Fire History in African Savannas

Fires Burning in a savanna in Africa. Source: blendspace

On an average day in August, 70% of the world’s fires will be burning in Africa. Maps that show the number of fires burning globally, like the one pictured below produced by NASA using satellite data, seem to show most of Sub-Saharan Africa on fire. Currently 2.56 Million square kilometers of land is burned in Africa each year1. This certainly would be concerning if these were forest fires akin to the type seen in the Amazon Rainforest. Luckily this is not the case because most of these fires are in savannas and grasslands. 

Map developed by NASA FIRMS showing fire events in Africa on January 31, 2022. Each red dot represents a fire event registered by the satellite. The map clearly indicates the large number of fire events happening in Sub-Saharan Africa. Source: NASA FIRMS  

Fires in savannas and grasslands are of much lesser concern and are a natural part of ecosystem cycling. The most fire prone ecosystem in the savanna, producing 65% of the global mean fire emissions2. Between 40-50% of Africa is savanna2. Savanna ecosystems are characterized by expanses of C4 grasses (referring to the type of photosynthetic pathway used by these grasses in order to improve water retention) interspersed with spotting of trees and shrubs3. This is very similar to the grasslands ecosystems which make up an additional 20-30% of Africa’s ecosystems and have the C4 grasses without the trees and shrubs. These grassy biomes often occur in mosaics across the landscapes along with agricultural fields of sorghum, millet, and cotton4. The complex landscape mosaics are an important factor in the spread of fire.

In modern day, fires are intentionally started in these landscapes by people, often taking the form of controlled burnings to clear croplands and dry season savanna burning to manage fire intensity. However, the use of fire as a landscape management tool for grassy biomes in Africa is in no way new. 

History of Fire Use in Africa

Africa has a longer history of human and hominin landscape interaction than any other continent, as it is the place where humans originated. These interactions have long included the use of fire by people. Archibald et al. (2012) outlined six stages in the evolution of hominin fire use in Africa5. 

Starting around 1.5 Mya ago, we see the first evidence of hominins harnessing fire. This fire naturally occurred and was caused by lightning, so while hominins could harness it to potentially increase the number of fires started, they were still limited to only starting fires during the wet season when lightning fires were occurring anyway. During this time fire was likely being used to improve resource acquisition by using it to dive out animals from brush for hunting6. We see evidence for this early fire use in a variety of sites in Kenya, Ethiopia, and South Africa. 

Fire burning in South Africa in boundary between grass
lands and forests. Credit: Carla Staver

Between 300,000 or 400,000 years ago and 70,000 years ago,  hominins gained the ability to start their own fires. It is during this period that hominins gain the ability to start fires both during the wet season like usual and also in the dry season5. Fire during this time also altered much of human culture, allowing for new tool productions and food production practices, as well as changes to their social lives, as harnessed fire allowed for the day to be extended into the night and provided a stage for activities such as storytelling, singing, and other higher thoughts 6. 

From 70,000 to 4,000 years ago there were substantial human population expansion which is associated with more fire starting events5. Around 4000-200 B.P. agropastoralism became more dominant in Africa 5. With people now cultivating plants and rearing grazing animals, there were changes in the amount of plant material available for burning during the fire season. The domestication of cattle led to more grass consumed, reducing the amount of grass that is available to burn. Cultivation means fields are now being cleared for agriculture, the connectivity of the land is decreasing, which makes it harder for fires to spread. 

The final stage from 200 B.P. until present where we see the large population expansion of modern Africa. During all five of these phases hominins were altering their landscape through their harnessing of fire in different ways and to different degrees. 

Trends in Wildfires in Africa

Map showing the vegetation types of Africa. This map shows
that most of Sub-Saharan Africa is savanna, and most of the
 areas which experience the high levels of burning shown in the
NASA map are in regions dominated by savannas, croplands,
and grasslands, which comprise the open grass mosaics of Africa.
 Source: Machete and Dintwe 2023

Contrary to what many people might expect given the current climate trends, the extent of fire burning in Africa is actually at its lowest point in 40,000 years and has been decreasing within recent history as well at a rate of roughly 36,000 square kilometers per year2. A recent study by Machete and Dintwe (2023) found that most of the decreasing burned area in Sub-Saharan Africa was happening in grasslands and savannas, while forests were increasing in the amount burnt and shrublands and croplands were remaining relatively constant in area burnt2. 

This doesn’t mean that anthropogenic climate change is not affecting Africa. Instead, the drying and increasing temperatures that are the result of climate change will continue this trend of decreasing fire. This is because unlike other areas such as the Amazon, where at the moment, the increased heat and dryness hasn’t been decreasing fuel loads but has been making them more flammable3. In the grass biomes, increasing temperatures and aridity will decrease the amount of grasses grown in a season, and thus reduce fuel loads. 

Despite the fact that fire is a necessary part of the ecosystem, recent talks about climate change have focused on the negative effects of green house gas emissions from these grassland fires. It should be noted that human involvement results in fires that release more greenhouse gases than late season fires that are often naturally igited1. That being said the long history of the savanna fires hardly pins them as central causes or side effects of global warming.

Since the grass biomes of Africa have been a dominant force in global fire regimes, it is worth analyzing that history and humans' role in it to better understand what modern best practice can look like. Research in the past couple decades has done a lot to expand our understanding of savannas in Africa, how they are formed and maintained, and humans' role in both of those processes. 

