Arctic Moss Research Articles

High Arctic Vegetation Communities With a Thick Moss Layer Slow Active Layer Thaw

AbstractSvalbards permafrost is thawing as a direct consequence of climate change. In the Low Arctic, vegetation has been shown to slow down and reduce the active layer thaw, yet it is unknown whether this also applies to High Arctic regions like Svalbard where vegetation is smaller, sparser, and thus likely less able to insulate the soil. Therefore, it remains unknown which components of High Arctic vegetation impact active layer thaw and at which temporal scale this insulation could be effective. Such knowledge is necessary to predict and understand future changes in active layer in a changing Arctic. In this study we used frost tubes placed in study grids located in Svalbard with known vegetation composition, to monitor the progression of active layer thaw and analyze the relationship between vegetation composition, vegetation structure and snow conditions, and active layer thaw early in summer. We found that moss thickness, shrub and forb height, and vascular vegetation cover delayed soil thaw immediately after snow melt. These insulating effects attenuated as thaw progressed, until no effect on thaw depth was present after 8 weeks. High Arctic mosses are expected to decline due to climate change, which could lead to a loss in insulating capacity, potentially accelerating early summer active layer thaw. This may have important repercussions for a wide range of ecosystem functions such as plant phenology and decomposition processes.

The genus Brachysira (Brachysiraceae, Bacillariophyceae) in the Arctic with the description of two new species

ABSTRACT Two unknown diatoms belonging to the genus Brachysira were found during a survey of the moss-inhabiting diatom flora in Canada, Greenland, Iceland and Spitsbergen. Detailed light and scanning electron microscopy revealed sufficient morphological differentiation in valve dimensions and shape, striae density, and ultrastructure for them to be described as new species. The description of Brachysira louiseboydiana sp. nov. and B. hutchisoniana sp. nov. brings the total number of Brachysira species globally to 132. Formal descriptions are presented, each illustrated by light and scanning electron micrographs, and the new species are compared with similar Brachysira taxa worldwide. Additionally, four previously described species which were also observed in the Arctic mosses are discussed. Our findings contribute to a better understanding of the Arctic terrestrial diatom flora.

De Novo Transcriptome Assembly and Comparative Analysis of Differentially Expressed Genes Involved in Cold Acclimation and Freezing Tolerance of the Arctic Moss Aulacomnium turgidum (Wahlenb.) Schwaegr.

Cold acclimation refers to a phenomenon in which plants become more tolerant to freezing after exposure to non-lethal low temperatures. Aulacomnium turgidum (Wahlenb.) Schwaegr is a moss found in the Arctic that can be used to study the freezing tolerance of bryophytes. To improve our understanding of the cold acclimation effect on the freezing tolerance of A. turgidum, we compared the electrolyte leakage of protonema grown at 25 °C (non-acclimation; NA) and at 4 °C (cold acclimation; CA). Freezing damage was significantly lower in CA plants frozen at -12 °C (CA-12) than in NA plants frozen at -12 °C (NA-12). During recovery at 25 °C, CA-12 demonstrated a more rapid and greater level of the maximum photochemical efficiency of photosystem II than NA-12, indicating a greater recovery capacity for CA-12 compared to NA-12. For the comparative analysis of the transcriptome between NA-12 and CA-12, six cDNA libraries were constructed in triplicate, and RNA-seq reads were assembled into 45,796 unigenes. The differential gene expression analysis showed that a significant number of AP2 transcription factor genes and pentatricopeptide repeat protein-coding genes related to abiotic stress and the sugar metabolism pathway were upregulated in CA-12. Furthermore, starch and maltose concentrations increased in CA-12, suggesting that cold acclimation increases freezing tolerance and protects photosynthetic efficiency through the accumulation of starch and maltose in A. turgidum. A de novo assembled transcriptome can be used to explore genetic sources in non-model organisms.

