VOA标准英语2013--Fighting Fire with Fire（在线收听）

Fighting Fire with Fire

In African savannahs, or grasslands, wildfires can either be destructive or beneficial. It all depends on when those fires occur. Scientists at the World Agroforestry Center have developed a system to determine the best time to set the savannahs ablaze.

Savannahs stretch across many parts of Africa. But scientists concentrated their research on grassland conditions similar to the Sahel and South Sudan. When wildfires spread uncontrolled, the heat and flames can cause damage on several levels.

“Many components of the ecosystem can be affected. The biodiversity itself, the soil composition and structure and gas emission and greenhouse gas emission into the atmosphere,” said Cheikh Mbow, senior climate change scientist with the World Agroforestry Center in Nairobi, Kenya.

“Fire is seen as one of the biggest drivers of deforestation in some ecosystems. And it contributes widely to the reduction of the ecosystem services, which [are] the basis of most of the livelihoods in Africa in poverty conditions. So we are worrying about fire because of the many impacts and implications for the human beings.”

Fires in savannahs are common.

“If you look at African images from [a] satellite in the dry season, it appears that most of the ecosystem, which has some level of dryness, will have fires on a regular basis. There have been many attempts in the past, since colonial time actually, to, sort of, ban fire in our ecosystem, but they never succeeded. They never succeeded because fire is a tool,” said Mbow.

Fire is used as a tool by local populations, for instance, when they’re gathering honey, clearing land or desiring new vegetation growth. Mbow says nearly all of the fires in the savannahs are caused by humans. Nature plays a very small role.

“The conditions in which natural fire can occur [are] when lightning happens, for instance. In Africa, when lightning happens in these tropical areas that’s a period during which we have rain. It’s wet. When it’s raining, that’s the time we have lightning, and it’s very unlikely that when it’s raining fire can take place,” he said.

With climate change, he said, there’s concern the savannahs will become even more dry, making destructive wildfires much more likely.

That’s why scientists at the World Agroforestry Center and their partners developed a system to pinpoint the best times to intentionally set fires. It’s known as early burning. It consumes layers of biomass before they build-up to highly combustible levels. Efforts to prevent fires altogether can actually make things worse.

“If you protect this area for five years, let’s say, or 10 years, you’ll have a dangerous amount of biomass. Fire not only consumes the biomass, it also destroys all the characteristics of the ecosystem, which makes this ecosystem viable. I’m thinking about microorganisms in the soil, small animals. So it’s a bit dangerous to over protect the ecosystem,” he said.

So an early burn, Mbow said, needs to be done when the conditions are just right – not too wet or not too dry. Fires intentionally set at that time not only consume the biomass, but prevent dangerous fires later in the season.

“There are many factors in determining fires. The one factor is the grass moisture, but also the grass load. The biomass load is extremely important. The second important factor is the atmospheric parameters – air temperature and wind. And the third one is the topography. If you are in heavy conditions, it’s very risky sometimes to use fire. As they go uphill they become stronger and very difficult to control. So there are many, many aspects which should be in consideration. And there is no silver bullet or one-size-fits-all situation.”

The early burns are generally fast moving and stay mainly on the surface. They cause only minor damage to trees, soil nutrients or microorganisms.

The recommendations for controlled fires in African grasslands can be found in the February issue of the Journal for Arid Environments. Mbow said that he hopes African governments will consider implementing them. However, he added, few countries on the continent currently have resources for fire management.