Illustration by Adelaide Tyrol
Have you ever been walking through the woods on a snowy afternoon and seen a bit of green poking up through the snow? Seeing the green fronds of a fern at this time of year may seem like an uncommon occurrence, but there are actually three common evergreen ferns that survive in the harsh cold of a northern New England winter. Seeing one brings to mind the question: why do some ferns choose to stay green while most others have shed their leaves for the winter?
The simple answer to this question is that evergreen ferns want to get a jump-start on photosynthesis. Staying green throughout the winter gives them a few more weeks of photosynthesis in the early spring before other plants leaf out. They are also able to conduct photosynthesis longer in the fall, after the leaves on other plants have fallen to the ground.
Despite the advantage of a longer growing season, most of our local fern species are deciduous and lose their leaves in the fall. That’s because the benefits of a longer period of photosynthesis bring with them some costs. Because evergreen fern fronds stay green throughout the winter, they must be able to withstand harsh temperatures and the increased likelihood of predation by winter-starved animals. Fern fronds in winter are relatively tough and have less nutritional value than in the summer, so the cost to an animal of digesting an evergreen leaf usually outweighs the nutritional benefit. But this may not stop a starving animal from sampling the fare. Some evergreen ferns go so far as to give off noxious fumes to deter predators from sampling their leaves.
Unappetizing leaves are not the only defense that evergreen ferns need in order to stay green during winter. Unlike most plants, the leaves of evergreens must be able to tolerate temperatures well below freezing, especially during those first few harsh days of winter before they are buried under the snow. Once there is a thick covering of snow on the ground, the leaves are protected in a comfortable, 32ºF blanket.
Deciduous plants, faced with impending winter, pull as much sugar in from their leaves as possible before abandoning them to wither and fall off. Evergreen plants do just the opposite. As the days shorten, evergreens begin to store ever-more sugar in their leaves. The sugar serves as antifreeze, protecting the leaves as temperatures fall 5 to 10 degrees below freezing. When the temperature drops still further, the plants resort to the same method used by insects that survive in deep cold: stopping the formation of ice crystals in their tracks.
Ice begins with a single frozen crystal and then grows as other ice crystals bond with the first. Evergreens stop this growth by synthesizing antifreeze proteins that bond to the initial ice crystal and block the bonding of other crystals. In our part of New England, this mechanism will protect a plant down to around -20ºF. In other regions, where the temperature can stay below -20ºF for many weeks, even evergreens will freeze solid. They withstand this temperature extreme by evacuating all the water from their cells, insuring that, when ice crystals do form, they end up outside the cell walls, preventing the cells from rupturing.
In deep winter, when evergreens are using these mechanisms to prevent freezing, photosynthesis halts. If it didn’t, the process would create free radicals in the dormant cells, which would damage plant tissue much the way they damage human tissue. Being able to halt photosynthesis is one of the most important adaptations in evergreen plants, ensuring their survival during the winter months.
The woods of Vermont and New Hampshire are home to three common evergreen ferns, which can be told apart by examining the shape of the leaves. The aptly named Christmas fern (Polystichum acrostichoides) is the most common, with fronds up to 2 feet tall and leaves that are so-called “singly cut”: they grow directly from the stem and have no notches in them. The marginal woodfern (Dryopteris marginalis) is of similar size but has doubly cut leaves: each leaf is deeply notched so that it appears at first glance to be a compound leaf. Finally there is the smaller spinulose woodfern (Dryopteris spinulos), whose fronds average only 12 inches in length and whose leaves are triply cut: the notches on the leaves are themselves notched, giving the appearance of compound leaves growing from compound leaves.
Even in the harsh mid-winter months, there are green plants to be found in our local woods. Being graced with the sight of one makes winter much easier to bear.
Carrie Chandler works at Northern Woodlands magazine in Corinth, Vermont.