[Show abstract][Hide abstract] ABSTRACT: Ecology Letters (2014) 17: 260 In Elmendorf et al. (2012), the contributions of Marilyn Walker and Juha Alatalo to the initial sampling of the warm-ing experiments were overlooked. The corrected author list is as follows:. et al. (2012). Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time. Ecol. Lett., 15, 164–175.
[Show abstract][Hide abstract] ABSTRACT: The Arctic is one of the ecosystems most affected by climate change; in particular, winter temperatures and precipitation are predicted to increase with consequent changes to snow cover depth and duration. Whether the snow-free period will be shortened or prolonged depends on the extent and temporal patterns of the temperature and precipitation rise; resulting changes will likely affect plant growth with cascading effects throughout the ecosystem. We experimentally manipulated snow regimes using snow fences and shoveling and assessed aboveground size of eight common high arctic plant species weekly throughout the summer. We demonstrated that plant growth responded to snow regime, and that air temperature sum during the snow free period was the best predictor for plant size. The majority of our studied species showed periodic growth; increases in plant size stopped after certain cumulative temperatures were obtained. Plants in early snow-free treatments without additional spring warming were smaller than controls. Response to deeper snow with later melt-out varied between species and categorizing responses by growth forms or habitat associations did not reveal generic trends. We therefore stress the importance of examining responses at the species level, since generalized predictions of aboveground growth responses to changing snow regimes cannot be made.
PLoS ONE 01/2014; 9(2):e86281. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Droppings of Svalbard reindeer (Rangifer tarandus platyrhynchus) could affect the carbon and nitrogen cycles in tundra ecosystems. The aim of this study was to evaluate the potential of reindeer droppings originating from the winter diet for emission and/or absorption of methane (CH4) and nitrous oxide (N2O) in summer. An incubation experiment was conducted over 14 days using reindeer droppings and mineral subsoil collected from a mound near Ny-Ålesund, Svalbard, to determine the potential exchanges of CH4 and N2O for combinations of two factors, reindeer droppings (presence or absence) and soil moisture (dry, moderate, or wet). A line transect survey was conducted to determine the distribution density of winter droppings at the study site. The incubation experiment showed a weak absorption of CH4 and a weak emission of N2O. Reindeer droppings originating from the winter diet had a negligible effect on the exchange fluxes of both CH4 and N2O. Although the presence of droppings resulted in a short-lasting increase in N2O emissions on day 1 (24 h from the start) for moderate and wet conditions, the emission rates were still very small, up to 3 μg N2O m−2 h−1.
[Show abstract][Hide abstract] ABSTRACT: The High Arctic winter is expected to be altered through ongoing and future climate change. Winter precipitation and snow depth are projected to increase and melt out dates change accordingly. Also, snow cover and depth will play an important role in protecting plant canopy from increasingly more frequent extreme winter warming events. Flower production of many Arctic plants is dependent on melt out timing, since season length determines resource availability for flower preformation. We erected snow fences to increase snow depth and shorten growing season, and counted flowers of six species over 5 years, during which we experienced two extreme winter warming events. Most species were resistant to snow cover increase, but two species reduced flower abundance due to shortened growing seasons. Cassiope tetragona responded strongly with fewer flowers in deep snow regimes during years without extreme events, while Stellaria crassipes responded partly. Snow pack thickness determined whether winter warming events had an effect on flower abundance of some species. Warming events clearly reduced flower abundance in shallow but not in deep snow regimes of Cassiope tetragona, but only marginally for Dryas octopetala. However, the affected species were resilient and individuals did not experience any long term effects. In the case of short or cold summers, a subset of species suffered reduced reproductive success, which may affect future plant composition through possible cascading competition effects. Extreme winter warming events were shown to expose the canopy to cold winter air. The following summer most of the overwintering flower buds could not produce flowers. Thus reproductive success is reduced if this occurs in subsequent years. We conclude that snow depth influences flower abundance by altering season length and by protecting or exposing flower buds to cold winter air, but most species studied are resistant to changes. Winter warming events, often occurring together with rain, can substantially remove snow cover and thereby expose plants to cold winter air. Depending on morphology, different parts of the plant can be directly exposed. On this picture, we see Dryas octopetala seed heads from the previous growing season protrude through the remaining ice layer after a warming event in early 2010. The rest of the plant, including meristems and flower primordia, are still somewhat protected by the ice. In the background we can see a patch of Cassiope tetragona protruding through the ice; in this case, the whole plant including flower primordia is exposed, which might be one reason why this species experienced a loss of flowers the following season. Photograph by Philipp Semenchuk.
