The snow sparkled in the sunlight and crunched under foot. Gulls periodically punctuated the wintry hush. At Moose Point State Park in Searsport, on New Year’s Day a few years ago, we chose to take a trail that remained close to the ocean’s edge. Soon we discovered two curiosities: one of human making and the other a mysterious wonder from the plant world.
The first curiosity was a young balsam fir (Abies balsamea) adorned with a half dozen or so non-breakable ornaments. Although a delight to see, it seemed out of place.
Not far from this first discovery came the second and more puzzling encounter. Green was protruding through the snow, when most other plants had withered into dry brown lifeless remnants of their former selves. Yes, there are forest floor evergreen plants, such as trailing arbutus (Epigaea repens) and wintergreen (Gaultheria procumbens). However, those were well insulated by the snow cover; the snow keeping the temperature around 32 degrees Fahrenheit at ground level. These puzzling three leaves were exposed to the crisp wintry air. When standing next to the three leaves, I could see they were blades of a fern.
At that time, I wasn’t as familiar with ferns as I am now, so I couldn’t identify it by name, nor did I know that some ferns are evergreen. After taking some detailed pictures and making notes in my mind, we continued our walk. Later, in the warmth of home, I began to do some digging. What species of fern are evergreen? And how was it that a fern can still be green in 25-degree temperatures? What made that possible? Was it something similar to how the trailing arbutus and wintergreen are adapted to winter even if they are under the snow?
What I discovered was remarkable. There are several ferns in northern New England that are “evergreen.” The fern we saw is commonly known as an intermediate wood fern (Dryopteris intermedia). The others are the Christmas fern (Polystichum acrostichoides), the marginal fern (Dryopteris marginalis) and the spinulose woodfern (Dryopteris spinulos).
So, how have they adapted to pull off this wintry feat? Without some change within the plant, the water in their cells would freeze and cause cell rupture, killing the plant. It’s like forgetting to winterize your car, camp or RV with antifreeze; frozen engines and pipes can cause lots of damage. Well, it turns out that ferns do have their own antifreeze process.
Like insects, frogs, trailing arbutus and wintergreen, evergreen ferns increase their storage of sugars. The sugar acts as an antifreeze, protecting the plant tissue. This works well down to about 5 to 10 degrees, but extra measures kick in below that. Like insects, evergreen ferns have developed an adaptation where a single ice crystal is prevented from bonding with other ice crystals, hence preventing damage to the plant cells to 20 below zero. Beyond that, they freeze solid. Yet, they survive because water is removed from the cells.
So, what’s the advantage of being a fern that is evergreen? Why go through all this? It’s about food. Evergreen ferns are able to continue to make sugars for food through using the sun’s energy (photosynthesis). Being evergreen they photosynthesize later in the fall and earlier in the spring. How remarkable is that? I find it incredibly amazing.
On your next fall or winter walk in the woods, keep an eye out for ferns that are still green. They’ll be there. It is such a treat to see these hardy plants.
Grace Bartlett is a Maine Master Naturalist, who lives in Bangor, Maine, and finds pleasure in exploring the forests and fields nearby. She is especially interested in trees, lichens, and other forest floor vegetation. Grace volunteers with Bangor Land Trust, the Orono Bog Boardwalk and Hirundo Wildlife Refuge.