Peat Moss: An Exceptional Ecosystem with Unique Traits

Characteristics of Sphagnum Mosses

From Fritz et al. (2014):

Bogs (ombrotrophic peatlands, i.e. predominantly fed by rainwater, from Greek ombros, rain, and trephein, to feed) are exceptional ecosystems that may show high storage rates for nutrients and carbon, while nutrient availability is very low due to ombrotrophic conditions (water and nutrient input solely by rain) limiting the growth of vascular plants. A set of traits, unique to Sphagnum mosses, enable their dominance in bog ecosystems. Sphagnum has evolved a high nutrient use efficiency to cope with low input rates of nutrients [1,2]. The atmospheric input of nutrients is efficiently retained in moss peat and decomposition rates are low, due to the high retention of rainwater, acidic conditions and poorly degradable organic matter [3]. In addition, a substantial part of the C [carbon] losses (including CH4) are refixed by Sphagnum as growth and photosynthesis have been shown to increase upon elevated CO2 in porewater [4–6]. This combination of traits enables Sphagnum to avoid being outcompeted by vascular plants. However, increased availability of nitrogen, e.g. by high airborne inputs, favours vascular plants at the expense of Sphagnum mosses. Displacement of Sphagnum by vascular bog plants often leads to reduced storage of nutrients, carbon (peat) and water [7–9]. In comparison to vascular plants, Sphagnum spp. show low decomposition rates due their chemical composition [2,3], in addition to anoxic and acidic conditions, and may still have similar primary production rates [10,11]. Sphagnum peat may therefore accumulate substantial amounts of nutrients, if considered m^-2 y^-1, even though nutrient concentrations in Sphagnum are lower. …

An in-text citation from this source would look like: Sphagnum sp. are adapted to low nutrient conditions and show low rates of nutrient release from decomposition (Fritz et al. 2014), and the citation would look like:

Recent research on the organo-chemical composition of Sphagnum and on the fate of its litter has further clarified how this plant builds acidic, nutrient-poor, cold and anoxic peat bogs. The bog environment helps Sphagnum to outcompete other plants for light. Its morphology, anatomy, physiology and composition make it an effective ecosystem engineer and at the same time benefit the plant in the short term. This may have facilitated the evolution of the genus.

Fresh Sphagnum mainly consists of polysaccharides⁵, made up of glucose and galacturonic acid units. The latter are sugars in which the CH2OH side-chain at C6 has been replaced by a carboxylic acid group, which give Sphagnum its high cation exchange capacity and are largely responsible for its acidic character⁶.

The Organo-Chemical Composition of Peat Moss

Sphagnum lacks lignin (early reports about lignin in Sphagnum are now attributed to contamination with vascular plants²⁰). Sphagnum is rich in phenols, including the genus-specific, very stable Sphagnum acid [p- hydroxy-beta-(carboxymethyl)-cinnamic acid]²¹. Sphagnum acid is present in a polyphenolic network polymer that is probably linked covalently to cell wall biopolymers²⁰. This combination confers one lignin- like property (poor substrate quality) to Sphagnum tissue, without providing the structural strength typical of woody tissue. Indeed, in spite of its lack of lignin and its high polysaccharide content 'almost nothing eats

Sphagnum '¹. Anatomical and biochemical details of Sphagnum may be preserved in peat for millennia, with the bulk of the polysaccharide still present after 70 000 years²⁰. In the acrotelm, Sphagnum litter decomposes more slowly (mass loss 10-20% yr^-1) than leaves of most other plants in their natural habitat (40-80% yr^-1). … Sphagnum peat is famous for its excellent preservation, not only of Sphagnum, but also of remains of human and animal bodies, and of organic artifacts. This was attributed to a tanning-like process involving 5-keto-D-mannuronic acid, associated with sphagnan (a complex pectin-like material that is covalently linked to cellulosic and amyloid-like chains in Sphagnum²⁹). Sphagnan would suppress microbial activity by strongly binding N, by inactivating exo-enzymes and by sequestering essential multivalent metal cation by chelation. While Painter²⁹ found no polyphenols responsible for tanning in Sphagnum peat, tannin-like compounds have been observed in Sphagnum more recently²⁰. Continuous waterlogging is essential for preserving peat. Aeration and decomposition following drainage irreversibly increases the permeability of the peat, which then becomes unsuitable as a substrate for Sphagnum³⁰. … The morphological, anatomical, physiological and organochemical properties of Sphagnum give it attributes (see Fig. 4) that help form acidic, nutrient-poor, heat-insulating and slowly permeable peat. Depressed growth of vascular plants increases (1) light availability and (2) wetness, via decreased evapotranspiration¹⁰, both of which positively feed back to the growth of Sphagnum, and thus to peat growth. Accumulation of peat is further promoted by feedbacks involving physico-chemical processes and depression of decomposers (Fig. 4). … In conclusion, the available literature gives some direct and much circumstantial evidence that four factors are mainly responsible for adverse conditions for vascular plants on raised bogs. They are (1) low nutrient availability (2) anoxia, (3) low temperatures and (4) high acidity.

