Signs of Fall 9: Ferns

Photo by D. Sillman

Deborah and I have always loved ferns. Our hikes through the woods here in Pennsylvania and prior to that up in New York State have always been highlighted by abundant fern landscapes and meadows. Even though we recognize that these extensive fern areas are poor resources for many browsers and grazers and may even be tipping forest succession away from healthy, stable climax ecosystems, we still appreciate the Mesozoic appearance and lushness of these glorious fern fields!

The Native Plant Information Network states that there are sixty-five species of ferns in Pennsylvania. That seems like a good number until you consider that there are over twelve thousand fern species worldwide! Deborah and I have seen fifteen or twenty of these Pennsylvania species over the years, but there are nine species (all listed in the Hiking Narratives on our web site “Between Stones and Trees”) that show up abundantly and quite regularly. I have written species pages on four of these nine common ferns for our “Virtual Nature Trail” web site (Christmas fern (Polystichum acrostichoides), evergreen wood fern (,Dryopteris intermedia), polypody fern (Polypodium virginanum), and sensitive fern (Onocela sensibilis)).

Evergreen wood fern (photo by D. Sillman)

Ferns occupy a very important stage in the evolution of plants. They are very complex entities with internal vascular tubes that transport water throughout their bodies, hearty stems that help them grow to significant heights, and roots and rhizomes that anchor them into soil, gather up water and nutrients, and help them spread throughout their ecosystems. They also have complex leaves (“fronds”) that are efficient organs for photosynthesis. Ferns, though, don’t have flowers and must reproduce instead via spores and gametic cells (sperm) that have to swim from fern to fern in surface water films in order to fertilize the ova. This necessity of water transfer of sperm cells greatly limits where most fern species can grow!

Polypody fern (Photo by D. Sillman)

Ferns are first seen in the fossil record some 360 million years ago, but they truly began to flourish about 145 million years ago in the Cretaceous Period of the Mesozoic Era. Maybe it was the competition with the newly evolved “flowering plants” that stimulated the extensive species radiation of the ferns at this time, or maybe it was the ferns themselves evolving complex, chemical protections against the huge Mesozoic herbivorous dinosaurs that triggered their new level of species diversity. Many of these Mesozoic fern species are still recognizable today, and one (a fern called Osmunda claytoniana) has been unchanged all the way down to its chromosomes for the past 180 million years! They are a persistent form of life!

Some great signs of fall (and then winter …. yes, winter is coming soon!) include three of those four very common, Nature Trail fern species that I listed above. At first frost, the sensitive fern will wilt and die back to its roots and rhizomes. This is how this species got its common name: even the slightest touch of frost on this sensitive plant drives it into its winter senescence. The Christmas fern, though, and its very hardy counterpart, the evergreen wood fern, will stay lush and green throughout the winter! Christmas fern has even been used for holiday greenery and wreaths! And, “evergreen” in the wood fern’s name clearly describes its appearance in the winter. Often these two fern species provide the only glimpse of green along our hiking trails throughout the long winter months. Their green fronds poke up out of the snow and remind us of all of the plants to come once spring finally arrives!

Photo by D. Sillman

Our eastern forests probably did not originally have such extensive fern growth in their understories. The current abundance may be due to the late Nineteenth and early Twentieth Century clear-cutting of the forests that opened up niches for the fast growing fern species and also to the presence of unsustainably large numbers of white-tailed deer. These deer extensively browse on almost every type of plant that grows on the forest floor with the notable exception of ferns! Deer browsing may be increasingly selecting for greater and greater fern densities in our forests!

There is some evidence that dense growths of ferns inhibit the seed germination and seedling survivals of many species of trees. There are models that show this inhibition (which may be due to either extensive shading of the forest floor by the fern cover or to a buildup of toxic chemicals in the soils under and around the growing ferns) will alter the successional direction of our forests. Only those tree species (if any) that can tolerate the ferns (and which can also avoid deer browsing!) will be able to grow and thrive in these altered ecosystems. It is also possible that these ferns (and their deer allies) will stop forest succession altogether! Maybe our eastern ecosystems will become simple, fern-filled moorlands instead of rich, complex deciduous forests.

This musing about ferns came about because of an email I got a couple of weeks ago. The emailer had been looking on-line for information about sensitive fern because of a tragic interaction that fern species had with one of her horses. She had just set up a new pasture after setting her horses out to graze soon noticed that one of her older animals was showing signs of distress. Within thirty days the horse had become emaciated and the next month had to be euthanized. She walked the pasture and found some species of ferns that included sensitive fern and then located the following paragraph from my sensitive fern species page:

Photo by D. Sillman

“Sensitive fern like most other fern species contains numerous, toxic chemicals in its tissues that are excellent defenses against both invertebrate and vertebrate herbivory. Deer seldom browse sensitive fern and may actually work to accelerate the fern’s growth and spread by their browse removal of less toxic, understory plant competitors. The toxic and sometimes fatal effect of sensitive fern on livestock and horses has been extensively documented. Avoidance of sensitive fern by grazers and care not to allow sensitive fern fronds to be incorporated into hay is essential.”

I replied to this woman immediately and included several links to references that described the chemical composition and impacts on grazing animals of sensitive fern and also bracken fern (a very common, open field dwelling fern species). They are very dangerous plants to have in pastures or in hay fields. They are full of chemicals that their ancestors used to fight off (and maybe even help drive into extinction) the dinosaurs! The beauty and elegance of these plants does hide a very toxic side of their existence.