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

人们通常认为，农林中出现遮荫树会通过竞争用水对可可的生长和产量产生负面影响。相比之下，之前一个合作项目的结果表明，由于可可和遮荫树之间土壤水分吸收的垂直分配存在明显差异，因此没有这种负面影响。我们之前的项目确实显示了不同的垂直生根模式，以及遮荫和可可树对土壤干旱的不同反应。然而，关于不同土壤水分条件下可可和遮荫树种细根生物量、垂直生根模式和地下生产力的系统差异的信息仍然很少。该项目旨在检验如下假设：(1)在水分供应较低的情况下，可可和遮荫树的根系垂直分离比在充足的供水条件下更明显；(2)Gliricidia和本地遮荫树都有有助于补充土壤水分吸收的深层根系，以及(3)在干旱地区的可可农林中，地下碳分配到细根生物量和周转率较高。该项目将比较没有遮荫树的可可林、遮荫树和当地遮荫树在不同降雨量下的情况。调查林分细根生物量、根系垂直分布和深根发生情况以及地上、地下生产力。稳定同位素(氚)技术的应用将有助于阐明单一深层根系对树木水分吸收的重要性。我们将特别与一个解决这些林分的生态水文学问题的项目合作。

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.