Did humans cause the formation of savannas and grasslands in Africa?

The traditional interpretation of the savannas in Africa was that they were the result of deforestation by humans. This was primarily based on the European ideal of forests being the ultimate and optimal state for ecosystems3. Savannas and grasslands in Africa are in areas that have the right climatic conditions to support forests as they are wet and cool enough3. The traditional logic was that in an untampered environment, the trees would beat out the grasses for resources such as sunlight, thus the trees replace the grasses and a forest develops. Since the European colonizers who arrived in Africa saw people burning the landscape and predominantly grassy landscape with scattered trees, they assumed that this was the story for African landscapes. 

Typical dry African Savanna with flat-topped
Acacia tree. Credit: A. C. Staver 

But is this really true? Bond and Zaloumis (2016), argue that no this is not the case and African savannas predate human activity and are not the result of deforestation3. The last time that Africa was predominantly forested was more than 10 million years ago, and by 2.6 million years ago savannas and grassy biomes covered much of Africa, and were associated with charcoal from fires as well. It wasn’t until the Holocene 12,000 years ago that forests began to become more common on the continent3. This indicates that not only do the savannas, and the fires in them, predate human involvement, they are also more established in the continent as a long lasting stable biome than the forests that it was previously believed they came from. If humans weren’t responsible for the formation of savannas and grasslands or for introducing fire into those ecosystems, then what was their role in influencing these grassland ecosystems?

How are humans altering savanna landscapes with fire?

Just because humans aren’t the cause for the existence of savanna landscapes doesn’t mean they haven’t been altering and maintaining savannas through fire use. Humans have had a substantial impact on the fire regimes in the grasslands throughout history and in the present. Looking back at the 6 stages of human fire regimes’ development that was outlined above, the largest amount of burning was around 300,000 to 4,000 years ago which is when hominins began to produce their own fire without the need for a lightning source5. It is in this period when humans were able to start lighting fires in the savanna during the dry period, changing the seasonality of the fires affecting the savannas and grasslands. This had effects on the types of fires, their extent, and plant make-up. While this period didn’t see an increase in a total area burned, there was a change in the seasonality of that bruning5. Being able to independently start fires meant fires could be lit at the start of the dry seasons, when the grasses are dry, but the trees have yet to dry out. The dry grass will burn fast and quickly without causing much risk to the trees which have too much moisture to ignite. 

Fire in African savanna. Credit: A. C. Staver

It is also possible that this type of fire regime, as has been indicated by long term fire experiments, that lighting fires earlier in the dry season would increase the amount of fire resistant trees in the savanna3. Rather than the deforestation hypothesis discussed in the earlier section, it is more likely that human controlled fire regimes in the African savannas increased trees. The shift to earlier dry season fires also changes the intensity of the fires, making them cooler, smaller, and having less of a risk of spreading to forests. 

Humans have also influenced the fire regimes in the savannas by increasing the disconnectedness of fuels sources. This happened predominantly in the later phase of human fire development after 4,000 years ago, when agropastoralism became more common5. Cultivation broke up grassy patches and grazing reduced fuel loads. This meant that even as populations increased and ignition events increased with them, we did not see an accompanying increase in land burned. In fact we see a decrease as the fires started can not spread as easily and there is less to burn. This all points of human involvement in seasonal fires in savannas reducing the amount of land burned which is supported by the observed data for recent years. 

Humans have also effected the broader environmental impacts that these seasonal fires have. Early season fires, while cooler, less intense, and more manageable, release more green house gases (twice the carbon monoxide and three times the methane) than late season fires do1. Even though these late season fires release less greenhouse gases, they are much more destructive ad damaging the forests and local wild life. As such, early season fires promote local environmental stability, but might have different effects on global climate stability. 

Knowing the ways past humans have influenced savannas through the use of controlled fires would be used today to influence and change policies around burnings. 

(More on this in a later blog post!).

References

1. Archibald, Sally.  "Managing the human component of fire regimes: lessons from Africa" Phil. Trans. R. Soc. B 371, no. 1696 (2016): 20150346. http://doi.org/10.1098/rstb.2015.0346

2. Machete, Reason L. and Dintwe, Kebonyethata. "Cyclic Trends of Wildfires over Sub-Saharan Africa," Fire 6, no. 2 (2023): 71. https://doi.org/10.3390/fire6020071

3. Bond, William and Zaloumis, Nicholas P. “The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes.” Phil. Trans. R. Soc. B 371, no. 1696 (2016): 20150170 http://doi.org/10.1098/rstb.2015.0170

4. Caillault, Sébastien, Paul Laris, Cyril Fleurant, Daniel Delahaye, and Aziz Ballouche. "Anthropogenic Fires in West African Landscapes: A Spatially Explicit Model Perspective of Humanized Savannas" Fire 3, no. 4 (2020): 62. https://doi.org/10.3390/fire3040062

5. Archibald, Sally, A. Carla Staver, and Simon A. Levin. “Evolution of Human-Driven Fire Regimes in Africa.” Proceedings of the National Academy of Sciences 109, no. 3 (2012): 847–852. https://doi.org/10.1073/pnas.1118648109.

6. Pooley, Simon. "Fire in African Landscapes." Oxford Research Encyclopedia of African History. 29 Oct. 2021; Accessed 21 Apr. 2023. https://doi.org/10.1093/acrefore/9780190277734.013.984