Feather Moss Research Articles

Broadleaf Litter Controls Feather Moss Growth in Black Spruce and Birch Forests of Interior Alaska

Plant–soil feedbacks can maintain or reinforce alternative states within ecological systems. In Alaskan boreal forests, changes in fire characteristics have stimulated the replacement of needle-leaf black spruce (Picea mariana) by broadleaf deciduous trees. Feather mosses have strong associations with forest type: They dominate black spruce forest understories and are uncommon in broadleaf stands, with consequences for nutrient cycling and carbon storage. Here we test a long-standing hypothesis that broadleaf litter directly excludes mosses with a field experiment in broadleaf paper birch (Betula neoalaskana) and black spruce stands. We established 30 plots (15 each in birch and spruce dominated areas) with three Hylocomium splendens transplants treated with one of three treatments in each plot (ambient leaf litter deposition, birch leaf litter exclusion or addition), and 30 natural H. splendens areas. We measured moss growth and reproductive potential over 3 years. A 1-year experiment assessed leaf leachate and physical structure impacts on moss growth. Moss shoot growth in natural patches was larger in spruce than in birch stands (24.8 vs. 17.3 mg) and H. splendens made large contributions to ecosystem productivity in spruce stands. In both stand types, we observed a 40% reduction in moss biomass between litter addition and exclusion treatments and litter additions decreased sporophyte production. We found no difference in growth for mosses treated for 1 year with leaf leachates or physical litter structures. Leaf litter effects appear strong enough to exclude mosses from broadleaf forests, providing experimental support for hypothesized plant–soil interactions that may stabilize alternate forest types.

Terrestrial Bryophyte and Lichen Responses to Canopy Opening in Pine-Moss-Lichen Forests

Pinus contorta-dominated montane forests of western Canada with relatively dense tree canopies have ground layers with abundant bryophytes, especially the feather mosses (Pleurozium schreberi and Hylocomium splendens), while those with more open canopies are dominated by species of reindeer lichens, especially Cladonia arbuscula s.l. and C. rangiferina s.l. Woodland caribou (Rangifer tarandus caribou), which are a threatened species in Alberta, prefer open, Cladonia-dominated forests for their winter food supply. This study investigated if opening the canopy by thinning mature montane forests of the Canadian Rocky Mountain foothills would change the abundance of lichens and bryophytes. In 1997, forests were thinned by removing 20%, 40%, and 60% by volume. In 2016, 19 years after treatment, we re-surveyed a subset of these plots (n = 97) for lichen and bryophyte abundance and species richness by utilizing the amount of canopy opening at the plot level as our prime gradient. We then used ordination to determine the relationship of control plots to treatment plots. In uncut forest, the control plots were highly variable, but were mostly dominated by feather mosses, with little or no bare ground. Feather moss abundance was lower in treatment plots when compared to control plots, while cover of bare ground was greater. Overall, 19 years after treatment, we found that, in treatment plots, lichen abundance remained stable or slightly increased, feather mosses decreased markedly, and unoccupied space was double that of the control plots. We conclude that the canopy opening had little effect on understory and ground layer diversity, but considering species abundance (1) bryophytes have not recovered after canopy opening, (2) populations of reindeer lichens increased marginally, but have not colonized areas left bare from bryophyte dieback, and (3), after 19 years there, remains unoccupied areas of bare ground in plots with a reduced canopy cover. Our study demonstrated that, with canopy cover reduction resulting from forest thinning operations, the ground layer diversity is maintained, but recovery of ground layers in old-growth pine-dominated forests is not promoted. Therefore, timber harvest that partially opens the tree canopy is unlikely to benefit caribou by augmenting or accelerating winter food availability and habitat suitability for caribou.

