The leaves of evergreens are pretty special. Here’s why.

Sep 29, 2023

Every other Friday, the Outside/In team answers a listener question about the natural world.

Alyssa submitted this week's question to us on Instagram: "How/why do some plants stay green in the winter? And what's the benefit of being evergreen?"

An evergreen plant keeps its leaves year after year, versus a deciduous plant, which sheds its leaves after each growing season.

While evergreens grow all over the world, the first that might come to mind for a lot of folks in New Hampshire is probably a conifer, the tree with needle-like leaves and cones.

"Maybe a common one for most people are pines," said Georgia Silvera Seamans, an urban forester and founder of the Local Nature Lab in New York City. "But you also have broadleaf trees that are evergreen. I'm in New York City, so a broadleaf tree that's evergreen [here] would be a holly."

Because evergreens keep their leaves all year, we might assume that a major benefit to staying green would be the opportunity to photosynthesize year-round — but not necessarily.

"It's not to say that there is no photosynthesis happening, but you have to think about it from the tree's perspective," said Silvera Seamans.

In order to photosynthesize, the tree would need to open the pores on its leaves to take in carbon dioxide, but that would also make it vulnerable to losing water. It's a balancing act.

It's not that being an evergreen is necessarily more beneficial than being deciduous — it's just a different life strategy. Each strategy involves trade-offs as to where plants spend their energy.

Since evergreens keep their leaves for more than one season, they tend to heavily invest in them. Evergreen leaves are often thick with a protective waxy coating, with resilient needle-like shapes, which also serve to minimize surface area.

"They're expensively made, like spending money on a really expensive sweater," said Silvera Seamas. "You know you're going to wear it for many years, versus like fast fashion, which is inexpensively made and the next season there's gonna be something else for you to wear."

That's not to say that a deciduous life strategy is a lesser one. It can be an advantage to start fresh every year, especially because growing seasons can be unpredictable. Storms and insects can batter leaves, and for an evergreen, losing an expensively made leaf before its regular renewal cycle can be a significant loss, because maybe a big storm batters your leaves or a hungry caterpillar is coming into your neck of the woods.

Evergreens eventually shed their leaves as well, though they just don't shed them all in a single season, and that shedding cycle varies across species. An eastern white pine might shed its leaf every two to three years, while the needles of the bristlecone pine can last for half a century.

"I think you can look at a tree and see it as the embodiment of its evolution and the 3D resources that have been available to that species," said Silvera Seamans.

Submit your question about the natural world to the Outside/In team. You can record it as a voice memo on your smartphone and send it to [email protected] or call the hotline, 1-844-GO-OTTER. We also accept questions sent to Twitter or Instagram. We’re @OutsideInRadio in both places.