Productivity–diversity patterns in arctic tundra vegetation

Productivity has long been argued to be a major driver of species richness patterns. In the present study we test alternative productivity–diversity hypotheses using vegetation data from the vast Eurasian tundra. The productivity–species pool hypothesis predicts positive relationships at both fine and coarse grain sizes, whereas the productivity–interaction hypothesis predicts unimodal patterns at fine grain size, and monotonic positive patterns at coarse grain size. We furthermore expect to find flatter positive (productivity–species pool hypothesis) or more strongly negative (productivity–interaction hypothesis) relationships for lichens and bryophytes than for vascular plants, because as a group, lichens and bryophytes are better adapted to extreme arctic conditions and more vulnerable to competition for light than the taller‐growing vascular plants. The normalised difference vegetation index (NDVI) was used as a proxy of productivity. The generally unimodal productivity–diversity patterns were most consistent with the productivity–interaction hypothesis. There was a general trend of decreasing species richness from moderately to maximally productive tundra, in agreement with an increasing importance of competitive interactions. High richness of vascular plants and lichens occurred in moderately low productive tundra areas, whereas that of bryophytes occurred in the least productive tundra habitats covered by this study. The fine and coarse grain richness trends were surprisingly uniform and no variation in beta diversity along the productivity gradient was seen for vascular plants or bryophytes. However, lichen beta diversity varied along the productivity gradient, probably reflecting their sensitivity to habitat conditions and biotic interactions. Overall, the results show evidence that productivity–diversity gradients exist in tundra and that these appear to be largely driven by competitive interactions. Our results also imply that climate warming‐driven increases in productivity will strongly affect arctic plant diversity patterns.     

Frequent fire promotes diversity and cover of biological soil crusts in a derived temperate grassland

The intermediate disturbance hypothesis (IDH) predicts that species diversity is maximized at moderate disturbance levels. This model is often applied to grassy ecosystems, where disturbance can be important for maintaining vascular plant composition and diversity. However, effects of disturbance type and frequency on cover and diversity of non-vascular plants comprising biological soil crusts are poorly known, despite their potentially important role in ecosystem function. We established replicated disturbance regimes of different type (fire vs. mowing) and frequency (2, 4, 8 yearly and unburnt) in a high-quality, representative Themeda australis–Poa sieberiana derived grassland in south-eastern Australia. Effects on soil crust bryophytes and lichens (hereafter cryptogams) were measured after 12 years. Consistent with expectations under IDH, cryptogam richness and abundance declined under no disturbance, likely due to competitive exclusion by vascular plants as well as high soil turnover by soil invertebrates beneath thick grass. Disturbance type was also significant, with burning enhancing richness and abundance more than mowing. Contrary to expectations, however, cryptogam richness increased most dramatically under our most frequent and recent (2 year) burning regime, even when changes in abundance were accounted for by rarefaction analysis. Thus, from the perspective of cryptogams, 2-year burning was not an adequately severe disturbance regime to reduce diversity, highlighting the difficulty associated with expression of disturbance gradients in the application of IDH. Indeed, significant correlations with grassland structure suggest that cryptogam abundance and diversity in this relatively mesic (600 mm annual rainfall) grassland is maximised by frequent fires that reduce vegetation and litter cover, providing light, open areas and stable soil surfaces for colonisation. This contrasts with detrimental effects of 2-year burning on native perennial grasses, indicating that this proliferation of cryptogams has potentially high functional significance for situations where vegetation cover is depleted, particularly for reducing soil erosion.     

Long‐term experimental warming alters community composition of ascomycetes in Alaskan moist and dry arctic tundra

Arctic tundra regions have been responding to global warming with visible changes in plant community composition, including expansion of shrubs and declines in lichens and bryophytes. Even though it is well known that the majority of arctic plants are associated with their symbiotic fungi, how fungal community composition will be different with climate warming remains largely unknown. In this study, we addressed the effects of long‐term (18 years) experimental warming on the community composition and taxonomic richness of soil ascomycetes in dry and moist tundra types. Using deep Ion Torrent sequencing, we quantified how OTU assemblage and richness of different orders of Ascomycota changed in response to summer warming. Experimental warming significantly altered ascomycete communities with stronger responses observed in the moist tundra compared with dry tundra. The proportion of several lichenized and moss‐associated fungi decreased with warming, while the proportion of several plant and insect pathogens and saprotrophic species was higher in the warming treatment. The observed alterations in both taxonomic and ecological groups of ascomycetes are discussed in relation to previously reported warming‐induced shifts in arctic plant communities, including decline in lichens and bryophytes and increase in coverage and biomass of shrubs.     

