Mosses in Winter

by Margie Hunter

A good friend of mine who introduced me to the beauty of native plants in 1994, Paul Moore, has an alter ego — Moss Man. With patches of moss dangling from his head, he peers around the trunk of a tree sporting a big grin and wide eyes, a grown man totally grooving on nature. He banished the lawn in his backyard and cultivated a moss garden that is his pride and joy. Years ago I visited a magnificent, mostly natural, carefully tended moss garden tucked in the mountains of North Carolina — stunning, hushed, reverent — unlike anything I'd ever seen before. On the upper slopes of Mt. Sterling in the Great Smoky Mountains, soil, boulders and downed logs are blanketed in continuous carpets of delicate fern moss (Thuidium delicatulum). Exceptions like these aside, mosses rarely enjoy a starring role.

Mosses dominate the understory in the higher elevation forests on Mt. Sterling’s Baxter Creek Trail in the Great Smoky Mountains National Park. 
Mosses are easier to appreciate in winter.  Bright emerald cushions of windswept broom moss (Dicranum scoparium) enliven the forest floor at Cedars of Lebanon State Park.

Three seasons of the year, they take a back seat to other flora — outshone among wildflowers in spring, overwhelmed amid unrelenting green in summer, and literally covered in autumn’s colorful castoffs. In winter, however, mosses assume center stage, providing welcome relief from that season’s drab dormancy. Without bigger, showier distractions, plant nerds can expand their horizons exploring the miniature world of mosses, both fascinating and challenging. To get started, a quick overview might be helpful.

One of three non-vascular phyla in the plant kingdom, the mosses of Bryophyta are divided into three classes: Sphagnum mosses, granite mosses, and true mosses, by far the largest group of the three. Typically found in areas of sufficient moisture and shade, mosses will grow on almost any surface: soil, tree bark, rock, sidewalk, asphalt driveway. Mosses and their nonvascular cousins, liverworts and hornworts, represent plants’ first foray onto dry land, initiating the evolutionary journey from algae to the vast array of flowering plants present today.

Sporophytes of pocket moss (Fissidens sp.)

Despite their non-vascular designation, some mosses do have rudimentary conducting tissues for water and even food, but the cells do not have lignin in their walls. This important plant polymer gives vascular plants' xylem and phloem greater strength and impermeability allowing greater height. Another difference from their vascular relations — the independent, free-living, photosynthesizing green moss plants are gametophytes, representing the haploid generation and carrying only one-half of that particular species' genetic material. Some moss gametophytes are unisexual, some bisexual. Once eggs are fertilized by swimming sperm, plants or shoots with female gametes generate the small, dependent, diploid sporophytes — little capsules on thin stalks that contain haploid spores to germinate and develop new moss gametophytes. In seed-bearing vascular plants by contrast, sporophytes are the large plants all around us, and their gametophytes are tiny pollen grains and ovules.

Rose moss (Rhodobryum ontariense)

Specific physical characteristics of the sporophytes aid species identification along with leaves, growth habit, substrate, and habitat. Mosses may easily be confused with leafy liverworts, the other common non-vascular plants in moist environments. Moss leaves taper to a single point and are arranged spirally on the stem. Most have midribs. Leafy liverwort leaves are rounded or lobed, have no midrib, and are "two-ranked," arranged to either side of the stem and overlapping like shingles. Mosses have root-like structures called rhizines, that attach plants to the substrate but do not absorb water or nutrients.

Mosses derive their nutrition from rainwater and airborne deposits absorbed directly through their leaves, which are only one cell layer thick. Their spiral arrangement catches and holds water within the plant as long as possible, plumping the leaves and promoting photosynthesis. During dry spells the leaves curl up to minimize water loss and suspend food manufacture until the next rainfall. 

Many mosses also have relationships with cyanobacteria to obtain nitrogen. Symbiotic interactions with several different types of fungi have been noted, most either parasitic or commensal in nature, though some may impart mutualistic benefits. Mosses are the center of the universe for a diverse array of organisms, from the spruce-fir moss spider (Microhexura montivaga) of high altitudes in the Southern Appalachians to microscopic waterbears (Tardigrada), also known as moss piglets, lumbering through the watery depths of mosses and lichens.

Two growth patterns are commonly seen among the true mosses: cushion (acrocarp) and feathery (pleurocarp). Cushiony plants have simple or sparsely forked stems packed into more or less dense tufts with terminal sporophytes. Good examples are windswept broom moss (Dicranum scoparium), haircap mosses (Polytrichum spp.), pincushion mosses (Leucobryum spp.), glade moss (Pleurochaete squarrosa), and rose moss (Rhodobryum ontariense).

Feathery plants branch freely into creeping mats with lateral sporophytes. Common species in Tennessee include worm moss (Bryoandersonia illecebra), tree moss (Climacium americanum), brocade moss (Hypnum imponens), train tracks moss (Thelia hirtella), and kilt fern moss (Thuidium recognitum).

The Princeton Field Guide Common Mosses of the Northeast and Appalachians helps aspiring amateur bryologists take the plunge. The main drawback to moss ID is the necessity of magnification well beyond a 10X hand lens. A 30 to 40X microscope works well, and to adequately view leaf cells, a compound microscope may be necessary.

Kilt fern moss (Thuidium recognitum) prefers basic soils, growing beneath eastern red-cedars at Cedars of Lebanon State Park.

I highly recommend Gathering Moss: A Natural and Cultural History of Mosses by Robin Wall Kimmerer. It is beautifully written and a delight to read. Also the "Feb. 16th" chapter in David Haskell's The Forest Unseen explores moss life.

One final note: Harvesting moss for commercial sale or private use is not allowed on public land without a permit from the appropriate agency and should follow best practices to minimize habitat damage and promote sustainable regrowth.

Mosses in Winter

Photography by Margie Hunter