Kendall’s rank correlation did not show presence of bias for crop yield (tau = 0.050, N = 389, P = 0.200), total N (tau = − 0.093, N = 389, P = 0.109), SOC (tau = − 0.054, N = 389, P = 0.329), and available P (tau = − 0.138, N = 502, P = 0.018), but showed presence of bias for water regulation (tau = -0.234, N = 96, P < 0.001), and erosion control (tau = − 0.107, N = 72, P < 0.001). The significant correlations found for water regulation and erosion control indicate that studies with non-significant effects were less likely to have been published. It is also possible that the bias emerges due to the fact that some studies could have been deemed “failures” because the trees did not establish properly. For example, in an earlier meta-analysis Sileshi et al. (2008) noted that out of 93 sites where improved fallow trials were established in southern Africa, maize was harvested from only 72 sites as a result of poor establishment of the legumes. The difficulty to capture such studies is one of the limitations of this analysis, and indeed any other similar meta-analysis (Sileshi et al. 2008).
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). Average crop yield was almost twice as high in agroforestry as in non-agroforestry systems; soil fertility was improved by a factor of 1.2, control of runoff and soil loss was five and nine times better with agroforestry, and infiltration was three times higher in agroforestry compared to the control. These are important insights into agroforestry, which is a land use option that is very common in SSA, where smallholder farms constitute ~ 80% of all farms, and roughly 70% of the population depend on agriculture for their livelihoods (Alliance for a Green Revolution 2014). At a farm scale, farmers are likely to invest in trees that provide food (fruits and nuts), fodder, fiber, or fuel while at the same time improving soil fertility, erosion control, and water regulation for sustainable production. On a larger geographic scale, agroforestry trees accrue benefits for many people and the environment, and farmers providing the services receive them as co-benefits.
Significant positive effects of agroforestry on ecosystem services were found across ecological and management conditions (Table 1). Exceptions were detected for some agroforestry practices (e.g., hedgerows) and some soil types where agroforestry had negative effects. This suggests that agroforestry’s potential for ecosystem service delivery cuts across the different ecological and management conditions involved. The 126 publications we reviewed present a mix of ecological, management, and biological characteristics that typify smallholder farming systems in SSA. The overall positive effects across contexts can be attributed to advances in the knowledge and practice of agroforestry. With decades of research and centuries of practice, agroforestry practitioners can now match some tree species to ecological conditions, select the right combinations of trees and crops, and productively manage trees on farms.
Table 1 The effects of agroforestry on crop yield, total nitrogen, available phosphorus, soil organic carbon (SOC), and water regulation (infiltration and soil moisture). Table values are the weighted mean response ratio (RR) and the 95% confidence interval (CI). Effects are significantly different from 0, if the 95% CI does not include 1. NA not available Crop yield
Crop yield was analyzed for 397 observations from 61 publications for studies conducted in 17 countries (Fig. 3). Close to half of the observations were from studies conducted in Kenya (10 studies, 108 observations) and Nigeria (10 studies, 77 observations). Other than agroforestry practice and soil type, there were no differences between any of the categories of agro-ecological zone, elevation, type of trial, growth form, or nitrogen fixation. Crop yield was higher in both humid and semi-arid situations compared to the control (Table 1). A similar pattern was observed for elevation, where agroforestry increased crop yield for trials at lowland and highland locations compared to the control. With regard to soil types, yields were two times higher under agroforestry with Acrisols, Cambisols, Lixisols, Luvisols, and Nitisols compared to controls (Fig. 3). These soils also had the highest number of cases with RR > 1. On the contrary, Arenosols and Andosols had some occurrences where the RR was less than 1 (Fig 3). Low crop yield associated with Arenosols and Andosols could be attributed to differences in soil quality. Arenosols have low nutrient and water storage capacity because of their course texture, which presents a limitation on crop growth (Hartemink and Huting 2008; IUSS 2014). Moreover, Arenosols generally occur in regions that are characterized by arid and semi-arid climates, where rainfall is erratic (Hartemink and Huting 2008). Andosols have high P retention capacity that makes applied P fertilizer unavailable for crop uptake (Batjes 2011). In addition, Andosols are nutrient-rich, and the risk of non-response to applied nutrients on fertile soil is known to be high due to a phenomenon termed “saturated fertility” effect (Sileshi et al. 2010).
Agroforestry increased crop yield for trials conducted on both farms and research stations in 77 and 68% of all cases (Fig. 3 and Table 1). Among agroforestry practices, crop yield was higher than controls when alley cropping, biomass transfer, and planted fallows were used, but not for hedgerows (Table 1). Alley cropping, biomass transfer, and planted fallows increased crop yield in 77, 93, and 85% of all cases, while hedgerows increased crop yield in 54%. Agroforestry increased crop yield when either trees or shrubs were grown compared to controls. Similarly, crop yield was enhanced when both nitrogen-fixing or non-fixing species were grown compared to controls.
