The number of dead trees in California’s drought-stricken forests has risen dramatically to more than 102 million in what officials described as an unparalleled ecological disaster that heightens the danger of massive wildfires and damaging erosion.
Officials said they were alarmed by the increase in the number of dead trees, which they estimated to have risen by 36 million since the government’s last aerial survey in May. The US Forest Service, which performs such surveys of forest land, says that 62 million trees have died this year alone.
“The scale of die-off in California is unprecedented in our modern history,” said Randy Moore, forester for the region of the US Forest Service that includes California. Trees are dying “at a rate much quicker than we thought”.
Scientists say five years of drought are to blame for much of the destruction. The lack of rain has put California’s trees under considerable stress, making them more susceptible to the organisms that can kill them, such as bark beetles.
Bark beetles are small hard-shelled insects about the size of a grain of cooked rice. The beetles can tunnel under bark, cutting off the supply of nutrients the tree needs to survive and can kill a tree in as little as two to four weeks.
Unusually high temperatures have added to the trees’ demand for water, exacerbating an already grim situation.
The majority of the dead trees are in the southern and central Sierra Nevada region, officials said, though they warned that high mortality levels are also creeping into forests in Northern California.
Adrian Das, an ecologist with the US Geological Survey, needs only to step outside his office in Sequoia National Park to see the extent of the damage.
“You look across the hillside on a side of the road, and you see a vast landscape of dead trees,” said Adrian Das, an ecologist with the US Geological Survey based at the Sequoia National Park. “It’s pretty startling.”
Das said the parts of the forest in the lower elevations--about 1500 to 1800 metres--continue to get hit the hardest. At higher elevations, it can sometimes appear as if there is no drought and the trees are much healthier.
“We have sugar pines here--grand trees that can live for 500 years,” he said. “Everywhere you walk, through certain parts of these forests, at least half of these big guys are dead.”
Although Northern California enjoyed a wet start to the water year, the central and southern parts of the state remain locked in what federal officials classify as “extreme” and “exceptional” drought.
“This staggering and growing number of tree deaths should be concerning for everyone,” said Max Gomberg, climate and conservation manager at the State Water Resources Control Board. “It helps us realise just how intense and extreme this drought has been--particularly for Central and Southern California.”
A single year of average precipitation, which parts of the state got last year, would not end the drought or stop trees from dying, experts said.
Even with a historic deluge this winter, Moore said, die-off would continue for at least a year or two.
Dead trees create various hazards for Californians. For example, such trees are weak and can fall more easily than healthy trees.
Then there is the wildfire danger.
California was struck this summer by a series of deadly wildfires that destroyed hundreds of homes and forced thousands to flee. Officials at the time said dead trees fuelled some of the fires.
When a lot of dead fuel remains on the ground, fires burn hotter and damage the soil, experts said. Whenever rain eventually arrives, the water cannot filter through the soil as easily, so it moves the top layer, creating the potential for mudslides and destroying root systems.
“Like a sheet of glass,” said Scott McLean, a battalion chief with the California Department of Forestry and Fire Protection. “Where’s that water going to go? Quickly downhill.”
Once fires burn through the fuel on the ground, they can climb up a “ladder” of dry branches and timber until they get into the crown of the tree, McLean said. And once a fire gets to the top of a tree, it can spread quickly--hopping from tree to tree rather than winding more slowly across the ground.
The forest service said longer, hotter fire seasons are likely to continue for years to come. Officials said this and increased development in forested areas is driving up the cost of fighting fires.
A blaze in Monterey County earlier this year burned for months, making it one of the costliest fires to fight in US Forest Service history.