Feather Moss Research Articles

An Interseasonal Comparison of Soil Respiration in Xeric and Mesic Pine Forest Ecosystems in Central Siberia

An understanding of how boreal forest composition responds to global environmental changes is an important challenge to predicting the future global carbon balance. Boreal forests are the most significant sink for atmospheric carbon dioxide; however, their sequestration capacity is highly sensitive to ongoing climate changes. The combination of the hydrothermal conditions of a territory strongly regulates its biogeochemical processes. The carbon fluxes in boreal forests are strongly mediated by the ground vegetation cover, composed of mosses (mesic) and lichens (xeric). Despite the concurrence of xeric and mesic vegetation types, their responses to climate variations varies significantly. Soil emission is an informative indicator of ecosystem functioning. In this study, we focused on the soil CO2 dynamics during frost-free seasons with different precipitation regimes in the xeric and mesic boreal ecosystems of Central Siberia. Seasonal measurements of soil CO2 emissions were conducted during frost-free seasons using the dynamic chamber method. Our findings reveal that the precipitation regimes of each year may control the seasonal soil emission dynamics. The soil moisture is the most important driver of emissions growth in the water-limited lichen pine forest (R2adj. = 18%). The soil temperature plays the largest role in the feather moss pine forest during the dry (R2adj. = 31%) seasons, and in the lichen pine forest during the wet (R2adj. = 41%) seasons. The cumulative efflux for the xeric and mesic sites is mostly related to the hydrothermal conditions, and not to the differences in ground vegetation cover. During the dry seasons, on average, the soil CO2 emissions are 45% lower than during the wet seasons for both sites. These findings emphasize the need for estimating and including the hydrothermal characteristics of the growing season for detailed emission assessments.

A first look into moss living tardigrades in boreal peatlands.

Tardigrades (Tardigrada) are a phylum of micrometazoans found in all biomes on Earth, but their ecology and habitat preferences remain vastly understudied. Boreal peatlands include a diversity of habitat types and high structural heterogeneity that represents an interesting system to study some of the poorly known habitat preferences of tardigrades. Here, we investigate for the first time tardigrade communities in peatland mosses and the latter's potential associations with key environmental variables. We collected 116 moss samples from 13 sites representing different peatland types and management histories. We found that tardigrades are common and diverse in boreal peatlands, as tardigrades were present in 72% of the collected samples and we identified 14 tardigrade genera. Tardigrade abundance seemed to increase alongside the increasing tree basal area and the density was higher in the microtopographic level further from the water table level, that is, hummocks (mean 117/moss gram) than in lawns/hollows (mean 84/moss gram). Furthermore, the highest tardigrade density was found in the moss taxa that are associated with forested peatland types (i.e., feather mosses) (321 mean/moss gram). Finally, we found interesting patterns regarding tardigrade functional diversity, as carnivorous tardigrades were found only in peatlands with tree basal area > 20 m2 and mostly in hummocks. Our study demonstrates that the habitat heterogeneity of peatlands (e.g., variation in moisture and vegetation cover) represents an interesting system to study tardigrade ecology and habitat preferences. However, since we found variation in tardigrade abundance and communities across peatland types and microhabitats within peatlands, our results highlight that such studies should be conducted with numerous replicate samples and a systematic study design that properly addresses the habitat heterogeneity between and within different peatland types.

Moss species and precipitation mediate experimental warming stimulation of growing season N2 fixation in subarctic tundra.

