Climate change, fallen trees deadly recipe for US forests

Laurent Banguet -

Severe drought, insect infestation and poor forest management have combined in recent years to kill millions of trees in the American West — 130 million in California alone — and provide fuel for huge wildfires.

The crisis is all the more alarming as hundreds of millions of hectares of land were scorched this summer in several states, causing a dozen or so deaths.

Early this year experts had warned of the risk of a new, potentially much more dangerous kind of forest fire.

They blame the rampant mortality of trees, mainly conifers, which has ravaged forests as a result of drought and beetle infestation.

In the Sierra Nevada mountains of California, some forests have lost 90 per cent of their trees, prompting authorities to declare a state of emergency.

As for the link between tree deaths and the rise in forest fires, the factors at play are complex and sometimes deceiving, said Brandon Collins, a researcher who has co-authored a book on the issue.

“The simplest sort of interpretation is that having dead trees just means you have a lot more dry fuel for fires. You might expect fires to burn more intensely, quickly,” Collins said.

But the nature and behaviour of these fires does not fundamentally change, said Collins, who is a researcher with the US Forest Service and the University of California at Berkeley.

Over time, the needles from dead trees fall to the ground, denying “fuel continuity” that helps fire spread from treetop to treetop.

This can lead to a reduction in so-called “crown fires,” said Collins.

However, over a longer time scale of 10 to 15 years, the problem becomes the presence of lots of dead trees that have fallen to the ground.

“So the whole trunks are on the ground, and then you have a completely different fire potential. That’s potential for ‘mass fires, fires that really can create sort of interactions with the atmosphere, and they really are explosive types of fires,” said Collins.

These are “eruptive and unpredictable” blazes that will be altogether new.

This kind of fire may not spread in a linear fashion via the

tree tops but rather on the ground through embers and pockets of dead fuel.

This is “not a type of fire spread that we’re used to. We can’t even handle it in our models,” said Collins. — AFP