Severe wildfire exposes remnant peat carbon stocks to increased post-fire drying

The potential of high severity wildfires to increase global terrestrial carbon emissions and exacerbate future climatic warming is of international concern. Nowhere is this more prevalent than within high latitude regions where peatlands have, over millennia, accumulated legacy carbon stocks comparable to all human CO2 emissions since the beginning of the industrial revolution. Drying increases rates of peat decomposition and associated atmospheric and aquatic carbon emissions. The degree to which severe wildfires enhance drying under future climates and induce instability in peatland ecological communities and carbon stocks is unknown. Here we show that high burn severities increased post-fire evapotranspiration by 410% within a feather moss peatland by burning through the protective capping layer that restricts evaporative drying in response to low severity burns. High burn severities projected under future climates will therefore leave peatlands that dominate dry sub-humid regions across the boreal, on the edge of their climatic envelopes, more vulnerable to intense post-fire drying, inducing high rates of carbon loss to the atmosphere that amplify the direct combustion emissions.

N2 fixation associated with the bryophyte layer is suppressed by low levels of nitrogen deposition in boreal forests

Biological fixation of atmospheric nitrogen (N2) by bryophyte-associated cyanobacteria is an important source of plant-available N in the boreal biome. Information on the factors that drive biological N2 fixation (BNF) rates is needed in order to understand the N dynamics of forests under a changing climate. We assessed the potential of several cryptogam species (the feather mosses Hylocomium splendens and Pleurozium schreberi, a group of Dicranum bryophytes, two liverworts, and Cladina lichens) to serve as associates of cyanobacteria or other N2-fixing bacteria (diazotrophs) using acetylene reduction assay (ARA). We tested the hypotheses that the legacy of chronic atmospheric N deposition reduces BNF in the three bryophyte species, sampled from 12 coniferous forests located at latitudes 60–68° N in Finland. In addition, we tested the effect of moisture and temperature on BNF. All species studied showed a BNF signal in the north, with the highest rates in feather mosses. In moss samples taken along the north–south gradient with an increasing N bulk deposition from 0.8 to 4.4 kg ha−1 year−1, we found a clear decrease in BNF in both feather mosses and Dicranum group. BNF turned off at N deposition of 3–4 kg ha−1 year−1. Inorganic N (NH4-N + NO3-N) best predicted the BNF rate among regression models with different forms of N deposition as explanatory variables. However, in southern spruce stands, tree canopies modified the N in throughfall so that dissolved organic N (DON) leached from canopies compensated for inorganic N retained therein. Here, both DON and inorganic N negatively affected BNF in H. splendens. In laboratory experiments, BNF increased with increasing temperature and moisture. Our results suggest that even relatively low N deposition suppresses BNF in bryophyte-associated diazotrophs. Further, BNF could increase in northern low-deposition areas, especially if climate warming leads to moister conditions, as predicted.

Lichens Contribute to Open Woodland Stability in the Boreal Forest Through Detrimental Effects on Pine Growth and Root Ectomycorrhizal Development

In the boreal forest, open lichen woodlands have been described as an alternative stable state to closed-crown feather moss forest. In this study, we addressed the role of terricolous lichens in stabilizing open woodlands by hindering tree regeneration and/or growth. Based on field and greenhouse experiments, we compared germination and growth of jack pine (Pinus banksiana) on feather mosses (primarily Pleurozium schreberi) and lichens (primarily Cladonia stellaris), using bare mineral soil as a control. Drivers were investigated by (1) manipulating nutrient supply, (2) simulating shade of a closed canopy on the ground layer with the assumption this would mitigate lichen influence on pine growth, and (3) examining pine root ectomycorrhizal colonization and diversity as indicators of pine ability to take up nutrients. Total growth of 6-month-old greenhouse and 2–3-year-old field seedlings, as well as belowground growth of 2-year-old greenhouse seedlings, was significantly greater in moss than in lichen. Seed germination was not affected by ground cover type. Although field phosphorus and base cation availability was greater in mosses than in lichens, fertilization did not entirely compensate for the negative effects of lichens on pine growth in the greenhouse. Ground layer shading had no impact on pine growth. Lichens were associated with reduced abundance and modified composition of the root ectomycorrhizal community. By suggesting that terricolous lichens constitute a less favorable growth substrate than mosses for pine, our results support the hypothesis that lichens contribute to open woodland stability in the potentially closed-crown feather moss forest.

