# Sphagnum rubellum

Sphagnum rubellum, commonly known as the red peat moss, is a species of peat moss in the family Sphagnaceae. It forms low, reddish cushions in wet areas like bogs and poor fens across North America and Eurasia, particularly in regions with oceanic climates. The species can vary in colour from green to pink or deep crimson, and grows up to 10 cm (3.9 in) tall. Like other peat mosses, it plays an important role in forming and maintaining peatlands.
This species is closely related to S. capillifolium, with which it was historically confused, though genetic studies have confirmed them as distinct species. While they can occasionally interbreed where their populations overlap, they typically occupy different habitats – S. rubellum preferring open, wet bogs while S. capillifolium grows in drier, more shaded areas.
Sphagnum rubellum is particularly well-adapted to wet, nutrient-poor conditions. It can transport water efficiently through its stems and maintain stable growth even when raised above the water table. While it grows more slowly than many related species, it is very resistant to decay. The species provides good conditions for tree seedlings to germinate, though its continued growth can later prevent young trees from establishing.
Though classified as a least-concern species in Europe and secure in North America, S. rubellum faces threats from peatland drainage, commercial harvesting, and air pollution, particularly nitrogen deposition. It is considered vulnerable or threatened in parts of its range, especially in central and southeastern Europe. Genetic studies suggest the species survived the last ice age in at least two separate refugia in Europe, which has influenced its current patterns of genetic diversity.

## Taxonomy
### Original description

Sphagnum rubellum was originally described by the English bryologist William Wilson in 1855, who referred to it as "red dwarf bog-moss" due to its slender stems and small, reddish appearance. Wilson noted that it is dioicous, meaning it has separate male and female plants, and he characterised its leaves as elliptical, obtuse, and subsucculent, with short, deflexed, attenuated branches. The species' capsules, which contain the spores, were described as "subexserted"—barely protruding from the protective covering (perichaetium). Wilson observed S. rubellum growing in peat mosses, specifically mentioning its presence at Risley Moss near Warrington, where it often appeared in moderately moist areas alongside Sphagnum molluscum.
Wilson also distinguished S. rubellum from other Sphagnum species based on morphological traits. Compared to S. molluscum, S. rubellum has slightly smaller stems and is less branched. Wilson noted that the species' stem leaves are round-ovate and slightly concave with a pellucid (transparent) margin, while branch leaves are elliptic-obtuse and ovate. Although S. rubellum bears some resemblance to S. acutifolium (now generally considered part of a broader species complex), Wilson distinguished it by its smaller size, neat appearance, and distinctive leaf shape.

### Taxonomic history and synonyms

Sphagnum rubellum belongs to the subgenus Acutifolia. The species is part of the "red Acutifolia" group, which includes the closely related species S. warnstorfii and S. capillifolium. It has been referred to by several synonyms over time, including Sphagnum tenellum (Schimp.) Klinggräff, and Sphagnum acutifolium var. tenellum Schimper. Historically, there has been debate regarding its classification; some early botanists, such as Albert LeRoy Andrews, treated S. rubellum as a mere variety of Sphagnum nemoreum (now commonly known as Sphagnum capillaceum tenellum) and grouped it with closely related species, including S. subtile. Despite this, most modern taxonomists recognise S. rubellum as an independent species, distinguishable by its unique morphological and ecological traits under optimal growth conditions. Additionally, the distribution of S. rubellum differs from that of S. nemoreum, further supporting its classification as a separate species.

