AUDIE CORNISH, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Audie Cornish. We hear a lot about this time of year, in stories and songs. Now, some actual science about mistletoe. For Druids, it was sacred. For many of us, it's a cute ornament and maybe an excuse to steal a kiss. But for a forest, mistletoe might be more important. NPR's Sabri Ben-Achour has this story about the hidden power of the small leafy garland.

SABRI BEN-ACHOUR, BYLINE: Mistletoe has a way of prying its way into the holidays.

(SOUNDBITE OF MUSIC)

UNIDENTIFIED WOMAN: (Singing) (Unintelligible) in the winter snow, but I'm be under the mistletoe.

BEN-ACHOUR: It also has a way of prying into tree bark. It's a parasite. It shows up on tree branches and looks like an out-of-place evergreen bush hanging in the air. The seeds drill through bark with a threadlike probe and then grow by sapping the energy out of its host. But it may actually be useful, and more than just as an excuse to make out with somebody.

DAVID WATSON: Oh, so much more, let me count the ways.

BEN-ACHOUR: David Watson had long suspected this. He's an ecologist at Charles Sturt University in Australia, but nobody had really proved it experimentally. So he did an experiment in an Australian forest. He just took all the mistletoe out.

WATSON: Me and a team of 12 volunteers and cherry pickers, we removed just over 41 tons of mistletoe.

BEN-ACHOUR: And I mean, it's just mistletoe, right? How much of a deal could that be? Well, three years later ...

WATSON: The main result was on birds. The simple act of removing mistletoe and nothing else led to losses of over a third of the woodland dependent species.

BEN-ACHOUR: The birds just left. And weirdly, the birds that took the biggest hit were insect-eating birds.

WATSON: Especially the insectivores that do their feeding on the forest floor.

BEN-ACHOUR: So how does that work?

WATSON: That's actually sort of a byproduct of how parasitic plants do their parasitizing.

BEN-ACHOUR: Parasitic plants are packed with nutrients that they gobble up from their hosts. And because they're moochers, they don't really care about conserving their resources, they can just suck out more. So mistletoes are like, whatever, and drop their leaves with all the vitamins inside.

WATSON: So there is this rain of enriched litter coming down. It's a bit like a fertile mulch.

BEN-ACHOUR: More goodies on the soil, more bugs, more birds that eat the bugs might mean more lizards and more mammals too, says Watson. He published earlier this year in the Proceedings of the Royal Society B. Brian Geils is a retired forest pathologist for the U.S. Forest Service. He says...

BRIAN GEILS: The jury is still out.

BEN-ACHOUR: In the American West, he points out, some mistletoes can kill trees.

GEILS: It's quite complex to come up with a simple, oh, it's good or bad, it's more or less.

BEN-ACHOUR: Either way, being under a mistletoe is a big deal and not just for people. For NPR News, I'm Sabri Ben-Achour.

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