What Gives Christmas Trees Their Crisp, Cozy Scent?

What gives Christmas trees that cozy, crisp smell?

Who Meghan Bartels edited by Lauren J. Young

Nothing smells like a Christmas tree—but where does that woody but pleasant smell come from? You can thank the ubiquitous category of chemicals called terpenes.

“Terpenes are the largest class of naturally produced chemicals in the world,” says Justin Whitehill, a plant pathologist who studies Christmas trees full-time at North Carolina State University. “They are found in almost all plant species.”

Terpenes can have many beneficial ecological functions: they can protect plants from hungry predators and parasites, attract pollinators, and help plants cope with stressful conditions. The specific terpenes produced—as well as the purposes they serve and the odors they produce—vary among different species.

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But terpenes can also be dangerous to a plant, Whitehill says, so in spruces, for example, these chemicals are carried away from the rest of the tree’s tissues in a protective liquid that botanists call resin. The thick liquid is stored in specialized tubes in the outer bark and needles where it can be drained as needed by the tree, for example to patch up an injury and reduce the likelihood of infection. It hardens when exposed to air, and is what makes your hands sticky after handling the Christmas tree. Resin is often mistakenly called sap, which refers to the watery liquid that carries nutrients and sugar throughout a tree. Sap is present in all trees, while resin is produced only by certain species.

Small terpenes have relatively low boiling points, where they turn into gas and create an odor that our nose can detect, and it’s these small terpenes that create the distinctive woody Christmas scent when a tree is placed indoors and warmed up a bit, Whitehill said. as he says . But a tree has a lot of terpenes to release, so the smell is limited. This is one reason why it is recommended to display the Christmas tree in a cooler part of your home, and it is keep the tree—and its scent—fresher.

Even people with a keen nose will notice that the scent of their tree changes throughout the interior of their residence, Whitehill notes. “There are some individual compounds that have different boiling points, so the range of those trees will change over time” from a strong pine scent to one that’s maybe a little sweeter, he says.

Whitehill is familiar with the range of terpenes produced by Fraser firs, which dominate the North Carolina Christmas tree farms where he works. And he says careful sniffing can pick up subtle differences in terpene profiles between those spruces and other Christmas tree species. “I’ve noticed that what makes the Fraser fir a little bit special is that it has a sweet scent in addition to that holiday bouquet,” says Whitehill. “A sweet, almost piney, woody scent that’s really delicious.”

(Why do it do we really like the smell? (Whitehill says he suspects it’s a combination of the smell itself and the memories it evokes from past vacations.)

While the smaller terpenes create the tree’s scent, the larger terpenes can help it last, Whitehill says. In balsam firs, scientists have discovered a particularly interesting large terpene called cis-abienol, which is surprisingly similar in structure to a chemical long used by the perfume industry to make perfumes last longer. Whether cis-abienol plays a similar role in Christmas trees, and whether it could be harvested for the perfume industry, remains to be determined.

Another terpene topic that Whitehill and his colleagues are investigating is whether genetically modifying a tree can change its aroma profile. “Can we start developing trees that go a little crazy other than that classic Fraser fir range and develop something like a mocha mint or a mint Christmas tree?” says Whitehill. “How far can we go with it?”