Effects of temperature and nutrient enrichment on the Arctic moss Hygrohypnella ochracea growth and productivity

Effects of temperature and nutrient enrichment on the Arctic moss Hygrohypnella ochracea growth and productivity

Host-specificity of moss-associated fungal communities in the Ny-Ålesund region (Svalbard, High Arctic) as revealed by amplicon pyrosequencing

Fungal communities play a significant role in regulating ecological processes in the Arctic tundra. However, the extent to which the Arctic moss species and host types (moss, lichen and vascular plant) determine the richness, diversity, and composition of fungal communities at a local scale has not been quantitatively explored. Using 454 pyrosequencing in the current study, we characterized the fungal communities associated with six moss species (Andreaea rupestris, Bryum pseudotriquetrum, Hymenoloma crispulum, Polytrichastrum alpinum, Racomitrium lanuginosum, and Sanionia uncinata) and compared them with fungal communities associated with lichens and vascular plants in the Ny-Ålesund region (High Arctic). Host-species preference had greater explanatory power than geographical factors (longitude, latitude, elevation) in shaping moss-associated fungal communities. Fungal communities associated with mosses differed significantly from those associated with vascular plants and lichens, suggesting specificity of the fungal communities among three host types. Pairwise comparison analysis also indicated that the relative abundance of many taxonomic groups (e.g., Chaetothyriales, Leotiales, Catenulifera, Alatospora, and Toxicocladosporium) significantly differed between mosses and the other two host types. These results suggest host factors significantly affect the distribution of the fungal species associated with these moss species in the local-scale Arctic tundra.

Cynodontium luthii sp. nov.: a permineralized moss gametophyte from the Late Cretaceous of the North Slope of Alaska.

Mosses are a major component of Arctic vegetation today, with >500 species known to date. However, the origins of the Arctic moss flora are poorly documented in the fossil record, especially prior to the Pliocene. Here, we present the first anatomically preserved pre-Cenozoic Arctic moss and discuss how the unique biology of bryophytes has facilitated their success in polar environments over geologic time. A permineralized fossil moss gametophyte within a block of Late Cretaceous terrestrial limestone, collected along the Colville River on the North Slope of Alaska, was studied in serial sections prepared using the cellulose acetate peel technique. The moss gametophyte is branched and has leaves with a broad base, narrow blade, and excurrent costa. We describe this fossil as Cynodontium luthii sp. nov., an extinct species of a genus that is known from the High Arctic today. Cynodontium luthii is the oldest evidence of the family Rhabdoweisiaceae (by ≥18 Ma) and reveals that genera of haplolepideous mosses known in the extant Arctic flora also lived in high-latitude temperate deciduous forests during the Late Cretaceous. The occurrence of C. luthii in Cretaceous sediments, together with a rich Pliocene-to-Holocene fossil record of extant moss genera in the High Arctic, suggests that some moss lineages have exploited their poikilohydric, cold- and desiccation-tolerant physiology to live in the region when it experienced both temperate and freezing climates.

Transfection of Arctic Bryum sp. KMR5045 as a Model for Genetic Engineering of Cold-Tolerant Mosses.

Mosses number about 13,000 species and are an important resource for the study of the plant evolution that occurred during terrestrial colonization by plants. Recently, the physiological and metabolic characteristics that distinguish mosses from terrestrial plants have received attention. In the Arctic, in particular, mosses developed their own distinct physiological features to adapt to the harsh environment. However, little is known about the molecular mechanisms by which Arctic mosses survive in extreme environments due to the lack of basic knowledge and tools such as genome sequences and genetic transfection methods. In this study, we report the axenic cultivation and transfection of Arctic Bryum sp. KMR5045, as a model for bioengineering of Arctic mosses. We also found that the inherent low-temperature tolerance of KMR5045 permitted it to maintain slow growth even at 2°C, while the model moss species Physcomitrium patens failed to grow at all, implying that KMR5045 is suitable for studies of cold-tolerance mechanisms. To achieve genetic transfection of KMR5045, some steps of the existing protocol for P. patens were modified. First, protoplasts were isolated using 1% driselase solution. Second, the appropriate antibiotic was identified and its concentration was optimized for the selection of transfectants. Third, the cell regeneration period before transfer to selection medium was extended to 9 days. As a result, KMR5045 transfectants were successfully obtained and confirmed transfection by detection of intracellular Citrine fluorescence derived from expression of a pAct5:Citrine transgene construct. This is the first report regarding the establishment of a genetic transfection method for an Arctic moss species belonging to the Bryaceae. The results of this study will contribute to understanding the function of genes involved in environmental adaptation and to application for production of useful metabolites derived from stress-tolerant mosses.

