Why does the American west have so many wildfires?

This practice has resulted in increasingly dense forests and lush brush on the forest floor. As a result, forests are becoming “tinderboxes” for more explosive fires, said Jennifer Marlon, a research scientist at the Yale School of the Environment and creator of the Global Paleofire Database, a collection of fire history records.

“When you have fuels that are more densely packed, they burn hotter and faster and more violently,” said Dr. Marlon.

Experts say firefighting has also altered the forest floor, making the fires worse. There are now more fire-tolerant shrubs and tree species such as silver fir at lower elevations. Silver fir trees have needles running up their trunks that act as ladders to the canopy and create canopy fires that are the hardest to contain and deadliest to trees.

In recent years, firefighting has turned to the use of “prescribed” or controlled burns to treat fire-prone land by brush thinning. Last year, the Forest Service used mandated fires on a record 1.8 million acres of state. The agency hopes to expand operations nationwide in the coming years, but public backlash over the practice has mounted. Opponents point to prescribed fires that sometimes get out of hand, like those that burned through New Mexico earlier this year.

As the Western population has grown, there is also a risk of wildfire being ignited.

Half of all forest fires are started by lightning strikes. The other half is ignited by people, whether indirectly – felled power lines or sparks from a train when its wheels hit the rails – or directly, from discarded cigarettes, car backfires or campfires.

Wildfires ignited by human activity spread more than twice as fast and killed more trees than those ignited by lightning, according to a study presented at a 2020 American Geophysical Union meeting. “Where people live, we provide opportunities for fires,” said Dr. Tingley.

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