The Guineo-Congolian Rainforest and the Late Holocene Rainforests Crisis

The Guineo-Congolian rainforest. Source: Huntly, Brian John. (2023). The Guineo-Congolian Rain Forest Biome. Ecology of Angola.


In the previous blog post, I briefly touched upon discussions of deforestation in African forest, namely how they related to the history of savannas. It has been clearly established that savannas are ancient, existing for longer than the tropical forests near, and thus not the result of mass deforestation. However, the history of forests within Africa is much more complicated than that.

History of Rainforests in Africa 

During the last glacial maximum approximately 20,000 years ago, most of the rainforests in Central and West Africa that we see today did not exist, and were likely savannas or grasslands1. This means that the rainforests we see today in Africa didn’t start developing until roughly 12,000 years ago when the climate had warmed and central and West Africa had become humid enough to support a rainforest. At the peak of this period, the rainforest extended 400-500 kilometers further north into the Sahara than they do at present1. These warm and humid conditions  continued until roughly 4,000 years B.P. (This refers to years before present, with “present” being marked at 1950). 

The period from 4,000 to 2,000 B.P.,  is marked by a retreat of the forests and climatic changes referred to as the late Holocene rainforest crisis, and is a topic of substantial debate at the moment about the extent of human involvement in the forest retreat. I will go into more detail into this later in the post. After that period, the rainforest went through a regeneration, reaching an extent roughly approximate to today. 

Congo Basin Rainforest. Source: Unearthed
Recent studies have disproved claims of extreme widespread deforestation in the past 100 years, which were often based on erroneous assumptions or faulty data. Reanalysis of the historical record and ecological record have shown that the extent of deforestation in the past 100 years of African rainforests is substantially less than the 25.5 to 30.2 square kilometers traditionally suggested, and is likely more in the ballpark of 9.5-10.5 square kilometers, with some margins even seeing an expansion of forest2. This most recent deforestation is likely the result of population expansion and people encroaching on the forest margins, as well as changes in agricultural and industrial practices due to the modern age and colonialism. 

Other studies have found different numbers or deforestation in Africa, and noticed a difference in spatial distribution, finding that while the average for the content was 27.1% since 1900, areas such as West and East Africa had 80 to 90 percent deforestation, while Central Africa gained 1.4% forest. The extent of deforestation in East and West Africa may be inflated in this study due to conflicting historical accounts. This brings us to our current place where today, the rainforests in central and West Africa (the Guineo-Congolian region) are the second largest tropical rainforest block in the world, after Amazonia, spanning 1,998,290 square kilometres1. 

Recent History of Human Occupation of Central and West Africa

Alongside this history of forest development, there is a complementary tale of the development of human cultures. Around 40,000 years B.P. Late Stone Age cultures began popping up  in central and West Africa and remained prominent until roughly 3,500 B.P.. A wave of migration  of Neolithic farmers from Sahel began at roughly 3,500 B.P. and was soon followed by the spread of early Iron Age cultures between 2,800 and 2,500 B.P1. These last two events align with the period of retreat seen in the forests 4,000 to 2,000 B.P. and competing hypotheses about whether humans drove the retreat or the retreat drove human migration have been proposed. People from 3,500 B.P. and on were bringing with them new technologies like agriculture and metallurgy which dramatically altered the human environment relationships in the region. By 1600 to 1000 B.P. This region experienced a significant decrease in population size, potentially due to epidemics1. From 1000 B.P. on the history of the people in and around the rainforests gets very complicated owing to the arrival of European colonists and the ensuing atrocities of the Slave trade and colonial rule. 

Late Holocene Rainforest Crisis (LHRC)

Between 4,000 and 2,000 B.P., the rainforests in central and west Africa experienced substantial decline. During this period, there was a shift away from the dense mature rainforests to larger amounts of grasses and pioneer trees, trees that grow early on in the succession sequence of forests3. There is substantial evidence for the occurrence of widespread changes in forests during this time period. Although there is some minor regional variation, throughout the Guinea-Congolese rainforests, the forests better suited for humid conditions were replaced by pioneer trees and grasses4. 

