ReviewEcosystem services from woody vegetation on agricultural lands in Sudano-Sahelian West Africa
Introduction
There is an increasing scientific interest in understanding effects of woody vegetation on agricultural lands for different aspects of human well-being, including climate change mitigation and adaptation (Bayala et al., 2014, Luedeling and Neufeldt, 2012, Mbow et al., 2014), resilience (De Leeuw et al., 2014, Mbow et al., 2014), agricultural production (Bayala et al., 2014) and nutrition (Ickowitz et al., 2014). There has also been a rapidly growing interest in development and conservation initiatives that enhance livelihood contributions from woody vegetation in rural communities in international research and development organizations, see e.g. the Livelihoods and Landscapes Strategy of the International Union for Conservation of Nature (IUCN, 2012), and the large research program on Forests, Trees and Agroforestry of CGIAR (CGIAR, 2013).
In the Sudano-Sahelian zone of West Africa, management of woody vegetation as an integrated part of production landscapes has been a livelihood strategy for a long time. The rural population in the region are mainly smallholder farmers with integrated crop and livestock production, and pastoralists (Powell et al., 2004), who all benefit from multiple uses of scattered woody vegetation (see e.g. Boffa, 1999, Pullan, 1974). So-called parklands – cultivated or recently fallowed fields with scattered well grown trees – have been described from the region since the 18th century (Pullan, 1974).
The Sudano-Sahelian zone of West Africa has a strong north–south gradient in landcover, ranging from mainly sparse vegetation and grassland in the north, through cropland and grassland/shrubland/forest mosaics in the middle of the zone to cropland, shrubland and cropland/shrubland/forest mosaics in the south (Fig. 1). The scattered woody vegetation on cropland is a result of farmers’ selection of trees and shrubs to keep when preparing fields (Kessler, 1992, Lykke et al., 2002). Scattered trees and shrubs also characterize pastoral land especially in the northern part of the region (Akpo, 1997). These scattered trees that exist on agricultural lands, i.e. on fields, pasture lands and in fallows can have higher cash or consumption value than products from forests (Pouliot et al., 2012), illustrating the importance of these landscapes for livelihoods.
Different relatively recent development initiatives support and promote planting or natural regeneration of trees as a way to improve livelihoods and conserve important landscape functions in the region. For example, farmer managed natural regeneration (FMNR) is a movement to keep naturally regenerating woody vegetation on agricultural land, that has large reported success with increasing tree cover especially in parts of Niger and Burkina Faso (IFAD, 2011, Sendzimir et al., 2011, WRI, 2008). FMNR is promoted as beneficial for livelihoods and as a way to adapt to climate change and decrease poverty in the region (Reij, 2012, WRI, 2008), and has been strongly supported by the International Fund for Agricultural Development (IFAD, 2011). Another example is the Great Green Wall for the Sahara and the Sahel initiative, launched in 2011 as an African Union project with support from the EU, FAO and the Global Mechanisms of the UN Convention to Combat Desertification (FAO, 2013a). The initiative supports sustainable management of natural resources in drylands to improve food security and livelihoods of the people, with a main focus on trees. In West Africa, the increased on-farm tree cover is so wide-spread that it has been a suggested explanation behind the increased vegetation greenness (termed the “re-greening of the Sahel”; WRI, 2008) that has been observed for the region in analyses of trends in normalized difference vegetation index (NDVI; Herrmann et al., 2005, Olsson et al., 2005).
Despite the growing interest in large-scale investments to promote woody vegetation for improved livelihoods, the literature on how woody vegetation contributes with multiple benefits useful for people is scattered across many different disciplines. There does not, to the best of our knowledge, exist any systematic synthesis of the role of woody vegetation for livelihoods in the Sudano-Sahelian zone of West Africa, even though Bayala et al. (2014) recently covered literature on effects on crop yield, carbon sequestration and uses of tree products particularly in parklands.
Here, we use the concept of ecosystem services (MA, 2003) as a lens to integrate knowledge about the role of woody vegetation for people in the region, both in terms of direct use and income generation (provisioning services), and the influence of woody vegetation on ecosystem processes in the landscape (regulating services). We focus on the Sudano-Sahelian zone of West Africa specifically, asking what the status of current scientific knowledge is regarding (i) ecosystem services that are generated from woody vegetation, from both the perspective of specific individual woody species and at landscape scale; (ii) beneficiaries of ecosystem services from woody vegetation, and (iii) seasonal and inter-annual variation in generation of ecosystem services from woody vegetation.
Section snippets
Geographical zone, selection of ecosystem services and selection of species
This review covers the West African Sahelian and Sudano-Sahelian climatological zones (Fig. 1; from now ‘Sudano-Sahelian zone of West Africa’) defined by an average annual precipitation between 250 and 900 mm (FAO, 2004). Rain falls during 4–6 months between May and October, with a peak in August. It is during this period farmers grow rainfed staple crops. The rest of the year is dry, starting with cooler temperatures in November and becoming increasingly hot towards April. The focus of the
Literature groups
The studies included in the review come from a quite broad range of literatures focusing on different functions of woody vegetation. Only Heubes et al. (2012) framed the study in terms of ecosystem services, and Dossa et al. (2013) discuss their results as agro-ecosystem services, whereas the rest of the included studies do not use the specific term of ecosystem services. When coding the data, we identified five major literature groups: (I) studies on tree–soil–crop interactions (these include
Discussion
Using the concept of ecosystem services as a lens has allowed us to integrate knowledge from several disciplines, and to synthesize the current scientific understanding on livelihood benefits from woody vegetation in the Sudano-Sahelian zone of West Africa. In this section we first discuss two potential biases in our results: the extent to which the number of papers about a species actually reflects the importance of that species, and the representation of species from different parts of the
Conclusions
Woody vegetation in Sudano-Sahelian West Africa contributes with multiple direct benefits and income to livelihoods. When it comes to indirect benefits through regulating ecosystem services, woody vegetation has positive or no effect on soil nutrient, carbon and water content, whereas the reviewed studies show ambiguous results for effects on crop yield. To use ecosystem services as a lens was useful in this review to synthesize knowledge across widely dispersed fields. While this synthesis
Acknowledgements
This work was conducted within the project Adapting to changing climate in drylands: The re-greening in Sahel as a potential success case funded by research grants from Sida (SWE-2008-148) and Vetenskapsrådet (SWE-2012-115). We would like to thank Jennie Barron and Elin Enfors for valuable input on the review.
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