Wild species 2010: chapter 7

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Mosses

Bryophyta - Non-vascular terrestrial plant lacking roots, but having primitive stems and leaves, and a simple reproductive cycle.

Quick facts

• There are more than 10 000 moss species in the world; 1006 have been identified in Canada.
• When excluding species ranked as Extinct, Extirpated, Undetermined, Not Assessed, Exotic or Accidental, the majority (76%) of mosses in Canada have Canada General Status Ranks (Canada ranks) of Secure, while 13% have Canada ranks of Sensitive and 9% have Canada ranks of May Be At Risk. Ten mosses species (2%) have a Canada rank of At Risk following a detailed assessment by COSEWIC.
• One moss species that used to be present in Canada is now Extinct globally and one species is recorded as Extirpated from Canada.
• Exotic species are rare in the moss flora of Canada and so far include only four species.
• The smallest mosses are Seligeria and measure less than 2 mm tall. The largest freestanding mosses in Canada are Polytrichum, sometimes reaching 25 cm in height.

Background

Mosses belong to the plant division Bryophyta (Bryophytes), which includes also liverworts and hornworts. Bryophytes are green land plants which, like all plants produce food via photosynthesis (using sunlight to convert carbon dioxide to simple sugars). While this activity is common to all plants, mosses are classified with the bryophytes (as opposed to vascular plants) because of their simple reproductive cycle and their simple anatomical structure. Features that distinguish mosses are the simple structure of the leaves and stems, lack of woody tissue, external fertilization, small size, lack of roots, and reproduction by spores.

The reproductive cycle of mosses is an important distinguishing character. The cycle comprises of two phases, a sperm and egg producing (“gametophyte”) phase, and a spore producing phase (“sporophytes”). The sporophyte phase is most familiar to people as it is the green plant and comprises most of the lifetime of the moss. Under the right conditions, the gametophyte will produce male and female reproductive structures that hold sperm and eggs. These structures can be on the same plant or on different plants. When mature, the sperm are released from the male structures (called “antheridia”) and swim to the female structures (called “archegonia”) where they fertilize an egg. The fertilization of an egg marks the beginning of the sporophyte phase. This phase is totally dependent on the gametophyte relying on it for nutrients and water. A mature sporophyte consists of a stalk with a spore capsule at the end of the stalk. The capsule contains spores that are eventually released and disperse by air currents. Upon landing in suitable habitat, the spore will germinate and develop into a new gametophyte, thus completing the cycle.

There are several hurdles that a moss must overcome to successfully produce spores and colonize new territory. The first is that water is required for the fertilization of the eggs by sperm. Thus, in arid regions or habitats where rainfall is scarce or infrequent, mosses may not have the opportunity to develop sporophytes, and thus will not produce spores. An equally important hurdle is that successful spore dispersal depends on luck and timing. The challenge is to get the spores dispersed into air currents and carried to a potential germination site. For instance, the spores of species growing in open habitats can more readily enter air currents than those of species that grow in dense woods or rock crevices. But, as already stated, spore dispersal is only a small part of successful colonization. Once a spore reaches its destination, landing in a suitable habitat or on a suitable substrate will largely determine if the spore can germinate. The majority of mosses require very specific environmental conditions (both climatic and physical) to grow and reproduce, and usually, the requirements for germination are the most stringent.

A physiological feature that distinguishes mosses from other plants is the ability of many species to withstand periods of desiccation (lack of water) and to recover from them upon rehydration. This phenomenon is known as “desiccation tolerance”, and it is exemplified also by some insects and many lichens. Desiccation tolerance is an adaptation in response to the inability of mosses to actively manage water loss from leaves and stems and other exposed structures. The issue of desiccation is compounded by the lack of roots in mosses. Without roots to absorb moisture from the soil, it is not possible to replenish water lost from the leaves by evaporation. As a result, the moisture content of mosses closely follows humidity cycles. Significantly, it is only when mosses are wet or moist that they grow since water is a necessary requirement for metabolism. When the moss’ habitat or substrate dries out, so does the moss and all growth stops. Contrast the mosses plight with that of vascular plants, most whose leaves are typically 10’s of cells thick and have thick waxy cuticles that prevent water loss, and that have extensive root systems to replenish their water needs.

