On any given day, there's a wildfire burning somewhere in the U.S. And that's not necessarily a bad thing. In the West, many forests have evolved with fire, and they benefit from occasional burning. But as NPR's Christopher Joyce reports, scientists say some trees are losing their ability to survive even small fires.

CHRISTOPHER JOYCE, BYLINE: A nice, little ground fire every few decades cleans house in the forest. It burns the grass and brush and maybe some smaller trees, so-called ladder fuels that might carry a fire up into the canopy. Those canopy fires are the worst kind. They kill forests. But most trees survive and thrive with the small fires. Phillip van Mantgem now says that's changing in the West, and he's found a pattern to explain it.

PHILLIP VAN MANTGEM: As we came across areas that were warmer relative to the long-term average, they had a higher chance of mortality, those trees, after they got burned.

JOYCE: Now, trees are no strangers to heat and drought. They're pretty tough. But van Mantgem has found that the kind of heat we're seeing in the West recently has made some trees vulnerable, even in small fires. How that works is not exactly clear. Van Mantgem says it appears that drought and heat can create air bubbles inside the trees, like air bubbles that sometimes jam up an engine's fuel system.

VAN MANTGEM: A tree will essentially have a bunch of tubes, a bunch of straws going through them from the roots up to the leaves, and it's pulling water through those tubes. And if you get an air bubble in there, it won't pull water anymore.

JOYCE: Van Mantgem is with the US. Geological Survey. He found this fire vulnerability across numerous national parks in the West: Colorado, Arizona, New Mexico, Oregon. He found that small, controlled fires set on purpose to help forests were killing trees that normally would have survived. Scientists say this doesn't bode well for western forests. Lots of computer models for future climate agree that the West will get hotter.

And while both heat and drought are implicated here, van Mantgem says temperature is the main culprit. David Breshears is an ecologist at the University of Arizona. He explains that you can have a decent amount of rainfall. But how much of it the trees actually get depends a lot on how thirsty the atmosphere is.

DAVID BRESHEARS: How much moisture the air wants to suck out of the ground. And as temperature goes up, that demand gets higher, and it makes it much tougher for the trees.

JOYCE: The research is described in the journal Ecology Letters. Breshears says the breadth of the area studied suggests that this is not an isolated phenomenon.

BRESHEARS: It's just another domino in the whole pileup of events that are happening in the West. These western forests are getting hammered over and over again.

JOYCE: Breshears and other ecologists say if these forests get hammered enough, they could be replaced with shrubs and other more fire-tolerant species.

BRESHEARS: If we have a lot more mortality after these wildfires, that changes what those landscapes are going to look like in the future.

JOYCE: Christopher Joyce, NPR News.


BLOCK: This is NPR News.

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