Winter often seems bleak and white. There is little to nothing growing. Everything that was once bright and beautiful has died and faded to grays and browns. Except for the evergreens. They hold their color through the long winters.
Many enjoy greenery brought from the forests inside their homes in winter. Trees, boughs, and wreaths bring color and life to us on some of the shortest days of the year. These evergreen trees are our conifers: the Colorado blue spruce, subalpine fir, ponderosa pine, and Engelmann spruce. How do these trees keep their color? And why would they keep their needles and stay green when all the other plants die back or drop their leaves? Here in the high elevation alpine forests of Colorado, life is hard. The growing season is short. Snow settles on branches and the ground for months at a time, and the cold wind blows relentlessly. Standing on ridges and slopes overlooking valleys, these trees endure more than most. Holding on to their green needles provides an advantage in this harsh climate. This is often called the evergreen advantage.
The evergreen advantage is often thought of as trees being able to produce food all winter long. In cold climates, like here in Colorado, producing food, also called photosynthesis, is very rare in winter. Keeping needles year-round still provides some advantages. The short growing season is extended on either end. Trees begin photosynthesis as soon as it is warm enough to allow consistent water flow. These trees don’t have to wait to grow leaves before they can begin producing food. And they save energy by not having to grow all new leaves.
Keeping needles isn’t as simple as it may seem. The cold is part of life here, but everything will freeze, the lakes, creeks, your pipes, the dog’s water bowl, and even the tiniest little cells in trees. When water freezes and ice crystals form, they expand. You may know this from personal experience after leaving a beverage in the freezer a little too long or having pipes in your home freeze (what a mess!). When ice crystals form within the cell wall or membranes they rupture those walls and kill the cell. This happens to us with frostbite, killing the tips of our fingers or toes. Trees have no heat source to keep their cells warm so they use other strategies to prevent damage.
When temperatures drop in late summer and fall conifers acclimate themselves to the cold. As these trees are preparing for colder spells they change where they store sugars and lipids (fats). Having dissolved substances which lowers the freezing point of water. Salty water, like the ocean, takes lower temperatures to freeze than freshwater. The lower freezing point of salty water is also why we put salt on the road. Trees use this part of chemistry to their advantage.
The space between cells has a lower concentration of sugars and lipids. Inside the cells there are more dissolved substances. This means that ice crystals will form between cells before they form inside the cells. Controlling where sugars and lipids are stored allows trees to control what freezes.
As ice crystals form outside the cells, it draws even more water out of the cells. This leaves a more concentrated solution inside the cell that is less likely to freeze and damage the cell. Thanks to this chemistry, our evergreens can stay green!
The ability of trees to survive in this harsh habitat provides many benefits. Animals rely upon them for shelter and food. Evergreen trees help keep water in the mountains by shading snow and ice long into the summer. And we enjoy them for fuel to heat our homes, for shade in the summer, and for greenery in homes in the cold and dark of winter.
Hannah Fake is the Visitor Information Specialist Coordinator with the San Juan Mountains Association in partnership with the Rio Grande National Forest Service. She enjoys being out on the Forest identifying the local flora and fauna.