Testing reliability of short-term responses to predict longer-term responses of bryophytes and lichens to environmental change

Environmental changes are predicted to have severe and rapid impacts on polar and alpine regions. At high latitudes/altitudes, cryptogams such as bryophytes and lichens are of great importance in terms of biomass, carbon/nutrient cycling, cover and ecosystem functioning. This seven-year factorial experiment examined the effects of fertilizing and experimental warming on bryophyte and lichen abundance in an alpine meadow and a heath community in subarctic Sweden. The aim was to determine whether short-term responses (five years) are good predictors of longer-term responses (seven years). Fertilizing and warming had significant negative effects on total and relative abundance of bryophytes and lichens, with the largest and most rapid decline caused by fertilizing and combined fertilizing and warming. Bryophytes decreased most in the alpine meadow community, which was bryophyte-dominated, and lichens decreased most in the heath community, which was lichen-dominated. This was surprising, as the most diverse group in each community was expected to be most resistant to perturbation. Warming alone had a delayed negative impact. Of the 16 species included in statistical analyses, seven were significantly negatively affected. Overall, the impacts of simulated warming on bryophytes and lichens as a whole and on individual species differed in time and magnitude between treatments and plant communities (meadow and heath). This will likely cause changes in the dominance structures over time. These results underscore the importance of longer-term studies to improve the quality of data used in climate change models, as models based on short-term data are poor predictors of long-term responses of bryophytes and lichens.     

The nature of spatial transitions in the Arctic

Aim Describe the spatial and temporal properties of transitions in the Arctic and develop a conceptual understanding of the nature of these spatial transitions in the face of directional environmental change. Location Arctic tundra ecosystems of the North Slope of Alaska and the tundra‐forest region of the Seward Peninsula, Alaska Methods We synthesize information from numerous studies on tundra and treeline ecosystems in an effort to document the spatial changes that occur across four arctic transitions. These transitions are: (i) the transition between High‐Arctic and Low‐Arctic systems, (ii) the transition between moist non‐acidic tundra (MNT) and moist acidic tundra (MAT, also referred to as tussock tundra), (iii) the transition between tussock tundra and shrub tundra, (iv) the transition between tundra and forested systems. By documenting the nature of these spatial transitions, in terms of their environmental controls and vegetation patterns, we develop a conceptual model of temporal dynamics of arctic ecotones in response to environmental change. Results Our observations suggest that each transition is sensitive to a unique combination of controlling factors. The transition between High and Low Arctic is sensitive primarily to climate, whereas the MNT/MAT transition is also controlled by soil parent material, permafrost and hydrology. The tussock/shrub tundra transition appears to be responsive to several factors, including climate, topography and hydrology. Finally, the tundra/forest boundary responds primarily to climate and to climatically associated changes in permafrost. There were also important differences in the demography and distribution of the dominant plant species across the four vegetation transitions. The shrubs that characterize the tussock/shrub transition can achieve dominance potentially within a decade, whereas spruce trees often require several decades to centuries to achieve dominance within tundra, and Sphagnum moss colonization of non‐acidic sites at the MNT/MAT boundary may require centuries to millennia of soil development. Main conclusions We suggest that vegetation will respond most rapidly to climatic change when (i) the vegetation transition correlates more strongly with climate than with other environmental variables, (ii) dominant species exhibit gradual changes in abundance across spatial transitions, and/or (iii) the dominant species have demographic properties that allow rapid increases in abundance following climatic shifts. All three of these properties characterize the transition between tussock tundra and low shrub tundra. It is therefore not surprising that of the four transitions studied this is the one that appears to be responding most rapidly to climatic warming.     