Mosses Are Better than Leaves of Vascular Plants in Monitoring Atmospheric Heavy Metal Pollution in Urban Areas

Mosses and leaves of vascular plants have been used as bioindicators of environmental contamination by heavy metals originating from various sources. This study aims to compare the metal accumulation capabilities of mosses and vascular species in urban areas and quantify the suitability of different taxa for monitoring airborne heavy metals. One pleurocarpous feather moss species, Haplocladium angustifolium, and two evergreen tree species, Cinnamomum bodinieri Osmanthus fragrans, and substrate soil were sampled in the urban area of different land use types in Wuhan City in China. The concentrations of Ag, As, Cd, Co, Cr, Cu, Mn, Mo, Ni, V, Pb, and Zn in these samples were analyzed by inductively coupled plasma mass spectrometry. The differences of heavy metals concentration in the three species showed that the moss species was considerably more capable of accumulating heavy metals than tree leaves (3 times to 51 times). The accumulated concentration of heavy metals in the moss species depended on the metal species and land use type. The enrichment factors of metals for plants and the correlations of metals in plants with corresponding metals in soil reflected that the accumulated metals in plants stemmed mostly from atmospheric deposition, rather than the substrate soil. Anthropogenic factors, such as traffic emissions from automobile transportation and manufacturing industries, were primarily responsible for the variations in metal pollutants in the atmosphere and subsequently influenced the metal accumulation in the mosses. This study elucidated that the moss species H. angustifolium is relatively more suitable than tree leaves of C. bodinieri and O. fragrans in monitoring heavy metal pollution in urban areas, and currently Wuhan is at a lower contamination level of atmospheric heavy metals than some other cities in China.

Effects of substrate availability and competing vegetation on natural regeneration of white spruce on logged boreal mixedwood sites

Given a seed source, the quality of available substrates is a key factor in determining the success of white spruce (Picea glauca (Moench) Voss) natural regeneration. We examined the influence of substrate and competing vegetation on survival and growth of natural regeneration of white spruce up to 4 years following harvesting in deciduous-dominated upland boreal mixedwood sites. Feather moss, thick soil surface organic layers, litter, and solid wood were poor substrates for establishment. Early successional mosses establishing on mineral soil, thin organics, and rotten wood were generally favourable microsites but were not highly available on postharvest sites. Mineral soil substrates were not as suitable as expected, likely because on a postlogged site, they are associated with unfavourable environmental characteristics (e.g., low nutrient availability, exposure). There was some evidence that survival and growth of seedlings were improved by surrounding vegetation in the first years, but heavy competing vegetation had a negative impact on older seedlings. Burial by aspen litter greatly increased seedling mortality, especially when combined with a brief period of submergence due to heavy spring snowmelt. The results provide insight into conditions under which natural regeneration could be an option for establishing white spruce following harvesting of deciduous-dominated boreal mixedwood forests.

Range Extensions of 35 Bryophyte Species in the Black Spruce–Feather Moss Forest of Western Quebec, Canada

Although the North American bryophyte flora are relatively well known, bryophytes of the Black Spruce–feather moss forest in the Nord-du-Québec administrative region, especially its southern portion (49–51°N, 74–79°W), remain under-sampled. Here, we report 169 bryophyte taxa for this region, of which 35 (14 true mosses, 20 liverworts, one sphagnum) represent noteworthy records, including 20 taxa new for the region. These new occurrences close several gaps in distribution in the study area and, more broadly, in the boreal Black Spruce (Picea mariana) forest of adjacent ontario. Microhabitat preferences of the species are also documented. This work represents a substantial contribution to knowledge of the bryophyte flora, which will help refine protection priority ranks of species of Quebec and Labrador.

Differential effects of feather and Sphagnum spp. mosses on black spruce germination and growth

