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March 2010
 

Natural environment

Humans affect fire regimes more than climate in southern Africa


A savanna fire burning through an area of high fuel loads in the Kruger National Park

Members of the Working on Fire team involved in controlling an experimental burn in the Kruger National Park

Sally Archibald is an ecologist with CSIR Natural Resources and the Environment. Her main research interests are in fire ecology, savanna vegetation dynamics and the behaviour of complex systems
A changing climate might have little effect on fire regimes in southern Africa - contrary to what is predicted for other parts of the world.

Dryer climates and more extreme weather events have been predicted by the Intergovernmental Panel on Climate Change (IPCC) to increase the incidence of large dangerous fires in a range of ecosystems across the globe. But, according to CSIR ecologist Sally Archibald, there are two very important reasons why southern Africa is unlikely to show similar patterns.

"Firstly, most of southern Africa is occupied and utilised by people, who influence when fires are ignited, and also how easy it is for fires to spread. Research coming out of the CSIR shows that when people light most of the fires, they start earlier in the season, are smaller, and are therefore less subject to variable climatic conditions," she explains.

In protected areas however, where human activities have less effect on fires, the researchers showed that changes in the amount and timing of rainfall had a strong effect on the total area burned in a landscape. They also showed that large dangerous fires are associated with particular weather conditions. For example, the more it rains, the more grass grows, producing more 'fuel' for a wildfire. Grass needs only a few rainless weeks to become highly flammable.

These insights were made possible by newly-available satellite data on the location and extent of all wildfires on the globe. According to Archibald the CSIR has been collaborating for many years with the developers of these satellite products to ensure that the data for southern Africa are reliable and easily available to the researchers and policy makers of the region.

"With these new data we were able for the first time to quantify the massive difference in fire regimes between protected and unprotected systems in southern Africa", said Archibald. "Before, the only available data on fire were from national parks and game reserves where managers record the locations of all fires. This has led to a skewed idea of how fire works in most landscapes in southern Africa".

The contrast between humans and climate in regulating wildfires has also been shown in other parts of the globe. In Greece and Spain it appears that dangerous wildfires are on the increase in rural areas. This is because people are moving to the cities and the landscape is becoming depopulated, which means climatic drivers start to take control of the fire regimes again.

"Currently in southern Africa it appears that people are toning down the variation one would expect to see in fire regimes in the region. Rainfall can vary in these systems by several hundred millimetres a year but the feedback from climate to fire is not strong because human activities such as grazing cattle, building roads, lighting fire breaks, and planting crops all break up the landscape. Relative to these activities, a little more or less rain each year is negligible", says Archibald.

These findings hold important implications for policy decisions regarding fire management practices. There is also growing interest in managing fire to promote carbon sequestration in Africa, with large-scale projects to manage fire regimes already in the pipe-line. These projects require accurate risk assessment. Policy makers need to understand the relative importance of climate and humans in driving fire, and predict how this will change in the future.

The second important reason why southern African fires do not fit the IPCC predictions is related to the type of vegetation that burns. In south-east Australia, the Mediterranean and the western United States, dangerous wildfires are 'crown fires' burning in the tree and shrub canopies at high intensity, and consuming large amounts of dead woody material which dries out during droughts. In southern Africa fires usually burn only dry grass in the surface layers below tree canopies and woody fuel plays much less of a role.

Similar negative results for climate impacts on fire have been shown by CSIR senior ecologist Dr Brian van Wilgen in the fynbos system, which is surprising because in this system it has always been thought that dry hot windy conditions are important drivers of fire. However, when Van Wilgen analysed long-term fire records for the region he found that neither fuel age nor severity of fire weather affect fire return periods. Rather, the frequence of ignitions determined the frequence of fire. Ignitions in turn are related directly to human population density and fires are set to increase as human populations grow. In addition, invasion by alien trees and shrubs increases the fuel loads of the vegetation, increasing the intensity of fires and the damage that they do.

"The current understanding is that climate change will increase the incidence of extreme fires in many parts of the globe. We are not yet able to make such predictions for Africa," Archibald concluded.

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