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

Abstract

International audiencePast studies showed a gradual expansion of forest 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) exhibit comparable multi-strata structures. By combining measurements of cocoa yield, litterfall, 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 fertility and plant diversity) although the time needed to reach levels found in F-cAFS varied strongly amongst variables. In addition to the previous study, we compared in a second time the impacts of the presence of shade trees on soil functions and cocoa yield in cocoa farms set up after savannah. To do so, we sampled 5 cocoa monocultures and 8 cAFS. Cocoa yield did not vary between these systems. Nonetheless, the soil P availability was improved in cAFS compared to monocultures. Within cAFS, unshaded cocoa conditions showed the lowest soil functions associated with the low leaf litter quality of cocoa plants. Tree species had contrasting effects on soil functions. Cocoa association with the fruit trees Canarium schweinfurthii and Dacryodes edulis did not improve soil fertility compared with unshaded cocoa, while associations with the legume tree Albizia adianthifolia, or the timber trees Milicia excelsa and Ceiba pentandra significantly improved soil N and P availability. These differences could be explained by the quality of the recycled litter, since low litter recalcitrance was strongly associated with increases in soil N and P availability, while soil pH and soil C contents increased with litter Ca restitution from C. pentandra. Our results highlighted the positive impacts of cAFS establishment in savannah on soil fertility, species diversity and carbon storage in biomass and soil over time. Furthermore, we showed that 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. Keywords: Cocoa agroforestry; ecosystem services; forest-savannah transition, litter traits, soil fertilit