Climate change in high latitude regions leads to both higher temperatures and more precipitation but their combined effects on terrestrial ecosystem processes are poorly understood. In nitrogen (N) limited and often moss-dominated tundra and boreal ecosystems, moss-associated N2 fixation is an important process that provides new N. We tested whether high mean annual precipitation enhanced experimental warming effects on growing season N2 fixation in three common arctic-boreal moss species adapted to different moisture conditions and evaluated their N contribution to the landscape level. We measured insitu N2 fixation rates in Hylocomium splendens, Pleurozium schreberi and Sphagnum spp. from June to September in subarctic tundra in Sweden. We exposed mosses occurring along a natural precipitation gradient (mean annual precipitation: 571-1155 mm) to 8 years of experimental summer warming using open-top chambers before our measurements. We modelled species-specific seasonal N input to the ecosystem at the colony and landscape level. Higher mean annual precipitation clearly increased N2 fixation, especially during peak growing season and in feather mosses. For Sphagnum-associated N2 fixation, high mean annual precipitation reversed a small negative warming response. By contrast, in the dry-adapted feather moss species higher mean annual precipitation led to negative warming effects. Modelled total growing season N inputs for Sphagnum spp. colonies were two to three times that of feather mosses at an area basis. However, at the landscape level where feather mosses were more abundant, they contributed 50% more N than Sphagnum. The discrepancy between modelled estimates of species-specific N input via N2 fixation at the moss core versus ecosystem scale, exemplify how moss cover is essential for evaluating impact of altered N2 fixation. Importantly, combined effects of warming and higher mean annual precipitation may not lead to similar responses across moss species, which could affect moss fitness and their abilities to buffer environmental changes.

First evidence of nitrogen fixation associated with bryophytes from coastal Wabanaki–Acadian forests

Associations between bryophytes and dinitrogen (N2)-fixing bacteria are a significant source of exogenous N in unmanaged boreal and possibly temperate ecosystems. However, the extent to which biological N2-fixation (BNF) applies to the boreal–temperate ecotone remains elusive. The current focus on common species limits our understanding of BNF at the community level. Our objective was to characterize the presence of cyanobacteria and BNF activity associated with bryophytes in the coastal forests of Fundy National Park (New Brunswick, Canada). In 2021, we harvested three liverwort and 11 moss species from two sites (71 samples) and measured environmental covariates (e.g., canopy composition and soil pH). We used stable isotope incubations with 15N2 gas in growth chambers to quantify potential BNF activity and used phycocyanin extractions as a cyanobacteria abundance proxy. Many species presented detectable BNF rates, which were similar to or higher than those of well-studied feather mosses. These included species that have rarely been found to contribute to BNF. While cyanobacteria were present on most samples, we found no positive association between abundance and BNF. Our findings are among the first records for bryophyte-associated BNF in the boreal–temperate ecotone of eastern Canada and offer insights into the potential role of this process in N cycling in coastal conifer-dominated forests.

The Feather Moss Hylocomium splendens Affects the Transcriptional Profile of a Symbiotic Cyanobacterium in Relation to Acquisition and Turnover of Key Nutrients

Moss-cyanobacteria symbioses were proposed to be based on nutrient exchange, with hosts providing C and S while bacteria provide N, but we still lack understanding of the underlying molecular mechanisms of their interactions. We investigated how contact between the ubiquitous moss Hylocomium splendens and its cyanobiont affects nutrient-related gene expression of both partners. We isolated a cyanobacterium from H. splendens and co-incubated it with washed H. splendens shoots. Cyanobacterium and moss were also incubated separately. After 1 week, we performed acetylene reduction assays to estimate N2 fixation and RNAseq to evaluate metatranscriptomes. Genes related to N2 fixation and the biosynthesis of several amino acids were up-regulated in the cyanobiont when hosted by the moss. However, S-uptake and the biosynthesis of the S-containing amino acids methionine and cysteine were down-regulated in the cyanobiont while the degradation of selenocysteine was up-regulated. In contrast, the number of differentially expressed genes in the moss was much lower, and almost no transcripts related to nutrient metabolism were affected. It is possible that, at least during the early stage of this symbiosis, the cyanobiont receives few if any nutrients from the host in return for N, suggesting that moss–cyanobacteria symbioses encompass relationships that are more plastic than a constant mutualist flow of nutrients.

