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Plant traits reflect growth strategies and trade-offs in response to environmental conditions. Because of climate warming, plant traits might change, altering ecosystem functions and vegetation–climate interactions. Despite important feedbacks of plant trait changes in tundra ecosystems with regional climate, with a key role for shrubs, information on responses of shrub functional traits is limited. Here, we investigate the effects of experimentally increased permafrost thaw depth and (possibly thaw-associated) soil nutrient availability on plant functional traits and strategies of Arctic shrubs in northeastern Siberia. We hypothesize that shrubs will generally shift their strategy from efficient conservation to faster acquisition of resources through adaptation of leaf and stem traits in a coordinated whole-plant fashion. To test this hypothesis, we ran a 4 year permafrost thaw and nutrient fertilization experiment with a fully factorial block design and six treatment combinations – permafrost thaw (control, unheated cable, heated cable) × fertilization (no nutrient addition, nutrient addition). We measured 10 leaf and stem traits related to growth, defence and the resource economics spectrum in four shrub species (Betula nana, Salix pulchra, Ledum palustre and Vaccinium vitis-idaea), which were sampled in the experimental plots. The plant trait data were statistically analysed using linear mixed-effect models and principal component analysis (PCA). The response to increased permafrost thaw was not significant for most shrub traits. However, all shrubs responded to the fertilization treatment, despite decreased thaw depth and soil temperature in fertilized plots. Shrubs tended to grow taller but did not increase their stem density or bark thickness. We found a similar coordinated trait response for all four species at leaf and plant level; i.e. they shifted from a conservative towards a more acquisitive resource economy strategy upon fertilization. In accordance, results point towards a lower investment into defence mechanisms, and hence increased shrub vulnerability to herbivory and climate extremes. Compared to biomass and height only, detailed data involving individual plant organ traits such as leaf area and nutrient contents or stem water content can contribute to a better mechanistic understanding of feedbacks between shrub growth strategies, permafrost thaw and carbon and energy fluxes. In combination with observational data, these experimental tundra trait data allow for a more realistic representation of tundra shrubs in dynamic vegetation models and robust prediction of ecosystem functions and related climate–vegetation–permafrost feedbacks.

The majority of variation in six traits critical to the growth, survival and reproduction of plant species is thought to be organised along just two dimensions, corresponding to strategies of plant size and resource acquisition. However, it is unknown whether global plant trait relationships extend to climatic extremes, and if these interspecific relationships are confounded by trait variation within species. We test whether trait relationships extend to the cold extremes of life on Earth using the largest database of tundra plant traits yet compiled. We show that tundra plants demonstrate remarkably similar resource economic traits, but not size traits, compared to global distributions, and exhibit the same two dimensions of trait variation. Three quarters of trait variation occurs among species, mirroring global estimates of interspecific trait variation. Plant trait relationships are thus generalizable to the edge of global trait-space, informing prediction of plant community change in a warming world., It is unclear whether plant trait relationships found at the global scale extend to climatic extremes. Here the authors analyse six major aboveground traits to show that known plant trait relationships extend to the tundra biomes and exhibit the same two dimensions of variation detected at the global scale.

Thermokarst features, such as thaw ponds, are hotspots for methane emissions in warming lowland tundra. Presently we lack quantitative knowledge on the formation rates of thaw ponds and subsequent vegetation succession, necessary to determine their net contribution to greenhouse gas emissions. This study sets out to identify development trajectories and formation rates of small-scale (2), shallow arctic thaw ponds in north-eastern Siberia. We selected 40 ponds of different age classes based on a time-series of satellite images and measured vegetation composition, microtopography, water table, and thaw depth in the field and measured age of colonizing shrubs in thaw ponds using dendrochronology. We found that young ponds are characterized by dead shrubs, while older ponds show rapid terrestrialization through colonization by sedges and Sphagnum moss. While dead shrubs and open water are associated with permafrost degradation (lower surface elevation, larger thaw depth), sites with sedge and in particular Sphagnum display indications of permafrost recovery. Recruitment of Betula nana on Sphagnum carpets in ponds indicates a potential recovery toward shrub-dominated vegetation, although it remains unclear if and on what timescale this occurs. Our results suggest that thaw ponds display potentially cyclic vegetation succession associated with permafrost degradation and recovery. Pond formation and initial colonization by sedges can occur on subdecadal timescales, suggesting rapid degradation and initial recovery of permafrost. The rates of formation and recovery of small-scale, shallow thaw ponds have implications for the greening/browning dynamics and carbon balance of this ecosystem.

Background and aims: Permafrost degradation has the potential to change the Arctic tundra landscape. We observed rapid local thawing of ice-rich permafrost resulting in thaw pond formation, which was triggered by removal of the shrub cover in a field experiment. This study aimed to examine the rate of permafrost thaw and the initial vegetation succession after the permafrost collapse. Methods: In the experiment, we measured changes in soil thaw depth, plant species cover and soil subsidence over nine years (2007–2015). Results: After abrupt initial thaw, soil subsidence in the removal plots continued indicating further thawing of permafrost albeit at a much slower pace: 1 cm y−1 over 2012–2015 vs. 5 cm y−1 over 2007–2012. Grass cover strongly increased after the initial shrub removal, but later declined with ponding of water in the subsiding removal plots. Sedges established and expanded in the wetter removal plots. Thereby, the removal plots have become increasingly similar to nearby ‘natural’ thaw ponds. Conclusions: The nine years of field observations in a unique shrub removal experiment at a Siberian tundra site document possible trajectories of small-scale permafrost collapse and the initial stage of vegetation recovery, which is essential knowledge for assessing future tundra landscape changes.