### Modern species delimitation

The taxonomic relationship between S. rubellum and S. capillifolium has been particularly debated. In 1989, research showed that the two species could overlap morphologically, leading to debate about their distinctness. Through this period, botanists variously treated these taxa in three different ways: as separate species, as varieties of the same species, or as environmentally induced modifications of a single taxon.
The taxonomic status of S. rubellum was particularly debated in Britain and parts of Europe in the 1980s, where it was often treated as a variety of S. capillifolium (as S. capillifolium var. rubellum) due to the presence of intermediate forms and environmental variation making the distinction between the two taxa unclear. However, comprehensive DNA sequence analysis has unambiguously supported their separation as distinct species, even though they cannot always be distinguished morphologically. The molecular evidence shows that while occasional hybridisation occurs, both species maintain their genetic distinctness.
Genetic studies in the 1990s provided strong evidence for treating them as distinct species. The research revealed clear genetic differentiation between the two taxa, with only 1.6% of specimens showing mixed genetic markers. These rare intermediates were typically found in mixed populations from mire margins, where both species occur together. Subsequent molecular analysis using microsatellite markers has further supported their status as distinct taxa, though there is evidence of high genetic variation within the red Acutifolia group as a whole.
Additionally, molecular studies have shown that several taxa previously considered distinct species should be merged with S. rubellum. Both S. andersonianum and S. bartlettianum show no significant genetic differentiation from S. rubellum, despite some morphological variations in features such as branch leaf arrangement and pore size. While coastal plain populations of S. bartlettianum can appear distinct from northern S. rubellum, mountain populations show intermediate characteristics, and genetic evidence supports treating them as a single species.

## Identification

Sphagnum rubellum is a small peat moss, typically less than 1 cm in diameter. The species is variable in colour, ranging from green through pink to deep crimson, and occasionally appearing copper-coloured. It can be recognised by its distinctive growing tip (capitulum), where the apical bud is visible and slightly elevated above the surrounding inner branches. A key diagnostic feature is the tongue-shaped (lingulate) stem leaves, which measure 1.0–1.25 mm long.
The species is frequently confused with its close relative S. capillifolium, though there are several features that help distinguish them. When growing together, S. rubellum typically appears lighter red than S. capillifolium. The most reliable distinguishing features are the stem leaves and growing tips: S. rubellum has tongue-shaped stem leaves and a visible apical bud, while S. capillifolium has oblong stem leaves that gradually taper to a point and its apical bud is usually hidden from view. These features can be observed with a hand lens in the field.

## Description

Sphagnum rubellum belongs to the subgenus Acutifolia, which comprises small to medium-sized slender plants with narrow and elongated branches. The plants have a loose growth form with flat growing tips (capitula), varying in colour from green to pink or deep crimson. The branch clusters (fascicles) are usually spaced apart rather than densely packed, allowing the stem to be visible between them.
The stem leaves are tongue-shaped (lingulate), 1.0–1.25 mm long, weakly fibrillose or sometimes almost lacking fibrils, with numerous septa (cross-walls). The branch leaves are loosely arranged in five distinct rows and appear rather blunt, with concave tips and slightly curved edges.
The reproductive structures include spores with three radiating ridges forming a Y-shaped pattern (trilete spores). The spores are relatively large compared to related species in section Acutifolia, ranging from 19.4–33.3 micrometres (μm) in diameter with a mean of 26.1 μm. Both surfaces of the spore show uneven deposits of protective coating material. This spore morphology helps distinguish it from closely related species such as S. nemoreum.

## Distribution and habitat

Sphagnum rubellum is found across the cool temperate regions of North America and Eurasia, in a belt-like pattern around the Northern Hemisphere (circumboreal distribution). The species shows a distinct preference for oceanic conditions, which is reflected in both its distribution and habitat preferences, occurring from sea level to elevations of 2,200 m (7,200 ft). This oceanic affinity distinguishes it from its close relative S. capillifolium, which tolerates more continental climates and shows a broader distribution pattern.

### Regional variation

Population characteristics vary geographically, with more morphologically distinct forms occurring in continental areas compared to more variable forms in oceanic regions. In Britain, S. rubellum is particularly abundant in southern and western regions, where it largely displaces S. capillifolium, which is restricted primarily to upland areas such as the Pennines. While the species typically occupies very wet locations in continental Europe, it can thrive in relatively drier sites in Britain's oceanic climate. Unlike other major sections of Sphagnum which show high diversity in tropical regions, section Acutifolia (including S. rubellum) maintains its highest diversity in boreal and temperate zones, and is not diverse in tropical regions outside Central and South America. It was recorded as new to both southwest Asia and Turkey in 2013.