The findings provide evidence that agroforestry can significantly increase crop yield. The studies reviewed suggest a combination of causes for increased crop yield, for example improved soil fertility due to nitrogen input from biological nitrogen fixation and nutrient cycling in organic inputs from trees (Bayala et al. 2002; Sileshi and Mafongoya 2003), improved water regulation through increased infiltration and higher soil moisture content (Chirwa et al. 2003; Makumba et al. 2006), ,improved microclimate (Rhoades 1995), and better soil physical properties (Chirwa et al. 2004). In most of the studies, yield was increased sufficiently to offset reduction caused by the presence of trees. However, a few studies reported a yield reduction due to competition for water and nutrients when the trees were not pruned (Bayala et al. 2002; Muthuri et al. 2005; Ndoli et al. 2017). Reductions in crop yield were also attributed to effects of shading (Rao et al. 1998; Bayala et al. 2002). Further meta-analyses can test if pruning and shade levels are indeed factors that lead to reduced crop yield.
Total nitrogen, available phosphorus, and soil organic carbon
A total of 515 observations were identified from 92 publications that fulfilled the selection criteria for studies investigating the effects of agroforestry on soil fertility. Among these, 61 publications reported total N, 68 reported available P, and 73 reported SOC for studies conducted in 19 countries (Fig. 2). Agroforestry improved total N (RR 1.2; 95% CI 1.1–1.2), SOC (RR 1.2; 95% CI 1.2–1.3) and available P (RR 1.2; 95% CI 1.1–1.2) compared to the control. Agroforestry also improved total N, available P, and SOC for all categories of agro-ecological zones and elevation compared to controls (Table 1). Compared to controls, agroforestry improved total N, available P, and SOC for all soil types except on Acrisols and Luvisols in the case of total N, and Andosols in the case of SOC (Table 1). The lower effect of agroforestry on Acrisols could be attributed to their chemical and physical limitations, which also constrain tree growth. Acrisols suffer from soil acidity, aluminum toxicity, low nutrient reserves, nutrient imbalance, and multiple nutrient deficiencies (IUSS 2014). Although Luvisols are inherently fertile, they are susceptible to crusting, compaction, and low moisture-retention (IUSS 2014). These constraints could have limited tree growth thereby reducing litter inputs to the soil on both soils. On the other hand, the low response on Andosols could be attributed to the “saturated fertility” effect described under crop yield.
There were no significant differences among agro-ecological zones, elevation, and type of trial. Over 80% of the cases in humid and semi-arid environments, as well as lowland and highland sites had RR > 1 for studies investigating total N (Fig. 4) and SOC (Fig. 5); a smaller proportion was found for available P (Fig. 6). All observations for total N and SOC in agroforestry under Ferralsols had RR > 1 (Figs. 4 and 5). Agroforestry increased total N, SOC, and available P for trials conducted on farms as well as on stations compared to controls (Figs. 4, 5, and 6). A lower proportion of cases were determined for available P (about 60%) compared to over 80% for total N and SOC for trials conducted on farms and on stations. Other than intercropping (RR < 1 = 35%) in the case of total N (Fig. 4), and alley cropping (RR < 1 = 62) in the case of available P (Fig. 6), soil fertility improved with agroforestry for all practices tested compared to controls. Agroforestry with all types of woody vegetation had a significant effect on total N, SOC, and available P compared to controls, although the proportion of observations with RR > 1 was low for available P, ranging between 58 and 68% for the different variables (Fig. 6). The differences among agroforestry practices and woody perennials used were not statistically significant.
The analysis has demonstrated that soil was more fertile in agroforestry than in controls. SOC showed a stronger increase in agroforestry than other indicators of soil fertility. Trees increase SOC by photosynthetic fixation of carbon from the atmosphere, and by transferring this carbon to the soil via litter and root decay. We infer that trees were the main source of nitrogen and soil organic carbon, since crop residues are usually removed with the harvest. Some studies reported a strong correlation between total N and SOC (Jonsson et al. Jonsson et al. 1999a; Bayala et al. 2002). Trees improve nitrogen primarily through inputs from biological nitrogen fixation (Sileshi and Mafongoya 2003), and recycling of nitrogen from above (litter) and belowground (roots) organic inputs (Rhoades 1995; Jonsson et al. 1999a). A few cases of decline in total N were attributed to uptake by trees (Teklay et al. 2006; Isaac et al. 2007; Ndoli et al. 2017).