Response of the soil microbial communities to forest ground cover manipulation in a boreal forest

In eastern Canada, boreal forests are locally experiencing a shift between two alternative stable states, productive closed-canopy feather moss (Pleurozium schreberi (Brid.) Mitt.) forests to low-productivity open lichen (Cladonia spp.) woodlands. While this shift has important consequences for ecosystem structure and productivity, little is known about the changes occurring in the diversity and composition of the soil microbial community which may be driven by this process. We evaluated the effects of 10-year moss transplantation on soil microbial communities in an open-lichen woodland. Treatments included: 1) removal of the lichen cover, 2) removal of the lichen cover followed by transplantation of a feather moss cover, 3) a control with the lichen cover kept in place (lichen control), and 4) a natural forest site with a feather moss cover (moss control). We found that changing the forest ground cover has a significant impact on the diversity, composition and function of soil microbial communities. Fungal alpha diversity was more sensitive to changes in lichen and moss cover, compared to bacterial diversity. Soil microbial community composition showed significant differences among all forest ground covers, but with greater similarities between the moss transplantation and control moss treatments. More importantly, changes of forest ground cover significantly affected the structure of microbial communities and fungal functional groups. Moss transplantation significantly increased the relative abundance of the organic nitrogen-scavenging fungal genus, Piloderma. Furthermore, moss transplantation significantly increased the overall relative abundance of ectomycorrhizal fungi and decreased the proportion of ericoid mycorrhizal fungi. Soil moisture and temperature were the main environmental variables associated to the shift in microbial community composition. Our study points out that moss transplantation in open-canopy lichen woodlands contributes to regulate and modify the composition, structure, and function of the soil microbial communities with potential implications for explaining the changes in ecosystem processes associated with these two forest ecosystems.

Natural seeding as an alternative to planting in black spruce-lichen woodlands

Regeneration failures in the closed-crown boreal forest, which lead to the creation of open lichen woodlands, are expected to increase with climate change due to a rise in burn rates. Past studies have shown that it was possible to restore the productivity of those stands through planting. Since most lichen woodland stands are remotely located, afforestation through planting can be prohibitive. Therefore, the objective of this research was to measure the afforestation efficiency of lichen woodlands through scarification and natural seeding after clear-cutting. Our results showed that clear-cutting, when combined with scarification, promoted seedling establishment more strongly than clear-cutting alone. Scarification was also necessary for creating suitable microsites for germination, which were essentially bare mineral soil and decomposed organic matter. Last, seedling growth was higher in lichen woodlands when logging and scarification were combined, but lower than their counterpart feather moss stands. We conclude that afforestation of lichen woodlands in the closed-crown boreal forest is possible through natural seeding following scarification and could be a cost-effective option compared to planting.

Long-Term Simulated Nitrogen Deposition Has Moderate Impacts on Soil Microbial Communities across Three Bioclimatic Domains of the Eastern Canadian Forest

The soil microbiome plays major roles in the below-ground processes and productivity of forest ecosystems. Atmospheric nitrogen (N) deposition is predicted to increase globally and might create disturbances in soil microbial communities, essentially by modifying soil chemistry. However, the impacts of higher N deposition on the soil microbiome in N-limited northern forests are still unclear. For 16 years, we simulated N deposition by adding ammonium nitrate at rates of 3 and 10 times the ambient N deposition directly into soils located in three bioclimatic domains of the eastern Canadian forest (i.e., sugar maple–yellow birch, balsam fir–white birch, and black spruce–feather moss). We identified changes in the microbial communities by isolating the DNA of the L, F, and H soil horizons, as well as by sequencing amplicons of the bacterial 16S rRNA gene and the fungal ITS region. We found that long-term increased N deposition had no effect on soil microbial diversity, but had moderate impacts on the composition of the bacterial and fungal communities. The most noticeable change was the increase in ectomycorrhizal fungi ASV abundance, potentially due to increased tree root growth on fertilized plots. Our work suggests that, in N-limited northern forests, extra N is rapidly mobilized by vegetation, thus minimizing impacts on the soil microbiome.