### Habitat preferences

Sphagnum rubellum predominantly occurs in open peatlands (mires), typically in conditions ranging from ombrotrophic (receiving water and nutrients solely from precipitation) to weakly minerotrophic (receiving some nutrients from groundwater). In these habitats, it forms extensive carpets and occasionally develops into low hummocks. This habitat preference contrasts with S. capillifolium, which favours minerotrophic conditions and typically forms compact hummocks on relatively dry ground in shaded forests and open subarctic heaths. Within bog systems, S. rubellum is typically found on exposed and rather wet mire expanses, while avoiding the drier, shaded mire margins that are preferred by S. capillifolium—a differentiation that is particularly pronounced in continental regions. In areas where both species occur together, such as mire margins, S. capillifolium tends to be the more abundant.

## Ecology

Sphagnum rubellum grows in bogs and poor fens (wetlands with some groundwater input), occurring in habitats ranging from raised mounds (hummocks) to flat areas (lawns). It typically grows in distinctly acidic locations raised somewhat above the water table. The species is strictly ombrotrophic and typically forms large, uniform lawns and carpets on treeless bogs.

### Growth and biomass production

Growth rates vary both temporally and spatially, though the species can achieve growth rates of up to 20 mm (0.79 in) per year in favourable conditions. Laboratory studies have shown that Sphagnum rubellum grows significantly slower in axenic (sterile) in vitro culture compared to related species, achieving only a 4-fold biomass increase over six weeks, in contrast to up to 80-fold increases seen in other Sphagnum species, such as S. cuspidatum. This reduced productivity under standard lab conditions suggests that current growth media may not be fully optimised for S. rubellum, potentially reflecting its specific habitat adaptations. It is also considerably slower growing than its close relative S. capillifolium, from which it is sometimes difficult to distinguish genetically, although phenotypic traits can aid in differentiation.
In natural conditions, S. rubellum typically achieves biomass production of around 237 g (8.4 oz) per square metre per year, with relatively stable growth rates across varying environmental conditions. While growth can decline significantly in dry years, the species maintains more consistent growth across varying weather conditions compared to hollow species. This relatively low but stable growth rate, combined with high decay resistance, represents an evolutionary strategy characteristic of section Acutifolia species.

