Showing papers on "Tundra published in 2011"

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Abstract: Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Carbon loss from an unprecedented Arctic tundra wildfire

Abstract: In 2007, an area of more than 1,000 square kilometres of Alaskan tundra was destroyed by a single fire, more than doubling the cumulative area burnt in this region since 1950. Michelle Mack and colleagues now show that, in the process, 2.1 teragrams of carbon was released and about one-third of soil organic matter burned away, thereby potentially exposing permafrost soils to thaw. The amount of carbon released from the entire burn was comparable to the annual net carbon sink of the entire Arctic tundra biome during the past 25 years of the twentieth century. As tundra fires are expected to increase as the climate warms, combustion of 'old growth' tundra soil could constitute a positive climate feedback, by transferring surface soil carbon to the atmosphere and accelerating the thaw and decomposition of deeper permafrost carbon. Arctic tundra soils store large amounts of carbon (C) in organic soil layers hundreds to thousands of years old that insulate, and in some cases maintain, permafrost soils1,2. Fire has been largely absent from most of this biome since the early Holocene epoch3, but its frequency and extent are increasing, probably in response to climate warming4. The effect of fires on the C balance of tundra landscapes, however, remains largely unknown. The Anaktuvuk River fire in 2007 burned 1,039 square kilometres of Alaska’s Arctic slope, making it the largest fire on record for the tundra biome and doubling the cumulative area burned since 1950 (ref. 5). Here we report that tundra ecosystems lost 2,016 ± 435 g C m−2 in the fire, an amount two orders of magnitude larger than annual net C exchange in undisturbed tundra6. Sixty per cent of this C loss was from soil organic matter, and radiocarbon dating of residual soil layers revealed that the maximum age of soil C lost was 50 years. Scaled to the entire burned area, the fire released approximately 2.1 teragrams of C to the atmosphere, an amount similar in magnitude to the annual net C sink for the entire Arctic tundra biome averaged over the last quarter of the twentieth century7. The magnitude of ecosystem C lost by fire, relative to both ecosystem and biome-scale fluxes, demonstrates that a climate-driven increase in tundra fire disturbance may represent a positive feedback, potentially offsetting Arctic greening8 and influencing the net C balance of the tundra biome.

Satellite observations of high northern latitude vegetation productivity changes between 1982 and 2008: ecological variability and regional differences

Abstract: To assess ongoing changes in high latitude vegetation productivity we compared spatiotemporal patterns in remotely sensed vegetation productivity in the tundra and boreal zones of North America and Eurasia. We compared the long-term GIMMS (Global Inventory Modeling and Mapping Studies) NDVI (Normalized Difference Vegetation Index) to the more recent and advanced MODIS (Moderate Resolution Imaging Spectroradiometer) NDVI data set, and mapped circumpolar trends in a gross productivity metric derived from the former. We then analyzed how temporal changes in productivity differed along an evergreen–deciduous gradient in boreal Alaska, along a shrub cover gradient in Arctic Alaska, and during succession after fire in boreal North America and northern Eurasia. We find that the earlier reported contrast between trends of increasing tundra and decreasing boreal forest productivity has amplified in recent years, particularly in North America. Decreases in boreal forest productivity are most prominent in areas of denser tree cover and, particularly in Alaska, evergreen forest stands. On the North Slope of Alaska, however, increases in tundra productivity do not appear restricted to areas of higher shrub cover, which suggests enhanced productivity across functional vegetation types. Differences in the recovery of post-disturbance vegetation productivity between North America and Eurasia are described using burn chronosequences, and the potential factors driving regional differences are discussed.

Changes in forest productivity across Alaska consistent with biome shift

Abstract: Global vegetation models predict that boreal forests are particularly sensitive to a biome shift during the 21st century. This shift would manifest itself first at the biome!s margins, with evergreen forest expanding into current tundra while being replaced by grasslands or temperate forest at the biome!s southern edge. We evaluated changes in forest productivity since 1982 across boreal Alaska by linking satellite estimates of primary productivity and a large tree-ring data set. Trends in both records show consistent growth increases at the boreal‐tundra ecotones that contrast with drought-induced productivity declines throughout interior Alaska. These patterns support the hypothesized effects of an initiating biome shift. Ultimately, tree dispersal rates, habitat availability and the rate of future climate change, and how it changes disturbance regimes, are expected to determine where the boreal biome will undergo a gradual geographic range shift, and where a more rapid decline.