While desiccation tolerance might be viewed as a disadvantage, this adaptation has in fact enabled mosses to diversify into many habitats and onto a wide variety of substrates that are impossible to be colonized by rooted plants. For instance, mosses are able to grow directly on rocks and or the barks of trees, two microhabitats that are outside the realm of vascular plants. Furthermore, the ability to grow on such substrates has allowed mosses to avoid competition with the larger and taller vascular plants, against whom they would certainly lose in any bid to acquire sunlight and water.

The small size of most mosses has played a large role in where they grow. The smallest mosses in Canada can be less that 1.5 millimetres tall (e.g., Seligeria spp.), whereas the largest may be some peat mosses (Sphagnum) that attain lengths of one meter when growing in bog pools. However, most mosses are of moderate size, varying from four millimetres to 20 centimetres. Their small size is due to the fact that mosses do not produce woody tissue that would provide the rigidity and strength to enable the plants to attain any height. Nonetheless, like desiccation tolerance, the small size of mosses can be viewed as advantageous, allowing them to grow in “microhabitats” where most vascular plants cannot. As a result, mosses can grow as epiphytes on trees and shrubs, in small rock crevices, or in animal burrows, to name a few. Where mosses dominate ecosystems, for example, in bogs, it is because they have changed the physical environment of such ecosystems to suit their needs.

The microhabitat preference of mosses has promoted much research into their use as indicators of environmental conditions in some ecosystems, such as forests. Their indicator value is useful in the management of natural resources.

Although small in size, mosses play an important role is many ecosystems. Mosses are dominant plants in many wetlands, peatlands, boreal forests, and coastal rainforest. In these systems they play an important role in controlling runoff and nutrient cycling, and influencing soil temperatures. Mosses are especially important in peatlands, which are an important ecosystem in the boreal zone of Canada. Peatlands consist of deep deposits of partly decomposed peat moss which in many places may have resulted from more than 5000 years of accumulation! Mosses are also conspicuous in the Arctic, where they predominate in many habitats, and where their diversity exceeds that of vascular plants.

Since some mosses produce an abundance of very small spores (<10 μm) that can be carried by wind, it is generally assumed that mosses are widespread and can be found everywhere. Moreover, if the species shows a gap in its range, then this gap is thought by some to likely be due to the species having been overlooked in that area. This could not be farther from the truth. Mosses show geographical patterns similar to those found in vascular plants. Fewer that 25 species in Canada are truly found worldwide where they occur on every continent.

About 40% of Canadian mosses are boreal species that can be found and recognized in many northern ecosystems of the northern hemisphere. About half of the boreal mosses are circumboreal species, meaning that they show continuous ranges throughout much of the boreal forests. Temperate mosses are of equally importance as boreal mosses in Canada’s moss flora. Like the boreal mosses, many of the mosses with distributions primarily associated with the temperate zone have wide ranges in this biome in the northern hemisphere. Montane mosses, as the name implies, occur only in mountainous regions of eastern and western Canada. Some species in this group have a truly unique ecology, being restricted to growing in snow beds that persist through most of the summer, and in some years, surviving under snow for the entire growing season. Arctic mosses range primarily at latitudes above tree line, and most occur also in arctic regions outside of Canada. A number of species have wider ranges and occur at southerly latitudes where they are found primarily in mountainous regions. Endemics are species with very restricted ranges, usually found only in one well defined region. Few mosses are endemic only to Canada and most are North American endemics whose ranges include a portion of Canada. The majority occur mainly along the coast of British Columbia or in southernmost Ontario and Québec.

Most mosses have little economic importance. The exception to this is the moss genus Sphagnum, which is commonly known as peat moss. Sphagnum is harvested in several provinces where it is processed and packaged for many uses. Certainly the most common usage is as a soil conditioner for gardens. However, the uses of peat are much more varied, and it is also used as a medium for growing mushrooms, as an industrial chemical absorbent, and as the main absorbent in some brands of feminine napkins. The importance of Sphagnum as an effective absorbent has long been known, as much research was conducted on this moss during World War I to determine which species were most effective for use in surgical dressing.

