Boreal Bryophytes Research Articles

Projected shifts in deadwood bryophyte communities under national climate and forestry scenarios benefit large competitors and impair small species

AbstractAimClimate change and habitat loss are the main threats to forest biodiversity. Deadwood bryophyte communities are composed of species with different functional traits and are affected by these processes. Grouping species depending on their traits can help to anticipate community responses to global change, and to potential conservation actions.LocationNational scale of Sweden.TaxonDeadwood bryophyte species (15 liverworts and 8 mosses).MethodsGeneralized linear mixed‐effects models were applied to test for differences in projected relative changes in habitat suitability (matching species’ requirements) among species with contrasting traits under varying climate and forest scenarios during the years 2020–2100. Projections were based on ensembles of species distribution models (GLM, Poisson point‐process, MaxEnt), climate scenarios and national scenarios of forest management and conservation.ResultsShoot length was the best predictor of projected future changes in habitat suitability. Habitat suitabilities for small, short‐lived species will decline in a warmer and wetter macroclimate, whereas those for large, perennial species will increase. We expect stronger habitat suitability decreases for obligate than for facultative deadwood species. Increasing the proportion of set‐aside forests from 16% to 32%, and reducing the harvest levels in production forests, mitigated negative habitat trends of several sensitive species. However, the potential benefits of increased conservation were even larger for species with traits favoured by climate change, suggesting that these actions will also enhance the spread of these species.Main conclusionsClimate change is expected to lead to shifts in boreal bryophyte communities towards large, competitive species and to an overall decrease in diversity. High investment in conservation actions seems necessary to maintain diversity. This should include increasing the area of forest set aside beyond 16%, and reducing harvest levels in production forests. However, whilst these actions may prevent species extinctions, at least in the short term, changes in community structure seem inevitable.

Warm range margin of boreal bryophytes and lichens not directly limited by temperatures

Abstract Species at their warm range margin are potentially threatened by higher temperatures, but may persist in microrefugia. Whether such microsites occur due to more suitable microclimate or due to lower biotic pressure from, for example competitive species, is still not fully resolved. We examined whether boreal bryophytes and lichens show signs of direct climate limitation, that is whether they perform better in cold and/or humid microclimates at their warm range margin. We transplanted a moss, a liverwort and a lichen to 58 boreal forest sites with different microclimates at the species' southern range margin in central Sweden. Species were grown in garden soil patches to control the effects of competitive exclusion and soil quality. We followed the transplanted species over three growing seasons (2016–2018) and modelled growth and vitality for each species as a function of subcanopy temperature, soil moisture, air humidity and forest type. In 2018, we also recorded the cover of other plants having recolonized the garden soil patches and modelled this potential future competition with the same environmental variables plus litter. Species performance increased with warmer temperatures, which was often conditional on high soil moisture, and at sites with more conifers. Soil moisture had a positive effect, especially on the moss in the last year 2018, when the growing season was exceptionally hot and dry. The lichen was mostly affected by gastropod grazing. Recolonization of other plants was also faster at warmer and moister sites. The results indicate that competition, herbivory, shading leaf litter and water scarcity might be more important than the direct effects of temperature for performance at the species' warm range margin. Synthesis. In a transplant experiment with three boreal understorey species, we did not find signs of direct temperature limitation towards the south. Forest microrefugia, that is habitats where these species could persist regional warming, may instead be sites with fewer competitors and enemies, and with sufficient moisture and more conifers in the overstorey.

Bryophyte species composition at the stand scale (1 ha) – Differences between secondary stands half a century after clear-cutting and older semi-natural boreal forests