Climate warming is faster in the Arctic than the global average. Nutrient availability in the tundra soil is expected to increase by climate warming through (i) accelerated nutrient mobilization in the surface soil layers, and (ii) increased thawing depths during the growing season which increases accessibility of nutrients in the deeper soil layers. Both processes may initiate shifts in tundra vegetation composition. It is important to understand the effects of these two processes on tundra plant functional types. We manipulated soil thawing depth and nutrient availability at a Northeast-Siberian tundra site to investigate their effects on above- and below-ground responses of four plant functional types (grasses, sedges, deciduous shrubs and evergreen shrubs). Seasonal thawing was accelerated with heating cables at c. 15 cm depth without warming the surface soil, whereas nutrient availability was increased in the surface soil by adding slow-release NPK fertilizer at c. 5 cm depth. A combination of these two treatments was also included. This is the first field experiment specifically investigating the effects of accelerated thawing in tundra ecosystems. Deep soil heating increased the above-ground biomass of sedges, the deepest rooted plant functional type in our study, but did not affect biomass of the other plant functional types. In contrast, fertilization increased above-ground biomass of the two dwarf shrub functional types, both of which had very shallow root systems. Grasses showed the strongest response to fertilization, both above- and below-ground. Grasses were deep-rooted, and they showed the highest plasticity in terms of vertical root distribution, as grass root distribution shifted to deep and surface soil in response to deep soil heating and surface soil fertilization respectively. Synthesis. Our results indicate that increased thawing depth can only benefit deep-rooted sedges, while the shallow-rooted dwarf shrubs, as well as flexible-rooted grasses, take advantage of increased nutrient availability in the upper soil layers. Our results suggest that grasses have the highest root plasticity, which enables them to be more competitive in rapidly changing environments. We conclude that root vertical distribution strategies are important for vegetation responses to climate-induced increases in soil nutrient availability in Arctic tundra, and that future shifts in vegetation composition will depend on the balance between changes in thawing depth and nutrient availability in the surface soil.

AimsShrub expansion is common in the tundra biome and has been linked to climate warming. However, the underlying mechanisms are still not fully understood. This study aimed to investigate the seasonal and vertical rooting patterns of different plant functional types, which is important for predicting tundra vegetation dynamics.MethodsWe harvested root samples by soil coring and investigated seasonal changes in root biomass and vertical root distribution across a vegetation gradient, focusing on the differences between graminoids and dwarf shrubs, at a northeastern Siberian tundra.ResultsGraminoid fine root biomass increased significantly during the growing season, whereas that of shrubs was already high at the beginning and did not change later on. Shrubs had a much shallower rooting pattern than graminoids. Also, shrub roots did not respond to increases in permafrost thawing depth over the growing season, whereas graminoids grew fine roots in deeper, recently thawed soil layers during the growing season.ConclusionsOur results show that shrubs are predominantly shallow-rooted and grow roots earlier than graminoids, which allows shrubs to take advantage of the nutrient pulse after snowmelt in the early growing season. In contrast, the deep-rooted graminoids can access the nutrients in deeper soil and may profit from increasing permafrost thawing depth. The outcome of the competitive interactions between graminoids and shrubs in tundra may depend on the balance between the benefits associated with earlier root growth and deeper root distribution, respectively. The shrub expansion with climate warming observed in recent decades suggests that earlier root growth in the upper soil layer may be more important than increased rooting depth later in the growing season. Abstract

It is widely believed that deciduous tundra-shrub dominance is increasing in the pan-Arctic region, mainly due to rising temperature. We sampled dwarf birch (Betula nana L.) at a northeastern Siberian tundra site and used dendrochronological methods to explore the relationship between climatic variables and local shrub dominance. We found that establishment of shrub ramets was positively related to summer precipitation, which implies that the current high dominance of B. nana at our study site could be related to high summer precipitation in the period from 1960 to 1990. The results confirmed that early summer temperature is most influential to annual growth rates of B. nana. In addition, summer precipitation stimulated shrub growth in years with warm summers, suggesting that B. nana growth may be co-limited by summer moisture supply. The dual controlling role of temperature and summer precipitation on B. nana growth and establishment is important to predict future climate-driven vegetation dynamics in the Arctic tundra.

The above mentioned article was originally scheduled for publication in the special issue on Ecology of Tundra Arthropods with guest editors Toke T. Høye . Lauren E. Culler. Erroneously, the article was published in Polar Biology, Volume 40, Issue 11, November, 2017. The publisher sincerely apologizes to the guest editors and the authors for the inconvenience caused.

In tundra ecosystems, global warming is expected to accelerate litter decomposition and to lead to shifts in vegetation composition. To understand these shifts, it is important to understand the interactions between global warming, vegetation composition, litter quality and decomposition in the tundra. In addition, it is important to consider root litter since roots are the major part of plant biomass in the tundra. In order to increase our understanding of decomposition, and root decomposition in particular, we performed a litter transplant experiment in northeastern Siberia, in which we measured mass loss for leaf and root litter (live and dead material) of the two dominant plant species, graminoid Eriophorum vaginatum and shrub Betula nana , in three vegetation types ( E. vaginatum or B. nana dominated and mixed vegetation) during the growing season. Our results show that although leaf decomposition did not differ between the two species, root decomposition showed significant differences. Mass loss of live roots was higher for E. vaginatum than for B. nana , but mass loss of E. vaginatum dead roots was lowest. In addition, we found evidence for home-field advantage in litter decomposition: litter of a plant decomposed faster in vegetation where it was dominant. Mass loss rates of the litter types were significantly correlated with phosphorus content, rather than nitrogen content. This indicates that phosphorus limits decomposition in this tundra site. The low decomposition rate of B. nana live roots compared to E. vaginatum live roots suggests that the acceleration of decomposition in the Arctic may be partly counteracted by the expected expansion of shrubs. However, more information on litter input rates and direct effects of climate change on decomposition rates are needed to accurately predict the effects of climate change on carbon dynamics in tundra ecosystems.