In recent years, mosses have been harvested in the rainforests of the Pacific Coast for use as packing material to help retain soil moisture in plant pots. Mosses absorb water quickly and release it slowly. In coastal rainforests where mosses are abundant, their removal by large scale harvesting by the horticultural industry can have significant effects on drainage by increasing erosion and leading to slope instability.

Status of knowledge

The study of mosses has a long history in Canada. The first catalogue of Canadian mosses is that of John Macoun’s (dating from the late 1800’s), who summarized records from many of the early collectors and botanists. Since Macoun, there has been considerable research and inventory on mosses in many parts of the country. While much of the early and middle part of the 20^th century, the study of mosses included a large floristic component while the latter part of the 1900’s has seen a shift toward more ecological studies geared to understanding the effects of human activities on moss diversity in an effort to effect better management of some ecosystems, but especially the boreal ecosystem. This information has been supplemented by many surveys conducted for rare mosses in areas that are planned for resource extraction or other industrial uses.

Relative to the status and distribution of vascular plants, mosses are not as well known, owing to the fewer numbers of specialists studying the group. Nonetheless, the overall distributions of mosses in Canada are well known, lacking only in detailed knowledge of species occurrences at smaller geographic scales. Most regions in Canada have been well explored and documented, and this is especially true in British Columbia, the southern Rocky Mountains, some of the Arctic Islands, southern Ontario and Québec, the island of Newfoundland, and much of the Maritimes. However, information on moss diversity and distribution in large regions is generally lacking. These regions include Manitoba, large parts of the Prairies ecosystem, much of the mainland Northwest Territories and Nunavut, large parts of northern Québec and Labrador, and some Arctic Islands.

Richness and diversity in Canada

Relative to some groups covered in this report, the species richness of mosses is generally high across the country (figure 7), peaking in British Columbia (760 species) and high also in Québec (578 species), Newfoundland and Labrador (531 species), Ontario (522 species), and Alberta (522 species). The lowest diversity is in Prince Edward Island (204 species), followed by Saskatchewan (286 species) and Manitoba (335 species). Richness of mosses is closely tied to landscape diversity, with regions endowed with a high diversity of landforms and climates having the highest diversity. Lack of inventory work also impacts diversity and in some areas of the country the mosses are poorly known. As previously mentioned, this is particularly true of some northern regions but especially in southern Nunavut, the eastern Northwest Territories, and surprisingly, the province of Manitoba. These areas should be priorities to be targeted for future inventory work.

Species spotlight - Stairstep Moss

Stairstep Moss (Hylocomium splendens) is one of three feathermosses. Feathermosses are some of the easiest mosses to know by name, not only because their growth form resembles that of a feather, but also because they are large and conspicuous. Of the three feathermosses, the growth form of Stairstep Moss is most distinctive and not seen in other mosses. The plant consists of large, lacy fronds that emanate at intervals along a main stem, giving the plant a “stairstep” appearance. Of particular interest is that only one frond is produced each year, at the apex of the stem. This gives a way to determine a minimum age of a plant, with each frond representing one year of growth; this is akin to growth rings on a tree. As noted, counting fronds gives a minimum age for the plan; after six to seven years, older fronds will have decomposed beyond recognition. Nevertheless, under ideal conditions, it is sometimes possible to find plants with eight to nine years of growth.

The growth form and size of stairstep also makes this plant useful as an indicator of average moisture conditions in forests where it is found. The length of the stem between the fronds corresponds directly to moisture levels in the environment, with longer stem lengths indicating higher moisture availability. This provides an easy way for researchers to compare long term moisture conditions in contrasting forest types. The size of stairstep is also a good indicator of moisture. In the coastal rainforests of British Columbia for instance, the fronds of stairstep often measure 3 cm across, whereas in the drier boreal forests of the continental interior, the fronds will be only 1-1.5 cm.

Species spotlight - the dung mosses

Dispersal of spores for almost all mosses is dependent on being carried by air currents to reach new colonization sites. One family of moss, the dung mosses (family Splachnaceae), has evolved interesting adaptations that allow them to take advantage of a different means of dispersing spores - by insects!