How ecological neighbourhoods influence the structure of the scavenger guild in low arctic tundra

Aim How the ecological neighbourhoods of coast and forest affect arctic tundra ecosystems is a pressing question as the circumpolar tundra belt is shrinking under global warming. Mobile facultative scavengers are likely to negatively impact tundra biodiversity as dominant competitors or predators, if they spill over into tundra. Here, we provide the first quantitative assessments of the structure of a scavenger guild in low arctic tundra with emphasis on how it changes along spatial gradients from neighbouring ecosystems (i.e. forest and coast) and with altitude (i.e. productivity gradients). We also assess the likelihood of interactions between guild members that may negatively impact vulnerable tundra species. Location North‐eastern part of Norway. Methods Extensive records of scavenger prevalence were obtained by deploying automatic digital cameras at experimental carcasses in tundra regions covering several thousand square kilometres and three winters in northern Norway. Main conclusions We found short‐range neighbourhood effects of forest and coast within the tundra scavenger guild. Species richness declined steeply with decreasing distance from the neighbouring ecosystems, in particular subarctic forest, and with increasing altitude. Bird species with strongholds in forest (golden eagle Aquila chrysaetos and hooded crow Corvus cornix) or along the coast (white‐tailed eagle Haliaeetus albicilla) were mostly responsible for short‐range neighbourhood effects on guild structure. However, the two most abundant guild members, the common raven Corvus corax and the red fox Vulpes vulpes, exhibited no spatial patterns within the range of neighbourhoods and altitudes examined. There was a clear diurnal segregation in the use of carcasses between birds and mammals reducing the likelihood of direct interactions between these two taxa. Presence of red fox appeared to exclude the arctic fox Vulpes lagopus, the only endemic tundra species within the guild, from carcasses.     

Summer temperature increase has distinct effects on the ectomycorrhizal fungal communities of moist tussock and dry tundra in Arctic Alaska

Arctic regions are experiencing the greatest rates of climate warming on the planet and marked changes have already been observed in terrestrial arctic ecosystems. While most studies have focused on the effects of warming on arctic vegetation and nutrient cycling, little is known about how belowground communities, such as fungi root‐associated, respond to warming. Here, we investigate how long‐term summer warming affects ectomycorrhizal (ECM) fungal communities. We used Ion Torrent sequencing of the rDNA internal transcribed spacer 2 (ITS2) region to compare ECM fungal communities in plots with and without long‐term experimental warming in both dry and moist tussock tundra. Cortinarius was the most OTU‐rich genus in the moist tundra, while the most diverse genus in the dry tundra was Tomentella. On the diversity level, in the moist tundra we found significant differences in community composition, and a sharp decrease in the richness of ECM fungi due to warming. On the functional level, our results indicate that warming induces shifts in the extramatrical properties of the communities, where the species with medium‐distance exploration type seem to be favored with potential implications for the mobilization of different nutrient pools in the soil. In the dry tundra, neither community richness nor community composition was significantly altered by warming, similar to what had been observed in ECM host plants. There was, however, a marginally significant increase in OTUs identified as ECM fungi with the medium‐distance exploration type in the warmed plots. Linking our findings of decreasing richness with previous results of increasing ECM fungal biomass suggests that certain ECM species are favored by warming and may become more abundant, while many other species may go locally extinct due to direct or indirect effects of warming. Such compositional shifts in the community might affect nutrient cycling and soil organic C storage.     

Variable sensitivity of plant communities in Iceland to experimental warming

Facing an increased threat of rapid climate change in cold‐climate regions, it is important to understand the sensitivity of plant communities both in terms of degree and direction of community change. We studied responses to 3–5 years of moderate experimental warming by open‐top chambers in two widespread but contrasting tundra communities in Iceland. In a species‐poor and nutrient‐deficient moss heath, dominated by Racomitrium lanuginosum, mean daily air temperatures at surface were 1–2°C higher in the warmed plots than the controls whereas soil temperatures tended to be lower in the warmed plots throughout the season. In a species‐rich dwarf shrub heath on relatively rich soils at a cooler site, dominated by Betula nana and R. lanuginosum, temperature changes were in the same direction although more moderate. In the moss heath, there were no detectable community changes while significant changes were detected in the dwarf shrub heath: the abundance of deciduous and evergreen dwarf shrubs significantly increased (>50%), bryophytes decreased (18%) and canopy height increased (100%). Contrary to some other studies of tundra communities, we detected no changes in species richness or other diversity measures in either community and the abundance of lichens did not change. It is concluded that the sensitivity of Icelandic tundra communities to climate warming varies greatly depending on initial conditions in terms of species diversity, dominant species, soil and climatic conditions as well as land‐use history.     