Effects of experimental warming of air, soil and permafrost on carbon balance in Alaskan tundra

Abstract: The carbon (C) storage capacity of northern latitude ecosystems may diminish as warming air temperatures increase permafrost thaw and stimulate decomposition of previously frozen soil organic C. However, warming may also enhance plant growth so that photosynthetic carbon dioxide (CO2) uptake may, in part, offset respiratory losses. To determine the effects of air and soil warming on CO2 exchange in tundra, we established an ecosystem warming experiment – the Carbon in Permafrost Experimental Heating Research (CiPEHR) project – in the northern foothills of the Alaska Range in Interior Alaska. We used snow fences coupled with spring snow removal to increase deep soil temperatures and thaw depth (winter warming) and open-top chambers to increase growing season air temperatures (summer warming). Winter warming increased soil temperature (integrated 5–40 cm depth) by 1.5 1C, which resulted in a 10% increase in growing season thaw depth. Surprisingly, the additional 2 kg of thawed soil Cm 2 in the winter warming plots did not result in significant changes in cumulative growing season respiration, which may have been inhibited by soil saturation at the base of the active layer. In contrast to the limited effects on growing-season C dynamics, winter warming caused drastic changes in winter respiration and altered the annual C balance of this ecosystem by doubling the net loss of CO2 to the atmosphere. While most changes to the abiotic environment at CiPEHR were driven by winter warming, summer warming effects on plant and soil processes resulted in 20% increases in both gross primary productivity and growing season ecosystem respiration and significantly altered the age and sources of CO2 respired from this ecosystem. These results demonstrate the vulnerability of organic C stored in near surface permafrost to increasing temperatures and the strong potential for warming tundra to serve as a positive feedback to global climate change.

Vegetation and Production Ecology of an Alaskan Arctic Tundra

Abstract: This volume on botanical research in tundra represents the culmination of four years of intensive and integrated field research centered at Barrow, Alaska The volume summarizes the most significant results and interpretations of the pri mary producer projects conducted in the US IBP Tundra Biome Program (1970-1974) Original data reports are available from the authors and can serve as detailed references for interested tundra researchers Also, the results of most projects have been published in numerous papers in various journals The introduction provides a brief overview of other ecosystem components The main body presents the results in three general sections The summary chapter is an attempt to integrate ideas and information from the previous papers as well as extant literature In addition, this chapter focuses attention on pro cesses of primary production which should receive increased emphasis Although this book will not answer all immediate questions, it hopefully will enhance future understanding of the tundra, particularly as we have studied it in Northern Alaska"

What are the main climate drivers for shrub growth in Northeastern Siberian tundra

Abstract: . Deciduous shrubs are expected to rapidly expand in the Arctic during the coming decades due to climate warming. A transition towards more shrub-dominated tundra may have large implications for the regional surface energy balance, permafrost stability and carbon storage capacity, with consequences for the global climate system. However, little information is available on the natural long-term shrub growth response to climatic variability. Our aim was to determine the climate factor and time period that are most important to annual shrub growth in our research site in NE-Siberia. Therefore, we determined annual radial growth rates in Salix pulchra and Betula nana shrubs by measuring ring widths. We constructed shrub ring width chronologies and compared growth rates to regional climate and remotely sensed greenness data. Early summer temperature was the most important factor influencing ring width of S. pulchra (Pearson's r = 0.73, p

Long‐term experimental manipulation of climate alters the ectomycorrhizal community of Betula nana in Arctic tundra

Abstract: Climate warming is leading to shrub expansion in Arctic tundra. Shrubs form ectomycorrhizal (ECM) associations with soil fungi that are central to ecosystem carbon balance as determinants of plant community structure and as decomposers of soil organic matter. To assess potential climate change impacts on ECM communities, we analysed fungal internal transcribed spacer sequences from ECM root tips of the dominant tundra shrub Betula nana growing in treatments plots that had received long-term warming by greenhouses and/or fertilization as part of the Arctic Long-Term Ecological Research experiment at Toolik Lake Alaska, USA. We demonstrate opposing effects of long-term warming and fertilization treatments on ECM fungal diversity; with warming increasing and fertilization reducing the diversity of ECM communities. We show that warming leads to a significant increase in high biomass fungi with proteolytic capacity, especially Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. In contrast, fertilization treatments led to relatively small changes in the composition of the ECM community, but increased the abundance of saprotrophs. Our data suggest that warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size.