The Role of Forest Stands Characteristics on Formation of Exterior Migratory Outbreak Spots by the Siberian Silk Moth Dendrolimus sibiricus (Tschetv.) during Population Collapse

The characteristics of Abies sibirica Ledeb.- and Pinus sibirica Du Tour dominated forests stands in outbreak spots formed during a Dendrolimus sibiricus outbreak in 2014–2017 were studied at the stage of population collapse (east of the West Siberian Plain and western foothills of the Yenisei Range). The research was based on the data obtained during ground surveys conducted in 2016 when stands characteristics, the Siberian silk moth population density, and defoliation level were recorded. We classified the studied stands using decision trees and random forest algorithms to identify the key characteristics that determine the formation of outbreak spots. The classification results showed that the characteristics of the detected outbreak spots differ significantly from those previously described for dark coniferous stands of the southern taiga in Siberia. The highest probability of the outbreak spot occurrence in the study area was revealed for stands with the following characteristics: moderately moist site; Siberian stone pine and Siberian fir take 40% or more of stem volume; age of less than 105 years. Another group of forest stands under threat are those sharing the following characteristics: large area (more than 60 ha); stand homogeneity; ground vegetation is dominated by feather mosses; age of more than 120 years; Picea obovata Ledeb. takes a significant share in a stem volume. Such characteristics indicate that during the population collapse, the Siberian silk moth forms outbreak spots in stands that either undergo an early successional stage after previous outbreak or have been slightly damaged during the previous outbreaks due to unfavorable habitat conditions.

Understory functional groups and fire history but not experimental warming drive tree seedling performance in unmanaged boreal forests

IntroductionSurvival and growth of tree seedlings are key processes of regeneration in forest ecosystems. However, little is known about how climate warming modulates seedling performance either directly or in interaction with understory vegetation and post-fire successional stages.MethodsWe measured survival (over 3 years) and growth of seedlings of three tree species (Betula pubescens, Pinus sylvestris, and Picea abies) in a full-factorial field experiment with passive warming and removal of two plant functional groups (feather moss and/or ericaceous shrubs) along a post-fire chronosequence in an unmanaged boreal forest.ResultsWarming had no effect on seedling survival over time or on relative biomass growth. Meanwhile, moss removal greatly increased seedling survival overall, while shrub removal canceled this effect for B. pubescens seedlings. In addition, B. pubescens and P. sylvestris survival benefitted most from moss removal in old forests (>260 years since last fire disturbance). In contrast to survival, seedling growth was promoted by shrub removal for two out of three species, i.e., P. sylvestris and P. abies, meaning that seedling survival and growth are governed by different understory functional groups affecting seedling performance through different mechanism and modes of action.DiscussionOur findings highlight that understory vegetation and to a lesser extent post-fire successional stage are important drivers of seedling performance while the direct effect of climate warming is not. This suggests that tree regeneration in future forests may be more responsive to changes in understory vegetation or fire regime, e.g., indirectly caused by warming, than to direct or interactive effects of rising temperatures.

Out of site, out of mind: Changes in feather moss phyllosphere microbiota in mine offsite boreal landscapes.