### Water relations and environmental responses

Sphagnum rubellum shows strong resistance to changes in water level, being able to maintain growth even when water tables drop to 15 cm (5.9 in) below surface level. This adaptability is aided by its efficient capillary water transport system. Compared to other Sphagnum species, S. rubellum shows strong resistance to desiccation, maintaining its moisture content and appearance better than species from more nutrient-rich habitats during dry periods.
In ecological gradient studies, S. rubellum occupies an intermediate position between true hummock species like S. fuscum (which grow well above the water table) and hollow species (which grow at water level). Its growing tips (capitula) typically maintain water content around 1100–1400% of dry weight under optimal conditions, though this can drop to around 500% during dry periods. This moisture content remains relatively consistent at around 1200% (12 grams of water per gram of dry weight), regardless of position relative to the water table. The species' water transport capacity has important ecological implications. Water loss through evaporation can reach up to 6 kg per square meter per day (equivalent to 6 mm depth). Even under such rapid evaporation, the plant's efficient water transport system allows it to maintain stable growth in its characteristic low hummock habitat. This system also benefits neighbouring plants – S. rubellum shows a facilitative effect on other Sphagnum species growing nearby, helping maintain higher moisture levels in species with poorer water retention abilities.
The species shows optimal growth under waterlogged or shallowly inundated conditions, particularly in water depths less than 30 cm (12 in). Laboratory experiments have demonstrated that S. rubellum can recover well from temporary periods of drought or flooding, though prolonged deep inundation may impair growth, especially when carbon dioxide concentrations in the water layer are low. The presence of vascular plant clumps (tussocks) can create favourable microclimatic conditions that help S. rubellum survive drier periods.
Water content strongly influences the plant's functioning. When water content falls below 60%, photosynthesis is negatively affected, and if it drops below 10–20%, recovery becomes difficult. Even small amounts of precipitation (less than 1 mm) can increase the plant's water content and boost productivity. The species achieves higher photosynthetic rates when growing closer to the water table than at higher elevations.
Beyond water relations, temperature also affects growth. The species responds positively to increased temperatures, showing enhanced growth and biomass production at 20 °C (68 °F) compared to 15 °C (59 °F), aligning with its relatively southern distribution. However, it is sensitive to winter stress, displaying reduced growth and photosynthetic efficiency at sub-zero temperatures. It acclimates more slowly to cold than boreal species, likely due to its preference for oceanic regions with milder winter temperatures.
The species also shows distinct responses to air quality. When exposed to elevated ozone levels, S. rubellum experiences reduced growth and lower photosynthetic rates, though it maintains stable chlorophyll levels—indicating moderate resilience. Comparatively, it displayed greater growth in open environments than in charcoal-filtered chambers, suggesting that field conditions may buffer some effects of ozone exposure.

## Species interactions
### Interactions with other Sphagnum species

While S. rubellum and S. capillifolium can occur together and occasionally hybridise, molecular evidence shows they remain genetically distinct even where their populations overlap. This suggests strong reproductive barriers exist beyond their habitat preferences.

### Tree interactions

Sphagnum rubellum has a complex ecological relationship with Scots pine (Pinus sylvestris). The moss provides highly favourable conditions for pine seed germination, with success rates reaching 75% compared to typical rates of 5% in forest environments. However, continued growth of S. rubellum can overwhelm and suppress young pine seedlings. This relationship changes once pines reach about 20 mm in stem diameter, at which point the trees begin to inhibit S. rubellum growth and alter the local habitat conditions, often leading to the elimination of S. rubellum from their immediate vicinity.

### Microhabitat communities

In peat bogs, Sphagnum rubellum provides a relatively dry microhabitat supporting diverse communities of oribatid mites, small soil arthropods involved in decomposition processes. Studies show that S. rubellum and S. magellanicum support similar levels of mite diversity, higher than wetter mosses like S. cuspidatum. Local habitat characteristics, such as vascular plant diversity and groundwater level, strongly influence these mite communities, while geographic distance between bogs has minimal effect.

### Nutrient interactions

Sphagnum rubellum shows complex responses to atmospheric nitrogen deposition. While the species has evolved efficient nutrient recycling mechanisms and can store excess nitrogen, it is sensitive to increased nitrogen input. Under conditions of high nitrogen deposition, it exhibits several physiological changes: increased mass in the growing tip (capitulum), darker green colouration due to increased chlorophyll, and reduced shoot formation. These responses can ultimately lead to reduced growth and production, making the species potentially vulnerable to atmospheric pollution. The plant typically maintains about 0.6% nitrogen and 0.03% phosphorus by weight, with growth limited by either nutrient depending on atmospheric nitrogen levels.
The internal structure of S. rubellum is highly specialised for its habitat. Approximately two-thirds of the plant's dry matter are contained within its leaves, which are only one cell thick. The chlorophyll is primarily contained within an open network of long narrow cells that adjoin other chlorophyll-containing cells only at their ends. These photosynthetic cells are effectively suspended within a fixed structural framework, making the plant's organisation intermediate between planktonic algae and vascular plants. This architecture also influences the plant's relationship with water and carbon dioxide, as CO$_2$ diffusion in water can be a limiting factor for growth.
Like other Sphagnum species, S. rubellum can thrive in environments with uncommonly low concentrations of inorganic nutrients, particularly nitrogen and phosphorus compounds. The plant typically contains about 0.6% nitrogen and 0.03% phosphorus by weight, reflecting the low nutrient levels in its habitat. The species is also well-adapted to highly acidic conditions, commonly growing in environments with pH values below 4.