Hot spots for nitrous oxide emissions found in different types of permafrost peatlands

Abstract: Recent findings on large nitrous oxide (N2O) emissions from permafrost peatlands have shown that tundra soils can support high N2O release, which is on the contrary to what was thought previously. However, field data on this topic have been very limited, and the spatial and temporal extent of the phenomenon has not been known. To address this question, we studied N2O dynamics in two types of subarctic permafrost peatlands, a peat plateau in Russia and three palsa mires in Finland, including also adjacent upland soils. The peatlands studied have surfaces that are uplifted by frost (palsas and peat plateaus) and partly unvegetated as a result of wind erosion and frost action. Unvegetated peat surfaces with high N2O emissions were found from all the studied peatlands. Very high N2O emissions were measured from peat circles at the Russian site (1.40±0.15 g N2O m−2 yr−1). Elevated, sparsely vegetated peat mounds at the same site had significantly lower N2O release. The N2O emissions from bare palsa surfaces in Northern Finland were highly variable but reached high rates, similar to those measured from the peat circles. All the vegetated soils studied had negligible N2O release. At the bare peat surfaces, the large N2O emissions were supported by the absence of plant N uptake, the low C : N ratio of the peat, the relatively high gross N mineralization rate and favourable moisture content, together increasing availability of mineral N for N2O production. We hypothesize that frost heave is crucial for high N2O emissions, since it lifts the peat above the water table, increasing oxygen availability and making it vulnerable to the the physical processes that may remove the vegetation cover. In the future, permafrost thawing may change the distribution of wet and dry surfaces in permafrost peatlands, which will affect N2O emissions.

Responses of High Arctic wet sedge tundra to climate warming since 1980

Abstract: The global climate is changing rapidly and Arctic regions are showing responses to recent warming. Responses of tundra ecosystems to climate change have been examined primarily through short-term experimental manipulations, with few studies of long-term ambient change. We investigated changes in above- and belowground biomass of wet sedge tundra to the warming climate of the Canadian High Arctic over the past 25 years. Aboveground standing crop was harvested from five sedge meadow sites and belowground biomass was sampled from one of the sites in the early 1980s and in 2005 using the same methods. Aboveground biomass was on average 158% greater in 2005 than in the early 1980s. The belowground biomass was also much greater in 2005: root biomass increased by 67% and rhizome biomass by 139% since the early 1980s. Dominant species from each functional group (graminoids, shrubs and forbs) showed significant increases in aboveground biomass. Responsive species included the dominant sedge species Carex aquatilis stans, C. membranacea, and Eriophorum angustifolium, as well as the dwarf shrub Salix arctica and the forb Polygonum viviparum. However, diversity measures were not different between the sample years. The greater biomass correlated strongly with increased annual and summer temperatures over the same time period, and was significantly greater than the annual variation in biomass measured in 1980–1983. Increased decomposition and mineralization rates, stimulated by warmer soils, were likely a major cause of the elevated productivity, as no differences in the mass of litter were found between sample periods. Our results are corroborated by published short-term experimental studies, conducted in other wet sedge tundra communities which link warming and fertilization with elevated decomposition, mineralization and tundra productivity. We believe that this is the first study to show responses in High Arctic wet sedge tundra to recent climate change.

A High Arctic soil ecosystem resists long-term environmental manipulations

Abstract: We evaluated above- and belowground ecosystem changes in a 16 year, combined fertilization and warming experiment in a High Arctic tundra deciduous shrub heath (Alexandra Fiord, Ellesmere Island, NU, Canada). Soil emissions of the three key greenhouse gases (GHGs) (carbon dioxide, methane, and nitrous oxide) were measured in mid-July 2009 using soil respiration chambers attached to a FTIR system. Soil chemical and biochemical properties including Q10 values for CO2, CH4, and N2O, Bacteria and Archaea assemblage composition, and the diversity and prevalence of key nitrogen cycling genes including bacterial amoA, crenarchaeal amoA, and nosZ were measured. Warming and fertilization caused strong increases in plant community cover and height but had limited effects on GHG fluxes and no substantial effect on soil chemistry or biochemistry. Similarly, there was a surprising lack of directional shifts in the soil microbial community as a whole or any change at all in microbial functional groups associated with CH4 consumption or N2O cycling in any treatment. Thus, it appears that while warming and increased nutrient availability have strongly affected the plant community over the last 16 years, the belowground ecosystem has not yet responded. This resistance of the soil ecosystem has resulted in limited changes in GHG fluxes in response to the experimental treatments.