Under favourable external conditions, Sphagnum growth, once initiated, stimulates peat growth and forms raised bogs. The morphological, anatomical, physiological and organochemical properties of Sphagnum give it attributes (see Fig. 4) that help form acidic, nutrient-poor, heat-insulating and slowly permeable peat. Depressed growth of vascular plants increases (1) light availability and (2) wetness, via decreased evapotranspiration¹⁰, both of which positively feed back to the growth of Sphagnum, and thus to peat growth. Accumulation of peat is further promoted by feedbacks involving physico-chemical processes and depression of decomposers (Fig. 4). …

Preservation of Peat

An in-text citation from this source would look like: The chemical structure of Sphagnum creates acidic conditions that limit the growth of competing plants (van Breeman 1995), and the citation would look like

In general, mosses possess several key traits that allow them to persist in cold regions, including a high degree of phenotypic plasticity and a broad response of net assimilation rates to temperature. Mosses can be regarded as opportunistic in terms of CO2 exchange in that they are able to respond positively to favorable environmental conditions where and when they occur, with CO2 assimilation even during low temperature and irradiance (Kallio & Heinonen, 1975; Oechel & Sveinbjörnsson, 1978; Harley et al., 1989). Key functional traits include tolerance to dessication and the ability to switch quickly between metabolic activity and rest. While vascular plants avoid dessication during drought by actively regulating internal water content (i.e. homoiohydry) through morphological adaptations such as well-developed conducting systems, leaf stomata, cuticle, and roots, these features are all poorly developed or absent in mosses.

Instead, mosses tolerate periods of drought largely through physiological responses, such as by suspending metabolism (controlled cessation) when water is not available and by withstanding cell dessication.

In their Tansley review, Proctor & Tuba (2002) argued that the moss strategy of poikilohydry ‘is not merely the primitive starting point of plants that failed to achieve homoiohydry, but a highly evolved strategy of adaptation in its own right, optimal in some situations and at least a viable alternative in some others’. When faced with dessication, net assimilation rates in mosses usually decline rapidly as tissue water content falls below the threshold required to maintain full turgor (Longton, 1988). Their cytoplasm can survive at low water contents for long periods, and upon rehydration resume metabolic activity. This strategy, however, has metabolic costs, as recovery is preceded by a burst of respiration that results in a net energy loss (Skre & Oechel, 1981). Desiccation time affects the timing and completeness of recovery, and extended desiccation can cause intense respiration and death. While many moss species can tolerate dessication, there is a wide variety of adaptations in terms of water economy among mosses (Vitt & Glime, 1984), as well as large variation in the relation of desiccation tolerance to desiccation intensity (Proctor & Tuba, 2002).

In general, these traits are not considered to be specific adaptations to cold region environments, but almost certainly increase the fitness of mosses growing in harsh northern conditions.…

An in-text citation from this source would look like: Mosses have the ability to survive drying for periods of time and come return to growth when conditions improve (Turetsky et al. 2012), and the citation would look like:

Pick one of the traits described in the case study information posted to create a scientific argument  that the trait is adaptive to the species’environment. Provide evidence for your claim from the case study information by discussing how the trait increases fitness at the individual level (i.e. a heritable trait that  increasesreproductive success / survival of offspring), and you should further justify (rationalize) how the trait has persisted at the population level by describing how it is selected for within the population.