The complete mitogenome of the Arctic moss Aulacomnium turgidum (Wahlenb.) Schwaegr

The Arctic moss Aulacomnium turgidum (Wahlenb.) Schwaegr. is distributed widely above the Arctic Circle and can regenerate successfully after 400 years of ice entombment. Here, we report the complete mitogenome sequence of A. turgidum (103,937 bp). The genome contains 3 ribosomal RNAs, 24 transfer RNAs, and 40 protein-encoding genes. In a phylogenetic tree generated using the combined amino acid sequences of 32 mitochondrial genes from A. turgidum, 25 Bryophyta, and three Marchantiophyta, the phylogenetic position of A. turgidum (Rhizogoniales) is close to that of the Hypnales and Ptychomniales, forming a monophyletic clade with perfect supporting values.

Reconstruction of annual growth in relation to summer temperatures and translocation of nutrients in the aquatic moss Drepanocladus trifarius from West Greenland

Mosses often dominate the submerged vegetation in Arctic lakes and ponds, making them essential contributors to the primary production in these habitats. However, little is known about the factors controlling annual growth of Arctic mosses and their sensitivity to climatic changes. It has been suggested that nutrient translocation occurs in mosses, and that annual growth of mosses therefore depends strongly on weather conditions and less on local environmental conditions. In this study, we examined annual growth of Drepanocladus trifarius ((F. Weber and D. Mohr) Broth.) from two ponds in West Greenland in relation to weather conditions. A reconstruction of annual growth increments from 2009 to 2014 was made in 200 individual mosses, and biomass and length were related to different weather parameters. In addition, we examined whether there would be an indication of nutrient translocation across annual growth segments. We found a positive relationship between mean summer temperature and growth segment length, which indicates the importance of temperature during seasons with sufficient light levels for growth of the D. trifarius. Weather parameters associated with light conditions had no significant effect on growth, which probably reflect that D. trifarius in two shallow ponds were not light limited. The nutrient stoichiometry showed that phosphorus (P) contents in the tissue were low (0.04–0.11% DW), and nutrient resorption efficiencies of P amounted to 11–29%. This suggests that D. trifarius was P limited during its growth season, but appears capable of nutrient translocation across annual segments, possibly to maintain growth in oligotrophic environments. Despite low nitrogen (N) contents (0.94–2.09%), no resorption of N was found, which indicates that D. trifarius was not N-limited in order to sustain growth. In conclusion, this study shows that growth of D. trifarius in small high Arctic ponds are mainly controlled by summer temperatures.

Short term changes in moisture content drive strong changes in Normalized Difference Vegetation Index and gross primary productivity in four Arctic moss communities

Climate change is currently altering temperature and precipitation totals and timing in Arctic regions. Moss communities constitute much of the understory in Arctic vegetation, and as poikilohydric plants moss are highly sensitive to timing and duration of moisture levels. Here we investigate the role of moisture content on NDVI, red and near-infrared reflectance, and gross primary productivity (GPP) of two sphagnum and two pleurocarpus moss community types during two separate drying experiments. For both experiments, blocks of moss were collected near Imnavait Creek, Alaska, saturated to full water capacity, and then allowed to air dry before being re-saturated. Drying of blocks was conducted in a translucent outdoor tent during the first experiment and under indoor climate-controlled conditions during the second. Community NDVI (experiment 1 and 2), and GPP (experiment 2) were measured at regular intervals during the dry-down and after rewetting. In both experiments, moss NDVI sharply declined between 80% and 70% moisture content for sphagnum moss communities (NDVI change = −0.17 to −0.2), but less so for pleurocarpus moss communities (NDVI change = −0.06 to −0.12). Changes in NDVI were largely the result of increases in reflectance in red wavelengths. Peak GPP for all community types in the second experiment (1.31 to 2.08 μmol m−2 s−1) occurred at 80% moisture content and declined significantly as moisture content decreased. Rates of GPP continued to decline below 80% moisture content until near zero as moss reached a steady weight (air dry) over a period of 84 h, while NDVI values declined slowly between 70% hydration and fully air dry. Re-saturation caused NDVI to increase in both sphagnum (NDVI change = +0.18 to +0.23) and pleurocarpus (NDVI change = +0.10 to +0.17) communities. Only sphagnum communities showed GPP resuming (0.824 μmol m−2 s−1) after 24 h. The strong changes in NDVI and mismatch of moss NDVI values and GPP with moisture content fluctuations indicate that using NDVI as a proxy for productivity in Arctic vegetation communities may be problematic and underscores the need for quantification of moss community coverage, composition, and moisture content.

Metals in Racomitrium lanuginosum from Arctic (SW Spitsbergen, Svalbard archipelago) and alpine (Karkonosze, SW Poland) tundra.