A growing proportion of the boreal biome consists of managed even-aged secondary forest stands regenerated after clear-cutting. Many disturbance-intolerant species may not be able to recolonize or reach their original abundance in these stands before the next clear-cutting, potentially causing large-scale biodiversity losses. Boreal bryophytes (mosses and liverworts) include many species intolerant to clear-cutting, and at small spatial scales species richness and occupancy has been shown to remain changed in secondary stands half a century after logging. To assess if such persistent changes occur also at the stand scale, we listed and estimated cover of all bryophyte species in 1-ha plots, comparing 14 secondary stands originating from clear-cutting 40–60 years earlier with 14 older semi-natural stands. The large plots also made it possible to assess differences in occupancy and abundance for more bryophyte species than in previous studies. Species composition differed significantly for both mosses and liverworts, but unlike earlier studies, we could not detect any significant difference between stand types in species numbers. Thirteen species were significantly associated with semi-natural stands and the total cover of liverworts was less than half in secondary stands. Secondary stands had significantly fewer species typically occurring under shady conditions and/or mostly growing on “tree substrates” (dead wood and/or bases and stems of living trees). Ordination analysis further emphasized the importance of shade and suitable deadwood substrates; the among-plot variation in bryophyte species composition was related to amount of coarse deadwood as well as to gradients from shady spruce dominated to open pine dominated stands and from polar- to equator-facing slopes. Besides lack of suitable habitat conditions in secondary stands, dispersal limitation may have caused a colonization time lag for some species. The clear importance of stand scale habitat conditions for bryophyte species composition calls for management adaptions to facilitate life boating and/or recolonization by ensuring availability of shade, coarse decomposing logs, and specific deciduous tree species (Populus, Salix, Sorbus) in secondary stands.

Interannual variation in bryophyte dispersal: linking bryophyte phenophases and weather conditions

In the context of global changes that modify the distribution range of species, there is an urgent need to identify climate variables affecting species dispersal. We investigated patterns of aerial propagule release (sexual and asexual) of boreal bryophytes in response to weather. We present the first community-level study that examines the impact of weather on the phenology of bryophytes, and we divided it into phases. Bryophyte propagule rain was trapped in 2013 (summer and fall) and 2014 (spring and fall), and climatic variables were collated from the years 2012 to 2014. The phases of the phenology and the weather variables one season to two years preceding propagule release, which may influence the dispersal of propagules, were identified. Propagule release varies with weather conditions at the time of dispersal (direct effects), but is also associated with weather during the winter and summer one year preceding dispersal (indirect effects), which presumably influences survival, growth, and fertilization of the mother plant as well as propagule and sporophyte development. We suggest that propagule release is related to weather conditions occurring from one to several previous seasons, particularly humidity, temperature, and length (duration) of winter. Dividing the phenology into phases, we present an innovative method that should provide new insights into bryophyte dispersal dynamics in response to climate.

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.

Are post-fire residual forest patches refugia for boreal bryophyte species? Implications for ecosystem based management and conservation

Residual patches of forest remaining after natural or anthropogenic disturbance may facilitate regeneration of fragmented forest. However, residual patch function remains unclear, especially after natural wildfire. We investigate the role of residual boreal forest patches as refugia for bryophytes and ask the question, do they house bryophyte communities similar to those encountered in undisturbed forests? Bryophytes were sampled in three habitat types in black spruce boreal forests illustrating a gradient of disturbance severity: undisturbed forests, residual patches and burned matrices. Temporal, disturbance severity, spatial and structural variables of habitats were also recorded. Bryophyte community composition differed among habitat types with residual patches characterized by higher species richness, the loss of forest specialists and the addition of disturbance-prone species. The bryophyte community found in residual patches is at the interface between the communities of undisturbed forests and burned matrices. As residual patches did not conserve all species, particularly forest specialists, they were not refugia. However, we identify temporal, spatial and structural characteristics that can maintain bryophyte communities most similar to undisturbed forests and enhance residual patch “refugia potential”. Residual patches enhance bryophyte diversity of the landscape housing species that cannot survive in the burned matrix. As conclusion we discuss the use of retention patches in harvested stands, together with the preservation of undisturbed stands that house singular bryophyte communities and especially sensitive forest specialists.

Disappearance of boreal plants in southern Britain: habitat loss or climate change?

Vascular plant and bryophyte species whose geographical range is mainly in the boreal zone have declined markedly in southern Britain since records began. Some of this decline can be attributed to a loss of habitat rather than climate change. In the present study, we estimate the loss attributable to climate change by setting up paired comparisons. In the zone of Britain with July mean temperature greater than 15 °C in the 30-year period 1961–1990, each boreal species was matched with a more southern one having similar frequency, size, and ecological attributes. Because both members of each pair have similar ecological attributes, they should suffer equally from habitat loss and degradation, allowing change attributable to differences in phytogeographical character to be separated out. Hectad (10 km2) records of presence in the period 2000–2013 were compared with those from 1970–1999. Boreal vascular plants decreased by approximately 41%, whereas their matched more southern species decreased by approximately 24%. For 39 of 53 matched pairs, the boreal partner decreased more than its southern comparator, a disparity most unlikely to be a result of chance. Declines for bryophytes averaged 15% for boreal species and 15% for their partners. We conclude that climate change is now having an effect comparable to that of habitat loss on boreal vascular plants in southern Britain but not on boreal bryophytes.

