Page last updated at 09:34 GMT, Thursday, 3 December 2009

Learning to love forest fires in Yosemite National Park

By Peter Bowes
BBC News, Yosemite National Park


Peter Bowes met Cpt. Steve Shoemaker, who is in charge of a water-dropping helicopter in San Diego.

When southern California is consumed by fire, the state employs an arsenal of equipment and manpower to battle the inferno. The key priority is to save lives and homes.

An increasingly familiar image of America's Golden State is of water-bombing aircraft dousing the flames as they lick around million-dollar mansions on the hillsides.

But in Yosemite National Park, in central California, fire is viewed differently. The forest needs to burn to survive, although fire was once thought to be an enemy of the region's giant sequoia trees.

People used to think that the park's beautiful trees needed to be protected from fire, according to Gus Smith, a fire ecologist.

Gus Smith
I think that we need to see more fire and the benefits of fire
Gus Smith

"Fire looks destructive and dangerous and would kill organisms made out of wood," he says.

For decades, through public service messages, people were encouraged to believe that all fires were bad.

Aggressive measures were taken to fight fires, since the perception was that the flames were a wholly negative force in the national park.

The Smokey Bear campaign, which started in 1944, promoted the message: "Only You Can Prevent Forest Fires."

Using colourful posters, it was an attempt by the US forest service to educate Americans about the dangers of forest fires.

'Decades of fuel'

But scientists have come to realise that years of suppressing fire in Yosemite prevented the trees from reproducing.

Excluding fire from the ecosystem allowed leaves and other vegetation to build up around the trees. The litter stopped seeds from germinating in exposed soil and a dense canopy of foliage blocked the sunlight from reaching the forest floor.

Fire damaged trees in Yosemite
Some of the forest's plants benefit from the regular burn

"I think that we need to see more fire and the benefits of fire," says Mr Smith.

"Without fire we already know the forest gets too dense with trees. When they get too dense, as the litter builds up more and more, you end up with more and more fuel, decades and decades and of fuel.

"We know that the longer period of time between fires, there's more fuel, fires are burning hotter these days, but if we have frequent fires, it consumes those fuels, and then fire can never be this great destructive force."

Forest management techniques have changed in recent years.

Fires are often deliberately started, under controlled conditions, to burn away the excess debris on the forest floor.

Thinning out the vegetation also means that when a fires does break out, it cannot turn into a massive inferno.

When fires burn in Yosemite they are usually started by lightning strikes. The flames are rarely above a metre high and they move relatively slowly through the forest.

"In the lower elevations, where we have excluded fire for decades, we have started fires ourselves under very tight controls - prescribed conditions to try to mimic the natural fire regime," explains Dr Jan van Wagtendonk of the US Geological Survey's Yosemite Field Station.

Gary Wuchner
We have smoke in the valley, but without that smoke, we don't have a healthy forest
Gary Wuchner

"We do that under those very tight prescriptions of air temperature, relative humidity, moisture content and wind speed.

"But also, because we're dealing with decades of accumulated fuels, very often the fires that we set are not as hot or as intense or severe as the natural fire regime.

"Our goal is to bring back the fuels and forest structure to the point where we can allow natural fires to burn," says Dr Wagtendonk.

Whether or not to set fires poses a dilemma for the forest's managers. It also highlights the competing values of visitors and ecologists.

"We have smoke in the valley, but without that smoke, we don't have a healthy forest," says Gary Wuchner, fire information and education manager for Yosemite National Park.

They are constantly balancing the needs of nature to what visitors expect, he says.

"It's tough for our public affairs office to say; 'I understand you're having a wedding today but we're also burning the meadow today.' We have a balance there and it's a tough one to strike," explains Mr Wuchner.

Limited resources

In 2001 Smokey Bear's message was modified to: "Only You Can Prevent Wildfires."

Yosemite National Park
Scientists say they are learning how to preserve that vasts forests

The new mantra makes the distinction between wildfires, which are unwanted, unplanned and damaging, and forest fires which can often be beneficial.

Fine-tuning the message can be difficult when images of southern California burning are so frequently seen in the media and resources are stretched to the limit.

All methods cost money but Mr Smith believes the key is using a variety of methods to tackle and prevent the fires.

"I believe we need a toolbox that's bigger than just protection and suppression," he says.

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