Intentional fires do not slow down bush encroachment

Controlled fires in South Africa's Kruger National Park have not stopped the increasing encroachment of the savannah. This is what a team led by Brian van Wilgen from the South African Stellenbosch University writes in the journal "Journal of Applied Ecology". The researchers had studied how the vegetation had changed seven years after intentionally setting large fires. They found that the fires could not prevent shrubs and small trees from spreading. In the future there should be a change in strategy for the fires that have been set.

In times of climate change, the savannah is threatened as a grass and herb landscape with isolated trees. Among other things, bush-like trees and shrubs are taking up more and more space and are displacing grass areas. According to a statement by Stellenbosch University, this development gives cause for concern because animals would then have less grass available for food and habitat. The causes of the bush encroachment are varied and not yet entirely clear. In savannas, a decreasing number of fires and a higher CO₂ concentration in the air are likely to play a role.

After trials with fires of various intensities in Kruger Park in 2010 and 2013, it initially looked as if the bushes could have been pushed back. But a review in 2020 showed that the bushes have become widespread again, but the number of large trees has decreased significantly.

Since there are occasional fires in savannas due to lightning strikes, there have already been a number of scientific attempts across continents to prevent bush encroachment by setting savanna fires in a targeted manner.

In 2010, those responsible and scientists selected three areas in the Kruger National Park. In these, they made precise counts of shrubs and trees more than ten meters high in several 20 by 20 meter areas. Then they caused fires in two areas with an output of more than 4000 kilowatts per square meter in the fire line, in the third area the output was almost 2000 kilowatts per square meter.

Three years later, the scientists again let the savannah burn in these areas with graduated intensities, resulting in areas with high, medium and low fire intensity

A survey in 2014 - i.e. one year after the fires - showed that after two high-intensity fires, the ground cover by trees less than ten meters high had decreased by 60 to 70 percent (depending on the size class). However, taller trees were hardly affected. "Tall trees are usually able to withstand frequent savannah fires because of their thick bark," study co-author Tercia Strydom of South African National Parks in Skukuza is quoted as saying in the statement. In the medium-intensity fires, the decline in low shrubs was less, in those of low-intensity, bush cover had actually increased.

2020 showed a completely different picture: in all three areas, the ground cover by bushes and trees was around 55 percent of the area, while in 2010 it was still between 25 and 35 percent. However, the number of large trees had decreased by 65 percent in all three areas. The researchers believe that the main reason for this is that elephants nibble off the tree bark, which makes the trees less resistant to savannah fires and increases the rate of death. The study shows "how complex things can be and that there is no simple solution to bush encroachment," says van Wilgen, according to the release.

The people in charge of the Kruger National Park and the scientists have therefore decided on a change of strategy: previously, the savannah fires were set in the dry season, when the trees are in a resting phase and many important substances have been stored in their roots. "If a fire consumes the above-ground parts during this period, the plant still has plenty of underground resources from which it can sprout again if the conditions are right," explains Izak Smit of South African National Parks. Therefore, future fire tests should be undertaken in the spring, when the trees are already in leaf and are therefore more susceptible to fire damage.

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