Abstract The composition of the bryophyte layer influences boreal forest regeneration and growth through its effect on soil conditions. Canopy openings in boreal spruce stands can favor the expansion of Sphagnum spp. in the understory at the expense of feather mosses. We use an experimental approach in both paludified field and greenhouse fully-randomized conditions to examine the differential effects of these two ground cover types on black spruce germination and growth, specifically the role of nutrient limitation in generating these effects. We also tested the impact of ground cover shading, simulating the effect of a closed forest canopy on the ground layer, with the assumption that the stress induced to mosses, especially to Sphagnum spp., would have a beneficial impact on tree growth. The two moss types had no differential effects on spruce germination and 0–6-month-old seedling growth in the greenhouse. However, the growth of 2-year-old seedlings in the greenhouse was lower in Sphagnum spp. than in feather mosses. This negative effect was removed by fertilization, suggesting that soil nutrient availability could explain the seedling growth difference between moss types. Greenhouse 2-year-old seedlings also allocated a greater proportion of biomass to roots in Sphagnum spp. than in feather mosses. In the field, feather and Sphagnum spp. mosses had no differential effects on 3-year-old seedling growth, and ground cover shading did not have any short-term positive impact on spruce growth. Although they were not validated in the field, the results we obtained in the greenhouse suggest that the replacement of feather mosses by Sphagnum spp. mosses do not only affect spruce growth through the build-up of an organic layer often associated with low soil temperature and excess water, but also through more direct effects on nutrient availability. Therefore, silvicultural treatments that would favor Sphagnum spp. expansion at the expense of feather mosses, such as partial or total harvesting with protection of regeneration and soils, may result in subsequent tree growth problems even in sites with moderate organic layer accumulation.

Anthropogenic deposition of heavy metals and phosphorus may reduce biological N2 fixation in boreal forest mosses

A study was undertaken to test the effects of molybdenum (Mo) and phosphorus (P) amendments on biological nitrogen (N) fixation (BNF) by boreal forest moss-associated cyanobacteria. Feather moss (Pleurozium schreberi) samples were collected on five sites, on two dates and at different roadside distances (0–100m) corresponding to an assumed gradient of reactive N deposition. Potential BNF of Mo and P amended moss samples was measured using the acetylene reduction assay. Total N, P and heavy metal concentrations of mosses collected at 0 and 100m from roadsides were also measured. Likewise, the needles from Norway spruce trees (Picea abies) at different roadside distances were collected in late summer and analyzed for total N, P and heavy metals. There was a significant increase in BNF with roadside distance on 7-of-10 individual Site×Date combinations. We found no clear evidence of an N gradient across roadside distances. Elemental analyses of feather moss and Norway spruce needle tissues suggested decreasing deposition of heavy metals (Mo-Co-Cr-Ni-V-Pb-Ag-Cu) as well as P with increasing distance from the roadside. The effects of Mo and P amendments on BNF were infrequent and inconsistent across roadside distances and across sites. One particular site, however, displayed greater concentrations of heavy metals near the roadside, as well as a steeper P fertility gradient with roadside distance, than the other sites. Here, BNF increased with roadside distance only when moss samples were amended with P. Also at this site, BNF across all roadside distances was higher when mosses were amended with both Mo and P, suggesting a co-limitation of these two nutrients in controlling BNF. In summary, our study showed a potential for car emissions to increase heavy metals and P along roadsides and underscored the putative roles of these anthropogenic pollutants on BNF in northern latitudes.

Symplasmic and apoplasmic transport inside feather moss stems of Pleurozium schreberi and Hylocomium splendens.

The ubiquitous feather mosses Pleurozium schreberi and Hylocomium splendens form a thick, continuous boundary layer between the soil and the atmosphere, and play important roles in hydrology and nutrient cycling in tundra and boreal ecosystems. The water fluxes among these mosses and environmental factors controlling them are poorly understood. The aim of this study was to investigate whether feather mosses are capable of internal transport and to provide a better understanding of species-specific morphological traits underlying this function. The impacts of environmental conditions on their internal transport rates were also investigated. Cells involved in water and food conduction in P. schreberi and H. splendens were identified by transmission electron microscopy. Symplasmic and apoplasmic fluorescent tracers were applied to the moss stems to determine the routes of internal short- and long-distance transport and the impact of air humidity on the transport rates. Symplasmic transport over short distances occurs via food-conducting cells in both mosses. Pleurozium schreberi is also capable of apoplasmic internal long-distance transport via a central strand of hydroids. These are absent in H. splendens. Reduced air humidity significantly increased the internal transport of both species, and the increase was significantly faster for P. schreberi than for H. splendens. Pleurozium schreberi and Hylocomium splendens are capable of internal transport but the pathway and conductivity differ due to differences in stem anatomy. These results help explain their varying desiccation tolerance and possibly their differing physiology and autecology and, ultimately, their impact on ecosystem functioning.