Arctic-alpine tundra habitats are very vulnerable to the input of relatively small amounts of xenobiotics, and thus their level in such areas must be carefully controlled. Therefore, we collected the terrestrial widespread moss Racomitrium lanuginosum (Hedw.) Brid. in Spitsbergen in the Arctic moss lichen tundra and, for comparison, in the Arctic-alpine tundra in the Karkonosze (SW Poland). Concentrations of the elements Cd, Co, Cr, Cu, Fe, Hg, Li, Mn, Mo, Na, Ni, Pb, V, and Zn in this species and in the parent rock material were measured. We tested the following hypothesis: R. lanuginosum from Spitsbergen contains lower metal levels than the species from the Karkonosze collected at altitudes influenced by long-range transport from former Black Triangle industry. Principal component and classification analysis (PCCA) ordination revealed that mosses of Spitsbergen were distinguished by a significantly higher Na concentration of marine spray origin and mosses of Karkonosze were distinguished by significantly higher concentrations of Cd, Cr, Cu, Fe, Hg, Li, Mn, Pb, V, and Zn probably from long-range atmospheric transport. The influence of the polar station with a waste incinerator resulted in significantly higher Co, Li, and Ni concentrations in neighbouring mosses in comparison with this species from other sites. This investigation contributes to the use of R. lanuginosum as a bioindicator for metal contamination in Arctic and alpine tundra regions characterised by severe climate habitats with a restricted number of species. This moss enables the control of pollution usually brought solely by long-range atmospheric transport in high mountains as well as in Arctic areas.

Abundance, richness, and succession of microfungi in relation to chemical changes in Antarctic moss profiles

Little is known in continental Antarctic about patterns of abundance, diversity, and succession of microfungi within moss profiles consisting of live, senescent, and dead tissues in different stages of decomposition. In the present study, vertical patterns of the abundance and diversity of microfungi and their relationship with chemical compositions were examined within moss colonies collected from coastal outcrops in the Lutzow–Holm Bay area (Queen Maud Land), East continental Antarctica. Total and darkly pigmented hyphal length, the richness of molecular operational taxonomic units (MOTUs) of microfungi, and the occurrence of Phoma herbarum and Pseudogymnoascus pannorum increased with the depth of moss profiles. The content of organic chemical components and nitrogen in moss tissues decreased, whereas ash content increased with the depth of moss profiles. Relative amount of recalcitrant compounds and total carbohydrates did not significantly differ among the vertical layers. The downward increase of the microfungal richness and occurrence in the Antarctic moss profiles without MOTU replacement was consistent with the directional-nonreplacement model of succession, indicative of the high environmental resistance, which represents the sum of the adverse factors hindering the success of species establishment. This contrasted with the fungal succession in arctic moss profiles, which accorded with the directional-replacement model, in which species replacement took place due to modification of habitat and competition. More hostile environmental conditions than those in the Arctic characterized the fungal succession and limited the fungal decomposition of moss in continental Antarctica.

Latitudinal zonation of arctic moss flora of the Koryakskoye Upland

Latitudinal zonation of arctic moss flora of the Koryakskoye Upland

Abundance and diversity of fungi in relation to chemical changes in arctic moss profiles

Mosses are a dominant component of high-arctic terrestrial ecosystems, yet little is known regarding the abundance and diversity of fungi associated with these abundant plants. We investigated vertical patterns of abundance and diversity of fungi and their relationship with chemical properties within profiles of Hylocomium splendens and Racomitrium lanuginosum collected in the Oobloyah Bay area on Ellesmere Island, Canada. The moss profiles were divided into 6 (H. splendens) and 5 (R. lanuginosum) layers according to the color and texture, and hyphal length, fungal assemblages, and contents of organic chemical components (acid-unhydrolyzable residues, total carbohydrates, extractives) and nutrients (N, P, K, Ca, Mg) were measured. Total hyphal length was greatest at the middle layers of H. splendens and at the deepest layers of R. lanuginosum and was significantly affected by moss species and nutrient contents. A total of 18 and 19 fungal taxa was isolated from the profiles of H. splendens and R. lanuginosum, respectively, with 11 taxa being common to both moss species. Moss species significantly affected the species distribution of fungi. Individual fungal taxa showed patterns of vertical distribution within the moss profiles. The contents of acid-unhydrolyzable residues and nutrients increased and the content of total carbohydrates decreased down the profile, which was attributable to the ability of fungi to decompose carbohydrates selectively and to immobilize nutrients in decomposed moss residues.

