Polytrichum is a genus of mosses known for its relatively large size and distinctive appearance. Often forming dense, vibrant green carpets, this moss is a common sight in various natural landscapes. Its upright growth habit makes it easily noticeable among other smaller moss species.
Understanding Polytrichum Moss
Polytrichum moss is commonly known as “haircap moss,” “hair moss,” or “common haircap,” names referring to the hairy covering on its spore capsules. These mosses typically grow upright, with stems reaching impressive heights for a moss, often between 5 to 10 centimeters, but sometimes up to 20 centimeters or rarely 70 centimeters tall. The leaves are lance-shaped and possess a distinct midrib, aiding in water retention.
A unique feature is the presence of lamellae, vertical ridges of photosynthetic cells on the upper surface. These lamellae trap moist air, helping prevent the leaves from drying out. The sporophytes, or spore-producing structures, consist of a stalk (seta) and a capsule. The capsule is often sharply four-winged and rectangular to cubic, turning brown as it matures. Common species include Polytrichum commune, widely distributed in temperate and boreal regions, and Polytrichum juniperinum, which often grows on drier, exposed sites.
Where Polytrichum Calls Home
Polytrichum species are widely distributed across the globe, thriving in temperate and cold regions of the Northern Hemisphere, with occurrences also noted in Mexico, New Zealand, Australia, and parts of Africa. This moss shows a preference for acidic, moist soils, often forming extensive mats in diverse habitats. These include moist forests, bogs, wetlands, heathlands, and open woodlands.
In moist forests, the consistent humidity and stable temperatures provide an ideal environment for Polytrichum to flourish, often forming lush green mats on the forest floor, around tree bases, or on fallen logs. Its adaptability allows it to tolerate waterlogged and acidic conditions found in bogs and wetlands, where many other plants struggle. Some species, like Polytrichum juniperinum, can also be found in drier, exposed areas and disturbed sites, including old fields and urban areas, demonstrating a broader range of habitat tolerance than many other mosses. It can grow on gravelly soil, steep banks of road cuts, and even soil over rock outcrops.
The Unique Life Cycle of Polytrichum
Polytrichum, like all mosses, exhibits an alternation of generations, cycling between a haploid gametophyte and a diploid sporophyte stage. The dominant part of the plant is the green, leafy gametophyte, which produces gametes. Polytrichum is dioecious, meaning male and female reproductive organs are on separate plants.
Male gametophytes develop antheridia, producing flagellated sperm, while female gametophytes develop archegonia, each containing an egg. Water is required for sperm to reach the egg. After fertilization, the diploid zygote develops into the sporophyte, growing directly from the female gametophyte and relying on it for nourishment.
The sporophyte consists of a stalk (seta) and a spore-producing capsule. The capsule is covered by a hairy cap called a calyptra, the origin of the common name “haircap moss.” Inside the capsule, cells undergo meiosis to produce haploid spores. The capsule releases spores, which germinate to form a protonema, eventually developing into a new gametophyte, completing the cycle.
Polytrichum’s Role in Ecosystems
Polytrichum species play several roles within their ecosystems. They often act as pioneer species, colonizing disturbed or barren areas. Their establishment helps stabilize soil, reducing erosion and initiating ecological succession.
This moss is known for its high capacity for water retention, forming dense mats that hold substantial moisture. This helps regulate local hydrology and creates stable microclimates. Polytrichum also contributes to soil formation and nutrient cycling. As it decomposes, it adds organic matter to the soil, enriching its composition. Its dense growth provides habitat and cover for small invertebrates, such as insects, and can be a food source for some microfauna. For instance, a moss aphid, Myzodium modestus, feeds on Polytrichum mosses. The American Robin has also been observed utilizing pieces of common haircap moss for nest construction.