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

在可可农林业中，可可树和遮荫树之间对有限资源的竞争可能会降低产量。特别是水分供应被认为是可可产量的主要决定因素。然而，我们之前的实验研究结果表明：(1)可可树和遮荫树之间的土壤水分吸收存在很强的垂直分区；(2)在少雨期，主要是可可树吸收了到达土壤的小体积水分；(3)遮荫树冠盖度的增加导致可可树的水分利用增加而不是减少。本研究在一个地点进行，使用单一类型的遮荫树混合物。因此，我想进一步检验这些论点，以期得出更一般的结论。该项目旨在研究三种不同遮荫树混合类型（无遮荫树、gliriicidia遮荫树、原生遮荫树）的可可种植，并在两个地点复制地块，对比降雨。这些方法强调稳定同位素技术（氘和18O）的应用，利用自然丰度和示踪实验，辅以树液通量研究。计划特别与一个项目合作，处理这些林分的生产力和特定物种的根系分布。

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.