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Agroforestry for sustainable cocoa production in the forest¿savannah transition zone in the north of the Congo Basin including Cameroon

Harmand J.M., Nijmeijer A., Sauvadet M., Enock S., Essobo Nieboukaho J.D., Fonkeng E.E., Lauri P.E., Jagoret P., Saj S.. 2020. In : Gitz V., Meybeck A., Ricci F., Belcher B., Brady M.A., Coccia F., Elias M., Jamnadass R., Kettle C., Larson A., Li Y., Louman B., Martius C., Minang P., Sinclair F., Sist P., Somarriba E. (editors). FTA 2020 Science Conference: Forests, trees and agroforestry science for transformational change: Book of abstracts. Bogor : CGIAR Research Program on Forests, Trees and Agroforestry (FTA), p. 108-108. FTA 2020 Science Conference: Forests, trees and agroforestry science for transformational change, 2020-09-14/2020-09-25, Bogor (Indonésie).

Past studies showed a gradual expansion of tree cover over savannah in the forest¿savannah boundary zone of Cameroon (Gillet et al. 2001). While the encroachment of savannah by forest is more and more impeded by human activities, farmers have proven that afforestation at the border of the forest is achievable using cocoa and specific techniques to build up an associated tree canopy (Jagoret et al. 2012). Furthermore, mature cocoa agroforestry systems created on savannah (S-cAFS) and in forest (F-cAFS) seem to exhibit comparable multi-strata structure linked to a multi-purpose objective of farmers in terms of livelihoods and long-term sustainable management of cocoa. By combining measurements of cocoa production, 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 generally tended to comparable levels after several decades. Results also emphasized the ability of S-cAFS to increase most of the ecosystem services (ES) although the time needed to reach levels found in F-cAFS varied strongly amongst variables (Nijmeijer et al. 2019a,b). We also compared the impact of five shade tree species (Canarium schweinfurthii, Dacryodes edulis, Milicia excelsa, Ceiba pentandra, Albizia adianthifolia) and unshaded conditions on soil functions and cocoa yield in relation to plant functional traits and leaf litterfall within 8 cocoa farms 20 to 60 years old (Sauvadet et al. 2020). While no difference in cocoa yields could be detected between the different tree species and unshaded conditions because of high variability of data, the effects on soil functions varied largely among species. Shade tree species with the most dissimilar litter traits to cocoa (cocoa showing the lowest leaf litter quality) showed the largest improvement of soil functions. Low litter recalcitrance was strongly associated with increases in soil fertility indicators such as N and P availability, while pH, soil C and N contents increased with litter Ca restitution. Improvements of soil functions were low under the two fruit trees (Canarium and Dacryodes), medium under the legume tree Albizia, and high under the two timber trees (Milicia and Ceiba). According to the tree species and nutrient, nutrient recycling through litterfall could barely to largely offset the nutrient removal by cocoa beans and husks. This study corroborates that the two tall timber trees (Milicia and Ceiba) are some of the most appreciated companion species for cocoa production by farmers, alleging desirable light shade, higher soil fertility and cocoa yield. In order to better assess the role of shade trees in these cocoa systems, future research will need to extend these approaches, especially to understand how plant diversity can help to adapt to climate change, including higher temperature and longer dry seasons.

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