Plant-microbe interactions play a crucial role in maintaining biodiversity and ecological services in boreal forest biomes. Mining for minerals, and especially the emission of heavy metal-enriched dust from mine sites, is a potential threat to biodiversity in offsite landscapes. Understanding the impacts of mining on surrounding phyllosphere microbiota is especially lacking. To investigate this, we characterized bacterial and fungal communities in the phyllosphere of feather moss Pleurozium schreberi (Brid). Mitt in boreal landscapes near six gold mine sites at different stages of the mine lifecycle. We found that (1) both mining stage and ecosystem type are drivers of the phyllosphere microbial community structure in mine offsite landscapes; (2) Bacterial alpha diversity is more sensitive than fungal alpha diversity to mining stage, while beta diversity of both groups is impacted; (3) mixed and deciduous forests have a higher alpha diversity and a distinct microbial community structure when compared to coniferous and open canopy ecosystems; (4) the strongest effects are detectable within 0.2 km from operating mines. These results confirmed the presence of offsite effects of mine sites on the phyllosphere microbiota in boreal forests, as well as identified mining stage and ecosystem type as drivers of these effects. Furthermore, the footprint was quantified at 0.2 km, providing a reference distance within which mining companies and policy makers should pay more attention during ecological assessment and for the development of mitigation strategies. Further studies are needed to assess how these offsite effects of mines affect the functioning of boreal ecosystems.

Initiating the transition from open-canopy lichen woodland to productive forest by transplanting moss, results from a 10-year experiment

Background and aimsLow productivity open lichen (Cladonia spp.) woodlands have been rapidly expanding in the closed-crown feather moss (Pleurozium schreberi (Brid.) Mitt.) boreal forest of eastern Canada. While open-woodland areas are progressing, there is little information on the recoverability of open lichen woodlands back to closed-canopy forests.MethodsAn experimental set-up using moss transplantation was installed on a poor jack pine (Pinus banksiana Lamb.) stand with a lichen ground cover in 2011. Treatments included: 1) lichen cover removed, 2) lichen cover removed and transplantation of a feather moss cover, 3) lichen control, and 4) a natural jack pine site with feather moss cover (moss control). We extracted tree stem increment cores and collected needles and soil samples for nutrient analysis.ResultsThe transplanted-moss treatment can counteract the adverse effects of lichen on jack pine growth. This treatment enhanced foliar nutrition and soil nutrients, especially ammonium (N-NH4+) and nitrate (N-NO3−). With this treatment, the soil conditions (e.g., soil nutrients, soil moisture) and foliar nutrition were closer to that of moss control. Surprisingly, lichen removal treatment did not improve growth and resulted in poorer jack pine growth and harsher soil conditions.ConclusionFeather moss can establish, survive, and remain healthy in an environment previously occupied by lichen. The replacement of lichen by feather moss establishes soil conditions that appear conducive to better tree growth and have the potential of restoring the productivity of boreal forests in open-canopy lichen woodlands.

Nitrogen fixation and nifH gene diversity in cyanobacteria living on feather mosses in a subalpine forest of Mt. Fuji

In the boreal forests, feather mosses such as Hylocomium splendens and Pleurozium schreberi are colonized by cyanobacteria, which provide large amounts of nitrogen to forest ecosystems through nitrogen fixation. Although these feather mosses are also ubiquitous in subalpine forests of East Asia, little is known regarding their associated cyanobacteria and their ability to fix nitrogen. In this study, we investigated (1) whether cyanobacteria co-exist and fix nitrogen in the two species of feather mosses that cover the ground surface in a subalpine forest of Mt. Fuji, (2) whether cyanobacteria belonging to a common cluster with boreal forests are found in feather mosses in Mt. Fuji, and (3) whether moss-associated nitrogen fixation rates differed among moss growing substrates, canopy openness, and moss nitrogen concentrations in the same forest area. Our results showed that cyanobacteria colonized feather mosses in the subalpine forests of Mt. Fuji and acetylene reduction rates as an index of nitrogen fixation tended to be higher in H. splendens than in P. schreberi. Based on analysis of the nifH gene, 43 bacterial operational taxonomic units (OTUs) were identified, 28 of which represented cyanobacteria. Among the five clusters of cyanobacteria classified based on their nifH gene and identified in northern Europe, four (Nostoc cluster I, Nostoc cluster II, Stigonema cluster, and nifH2 cluster) were also found at Mt. Fuji. The acetylene reduction rate differed depending on the moss growing substrate and the total nitrogen concentration of moss shoots, and a strong negative correlation was observed with the total nitrogen concentration.

