Agroforestry parks are emblematic of sub-Saharan Africa, and are an integral part of African agricultural landscapes. The trees provide a range of ecosystem services to the farmers who keep them, and to the crops with which they share agricultural plots. However, those services sometimes enter into competition, with adverse effects on agricultural output. Those effects can be mitigated by adapting farming practices.
Agroforestry parks are agricultural spaces with a few trees, chosen by farmers through successive crop-fallow rotations. They provide food products (for both people and livestock), fuel, and craft materials. The cover they provide makes farming operations more resilient to climate and health hazards (shade, moisture, soil health). These advantages, known as “ecosystem services” are generally studied separately and on a tree scale.
A new study, published in the December issue of the journal Science of The Total Environment, suggests for the first time an analysis of the relationships between such services on a landscape scale:
“Of the 11 ecosystem services studied in the agroforestry parks of Senegal, we observed competition between those services across more than 50% of the area concerned”, says Louise Leroux, a geographer with CIRAD and lead author of the study. “This is a major result because it serves to qualify the known advantages on a tree scale. Depending on the relationships between those services, we will be able to pinpoint suitable levers for adapting agricultural practices.”
The relationships between the services rendered by trees: a new point of view
Even now, most studies still analyse the ecosystem services that trees render to crops without taking account of the relationships between those services, or of the heterogeneity of parks in terms of tree density or species diversity.
Analyses of individual services are vital for understanding the impact of trees on agricultural operations. For instance, Faidherbia albida, a tree species widely grown in the agroforestry parks of Senegal because of the nutritious fodder it provides, fosters atmospheric nitrogen fixation in soils. This serves to restore soil fertility and reduce farmers’ use of synthetic inputs. However, such analyses are inadequate for understanding the relationships between the different services and their variability within the ecosystem as a whole, as Louise Leroux stresses:
“When we looked at the relationships on a landscape scale, we saw that services were competing with each other in some parts of the park studied, whereas they worked in synergy in other zones. The benefits cancelled each other out, which means we need to rethink how we manage crops, and the agroforestry park as a whole, to limit competition between services. To boost the “agronomic” efficacy of these parks, we need to identify the zones where there is synergy and analyse them to understand why.”
Playing on agricultural practices to boost synergy
In particular, the researchers observed that there were more synergies between ecosystem services within a ten-metre radius around trees. They therefore suggest that farmers concentrate organic matter applications outside that area, to optimize soil management.
“These parks are very old”, says Josiane Seghieri, an ecologist with IRD and coordinator of the RAMSES II project. “The aim of this study was not just to say that more trees should be added, but that there is room for manoeuvre in terms of improving agricultural plot management in these zones.”
Reference: L. Leroux, C.Clermont-Dauphin, M.Ndienor, C.Jourdan, O.Roupsard, J.Seghieri, Science of The Total Environment, décembre 2022. A spatialized assessment of ecosystem service relationships in a multifunctional agroforestry landscape of Senegal.
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