In Canada, the Splachnaceae consists of four genera: Aplodon, Tayloria, Tetraplodon, and Splachnum. The species in these genera are specialized to grow on dung, carrion, stomach pellets of birds, and bone. As can be imagined, because of their size and random locations on the landscape, these substrates are not easy “targets” for spores to land on when spores are carried by air. However, flies and other insects are naturally attracted to these substrates. Consequently, dung moss species have evolved adaptations to attract insects so that spores may be attached to them and carried to the appropriate substrate. The adaptations involve the spore capsule, and include enlargement and development of coloration of the neck of the capsule (hypophysis), the production of odiferous (smelly) capsules, and the development of sticky spores dispersed in clumps.

These adaptations in Splachnaceae allow the mosses to continually colonize new substrates as they appear in an area, ensuring their long time survival there.

Results of general status assessment

Wild Species 2010 marks the first national assessment for mosses in Canada. Results of this assessment indicated that 58% of mosses have Canada ranks of Secure, while 10% have Canada ranks of Sensitive and 7% have Canada ranks of May Be At Risk (figure 7 and table 7). Ten mosses species (2%) have a Canada rank of At Risk following a detailed assessment by COSEWIC. One moss species (Neomacounia nitida) that used to be present in Canada is now Extinct globally. One species of moss (Ptychomitrium incurvum) is recorded as Extirpated from Canada. To date, four Exotic moss species have been recorded in Canada.

Threats to Canadian mosses

Like many other plants and animals, mosses require both terrestrial and aquatic habitats in order to survive and maintain their populations. Most mosses are vulnerable to both habitat degradation and destruction as a result of human activities. Important habitats for mosses at risk include forests, cliffs, as well as wetlands. Climate warming is often considered as a threat to many wildlife species, and certainly this factor will have an effect on mosses. Of special importance in this regard are many mosses that grow in mountains or in arctic regions in association with late snow beds or habitats whose existence relies on colder temperatures.

Conclusion

This first general status assessment of the mosses of Canada is an important achievement as it marks a milestone in highlighting the importance of mosses in Canada. With more than 1000 species, this is one of the largest groups of organisms assessed. While the report shows that at least 7% of mosses have a Canada Rank of May Be At Risk, it also shows that the status of a large proportion (23%) are ranked Undetermined. The latter is a reflection how much more there is to learn about this fascinating group and this first assessment provides a solid foundation for further research and conservation effort.

Further information

Crum, H. A. and Anderson, L. E. 1981. The mosses of Eastern North America. Two volumes. Columbia University Press, New York.

Ireland, R. R. 1982. Mosses of the Maritime Provinces. Publ. Botany 13, National Museum of Natural Sciences, National Museums of Canada. Ottawa.

Ireland, R. R. and Ley, L. M. Atlas of Ontario mosses. 1992. Syllogeus 70: 1-138. Canadian Museum of Nature. Ottawa.

Lawton, E. 1971. Moss flora of the Pacific Northwest. Hattori Botanical Lab. Nichinan, Japan.

Schofield, W. B. 1985. Introduction to Bryology. The Blackburn Press, Caldwell, New Jersey: 431 pp.

Schofield, W. B. 1992. Some common mosses of British Columbia, second edition. Royal British Columbia Museum, Victoria: 394 pp.

References

Anderson, L. E., Crum, H. A. and Buck, W. R. 1990. List of mosses of North America north of Mexico. Bryologist 93: 448-499.

Belland, R. J. 1987. The moss flora of the Gulf of St. Lawrence Region (Canada): ecology and phytogeography. Journal of the Hattori Botanical Laboratory 62: 205-268.

Brassard, G. R. 1983. Checklist of the mosses of the island of Newfoundland, Canada. Bryologist 86: 54–63.

Brassard, G. R. and D. P. Weber. 1978. The mosses of Labrador, Canada. Canadian Journal of Botany 56: 441-466.

Flora of North America Editorial Committee. 2007. Flora of North American North of Mexico, volume 27. Bryophytes: mosses. Part 1. Oxford University Press. New York and Oxford, U.K.

Ireland, R. R., Brassard, G. R., Schofield, W. B. and Vitt, D. H. 1987. Checklist of the mosses of Canada II. Lindbergia 13: 1-62.