Farming with trees: a balancing act in the shade

The smallholder agriculture sector in East Africa is the dominant economic and social activity for millions of farm households who are often resource-poor, food-insecure and most vulnerable to climate change. In this region, population pressure has led to shorter fallow periods or continuous cropping even on hillslopes causing erosion and leading to reduced soil organic matter content and nutrient mining without replenishment. Consequently, poor agricultural productivity has led to food shortages and these problems are likely to intensify in the region, as the human population is growing faster than in other parts of the world. Agroforestry, a low-input technology, was shown to contribute to the enhancement of food production while ensuring sustainability in sub-Saharan Africa. Agroforestry may improve food security by increasing soil fertility and providing additional income from tree products. Thus, agroforestry is now receiving increasing attention as a sustainable land-management option and some countries in East Africa (e.g. Rwanda) have pledged to restore up to 100% of their agricultural land mainly through agroforestry by the year 2020. Nevertheless, crop yields reduction in agroforestry are frequent due to competition for resources among trees and crops. In recent studies, tree canopy and root pruning were tested to improve light availability and resource use efficiency but studies that tackle crop management and tillage options to optimize crop productivity in the agroforestry systems are scarce. This thesis aims to assess the importance of agroforestry across Rwanda and its implication on crop productivity and food security of farm households, explore and recommend the maize varieties and tillage options that could minimize tree-crop competition in the equatorial savannah of Rwanda and Ethiopia. The approach combined household survey on the contribution of trees on household income and food security in six agroecologies of Rwanda, experiments on the microclimate and fertility effects of trees on crops in sub-humid region of Rwanda, maize variety testing in agroforestry systems and trials on conservation agriculture with trees in the equatorial savannah of two East African countries: Rwanda (Bugesera site) and Ethiopia (Meki site). The survey in Rwanda found that food security increases with increasing farm size and farmers with more trees tended to be wealthier (e.g. with larger land and more often higher crop and livestock income) and therefore tended to be more food secure in half of the agroecologies. The proportion of household income that came from tree products was the least among sources of income suggesting that most tree products are not sold but kept by farmers for their own use. Yet tree income was important for about 12% of the farmers, contributing more than 20% of their overall income. Households having low food security relied more on income from tree products than those with higher food security status. Therefore, income from tree products can be seen as a ‘safety net’ for the poorest households. Experiments in the sub-humid environment of Rwanda assessed the effects of mature Alnus acuminata (Kunth) and Markhamia lutea (Seem.) on maize at different distances from tree trunk for four consecutive seasons. Nutrients availability was higher under A. acuminata compared with M. lutea, because of higher litter fall but maize nutrient uptake increased only under A. acuminata 3 m from tree trunk during a wetter season. None of tree species affected water availability for maize in the topsoil. Total solar radiation, photosynthetically active radiation (PAR), and day air temperature were reduced by both tree species. Whereas crops consistently underperformed in M. lutea system, the competitive effect of A. acuminata for light was to some extent compensated by extra N input in the wetter seasons (2015 A and 2015 B) at 3 m but not at 1 m from the tree trunk. In an APSIM modelled scenario under low N fertilization, larger N input from trees could compensate for yield loss caused by reduction in radiation and temperature in about 60% of the seasons. This study suggested that adequate pruning and high leaf litter recycling can reduce the negative effect of shade in low intensity farming systems. The low competition of A. acuminata with crops was also perceived by Rwandan farmers, who ranked this tree species as the least competing among all the other upper story trees grown on-farm in the highlands. Experiments compared the performance of four maize hybrids and four OPVs was compared in sole crop and under mature Grevillea robusta and Senna spectabilis – in Bugesera, Rwanda or Acacia tortilis – in Meki, Ethiopia. In Bugusera, grain yields of hybrids (2 t ha-1) was significantly better than OPVs (1.5 t ha-1). Further, the presence of trees significantly reduced maize grain yield and total biomass in both hybrids and OPVs in the same manner. However, trees reduced harvest index significantly more in OPVs than in hybrids, suggesting that competition had a greater impact on grain yield of OPVs than on biomass production. In the experiments in Meki, the grain yield of OPVs (2.08 t ha-1) and hybrids (2.04 t ha-1) did not significantly differ and the presence of trees reduced their grain yields in the same manner. We concluded that agroforestry farmers could benefit from growing hybrids in the equatorial savannahs of Rwanda, but not in the equatorial savannahs of Ethiopia. It appears that the relevance of using either hybrids or OPVs in agroforestry systems depends on local conditions and the comparative advantages in seed costs. Experiments in the same regions of Rwanda and Ethiopia were carried out to assess the effect of conservation agriculture with trees (CAWT) on crop productivity as compared to conventional tillage with trees (CTWT) in the equatorial savannah. Crop emergence was significantly reduced under CAWT compared with CTWT. Maize emergence rates in CAWT and CTWT were respectively 46.9% and 70.1%, compared with 74.7% and 79.8% in sole maize under conservation agriculture (CA) and conventional tillage (CT). Grain yield in CAWT and CTWT were respectively 0.37 t dry matter (DM) ha-1 and 1.18 t DM ha-1 as compared with 1.65 t DM ha-1 and 1.95 t DM ha-1 in CA and CT. It was concluded that CAWT likely exacerbates tree-crop competition for water and nutrients and reduce crop yields and was therefore not considered as a viable alternative to CTWT or to CT in the studied systems. Overall, this study found that mixing trees and crops produced a worthwhile, if somewhat reduced, crop yield, and that on-farm trees can provide substantial income for the poorest households of Rwanda.</p