Tundra shrubs are slow-growing species limited by low air temperature and scarce nutrient availability. However, shrub expansion has been widely observed in the Arctic during the last decades and attributed to climate warming. Shift in shrub growth, wood structure and abundance affects the surface albedo and permafrost thawing and these changes may feedback to climate. Despite the importance of shrub–climate feedbacks, uncertainties about shrub growth sensitivity to climate remain. Here, we explored the indirect effects of climate warming on shrub growth (vertical and radial), bark thickness, and bark investment in four arctic shrub species. We combined a field experiment addressing two suggested growth drivers – thawing depth and nutrient availability – with dendroecology in a Siberian tundra ecosystem. We used heating cables to increase the thawing depth. To enhance the nutrient availability, we fertilized the surface soil layers. We found that shrub growth was mainly limited by nutrient availability, as indicated by the fertilization treatment effects on shrub growth ring widths. We also found a bark thickness decrease with the combined soil heating and nutrient addition treatment and a negative correlation between bark investment and growth rate for two of the species. These findings suggest that tundra shrubs, especially deciduous species, will grow faster and taller driven by an increasing nutrient availability in the surface soil layers. However, shrubs might become more vulnerable to pests, herbivory, and climate extremes, such as frost or drought events, due to thinner bark and lower bark investment. Using dendroecological approaches in field experiments simulating projected climate scenarios for the Arctic, and an increasing number of study species and locations will reduce uncertainties related to shrub growth sensitivity to climate and other processes driving shrub dynamics.

Chronic, low intensity herbivory by invertebrates, termed background herbivory, has been understudied in tundra, yet its impacts are likely to increase in a warmer Arctic. The magnitude of these changes is however hard to predict as we know little about the drivers of current levels of invertebrate herbivory in tundra. We assessed the intensity of invertebrate herbivory on a common tundra plant, the dwarf birch (Betula glandulosa-nana complex), and investigated its relationship to latitude and climate across the tundra biome. Leaf damage by defoliating, mining and gall-forming invertebrates was measured in samples collected from 192 sites at 56 locations. Our results indicate that invertebrate herbivory is nearly ubiquitous across the tundra biome but occurs at low intensity. On average, invertebrates damaged 11.2% of the leaves and removed 1.4% of total leaf area. The damage was mainly caused by external leaf feeders, and most damaged leaves were only slightly affected (12% leaf area lost). Foliar damage was consistently positively correlated with mid-summer (July) temperature and, to a lesser extent, precipitation in the year of data collection, irrespective of latitude. Our models predict that, on average, foliar losses to invertebrates on dwarf birch are likely to increase by 6–7% over the current levels with a 1 °C increase in summer temperatures. Our results show that invertebrate herbivory on dwarf birch is small in magnitude but given its prevalence and dependence on climatic variables, background invertebrate herbivory should be included in predictions of climate change impacts on tundra ecosystems. This study is a joint contribution of the Herbivory Network (http://herbivory.biology.ualberta.ca) and the Network for Arthropods of the Tundra (NeAT; https://tundraarthropods.wordpress.com/). Dwarf birch distribution maps were kindly provided by Kyle Joly. Sample collection during 2014 was facilitated by INTERACT (http://www.eu-interact.org/). ICB was supported by a postdoctoral fellowship funded by the Icelandic Research Fund (Rannsóknasjóður, grant nr 152468-051) and AXA Research Fund (15-AXA-PDOC-307); MtB and EK were supported by the Nordic Centre of Excellence TUNDRA, funded by the Norden Top-Level Research Initiative ‘‘Effect Studies and Adaptation to Climate Change’’; EMS and KAB were supported by COAT (Climate-ecological Observatory of the Arctic Tundra); AB was supported by MOBILITY PLUS (1072/MOB/2013/0) and the Polish-American Fulbright Commission; CGB was supported by IUT 20-28, EcolChang e Center of Excellence; BCF and TK were supported by the Academy of Finland (project 256991); MMPDH was supported by The Netherlands Organization for Scientific Research (NWO-ALW, VIDI grant 864.09.014); DSH was supported by the Natural Sciences and Engineering Research Council of Canada; AH was supported by the Research Council of Norway (grant 244557/E50); JL was funded by the German Research Foundation DFG (project WI 2680/8-1); MM-F was supported by a NERC IRF fellowship NE/L011859/1; SN was supported by the Villum foundation’s Young Investigator Programme (VKR023456); JS was supported by Kempestiftelserna and the Research Foundation Flanders (FWO); AS and NS were supported by the grant of RFBR (project 16-44-890108), grant of UB of RAS (project 15-15-4-35) and IEC “Arctic” of Yamal Government Department of Science and Innovation; LES and PAW were supported by the UK Natural Environment Research Council (NERC) grant NE/K000284/1; MVK and VZ were supported by the Academy of Finland (project 276671). Scopus

This special issue comprising 14 articles emerged from the PAGES supported meeting: Holocene Circum Arctic Peatland Carbon Dynamics Community Wide Data Synthesis and Modeling Initiatives which took place from the 12 16 October 2013 in Bethlehem Pennsylvania. It is a precursor product of PAGES' C PEAT Working Group. ABSTRACT: Historic carbon accumulation rates in four bogs on a north to south transect from Sweden to Germany were calculated by using the bulk densities and carbon concentrations of 1 cm peat layers and a fine resolution radiocarbon chronology. Carbon accumulation rates were compared to environmental data to explore the effects of climatic factors. Carbon accumulation rates in a period without clear human impact on the bog ecosystems (c.ad 1700–ad 1800) ranged from 25 g C/m2/yr in the most northern site to 50 g C/m2/yr in the southernmost site which coincided with increasing annual temperatures from north to south. This suggests that temperature or growing season length is a major factor influencing carbon accumulation rates at different geographical sites. The temporal variations in carbon accumulation rates within the sites tentatively suggest that carbon accumulation rates may still increase with further warming in northern peat bogs but decrease in southern peat bogs.

