Potential of Alnus acuminata based agroforestry for carbon sequestration and other ecosystem services in Rwanda

Abstract

Alnus acuminata Kunth. (alnus) is widely used in agroforestry systems across the globe and is believed to provide multiple ecosystem services; however, evidence is lacking in agroforestry literature to support the perceived benefits, particularly in Rwanda. To understand carbon sequestration potential and other benefits of alnus, a household survey, tree inventory and destructive sampling were conducted in north-western Rwanda. Over 75% of the respondents had alnus trees in their farms. The trees provide stakes for climbing beans, firewood and timber. They also improve soil fertility and control soil erosion. Farmers had between 130 and 161 alnus trees per hectare with an average height of 7.7 ± 0.59 m and diameter at breast height of 16.3 ± 1.39 cm. The largest biomass proportion was found in stems (70.5%) while branches and leaves stock about 16.5 and 13% of the total biomass, respectively. At farm level, aboveground biomass of alnus trees was estimated to be 27.2 ± 0.7 Mg ha−1 representing 13.6 Mg of carbon (C) per hectare. Biomass carbon increased with tree size, from 7.1 ± 0.2 Mg C ha−1 in 3 years old trees to 34.4 ± 2.2 Mg C ha−1 in 10 years old trees. The converse was observed with elevation; biomass carbon decreased with increasing elevation from 21.4 ± 1.29 Mg C ha−1 at low (2011–2110 m) to 9.6 ± 0.75 Mg C ha−1 in the high elevation (> 2510 m). In conclusion, alnus agroforestry significantly contributes to carbon sequestration, although the magnitude of these benefits varies with tree age and elevation. Planting alnus trees on farms can meet local needs for stakes for climbing beans, wood and soil fertility improvement, as well as the global need for regulation of climate change.

Variation in Tree Density, Biomass and Carbon Stock with Altitude under the Large Cardamom-based Traditional Agroforestry System of Darjeeling

“…Similarly, above and below-ground biomass allocation of many individual tree species was reported with a major contribution by above-ground biomass (Justine et al, 2015;Singh and Singh, 2017). Large cardamom based traditional agroforestry systems in the present study exhibited many shade-tree including Alnus nepalensis which provide multiple ecosystem services from provisions like fuelwood and fodder to regulatory like maintaining soil fertility and productivity (Lepcha and Devi, 2020;Cyamweshi et al, 2021). Alnus nepalensis was reported as dominant shade tree species in large cardamom based traditional agroforestry systems of Sikkim Himalayas Singh et al, 2018;Lepcha and Devi, 2020) but in present study the species was found as one of the many species present as shadetrees in the large-cardamom systems.…”

A quantitative appraisal of selected agroforestry studies in the Sub-Saharan Africa

“…(2021) Fertility trees/shrubs systems Development of tree and shrub in farmlands (for example Glyricidia/Cassia siamea) to enhance soil fertility through nitrogen-fixing to bring nutrients closer to the soil surface multiple agroecological zones, especially those with degraded soils Food, fodder, and fuelwood production Soil fertility, nitrogen-fixing Increased farm yield resulting in farm profitability Teklehaimanot, (2004) , Carsan et al. (2014) , Cyamweshi et al. (2021) Improved tree fallows technology A technology of planting legume trees and crops in rotation arrangement within the farmland Different agroecological niches depending on trees-crops arrangement Food, fodder, timber and non-timber products, biomass provision Improved land productivity, soil fertility, soil carbon sequestration Rural income diversification Crop-trees competition for nutrients, water, and other resources Onoja et al.…”

“…Carbon sequestration was noted as the dominant regulatory ecosystem service generated from agroforestry technologies at 31% of the mentioned regulatory services as summarized in Figure 6 . The studies captured this role differently, including carbon sequestration, carbon storage, carbon sinking, greenhouse gases emissions reduction, methane reduction, aboveground tree carbon storage and increased carbon stock [see for example: Nair et al., 2009 ; Luedeling et al., 2014 ; Cyamweshi et al., 2021 ]. A study by Balasubramanian (2019) estimated the economic value of global regulating ecosystem services at US$29.085 trillion for 2015, with climate and water regulation services taking the lead.…”

“…Agroforestry technologies also provide a critical regulatory service of erosion control, which was cited by 16% of the total regulatory services. This service was captured differently by the studies, including soil erosion prevention/control, surface water runoff control, wind erosion regulation/control, and reduced soil loss [see for example : Jose, 2009;Kiptot and Franzel, 2011;Kaczan et al, 2013;Andres et al, 2016;and Cyamweshi et al, 2021]. Further, adaptation and mitigation to climate change, and enhanced water and air quality regulation were identified as key regulatory services provided by agroforestry technologies at 12% of the regulatory services mentioned.…”

Pruning and green manure from Alnus acuminata improve soil health, and potato and wheat productivity in Rwanda

“…The improvement in SOC stocks due to green manure application under trees in the study site corresponded to SOC stock of 40-43 t C ha − 1 which is in the range of SOC stocks reported in agroforestry systems (30-300 t C ha − 1 )(Nair et al 2010) and that reported for tropical agroforestry (7.9-105 t C ha − 1 )(Montagnini and Nair 2004). In addition to the carbon stored in alnus biomass (17 t C ha − 1 ) in Rwanda(Cyamweshi et al 2021), the build-up of SOC stocks underlines the contribution of alnus agroforestry in climate change adaptation and mitigation of greenhouse gas emissions. Therefore, it is worthy to be considered in the country's national carbon accounting system and be included in the Nationally Determined Contributions to the Paris Agreement on combatting global climate change.…”