Mounding treatments set back bryophyte recovery on linear disturbances in treed peatlands

Treed peatlands are dominated by bryophytes, particularly Sphagnum spp., which create the characteristic hummock‐hollow microtopography. This, in turn, shapes the distribution of bryophyte communities. Disturbances can lead to a loss of this microtopographic variation, impacting the bryophyte community. Seismic lines are deforested linear disturbances (approximately 3–8 m wide) created during oil and gas exploration and very extensive in Alberta, Canada. Loss of microtopographic variation leads to slow tree recovery on seismic lines in treed peatlands. To facilitate tree establishment, mounding is being applied to restore the microtopography on seismic lines. We compared bryophyte communities on unmounded and mounded seismic lines (18 years after seismic line creation, 3 years after mounding), and in adjacent treed fens as the reference, to evaluate how recovery varies with microtopographic position (i.e., top of hummock, slope or side of the hummock, and level ground). We found that recovery on the unmounded seismic lines was underway 18 years after seismic lines were created, while mounding redisturbed the sites and thus set back recovery, at least in the short term (3 years post‐mounding). In unmounded seismic lines, Sphagnum cover was similar to that of reference treatments at all microtopographic positions. In contrast, mounded seismic lines had lower Sphagnum cover and higher cover of true mosses (sans feather mosses) than reference and unmounded treatments. Overall, these results show mounding sets back the recovery of bryophyte communities—the very organisms responsible for the microtopographic variation we are trying to establish. Further monitoring is needed to understand how this changes over time.

Synergistic effects in mine offsite landscapes: Predicted ecosystem shifts could exacerbate mining effects on bryophyte community structure

Global change is shifting ecosystem type relative abundance in boreal forests, while the green energy transition results in increased mining activities around the globe. The interaction and consequent effects of these two trends on biodiversity have not been examined in depth. Bryophytes species can be used as indicators to measure these effects because they play key ecological roles in boreal forests. We identified and evaluated the interaction between ecosystem type (i.e., coniferous, deciduous, mixed forest and open canopy) and mining on microhabitat scale bryophyte diversity and composition in 1-km landscapes surrounding six mine sites at different stages of the mining lifecycle in the Canadian boreal forest. Irrespective of microhabitat type, the combined effects of ecosystem type and mining stage were interactive on bryophytes. Bryophyte richness and community composition were negatively affected by offsite effects of mines in only deciduous and mixed forests. The interacted effects on bryophyte richness mainly occurred on the ground r microhabitats. We also found that deciduous, mixed forests (coniferous forest as a reference) and mines had a negative impact on the abundance of feather mosses and sphagna. Furthermore, indicator species were identified for areas affected by mines (Pohlia nutans and Dicranum polysetum) and for control areas (Sphagnum angustifolium and Plagiomnium cuspidatum). Our results suggest the predicted ecosystem shifts with global changes, from coniferous to deciduous forests, could potentially increase the effects of mining on forest ecosystem resistance through the changes in bryophyte community structure. Adding microhabitats (i.e., adding coarse woody debris) near mine sites is a potential strategy to maintain species richness. Collectively, these findings advance our understanding of how mining affects biodiversity and highlight the importance of considering mine offsite landscapes in future environmental evaluations of development projects in the context of global changes.

Unraveling global and diazotrophic bacteriomes of boreal forest floor feather mosses and their environmental drivers at the ecosystem and at the plant scale in North America.