The response of Arctic vegetation to the summer climate: relation between shrub cover, NDVI, surface albedo and temperature

Abstract: 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

Multi-Decadal Changes in Tundra Environments and Ecosystems: Synthesis of the International Polar Year-Back to the Future Project (IPY-BTF)

Abstract: Understanding the responses of tundra systems to global change has global implications. Most tundra regions lack sustained environmental monitoring and one of the only ways to document multi-decadal change is to resample historic research sites. The International Polar Year (IPY) provided a unique opportunity for such research through the Back to the Future (BTF) project (IPY project #512). This article synthesizes the results from 13 papers within this Ambio Special Issue. Abiotic changes include glacial recession in the Altai Mountains, Russia; increased snow depth and hardness, permafrost warming, and increased growing season length in sub-arctic Sweden; drying of ponds in Greenland; increased nutrient availability in Alaskan tundra ponds, and warming at most locations studied. Biotic changes ranged from relatively minor plant community change at two sites in Greenland to moderate change in the Yukon, and to dramatic increases in shrub and tree density on Herschel Island, and in sub-arctic Sweden. The population of geese tripled at one site in northeast Greenland where biomass in non-grazed plots doubled. A model parameterized using results from a BTF study forecasts substantial declines in all snowbeds and increases in shrub tundra on Niwot Ridge, Colorado over the next century. In general, results support and provide improved capacities for validating experimental manipulation, remote sensing, and modeling studies.

Progressive Cenozoic cooling and the demise of Antarctica’s last refugium

Abstract: The Antarctic Peninsula is considered to be the last region of Antarctica to have been fully glaciated as a result of Cenozoic climatic cooling As such, it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the Gondwana supercontinent Drill cores and seismic data acquired during two cruises (SHALDRIL I and II) in the northernmost Peninsula region yield a record that, when combined with existing data, indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years Mountain glaciation began in the latest Eocene (approximately 37–34 Ma), contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric CO2 concentrations This climate cooling was accompanied by a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern peninsula during the Eocene A mosaic of southern beech and conifer-dominated woodlands and tundra continued to occupy the region during the Oligocene (approximately 34–23 Ma) By the middle Miocene (approximately 16–116 Ma), localized pockets of limited tundra still existed at least until 128 Ma The transition from temperate, alpine glaciation to a dynamic, polythermal ice sheet took place during the middle Miocene The northernmost Peninsula was overridden by an ice sheet in the early Pliocene (approximately 53–36 Ma) The long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the Antarctic margin, involving the opening of ocean passageways and associated establishment of circumpolar circulation

The Cooling Capacity of Mosses: Controls on Water and Energy Fluxes in a Siberian Tundra Site

Abstract: 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.

Longer growing seasons do not increase net carbon uptake in the northeastern Siberian tundra

Abstract: With global warming, snowmelt is occurring earlier and growing seasons are becoming longer around the Arctic. It has been suggested that this would lead to more uptake of carbon due to a lengthening of the period in which plants photosynthesize. To investigate this suggestion, 8 consecutive years of eddy covariance measurements at a northeastern Siberian graminoid tundra site were investigated for patterns in net ecosystem exchange, gross primary production (GPP) and ecosystem respiration (R-eco). While GPP showed no clear increase with longer growing seasons, it was significantly increased in warmer summers. Due to these warmer temperatures however, the increase in uptake was mostly offset by an increase in R-eco. Therefore, overall variability in net carbon uptake was low, and no relationship with growing season length was found. Furthermore, the highest net uptake of carbon occurred with the shortest and the coldest growing season. Low uptake of carbon mostly occurred with longer or warmer growing seasons. We thus conclude that the net carbon uptake of this ecosystem is more likely to decrease rather than to increase under a warmer climate. These results contradict previous research that has showed more net carbon uptake with longer growing seasons. We hypothesize that this difference is due to site-specific differences, such as climate type and soil, and that changes in the carbon cycle with longer growing seasons will not be uniform around the Arctic. (Less)

The Influence of Vegetation Type on the Dominant Soil Bacteria, Archaea, and Fungi in a Low Arctic Tundra Landscape