Sensitivity of carbon stores in boreal forest moss mats - effects of vegetation, topography and climate

In northern regions, moss and lichen mats are the major carbon-cycling interface between soils and the atmosphere. We aimed to quantify sensitivity of ground layer nutrient stores to environmental predictors, to better understand interactions with vegetation, topography and climatic conditions. With non-destructive forest inventory techniques, we estimated distributions of biomass, carbon and nitrogen among moss/lichen ground layers in a 1.1 million-ha watershed within Alaska’s boreal forest region. Using nonparametric multiplicative regression, we fit response surfaces and quantified sensitivity to environmental predictors. Across 96 sites, half the ground layer biomass values were in the range 4750–18,900 kg ha−1 (25th to 75th percentiles). Carbon and nitrogen stores peaked in older stands and those with little forb cover (suggesting low disturbance) and low incident radiation. Among functional groups, the most abundant were nitrogen-fixing feather mosses, which formed extensive carpets. Nutrient stores were most sensitive to local vegetation and topography predictors, but less sensitive to regional climate. Moss and lichen mats in boreal forests are substantial carbon and nitrogen stores, with consequences for carbon sequestration and ecosystem productivity. Their environmental sensitivity suggests that ground layer nutrient stores could decrease if global changes promote vascular vegetation expansions and intensifying wildfire regimes.

Low Evapotranspiration Enhances the Resilience of Peatland Carbon Stocks to Fire

AbstractBoreal peatlands may be vulnerable to projected changes in the wildfire regime under future climates. Extreme drying during the sensitive postfire period may exceed peatland ecohydrological resilience, triggering long‐term degradation of these globally significant carbon stocks. Despite these concerns, we show low peatland evapotranspiration at both the plot‐ and landscape‐scale postfire, in water‐limited peatlands dominated by feather moss that are ubiquitous across continental western Canada. Low postfire evapotranspiration enhances the resilience of carbon stocks in such peatlands to wildfire disturbance and reinforces their function as a regional source of water. Near‐surface water repellency may provide an important, previously unexplored, regulator of peatland evapotranspiration that can induce low evapotranspiration in the initial postfire years by restricting the supply of water to the peat surface.

Fine‐scale distribution of moisture in the surface of a degraded blanket bog and its effects on the potential spread of smouldering fire

AbstractDuring rain‐free periods, the water table in peatlands falls and the moisture content (MC) of top 10 cm, including the moss layer, depends upon the hydrophysical properties and the responses of the local composition of species to water deficit. Thus, the ecosystem becomes vulnerable to surface peat fires. The spread of such fires is often irregular; however, there is little understanding of how the fine‐scale variation of peatMCcan affect the spread of smouldering fire.We analyse the fine‐scale distribution of surfaceMCfrom a degraded blanket bog (i.e., horizontal intervals of 10 cm), thus indirectly analysing the effect of fine‐scaleMCdistribution on peat fire spread. We determine the relationship of vegetation and microtopography to the spatial distribution of near surfaceMCand the moisture gradient around patches of dry peat (less than 250%MC, moisture content in a dry mass basis). We found that theMCof the surface peat was distributed in clusters with dry patches (less than 250%MC) averaging 40 ± 15 cm in size. TheMCgradient surrounding dry patches was 10 ± 7%MC∙cm−1. A mixed‐effects model showed that dry patches were associated with lawns (of feather moss orSphagnum) and hummocks of feather moss. Our model showed that wet patches were associated with both hummocks and hollows ofSphagnum. Therefore, the characterization of the surfaceMCin a fine‐scale is critical if we are to understand the spread of fires in peatlands.

