Managing climatic risk in cocoa by designing resilient agroforestry systems based on a functional trait approach

Abstract

Climate change projections indicate that significant areas of the current cocoa cultivation areas in West Africa are likely to experience unfavorable climatic conditions by 2050. Water use efficient agroforestry systems are considered to be an important option to adapt cocoa to climate change. Water use efficiency and complementary soil water use between cocoa and shade trees have been reported in previous studies in Indonesia but in our previous 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 experienced during 2015/16. Cocoa plants under no shade were relatively more resilient with higher survival rate and post drought recovery.While past research studies   were each limited to few selected shade tree species being investigated despite the huge numbers within cocoa landscapes. To overcome this limitation, 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 systems rather than describing individual species. We apply this concept across temporal and spatial scales in a marginally suitable cocoa climate in Ghana. Shade tree species have been categorized into phenological trait groups (evergreen, deciduous, or brevi-deciduous) under which detailed above- and belowground traits interactions with cocoa plant and effect on water use has been evaluated.The following hypotheses are being 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 above- and belowground morphological and physiological traits are influenced by shade tree phenological and morphological traits and their modification by micro-climatic (light, temperature, relative humidity and VPD), and soil (water and nutrients) conditions over different seasons.Replicated plots of 19 shade tree species distributed across the three phenological trait groups have been established for analysis of functional traits interactions in such a multi-species agroforestry system. We determine root specific traits through direct sampling, stable isotope analysis for assessment of water use portioning between the various shade tree groups and cocoa plants. Soil moisture, temperature and relative humidity and light sensors were installed in each plot. Cocoa plants under different micro-climatic impact zones of the shade trees have been monitored for a full production cycle.  The shade trees impact on the cocoa plant productivity (morphology and yield traits) has been evaluated. Complementary and non-complementary shade tree species with respect to their trait interactions and effect on cocoa plant productivity have been identified.