Vegetation changes, such as shrub encroachment and wetland expansion, have been observed in many Arctic tundra regions. These changes feed back to permafrost and climate. Permafrost can be protected by soil shading through vegetation as it reduces the amount of solar energy available for thawing. Regional climate can be affected by a reduction in surface albedo as more energy is available for atmospheric and soil heating. Here, we compared the shortwave radiation budget of two common Arctic tundra vegetation types dominated by dwarf shrubs (Betula nana) and wet sedges (Eriophorum angustifolium) in North-East Siberia. We measured time series of the shortwave and longwave radiation budget above the canopy and transmitted radiation below the canopy. Additionally, we quantified soil temperature and heat flux as well as active layer thickness. The mean growing season albedo of dwarf shrubs was 0.15 ± 0.01, for sedges it was higher (0.17 ± 0.02). Dwarf shrub transmittance was 0.36 ± 0.07 on average, and sedge transmittance was 0.28 ± 0.08. The standing dead leaves contributed strongly to the soil shading of wet sedges. Despite a lower albedo and less soil shading, the soil below dwarf shrubs conducted less heat resulting in a 17 cm shallower active layer as compared to sedges. This result was supported by additional, spatially distributed measurements of both vegetation types. Clouds were a major influencing factor for albedo and transmittance, particularly in sedge vegetation. Cloud cover reduced the albedo by 0.01 in dwarf shrubs and by 0.03 in sedges, while transmittance was increased by 0.08 and 0.10 in dwarf shrubs and sedges, respectively. Our results suggest that the observed deeper active layer below wet sedges is not primarily a result of the summer canopy radiation budget. Soil properties, such as soil albedo, moisture, and thermal conductivity, may be more influential, at least in our comparison between dwarf shrub vegetation on relatively dry patches and sedge vegetation with higher soil moisture., Environmental Science & Technology, 13 (2016), ISSN:0013-936X, ISSN:1520-5851

International audience; Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. * Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. * We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m)2 yr)1 for each 1°C increase. * Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.

Questions: What are the long-term effects of climate change on the plant species composition and carbon sequestration in peat bogs?Methods: We developed a bog ecosystem model that includes vegetation, carbon, nitrogen and water dynamics. Two groups of vascular plant species and three groups of Sphagnum species compete with each other for light and nitrogen. The model was tested by comparing the outcome with long-term historic vegetation changes in peat cores from Denmark and England. A climate scenario was used to analyse the future effects of atmospheric CO2, temperature and precipitation.Results: The main changes in the species composition since 1766 were simulated by the model. Simulations for a future warmer, and slightly wetter, climate with doubling CO2 concentration suggest that little will change in species composition, due to the contrasting effects of increasing temperatures (favouring vascular plants) and CO2 (favouring Sphagnum). Further analysis of the effects of temperature showed that simulated carbon sequestration is negatively related to vascular plant expansion. Model results show that increasing temperatures may still increase carbon accumulation at cool, low N deposition sites, but decrease carbon accumulation at high N deposition sites.Conclusions: Our results show that the effects of temperature, precipitation, N-deposition and atmospheric CO2 are not straightforward, but interactions between these components of global change exist. These interactions are the result of changes in vegetation composition. When analysing long-term effects of global change, vegetation changes should be taken into account and predictions should not be based on temperature increase alone.

Despite their low primary production, ombrotrophic peatlands have a considerable potential to store atmospheric carbon as a result of their extremely low litter decomposition rates. Projected changes in temperature and nitrogen (N) deposition may increase decomposition rates by their positive effects on microbial activity and litter quality, which can be expected to result in enhanced mass loss and N release from Sphagnum and vascular plant litter. This is the first study that examines the combined effects of increased temperature and N deposition on decomposition in bogs. We investigated mass loss and N release at four bog sites along a gradient from north Sweden to northeast Germany in which both temperature and N deposition increased from north to south. We performed two litterbag experiments: one reciprocal experiment with Eriophorum vaginatum litter and one experiment using recalcitrant (Sphagnum fuscum) and more degradable (Sphagnum balticum) Sphagnum litter collected from the most northern site. We measured mass loss and N release during two (Sphagnum) and three (E. vaginatum) years. The N concentration and decomposability of the E. vaginatum litter did not differ between the sites. Mass loss from E. vaginatum litter increased over the gradient from north to south, but there was no such effect on Sphagnum litter. N loss of all litter types was affected by collection site, incubation site and time and all interactions between these factors. N release in Sphagnum was positively related to N concentration. We conclude that decomposition of vascular plants and Sphagnum litter is influenced by different environmental drivers, with enhanced temperatures stimulating mass loss of vascular plant litter, but not of Sphagnum. Enhanced N deposition increases Sphagnum litter N loss. As long-term consequences of climate change will presumably entail a higher vascular plant production, overall litter decomposition rates are likely to increase, especially in combination with increased temperature.