Restoration of reindeer lichen pastures after forest fire in northern Sweden: Seven years of results

Fire suppression since the 19th century has modified the functioning of boreal forest ecosystems in northern Sweden. In the long run, it induces changes in understory vegetation that result, especially on mesic sites, in feather mosses and Ericaceous dwarf shrubs outcompeting ground lichens, thus decreasing winter pastures for reindeer husbandry. In 2008, a field experiment was established in which, two years after a large forest fire, reindeer lichen (Cladonia spp.) was transplanted across various plots. The general objective was to accelerate the recovery of lichen-rich reindeer pastures, and test the ability of lichen thalli to establish on burnt surfaces following different post-fire treatments (tree harvest and standing trees retained), in different transplantation seasons (summer and winter) and at different doses (0.45Lm−2 and 2.25Lm−2). The abundance and level of occupancy of viable and established lichen fragments was determined in 0.25m2 quadrats in 2010, 2013 and 2015. There was a continuous increase in lichen establishment over time, and seven years after transplantation, established lichen occupied, on average, 55% and 83% of the 0.25m2 quadrats treated with the lower and higher doses, respectively. Nine years after fire, no lichen had colonized the control, i.e. the burnt surface outside the experimental area. At the site with standing trees retained, lichen had already formed a well-established mat with a significantly higher lichen occupancy and abundance than in the open, clear-cut sites, where lichen agglomerated in proto-mats. Lichen transplanted in late-summer exhibited higher abundance and occupancy than that transplanted in late-winter. On average, the difference in lichen occupancy and abundance between different doses after seven years was of lower magnitude than between the doses of lichen transplanted initially. The experiment reveals useful results for the restoration of reindeer pastures and for specifying fire management regimes adapted to both forestry and reindeer husbandry.

Boreal bryophyte response to natural fire edge creation

AbstractQuestionChanges in species richness and composition in plant communities as a result of edge creation are well documented in anthropogenically modified landscapes, but what happens after natural disturbance? We investigate the responses of boreal bryophytes to edge creation in a post‐wildfire residual forest patch system.LocationBoreal black spruce–feather moss forest, western Québec, Canada.MethodsBryophyte community was sampled in 50‐m2 plots: 117 plots in 39 undisturbed forest cores (control) and 108 plots in 30 residual forest patches from wildfire, divided into 48 core plots, 30 north‐facing edge plots and 30 south‐facing edge plots. Temporal, severity, spatial and structural characteristics of the stands were also recorded to explain bryophyte community composition.ResultsResidual edges and cores were more species‐rich than undisturbed cores, particularly north‐facing edges, but community composition differed between undisturbed and residual cores. Indicator species of undisturbed cores and residual edges differed in their species traits. Spatial variables explained variations in composition, with a larger difference between core and edge communities in patches over 3 ha compared to smaller patches.ConclusionWe observed community composition change in response to natural edge creation. While edge influence is reduced in large patches, the bryophyte community remains distinct from undisturbed cores. This community change is a ‘control’ in studies of boreal bryophyte response to anthropogenic edge creation, and suggests that we should consider the natural range of variability. Finally, this study emphasizes that forest remnants do not substitute for continuous and undisturbed forests.

Methane emission from feather moss stands.

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH4 . However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH4 turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH4 cycling. Therefore, we investigated methanogenic rates and pathways together with the methanogenic microbial community composition in feather moss stands from temperate and boreal forests. Potential rates of CH4 emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH4 h-1 gdw-1 . Temperature and water content positively influenced CH4 emission. Methanogenic potentials determined under N2 atmosphere in darkness ranged between 22 and 157 pmol CH4 h-1 gdw-1 . Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH4 was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 104 and 105 copies per gram dry weight moss of the mcrA gene coding for a subunit of the methyl CoM reductase. Archaeal 16S rRNA and mcrA gene sequences in the moss stands were characteristic for the archaeal families Methanobacteriaceae and Methanosarcinaceae. The potential methanogenic rates were similar in incubations with and without methyl fluoride, indicating that the CH4 was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH4 produced was depleted in 13 C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3-62 mM). The δ13 C of acetate was similar to that of the moss biomass, indicating acetate production by fermentation. Our study showed that the feather moss stands contained active methanogenic microbial communities producing CH4 by hydrogenotrophic methanogenesis and causing net emission of CH4 under ambient conditions, albeit at low rates.

Molybdenum and phosphorus limitation of moss-associated nitrogen fixation in boreal ecosystems.