Das Verhalten von Phosphor in ausgewählten Böden eines Bergbaugebiets in Ruanda

In den stark verwitterten, sauren Böden der immerfeuchten Tropen ist Phosphor häufig ein wichtiger limitierender Faktor für das Pflanzenwachstum. Die P-Verfügbarkeit wird hier abgesehen von den vorherrschenden klimatischen Bedingungen insbesondere von den spezifischen chemischen und phy-sikalischen Bodenparametern beeinflusst. Bei den untersuchten Böden aus dem Ga-tumba Mining District (GMD) handelt es sich sowohl um in situ entwickelte Böden als auch um landwirtschaftlich bewirtschaftete Technosole aus Haldenmaterial. Aufgrund der Vielfalt der untersuchten Böden und deren Eigenschaften verwendeten wir vier Prüfverfahren zur Bestimmung der P-Verfügbarkeit: Bray 1, Mehlich 3, Olsen und Pi-Test. Die Gehalte an verfügbarem P wa-ren bei allen Verfahren unterhalb empfohlener Bereiche. Der Vergleich aller Methoden zeigte jeweils hohe Korrelationen auf. Die verfügbaren P-Gehalte nahmen in der Reihe Mehlich 3 > Bray 1 > Olsen > Pi ab. Die Güte der Tests war aufgrund statistischer Untersuchungen und ausschließender bodenspezifischer Faktoren für die Böden des GMD bei Bray 1 am besten und bei Olsen am schlechtesten. Ausschlaggebende Faktoren auf die Güte eines Tests waren vor allem der Einsatz von Rohphosphat, Kalk und organischen Substraten, der Boden-pH-Wert und der Bewuchs

Spurenelementbelastung von Böden im Bergbaugebiet Gatumba, Ruanda

Die Böden des Coltan-Bergbaugebietes Gatumba Mining District in Ruanda sind potenzielle Quellen für die Freisetzung toxischer Spurenelemente. Insbesondere Technosole auf Haldenabraum, Hangwasser-beeinflusste Gleysole sowie Fluvisole in der Bergbauregion wurden auf pH-Wert, Kationenaustauschkapazität, Korngrößenzusammensetzung, Kohlenstoff- und Stickstoffgehalt sowie maßgeblich auf den Gehalt an Schwermetallen untersucht. Es handelt sich um saure und nährstoffarme, tropische Böden, deren Gehalte an toxischen Elementen unterhalb der Maßnahmewerte der Bundesboden-schutzverordnung (1999) und der Zielwerte der internationalen Hollandliste liegen. Von einer Gefährdung der Bevölkerung ist in diesem Gebiet nicht auszugehen, was aber in benachbarten Bergbaugebieten und speziellen hydrothermalen Zonen im Untergrund nicht auszuschließen ist

Disentangling the positive and negative effects of trees on maize performance in smallholdings of Northern Rwanda

