Shade trees in cacao agroforestry systems: influence on water resource use

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In cacao agroforestry, competition between cacao and shade trees for limited resources may reduce yields. In particular water availability is regarded a main determinant of cacao yield. Results of our previous experimental study however suggest that (1) there was a strong vertical partitioning of soil water uptake among cacao and shade trees, (2) during low rainfall periods, mainly cacao trees took up the small volumes of water reaching the soil, and (3) an increased shade tree canopy cover led to an increased rather than a decreased water use of cacao trees. This study was conducted at one site and with a single type of shade tree admixture. I thus want to further test these contentions aiming at more general conclusions. The proposed project intends to study cacao cultivation at three different shade tree admixture types (no shade trees, Gliricidia shade trees, native shade trees) with replicated plots at two sites with contrasting rainfall. The methods emphasize the application of stable isotope techniques (deuterium and 18O), using natural abundances as well as tracer experiments, supplemented by tree sap flux studies. Cooperation is planned particularly with a project addressing the productivity and species-specific root distribution in these stands.