Although recent eddy covariance measurements in boreal forests provide CO2 and energy exchange data for the whole ecosystem, very little is known about the role of the understory vegetation. We conducted chamber flux measurements in an Alaskan black spruce forest in order to compare CO2 and water vapour exchange among patches of understory vegetation dominated by feathermoss (Hylocomium), peatmoss (Sphagnum), vascular plants (mainly low shrubs), or lichens. We found large differences among understory vegetation types with respect to midday net CO2 exchange and its seasonal pattern of variation. Sphagnum and vascular-plant plots showed net CO2 uptake, with most uptake on days of high light availability. In contrast, Hylocomium and lichen plots lost CO2 during the middle of the growing season, but showed net uptake at the end of the season when the soil had cooled down. Spatial variation in net CO2 exchange was related more to biotic variables like soil organic matter than to environmental variables. The differences among vegetation types with respect to water vapour fluxes were smaller, because evapotranspiration was more constrained by climatic variables like solar radiation. Net CO2 uptake in Hylocomium plots was negatively related to evapotranspiration, because Hylocomium photosynthesis was very sensitive to evaporative stress, whereas evapotranspiration and net CO2 uptake in Sphagnum were not limited by moisture conditions. These differences suggest that species composition of the understory should be taken into account when discussing understory contributions to CO2 and water vapour exchange. Author Keywords: Climate change; CO2; Evapotranspiration; Hylocomium splendens; Sphagnum capillifolium; Understory vegetation

The response of plant growth to rising CO2 levels appears todepend on nutrient availability, but it is not known whether the growth of bogplants reacts similarly. We therefore studied the effects of elevatedCO2 in combination with N supply on the growth ofSphagnum mosses and vascular plants in ombrotrophic bogvegetation. Because the growth of Sphagnum is lessnutrient-limited than that of vascular plants, we hypothesized thatSphagnum would benefit from elevated CO2. In ourgreenhouse experiment, peat monoliths (34 cm diameter, 40cm deep) with intact bog vegetation were exposed to ambient (350ppmv) or elevated (560 ppmv) atmosphericCO2 combined with low (no N addition) or high (5 g Nm−2 yr−1 added) N deposition for twogrowing seasons. Elevated atmospheric CO2 had unexpected deleterious effectson the growth of Sphagnum magellanicum, the dominant Sphagnumspecies. Growth was greatly reduced, particularly in the second growing seasonwhen, regardless of N supply, the mosses looked unhealthy. The negativeCO2 effect was strongest in the warmest months, suggesting a combinedeffect of elevated CO2 and the raised temperatures in the greenhouse.High N deposition favored Rhynchospora alba, which became the dominant vascular plant speciesduring the experiment. Biomass increased more when N supply was high. There wereno significant effects of elevated CO2 on vascular plants, althoughelevated CO2 combined with high N supply tended to increase theaboveground vascular plant biomass. As Sphagnum is the maincarbon-sequestrating species in bogs and rising atmospheric CO2levels are likely to be followed by increases in temperature, there is an urgentneed for further research on the combined effects of elevated CO2 andincreased temperature on Sphagnum growth in bog ecosystems.

We determined evapotranspiration in three experiments designed to study the effects of elevated CO2 and increased N deposition on ombrotrophic bog vegetation. Two experiments used peat monoliths with intact bog vegetation in containers, with one experiment outdoors and the other in a greenhouse. A third experiment involved monocultures and mixtures of Sphagnum magellanicum and Eriophorum angustifolium in containers in the same greenhouse. To determine water use of the bog vegetation in JulyAugust for each experiment and each year we measured water inputs and outputs from the containers. We studied the effects of elevated CO2 and N supply on evapotranspiration in relation to vascular plant biomass and exposure of the moss surface (measured as height of the moss surface relative to the container edge). Elevated CO2 reduced water use of the bog vegetation in all three experiments, but the CO2 effect on evapotranspiration interacted with vascular plant biomass and exposure of the moss surface. Evapotranspiration in the outdoor experiment was largely determined by evaporation from the Sphagnum moss surface (as affected by exposure to wind) and less so by vascular plant transpiration. Nevertheless, elevated CO2 significantly reduced evapotranspiration by 910␒n the outdoor experiment. Vascular plants reduced evapotranspiration in the outdoor experiment, but increased water use in the greenhouse experiments. The relation between vascular plant abundance and evapotranspiration appears to depend on wind conditions; suggesting that vascular plants reduce water losses mainly by reducing wind speed at the moss surface. Sphagnum growth is very sensitive to changes in water level; low water availability can have deleterious effects. As a consequence, reduced evapotranspiration in summer, whether caused by elevated CO2 or by small increases in vascular plant cover, is expected to favour Sphagnum growth in ombrotrophic bog vegetation.

Part of the missing sink in the global CO2 budget has been attributed to the positive effects of CO2 fertilization and N deposition on carbon sequestration in Northern Hemisphere terrestrial ecosystems. The genus Sphagnum is one of the most important groups of plant species sequestrating carbon in temperate and northern bog ecosystems, because of the low decomposability of the dead material it produces. The effects of raised CO2 and increased atmospheric N deposition on growth of Sphagnum and other plants were studied in bogs at four sites across Western Europe. Contrary to expectations, elevated CO2 did not significantly affect Sphagnum biomass growth. Increased N deposition reduced Sphagnum mass growth, because it increased the cover of vascular plants and the tall moss Polytrichum strictum. Such changes in plant species composition may decrease carbon sequestration in Sphagnum-dominated bog ecosystems.