Available P was the least improved indicator of soil fertility. Unlike nitrogen and carbon, trees do not provide phosphorus but improve its availability and uptake by recycling the nutrient from organic inputs. This occurs when tree roots retrieve nutrients that have leached to soil layers not accessed by crop roots and recycle them to the topsoil as litter (Sileshi et al. 2014). However, trees may fail to improve phosphorus availability when the nutrient is not recycled and released in accessible form. This may explain some of the situations where available P was lower in tree-based compared to tree-less systems (Kho et al. 2001; Bayala et al. 2002; Isaac et al. 2007).
Erosion control
Out of seven studies conducted on erosion control, 49 observations were identified for runoff and 49 for soil loss. The studies were conducted in Kenya, Nigeria, and Zimbabwe. Our findings show that agroforestry performed best in terms of erosion reduction ecosystem services, five and ten times better than controls for runoff (RR: 5.0; 95% CI: 3.3-7.9) and soil loss (RR: 9.7; 95% CI: 5.9-17.3). However, these very large effect sizes could also be due to publication bias as demonstrated by the high Kendall’s rank correlation coefficients. If we had found more published studies (larger sample sizes), we expect the effect sizes to be more modest than the figures we reported here. Erosion control with agroforestry was more effective in both humid (RR 7.2; 95% CI 4.8 to 13.9) and semi-arid zones (RR 8.0; 95% CI 4.8 to 16.7) compared to controls. Similarly, erosion control with agroforestry was more effective when either shrubs (RR 6.9; 95% CI 4.6 to 11.4) or trees (RR 11.1; 95% CI 6.1 to 24.7) were planted. There were no significant differences in the effects between humid and semi-arid sites, or between trees and shrubs. Comparisons for soil erosion were not performed for elevation, soil type, site of trial, agroforestry practice, and growth form due to a low number of studies in those categories.
Trees have been shown to reduce soil loss by forming barriers that slow runoff and capture sediments (Angima et al. 2000, 2002), protecting soil aggregates from direct raindrops (Lal 1989a; Omoro and Nair 1993; Nyamadzawo et al. 2003), and improving soil structure (Lal 1989a). Without soil cover, direct raindrops on bare soils increase detachment of soil particles, which lowers infiltration and can stimulate runoff and soil loss. Carbon inputs from decomposing litter and decaying tree roots can be increased to stabilize soil structure (Salako et al. 2001). Runoff rates were low on plots with trees because of reduced overland flow (Omoro and Nair 1993) and increased infiltration (Nyamadzawo et al. 2003). A study at Domboshawa in Zimbabwe showed that vegetation reduces the amount of rainfall transformed into runoff by increasing the time to ponding and runoff (Nyamadzawo et al. 2003).
Water regulation
Studies on water regulation were conducted in 12 countries. In total, 96 observations were identified from 38 studies that fulfilled the selection criteria. Out of the 38 studies, 11 had 34 observations reporting on infiltration rates, while 27 studies with 62 observations reported on soil moisture content (Fig. 2). Agroforestry improved infiltration and soil moisture content compared to the control (Fig. 2). However, the effect of agroforestry on infiltration (RR 2.7, 95% CI 2.1–.5) was greater than that on soil moisture (RR 1.6; 95% CI 1.1–1.2). Over 90% of all the observations had RR > 1 compared to 70% for soil moisture. However, the large effect sizes found for infiltration rates could be due to publication bias.
The effect of agroforestry on water regulation was greater across agro-ecological zones, elevations, soil types, type of trials, agroforestry practices, and woody species compared to controls (Table 1). Water regulation was more strongly improved under agroforestry in semi-arid than in humid locations (Table 1). There were no significant differences among elevations and types of trial. The effects of agroforestry on water regulation were significantly greater on Lixisols (RR > 1 = 100%) compared to Luvisols and Nitisols. This is probably due to smaller effects of agroforestry in more fertile, free-draining Nitisols and Luvisols in humid and subhumid areas. Lixisols were mainly associated with experiments in semi-arid areas, e.g., in Machakos in eastern Kenya (Jackson and Wallace 1999) and Domboshawa in Zimbabwe (Nyamadzawo et al. 2008a), where trees have been shown to improve water infiltration and soil moisture; while Nitisols were associated with experiments in humid areas, e.g., Ibadan in Nigeria (Adejuyigbe et al. 1999; Salako et al. 2001), Embu in Kenya (Angima et al. 2002), and Ginchi in Ethiopia (Kidanu et al. 2004), where the effect of agroforestry on water regulation was low. Water regulation by agroforestry was higher in planted fallows than in intercropping situations or in experiments under a canopy. No differences were detected for the effects of agroforestry when trees or shrubs were planted. The effect of agroforestry was greater when nitrogen-fixing species were used than when non-nitrogen-fixing species were planted.
