Afforestation of savannah using cocoa agroforestry: impacts on ecosystem services and effects of associated tree species on soil fertility

Past studies showed a gradual expansion of tree cover over savannah in forest-savannah transition zones in Central Africa. While the natural encroachment of savannah by forest is more and more impeded by human activities, farmers in Cameroon have proven that afforestation of savannah is achievable using cocoa and specific technics to build up an associated tree canopy. Furthermore, mature cocoa agroforestry systems created on savannah (S-cAFS) or in forest (F-cAFS) seem to exhibit comparable multi-strata structures. By combining measurements of cocoa yield, litterfall and cycling, soil quality, carbon storage and tree species diversity along an age gradient (1 to 70 years), we showed that those variables in S- and F-cAFS tended to comparable levels after several decades. Results also emphasized the ability of S-cAFS to increase most of the ecosystem services (including soil carbon and nutrients storage) although the time needed to reach levels found in F-cAFS varied strongly amongst variables. We also compared the impact of five associated shade tree species and unshaded conditions on soil functions and cocoa yield in relation to plant functional traits and leaf litterfall within 8 cocoa farms set up after savannah. Unshaded cocoa conditions showed the lowest soil functions associated with the low leaf litter quality of cocoa plants. Soil functions varied largely among species. Low litter recalcitrance was strongly associated with increases in soil N and P availability, while soil pH, soil C and N contents increased with litter Ca restitution. Improvements of soil functions were the lowest under the fruit trees (Canarium schweinfurthii and Dacryodes edulis), intermediate under the legume tree Albizia adianthifolia, and the highest under the timber trees (Milicia excelsa and Ceiba pentandra). We detected no change in cocoa yield between the different treatments because of high variability of data but the P nutritional status of cocoa plants was positively related to soil P availability which varied under tree species. According to the tree species, N, P and K recycling through litterfall was equal to or greater than nutrient removal by cocoa beans and husks. Establishing cAFS in savannah can increase soil fertility, species diversity and carbon storage in biomass and soil over time. Using specific trees for soil quality and compatible with cocoa, and other trees for the provision of goods, can ensure cocoa AFS multifunctionality and sustainability. Future research will need to understand how plant diversity in these cocoa systems, can help to adapt to climate change.