Christmas Fern

Polystichum acrostichoides
Wednesday, June 25, 2014

     There is good speculation that the recent increase in deer populations over the entire eastern seaboard has, rather counterintuitively, given fern populations a competitive advantage over other groundcover types. Grasses and lowbush blueberries, the other two main groundcovers on the mountain, are both more heavily grazed by deer than are most species of ferns. Furthermore, the chemical composition of Christmas ferns makes them especially distasteful to deer. Researchers working in Indiana examined the contents of deer rumens (a ruminant’s first stomach, where cellulose is broken down) to determine the relative importance of various species in their diet in each season. Polystichum acrostichoides had been recently consumed by 4 of the 21 individuals surveyed in the spring, and it made up only 1% of total consumption by volume. It was largely absent from their diets during the summer and fall, but it was found in 21 of the 33 individuals examined in the winter, and composed 3% of rumen content. This pattern is likely found because it remains green year-round in mostly deciduous forests, making it an option of last resort. For perspective, in the spring, greater quantities of poison ivy and moth wings were found in rumen than P. acrostichoides, and more deer were found to have eaten bramble leaves and twigs than the comparatively painless Christmas fern.

            Christmas ferns are so named because their fertile fronds—those involved in spore production—remain green year-round and are popular greenery to use in holiday decorations. Their fronds, which can grow to about 90 cm in length, have a dark green color and a leathery texture. They are attached to a short stalk, which is supported by the underground, perennial rhizome. Fertile fronds are more well-supported by the stalk, and are held erectly in the center of the plant and ringed by less-erect sterile fronds. Unlike the fertile fronds, sterile fronds do die back in the winter.

            The sori, clusters of spore-producing sporangia, are located on the undersides of the leaflets that make up the upper third to half of the fertile fronds. During favorable conditions in June-October, the sporangia open and release haploid spores into the air, which settle on the ground and germinate into minute gametophytes. These gametophytes, in turn, produce eggs and sperm. In wet conditions, the sperm emerge from the antheridia, where they are produced, and swim off in search of eggs to fertilize. Once fertilized, the zygote grows into a diploid sporophyte, the plant we recognize as a Christmas fern.

            Greer and McCarthy (2000) noted that there appears to be a significant cost of reproduction to growth in these ferns. Over a two-year study, they found that above-ground growth rates only increased in ferns that reproduced the first year, but not the second. They concluded that “reproduction in P. acrostichoides only occurs when resources are sufficient to offset it’s [sic] cost to future growth; a life history that may optimize the advantages of early reproduction and life-time fecundity in a species whose colonizing phases (i.e., gametophyte and sporophyte) have high risks of mortality.”

            Christmas ferns are typically found on shaded, forested hills, often near streams. They do not occur where the ground is too waterlogged or rocky. They can be found throughout the north-eastern and north-central US, as far south as South Carolina. This cutting was taken near the north door of Lewis.

Hazel Galloway

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