Extent of the rainforests in the Guineo-Congolian region.
Source: Mayaux et al. (2013) Philosophical Transactions of the Royal Society B 368

Climatic Drivers for Late Holocene Rainforest Crisis (LHRC)

Substantial debate exists within the archaeological and paleoecological community
about what caused this wide-spread forest retreat [3,5-9]. Traditionally, the view has been that the forest retreat was the result of climatic changes, either aridification due to reduced precipitation or the more widely accepted idea now which is that precipitation remained constant, but slight changes in climate dynamics meant that rain was now split seasonally, causing the introduction of a dry season to these environments which they didn't have prior to the LHRC 5,6. The dry season would be enough to cause an environmental shift away from rainforest trees because these trees require moisture all of the time. The savannas in central and west Africa today do not require substantially less water than the surrounding forests, but they do need a dry season. As such, it is very feasible that a shift that caused t
he formation of the dry season would have resulted in the expansion of savanna landscapes. This dry season is proposed to have been the result of changes in the ocean temperatures offshore of West Africa which control the seasonality of the rains. 

Fragments of oil palm seeds found in
archaeological site in South Cameroon.
Source: Neumann et al. (2012). [10] 

The time frame of the LHRC also coincides with the period of expansion in the area of agriculturalists and early iron age peoples, referred to as the Bantu expansion, referring to the Bantu speaking people who were expanding across central Africa at this point. People who support a climatic change model suggest that the expansion of the Bantu speaking people into these regions was fueled by this change in environments5. The receding forests and expanding grasslands gave greater access to resources, allowing more area to graze and to cultivate. The retreat of the forest coincides with increases in grass seeds belonging to  pearl millet, which was cultivated by the Bantu speaking people5. The climate catalysts supporters argue that the agriculture wasn’t what caused the forest retreat, but the retreating forest allowed for light penetration and thus the growth of grasses. 

The Bantu speaking people brought with them  pearl millet and took advantage of the new climatic conditions to cultivate it. The receding forest also likely led to the expansion of palm trees along the margin that may also have been cultivated by the Bantu migrants5. Along with the cultivation, the Bantu people also brought with them iron smelting and metallurgy. Processing iron requires a substantial amount of wood resources to burn, although there is also debate about whether the demands for wood resources for metallurgy is significant enough to cause deforestation9. 

Human Driven Causes for the Late Holocene Rainforest Crisis

While this stance has long been the one taken by archaeologists, recent dissenting voices have proposed that the shift was not the result of changes in precipitation patterns, but was instead induced by human migration into the region. Garcin et al. argue that the explanations based on the amount or timing of precipitation are founded3. They included a new line of evidence beyond just the pollen data which had been previously used to try to reconstruct climate conditions around the LHRC by measuring the relative proportions of heavy and light stable isotopes of carbon and hydrogen in plant waxes.

Modern day farmer in pearl millet field.
Source: Neumann et al. (2012) [10]
Isotopes are atoms of the same element that have different weights because of a different amount of neutrons in the nucleus. In this case, the  isotopes that are heavy will have one more neutron in their nucleus than the lighter isotope. The ratio between the light and heavy isotope can be very insightful because different processes in the environment and in organisms can cause something called fractionation, which basically means that either the light or heavy isotopes in prefenced in the process skewing the product of the process either lighter or heavy than the original source material. The amount these ratios are skewed and in which direction is process dependent, varies between types of organisms, and can be temperature dependent. 

The stable isotope ratios from the plant waxes showed that there was a change from trees to grasses during the LHRC which aligns with pollen record3. However, they did not indicate the necessary reduction in precipitation needed for the extent of aridification required to produce the scale of forest retreat observed during this period. They also found by looking at offshore marine cores that there was no change in the ocean surface temperature which would be expected if there was going to be a change in the seasonality of rains.

 Both of these findings, if accurate, would completely dismantle the climate hypothesis for the LHRC8. However, the methodology is under debate by others in the community5. Garcin et al. then correlates these changes with the arrival for people to the area with pearl millet farming and other anthropogenic markers such as charcoal as evidence that the rainforest was cleared for the purpose of farming the pearl millet. At the end of the LHRC, the isotopes indicate a shift back to tree plants, though there is no way of knowing if these were rainforest trees or periphery palm oil trees being cultivated and used by the local people3. 