To study the relationships between groups of organisms and the degree to which these relationships are consistent across major climatic gradients, we analysed the testate amoeba (Protozoa) communities, vegetation and water chemistry of one peatland in five countries: Switzerland, The Netherlands, Great Britain, Sweden and Finland, as part of the BERI (Bog Ecosystem Research Initiative) project. The relationships between the different data sets and subsets were investigated by means of detrended correspondence analysis, canonical correspondence analysis and Mantel permutation tests. The comparison of data on vegetation and testate amoebae showed that inter-site differences are more pronounced for the vegetation than for the testate amoebae species assemblage. Testate amoebae are a useful tool in multi-site studies and in environmental monitoring of peatlands because: (1) the number of species in Sphagnum-dominated peatlands is much higher than for mosses or vascular plants; (2) most peatland species are cosmopolitan in their distributions and therefore less affected than plants by biogeographical distribution patterns, thus differences in testate amoeba assemblages can be interpreted primarily in terms of ecology; (3) they are closely related to the ecological characteristics of the exact spot where they live, therefore they can be used to analyse small-scale gradients that play a major role in the functioning of peatland ecosystems. This study revealed the existence of small-scale vertical gradients within the vegetation and life-form niche separation in response to water chemistry. The deep-rooted plants such as Carex spp. and Eriophorum spp. are related to the chemistry of water sampled at or near the ground water table, whereas the mosses are not. Testate amoebae were shown to be ecologically more closely related to the chemistry of water sampled at or near the water table level and to the mosses than to the deep-rooted plants.

Peatlands store approximately 30% of global soil carbon, most in moss-dominated bogs. Future climatic changes, such as changes in precipitation patterns and warming, are expected to affect peat bog vegetation composition and thereby its long-term carbon sequestration capacity. Theoretical work suggests that an episode of rapid environmental change is more likely to trigger transitions to alternative ecosystem states than a gradual, but equally large, change in conditions. We used a dynamic vegetation model to explore the impacts of drought events and increased temperature on vegetation composition of temperate peat bogs. We analyzed the consequences of six patterns of summer drought events combined with five temperature scenarios to test whether an open peat bog dominated by moss (Sphagnum) could shift to a tree-dominated state. Unexpectedly, neither a gradual decrease in the amount of summer precipitation nor the occurrence of a number of extremely dry summers in a row could shift the moss-dominated peat bog permanently into a tree-dominated peat bog. The increase in tree biomass during drought events was unable to trigger positive feedbacks that keep the ecosystem in a tree-dominated state after a return to previous ‘normal’ rainfall conditions. In contrast, temperature increases from 1 °C onward already shifted peat bogs into tree-dominated ecosystems. In our simulations, drought events facilitated tree establishment, but temperature determined how much tree biomass could develop. Our results suggest that under current climatic conditions, peat bog vegetation is rather resilient to drought events, but very sensitive to temperature increases, indicating that future warming is likely to trigger persistent vegetation shifts. Keywords: alternative states, climate change, ecosystem model, extreme events, peatlands, pulse, rainfall, Sphagnum, temperature increase, vegetation shift

Recently observed Arctic greening trends from normalized difference vegetation index (NDVI) data suggest that shrub growth is increasing in response to increasing summer temperature. An increase in shrub cover is expected to decrease summer albedo and thus positively feed back to climate warming. However, it is unknown how albedo and NDVI are affected by shrub cover and inter-annual variations in the summer climate. Here, we examine the relationship between deciduous shrub fractional cover, NDVI and albedo using field data collected at a tundra site in NE Siberia. Field data showed that NDVI increased and albedo decreased with increasing deciduous shrub cover. We then selected four Arctic tundra study areas and compiled annual growing season maximum NDVI and minimum albedo maps from MODIS satellite data (2000–10) and related these satellite products to tundra vegetation types (shrub, graminoid, barren and wetland tundra) and regional summer temperature. We observed that maximum NDVI was greatest in shrub tundra and that inter-annual variation was negatively related to summer minimum albedo but showed no consistent relationship with summer temperature. Shrub tundra showed higher albedo than wetland and barren tundra in all four study areas. These results suggest that a northwards shift of shrub tundra might not lead to a decrease in summer minimum albedo during the snow-free season when replacing wetland tundra. A fully integrative study is however needed to link results from satellite data with in situ observations across the Arctic to test the effect of increasing shrub cover on summer albedo in different tundra vegetation types.

Arctic tundra vegetation composition is expected to undergo rapid changes during the coming decades because of changes in climate. Higher air temperatures generally favor growth of deciduous shrubs, often at the cost of moss growth. Mosses are considered to be very important to critical tundra ecosystem processes involved in water and energy exchange, but very little empirical data are available. Here, we studied the effect of experimental moss removal on both understory evapotranspiration and ground heat flux in plots with either a thin or a dense low shrub canopy in a tundra site with continuous permafrost in Northeast Siberia. Understory evapotranspiration increased with removal of the green moss layer, suggesting that most of the understory evapotranspiration originated from the organic soil layer underlying the green moss layer. Ground heat flux partitioning also increased with green moss removal indicating the strong insulating effect of moss. No significant effect of shrub canopy density on understory evapotranspiration was measured, but ground heat flux partitioning was reduced by a denser shrub canopy. In summary, our results show that mosses may exert strong controls on understory water and heat fluxes. Changes in moss or shrub cover may have important consequences for summer permafrost thaw and concomitant soil carbon release in Arctic tundra ecosystems. Key words: moss; evaporation; ground heat flux; shrub; permafrost; tundra; Arctic; climate change.

A large proportion of northern peatlands consists of Sphagnum-dominated ombrotrophic bogs. In these bogs, peat mosses (Sphagnum) and vascular plants occur in an apparent stable equilibrium, thereby sustaining the carbon sink function of the bog ecosystem. How global warming and increased nitrogen (N) deposition will affect the species composition in bog vegetation is still unclear. We performed a transplantation experiment in which mesocosms with intact vegetation were transplanted southward from north Sweden to north-east Germany along a transect of four bog sites, in which both temperature and N deposition increased. In addition, we monitored undisturbed vegetation in control plots at the four sites of the latitudinal gradient. Four growing seasons after transplantation, ericaceous dwarf shrubs had become much more abundant when transplanted to the warmest site which also had highest N deposition. As a result ericoid aboveground biomass in the transplanted mesocosms increased most at the southernmost site, this site also had highest ericoid biomass in the undisturbed vegetation. The two dominant Sphagnum species showed opposing responses when transplanted southward; Sphagnum balticum height increment decreased, whereas S. fuscum height increment increased when transplanted southward. Sphagnum production did not differ significantly among the transplanted mesocosms, but was lowest in the southernmost control plots. The dwarf shrub expansion and increased N concentrations in plant tissues we observed, point in the direction of a positive feedback toward vascular plant-dominance suppressing peat-forming Sphagnum in the long term. However, our data also indicate that precipitation and phosphorus availability influence the competitive balance between Sphagnum, dwarf shrubs and graminoids.

