Promotion of agroforestry practices in sub-Sahara Africa may help sustain subsistent food and wood production by integrating trees and crops on farmlands to replenish soil fertility and improve crop yield. Using rotational woodlot and pigeonpea intercropping systems in semi-arid Tanzania as case studies, my research screened suitable tree species to increase fuelwood supply and examined mechanisms for reducing tree-crop competition. By adopting nutrient use efficiency (the ratio of biomass yield to nutrient uptake) as a criterion, I found that selecting tree species of low wood nutrient concentrations would minimize nutrient exports by 42 – 60 %, thus reducing soil nutrient depletion while concurrently sustaining local fuelwood supply harvested from rotational woodlots. Currently smallholder farmers cannot afford to replenish soil fertility because of high fertilizer costs. However, 5-year tree fallowing raised soil N and P levels for maize culture as high as those from recommended fertilizer applications. Post-fallow maize yield was also increased significantly over natural fallow practices. Apparently there is a trade-off between yields of maize and fuelwood under rotational woodlot culture providing farmers the choice to proportion tree and crop composition based on priority demands. An alternative practice of intercropping pigeonpea with maize may also rapidly replenish soil fertility as well as enhance maize yield when competitive interactions between trees and crops are controlled. Vector analysis revealed that such interactions suppressed biomass yields of maize and pigeonpea by 30 % and 60 %, respectively, due to limited soil nutrients and/or moisture. Optimizing yields of both crops would require prescribed fertilizer addition when intercropped, but dose rates can be lowered by half under the improved fallow system due to alleviating interspecific competition. My findings form the basis of a plea for greater use of rotational woodlot and pigeonpea intercropping systems in semi-arid areas. I conclude that smallholder farm management of rotational agroforestry systems can be significantly improved by refining tree selection criteria and mitigating nutrient competition between trees and crops to maintain food and fuelwood production.

Promotion of agroforestry practices in sub-Sahara Africa may help sustain subsistent food and wood production by integrating trees and crops on farmlands to replenish soil fertility and improve crop yield. Using rotational woodlot and pigeonpea intercropping systems in semi-arid Tanzania as case studies, my research screened suitable tree species to increase fuelwood supply and examined mechanisms for reducing tree-crop competition. By adopting nutrient use efficiency (the ratio of biomass yield to nutrient uptake) as a criterion, I found that selecting tree species of low wood nutrient concentrations would minimize nutrient exports by 42 – 60 %, thus reducing soil nutrient depletion while concurrently sustaining local fuelwood supply harvested from rotational woodlots. Currently smallholder farmers cannot afford to replenish soil fertility because of high fertilizer costs. However, 5-year tree fallowing raised soil N and P levels for maize culture as high as those from recommended fertilizer applications. Post-fallow maize yield was also increased significantly over natural fallow practices. Apparently there is a trade-off between yields of maize and fuelwood under rotational woodlot culture providing farmers the choice to proportion tree and crop composition based on priority demands. An alternative practice of intercropping pigeonpea with maize may also rapidly replenish soil fertility as well as enhance maize yield when competitive interactions between trees and crops are controlled. Vector analysis revealed that such interactions suppressed biomass yields of maize and pigeonpea by 30 % and 60 %, respectively, due to limited soil nutrients and/or moisture. Optimizing yields of both crops would require prescribed fertilizer addition when intercropped, but dose rates can be lowered by half under the improved fallow system due to alleviating interspecific competition. My findings form the basis of a plea for greater use of rotational woodlot and pigeonpea intercropping systems in semi-arid areas. I conclude that smallholder farm management of rotational agroforestry systems can be significantly improved by refining tree selection criteria and mitigating nutrient competition between trees and crops to maintain food and fuelwood production.

This study attempts to explain the inability of resource constrained farmers in Zambia to invest in soil fertility enhancing improved fallows, a sustainable land use practice developed by the World Agroforestry Centre (ICRAF) in the 1980s. Although several studies in the laboratory and field have shown that improved fallows positively impact on farmers’ welfare, the reliability of such conclusions comes into question given their use of improper identification strategies. Secondly, although there is general consensus that improved fallows additionally co-produce environmental services, the literature acknowledges that such services are not only imprecisely defined but also rarely quantified. Most estimates for environmental services have been confined to controlled field trials and laboratory experiments. Consequently, this research was designed to answer the following questions: 1) Would the use of randomisation procedures to estimate impact provide additional support to the foregone conclusions by most literature regarding the positive impact of improved fallows on farmer welfare? 2) Studies from on-station experiments show that improved fallows provide environmental services; do such conclusions hold for improved fallows planted on-farm where the near ideal experimental conditions are not guaranteed? A structured questionnaire was used to interview 324 randomly selected small scale farmers in Chongwe district of Zambia between November and December 2011. The data was analysed using well-grounded and robust matching and switching regression counter factual analysis tools. The rigorous econometric methods confirmed the positive impact of improved fallows on household maize yields, maize productivity, per capita maize yield and maize income. Insignificant impact results were however obtained when broader welfare indicators – overall per capita, crop income and value of crop production were considered. The study attributes these later results to two possible areas; first, most of the maize sold that contributes to crop income may be coming from other input sources such as the inorganic fertiliser that is common in the study area. Second, the non-use of the technology on cash crops (for example cotton) in subsequent periods after a year or two of maize cropping reduces the technology’s contribution to the households’ cash crop income portfolio. Had the study only used maize income or value of maize income to measure overall crop income (or value of crop production), or had it just made a simple comparison between adopters and non adopters, the likelihood of not finding any insignificant results on the efficacy of improved fallows would have been high. The study thus concludes that the use of improved fallows should be diversified to cover the entire cash crop portfolio especially a year or so after maize cropping when most of the nitrogen supplied by technology has been used up. More importantly, the study recommends use of better and more robust methodologies in evaluating impact of interventions. The positive effects of improved fallows on on-farm environmental quality, controlling for farmers’ biophysical and socio-economic characteristics were confirmed. Estimates from OLS regression, matching and the more robust endogenous switching regression showed that the technology had a significant causal effect on households’ consumption of fuel wood obtained from natural forests. The technology can provide up to 1,086 kg or about 51% of annual household fuel wood requirements in the year the fallows are terminated. This amount is substantial enough to make a positive contribution towards reducing encroachment on public forests and thus control the rate of deforestation. In addition to promoting the technology for soil fertility improvement (the role which is widely accepted by the farmers), explicit extension messages conveying the technology’s capacity to provide various products that contribute to farmer welfare as well as provide on farm environmental quality should be made available. / Thesis (PhD)--University of Pretoria, 2014. / lk2014 / Agricultural Economics, Extension and Rural Development / PhD / Unrestricted