Impacts of long-term enhanced UV-B radiation on bryophytes in two sub-Arctic heathland sites of contrasting water availability.

Anthropogenic depletion of stratospheric ozone in Arctic latitudes has resulted in an increase of ultraviolet-B radiation (UV-B) reaching the biosphere. UV-B exposure is known to reduce above-ground biomass and plant height, to increase DNA damage and cause accumulation of UV-absorbing compounds in polar plants. However, many studies on Arctic mosses tended to be inconclusive. The importance of different water availability in influencing UV-B impacts on lower plants in the Arctic has been poorly explored and might partially explain the observed wide variation of responses, given the importance of water in controlling bryophyte physiology. This study aimed to assess the long-term responses of three common sub-Arctic bryophytes to enhanced UV-B radiation (+UV-B) and to elucidate the influence of water supply on those responses. Responses of three sub-Arctic bryophytes (the mosses Hylocomium splendens and Polytrichum commune and the liverwort Barbilophozia lycopodioides) to +UV-B for 15 and 13 years were studied in two field experiments using lamps for UV-B enhancement with identical design and located in neighbouring areas with contrasting water availability (naturally mesic and drier sites). Responses evaluated included bryophyte abundance, growth, sporophyte production and sclerophylly; cellular protection by accumulation of UV-absorbing compounds, β-carotene, xanthophylls and development of non-photochemical quenching (NPQ); and impacts on photosynthesis performance by maximum quantum yield (F(v) /F(m)) and electron transport rate (ETR) through photosystem II (PSII) and chlorophyll concentrations. Responses were species specific: H. splendens responded most to +UV-B, with reduction in both annual growth (-22 %) and sporophyte production (-44 %), together with increased β-carotene, violaxanthin, total chlorophyll and NPQ, and decreased zeaxanthin and de-epoxidation of the xanthophyll cycle pool (DES). Barbilophozia lycopodioides responded less to +UV-B, showing increased β-carotene and sclerophylly and decreased UV-absorbing compounds. Polytrichum commune only showed small morphogenetic changes. No effect of UV-B on bryophyte cover was observed. Water availability had profound effects on bryophyte ecophysiology, and plants showed, in general, lower growth and ETR, together with a higher photoprotection in the drier site. Water availability also influenced bryophyte responses to +UV-B and, in particular, responses were less detectable in the drier site. Impacts of UV-B exposure on Arctic bryophytes were significant, in contrast to modest or absent UV-B effects measured in previous studies. The impacts were more easily detectable in species with high plasticity such as H. splendens and less obvious, or more subtle, under drier conditions. Species biology and water supply greatly influences the impact of UV-B on at least some Arctic bryophytes and could contribute to the wide variation of responses observed previously.

Characterisation of cold-tolerant fungi from a decomposing High Arctic moss

Given that changes in the patterns of vegetation and size of carbon (C) pools in the Arctic are likely to be profound by the end of this century, it is necessary to characterise the identity and ecological groupings, in terms of temperature response and C substrate utilisation, of saprotrophic (decomposer) fungi in organic matter in Arctic soils. Thus, the aims of this study were: (1) to identify the fungi isolated from standing-dead material of Schistidium apocarpum, as an example of a High Arctic moss, (2) to determine mycelial extension rates of these fungi at a range of temperatures (4, 10 and 25 °C), and (3) to characterise the functional potential, defined by C substrate utilisation at 6 °C, of fungal taxa. Fungi were isolated at 4 °C from standing-dead material of S. apocarpum from an area of polar semi-desert (79 degrees N), close to Ny-Ålesund, Svalbard, in the High Arctic. From a collection of 662 isolates, 43 pure cultures were identified by DNA extraction, amplification, and sequencing. Phoma sclerotioides, previously known as a temperate snow mould, was isolated most frequently. The ecology of fifteen fungal isolates was characterised in detail. In terms of temperature response, two groups were apparent, one of truly psychrophilic/psychrotrophic fungi and one of more mesophilic fungi which were generally less frequently isolated. In terms of carbon substrate utilisation in semi-defined solid media, most fungi could utilise a variety of carbon substrates (degradation of casein, cellulose and starch was widespread), except for tannic acid (degraded by only two of the five P. sclerotioides isolates and Scytalidium lignicola) and lignin and chitin (not decomposed by any isolate). The majority of isolates had been recorded previously from polar environments and/or as being able to survive at low temperatures. Fungi in tundra ecosystems, therefore, have significant potential to mineralise C at temperatures below 10 °C. A better understanding of the ecology of these fungi will allow us to improve predictions of C dynamics in arctic biomes in the future.