Comparative cryptogam ecology: a review of bryophyte and lichen traits that drive biogeochemistry

BACKGROUND: Recent decades have seen a major surge in the study of interspecific variation in functional traits in comparative plant ecology, as a tool to understanding and predicting ecosystem functions and their responses to environmental change. However, this research has been biased almost exclusively towards vascular plants. Very little is known about the role and applicability of functional traits of non-vascular cryptogams, particularly bryophytes and lichens, with respect to biogeochemical cycling. Yet these organisms are paramount determinants of biogeochemistry in several biomes, particularly cold biomes and tropical rainforests, where they: (1) contribute substantially to above-ground biomass (lichens, bryophytes); (2) host nitrogen-fixing bacteria, providing major soil N input (lichens, bryophytes); (3) control soil chemistry and nutrition through the accumulation of recalcitrant polyphenols (bryophytes) and through their control over soil and vegetation hydrology and temperatures; (4) both promote erosion (rock weathering by lichens) and prevent it (biological crusts in deserts); (5) provide a staple food to mammals such as reindeer (lichens) and arthropodes, with important feedbacks to soils and biota; and (6) both facilitate and compete with vascular plants. APPROACH: Here we review current knowledge about interspecific variation in cryptogam traits with respect to biogeochemical cycling and discuss to what extent traits and measuring protocols needed for bryophytes and lichens correspond with those applied to vascular plants. We also propose and discuss several new or recently introduced traits that may help us understand and predict the control of cryptogams over several aspects of the biogeochemistry of ecosystems. CONCLUSIONS: Whilst many methodological challenges lie ahead, comparative cryptogam ecology has the potential to meet some of the important challenges of understanding and predicting the biogeochemical and climate consequences of large-scale environmental changes driving shifts in the cryptogam components of vegetation composition.     

Mosses modify effects of warmer and wetter conditions on tree seedlings at the alpine treeline

Climate warming enables tree seedling establishment beyond the current alpine treeline, but to achieve this, seedlings have to establish within existing tundra vegetation. In tundra, mosses are a prominent feature, known to regulate soil temperature and moisture through their physical structure and associated water retention capacity. Moss presence and species identity might therefore modify the impact of increases in temperature and precipitation on tree seedling establishment at the arctic‐alpine treeline. We followed Betula pubescens and Pinus sylvestris seedling survival and growth during three growing seasons in the field. Tree seedlings were transplanted along a natural precipitation gradient at the subarctic‐alpine treeline in northern Sweden, into plots dominated by each of three common moss species and exposed to combinations of moss removal and experimental warming by open‐top chambers (OTCs). Independent of climate, the presence of feather moss, but not Sphagnum, strongly supressed survival of both tree species. Positive effects of warming and precipitation on survival and growth of B. pubescens seedlings occurred in the absence of mosses and as expected, this was partly dependent on moss species. P. sylvestris survival was greatest at high precipitation, and this effect was more pronounced in Sphagnum than in feather moss plots irrespective of whether the mosses had been removed or not. Moss presence did not reduce the effects of OTCs on soil temperature. Mosses therefore modified seedling response to climate through other mechanisms, such as altered competition or nutrient availability. We conclude that both moss presence and species identity pose a strong control on seedling establishment at the alpine treeline, and that in some cases mosses weaken climate‐change effects on seedling establishment. Changes in moss abundance and species composition therefore have the potential to hamper treeline expansion induced by climate warming.     

Cryptogams on decaying wood in old-growth forests of southern coastal British Columbia

Abstract. We studied the floristic composition of lignicolous cryptogams (i.e. bryophytes and lichens inhabiting decaying wood) in the old-growth coniferous forests of southern coastal British Columbia. The composition of the cryptogams was related to forest communities (described by vegetation units using forest floor cryptogams and vascular plants) and regional climates (described by biogeoclimatic zones). The study is based on a total of 247 sample plots, each of 0.04 ha in size. The plots were located in three different biogeoclimatic zones and were classified into 12 alliances and four orders in a previous study. We used indicator species analysis to determine cryptogam indicator species for each vegetation unit and biogeoclimatic zone, and used similarity analysis and multi-variate analyses (discriminant analysis and detrended correspondence analysis) to detect differences in the floristic composition of the cryptogams among sample plots, vegetation units, and biogeoclimatic zones. Most of the cryptogams in this study had a narrow distribution, and only < 5% of the species were present across all the vegetation units and biogeoclimatic zones. The overall means of Jaccard coefficients between two sample plots from the same vegetation unit (alliance or order) or biogeoclimatic zone were significantly higher (P < 0.05) than those from different vegetation units or biogeoclimatic zones. The difference in the mean Jaccard coefficients within- and between-units was highest for zones. The results of detrended correspondence analysis and discriminant analysis suggest that the composition of lignicolous cryptogams changes with the change in the floristic composition of forest floor vegetation. When the first axes of detrended correspondence analysis were compared, the Pearson's correlation coefficients between the first axes of lignicolous cryptogams and forest floor vascular plants and between the first axes of lignicolous cryptogams and forest floor cryptogams were 0.78 and 0.87, respectively. The degree of correspondence in the composition of lignicolous cryptogams and forest floor vegetation increased from alliance to order to biogeoclimatic zone. This trend suggests that the floristic composition of lignicolous cryptogams is influenced at the community level mainly by edaphic conditions and at the regional level by climatic conditions.     

