Tropical forests in Africa’s mountains store more carbon than previously thought – but are disappearing fast
Scientists studying tropical forests in Africa’s mountains were surprised to uncover how much carbon they store, and how fast some of these forests are being cleared.
The mountain forest in Nyungwe (Rwanda) is home to many species (including L'Hoest's monkey) that are only found in the East African Rift Zone. © Wannes Hubau, Royal Museum for Central Africa
The international study reported in Nature, found that intact tropical mountain (or montane) forests in Africa store around 150 tonnes of carbon per hectare. This means that keeping a hectare of forest standing saves CO2 emissions equivalent to powering 100 homes with electricity for one year.
The study found that African mountain forests store more carbon per unit area than the Amazon rainforest and are similar in structure to lowland forests in Africa. Existing guidelines for African mountain forests – which assume 89 tonnes of carbon per hectare – greatly underestimate their role in global climate regulation.
The international team, including two scientists from the Royal Museum for Central Africa, also investigated how much tropical mountain forest had been lost from the African continent in the past 20 years. They found that 0.8 million hectares have been lost, mostly in the Democratic Republic of the Congo, Uganda and Ethiopia, emitting over 450 million tonnes of CO2 into the atmosphere. If current deforestation rates continue, a further 0.5 million hectares of these forests would be lost by 2030.
African mountain forests: astonishingly carbon rich
Lead author Dr Aida Cuni-Sanchez, from the University of York’s Department of Environment and Geography and at Norwegian University of Life Sciences, said: “The results are surprising because the climate in mountains would be expected to lead to low carbon forests. The lower temperatures of mountains and the long periods they are covered by clouds should slow tree growth, while strong winds and steep unstable slopes might limit how big trees can get before they fall over and die.
But unlike other continents, in Africa we found the same carbon store per unit area in lowland and mountain forests. Contrary to what we expected, large trees remain abundant in mountain forests, and these large trees (defined as having diameters over 70 cm) store a lot of carbon”.
Scientists measured 72,000 trees in 44 mountain sites in 12 African countries, from Guinea to Ethiopia, and south to Mozambique. In each mountain site they established plots where they recorded the diameter, height and species of every tree. Researchers said that better knowledge about how much carbon mountain forests store is especially important for the ten African nations where the only tropical forests they have are those found on mountains.
“While we know what makes African forests special, we don’t yet know why they are different. It is possible that in Africa, the presence of large herbivores such as elephants plays an important role in mountain forest ecology, as these large animals disperse seeds and nutrients, and eat small trees creating space for others to grow larger, but this requires further investigation” Dr Cuni-Sanchez added.
Co-author Wannes Hubau (Royal Museum for Central Africa and professor at Ghent University): “From previous research we already knew how important tropical African lowland forests are for the global carbon cycle, but now it is becoming clear that also the role of the African tropical mountain forests should in no way be underestimated. Moreover, these ecosystems harbour a particularly high and unique biodiversity, and they are home to many rare and endangered animal and plant species.” Co-author Dr Phil Platts, from York’s Department of Environment and Geography and the IUCN’s Climate Change Specialist Group, said: “Besides their importance for biodiversity and climate regulation, these forests provide very important water services to millions of people downstream. However, about 5 per cent of Africa’s tropical mountain forests have been cleared since 2000, and in some countries the rate exceeds 20 per cent.”
Most African nations have committed large amounts of land to forest restoration under the Bonn Challenge. Although forest restoration is important to mitigate climate change, avoiding deforestation is a greater priority. Co-author Dr Martin Sullivan, at the Department of Natural Sciences, Manchester Metropolitan University, added: “Previous carbon estimates for tropical mountain forests in Africa were much lower than the values we report in our study. We hope that these new data will encourage carbon finance mechanisms towards avoided deforestation in tropical mountains. As outlined in the Paris Agreement, reducing tropical deforestation, in both lowland and mountain forests must be a priority”. Co-author Dr Gerard Imani, at the Department of Biology, Université Oficielle de Bukavu in DR Congo, added: “Carbon finance mechanisms could help improve conservation interventions on the ground – even within protected areas, deforestation, forest degradation and defaunation remain a challenge”.
Scientific article: Aida Cuni-Sanchez, Martin J. P. Sullivan, Phil Platts, Simon L. Lewis, Rob Marchant, Gérard Imani, Wannes Hubau et.al., High above-ground carbon stock of African tropical montane forests, Nature, 25 August 2021. DOI: 10.1038/s41586-021-03728-4. https://www.nature.com/articles/s41586-021-03728-4