PH.D Thesis

“Coffee Productivity and Water Use in Open vs Shaded Systems along an Altitudinal Gradient at Mt. Elgon, Uganda”

Coffee is one of the most important agricultural products worldwide. It provides the livelihood of 25 million farmers in tropical countries and of approximately 125 million people along the production chain. Furthermore, coffee demand has steadily increased in the last decades; a trend that is projected to continue in the coming years. Although coffee production is a crucial source of income for several million people, it also contributes to the current environmental crisis. Coffee cultivation and boom and bust cycles lead to deforestation, negatively affecting carbon and water cycles and biodiversity. On the other hand, coffee, as a shade tolerant species, can be intercropped with shade trees in agroforestry systems (AGF). And if appropriately managed, AGF can provide several ecosystem services, such as climate protection, microclimate regulation, biodiversity conservation, soil protection, and income diversification among others.

At the smallholder coffee farmers’ scale, the intercropping of coffee and shade trees can generate a range of benefits (i.e. more biodiversity, improved pest control and income diversification) as well as trade-offs such as competition for water and nutrients between coffee and shade trees. The degree to which these benefits and trade-offs develop depends on the specific environmental conditions, management practices and cropping systems. This study aimed to understand the functioning of three coffee cropping systems on the slopes of Mt. Elgon Uganda, in particular to gain insights on how these cropping systems affected coffee productivity, water use and microclimate regulation at different altitudes – ranging beween 1100 and 2100 m.a.s.l. The cropping systems studied were coffee open (i.e. shade cover < 20 %, CO), coffee intercropped with bananas (CB) and coffee intercropped with shade trees (CT). The data collection consisted of two main components: (i) a field experiment on water use, and (ii) a coffee tree inventory and monitoring of reproductive and vegetative growth. The results were structured in three research articles as presented below.

Coffee yield (kg ha-1) and coffee yield component performance in different coffee cropping systems along an altitudinal gradient and shade cover gradient was evaluated in the first article: “Effect of cropping system, shade cover along and altitudinal gradient on coffee yield components at Mt. Elgon, Uganda”. Fruit load per branch, productive nodes per branch and number of productive branches per stem were monitored on 810 coffee stems distributed over 27 plots (9 belonging to each cropping system). Additionally, coffee cherry weight, productive stems per ha and shade cover was monitored in each plot for two harvest seasons (2015 and 2016). CB system had higher yields per ha (1086 ± 736 kg green beans) than CO (670 ± 457 kg green beans) and CT (428 ± 259 kg green beans). Fruit load per branch and number of productive branches per stem were the most important yield components. Both decreased with shade cover above 30 % and were negatively correlated with the number of stems per coffee tree. Overall, we did not find differences in cherry weight or productive stems per ha across cultivation systems, nor did altitude show a clear effect on yield components.

In the second article, “Water use of Coffea arabica in open versus shaded systems under smallholder’s farm conditions in Eastern Uganda” we explored the water relationships of the three previously mentioned cropping systems (CO, CB and CC). We found that (i) coffee water use rates did not differ across systems, (ii) coffee trees benefited from the microclimate provided by shade trees (banana and C. africana), and (iii) CB is an attractive system for smallholder farmers, as it also provides food. Soil water content was reduced in shaded systems (CB and CC) compared to CO), especially in coffee intercropped with C. africana. This suggested that under harsher conditions (hotter and dryer) than the ones recorded in our study, water competition between coffee and shade trees could become a problem.

In the third article, “Disentangling effects of altitude and shade cover on coffee fruit dynamics and vegetative growth in smallholder coffee systems”, we investigated coffee fruit development (from fruit initiation to harvest) and vegetative growth during two production cycles (2015 and 2016) in 810 coffee stems distributed over 27 coffee plots. Additionally, microclimate and soil water content were monitored (in 18 plots and 16 plots respectively). Shaded systems buffered microclimate. Fruit set was not limited by temperature but reduced by increases in shade cover. Whilst fruit drop was similar along the shade gradient and was positively related to initial fruit set. Finally, leaf set was the most important variable to ensure vegetative and reproductive growth along several production cycles.

Coffee cultivation systems at the slopes of Mt. Elgon are not intensively managed and mostly have low yields; but as such they fall within the average range of conditions faced by smallholders in Eastern African. There is scope to improve yields by reducing the number of stems per coffee tree (pruning) and increasing coffee tree density. Furthermore, a certain level of shade should be maintained to protect coffee from increased maximum temperatures and avoid high temperature amplitudes, regardless of the system type. Coffee intercropped with bananas showed an optimal balance between microclimate regulations, fruit set, fruit drop and yields, and provide staple food and an extra source of income.

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