Ecology and Evolution 08/2013; 3(8):2586-99. · 1.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently there have been several studies using open top chambers (OTCs) or cloches to examine the response of Arctic plant communities to artificially elevated temperatures. Few, however, have investigated multi-trophic systems, or the effects of both temperature and vertebrate grazing treatments on invertebrates. This study investigated trophic interactions between an herbivorous insect (Sitobion calvulum, Aphididae), a woody perennial host plant (Salix polaris) and a selective vertebrate grazer (barnacle geese, Branta leucopsis). In a factorial experiment, the responses of the insect and its host to elevated temperatures using open top chambers (OTCs) and to three levels of goose grazing pressure were assessed over two summer growing seasons (2004 and 2005). OTCs significantly enhanced the leaf phenology of Salix in both years and there was a significant OTC by goose presence interaction in 2004. Salix leaf number was unaffected by treatments in both years, but OTCs increased leaf size and mass in 2005. Salix reproduction and the phenology of flowers were unaffected by both treatments. Aphid densities were increased by OTCs but unaffected by goose presence in both years. While goose presence had little effect on aphid density or host plant phenology in this system, the OTC effects provide interesting insights into the possibility of phenological synchrony disruption. The advanced phenology of Salix effectively lengthens the growing season for the plant, but despite a close association with leaf maturity, the population dynamics of the aphid appeared to lack a similar phenological response except for the increased population observed. This article is protected by copyright. All rights reserved.
Global Change Biology 06/2013; · 8.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Environmental manipulation studies are integral to determining biological consequences of climate warming. Open Top Chambers (OTCs) have been widely used to assess summer warming effects on terrestrial biota, with their effects during other seasons normally being given less attention even though chambers are often deployed year-round. In addition, their effects on temperature extremes and freeze-thaw events are poorly documented. To provide robust documentation of the microclimatic influences of OTCs throughout the year, we analysed temperature data from 20 studies distributed across polar and alpine regions. The effects of OTCs on mean temperature showed a large range (-0.9 to 2.1 °C) throughout the year, but did not differ significantly between studies. Increases in mean monthly and diurnal temperature were strongly related (R(2) = 0.70) with irradiance, indicating that PAR can be used to predict the mean warming effect of OTCs. Deeper snow trapped in OTCs also induced higher temperatures at soil/vegetation level. OTC-induced changes in the frequency of freeze-thaw events included an increase in autumn and decreases in spring and summer. Frequency of high-temperature events in OTCs increased in spring, summer and autumn compared with non-manipulated control plots. Frequency of low-temperature events was reduced by deeper snow accumulation and higher mean temperatures. The strong interactions identified between aspects of ambient environmental conditions and effects of OTCs suggest that a detailed knowledge of snow depth, temperature and irradiance levels enables us to predict how OTCs will modify the microclimate at a particular site and season. Such predictive power allows a better mechanistic understanding of observed biotic response to experimental warming studies and for more informed design of future experiments. However, a need remains to quantify OTC effects on water availability and wind speed (affecting, for example, drying rates and water stress) in combination with microclimate measurements at organism level.
