Deepening the understanding of water use dynamics and drought resilience in cocoa agroforestry by improving the prediction of shade tree effects

• Applicant: Issaka Abdulai, Ph.D.
• Subject Area: Ecology of Land Use
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449815483

Water use efficiency and complementary soil water use between cocoa and shade trees have been reported in previous studies in Indonesia. In our study in Ghana, popular native shade tree species Albizia ferruginea and Antiaris toxicaria were found to have a strong competitive water use advantage over cocoa plants during an extreme drought. Cocoa plants under no shade were rather resilient with higher survival rate and post drought recovery. The key problem in past research is the focus at species level where only few shade tree species are studied despite the huge numbers within the cocoa landscape. To overcome this problem, an approach based on functional traits of trees in terms of water use is proposed to understand their effect on water use efficiency and drought resilience in cocoa agroforestry system over temporal and spatial scale in a marginal cocoa climate in Ghana. Shade tree species have been categorized into phenological trait groups of evergreen, deciduous, or brevi-deciduous under which detailed above- and belowground traits interaction with cocoa plant and effect on water use will be evaluated. The following sub-hypothesis will be tested: (i) shade trees in the three phenological trait groups exhibit significant difference in their root and water uptake depth, and, thus, affect environmental conditions relevant for cocoa (ii) cocoa plant morphological and physiological traits are significantly influenced by shade tree specific phenological trait group and (iii) above- and belowground traits, water use, productivity and drought resilience of cocoa plants are influenced by shade tree trait-specific modification of micro-climatic and root morphology-related hydraulic conditions over different seasonsReplicated plots of 15 shade tree species distributed across the three phenological trait groups will be established functional traits analysis. Complementary and non-complementary water use shade tree species will be identified and tested for effect on water use and productivity of cocoa plants over different seasons and shade tree impact zones. Root depth, growth/ biomass and water uptake depth will be assessed through manual excavation, imaging and stable isotope techniques. The WaNuLCAS (Water, Nutrient and Light Capture in Agroforestry Systems) agroforestry model, with the capability for simulating above and below ground interaction between trees and crops, will be calibrated and validated with experimental data. The validated WaNuLCAS model will be applied to deepen the understanding of the effects of specific shade tree characteristics and to evaluate different scenarios of shade tree combinations (upper scale to parcel level) and drought regimes on water use, drought resilience and productivity of the cocoa agroforestry system.

• DFG Programme: Research Grants