Plant–herbivore interactions in Alaskan arctic tundra change with soil nutrient availability

Herbivores in nutrient‐limited systems such as arctic tundra have been suggested to play a minor role in controlling plant growth simply because they are relatively few in number. However, theory predicts that as net primary productivity (NPP) increases because of greater inputs of nutrients or energy, herbivores may have greater effects on plant growth. This prediction has not been tested in the context of climate warming in arctic tundra, which may increase soil nutrient availability and thus NPP. We examined a long‐term experiment that excluded small and large mammalian herbivores and increased soil nutrients in two arctic Alaskan tundra communities: dry heath (DH) and moist acidic tussock (MAT). In the ninth year of manipulations, we measured weekly growth of three plant species of three growth forms: tussock‐forming graminoid, rhizomatous graminoid, and dwarf deciduous shrub, in each community. All species grew better when fertilized. In DH, this increase in growth was exaggerated when plants were protected from herbivores, confirming that herbivory had a negative effect on plant growth under increased nutrient conditions, but was unimportant under ambient soil conditions. However, in MAT, the importance of herbivory differed among species with fertilization. The tussock‐forming sedge at MAT, Eriophorum vaginatum, grew better and flowered more when fenced under both ambient and amended nutrients compared to plants exposed to herbivores. This species decreases in abundance in long‐term fertilized plots when mammals are present, and our results suggest that herbivory may be accounting for at least some of that loss, in addition to shifts in competitive relationships. Although we only focused on individual plants here rather than the entire community, our results suggest that under the increased soil nutrient conditions expected with continued climate warming in the Arctic, herbivores may become more important in affecting several abundant tundra plant populations, and should not be ignored.     

Long-term ecosystem level experiments at Toolik Lake, Alaska, and at Abisko, Northern Sweden: generalizations and differences in ecosystem and plant type responses to global change

Long‐term ecosystem‐level experiments, in which the environment is manipulated in a controlled manner, are important tools to predict the responses of ecosystem functioning and composition to future global change. We present the results of a meta‐analysis performed on the results of long‐term ecosystem‐level experiments near Toolik Lake, Alaska, and Abisko, Sweden. We quantified aboveground biomass responses of different arctic and subarctic ecosystems to experimental fertilization, warming and shading. We not only analysed the general patterns but also the differences in responsiveness between sites and regions. Aboveground plant biomass showed a broad similarity of responses in both locations, and also showed some important differences. In both locations, aboveground plant biomass, particularly the biomass of deciduous and graminoid plants, responded most strongly to nutrient addition. The biomass of mosses and lichens decreased in both locations as the biomass of vascular plants increased. An important difference between the two regions was the smaller positive aboveground biomass response of deciduous shrubs in Abisko as compared with Toolik Lake. Whereas in Toolik Lake Betula nana increased its dominance and replaced many of the other plant types, in Abisko all vascular plant types increased in abundance without major shifts in relative abundance. The differences between the responses of the dominant vegetation types of the Toolik Lake region, i.e. tussock tundra systems, and that of the Abisko region, i.e. heath systems, may have important implications for ecosystem development under expected patterns of global change. However, there were also large site‐specific differences within each region. Several potential mechanistic explanations for the differences between sites and regions are discussed. The response patterns show the need for analyses of joint data sets from many regions and sites, in order to uncover common responses to changes in climate across large arctic regions from regional or local responses.     

