Adaptations and constraints
Why do bryophytes succeed in the arctic?
In general there are two major characteristics of bryophytes restricting their distribution and their ability to compete with higher plants. The arctic environment seems to provide many habitats that compensate for these shortcomings of bryophytes.
Lack of lignin
This prevents them from reaching a significant height. The Arctic is characterized by a simple vegetation structure: There are no trees, shrubs are small, and other vascular plants such as herbs and grasses usually have a scattered distribution with low cover. As a result, competition for light is no longer a major limiting factor.
Dependence on moisture
Bryophytes are more dependent on moisture than higher land plants to complete their life cycle. Arctic soils are usually well saturated with moisture. The permafrost below the shallow active soil layer prevents water from draining away and keeps it available to plants. Tundra ecosystems, for example, are characterised by large areas of moist to wet soils.
Ecophysiological constraints and adaptations of arctic bryophytes
Bryophytes are very resilient and have a unique ability to recover from long-lasting extreme environmental conditions (La Farge et al. 2013, Procter et al. 2007). Bryophytes show a high degree of phenotypic plasticity as well as a remarkable ability to photosynthesise whenever conditions are favourable. Both these traits enable them to survive in cold regions (Turetsky et al. 2012). They are known to survive droughts, shutting down all metabolic processes, and reviving under favourable conditions (Glime 2007).
Example 1: Plant communities exposed by retreating glaciers in arctic Canada.
The vegetation studied was buried below glaciers since the Little Ice Age some 400 years ago. Of the subglacial bryophytes that were analysed, four taxa were able to regenerate in culture. The regenerated taxa came from habitats that ranged from hydric to xeric (La Farge et al. 2013).
Moisture and nutrients
Bryophytes are considered poikilohydric plants. This means their state of hydration is controlled by environmental conditions. Water is freely gained and lost over their surface and they tend to quickly reach equilibrium with the water potential in their direct surrounding, changing between being either hydrated and metabolically active or desiccated (air-dried) and metabolically inactive (Proctor et al 2007). The process is facilitated by a large surface area to volume ratio, and a low surface resistance due to the limited development of waxy leaf coating called cuticle. Vegetative desiccation-tolerance (DT), “the ability to revive from an air-dried state”, is a common trait in bryophytes (Wood 2007) . Most bryophytes seem to be able to survive short periods of moderate desiccation, while some can survive extended periods of severe desiccation as for example herbarium specimens that were revived after many years (Proctor and Pence 2002). So far, DT-species have been shown to exist within at least 6 of 13 bryophyte classes and 15 of 27 moss orders (Wood 2007). As a primitive trait, DT in green plants likely allowed freshwater algae to successfully colonise the land (Oliver et al. 2005). Many factors like drying rate, length of desiccation, intensity of desiccation and more have an effect on recovery (Oliver et al. 2005). Recovery rates are fast for respiration, photosynthesis and protein synthesis, between minutes and a few hours, while other processes take longer to recover (Proctor et al 2007).
The bryophytes’ poikilohydric water regulation both limits and enables their nutrient supply, since bryophytes absorb not only water, but also mineral nutrients, over their entire surface. This is further enhanced within the bryophyte by typically having leaves of only one cell layer in thickness, hence exposing every leaf cell directly and immediately to the nutrient supply (Glime 2007).
Generally, bryophytes require the same nutrient elements as vascular plants, but in lower concentrations. As for vascular plants, the major elements restricting bryophyte growth and productivity are nitrogen and phosphorus, elements that are generally available in low concentrations in the Arctic. Bryophytes predominantly receive nutrients dissolved in rainwater during summertime or as accumulated nutrient released during spring snowmelt. Sources of nutrient deposition and contamination in the Arctic are shipping emissions, and the long-range transport by rain and snow. Some uniquely nutrient-rich habitats in the otherwise nutrient-poor arctic environment exist below bird-nesting cliffs. Throughout the growing season, these areas receive a constant input of nutrients directly as droppings from the birds or dissolved in water draining from the bird cliffs.
Although bryophytes lack tracheids and vessel elements present in vascular plants, many of them possess structures that mimic the vascular tissue of flowering plants. Among the acrocarp mosses, a more complex stem can sometimes be found, with cells called hydroids (water-conducting) and stereids (conducting sugar). This indicates a certain ability to actively distribute water and carbohydrates.
In general, bryophytes are able to grow over a wider temperature range than vascular plants, particularly at the low end of the scale. Most bryophytes seem to grow optimally between 15 and 25°C, but the lower limit can drop considerably in cold habitats. Ceratodon purpureus and Bryum pseudotriquetrum for example had maximum rates of net photosynthesis at saturating light levels at approximately 10ºC in East Antarctic populations (Ino 1990). Racomitrium lanuginosum, a cosmopolitan species, had its photosynthetic optimum in high light intensities at 5ºC, with a minimum net gain at -8 to -10ºC (Kallio and Heinonen 1973). Since bryophytes utilise the metabolic C3 pathway for carbon fixation in photosynthesis, they are adapted to have a net photosynthetic gain at a relatively low temperature and light saturation. In fact, some mosses are able to photosynthesise at temperatures well below 0°C, with extremes of photosynthetic activity around -15°C (Glime 2007). This enables moss to grow for example at nunataks of Queen Maud Land, Antarctica, where air temperature usually are below 0°C, as long as sufficient water is available (Gjessing & Øvstedal 1989). Cold adapted bryophytes seem to survive and thrive at higher temperatures as well but seldom have a net gain at temperatures above 25ºC. Rather, they typically become dormant in summer heat and drought. Most species, including those from the tropics, seem to be pre-adapted to cold and survive temperatures ranging from -10° to -27° C (for more details see Glime 2007).
Tolerance to desiccation and desiccation itself, that prevents or reduces crystal formation inside the cells, is one feature that helps bryophytes to survive freezing. Several chemical components inside the cell, such as certain proteins, calcium, abscisic acids and altered fatty acids, and lipids in the cell wall, increase cold tolerance. Many of those substances help to increase both cold and desiccation tolerance. Coloured pigments and down-regulation of photosystem II seems to prevent photo-inhibition at low temperatures and high light intensities. Their structure itself, which is only one cell layer thick and therefore does not have internal air spaces, seems to help tolerate sub-zero temperatures. Ice forms on the surface instead of as ice crystals and helps to insulate the cell. For more details, you can read Glime 2007 (chapter 7-9 &10-2).
Light, water availability and temperature are closely linked in determining the photosynthetic capacity of bryophytes. Bryophytes generally grow maximally at less than full sunlight, because photo-oxidative processes limit moss production at maximum sunlight (Tenhunen et al. 1992). As such, they are typical shade-tolerant plants. They have low light compensation points (here photosynthetic energy gain equals energy loss through respiration) and light saturation points (level of light above which net photosynthetic gain stops to increase) (Glime 2007). Saturation levels around 20% of full sunlight have been found for a wide range of bryophytes, including species of open, brightly lit habitats such as Bryum argenteum, Racomitrium lanuginosum, Grimmia pulvinata and Tortula ruralis (Marschall and Proctor 2004). Most of their photosynthesis takes place in rainy or cloudy weather. During a bright sunny day bryophytes will generally be dry and metabolically inactive.