Abstract
Agricultural landscapes are increasingly being managed with the aim of enhancing the provisioning of multiple ecosystem services and sustainability of production systems. However, agricultural management that maximizes provisioning ecosystem services can often reduce both regulating and maintenance services. We hypothesized that agroforestry reduces trade-offs between provisioning and regulating/maintenance services. We conducted a quantitative synthesis of studies carried out in sub-Saharan Africa focusing on crop yield (as an indicator of provisioning services), soil fertility, erosion control, and water regulation (as indicators of regulating/maintenance services). A total of 1106 observations were extracted from 126 peer-reviewed publications that fulfilled the selection criteria for meta-analysis of studies comparing agroforestry and non-agroforestry practices (hereafter control) in sub-Saharan Africa. Across ecological conditions, agroforestry significantly increased crop yield, total soil nitrogen, soil organic carbon, and available phosphorus compared to the control. Agroforestry practices also reduced runoff and soil loss and improved infiltration rates and soil moisture content. No significant differences were detected between the different ecological conditions, management regimes, and types of woody perennials for any of the ecosystem services. Main trade-offs included low available phosphorus and low soil moisture against higher crop yield. This is the first meta-analysis that shows that, on average, agroforestry systems in sub-Saharan Africa increase crop yield while maintaining delivery of regulating/maintenance ecosystem services. We also demonstrate how woody perennials have been managed in agricultural landscapes to provide multiple ecosystem services without sacrificing crop productivity. This is important in rural livelihoods where the range of ecosystem services conveys benefits in terms of food security and resilience to environmental shocks.
Highlights
The smallholder agricultural sector in sub-Saharan Africa (SSA) is heavily constrained by declining per capita land holdings (Montpellier Panel 2013) and loss of soil fertility and productivity (Sanchez 2015)
This study aims to determine the overall effect of agroforestry on these ecosystem services and tries to answer the following questions: (1) What is the impact of agroforestry on crop yield, soil fertility, erosion control, and water regulation? (2) Under which ecological conditions does agroforestry have a positive or a negative effect? (3) What is the impact of management on agroforestry’s effect on crop yield, soil fertility, erosion control, and water regulation? (4) How do different shrub and tree species differ regarding their potential to regulate these ecosystem services?
Despite the publication bias revealed above, overall the analysis showed that agroforestry can increase crop yield, and improve soil fertility, erosion control, and water regulation compared to the control (Fig. 2)
Summary
The smallholder agricultural sector in sub-Saharan Africa (SSA) is heavily constrained by declining per capita land holdings (Montpellier Panel 2013) and loss of soil fertility and productivity (Sanchez 2015). Sustainable intensification of smallholder agriculture has been recognized as a crucial component of the strategy towards increasing food production in the region (Snapp et al 2010; Montpellier Panel 2013). It is apparent that intensification of agriculture in SSA will continue to play a role in feeding the growing population (Tully et al 2015; van Ittersum et al 2016). This poses the challenge of creating conditions for sustainable agriculture that can harness regulating/maintenance services (Bommarco et al 2013). Agroforestry is considered as one of the sustainable intensification practices, and widely promoted in SSA as it provides low-input, resource-conserving farming approaches that are socially relevant and relate well to livelihood and ecosystem functions (Carsan et al 2014)
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