Biological nitrogen fixation (BNF) performed by moss-associated cyanobacteria is one of the main sources of new nitrogen (N) input in pristine, high-latitude ecosystems. Yet, the nutrients that limit BNF remain elusive. Here, we tested whether this important ecosystem function is limited by the availability of molybdenum (Mo), phosphorus (P), or both. BNF in dominant mosses was measured with the acetylene reduction assay (ARA) at different time intervals following Mo and P additions, in both laboratory microcosms with mosses from a boreal spruce forest and field plots in subarctic tundra. We further used a 15 N2 tracer technique to assess the ARA to N2 fixation conversion ratios at our subarctic site. BNF was up to four-fold higher shortly after the addition of Mo, in both the laboratory and field experiments. A similar positive response to Mo was found in moss colonizing cyanobacterial biomass. As the growing season progressed, nitrogenase activity became progressively more P limited. The ARA:15 N2 ratios increased with increasing Mo additions. These findings show that N2 fixation activity as well as cyanobacterial biomass in dominant feather mosses from boreal forests and subarctic tundra are limited by Mo availability.

Long-Term Recovery of Microbial Communities in the Boreal Bryosphere Following Fire Disturbance

Our study used a ∼360-year fire chronosequence in northern Sweden to investigate post-fire microbial community dynamics in the boreal bryosphere (the living and dead parts of the feather moss layer on the forest floor, along with the associated biota). We anticipated systematic changes in microbial community structure and growth strategy with increasing time since fire (TSF) and used amplicon pyrosequencing to establish microbial community structure. We also recorded edaphic factors (relating to pH, C and N accumulation) and the physical characteristics of the feather moss layer. The molecular analyses revealed an unexpectedly diverse microbial community. The structure of the community could be largely explained by just two factors, TSF and pH, although the importance of TSF diminished as the forest recovered from disturbance. The microbial communities on the youngest site (TSF = 14years) were clearly different from older locations (>100years), suggesting relatively rapid post-fire recovery. A shift towards Proteobacterial taxa on older sites, coupled with a decline in the relative abundance of Acidobacteria, suggested an increase in resource availability with TSF. Saprotrophs dominated the fungal community. Mycorrhizal fungi appeared to decline in abundance with TSF, possibly due to changing N status. Our study provided evidence for the decadal-scale legacy of burning, with implications for boreal forests that are expected to experience more frequent burns over the course of the next century.

Persistent reduction of segment growth and photosynthesis in a widespread and important sub‐Arctic moss species after cessation of three years of experimental winter warming

SummaryWinter is a period of dormancy for plants of cold environments. However, winter climate is changing, leading to an increasing frequency of stochastic warm periods (winter warming events) and concomitant reductions in snow cover. These conditions can break dormancy for some plants and expose them to freeze‐and‐thaw stress. Mosses are a major component of high‐latitude ecosystems, yet the longer‐term impacts of such winter warming events on mosses remain unknown.In order to determine the longer‐term legacy effects of winter warming events on mosses, we undertook a simulation of these events over three consecutive winters in a sub‐Arctic dwarf shrub‐dominated open woodland. The mat‐forming feather moss,Hylocomium splendens(the most abundant cryptogam in this system), is one of the most widespread Arctic and boreal mosses and plays a key functional role in ecosystems. We studied the ecophysiological performance of this moss during the summers of the experimental period (2007–2009) and in the following years (2010–2013).We show that the previously reported warming‐induced reduction in segment growth and photosynthesis during the experimental years was persistent. Four years after the last event, photosynthesis and segment growth were still 30 and 36% lower than control levels, which was only a slight improvement from 44 and 43% 4 years earlier. Winter warming did not affect segment symmetry. During the years after the last simulated event, in both warmed and control plots, chlorophyll fluorescence and segment growth, but not net photosynthesis, increased slightly. The increases were probably driven by increased summer rainfall over the study years, highlighting the sensitivity of this moss to rainfall change.Overall, the legacy effects shown here demonstrate that this widespread and important moss is likely to be significantly disadvantaged in a future sub‐Arctic climate where frequent winter warming events may become the norm. Given the key importance of mosses for soil insulation, shelter and carbon sequestration in high‐latitude regions, such persistent impacts may ultimately affect important ecosystem functions.ALay Summaryis available for this article.