Bryophytes as Heavy Metal Biomonitors in the Canadian High Arctic

Mosses are a major component of the tundra flora in the Canadian Arctic, yet their use in arctic contaminant research is lacking. Biomonitoring of atmospheric heavy metal deposition using mosses has been extensively employed in Europe, providing a higher sampling density than precipitation monitoring. Temporal, spatial, and habitat gradients of concentrations and enrichment factors of As, Cd, Cr, Cu, Ni, Zn, and Pb (and its stable isotopes) in mosses from Ellesmere Island are examined. Anthropogenically influenced concentrations of As, Cr, Cu, Ni, and Zn in samples collected in 2007 were observed. Concentrations of heavy metals in hydric taxa were larger than those observed in xeric or mesic taxa, though non-significant. Generally, heavy metal concentrations decreased from 1983 to 2007 in a single high arctic locality, though non-significant. Pb-isotope ratios were radiogenic and characteristic of the High Arctic Islands. Trends in high arctic moss data corresponded with environmental proxies such as glacial ice cores, lake sediments, and atmospheric aerosols illustrating the usefulness of bryophytes as biomonitors. This paper outlines the utility of using mosses as biomonitors of heavy metal depositions in the Canadian High Arctic.

Turnover of recently assimilated carbon in arctic bryophytes

Carbon (C) allocation and turnover in arctic bryophytes is largely unknown, but their response to climatic change has potentially significant impacts on arctic ecosystem C budgets. Using a combination of pulse-chase experiments and a newly developed model of C turnover in bryophytes, we show significant differences in C turnover between two contrasting arctic moss species (Polytrichum piliferum and Sphagnum fuscum). (13)C abundance in moss tissues (measured up to 1 year) and respired CO(2) (traced over 5 days) were used to parameterise the bryophyte C model with four pools representing labile and structural C in photosynthetic and stem tissue. The model was optimised using an Ensemble Kalman Filter to ensure a focus on estimating the confidence intervals (CI) on model parameters and outputs. The ratio of aboveground NPP:GPP in Polytrichum piliferum was 23% (CI 9-35%), with an average turnover time of 1.7 days (CI 1.1-2.5 days). The aboveground NPP:GPP ratio in Sphagnum fuscum was 43% (CI 19-65%) with an average turnover time of 3.1 days (CI 1.6-6.1 days). These results are the first to show differences in C partitioning between arctic bryophyte species in situ and highlight the importance of modelling C dynamics of this group separately from vascular plants for a realistic representation of vegetation in arctic C models.

Quantifying the role of deterministic assembly and stochastic drift in a natural community of Arctic mosses

The interpretation of natural plant communities frequently invokes species-sorting controlled by niche differences along spatial environmental gradients. This process of niche structuring can be explained by reference to functional traits, which provide a mechanistic explanation for community structure. In contrast, models explaining species coexistence obviate the limiting effect of niche difference, by invoking processes which cause species-level drift, e.g. demographic stochasticity. This paper investigates a simple habitat with strong gradients (moss communities in a patterned arctic wetland) to identify signature-patterns under-pinning the relative importance of deterministic assembly and stochastic drift in a natural community. First, ordination analysis was used to confirm community composition structured by a range of nine carefully selected functional traits. Second, to determine whether traits explaining community composition might also explain species richness, local species richness (sR) was compared to (1) observed trait diversity and (2) expected trait diversity based on permutation tests, which are used to simulate null community assembly for different values of sR. Traits explaining species composition, consistent with deterministic niche structuring, do not appear to maintain sR. This surprising result was explained by decomposing the community into individual pair-wise comparisons, i.e. species niche-differences and association (χ 2 ). Results support deterministic processes via the sorting of species with similar and contrasting niches, at opposite ends of a composite environmental gradient. Nevertheless, stochastic drift is apparent in the random structure of a majority of pair-wise associations; in addition, a species' abundance was in general not related to environmental distance from response-optima. We suggest therefore that spatial pattern in the moss community is a balance between deterministic forces with respect to species traits and controlling environmental gradients, and stochastic drift, which weakens this deterministic structure.

Type specimens of Arctic mosses in Robert Brown's herbarium (BM)

Type specimens of Arctic mosses in Robert Brown's herbarium (BM)