Stochastic processes dominate during boreal bryophyte community assembly

Why are plant species found in certain locations and not in others? The study of community assembly rules has attempted to answer this question, and many studies articulate the historic dichotomy of deterministic (predictable niches) vs. stochastic (random or semi-random processes). The study of successional sequences to determine whether they converge, as would be expected by deterministic theory, or diverge, as stochastic theory would suggest, has been one method used to investigate this question. In this article we ask the question: Do similar boreal bryophyte communities develop in the similar habitat created by convergent succession after fires of different severities? Or do the stochastic processes generated by fires of different severity lead to different communities? Specifically we predict that deterministic structure will be more important for large forest-floor species than stochastic processes, and that the inverse will be true for small bryophyte species. We used multivariate regression trees and model selection to determine the relative weight of structure (forest structure, substrates, soil structure) and processes (fire severity) for two groups of bryophyte species sampled in 12 sites (seven high-severity and five low-severity fires). Contrary to our first hypothesis, processes were as important for large forest-floor bryophytes as for small pocket species. Fire severity, its interaction with the quality of available habitat, and its impact on the creation of biological legacies played dominant roles in determining community structure. In this study, sites with nearly identical forest structure, generated via convergent succession after high- and low-severity fire, were compared to see whether these sites supported similar bryophyte communities. While similar to some degree, both the large forest-floor species and the pocket species differed after high-severity fire compared to low-severity fire. This result suggests that the "how," or process of habitat generation, influences community composition in this system and that a snapshot of habitat conditions taken at only one point in time is insufficient to explain species distribution.

Linking the biological traits of boreal bryophytes to forest habitat change after partial harvesting

Biological traits are potentially important for understanding mechanisms of plant species responses to alteration of local habitat conditions through natural and anthropogenic disturbance. Forest harvesting is a prominent disturbance in the circumpolar boreal biome, influencing stand- and landscape-scale patterns of forest structure and biodiversity. We examined a range of variable retention harvesting intensities (10%, 50%, and 75% dispersed green-tree retention harvesting and unharvested controls) in terms of their effectiveness for maintaining mosses and liverworts with differing biological traits. Bryophytes were sampled in 20m radius plots 5–6years post-harvest in 24 forest stands (each 10ha) of two forest types (broadleaf-coniferous mixedwood, coniferous-dominated). We first examined the environmental factors that were the strongest predictors of species composition across the forest types and retention levels. We then used fourth-corner analysis to relate differences in the forest environment to species traits. Selected traits included bryophyte group, life form, habitat requirements, and reproductive and dispersal characteristics. The strongest predictors of species composition were ground-level moisture (estimated using growth of the moss Hylocomium splendens) and degree of canopy cover. Fourth-corner analysis showed that forest type, retention level, and their associated moisture conditions were closely related to the abundances of species characterized by different biological traits. Species with rare sporophyte production, larger spores, dioicous sexuality, or that require greater moisture or shade, were affiliated with higher retention and forest moisture. Reduced abundances of species with these traits after harvesting may detrimentally affect their capacity to disperse and re-establish, and suggests that moisture limitation is an important environmental filter that may restrict their representation at harvested sites. Coniferous-dominated forests supported higher abundances of several species types compared to mixed forests, including liverworts, acrocarpous mosses, and species that have greater moisture requirements, dioicous sexuality, or infrequent sporophyte production. This conveys the importance of coniferous forests as bryophyte habitat in mixedwood landscapes and the influence of canopy composition on regional species distributions. Understanding the tolerances of species exhibiting particular traits after harvesting may improve predictions about species extirpation risk and inform approaches to ensure their continued survival.

Effects of microhabitat and growth form on bryophyte mortality associated with leaf litter burial in a boreal spruce forest