Feather mosses are abundant cryptogams of the boreal forest floor and shelter a broad diversity of bacteria who have important ecological functions (e.g., decomposition, nutrient cycling). In particular, nitrogen (N2-) fixation performed by feather moss-associated diazotrophs constitutes an important entry of nitrogen in the boreal forest ecosystem. However, the composition of the feather moss bacteriome and its environmental drivers are still unclear. Using cDNA amplicon sequencing of the 16S rRNA and nifH genes and cyanobacterial biomass quantification, we explored the active global and diazotrophic bacterial communities of two dominant feather moss species (i) at the ecosystem scale, along a 500-km climatic and nutrient deposition gradient in the North American boreal forest, and (ii) at the plant scale, along the moss shoot senescence gradient. We found that cyanobacteria were major actors of the feather moss bacteriome, accounting for 33% of global bacterial communities and 65% of diazotrophic communities, and that several cyanobacterial and methanotrophic genera were contributing to N2-fixation. Moreover, we showed that bacteria were occupying ecological niches along the moss shoot, with phototrophs being dominant in the apical part and methanotrophs being dominant in the basal part. Finally, climate (temperature, precipitation), environmental variables (moss species, month, tree density) and nutrients (nitrogen, phosphorus, molybdenum, vanadium, iron) strongly shaped global and diazotrophic bacteriomes. In summary, this work presents evidence that the feather moss bacteriome plays crucial roles in supporting moss growth, health, and decomposition, as well as in the boreal forest carbon and nitrogen cycles. This study also highlights the substantial effects of climate and nutrients on the feather moss bacteriome, suggesting the importance of understanding the impacts of global change on moss-associated bacterial growth and activity.

Chemical Stimulation of Heterocyte Differentiation by the Feather Moss Hylocomium splendens: a Potential New Step in Plant-Cyanobacteria Symbioses.

Cyanobacteria associated with mosses play a key role in the nitrogen (N) cycle in unpolluted ecosystems. Mosses have been found to release molecules that induce morphophysiological changes in epiphytic cyanobionts. Nevertheless, the extent of moss influence on these microorganisms remains unknown. To evaluate how mosses or their metabolites influence N2 fixation rates by cyanobacteria, we assessed the nitrogenase activity, heterocyte frequency and biomass of a cyanobacterial strain isolated from the feather moss Hylocomium splendensand a non-symbiotic strain when they were either growing by themselves, together with H. splendens or exposed to H. splendens water, acetone, ethanol, or isopropanol extracts. The same cyanobacterial strains were added to another moss (Taxiphyllum barbieri) and a liverwort (Monosolenium tenerum) to assess if these bryophytes affect N2 fixation differently. Although no significant increases in nitrogenase activity by the cyanobacteria were observed when in contact with H. splendens shoots, both the symbiotic and non-symbiotic cyanobacteria increased nitrogenase activity as well as heterocyte frequency significantly upon exposure to H. splendens ethanol extracts. Contact with T. barbieri shoots, on the other hand, did lead to increases in nitrogenase activity, indicating low host-specificity to cyanobacterial activity. These findings suggest that H. splendens produces heterocyte-differentiating factors (HDFs) that are capable of stimulating cyanobacterial N2 fixation regardless of symbiotic competency. Based on previous knowledge about the chemical ecology and dynamics of moss-cyanobacteria interactions, we speculate that HDF expression by the host takes place in a hypothetical new step occurring after plant colonization and the repression of hormogonia.

Bryophyte species differ widely in their growth and N2-fixation responses to temperature

Bryophytes are abundant in tundra ecosystems, where they affect carbon and nitrogen cycling through primary production and associations with N2-fixing bacteria. Bryophyte responses to climate warming are inconclusive, likely because species-specific responses are poorly understood. Here we investigated how warming affects the growth and nitrogenase activity of 10 tundra bryophyte species in two tundra landscapes. Collected bryophyte samples were grown in temperature-controlled growth chambers for 12 weeks at five temperatures from 3 to 18 °C. We measured growth, N concentration, δ15N, and δ13C after 3 months and nitrogenase activity after 5 and 12 weeks. Bryophyte growth and associated nitrogenase activity generally increased with temperature, but species differed in their optima. Bryophyte N concentration and δ15N indicated that, for some species, increased N2-fixation could not compensate for growth-induced N limitation. High landscape coverage and large positive warming effects on feather mosses and Sphagnum species highlight their competitive advantages, confirm earlier field observations, and contribute to the mechanistic understanding of differential bryophyte growth in response to warming. We suggest that indirect effects of climate change, such as surface drying and shrub expansion, are likely main threats to slow-growing bryophytes across the Arctic, with consequences for biodiversity and C balance.

