Effects of tree spacing and species composition in agroforestry systems in Rwanda
Project info
Status: Active
Span: 2025-2026
Location: Nyagatare District, Rwanda
Contact: Ulrik Ilsted & Jose Gutierrez Lopez (Ulrik.Ilsted@slu.se, Jose.Lopez@slu.se)
Summary
Water availability limits primary production in tropical agroforestry systems, affecting three billion people globally. In East Africa, exotic, fast-growing tree species such as Eucalyptus sp. and Grevillea sp. are commonly used in agroforestry. Despite their widespread and documented benefits, fast-growing exotic tree species are known to consume large amounts of water, potentially reducing water available for crops. Yet, relevant studies assessing the impact of tree management practices such as pruning on water uptake are scarce. In this study, we established a study to monitor the water uptake in Eucalyptus tereticornis and Grevillea robusta trees in a farm in Kayonza, Rwanda, to assess the effects of different pruning intensities on their transpiration, and to understand water uptake characteristics across the wet and dry seasons.
Agroforestry in the Rwandan context
Development of plantation forestry is increasing globally to meet the demand for forest products, largely due to an increase in the global human population and the decline in natural forests (FAO, 2015). Globally, the area of plantation forests increased by 55.8 million ha between 1990 and 2020, with the biggest jump (21.2 million ha) occurring between 2000 and 2010 (FAO, 2020).
Worldwide, Eucalyptus is the second most planted genus next to Pinus and grown in over 100 countries. Eucalyptus is native to Australia, Philippines, Papua New Guinea, Indonesia and Timor where more than 700 species are present (Boland et al., 2006; Grattapaglia et al., 2012; Wingfield et al., 2015; Brett et al., 2017; Sappi, 2020).
In East Africa, where there is a huge gap between demand and supply of wood, the use of productive tree species such as Eucalyptus and Grevillea is increasing rapidly (Nduwamungu et al., 2008; Pedercini et al., 2022). The history of growing Eucalyptus in East African countries dates back to late 19th and early 20th century.
Subsequently, Eucalyptus plantings have expanded dramatically in countries such as Rwanda, Ethiopia, Kenya, Burundi, Uganda, Sudan, and Tanzania, to the extent where Eucalyptus is a prominent part of the urban and rural landscapes of these countries (Nduwamungu, 2011a; Nduwamungu, 2011b; Mathu, 2011; Dessie and Erkossa, 2011).
The extensive cultivation of Eucalyptus in East Africa is justified by the increasing demand for wood, which is propelled by the growing human population and economy, and drastic decline in natural forest cover (Dessie and Erkossa, 2011). Some of the merits of growing Eucalyptus include its fast growth, minimum requirement of care, ability to grow in wide ecological zones and under poor environmental conditions, ability to coppice after harvest and resist environmental stresses, ease to collect and store seeds and suitability to limited resources of smallholder growers. In addition to providing building materials, fuel wood, poles, farm timber and essential oils, monetary income from Eucalyptus has helped in buffering financial crisis for many poor farmers (Brett et al., 2017; Botho Emerging Markets Group, 2019).
The effects of tree spacing in agroforestry systems
Rwanda has over 12.6 million people living on just 26,338 Km2 with a pop density of 525 per Km2. Agricultural land covers 59% of the country. The Strategic Plan for Agriculture Transformation (PSTA) in Rwanda identified Agroforestry as the best strategy to meet the demand for wood biomass and agricultural production (ROR, 2024). Eucalyptus is the most widely planted hardwood genus in the world due to its high productivity (Albaugh et al., 2013). In Rwanda Eucalyptus occupy 80 % and 50 % of non-cultivated and cultivated areas (Winowiecki et al., 2018). Grevillea is another popular agroforestry tree species found on farms (Bucagu et al., 2013) and praised for its climatic tolerance, less competition, less water use and well-integrated in different cropping systems (Pedercini et al.,2022). In agroforestry, water use can be managed by tree species selection, pruning and control of tree spacing; these can reduce tree water use and impact ecosystem water balance (Chavan et al., 2018). In a study conducted in Burkinafaso it was concluded that an intermediate tree cover increases ground water recharge (Ilstedt et al., 2016), but we do not know which one can best fit in Rwandan AF system. Thus, we established a split-split-plot design experiment (Nyagatare District, Rwanda) to measure how tree density (spacings of 9, 6 and 3 m) and tree species affect tree water use. We expect that in the end we will have one planting spacing which will optimise tree water use in the study area.
Installing sap flow sensors in Eucalyptus trees
Installing sap flow sensors in Eucalyptus trees
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