美国国家公共电台 NPR Getting Fire From A Tree Without Burning The Wood（在线收听）

MICHEL MARTIN, HOST:

At the Oak Ridge National Laboratory in Tennessee, scientists have been investigating something very strange - a flammable gas leaking from the inside of a tree. NPR science correspondent Joe Palca recently went to Oak Ridge to learn more.

JOE PALCA, BYLINE: There's a 40 or 50-foot-tall cottonwood tree growing right behind one of the research buildings on the national lab campus. Cottonwoods are tall, straight trees with leaves the shape of spades. Chris Schadt is an environmental microbiologist. He offered to try to demonstrate the phenomenon. He starts assembling a tool he's brought out to the tree.

So hang on. Tell me what you just put together there.

CHRIS SCHADT: So I put together what we call an increment borer. It's got a hollow tube and a handle. It's a T-formation. And you can just pick a spot in the tree.

PALCA: Schadt picks a spot about chest-high.

SCHADT: So once we get to what would be approximately the center, you take the extractor...

PALCA: The extractor is basically a long, thin spatula that slides inside the borer.

SCHADT: Insert it in here, give a couple of turns backwards, you can pull out a core of a tree.

PALCA: In this case, we're not interested in the core. We're interested in what might be coming out of the hollow tube that's now stuck into the tree.

SCHADT: Some of the trees produce methane in the tree.

PALCA: Methane is the gas in natural gas.

SCHADT: And you could actually light the methane on fire coming out of it. This one does not appear to have any trapped methane.

PALCA: But many do. So what's going on here? Why is methane coming out of a tree? Well, the answer lies with a property of cottonwoods.

SCHADT: The wood in this particular species sort of naturally has this condition called wetwood where it's saturated within the trunk of the tree.

PALCA: And this wetwood makes for a welcoming home for all sorts of microorganisms.

MELISSA CREGGER: You can't actually see a lot of the organisms because we can't grow a lot of these organisms, so we're able to identify them using their gene sequences.

PALCA: Melissa Cregger is an ecologist at Oak Ridge. She says some of those organisms called archaea turned out to be known methane producers. So it's not the trees themselves producing the methane. It's the microbes living in the tree. Cregger says scientists have known for a while that these organisms existed in forests but not in the trees themselves.

CREGGER: We had historically worked below ground looking at soil communities, root communities.

PALCA: Cregger says because methane is a potent greenhouse gas, it's important to see how much the trees are actually producing. And that raises a surprising notion - yes, trees take carbon dioxide out of the atmosphere, a good thing. But they also add methane, a potentially bad thing. But University of Delaware ecologist Rodrigo Vargas says trees are most likely doing more good than harm.

RODRIGO VARGAS: It's not like now trees are the bad actors, and now they're emitting methane, and now we're seeing a big source of that. It's not like that.

PALCA: Vargas says that's because the amount of methane coming from the trees is small compared with other sources. Vargas is interested in getting a better understanding of the gases trees produce. He's developing an automated system because doing the measurements manually would be a giant pain.

VARGAS: Because then you would need to be there in the tree and measuring (laughter) every half an hour forever, right? And if it rains, you have to be there. If it's - you actually have to be there. No, if you're uncomfortable, hungry, you have to be there.

PALCA: Once his automated system is up and running, he's hoping to be able to see if the amount of methane increases from time to time, for example after a big storm or when the temperature suddenly changes.

VARGAS: We have the opportunity to see these patterns that no one has seen before.

PALCA: Perhaps this is one of those cases where it's more important to see the trees than the whole forest.

Joe Palca, NPR News.