Climate change is expected to cause extensive vegetation changes in the Arctic: deciduous shrubs are already expanding, in response to climate warming. The results from transect studies suggest that increasing shrub cover will impact significantly on the surface energy balance. However, little is known about the direct effects of shrub cover on permafrost thaw during summer. We experimentally quantified the influence of Betula nana cover on permafrost thaw in a moist tundra site in northeast Siberia with continuous permafrost. We measured the thaw depth of the soil, also called the active layer thickness (ALT), ground heat flux and net radiation in 10 m diameter plots with natural B. nana cover (control plots) and in plots in which B. nana was removed (removal plots). Removal of B. nana increased ALT by 9% on average late in the growing season, compared with control plots. Differences in ALT correlated well with differences in ground heat flux between the control plots and B. nana removal plots. In the undisturbed control plots, we found an inverse correlation between B. nana cover and late growing season ALT. These results suggest that the expected expansion of deciduous shrubs in the Arctic region, triggered by climate warming, may reduce summer permafrost thaw. Increased shrub growth may thus partially offset further permafrost degradation by future temperature increases. Permafrost models need to include a dynamic vegetation component to accurately predict future permafrost thaw.

Climate warming is likely to increase nutrient mineralization rates in bog ecosystems which may change the plant species composition. We examined the competitive relationships between two graminoid species, Eriophorum vaginatum and Rhynchospora alba, and two ericoid species, Calluna vulgaris and Vaccinium oxycoccus, at different nutrient supply rates. In a greenhouse, the plants were grown in monocultures and mixtures at four nutrient treatments: control, high N, high P, and high N + P. The results show that the ericoids responded more strongly to the nutrient treatments than the graminoids. The dwarf shrubs showed higher growth rates and reduced root:shoot ratio at high N + P supply. When grown in mixture the ericoids increased their growth, while graminoids decreased in biomass or showed signs of nutrient limitation compared to their monoculture plants. This suggests that under increased nutrient availability, bogs are more likely to turn into dwarf shrub dominated ecosystems and not grassland.

Climate change can be expected to increase the frequency of summer droughts and associated low water tables in ombrotrophic peatlands. We studied the effects of periodic water table drawdown in a mesocosm experiment. Mesocosms were collected in Southern Sweden, and subsequently brought to an experimental field in the Netherlands. Two water table treatments were applied: one with constant water tables at 5 cm below the moss surface, and one in which the water table was allowed to drop, resulting in water tables fluctuating between 5 and 21 cm below the moss surface. Sphagnum growth, as well as Sphagnum and vascular plant abundance, were assessed for 2 1 2 years. Our results show that the abundance of graminoid species increased most in the constant water table treatment. In contrast, ericoid species cover increased when water tables were allowed to fluctuate. Furthermore, Sphagnum cuspidatum production decreased with fluctuating summer water tables, while Sphagnum magellanicum responded oppositely. From these results we conclude that increased occurrence of periods with low water tables may bring about a shift in dominant Sphagnum species as well as a shift from graminoid to ericoid vascular plant cover, resembling the shift from hollow to lawn or hummock vegetation. The difference in response within functional groups (vascular plants, Sphagnum) may add to the resilience of the ecosystem.

Increased N deposition in Europe has affected mire ecosystems. However, knowledge on the physiological responses is poor. We measured photosynthetic responses to increasing N deposition in two peatmoss species (Sphagnum balticum and Sphagnum fuscum) from a 3-year, north-south transplant experiment in northern Europe, covering a latitudinal N deposition gradient ranging from 0.28 g N m(-2) year(-1) in the north, to 1.49 g N m(-2) year(-1) in the south. The maximum photosynthetic rate (NP(max)) increased southwards, and was mainly explained by tissue N concentration, secondly by allocation of N to the photosynthesis, and to a lesser degree by modified photosystem II activity (variable fluorescence/maximum fluorescence yield). Although climatic factors may have contributed, these results were most likely attributable to an increase in N deposition southwards. For S. fuscum, photosynthetic rate continued to increase up to a deposition level of 1.49 g N m(-2) year(-1), but for S. balticum it seemed to level out at 1.14 g N m(-2) year(-1). The results for S. balticum suggested that transplants from different origin (with low or intermediate N deposition) respond differently to high N deposition. This indicates that Sphagnum species may be able to adapt or physiologically adjust to high N deposition. Our results also suggest that S. balticum might be more sensitive to N deposition than S. fuscum. Surprisingly, NP(max) was not (S. balticum), or only weakly (S. fuscum) correlated with biomass production, indicating that production is to a great extent is governed by factors other than the photosynthetic capacity.

