Addendum: Evergreen ferns

Marginal Shield Fern fruitdots

While out walking Raven this afternoon I had another, closer look at the ferns that were mentioned in yesterday’s post, specifically the Marginal Shield Fern, Dryopteris marginalis. When I took the first set of photos, I hadn’t thought to check the underside of the fronds for the spore pores, so I did that today. One of the references I used for the last post was A Field Guide to the Familiar by Gale Lawrence. In it she indicates that an easy way to tell the three species apart is by the patterns of the spore cases on the underside of the subleaflets. The pattern of these dots – actually called sori (singular sorus) but also commonly referred to as fruitdots, which I find inexplicably amusing – is unique to each species (at least each of these three). As I stated yesterday, those of the Marginal Shield Fern are lined up along the margins of the subleaflet, giving the fern both its common and scientific names. You can see that in the photo above. The fruitdots are empty now, the ferns having cast out their spores to the wind in late fall, but the cases remain affixed to the evergreen fronds. Not all species of fern have fruitdots, some curl over the edges of the subleaflets or other means of protecting the spore cases.

Winter green

Rock Polypody, Polypodium virginianum

It’s getting harder to spot things in the woods to write about. From spring through fall life is so abundant and so vibrant that you could write five posts a day and still not run out of subject matter (true, four of those five would be about plants or invertebrates, but that’s beside the point). Come winter, though, things slow down. Animals migrate south or go into hibernation. Herpetiles bury themselves in the muck, fish swim down to the deeper parts of the lake. Insects die off, leaving just their eggs or cocoons to carry on the species come spring. Plants die back, trees and shrubs lose their leaves, and non-woody plants disappear altogether. What remains in January is just an empty shell of what was here in July.

That doesn’t mean, however, that there is nothing here. Although the winter complement is a far cry from the 100+ species recorded around here during the summer, our winter bird list will probably include some two dozen species. If we’re lucky, we may record half a dozen mammal species. Herpetiles and fish are in hiding, of course, but there is still evidence of insects if you know where to look. Woody plants – trees and shrubs – are easy to find, even if they have dropped their leaves. And, there are still some green plants around.

Rock Polypody, Polypodium virginianum

It’s an interesting juxtaposition, seeing perky green leaves surrounded by snow and leafless trees. It wouldn’t be too abnormal if this was an early snow in fall, when there were other plants that were still in the process of winterization. But the rest of the forest was barren and quiet, and the green leaves seemed somewhat out of place. These are evergreen ferns. Like in trees and some other types of plants, there is a subset of ferns that remain green year round. Here in the northeast there are three such species. The little ones in the above photos are Rock Polypody, Polypodium virginianum. It’s sometimes also called Common Polypody, but this is more often used for P. vulgare, a species of the Pacific northwest. The name Rock Polypody comes from the species’ habit of growing primarily on rock surfaces covered with a thin layer of soil. Indeed, virtually all of the plants I saw were on exposed rock surfaces or the sides of rock piles.

Marginal Shield Fern, Dryopteris marginalis, and Rock Polypody, Polypodium virginianum

The lower fern in this photo is Rock Polypody, but the upper one is Marginal Shield Fern, Dryopteris marginalis. Like fungi, ferns reproduce through spores rather than seeds, and the pores that release the spores are found either on specialized spikes that the plants put up, or on the underside of the plant’s leaves. The pattern of the pores is often helpful in identifying the species. In the case of the Marginal Shield Fern, the first part of its name comes from the arrangement of the pores along the margins of the fern’s subleaflets (the frilly edges on the sides of each of the fern frond’s “fingers”). In fact, at this time of year it’s difficult to confuse it with anything else. The only other commonly found evergreen fern to grow around here is Christmas Fern, Polystichum acrostichoides, which resembles a Boston Fern in the long, narrow shape of the fronds.

Rock Polypody, Polypodium virginianum

Evergreen ferns have an interesting adaptation that allows them to remain green all winter without the leaves freezing and dying. While most plants will pull the sugars out of their leaves and into storage in their stems and roots, evergreen ferns do the opposite, instead packing the leaves with extra sugars, which act as a sort of anti-freeze, protecting the cells of the leaves from freezing and rupturing by ice crystals. A thick blanket of snow helps provide snug insulation against the dessicating effects of the cold, dry winter wind (although the species are also remarkably resilient to drying out, able to revive even after losing over half of their internal water content). By remaining green over the winter they extend the period that they’re able to photosynthesize, taking advantage of late fall sun before the snow falls, and getting a head start on other plants in the spring. Generally speaking, ferns aren’t a favourite foodstuff for many wild animals, but in the winter when food can be harder to find some animals, including deer, turkey and grouse, will nibble on evergreen fern fronds for nourishment.