Abstract: Arctic vegetation communities vary greatly over short distances due to landscape heterogeneities in topography and hydrological conditions, but corresponding patterns and controls for soil microbial communities are not well understood. We characterized and compared the most abundant phylotypes within replicate soil microbial communities (n = 4) underlying the four principal vegetation types in Canadian low Arctic tundra (dry heath, birch hummock, tall birch, and wet sedge) using denaturing gradient gel electrophoresis (DGGE) of small subunit rRNA genes. We identifi ed 10 major bacterial phylotypes. Although most were present in all soil samples, their relative abundances diff ered signifi cantly and consistently according to vegetation type. By contrast, the fungal communities of all vegetation types were dominated by two common phylotypes. Th e communities of major archaea (11 identifi ed) diff ered substantially among some of the vegetation types and even among replicate patches of the same vegetation type, indicating large spatial heterogeneities that could not be attributed to the infl uence of vegetation type. Bacterial and fungal communities in all vegetation types were dominated by Acidobacteria and Zygomycota, respectively. Archaeal communities were dominated by Euryarchaeota in tall birch and wet sedge although both Euryarchaeota and Th aumarchaeota were abundant in the birch hummock and dry heath soils. We conclude that vegetation type exerts a strong infl uence on soil bacterial community structure, and a relatively small and varying infl uence on archaeal and fungal communities in low Arctic tundra. Finally, variation in bacterial community structure among the vegetation types was correlated with soil soluble N and N mineralization potential, suggesting a close association between the relative abundances of dominant soil bacteria and N availability across low Arctic tundra.

Tundra vegetation effects on pan-Arctic albedo

Abstract: Recent field experiments in tundra ecosystems describe how increased shrub cover reduces winter albedo, and how subsequent changes in surface net radiation lead to altered rates of snowmelt. These findings imply that tundra vegetation change will alter regional energy budgets, but to date the effects have not been documented at regional or greater scales. Using satellite observations and a pan-Arctic vegetation map, we examined the effects of shrub vegetation on albedo across the terrestrial Arctic. We included vegetation classes dominated by low shrubs, dwarf shrubs, tussock-dominated graminoid tundra, and non-tussock graminoid tundra. Each class was further stratified by bioclimate subzones. Low-shrub tundra had higher normalized difference vegetation index values and earlier albedo decline in spring than dwarf-shrub tundra, but for tussock tundra, spring albedo declined earlier than for low-shrub tundra. Our results illustrate how relatively small changes in vegetation properties result in differences in albedo dynamics, regardless of shrub growth, that may lead to differences in net radiation upwards of 50 W m − 2 at weekly time scales. Further, our findings imply that changes to the terrestrial Arctic energy budget during this important seasonal transition are under way regardless of whether recent satellite observed productivity trends are the result of shrub expansion. We conclude that a better understanding of changes in vegetation productivity and distribution in Arctic tundra is essential for accurately quantifying and predicting carbon and energy fluxes and associated climate feedbacks.

Land use and land cover change in Arctic Russia: ecological and social implications of industrial development.

Abstract: Sizable areas in northwestern arctic Russia have undergone fundamental change in recent decades as the exploration of vast hydrocarbon deposits has intensified. We undertook two case studies on the influence of oil and gas activities within neighbouring federal districts in the tundra zone. Employing a strongly interdisciplinary approach, we studied the ecological, spatial and social dimensions of the visible and perceived changes in land use and land cover. Our data are derived from field sampling, remote sensing and intensive participant observation with indigenous Nenets reindeer herders and non-indigenous workers. Important trends include the rapid expansion of infrastructure, a large influx of workers who compete for freshwater fish, and extensive transformation from shrub- to grass- and sedge-dominated tundra. The latter represents an alternative ecosystem state that is likely to persist indefinitely. On terrain disturbed by off-road vehicle traffic, reindeer pastures’ vegetation regenerates with fewer species among which grasses and sedges dominate, thus reducing biodiversity. To have maximum forage value such pastures must be accessible and free of trash, petro-chemicals and feral dogs. We found that a wide range of direct and indirect impacts, both ecological and social, accumulate in space and time such that the combined influence is effectively regional rather than local, depending in part on the placement of facilities. While incoming workers commonly commit poaching, they also serve as exchange partners, making barter for goods possible in remote locations. In general, the same positive and negative impacts of the presence of industry were mentioned in each study region. Even using very high-resolution remote sensing data (Quickbird-2) it is not possible to determine fully the amount of degraded territory in modern oil and gas fields. With regard to policy, both biophysical and social impacts could be substantially reduced if information flow between herders and workers were to be optimized.

