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

农林间作中的遮荫树通常被认为通过竞争性用水对可可植物的生长和产量产生负面影响。相比之下，从以前的合作项目的结果表明，没有这样的负面影响，由于可可树和遮荫树之间的土壤水分吸收的垂直分区的明显差异。我们早期的项目确实显示了不同的垂直生根模式和不同的土壤干旱反应的阴影和可可树。然而，仍然有很少的信息系统的差异，细根生物量，垂直生根模式，地下生产力的可可和遮荫树种在不同的土壤水分制度。拟议的项目旨在测试以下假设：（1）可可和遮荫树根系的垂直分离在低供水条件下比在充足供水条件下更明显，（2）Gliricidia和本地遮荫树都有深根系统，有助于补充土壤水分吸收，（3）地下碳分配到细根生物量和营业额在干燥地点的可可农林中更高。本项目应在对比降雨量下比较无遮荫树、Gliricidia遮荫树和本地遮荫树的可可农林。我们将调查立细根生物量，垂直根分布和深根的发生，以及地上和地下生产力。稳定同位素（氘）技术的应用将有助于阐明单根深根对树木吸水的重要性。我们将特别与一个解决这些林分生态水文学的项目合作。

The presence of shade trees in agroforests is often assumed to negatively affect growth and yield of cacao plants through competitive water use. In contrast, 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 indeed showed different vertical rooting patterns and a different soil drought response of shade and cacao trees. However, there are still few information about systematic differences in fine root biomass, vertical rooting pattern, and below-ground productivity of cacao and shade tree species under different soil moisture regimes. 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 Gliricidia and native shade trees have deep root systems contributing to complementary soil water uptake, and (3) below-ground carbon partitioning into fine root biomass and turnover is higher in cacao agroforests at drier sites. The project shall compare cacao agroforests with no shade tress, Gliricidia shade trees, and native shade trees under contrasting rainfall. We will investigate standing fine root biomass, vertical root distribution and the occurrence of deep roots , and above- and below-ground productivity. The application of stable isotope (deuterium) techniques will help to clarify the importance of single deep roots for the trees water uptake. We will particularly cooperate with a project addressing ecohydrology of these stands.