Last month was the wettest May on record since 1895 in some parts of the western United States (dark green). Light green indicates areas with above-average precipitation, while lighter colors are near or below average.

Last month was the wettest May on record since 1895 in some parts of the western United States (dark green). Light green indicates areas with above-average precipitation, while lighter colors are near or below average.

National Oceanic and Atmospheric Administration

Record U.S. rains trigger wildfire fears. How’s that work?

Last month was the wettest May in more than a century in large chunks of the western United States. While the soggy weather ended lengthy droughts in some regions, the spring showers may have also boosted the chances of ferocious future wildfires. That’s because the rain means more grass and other vegetation that can help catalyze big fires, researchers say.

“The wet month has brought a reprieve, but not a sigh of relief—it means we’ll have to worry about fire in the future,” says Jennifer Balch, a fire ecologist at the University of Colorado (CU), Boulder. “The grasses that are growing now can be really flammable when they dry out later—just instantaneous fuel.”

The paradoxical idea that wet weather can catalyze fire isn’t new. Researchers have learned by studying tree rings, ancient charcoal found in soil, and other evidence that big fire seasons often occur a few years after especially wet periods. “We see this pattern going back hundreds of years,” says landscape ecologist Tania Schoennagel, also at CU Boulder. In many parts of western North America, she notes, it is linked to a much larger Pacific Ocean climate pattern known as the El Niño–Southern Oscillation, in which generally wet and dry periods alternate.

But the wet-dry oscillation doesn’t play out the same way everywhere; it can change according to geography, elevation, terrain, and vegetation type. This year’s record precipitation, for instance, has some researchers keeping a particularly close eye on the mosaic of relatively low-elevation forests and grasslands in the Rocky Mountains—and on a specific invasive plant called cheatgrass (Bromus tectorum).

Cheatgrass has spread widely since it first appeared in the United States in the mid-1800s. And recent research has confirmed that its presence can increase the risk of wildfire. In the 2000s, for instance, cheatgrass helped ignite more than one-half of the multiday fires in the Great Basin region centered on Nevada and Utah, Balch and colleagues reported in Global Change Biology in 2012. Cheatgrass “can start growing early, dry out quickly, and has virtually no root mass—so all of that biomass can become really flammable,” says Balch, who is now studying how the grass and fire are interacting across the western United States. “It also likes to fill in the spaces [between other plants], creating this continuous carpet of fuel.” And researchers are examining whether fire actually helps spread the grass by opening new areas to colonization—creating a cycle that feeds on itself.

Now, “it’s a matter of when and where, not if,” fires will occur, Balch says. All it will take is some extended hot, dry weather, and a random lightning strike or smoldering cigarette butt to ignite the vegetation. Still, some areas may see “a lag of 2 or 3 years” between wet weather and fires, Schoennagel says.

In the meantime, land managers are wondering what—if anything—they can do to prepare for the fire next time. In some places, controlled burns might temporarily reduce potential fuel loads, but experts say that tactic isn’t practical in remote or heavily populated areas. It may be more important to prevent people from building homes in fire-prone areas, they note, and help those who already live in danger zones fireproof their structures and learn how to flee safely if fire strikes.