Sustainable intensification of smallholder farming systems in Ethiopia : what roles can scattered trees play?

Abstract

Scattered trees dominate smallholder agricultural landscapes in Ethiopia, as in large parts of sub-Saharan Africa (SSA). While the integration of scattered trees with crops could provide a viable pathway for sustainable intensification of these farming systems, they also lead to trade- offs. Trade-off minimization and benefit maximization from these trees in the system require the processes that underlie tree-crop interactions to be unravelled. This study explored tree- based pathways for the sustainable intensification (SI) smallholder crop production systems in contrasting agroecologies of Ethiopia. Combination of methodologies from agronomy, socio- economics and conservation sciences were utilized to understand the potential roles of scattered trees in smallholder farming systems. Results indicated that farmers maintained on-farm trees because of their direct timber, fencing, fuelwood, and charcoal production values, regardless of their effect on crop productivity. A trade-off analysis revealed that economic gains from trees were not large enough to compensate for tree-induced crop yield penalties in tree-crop mixed farming systems. Under farmers’ practices, most scattered trees generally had a significant negative effect on maize yield. For example, mean maize grain yields were 59%, 42% and 26% less under the canopies of Cordia africana, Croton macrostachyus and Acacia tortilis, respectively, compared with corresponding open field yields. The yield reductions dropped to as low as 5% under ‘good agronomic practices’, such as early planting, variety selection, improved weed management, fine seedbed preparation and higher rates of nitrogen fertilizer. Similar yield reduction was observed in maize under the canopy of Grevillea robusta. Application of nitrogen and phosphorus fertilizers to under canopy maize in Grevillea robusta and Acacia tortilis improved crop yields, compared with non-fertilized maize under the canopies of these tree species. However, recommended rates of nitrogen and phosphorus fertilizers produced significantly less maize yields compared with the open fields. Faidherbia albida is an exceptional scattered tree species that improved soil water, nitrogen and phosphorus use efficiencies, leading to significantly higher yields in wheat gunder tree crown. Available N was 35-55% larger close to the crowns of Faidherbia compared with open fields, apparently contributing as much as 64 kg ha-1 yr-1 mineral N. In addition, this tree significantly reduced photosynthetically active radiation (PAR), reaching the canopy to optimum levels for wheat growth and development. Under the crowns, midday temperature was about 6oC less compared with nearby open fields. Regardless of the triple-win effects (crop production, adaptation and mitigation) of this tree species, over-utilization caused tree population decline. Under the current management, Faidherbia population would decline to a critical density of less than one tree ha-1 within six decades. The current study underlined that conservation of scattered trees can never be achieved through promotions based on neither the trade-offs nor crop productivity benefits involved. Scattered trees can be maintained even when trade-offs with crop production are overriding. Contrarily, these trees may be endangered even if they provide all-round benefits. Thus, a ‘whole sale’ approach that advocates scattered trees on their theoretical environmental and crop production values could jeopardize both conservation and crop production goals. A ‘process-based’ rather than ‘technology-based’ recommendation is required to harness the promising potential that scattered trees offer as a starting point for sustainable intensification of smallholder farming systems.