AbstractQuestionsDo mortality rates of boreal bryophytes associated with leaf litter burial vary with degree of shelter on forest floor microsites in spruce‐dominated forests? Do erect and prostrate species respond similarly? What is the relative importance of deciduous leaves (from aspen and birch) vs conifer needles as bryophyte mortality agents?LocationBoreal spruce forests of central Sweden.MethodsA transplant study examining mortality was set up with four different bryophyte species: two creeping, prostrate liverworts (Calypogeia integristipula and Lepidozia reptans) and two more erect mosses (Rhytidiadelphus triquetrus and Hylocomiastrum umbratum). We tested for differences in mortality rates associated with leaf litter burial in three microhabitats differing in degree of shelter on the forest floor, i.e. sheltered (large boulders, tree bases of spruce) and unsheltered (open forest floor).ResultsAfter two growing seasons, 12.7% of the bryophyte transplants (40 out of 316, of which 37 transplants were prostrate liverworts) were dead, presumably due to litter burial. These two prostrate liverworts displayed significantly higher mortality rates close to boulders and at tree bases compared to unsheltered forest floors. Furthermore, although only comprising a small portion of the canopy (<10%), deciduous litter accounted for 53% of all transplants that were buried by litter, compared to 47% for conifer needle litter.ConclusionWe find fine‐scale spatial patchiness in the responses of bryophytes to litter fall. Mortality of prostrate liverworts associated with litter burial varies among microhabitats on the forest floor, with significantly higher mortality at sheltered microsites (here exemplified by boulders and tree bases) compared to erect moss species. Bryophytes close to microtopographic structures (e.g. boulders and trees) are more strongly exposed to litter burial, but such structures can also function as refuges under disturbance events such as clear‐cuts and windthrows. In addition, they may contain convex surfaces that accumulate less litter than flat or convex forest floor surfaces. The observed large effect of litter burial associated with bryophyte mortality, and the variation among microhabitats and species growth forms, suggest that incorporation of litter fall is of vital importance for our understanding of the dynamics of forest bryophyte communities.

Periglacial microclimate in low-altitude scree slopes supports relict biodiversity

Air circulation through talus slopes creates unique microclimates, with some of the most interesting being low-elevation mid-latitude scree in areas with frequent snow-free < 0°C (e.g. “black frost”) days that allow for development of year-round ice accumulations. Here, we document this phenomenon on Kamenec Hill in North Bohemia (Czech Republic) located at an altitude of 330 m above sea level, where mean annual temperatures < 0°C are maintained in a narrow strip along the slope's lower margin. This microhabitat, as well as interstitial spaces between scree blocks elsewhere on this slope, supports an important assemblage of boreal and arctic bryophytes, pteridophytes and arthropods that are disjunct from their normal ranges far to the north. This freezing scree slope represents a classic example of a palaeorefugium that significantly contributes to protection and maintenance of regional landscape biodiversity.

Seasonal trends in the biomass and structure of bryophyte‐associated fungal communities explored by 454 pyrosequencing

Bryophytes are a dominant vegetation component of the boreal forest, but little is known about their associated fungal communities, including seasonal variation within them. Seasonal variation in the fungal biomass and composition of fungal communities associated with three widespread boreal bryophytes was investigated using HPLC assays of ergosterol and amplicon pyrosequencing of the internal transcribed spacer 2 (ITS2) region of rDNA. The bryophyte phyllosphere community was dominated by Ascomycota. Fungal biomass did not decline appreciably in winter (P=0.272). Significant host-specific patterns in seasonal variation of biomass were detected (P=0.003). Although seasonal effects were not the primary factors structuring community composition, collection date significantly explained (P=0.001) variation not attributed to locality, host, and tissue. Community homogenization and a reduction in turnover occurred with the onset of frost events and subzero air and soil temperatures. Fluctuations in the relative abundance of particular fungal groups seem to reflect the nature of their association with mosses, although conclusions are drawn with caution because of potential methodological bias. The moss-associated fungal community is dynamic, exhibiting seasonal turnover in composition and relative abundance of different fungal groups, and significant fungal biomass is present year-round, suggesting a winter-active fungal community.

Boulders increase resistance to clear-cut logging but not subsequent recolonization rates of boreal bryophytes

The extent to which a plant assemblage might recolonize a disturbed system is in general related to the availability of propagule sources and sites with appropriate conditions for establishment. Both these factors might be sensitive to aspects of spatial heterogeneity. Microtopographic variation may enhance initial resistance by reducing the impact of the disturbance and facilitating establishment of incoming propagules by providing shaded "safe-sites". This study explores the influence of microtopographic heterogeneity (caused by variation in surface boulder cover) on the recolonization of closed-canopy forest floor bryophytes using a chronosequence of 75 spruce-dominated forests in south-central Sweden (2-163 years after clear-cutting). We found that high boulder cover did increase survival and subsequent persistence in young forests at both investigated scales (i.e. 1,000 and 100 m(2)), although this pattern became less evident on the smaller spatial scale. Species accumulation in boulder-poor subplots was not different when surrounded by boulder-rich compared with boulder-poor subplots suggesting short-distance recolonization from boulder-created refugia to be of little importance during recolonization. To conclude, it seems that boulders increase initial resistance to clear-cutting for this bryophyte guild, but that the subsequent recolonization process is more likely to depend on external propagule sources and factors affecting establishment such as the microclimate in the developing stand.