Whether the Late Holocene Rainforest Crisis was caused by human activity or not is still up for debate, but regardless of the cause, it is clear that the relationship between people and the Guineo-Congolian rainforest has been very dynamic changing through time. In both cases, people took land which was previously thick rainforest and began modifying it by cultivating their pearl millet and extracting resources from the oil palm trees. 



References

  1. Malhi, Yadvinder, Stephen Adu-Bredu,Rebecca A. Asare, Simon L. Lewis, Phillippe Mayaux.”African rainforests: past, present and future.” Phil Trans R Soc B 368, (2013): 20120312. http://dx.doi.org/10.1098/rstb.2012.0312African rainforests

  2. Fairhead, James and Leach, Melissa. “Reconsidering the Extent of Deforestation in Twentieth Century West Africa.” Unasylva 49, no. 192 (1998): 38-42. https://www.fao.org/3/w7126E/w7126e06.htm#reconsidering%20the%20extent%20of%20deforestation%20in%20twentieth%20century%20west%20africa

  3. Garcin, Yannick, Pierre Deschamps, Guillemette Ménot, Geoffroy de Saulieu, Enno Schefuß,  David Sebag, Lydie M. Dupont, Richard Oslisly,  Brian Brademann, Kevin G. Mbusnum, et al. “Early anthropogenic impacts on Western Central African Rainforests 2,600 y ago.” Proceedings of the National Academy of Science 115, no. 13 (2018): 3261-3266. https://www.pnas.org/doi/abs/10.1073/pnas.1715336115

  4. Oslisly, Richard, Lee White, Ilham Bentaleb, Charly Favier, Michel Fontugne, Jean-François Gillet and David Sebag. “Climatic and cultural changes in the west Congo Basin forests over the past 5000 years.” Phil. Trans. R. Soc. B 368, no. 1625 (2013): 20120304. http://doi.org/10.1098/rstb.2012.0304

  5. Giresse, P, Maley, J., Doumenge, C., Philippon, N., Mahé, G., Chepstow-Lusty, A., Aleman, J., Lokonda, M., and Elenga, H. “Paleoclimatic changes are the most probable causes of the rainforest crisis 2,600 yr ago in Central Africa.”  Proceedings of the National Academy of Science 115, no. 29 (2018): E6672-E6673. https://doi.org/10.1073/pnas.1807615115

  6. Neumann, K., M. K. H. Eggert, R. Oslisly, B. Clist, T. Denham, P. de Maret, S. Ozainne, E. Hildebrand, K. Bostoen, U. Salzmann, D. Schwartz, B. Eichhorn, B. Tchiengu, and A. Hhn. “Comment on “Intensifying weathering and Land Use in Iron Age Central Africa.”” Science 337, no. 6098 (2012): 1040 https://www.science.org/doi/10.1126/science.1221747

  7. Garcin, Yannick, Pierre Deschamps, Guillemette Ménot, Geoffroy de Saulieu, Enno Schefuß,  David Sebag, Lydie M. Dupont, Richard Oslisly,  Brian Brademann, Kevin G. Mbusnum, et al. “Reply to Clist et al: Human activity is the most probable trigger of late Holocene rainforest crisis Western Central Africa.”  Proceedings of the National Academy of Science 115, no. 21 (2018): E4735-E4736. https://doi.org/10.1073/pnas.1805582115

  8. Malhi, Yadvinder. “Ancient Deforestation in the green heart of Africa.” Proceedings of the National Academy of Science 115, no. 13 (2018): 3202-3204. https://doi.org/10.1073/pnas.1802172115

  9. Maley, Jean. “A Catastrophic Destruction of African Forests about 2,500 Years Ago Still Exerts a Major Influence on Present Vegetation Formations.” IDS Bulletin 33, no. 1 (2009): 13-30. https://doi.org/10.1111/j.1759-5436.2002.tb00003.x

  10. Neumann, Katharina, Koen Bostoen, Alexa Hohn, Stefanie Kahlheber, Alfred Ngomanda, and Bathelemy Tchiengue. "First Farmers in the Central African rainforest: A view from Southern Cameroon." Quaternary International 249, no. 6 (2012): 53-62. https://doi.org/10.1016/j.quaint.2011.03.024

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