In the sub-humid parts of East Africa, high population density and pressure on land have led farmers to integrate multipurpose trees on farm. Although mixing trees and crops generates numerous benefits (e.g., fuelwood, timber), it often reduces crop yields. Whereas the effects of mature trees on crops are well studied in semi-arid parklands, there are only few studies for the sub-humid environment. The effects of mature Alnus acuminata (Kunth) and Markhamia lutea (Seem.) on crops were studied on-farm for four seasons in the sub-humid environment of northern Rwanda. Five sampling points for A. acuminata and M. lutea were: (i) 1 m from tree trunk without maize, (ii) 3 m from tree trunk without maize, (iii) 1 m from tree trunk with maize, (iv) 3 m from tree trunk with maize and (v) sole maize away from any tree. Nutrient availability and uptake, soil water, air temperature, solar radiation, crop growth and yields were measured. The APSIM-maize module was used to assess the sensitivity of maize yields to changes in these variables. Nutrients availability was higher under A. acuminata compared with M. lutea, because of higher litter fall but maize nutrient uptake increased only under A. acuminata 3 m from tree trunk during a wetter season. None of tree species affected water availability for maize in the topsoil. Photosynthetically active radiation (PAR), total solar radiation and day air temperature were reduced by both tree species. Maize crop at 1 m and 3 m from the tree trunk was shorter in height but had the same number and size of leaves when compared to sole maize plots. Crop yield was generally reduced more at 1 m than at 3 m from the tree trunk. A positive interaction between A. acuminata and maize was only apparent at 3 m from the tree in one of the four seasons following higher litter fall, suggesting that the negative effect of shade was offset by extra N input during that season. In a modelled scenario under low N fertilization, larger N input from trees could compensate for yield loss caused by reduction in radiation and temperature in about 60% of the seasons. Our findings suggest that adequate pruning and high leaf litter recycling can reduce the negative effect of shade in low intensity farming systems

How to increase the productivity and profitability of smallholder rainfed wheat in the Eastern African highlands? Northern Rwanda as a case study

As wheat demand is increasing in sub-Saharan Africa (SSA), domestic production is being encouraged. The potential to increase the productivity and profitability of wheat appears large in the region, but little is known about the concrete interventions needed to meet that potential. In this study, we selected a site in Northern Rwanda (representative of the cool humid climatic zone which accounts for most of the spring wheat production of SSA) and analysed the determinants of wheat productivity and profitability for 130 smallholder farms during two consecutive short rainy seasons, namely 2017A and 2018A (wheat is seldom grown during long rainy seasons: potato is the preferred crop then). Although wheat yields were found to be high when compared to typical yields in SSA (means of 3469 and 3052 kg ha −1 during the seasons 2017A and 2018A, respectively), large yield gaps were also found (1.977 t ha −1 on average, or 37.6% of the highest farmer's yield, defined as the average actual yields above the 90th percentile of this variable). Evidences presented in the paper suggest that wheat productivity could be increased through increased seeding rate (a 0.14% increase in wheat grain yield was found with a 1% increase in seeding rate), increased nitrogen (N) application combined with frequent weeding (a 0.02% increase in wheat grain yield was found with a 1% increase in N application and frequent weeding), and labour-saving technologies (e.g., herbicides and mechanization). If wheat profitability would also increase with frequent weeding and labour-saving technologies, it would decrease with increased input use in many cases. Indeed, seed, fertilizer and amendments represent most of the wheat production cost in the area. These results illustrate the importance of assessing the impact of narrowing the yield gap on profitability, not only productivity, as some yield-increasing technologies may not be desirable from an economic perspective. They also demonstrate that resource-saving technologies (input-saving e.g., precision agriculture, labour-saving e.g., mechanization) may be as much in demand by African smallholders as yield-increasing technologies, calling for a more balanced approach in current research and development initiatives on the continent. </p

Should fertilizer recommendations be adapted to parkland agroforestry systems? Case studies from Ethiopia and Rwanda

Aims: This study aimed to disentangle tree-crop-fertilizer interactions in agroforestry systems, which has been suggested as an entry point for sustainable intensification of smallholder farming systems in sub-Saharan Africa (SSA). Although tree-crop systems generate multiple economic and ecological benefits, tree-crop competition commonly occurs. We hypothesized that mineral fertilizers affect facilitative and competitive interactions differently in tree-crop systems. Methods: Tree-crop-fertilizer interactions were explored for wheat growing under Faidherbia albida, and maize growing under Acacia tortilis and Grevillea robusta through omission trials of nitrogen (N) and phosphorus (P) in open fields and fields under tree canopy, using a split plot design. The experiments were conducted in Ethiopia and Rwanda, replicated four times, and over two seasons. Results: Our results demonstrated that the presence of F. albida significantly improved N and P use efficiencies, leading to significantly higher (P −1 yr.−1 of mineral N. The P use efficiency of wheat under F. albida was double that of open field wheat. By contrast, G. robusta and A. tortilis trees lowered nutrient use efficiencies in maize, leading to significantly less maize grain yields compared with open fields receiving the same fertilization. Probabilities of critically low crop yields and crop failure were significantly greater for maize growing under the canopy of these species. Conclusions: Our results showed that recommended fertilizer rates led to facilitative interaction only with F. albida, highlighting that fertilizer recommendations need to be adapted to agroforestry systems.</p

Do open-pollinated maize varieties perform better than hybrids in agroforestry systems?

