Phil Pritchard: Oxford City Farm and its forest garden
My name is Julian Radford-Smith and I have been working as Programmes Volunteer at ITF. I took 6 months off from my studies at the University of Queensland in Australia, where I am very close to completing a Bachelor of Science (majoring in Ecology).
At ITF, a lot of people ask us about the amount of carbon sequestered by individual trees or per hectare of reforested land.
As Programmes Volunteer, I was put to the task of consolidating these figures. I started my research with an assumption that there would exist well-recognised, easily accessible values. This proved to not be the case!
Trees are much more than carbon sinks!
Before I go further, in considering the nature of our projects in Africa, I realised that these are perhaps not such pertinent questions. The vast majority of tree planting that we support takes place on a small scale. It is often either for community benefit on community owned land, or for household benefit on individual farms. And at this scale, trees are rarely planted for their carbon sequestering capacity. But there are a range of other reasons that have more tangible, direct benefits.
Here are just a few, we would argue more important, reasons for planting trees. Trees improve the daily lives of household and community members, producing fruits/nuts/leaves for consumption or trade, fodder for livestock, timber, firewood, as well as medicinal ingredients.
Agroforestry (integrating trees and crops) can also relieve the pressure on remnant or old-growth forests that are frequently affected by grazing, logging and burning for charcoal. Trees also carry out a range of local regulating services. Small-scale planting is capable of improving water infiltration and ground water recharge catchment function (leading to greater water security), increasing soil fertility for agriculture, controlling erosion, conserving soil and improving microclimate.
Ultimately, within communities, trees are valued for their localised functions. These include promoting diversification, increasing resilience, improving local economies and enhancing sustainability. It is only at a global scale that their role in carbon sequestration become recognized and appreciated.
Is measuring ‘per tree’ carbon sequestration realistic?
If you are still intent on asking how much carbon a tree sequesters – it’s quite a difficult question to reliably answer!
First off, having a ‘per tree’ carbon sequestration figure is not actually very useful. Planting specifically for carbon sequestration takes place over very large areas, and it is only at such scales that these projects make sense. As such, ‘per tree’ values are rarely attributed in the available literature. Furthermore, depending on the tree species of interest, the time period, the climate and the soil type, sequestration values will vary considerably. So narrowing down a possible range soon becomes a complex task.
During my research, I have delved into the available literature, with an aim to identify yearly carbon sequestration rates ‘per tree’ or ‘per hectare’ of forest. I targeted three regions in my search: Eastern Africa, Southern Africa and the Sahel, while also researching popular agroforestry trees such as Grevillea robusta. As expected, I found that determining sequestration values for individual trees was not practical. Yet for the regions above, tentative figures were identified.
For Eastern Africa, the carbon sequestration rate per hectare per year (Kg C ha-1 year-1) ranged from 500-3,000, encompassing agroforestry (e.g. intensification of windrows and tree biomass) and forest restoration. Sequestration rates for agroforestry and low-density planting and restoration in the Sahel ranged from 220 to 770. In Southern Africa, values ranged from 500-1,200 for natural woodland regeneration, agroforestry and plantations. A list of primary references has been provided below, with approximately ten other (secondary) journals and reports not included.
Other organisations similar to ITF were explored to see how they dealt with the issue of tree planting and carbon sequestration. An interesting example is Tree-Nation, who display annual carbon sequestration rates for individual tree species on their website (e.g. https://tree-nation.com/projects/20-million-trees-for-kenyas-forests/species). A simplified approach is taken, where the lifetime CO2 offset of the tree is divided by its maximum growth period to get a CO2 value ‘per year’.
Although carbon sequestration is a complex area, we have tried to calculate a sensible figure for our Centenary Campaign. With a number of different scenarios present on Mt Kenya, we think that an overall offset value of 4kg C/ tree/ year can be set for the first 20 years, after which the rate of carbon sequestration will decrease.
If this is an area of interest for you and you have knowledge of more definitive figures or evidence, please don’t hesitate to contact us.
Julian Radford-Smith
References
Luedeling, E, Sileshi, G, Beedy, T & Dietz, J 2011, ‘Carbon Sequestration Potential of Agroforestry Systems in Africa’, Advances in Agroforestry, vol. 8, no. 4, pp. 61-85.
Kimaro, A, Isaac, M & Chamshama, S 2011, ‘Carbon Pools in Tree Biomass and Soils Under Rotational Woodlot Systems in Eastern Tanzania’, Advances in Agroforestry, vol. 8, no. 4, pp. 129-145.
Paustian, K, Ravindranath, N & van Amstel, A 2006, Agriculture, Forestry and Other Land Use, Guidelines for National Greenhouse Gas Inventories Vol. 4, Intergovernmental Panel on Climate Change, Geneva, (https://www.ipcc.ch/meetings/session25/doc4a4b/vol4.pdf).
Batjes, NH 2004, ‘Estimation of Soil Carbon Gains Upon Improved Management within Croplands and Grasslands of Africa’, Enrichment, Development and Sustainability, vol. 6, no. 2, pp. 133-143.
Luedeling, E & Neufeldt, H 2012, ‘Carbon sequestration potential of parkland agroforestry in the Sahel’, Climatic Change, vol. 115, no. 4, pp. 443-461.
Rayden, T, Ramani, P, Lander, T, Ebeling, J & Nussbaum, R 2010, Terrestrial carbon: emissions, sequestration and storage in tropical Africa, FPAN African Tropical Forests Review, Proforest, Oxford, (http://www.proforest.net/proforest/en/files/terrestrial-carbon-emissions-sequestration-and-storage-in-tropical-africa.pdf).
Perez, C, Roncoli, C, Neely, C & Steiner, J 2007, ‘Can carbon sequestration markets benefit low-income producers in semi-arid Africa? Potentials and challenges’, Agricultural Systems, vol. 94, no. 1, pp. 2-12.
Jindal, R, Swallow, B & Kerr, J 2008, ‘Forestry-based carbon sequestration projects in Africa: Potential benefits and challenges’, Natural Resources Forum, vol. 32, no. 2, pp. 116-130.
Woomer, P, Toure, A & Sall, M 2004, ‘Carbon stocks in Senegal’s Sahel Transition Zone’, Journal of Arid Environments, vol. 59, no. 3, pp. 499-510.
Aune, J, Alemu, A & Gautam, K (2008), ‘Carbon Sequestration in Rural Communities’, Journal of Sustainable Forestry, vol. 21, no. 1, pp. 69-79.
Unruh, J 2008, ‘Carbon sequestration in Africa: The land tenure problem’, Global Environmental Change, vol. 18, no. 4, pp. 700-707.
UNFCCC 2009, Reforestation in grassland areas of Uchindile, Kilombero, Tanzania & Mapanda, Mufindi, Tanzania, Project Description Template for the Voluntary Carbon Standard ARR project activity, <https://s3.amazonaws.com/CCBA/Projects/Reforestation_in_Grassland_of_Uchindile_Kilomber_Tanzania/ufp_mfp_combined_validation_VCS_PDD-1.pdf>.
UNFCCC 2009, Humbo Ethiopia Assisted Natural Regeneration Project, Project design document for afforestation and reforestation project activities, <https://s3.amazonaws.com/CCBA/Projects/Humbo_Ethiopia_Assisted_Natural_Regeneration_Project/PDD+of+Humbo+project.pdf>.
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