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Even on a cold, winter’s day there is still some green to be seen even though it might just barely poking up through the upper crust of the snow cover. There are two very common ferns here in Western Pennsylvania that stay green all through the winter. They stand up to the snow and to the brutally cold temperatures of the season. They are Christmas fern (Polystichum acrostichoides) and evergreen wood fern (Dryopteris intermedia).
There are two different explanations for the origin of Christmas fern’s common name. The first emphasizes the persisting green of the fern’s fronds in the winter. It is, therefore, available to be used as greenery and edgings in Christmas decorations. The second explanation is based on the unusual shape of the fern’s leaflets: they have an angled crook in them that resembles the foot to ankle joint in a boot or a sock (in particular, a sock being used as a Christmas stocking!). These “Christmas stocking ferns,” then, were shortened down to “Christmas ferns.” It is hard to say which of these stories is the more credible or which one has historical precedence over the other. We also should consider the possibility that they both just might be figments of some botanist’s imagination.
Like most ferns, Christmas fern grows from a large, central, root-like structure called a rhizome. The fronds grow up in a cluster from the rhizome with larger, “fertile” fronds (those fronds with sporangia and the ability to make reproductive spores) forming a center of the cluster and smaller, “sterile” fronds (i.e. those fronds incapable of spore production) forming an encircling base. All of the fronds have leaflets that contain chlorophyll and are thus green and are capable of photosynthesis.
In late Fall as the shorter day-lengths and colder temperatures begin to settle in, the fertile fronds of the Christmas fern along with any of their unreleased spores wither and fall to the ground. The sterile fronds, though, remain intact and upright and retain their chlorophyll throughout the winter even if the entire fern in buried under a significant snow cover. Exactly how these sterile fronds keep their structural integrity in the face of potentially cell and tissue-destroying, cold temperatures, and what might be an evolutionary explanation for the preservation of these fern tissues are both related to some experimental observations on our second winter-persisting fern.
Evergreen wood fern gets its common name from its very straightforward tendency to stay green all winter long. Like the Christmas fern, the above ground fronds of the evergreen wood fern grow in bunches from a central rhizome. Some of these fronds are sterile and others are fertile, but all of these fronds are of similar size and outward appearance. With the onset of winter the fertile fronds of the evergreen wood fern, like those of the Christmas fern, lose their structural integrity and fall to the ground leaving the sterile fronds intact, erect and green.
Experiments looking at this transition from the fully fronded fern of summer to the selectively fronded fern of winter have shown that the persisting sterile fronds are not capable of significant photosynthesis during the winter months or even during the warming days of early spring. It has also been shown that these persisting fronds do not significantly gather or store inorganic nutrients (like phosphorus or nitrogen) that might then be utilized by the new frond growth of spring and summer. Instead, these persisting fronds accumulate carbon compounds (probably from the senescing fertile fronds) mostly in the form of plant sugars. These sugars accomplish two tasks: 1. They act as a natural antifreeze and keep the persisting fronds from freezing, and 2. They can be translocated from the persisting fronds to the new fronds growing in the spring and provide a much needed energy boost for the growth of these new tissues.
If the winter-persisting fronds of an evergreen wood fern are removed, subsequent spring frond growth is both delayed and significantly inhibited. The consensus is, then, that the evergreen wood fern uses these above-ground, persisting, winter fronds as an auxiliary energy storage system to augment the already significant storage of sugars in the fern’s underground rhizome. It is logical to assume that the Christmas fern is also employing sugars as its protection against freezing for its winter-persisting fronds and then also utilizes these stored sugars to help power the formation of its new spring growth.
Out on our old Nature Trail on the Penn State New Kensington Campus (a place that, sadly, is no longer an accessible or functioning nature trail), some students and I did a survey of the Christmas ferns and evergreen wood ferns growing along path. We found that Christmas fern was much more abundant down in the lower sections of the trail. Especially along what we called “The Ravine Trail” (the part of the path along the small creek that bisected the Nature Trail area).
Evergreen wood fern, though, was much more abundant and much more widely distributed in the upper regions of the trail where the paths cut through the mixed white ash, yellow poplar and oak forest. The genus name for evergreen wood fern “Dryopteris” is based on the Greek words for oak (‘drys”) and fern (“pteris”) emphasizing the association of the entire genus with oaks. They are especially abundant in primary (uncut) forested sites but have an accelerated rate of growth and reproduction in forests that have been cut or disturbed. These ferns have a wide range of light tolerances and soil moisture requirements and are capable of thriving in a wide range of habitats. Our observations indicate that evergreen wood fern grows in sites that are slightly drier than those that favor Christmas fern. Christmas ferns, then, have higher and possibly more narrow moisture requirements than evergreen wood ferns and are thus found more abundantly in the shadier and wetter regions of the Nature Trail ravine.
Winter is a great time to go hiking in the woods! No biting insects and very little chance of any tick encounters. The Christmas fern and the evergreen wood fern will be there waiting for you! Let’s hope we don’t get too much snow so that we are still able to see them!