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Evaluating ecosystem service trade-offs and synergies from slash-and-mulch agroforestry systems in El Salvador

S.P. Kearney, S.J. Fonte, E. García, P. Siles, K.M.A. Chan, S.M. Smukler
Ecological indicators 2019 v.105 pp. 264-278
agroforestry, biodiversity, disease control, ecosystem services, farmers, issues and policy, monitoring, mulches, pests, shifting cultivation, slash, soil composition, tropics, El Salvador
Agroforestry has been proposed as an alternative to slash-and-burn agriculture for smallholder farmers throughout the tropics in order to enhance ecosystem service (ES) supply and conserve biodiversity. Payments for ecosystem services (PES) have emerged as a promising tool to overcome socioeconomic barriers to agroforestry adoption, however PES policy remains nascent, in part due to the challenges associated with quantifying and monitoring ES provision. One key challenge stems from the need to simultaneously evaluate a suite of ES benefits and consider synergies and trade-offs among them, for example to address critiques that PES could have undesirable consequences when focused solely on an individual ES. Such evaluations are lacking, especially for smallholder systems, as are clear methods for carrying them out. Here we evaluate multiple ES in the context of the ‘slash and mulch’ agroforestry system (SMAS), a flexible alternative to conventional maize-bean farming currently practiced by some 11 million smallholders across Central America. We conducted on-farm trials in El Salvador comparing two variations of SMAS to conventional and organic management and forest-fallows to evaluate the adaptability of SMAS and its impact on multiple ES. We found that variability associated with SMAS made it difficult to statistically demonstrate the benefits of isolated individual ES indicators. However, when multiple indicators were evaluated simultaneously, both SMAS treatments outperformed conventional and organic management in nearly all ES categories. By developing composite indices of multiple ES we identified patterns indicating that SMAS enhances multiple ES and better capitalizes on synergies between regulating and provisioning ES compared to conventional management. Specifically, the SMAS treatments showed synergies between water regulation, pest and disease control, soil composition, belowground biodiversity and production value, while in conventional plots we found trade-offs between provisioning and regulating ES. Finally, we identified simple field proxies that correlate well with multiple ES, and discuss important management, monitoring and policy implications for adaptable agroforestry systems.