Widespread release of old carbon across the Siberian Arctic echoed by its large rivers

Abstract: . Over decadal-centennial timescales, only a few mechanisms in the carbon-climate system could cause a massive net redistribution of carbon from land and ocean systems to the atmosphere in response to climate warming. The largest such climate-vulnerable carbon pool is the old organic carbon (OC) stored in Arctic permafrost (perennially frozen) soils. Climate warming, both predicted and now observed to be the strongest globally in the Eurasian Arctic and Alaska, causes thaw-release of old permafrost carbon from local tundra sites. However, a central challenge for the assessment of the general vulnerability of this old OC pool is to deduce any signal integrating its release over larger scales. Here we examine radiocarbon measurements of molecular soil markers exported by the five Great Russian-Arctic Rivers (Ob, Yenisey, Lena, Indigirka and Kolyma), employed as natural integrators of carbon release processes in their watersheds. The signals held in estuarine surface sediments revealed that average radiocarbon ages of n-alkanes increased east-to-west from 6400 yr BP in Kolyma to 11 400 yr BP in Ob. This is consistent with westwards trends of both warmer climate and more degraded organic matter as indicated by the ratio of high molecular weight (HMW) n-alkanoic acids to HMW n-alkanes. The dynamics of Siberian permafrost can thus be probed via the molecular-radiocarbon signal as carried by Arctic rivers. Old permafrost carbon is at present vulnerable to mobilization over continental scales. Climate-induced changes in the radiocarbon fingerprint of released permafrost carbon will likely depend on changes in both permafrost coverage and Arctic soil hydraulics.

Changes in high arctic tundra plant reproduction in response to long‐term experimental warming

Abstract: We provide new information on changes in tundra plant sexual reproduction in response to long-term (12 years) experimental warming in the High Arctic. Open-top chambers (OTCs) were used to increase growing season temperatures by 1―2 °C across a range of vascular plant communities. The warming enhanced reproductive effort and success in most species; shrubs and graminoids appeared to be more responsive than forbs. We found that the measured effects of warming on sexual reproduction were more consistently positive and to a greater degree in polar oasis compared with polar semidesert vascular plant communities. Our findings support predictions that long-term warming in the High Arctic will likely enhance sexual reproduction in tundra plants, which could lead to an increase in plant cover. Greater abundance of vegetation has implications for primary consumers - via increased forage availability, and the global carbon budget - as a function of changes in permafrost and vegetation acting as a carbon sink. Enhanced sexual reproduction in Arctic vascular plants may lead to increased genetic variability of offspring, and consequently improved chances of survival in a changing environment. Our findings also indicate that with future warming, polar oases may play an important role as a seed source to the surrounding polar desert landscape.

Impact of shrub canopies on understorey vegetation in western Eurasian tundra

Abstract: Question: How does the composition and species richness of understorey vegetation associate with changing abundance of deciduous shrub canopies? What are the species-specific associations between shrubs and understorey plants? Location: Tundra habitats along an over 1000-km long range, spanning from NW Fennoscandia to the Yamal Peninsula in northwest Russia. Methods: The data from 758 vegetation sample plots from 12 sites comprised cover estimates of all plant species, including bryophytes and lichens, and canopy height of deciduous shrubs. The relationships between shrub volume and cover of plant groups and species richness of vegetation were investigated. In addition, species-specific associations between understorey species and shrub volume were analysed. Results: Shrub abundance was shown to be associated with the composition of understorey vegetation, and the association patterns were consistent across the study sites. Increased forb cover was positively associated with shrub volume, whereas bryophyte, lichen, dwarf shrub and graminoid cover decreased in association with increasing volume of deciduous shrubs. The total species richness of vegetation declined with increasing shrub volume. Conclusions: The results suggest that an increase of shrubs – due to climatic warming or a decrease in grazing pressure – is likely to have strong effects on plant–plant interactions and lead to a decrease in the diversity of understorey vegetation.

