizontally, root distributions of the two species with cocoa at 3 x 3 m and inga at 9 x 9 m spacing overlapped strongly, while inga did not cover all points at 18 x 18 m spacing. Both species seem to efficiently capture nutrients released from the litter layer on the soil surface, and nutrient leaching in shaded cocoa plantations is unlikely. The proximity of root systems may facilitate N exchange between the N-2-fixing inga and cocoa, but competition for other nutrients is likely.