Global Change Biology 01/2013; 19(1):64-74. · 8.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Temperature is increasing at unprecedented rates across most of the tundra biome 1 . Remote-sensing data indicate that con-temporary climate warming has already resulted in increased productivity over much of the Arctic 2,3 , but plot-based evidence for vegetation transformation is not widespread. We analysed change in tundra vegetation surveyed between 1980 and 2010 in 158 plant communities spread across 46 locations. We found biome-wide trends of increased height of the plant canopy and maximum observed plant height for most vascular growth forms; increased abundance of litter; increased abundance of evergreen, low-growing and tall shrubs; and decreased abundance of bare ground. Intersite comparisons indicated an association between the degree of summer warming and change in vascular plant abundance, with shrubs, forbs and rushes increasing with warming. However, the association was dependent on the climate zone, the moisture regime and the presence of permafrost. Our data provide plot-scale evidence linking changes in vascular plant abundance to local summer warming in widely dispersed tundra locations across the globe. Latitudinal gradients in tundra vegetation and palaeorecords of increases in the abundance of tundra shrubs during warm periods provide strong evidence of climate warming as an important moderator of plant composition in this biome 4 . The long life span of most tundra plants suggests that community-level responses to environmental change could occur over decades to centuries, but several lines of evidence indicate that climate-induced changes in tundra vegetation may already be detectable, portending more drastic changes in the coming decades. First, a systematic resurvey of European alpine plants found detectable decreases in cold-adapted species and increases in warm-adapted species over a five-year period, and that such changes were correlated with the degree of localized warming 5 . Second, warming experiments across the tundra biome have documented impacts of a 1–2 • C increase in summer temperature on the composition of tundra plant communities within a decade of warming in some regions, but also highlighted the resistance of tundra vegetation composition to climate warming in some locations 6,7 . Third, normalized difference vegetation index (NDVI) values have increased over the tundra biome in recent years, indicating a greening of the tundra ecosystem coincident with climate warming trends 2,3 . However, NDVI values are sensitive to a variety of ground-cover changes that can be difficult to tease apart, such as the amount and type of vegetation, litter, bare ground and soil-moisture status, and potentially influenced by non-vegetation changes such as atmospheric conditions and satellite drift 8 . Last, plot-based sampling, repeat aerial photography and annual-growth-ring studies have documented recent increases in biomass and shrub abundance in many, but not all, Arctic, high-latitude and alpine tundra ecosystems 9–13 . Attributing these results to climate patterns in a single region is tenuous because factors other than climate *A full list of authors and their affiliations appears at the end of the paper. could be responsible for the observed changes. Thus, despite these compelling lines of evidence, uncertainty remains as to the extent of change in vegetation that has occurred across the tundra biome owing to climate change. Cross-study synthesis offers an opportunity to take advantage of naturally occurring spatial variation in the rate and direction of climate change to test the association between site-specific environmental and biological change 14 . Here, we report on decadal scale vegetation changes that have occurred in Arctic and alpine tundra using the largest data set of plot-level tundra vegetation change ever assembled (Fig. 1; Supplementary Table S1). We hypothesized that tundra vegetation is undergoing directional change over time, with an increase in canopy height and abundance of vascular plants, particularly deciduous, tall and low-growing shrubs, and a corresponding decline in mosses, lichens and bare ground, similar to what has been observed in tundra warming experiments 6,7 . We anticipated that these changes would be greatest in the areas with the most pronounced increases in summer air temperature. Therefore, we examined biome-wide trends in vegetation change; whether vegetation change was spatially associated with local summer temperature trends; and whether the direction of observed changes was consistent with predictions based on warming experiments in tundra ecosystems. Across studies we found increases in mean canopy height; increases in the maximum height of shrubs (especially deciduous, dwarf and tall shrubs), graminoids (especially grasses) and forbs (Fig. 2a); increases in the abundance of litter and evergreen, low and tall shrubs; and declines in bare ground cover (Fig. 2b). Although not always statistically significant, general trends in the height and abundance of vascular and non-vascular plant groups were largely congruent with expectations based on warming experiments; litter and most vascular growth forms increased in height and abundance, whereas mosses showed decreasing trends. These patterns also align with satellite-derived