Herbivores inhibit climate-driven shrub expansion on the tundra

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures with different mesh sizes that exclude either only reindeer or all mammalian herbivores including voles and lemmings. The exclosures were replicated at three forest and tundra sites at four different locations along a climatic gradient (oceanic to continental) in northern Fennoscandia. Since the last 10 years have been exceptionally warm, we could study how warming has influenced the vegetation in different grazing treatments. Our results show that the abundance of the dominant shrub, Betula nana , has increased during the last decade, but that the increase was more pronounced when herbivores were excluded. Reindeer have the largest effect on shrubs in tundra, while voles and lemmings have a larger effect in the forest. The positive relationship between annual mean temperature and shrub growth in the absence of herbivores and the lack of relationships in grazed controls is another indication that shrub abundance is controlled by an interaction between herbivores and climate. In addition to their effects on taller shrubs (>0.3 m), reindeer reduced the abundance of lichens, whereas microtine rodents reduced the abundance of dwarf shrubs (<0.3 m) and mosses. In contrast to short-term responses, competitive interactions between dwarf shrubs and lichens were evident in the long term. These results show that herbivores have to be considered in order to understand how a changing climate will influence tundra ecosystems.     

Browsing‐mediated shrub canopy changes drive composition and species richness in forest‐tundra ecosystems

Changes in climate and in browsing pressure are expected to alter the abundance of tundra shrubs thereby influencing the composition and species richness of plant communities. We investigated the associations between browsing, tundra shrub canopies and their understory vegetation by utilizing a long‐term (10–13 seasons) experiment controlling reindeer and ptarmigan herbivory in the subarctic forest tundra ecotone in northwestern Fennoscandia. In this area, there has also been a consistent increase in the yearly thermal sum and precipitation during the study period. The cover of shrubs increased 2.8–7.8 fold in exclosures and these contrasted with browsed control areas creating a sharp gradient of canopy cover of tundra shrubs across a variety of vegetation types. Browsing exclusions caused significant shifts in more productive vegetation types, whereas little or no shift occurred in low‐productive tundra communities. The increased deciduous shrub cover was associated with significant losses of understory plant species and shifts in functional composition, the latter being clearest in the most productive plant community types. The total cover of understory vegetation decreased along with increasing shrub cover, while the cover of litter showed the opposite response. The cover of cryptogams decreased along with increasing shrub cover, while the cover of forbs was favoured by a shrub cover. Increasing shrub cover decreased species richness of understory vegetation, which was mainly due to the decrease in the cryptogam species. The effects were consistent across different types of forest tundra vegetation indicating that shrub increase may have broad impacts on arctic vegetation diversity. Deciduous shrub cover is strongly regulated by reindeer browsing pressure and altered browsing pressure may result in a profound shrub expansion over the next one or two decades. Results suggest that the impact of an increase in shrubs on tundra plant richness is strong and browsing pressure effectively counteracts the effects of climate warming‐driven shrub expansion and hence maintains species richness.     

Predicted changes in vegetation structure affect the susceptibility to invasion of bryophyte-dominated subarctic heath

Background and Aims

A meta-analysis of global change experiments in arctic tundra sites suggests that plant productivity and the cover of shrubs, grasses and dead plant material (i.e. litter) will increase and the cover of bryophytes will decrease in response to higher air temperatures. However, little is known about which effects these changes in vegetation structure will have on seedling recruitment of species and invasibility of arctic ecosystems.

Methods

A field experiment was done in a bryophyte-dominated, species-rich subarctic heath by manipulating the cover of bryophytes and litter in a factorial design. Three phases of seedling recruitment (seedling emergence, summer seedling survival, first-year recruitment) of the grass Anthoxanthum alpinum and the shrub Betula nana were analysed after they were sown into the experimental plots.

Key Results

Bryophyte and litter removal significantly increased seedling emergence of both species but the effects of manipulations of vegetation structure varied strongly for the later phases of recruitment. Summer survival and first-year recruitment were significantly higher in Anthoxanthum. Although bryophyte removal generally increased summer survival and recruitment, seedlings of Betula showed high mortality in early August on plots where bryophytes had been removed.

Conclusions

Large species-specific variation and significant effects of experimental manipulations on seedling recruitment suggest that changes in vegetation structure as a consequence of global warming will affect the abundance of grasses and shrubs, the species composition and the susceptibility to invasion of subarctic heath vegetation.     