Productivity of agricultural lands can be constrained by water availability in the soil, which largely depends on infiltration and retention. The effects of agroforestry were stronger for infiltration than for soil moisture, suggesting that the primary mechanism through which trees improve water regulation is improved infiltration, since effects of trees on soil moisture content are subject to uptake and transpiration by trees. Empirical studies attributed high infiltration rates in agroforestry to improved hydraulic conductivity of the soil and better porosity (Nyamadzawo et al. 2003, 2007). On the contrary, lower infiltration in controls was attributed to soil compaction due to degradation of soil structure (Salako et al. 2001; Chirwa et al. 2003; Sanou et al. 2010). For example, soils in planted fallows had more macropores and large pore sizes because of improved aggregation (Chirwa et al. 2004; Nyamadzawo et al. 2008a) and presence of channels formed when roots die and decompose (Chirwa et al. 2003). Agroforestry has been shown to improve soil moisture compared to control by reducing loss of water from the soil through evaporation and transpiration by crops (Rhoades 1995; Siriri et al. 2013), increasing water infiltration, and improving water storage capacity (Makumba et al. 2006; Nyamadzawo et al. 2012a). Trees with a dense canopy and intense litter fall can reduce evaporation from the soil surface by modifying microclimate (Rhoades 1995; Siriri et al. 2013).
Win-wins and trade-offs
Our findings suggest possibilities of both win-wins and trade-offs in agroforestry production. This confirms the proposition that win-win scenarios are possible between agricultural production and ecosystem services, and that trade-offs can also occur and may have the potential to be managed (Foley et al. 2009; Power 2010). Agroforestry improved both yield and soil fertility indicators leading to a win-win situation in 72, 76, and 53% of the pairwise observations for crop yield and total N, crop yield and SOC, and crop yield and available P, respectively (Fig. 7a–c). Win-win outcomes also dominated studies reporting both total N and SOC (80%), but were less common for total N and available P (55%) as well as SOC and available P (59%). Win-win scenarios occur in situations where trees improve soil fertility, and soil moisture is not limiting or trees are managed to minimize competition.
A small number of studies showed trade-offs and lose-lose outcomes between crop yield and total N (28%) and crop yield and SOC (24%). Close to half of the studies (47%) revealed trade-offs and lose-lose outcomes between yield and available P, while a third of the studies showed trade-offs between available P and total N (32%), and available P and SOC (31%). Trade-offs occur when competition for nutrients or water (or light) outweighs the benefits of improved yield or enhanced provision of an ecosystem service. For example, transpiration in agroforestry can exceed that of tree-less plots if trees are not pruned to reduce water demand (Jonsson et al. 1999a; Bayala et al. 2002; Muthuri et al. 2005; Ndoli et al. 2017). In this case, the benefits of modified microclimate and improved soil structure are negated by high transpiration and uptake by trees, leading to low soil moisture. Trade-offs involving available P and soil moisture indicate that improved yield does not necessarily signify that all other ecosystem services are provided at higher levels.
Spearman's rank correlation did not show a significant relationship between crop yield and total N (rs = 0.222, N = 29, P = <0.247) or crop yield and SOC (rs = 0.196, N = 38, P = 239). On the other hand, positive and significant correlations were found between crop yield and available P (rs = 0.360, N = 34, P < 0.05), suggesting that soil nutrient availability was a main driver of crop yield in this meta-analysis. Correlation between SOC and total N (rs 0.433, N = 45, P < 0.05), and SOC and available P (rs = 0.277, N = 49, P < 0.05) were positive and significant. However, the correlation between total N and available P was positive but not significant (rs = 0.277, N = 47, P < 0.060). The relationship between crop yield and soil moisture was negative but not significant (rs = − 0.294, N = 12, P = 0.354). The lack of significant relationships between crop yield and total N or crop yield and SOC indicates that yield may not consistently covary with soil fertility. This suggests that beneficial effects of agroforestry on yield do not primarily stem from improved total soil nitrogen and SOC, but from a set of complex interdependent relationships among resources (light, water, and nutrients). Holding other factors of production constant, soil fertility is known to improve yield. Therefore, the lack of significant correlation between crop yield and some indicators of soil fertility can be attributed to differences in ecological conditions, management, tree and shrub species, and crops included in the studies reviewed. Correlations between crop yield and runoff, soil loss, or infiltration were not tested because of an insufficient number of studies that did not allow pairwise comparison.