Shade trees in cacao agroforestry systems: influence on roots and net primary production

Summary
Various degrees of shade tree cover are common in cacao agroforestry systems. The presence of shade trees is often assumed to negatively affect growth and yield of cacao plants through competitive water use. However, results from a previous collaborating project suggest no such negative effects due to clear differences in vertical partitioning of soil water uptake among cacao and shade trees. Our earlier project showed different vertical rooting patterns and a different soil drought response of shade and cacao trees that coincided with this vertical water-use partitioning. However, there are still few information about systematic differences in standing fine root biomass, vertical rooting pattern, and below-ground productivity of cacao and shade trees as well as among planted and native shade trees at sites differing in soil water availability. The proposed project intends to test the hypotheses that (1) the vertical segregation of the root systems of cacao and shade trees is more pronounced under low than under ample water supply, (2) both planted and native shade trees have a complementary soil water use, and (3) below-ground carbon partitioning into fine root biomass and turnover is higher in cacao agroforests under lower soil moisture conditions. The project shall be conducted in a replicated matrix of cacao agroforests at three different shade tree admixture types (no shade tress, planted Gliricidia shade trees, native shade trees) at two study sites of contrasting rainfall. Our study approach will include investigations on standing fine root biomass, vertical root distribution (including the occurrence of 'deep roots' below 3 m soil depth), above- and below-ground productivity, as well as the application of stable isotope (deuterium) techniques to clarify the importance of single 'deep roots' for the trees' water uptake.