Arctic arthropod assemblages in habitats of differing shrub dominance

Recent climate warming in the Arctic has caused advancement in the timing of snowmelt and expansion of shrubs into open tundra. Such an altered climate may directly and indirectly (via effects on vegetation) affect arctic arthropod abundance, diversity and assemblage taxonomic composition. To allow better predictions about how climate changes may affect these organisms, we compared arthropod assemblages between open and shrub‐dominated tundra at three field sites in northern Alaska that encompass a range of shrub communities. Over ten weeks of sampling in 2011, pitfall traps captured significantly more arthropods in shrub plots than open tundra plots at two of the three sites. Furthermore, taxonomic richness and diversity were significantly greater in shrub plots than open tundra plots, although this pattern was site‐specific as well. Patterns of abundance within the five most abundant arthropod orders differed, with spiders (Order: Araneae) more abundant in open tundra habitats and true bugs (Order: Hemiptera), flies (Order: Diptera), and wasps and bees (Order: Hymenoptera) more abundant in shrub‐dominated habitats. Few strong relationships were found between vegetation and environmental variables and arthropod abundance; however, lichen cover seemed to be important for the overall abundance of arthropods. Some arthropod orders showed significant relationships with other vegetation variables, including maximum shrub height (Coleoptera) and foliar canopy cover (Diptera). As climate warming continues over the coming decades, and with further shrub expansion likely to occur, changes in arthropod abundance, richness, and diversity associated with shrub‐dominated habitat may have important ecological effects on arctic food webs since arthropods play important ecological roles in the tundra, including in decomposition and trophic interactions.     

Seasonal variation in nitrogen fixation and effects of climate change in a subarctic heath

Background and aims

Nitrogen fixation associated with cryptogams is potentially very important in arctic and subarctic terrestrial ecosystems, as it is a source of new nitrogen (N) into these highly N limited systems. Moss-, lichen- and legume-associated N₂ fixation was studied with high frequency (every second week) during spring, summer, autumn and early winter to uncover the seasonal variation in input of atmospheric N₂ to a subarctic heath with an altered climate.

Methods

We estimated N₂ fixation from ethylene production by acetylene reduction assay in situ in a field experiment with the treatments: long- vs. short-term summer warming using plastic tents and litter addition (simulating expansion of the birch forest).

Results

N₂ fixation activity was measured from late April to mid November and 33 % of all N₂ was fixed outside the vascular plant growing season (Jun–Aug). This substantial amount underlines the importance of N₂ fixation in the cold period. Warming increased N₂ fixation two- to fivefold during late spring. However, long-term summer warming tended to decrease N₂ fixation outside the treatment (tents present) period. Litter alone did not alter N₂ fixation but in combination with warming N₂ fixation increased, probably because N₂ fixation became phosphorus limited under higher temperatures, which was alleviated by the P supply from the litter.

Conclusion

In subarctic heath, the current N₂ fixation period extends far beyond the vascular plant growing season. Climate warming and indirect effects such as vegetation changes affect the process of N₂ fixation in different directions and thereby complicate predictions of future N cycling.     

Can antibrowsing defense regulate the spread of woody vegetation in arctic tundra?

Global climate warming is projected to promote the increase of woody plants, especially shrubs, in arctic tundra. Many factors may affect the extent of this increase, including browsing by mammals. We hypothesize that across the Arctic the effect of browsing will vary because of regional variation in antibrowsing chemical defense. Using birch (Betula) as a case study, we propose that browsing is unlikely to retard birch expansion in the region extending eastward from the Lena River in central Siberia across Beringia and the continental tundra of central and eastern Canada where the more effectively defended resin birches predominate. Browsing is more likely to retard birch expansion in tundra west of the Lena to Fennoscandia, Iceland, Greenland and South Baffin Island where the less effectively defended non‐resin birches predominate. Evidence from the literature supports this hypothesis. We further suggest that the effect of warming on the supply of plant‐available nitrogen will not significantly change either this pan‐Arctic pattern of variation in antibrowsing defense or the resultant effect that browsing has on birch expansion in tundra. However, within central and east Beringia warming‐caused increases in plant‐available nitrogen combined with wildfire could initiate amplifying feedback loops that could accelerate shrubification of tundra by the more effectively defended resin birches. This accelerated shrubification of tundra by resin birch, if extensive, could reduce the food supply of caribou causing population declines. We conclude with a brief discussion of modeling methods that show promise in projecting invasion of tundra by woody plants.