## Historical biogeography

Sphagnum rubellum underwent a significant range expansion during the Atlantic period (approximately 7500–5000 years before present), coinciding with the development of nutrient-poor (oligotrophic) plant communities across Central and Northern Europe. While the species' early Holocene habitats remain uncertain, fossil evidence shows that Sphagnum spores were abundant in several regions by around 9000 years before present, with notable deposits found in Western Great Britain, Northern France, Southeastern Germany, and Northern Austria.
Genetic studies suggest that S. rubellum likely survived the Last Glacial Period in at least two separate survival areas (refugia) – one in Central Europe and another in Western Europe. This separation has left a lasting legacy in modern populations: British populations show considerably higher genetic diversity than those in Central Europe and Scandinavia. This pattern of reduced genetic diversity in Central European populations is similar to that seen in other plant species that survived in restricted areas during glacial periods.
The influence of past climate conditions on the species' distribution is particularly evident in its glacial survival patterns. During glacial periods, the drier climate would have significantly restricted the available habitat for this moisture-dependent species, likely contributing to the formation of isolated populations in separate refugia. Modern genetic analysis indicates that the Neotropical populations of section Acutifolia (including S. rubellum) originated from a single northern (boreal) ancestor, though the exact relationships between groups remain unclear due to conflicting evidence from different genes.

## Conservation

Sphagnum rubellum is classified as a least-concern species in both Europe and the European Union, where it maintains a stable population trend across much of its range. The species has an extensive distribution, with an estimated extent of occurrence of 12 million square kilometres in Europe, of which approximately 6 million square kilometres are within EU member states.
Regional population patterns vary considerably. The species is frequent in southwestern coastal areas of Norway, Sweden, and Finland, though notably absent from northern regions and rare in Finland's lake district. While widely distributed across the Baltic states of Estonia, Latvia, and Lithuania, it becomes increasingly scarce in central and southeastern Europe.
Despite its overall stable status, the species faces several human-caused (anthropogenic) threats. Primary concerns include habitat degradation through peatland drainage for forestry and agricultural development, commercial harvesting of both peat and Sphagnum species, groundwater depletion, and peatland fires. These pressures are particularly significant in central and southeastern Europe, where the species is comparatively rare. The commercial exploitation of Sphagnum species and peat for agricultural purposes represents an ongoing pressure on populations. Of particular concern is the increasing abundance of invasive vascular plants such as Molinia caerulea in northern peatlands. Research has shown that when dead plant material from these species mixes with S. rubellum, it can accelerate decomposition, potentially undermining the peatland's ability to store carbon.
The conservation status of S. rubellum varies regionally, leading to its inclusion on several national Red Lists. It is categorised as vulnerable in Luxembourg, near threatened in Switzerland, Serbia, and Slovenia, and is given various protective designations in parts of Russia, including classification as a rare species in both the Mari El Republic and Ulyanovsk Oblast. In Germany and Austria, the species is noted as requiring monitoring, though specific threat categories vary between regions. While some populations occur within protected areas, conservation biologists recommend additional research to better understand population dynamics and distribution patterns, particularly in regions where mire habitats are limited or vulnerable.
The species' conservation is complicated by taxonomic uncertainty, as some specimens historically identified as S. rubellum may actually represent S. capillifolium var. rubellum. This taxonomic ambiguity has made it challenging to accurately map the species' distribution in some regions, including the United Kingdom, where the two taxa have not been consistently distinguished in historical records. Given the vast areas covered by Sphagnum species globally, particularly in Siberia and Canada, and the increasing abundance of colonising vascular plant species, better understanding of these species interactions is crucial for effective conservation planning.