Carbon balance of Arctic tundra under increased snow cover mediated by a plant pathogen

Abstract: Climate change is affecting plant community composition1 and ecosystem structure, with consequences for ecosystem processes such as carbon storage2, 3, 4. Climate can affect plants directly by altering growth rates1, and indirectly by affecting predators and herbivores, which in turn influence plants5, 6, 7, 8, 9. Diseases are also known to be important for the structure and function of food webs10, 11, 12, 13, 14. However, the role of plant diseases in modulating ecosystem responses to a changing climate is poorly understood15, 16. This is partly because disease outbreaks are relatively rare and spatially variable, such that that their effects can only be captured in long-term experiments. Here we show that, although plant growth was favoured by the insulating effects of increased snow cover in experimental plots in Sweden, plant biomass decreased over the seven-year study. The decline in biomass was caused by an outbreak of a host-specific parasitic fungus, Arwidssonia empetri, which killed the majority of the shoots of the dominant plant species, Empetrum hermaphroditum, after six years of increased snow cover. After the outbreak of the disease, instantaneous measurements of gross photosynthesis and net ecosystem carbon exchange were significantly reduced at midday during the growing season. Our results show that plant diseases can alter and even reverse the effects of a changing climate on tundra carbon balance by altering plant composition.

Methane oxidation associated with submerged brown mosses reduces methane emissions from Siberian polygonal tundra

Abstract: Summary 1. Methane (CH4) oxidation (methanotrophy) associated with submerged brown moss species occurs in polygonal tundra environments of the Siberian Arctic. Methanotrophic bacteria living in close association with mosses are thus not restricted to Sphagnum species and low-pH peatlands. 2. Moss-associated methane oxidation (MAMO) can be an effective buffer for CH4 emissions from permafrost-affected tundra, a region that is of high importance for the global greenhouse gas budget. Combining biogeochemical and molecular approaches revealed that MAMO in polygonal ponds exceeds methanotrophic activity in terrestrial sites by up to two orders of magnitude. 3. Moss-associated methane oxidation is not only promoted by submerged conditions but also by light exposure. Polygonal ponds covered by the brown moss Scorpidium scorpioides became a net sink for atmospheric CH4 (−1.7 mg CH4 m−2 day−1) when exposed to sunlight but a CH4 source (21.6 mg CH4 m−2 day−1) in the absence of light. 4. Based on stable isotope probing with 13CH4, carbon deriving from CH4 was incorporated into the bacterial fatty acids 16:1ω7 and 18:1ω9/ω7 common in methanotrophs and into plant phytol, sitosterol and stigmastanol, all of which are highly abundant in moss biomass. 5. Synthesis. A mutualistic symbiosis between methanotrophic bacteria and brown mosses reduces CH4 emissions from Arctic polygonal tundra by at least 5%. Both partners benefit from this association: the moss from the additional CO2 supplied through methane oxidation and the methane-oxidizing bacteria from the oxygen produced through photosynthesis. Considering that submerged mosses are widely abundant in the polar region, MAMO may have a major impact on carbon turnover rates in Arctic freshwater environments.

Microtopographic controls on ecosystem functioning in the Arctic Coastal Plain

Abstract: [1] The investigation of the microtopographic controls on thermal and hydrologic conditions of the soil and consequently the carbon dynamics from Arctic regions is of major importance. Ecosystem respiration (ER) between microsites of the same tundra type could differ more than ER in different tundra types even at different latitudes. In this study we investigated the microtopographic effect on soil temperature, thaw depth, pH, oxidation reduction potential (ORP), electrical conductivity (EC), dissolved CO2, vegetation types, and ER rates from different features forming the low-center polygon structure. Most of these environmental variables significantly differ particularly between areas with higher elevation (polygon rims) and with lower elevation (low-center polygons). Polygon rims presented the lowest water table and showed the lowest thaw depth and the highest ER (a seasonal average of 1 μmol CO2 m−2 s−1), almost double than the ER in the low-center polygons (a seasonal average of 0.6 μmol CO2 m−2 s−1). The microtopographic gradient from polygon rims to low-centers led to a very consistent pattern in pH, EC, ORP and dissolved CO2, with low-centers presenting the highest pH, the highest EC, the highest dissolved CO2, and the lowest ORP. Based on vegetation measurements, we also showed that microtopography controls the lateral flow of organic matter, and that vascular plant material accumulates as litter in the lower elevation areas, possibly contributing to the higher dissolved CO2 in the low-center polygons. Microtopography, and the ramifications discussed here, should be considered when evaluating landscape scale environmental controls on carbon dynamics in the Arctic.