Do you want to participate in the ongoing fight against climate change with the help of the 13 Icelandic Yule Lads?

Imagine waking up in Iceland on a dark December morning, bleary-eyed, then stumbling out of your door to see something strange in your shoe. You bend down to take a closer look. Sure enough, protruding from your shoe is a promise to plant a tree, left for you by a Yule Lad — one of Iceland’s Christmas elves — in the dead of night.

If you stay in one of Hosteling International’s three Reykjavík hostels during the month of December, this might just happen to you. Be sure to leave your shoes in the corridor, and in the morning, you’ll be gifted a promise to plant a tree.

The mischievous Yule Lads are known for pulling pranks on unsuspecting humans, throwing your holiday season into disorder—but this shoe stunt is no prank.

That’s because HI staff will take the saplings and plant them on the outskirts of Reykjavík as part of the company’s ongoing mission to combat climate change and promote sustainable tourism.

The “Plant a Tree” project is one way for you, as a tourist, to be green and give back to Iceland.

“For two years now, HI Iceland has planted trees to offset the carbon emissions from its operations in Reykjavík HI Hostels,” says Helena Óladóttir, Quality  and Sustainability Manager at HI Iceland. “It is very valuable for us to actively contribute to the fight against climate change, as well as for the staff to plant the trees themselves and thus be in direct contact with the land.”

HI’s partnership with the Icelandic Forestry Association (Skógræktarfélag Íslands) fosters a deep relationship with the natural world. Since 2016, hostel staff have planted over 2,500 native birches (Betula pubescens).

In Iceland, planting trees is particularly meaningful. Though up to 40% of the land was forested when the country was settled around 870 C.E., centuries of clearing, grazing and erosion destroyed the woods—while fragile soils and strong winds meant that the trees couldn’t grow back on their own. Today, only around 1% of Iceland’s landmass is forested. Meanwhile, erosion, desertification and climate change pose continual threats to the country’s plant life.

But that’s where you come in. Not only will the sapling left in your shoe help forests grow again, but it will also soak up carbon dioxide and breathe out the oxygen that all life needs.

You’ll also be taking part in a larger movement: reforestation is on the rise in Iceland thanks to a growing awareness of the importance of trees to the country’s ecology and the globe’s overall climate. Though leaving your shoes outside your hostel door may seem like a small gesture, it goes a long way—especially when other travelers do the same.

“At HI Iceland, we want our guests to experience a meaningful interaction with local communities in a way that enriches all parties and promotes mutual understanding,” says Helena.

Thus, your sapling connects you to Iceland just a little bit more.

And planting trees isn’t all that HI does. The company has been at the forefront of sustainability efforts in Iceland for years: robust waste management and water consumption systems promote recycling and responsible resource use at each hostel, and guests are encouraged to drink tap water—which is safe and clean—to cut down on the number of plastic bottles. What’s more, HI only uses eco-certified cleaners and soaps to help guests and staff alike avoid potentially hazardous chemicals.

There are numerous ways you can participate in HI’s circular, zero-waste economy. Instead of throwing extra food away when you leave, you can put it on designated shelves for other guests to eat. You can also participate in Swap Till You Drop, a once-a-month event at HI’s Loft Hostel in Reykjavík. Here, tourists and locals alike can swap clothes, books and other goods that might otherwise be trashed.

But the “Plant a Tree” program is perhaps the most creative approach to HI’s mission of sustainability. It adds a festive, elvish flare to the holiday season—and promises that, one day, forests might again stretch from the sea to the mountains.

“To us,” says Helena, “sustainability means continuously striving to find new solutions to minimize the negative environmental impact of our operations.”

Not only are the Yule Lads rushing around in the night to pull tricks, but they’re also helping you live a greener life. First, though, you have to give them your shoe.