Dominance of coniferous and broadleaved trees drives bacterial associations with boreal feather mosses.

The composition of ecologically important moss-associated bacterial communities seems to be mainly driven by host species but may also be shaped by environmental conditions related with tree dominance. The moss phyllosphere has been studied in coniferous forests while broadleaf forests remain understudied. To determine if host species or environmental conditions defined by tree dominance drives the bacterial diversity in the moss phyllosphere, we used 16S rRNA gene amplicon sequencing to quantify changes in bacterial communities as a function of host species (Pleurozium schreberi and Ptilium crista-castrensis) and forest type (coniferous black spruce versus deciduous broadleaf trembling aspen) in eastern Canada. The overall composition of moss phyllosphere was defined by the interaction of both factors, though most of the bacterial phyla were determined by a strong effect of forest type. Bacterial α-diversity was highest in spruce forests, while there was greater turnover (β-diversity) and higher γ-diversity in aspen forests. Unexpectedly, Cyanobacteria were much more relatively abundant in aspen than in spruce forests, with the cyanobacteria family Nostocaceae differing the most between forest types. Our results advance the understanding of moss-associated microbial communities among coniferous and broadleaf deciduous forests, which are important with the increasing changes in tree dominance in the boreal system.

New insights into the drivers of moss‐associated nitrogen fixation and cyanobacterial biomass in the eastern Canadian boreal forest

Abstract Moss‐associated cyanobacteria nitrogen (N2‐) fixation can contribute to support moss growth and constitutes a major source of new N in boreal forest ecosystems. The biomass of moss‐colonizing cyanobacteria and their N2‐fixation are usually considered linearly correlated. Yet, recent evidence has shown that cyanobacterial biomass and N2‐fixation can be decoupled, suggesting that they are not necessary affected by the same environmental and ecological drivers. Climate and nutrients were reported as affecting moss‐associated N2‐fixation, with equivocal results, whereas drivers of moss cyanobacterial biomass remain unclear. In addition, these drivers are often determined through manipulative experiments (e.g. fertilization and incubation) and remain to be validated with complementary observational studies to help us better understand future impacts of global change on the moss–cyanobacteria symbiosis. We hypothesized that moss‐associated cyanobacterial biomass is controlled in situ by factors affecting bacterial growth, whereas N2‐fixation is controlled by factors affecting enzymatic reactions. Using random forests, Spearman correlations and linear mixed‐effects models, we determined the main drivers of cyanobacterial biomass and N2‐fixation of two feather moss species, which were collected over 3 years along a 1000‐km latitudinal transect in the eastern Canadian boreal forest. We found that temperature, precipitation and phosphorus were the main positive drivers of moss cyanobacterial biomass and that temperature and molybdenum were the main positive drivers of N2‐fixation. Vanadium was a negative driver of N2‐fixation, suggesting the use of alternative nitrogenases by cyanobacteria. Both cyanobacterial biomass and N2‐fixation were strongly influenced by the moss species and were negatively correlated with moss C:N stoichiometry, highlighting the role of N2‐fixation in moss N enrichment. Synthesis. We identified for the first time some environmental drivers of moss‐associated cyanobacterial biomass and showed that they contrast with the drivers of N2‐fixation, which should be considered in further research and confirmed in other experimental settings. This is an important advance in our knowledge of moss–cyanobacteria associations, which would greatly help in better predicting the impacts of global change on this symbiosis and on nitrogen inputs in boreal forest ecosystems.