Saprobic and parasitic interactions of Coniochaeta velutina with mosses

During a survey of fungi associated with boreal bryophytes in Alberta, Canada, an isolate of Coniochaeta velutina (Coniochaetaceae, Coniochaetales), an opportunistic tree-pathogen and saprobe, was obtained from Hylocomium splendens (Hedw.) Schimp. (Hylocomiaceae, Hypnales). This isolate exhibited phylogenetic affinities to C. velutina isolated from vascular plants. In vitro characterization of its interactions with the model host Funaria hygrometrica Hedw. (Funariaceae, Funariales) demonstrated that it readily forms both anamorph and teleomorph states on the moss. Infected hosts exhibited no macroscopic symptoms or, occasionally, became weakly chlorotic, suggesting that the fungus is a parasite. However, cellulase and polyphenolic oxidase enzyme production suggest it may also be a latent saprobe. Vegetative hyphae of C. velutina directly penetrated rhizoids and frequently elicited a papilla-formation host response, neither of which have been reported in vascular plants infected with C. velutina. The Lecythophora anamorph of C. velutina was produced within host rhizoids, a new phenomenon among bryophilous fungi. Ascomata were produced superficially on F. hygrometrica, although the rhizoid the ascoma is initiated on frequently becomes embedded in the mature peridium. Given the ability of C. velutina to infect, persist, and reproduce on moss hosts, understory bryophytes may represent an inoculum reservoir that contributes to the disease cycle of C. velutina on tree species.

Succession of bryophyte assemblages following clear-cut logging in boreal spruce-dominated forests in south-central Sweden — Does retrogressive succession occur?

The recovery process of boreal bryophyte communities after clear-cutting was studied in a chronosequence in south-central Sweden. We hypothesized that high initial grass cover on clearcuts, high litter cover and low light levels during canopy closure, and shortage of coarse woody substrates would constrain recovery in different ways. Instead, both epigeic and epixylic guilds (i.e., species growing on forest floor and deadwood) displayed a gradual increase in similarity over time from the clear-cut phase, perhaps because of the absence of distinct peaks in needle litter and canopy cover. Epixylic species started to recover long before the accumulation of deadwood, indicating that microclimate rather than substrate availability was the most constraining factor during the first 50 years. Since we did not find any other bottlenecks during the succession after clear-cutting, conservation measures aiming at decreasing local extinction rates during clear-cutting may also increase long-term persistence. On the other hand, as the results from the epixylic guild suggest, other factors during the forest succession, such as the development of a suitable microclimate, might be more important for some organisms, thus possibly mitigating such long-term positive effects of adjusted management during the clear-cutting operation.

No increase in colonization rate of boreal bryophytes close to propagule sources

Knowledge of the process of recolonization, and which temporal and spatial scale it operates on, is central to our understanding of species distributions, metapopulation dynamics, regional extinction risks, and ecosystem resilience. In this study the recolonization pattern of boreal forest bryophytes was investigated in stands that had been clear-cut approximately 50 years ago. Species known to be sensitive to clear-cutting were inventoried in 23 mature forest stands and in adjacent young stands at 10, 20, 40, and 80 m from the former forest-clear-cut edge. Based on previous studies showing that bryophytes tend to be dispersal limited at local population levels, it was hypothesized that the recolonizaton of many bryophyte species should be higher closer to the mature forest edge. It was also hypothesized that some species would show full recovery, while for others the young stands would still be inhospitable. All these patterns were found for individual species, but the main pattern was, however, quite different. Most species had started to recolonize the young stands (i.e., little or much, depending on species), but without any tendency of a higher colonization rate close to the mature stands. Possible explanations for the limited signs of positive influence of local propagule sources might be microsite limitation or that the local propagule availability displays a rapid decline from its sources and is masked by a higher regional propagule rain. For organisms with light propagules able to build up a regional background level, the role of mature forest stands in the recolonization process of the matrix may rather be to contribute to the regional background level of spores in the landscape than to affect the adjacent stands directly.

