Wetenschap - 9 juni 2011

Wildfires cause erosion

Wildfires can increase soil erosion because the fire destroys the vegetation and makes the soil water-repellent. The water then runs off the surface, leading to erosion, says Cathelijne Stoof in the PhD thesis she is due to defend on Friday 10 June.


Foto: .

A blackened ashy expanse of land that was once a beautiful nature area is an increasingly common sight in the Netherlands. Nature managers are very concerned about the damage being caused by the increasing number of forest and heath fires in the country. But according to forest fire expert Cathelijne Stoof, the ecological damage done by a fire is not always as extreme as it seems, and varies with local conditions. 'An area that has been devastated by fire like that is a dramatic sight', agrees Stoof. 'But you cannot see from the surface what the damage is, nor do you know what will come to take its place.'

That fires can have a positive impact on nature is beyond doubt. Many ecosystems are even dependent on fire. 'Some plant seeds only germinate at temperatures above 60 degrees, and others only sprout after a fire', says Stoof. 'Fire can also rejuvenate vegetation, creating a dynamic ecosystem with a variety of vegetation types. On terrain where the ministry of defence starts controlled fires, for example, you see rare species coming back.' But little is known about the effects of wildfires on the soil. To research this, Stoof went to Portugal, where she started a controlled fire in an area of nine hectares. Sensors put in place beforehand measured the soil temperature among other things. When the blaze had been put out, the PhD researcher studied the effect it had had on the soil. She looked particularly at the soil humidity, the soil temperature during the fire, the change in the soil's water absorption capacity and the runoff of rainwater.

The impact of the fire on the soil was not as bad as expected. The temperature did not rise too high because the soil was fairly damp. 'When the soil is damp, only the top one or two centimetres gets hot, in this case no hotter than 100 degrees Celsius, explains the researcher. 'With a dry soil, the impact is bigger.' Because of the heat, some of the organic substances are released and they coat the grains of sand. This makes the soil water-repellent: rainwater no longer trickles slowly down through the soil, but streams off the surface, causing erosion.
But erosion also increases when trees and bushes are burned, Stoof's field tests revealed. She found that when the rain is no longer caught by the leaves, twice as much water runs off over the ground. 'Not only does this cause more erosion, but also the rivers can't cope with so much water and they may flood.'

Fire-sensitive nature
Wildfires in the Netherlands are likely to become more common, given that current climate scenarios predict more dry summers. But the Dutch fire brigade is inadequately equipped to deal with fires in nature areas. 'There is too little equipment and too little specific knowledge', says Stoof. In cases of wildfire, it is good to respond as fast as possible. A fire in low vegetation is quite easy to put out, but once it reaches the tree canopy it is a lot harder.

Besides drier summers, modern nature management also plays a role in the risks of wildfire. Many nature managers believe in leaving nature to its own devices. As a result, there is plenty of flammable material around and it can be difficult for the fire brigade to get into the forest. 'Wildfires should therefore be seen as an issue in nature area management', says Stoof. Measures such as preventive burning during the winter stop too much deadwood accumulating in the forest, reducing the risks of a big summer fire. The layout of an area can also help to minimize the chances of a big fire. Creating fire breaks and planting deciduous trees instead of conifers can make an area less vulnerable to fire and prevent fire from spreading. To some extent, nature managers can protect vulnerable or unique vegetation such as the juniper bush with open strips of land. 'In the southern European countries there is a lot of expertise in the area of fire prevention. In the Netherlands we should work more closely with those countries in order to learn how we could tackle wildfires better', concludes Stoof.