Water use and functional traits dynamics in cocoa agroforestry systems

Deepening the understanding of water use dynamics and drought resilience in cocoa agroforestry by improving the prediction of shade tree effects
Project number 449815483 - Deutsche Forschungsgemeinschaft (DFG)

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.

Fig. 1: Illustration of cocoa agroforestry system with evergreen, deciduous, or brevi-deciduous trees (Source: A. Sarmiento Solar)

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 seasons.

Fig. 2: Field measurements and sampling on cocoa agroforestry research in Ghana

Replicated 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. Agroforestry model with capability in simulating above- and belowground traits interaction between trees and crop plants will be calibrated and refined with experimental data and applied in simulation experiments to investigate the potential effect of different shade tree traits on water use in cocoa agroforestry system in space and time.

Fig. 3: Data and knowledge generation through remote sensing enables better characterization of agroforestry structures and management impacts. Pictures show RGB and NDVI mapping.

Specific focus

Influence of shade trees on soil nutrient characteristics in cocoa agroforestry systems
Microclimate and Physiological traits characterization
Complementarity in water use between cocoa and shade trees
Morphological and spectral traits characterisation of multiple shade tree species
Identification of positive (+) and negative (-) shade tree functional traits groups in cocoa agroforestry systems
Cocoa yield forecasting through multispectral image analysis
Seasonal (wet, dry and extreme dry) effect on cocoa plant physiology under shade tree functional traits groups
Below ground traits response to seasonal changes under different functional shade tree trait groups
Cocoa yield quantity and quality response to seasonal changes under different shade tree traits groups
Modelling cocoa tree above and below ground traits response to shade tree induced conditions

Issaka Projekt
Fig. 4: Structure of the proposal with the differently coloured boxes representing the three work packages. Arrows indicate WPs interrelationships.