Nitrogen uptake by Hylocomium splendens during snowmelt in a boreal forest

In most boreal regions snow composes a large portion of the annual precipitation. Although many boreal forest floor bryophytes depend largely on precipitation for their nitrogen (N) supply, bryophyte uptake of snow N is little explored. We studied chemical forms of plant-accessible N in snowmelt, as well as the temporal dynamics of their release. In conjunction we performed a N uptake experiment using the common boreal bryophyte Hylocomium splendens. The results demonstrated that the snowmelt N pool was dominated by NO3− (86%), followed by NH4+ (11%) and amino acid N (3%), in total providing ca 0.3 kg N·ha−1 to the forest floor vegetation. Hylocomium splendens was able to access both inorganic and organic 15N labelled N forms (NO3−, NH4+, and glycine) applied in situ to the snow covering the moss prior to snowmelt. Across all the N forms H. splendens took up ca 24% of the snow-deposited N. Nitrate uptake exceeded that of glycine, while NH4+ uptake was intermediate, reflecting the ambient distribution of the snowmelt N pool between plant-accessible N forms.

Facilitative succession in a boreal bryophyte community driven by changes in available moisture and light

AbstractQuestion:What are the drivers of bryophyte succession in paludifying boreal Picea mariana forests?Location:The Clay‐Belt of Quebec and Ontario, Canada.Methods:The bryophyte community and habitat variables (forest floor thickness, water table, stand density, canopy openness micro‐climate and presence of ericaceous species) were analysed in a chronosequence of 13 stands from 50 to more than 350 years since fire.Results:Across the chronosequence, feathermosses were replaced by shade and desiccation tolerant slower growing hummock Sphagna and then by faster growing hollow Sphagna. These changes were linked with both increasing light availability and the movement of the water table into the forest floor.Conclusions:As water table rise is dependent on forest floor thickness, which is in turn influenced by the presence of Sphagna, this successional sequence represents an example of facilitation. Furthermore, it emphasizes the importance of water table rise in determining stand level, and landscape level variables such as carbon balance.

Facilitative succession in a boreal bryophyte community driven by changes in available moisture and light

Abstract Question: What are the drivers of bryophyte succession in paludifying boreal Picea mariana forests? Location: The Clay-Belt of Quebec and Ontario, Canada. Methods: The bryophyte community and habitat variables (forest floor thickness, water table, stand density, canopy openness micro-climate and presence of ericaceous species) were analysed in a chronosequence of 13 stands from 50 to more than 350 years since fire. Results: Across the chronosequence, feathermosses were replaced by shade and desiccation tolerant slower growing hummock Sphagna and then by faster growing hollow Sphagna. These changes were linked with both increasing light availability and the movement of the water table into the forest floor. Conclusions: As water table rise is dependent on forest floor thickness, which is in turn influenced by the presence of Sphagna, this successional sequence represents an example of facilitation. Furthermore, it emphasizes the importance of water table rise in determining stand level, and landsc...

Element accumulation in boreal bryophytes, lichens and vascular plants exposed to heavy metal and sulfur deposition in Finland

Macronutrient (N, P, K, Mg, S, Ca), heavy metal (Fe, Zn, Mn, Cu, Ni, Cd, Pb) and Al concentrations in understorey bryophytes, lichens and vascular plant species growing in Scots pine forests at four distances from the Harjavalta Cu–Ni smelter (0.5, 2, 4 and 8 km) were compared to those at two background sites in Finland. The aim was to study the relationship between element accumulation and the distribution of the species along a pollution gradient. Elevated sulfur, nitrogen and heavy metal concentrations were found in all species groups near the pollution source. Macronutrient concentrations tended to decrease in the order: vascular plants>bryophytes>lichens, when all the species groups grew on the same plot. Heavy metal concentrations (except Mn) were the highest in bryophytes, followed by lichens, and were the lowest in vascular plants. In general, vascular plants, being capable of restricting the uptake of toxic elements, grew closer to the smelter than lichens, while bryophytes began to increase in the understorey vegetation at further distances from the smelter. A pioneer moss ( Pohlia nutans) was an exception, because it accumulated considerably higher amounts of Cu and Ni than the other species and still survived close to the smelter. The abundance of most of the species decreased with increasing Cu and Ni concentrations in their tissues. Cetraria islandica, instead, showed a positive relationship between the abundance and Cu, Ni and S concentrations of the thallus. It is probable that, in addition to heavy metals, sporadically high SO 2 emissions have also affected the distribution of the plant species.