A large body of evidence demonstrates the agronomic superiority of maize hybrids over open-pollinated varieties (OPVs) in intensive monoculture. However, comparisons of the performance of hybrids and OPVs in agroforestry systems are scarce. In this study, the performance of four maize hybrids and four OPVs is compared in sole crop and under mature trees. Experiments were conducted on-farm during four seasons in Bugesera, Rwanda and two seasons in Meki, Ethiopia. Two tree species were selected in Bugesera (Grevillea robusta and Senna spectabilis) and one in Meki (Acacia tortilis), and three farms were selected for each tree species, each including two plots with almost identical trees in their centre and two plots without tree. In Bugusera, grain yield was higher for hybrids (2 Mg ha-1) than for OPVs (1.5 Mg ha-1), and the presence of trees reduced the harvest index more in OPVs than in hybrids. In this region, the estimated reduction in grain yield due to the presence of trees was 0.9 and 1.1 Mg ha-1 in hybrids and OPVs, respectively, while estimated reduction in biomass was 1.5 and 1.7 Mg ha-1, respectively. In Meki, the grain yield of OPVs (2.08 Mg ha-1) and hybrids (2.04 Mg ha-1) did not differ and the presence of trees reduced their grain yields in the same manner. Our results showed that hybrids yielded more than OPVs under G. robusta and S. spectabilis in Bugesera but performed equally well under A. tortilis in Meki. We conclude that agroforestry farmers could benefit from growing hybrids in the equatorial savannahs of Rwanda, but that the choice between hybrid and OPV in equatorial savannahs of Ethiopia can simply be based on other factors such as seed costs and availability

Conservation agriculture with trees amplifies negative effects of reduced tillage on maize performance in East Africa

Conservation agriculture (CA) is widely promoted in sub-Saharan Africa both in open fields and in agroforestry where the practice is known as ‘conservation agriculture with trees’ (CAWT). Although advantages and disadvantages of CA are well studied under sole cropping, less is known about its impact in agroforestry systems. The performance of open pollinated maize varieties under CA, CAWT, sole maize under conventional tillage (CT) and conventional tillage with trees (CTWT) was compared on-farm in equatorial savannah areas over four consecutive seasons in Rwanda and two seasons in Ethiopia. The tree species considered in the study were mature Grevillea robusta (A. Cunn.) and Senna spectabilis (DC.) in Rwanda and mature Acacia tortilis (Forssk.) in Ethiopia. Both CA and the presence of trees consistently reduced maize emergence, leaf area (LA), plant height, and maize yields. Crop emergence was significantly reduced under CAWT compared with CTWT. Maize emergence rates in CAWT and CTWT were respectively 46.9% and 70.1%, compared with 74.7% and 79.8% in sole maize under CA and CT. Grain yield in CAWT and CTWT were respectively 0.37 t dry matter (DM) ha−1 and 1.18 t DM ha−1 as compared with 1.65 t DM ha−1 and 1.95 t DM ha−1 in CA and CT. We conclude that CAWT strongly reduces crop yield in the equatorial savannah of East Africa. CA is incompatible with agroforestry under the conditions of our study. There is an urgent need for rigorous research to revisit if, when and where CAWT can generate benefits for smallholder farmers.</p

Tree-Based Ecosystem Approaches (TBEAs) as Multi-Functional Land Management Strategies—Evidence from Rwanda

Densely populated rural areas in the East African Highlands have faced significant intensification challenges under extreme population pressure on their land and ecosystems. Sustainable agricultural intensification, in the context of increasing cropping intensities, is a prerequisite for deliberate land management strategies that deliver multiple ecosystem goods (food, energy, income sources, etc.) and services (especially improving soil conditions) on the same land, as well as system resilience, if adopted at scale. Tree based ecosystem approaches (TBEAs) are among such multi-functional land management strategies. Knowledge on the multi-functionality of TBEAs and on their scaling up, however, remains severely limited due to several methodological challenges. This study aims at offering an analytical perspective to view multi-functional TBEAs as an integral part of sustainable agricultural intensification. The study proposes a conceptual framework to guide the analysis of socio-economic data and applies it to cross-site analysis of TBEAs in extremely densely populated Rwanda. Heterogeneous TBEAs were identified across Rwanda&rsquo;s different agro-ecological zones to meet locally-specific smallholders&rsquo; needs for a set of ecosystem goods and services on the same land. The sustained adoption of TBEAs would be guaranteed if farmers subjectively recognize their compatibility and synergy with sustainable intensification of existing farming systems, supported by favorable institutional conditions