In a previous Storage Two article, we talked about the science behind why leaves change color in the fall. Now that most trees are wearing their bare winter branches, it’s time to talk about those that kept their leaves: Evergreens.
A tree is classified as an evergreen if it maintains its foliage year-round. While this may be easy to do in moist tropical environments, it is a far more complicated prospect in climates with temperatures that often fall below freezing, like Connecticut.
There are a number of reasons that make being an evergreen tree in a cold climate advantageous. The first comes down to where these trees grow. Evergreen trees often grow in nutrient-poor soil. If you’re a tree growing in an environment with few resources available, you don’t want to waste the resources you have in shedding your leaves every year. Evergreens keep each leaf as long as possible, which usually ends up being 2-3 years.
The structure of evergreen leaves tend to be quite different from deciduous trees as well. If you rub the needles of a pine tree, you may notice a waxy surface texture. This waxy coating is called cutin and it helps prevent water loss during winter months. Water conservation is exceptionally important when the weather gets cold. When the ground freezes, trees are no longer able to draw water up through their roots. If evergreen leaves become too dehydrated, they can begin to brown, or even die.
Of course, keeping water in its leaves isn’t the only concern for an evergreen in winter. They also need methods to keep water within leaves freezing and harming the cells. For some evergreens, this starts with transferring water out of leaf cells and into the space between cells so that if the water freezes, no cells will be harmed. Evergreens also increase concentration of sugars and anti-freeze proteins in their cells during the winter months to make it less likely that ice crystals will form.
If an evergreen is able to keep providing water to its cells, it can actually keep performing photosynthesis throughout winter, though at a much slower rate. Still, this small benefit is one more factor that has helped evergreens become so successful. About 15% of the Earth’s surface is covered in evergreen forests. Not a bad record for these hardy plants!
- Kate Dzikiewicz, Science Curatorial Associate and Seaside Center Manager