Question: Can mixing ratio and species affect the use of substrate-derived CO2 by Sphagnum? Location: Poor fen in south Sweden and greenhouse in Wageningen, The Netherlands. Methods: Two mixing ratios of Sphagnum cuspidatum and S. magellanicum were exposed to two levels of CO2 by pumping CO2 enriched and non-enriched water through aquaria containing the species mixtures in the greenhouse. Results: Enhanced CO2 stimulated the production of S. cuspidatum, but only in aquaria co-dominated by S. magellanicum, coinciding with higher CO2 concentrations in the water layer. The denser growing S. magellanicum seemed to reduce gas escape from the water, resulting in accumulation of dissolved CO2. Adding CO2 did not affect species replacement. Conclusions: The use of substrate-derived CO2 for Sphagnum production depended on species identity and mixing ratio. The effect of mixing ratio on CO2 concentrations in the water layer suggests that species composition may affect both the efficiency with which substrate-derived CO2 is trapped and subsequently used. This could result in hitherto unexplored feedbacks between vegetation composition and gas exchange.

Although numerous studies have addressed the effects of increased N deposition on nutrient-poor environments such as raised bogs, few studies have dealt with to what extent, and on what time-scale, reductions in atmospheric N supply would lead to recovery of the ecosystems in question. Since a considerable part of the negative effects of elevated N deposition on raised bogs can be related to an imbalance in tissue nutrient concentrations of the dominant peat-former Sphagnum, changes in Sphagnum nutrient concentration after excess N supply may be used as an early indicator of ecosystem response. This study focuses on the N and P concentrations of Sphagnum magellanicum and Sphagnum fallax before, during and after a factorial fertilization experiment with N and P in two small peatlands subject to a background bulk deposition of 2 g N m(-2) year(-1). Three years of adding N (4.0 g N m(-2) year(-1)) increased the N concentration, and adding P (0.3 g P m(-2) year(-1)) increased the P concentration in Sphagnum relative to the control treatment at both sites. Fifteen months after the nutrient additions had ceased, N concentrations were similar to the control whereas P concentrations, although strongly reduced, were still slightly elevated. The changes in the N and P concentrations were accompanied by changes in the distribution of nutrients over the capitulum and the stem and were congruent with changes in translocation. Adding N reduced the stem P concentration, whereas adding P reduced the stem N concentration in favor of the capitulum. Sphagnum nutrient concentrations quickly respond to reductions in excess nutrient supply, indicating that a management policy aimed at reducing atmospheric nutrient input to bogs can yield results within a few years.

The competition between peat mosses (Sphagnum) and vascular plants as affected by raised CO2 and increased N deposition was studied in a glasshouse experiment by exposing peat monoliths with monocultures and mixtures of Sphagnummagellanicum and Eriophorumangustifolium to ambient (350 ppmv) or raised (560 ppmv) atmospheric CO2 concentrations, combined with low (no N addition) or high (5 g m2 yr1 added) N deposition. Growth of the two species was monitored for three growing seasons. The presence of Eriophorum did not affect Sphagnum biomass, because Eriophorum density did not become high enough to severely shade the moss surface. In contrast, Sphagnum had a negative effect on Eriophorum biomass, particularly on the number of flowering stems. Possibly, the presence of a living Sphagnum layer decreased nutrient availability to Eriophorum by immobilising nutrients mineralised from the peat. Raised CO2 and/or increased N deposition did not change these competitive relationships between Sphagnum and Eriophorum, but had independent effects. Raised CO2 had a positive effect both on Sphagnum and Eriophorum biomass, though on Eriophorum the effect was transient, probably because of P limitation. Nitrogen addition had a direct negative effect on Sphagnum height growth in the first growing season, but by the third year an increased shoot density had cancelled this out, so no N effect on Sphagnum biomass was present at the end of the experiment. The response of Eriophorum to N addition was small; N availability appeared not to limit its growth.

To monitor global change, large scale long term studies are needed. Such studies often focus on vegetation, but most plant species have limited distribution areas. Micro-organisms by contrast are mostly cosmopolitan in their distributions. To study the relationships between organisation groups, we analysed the testate amoebae (Protozoa), vegetation, and water chemistry of five Sphagnum peatlands across Europe. Inter-site differences were more pronounced for the vegetation than for testate amoebae species assemblage. Testate amoebae represent a useful tool in multi-site studies and environmental monitoring of peatlands because: 1) the number of species is much higher than for plants, 2) most species are cosmopolitan and are therefore less affected by biogeographical distribution patterns than plants; thus differences in testate amoebae assemblages can be interpreted primarily in terms of ecology, 3) testate amoebae can be used to analyse and monitor small scale (cm) gradients that play a major role in the functioning of peatland ecosystems. We further studied the effect of elevated CO2 on microbial communities in the same peatlands. Elevated CO2 increased the biomass of heterotrophic bacteria and decreased the biomass of medium size protozoa (mostly small testate amoebae). These effects suggest changes in community functioning that may have feedback effects on other components of the ecosystem.

To predict the role of ombrotrophic bogs as carbon sinks in the future, it is crucial to understand how Sphagnum vegetation in bogs will respond to global change. We performed a greenhouse experi- ment to study the effects of two temperature treatments (17.5 and 21.7C) and two N addition treatments (0 and 4gNm -2 year -1 ) on the growth of four Sphagnum species from three geographically interspersed regions: S. fuscum, S. balticum (northern and central Sweden), S. magellanicum and S. cuspidatum (south- ern Sweden). We studied the growth and cover change in four combinations of these Sphagnum species during two growing seasons. Sphagnum height increment and production were affected negatively by high temper- ature and high N addition. However, the northern species were more affected by temperature, while the southern species were more affected by N addition. High temperature depressed the cover of the 'wet' species, S. balticum and S. cuspidatum. Nitrogen concentrations increased with high N addition. N:P and N:K ratios indicated P-limited growth in all treatments and co-limitation of P and K in the high N treatments. In the second year of the experiment, several containers suffered from a severe fungal infection, particularly affecting the 'wet' species and the high N treatment. Our findings suggest that global change can have negative consequences for the production of Sphagnum species in bogs, with impor- tant implications for the carbon sequestration in these ecosystems.