observations of greening across the tundra biome, which are typically thought to reflect increases in total photosynthetic biomass 15 , leaf area 16 and shrub biomass 17 . Summer temperature increased significantly over the study region, but the rate of change was spatially variable: mean study-period summer warming = 0.72 • C (standard error (s.e.m.) = 0.10); p < 0.0001 based on generalized estimating equations (GEEs), range = −1.47–2.29 • C. Taking advantage of the variability among studies, we compared local patterns of vegetation change with local temperature records to determine the sensitivity of tundra vegetation to summer temperature change. Although shrubs are thought to be increasing over much of the tundra biome, we did not find that all types of shrub were uniformly increasing where the summer climate was warming. Instead, we found that warming had a positive effect on the total abundance of shrubs primarily in study locations that were warmer to begin with (Supplementary Table S2; Fig. 3a),
[Show abstract][Hide abstract] ABSTRACT: Understanding the sensitivity of tundra vegetation to climate warming is critical to forecasting future biodiversity and vegetation feedbacks to climate. In situ warming experiments accelerate climate change on a small scale to forecast responses of local plant communities. Limitations of this approach include the apparent site-specificity of results and uncertainty about the power of short-term studies to anticipate longer term change. We address these issues with a synthesis of 61 experimental warming studies, of up to 20 years duration, in tundra sites worldwide. The response of plant groups to warming often differed with ambient summer temperature, soil moisture and experimental duration. Shrubs increased with warming only where ambient temperature was high, whereas graminoids increased primarily in the coldest study sites. Linear increases in effect size over time were frequently observed. There was little indication of saturating or accelerating effects, as would be predicted if negative or positive vegetation feedbacks were common. These results indicate that tundra vegetation exhibits strong regional variation in response to warming, and that in vulnerable regions, cumulative effects of long-term warming on tundra vegetation - and associated ecosystem consequences - have the potential to be much greater than we have observed to date.
[Show abstract][Hide abstract] ABSTRACT: Cooper, E.J., 2011. Polar desert vegetation and plant recruitment in Murchisonfjord, Nordaustlandet, Svalbard. Geografiska Annaler: Series A, Physical Geography, 93, 243–252. DOI: 10.1111/j.1468-0459.2011.00426.xABSTRACTVegetation descriptions of Nordaustlandet, Svalbard (80° N) are not well represented in published literature. Harsh climatic conditions in polar deserts may limit vascular plant growth and reproduction and inhibit germination and establishment. Rapid climatic changes in polar areas may precipitate vegetation change, so documentation of present vegetation as a baseline for the future is important. A vegetation survey in the vicinity of Kinnvika in Murchisonfjord, Nordaustlandet, was thus carried out at 23 sites ranging from sparsely vegetated to almost complete cover on the slopes under a birdcliff. In total, 36 vascular plant species were recorded in the area, 28 species under a birdcliff and 21 species at the other sites. Twenty-eight species were registered within quadrats as established plants, and at least 15 species as seedlings and small plants. Vascular plant cover was generally low and varied between sites of differing moisture and nutrient content. The Shannon-Wiener diversity index, H, was also low, ranging between 0 and 2.0, demonstrating uneven distribution of vascular species, with one dominating species at the dry and wet extremes, and several species contributing in the mesic and moist sites. Manured birdcliff sites with high vascular diversity and cover provide important reindeer and geese forage in otherwise sparsely vegetated areas. At moisture limited sites, recruiting plant density significantly correlated to cyanobacterial crust cover. This crust ameliorates the substrate surface providing moisture, warmth, nutrients and a more stable environment for establishing plants, and is thus of major importance for plant community development in High Arctic polar deserts.
Geografiska Annaler Series A Physical Geography 07/2011; 93(4):243 - 252. · 1.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Studies in the Canadian Arctic show dramatic effects of increased goose grazing on vegetation structure and soil conditions, but little is known of the role of goose grazing in the European Arctic. We focused on how geese might affect plant recruitment via effects on seed production and soil seed bank in High Arctic Svalbard. Experimental grazing by captive Barnacle geese (Branta leucopsis (Bechstein, 1803)) decreased flower densities both at normal and at high graz-ing pressure. Geese showed a clear preference for reproductive rather than vegetative shoots. Soil samples collected inside and outside 7-year-old exclosures in an intensively goose-grazed area revealed significant effects on the germinable soil seed bank. The density of viable seeds in the top soil layer inside exclosures was six times higher than in grazed plots. Lower densities of viable seeds occurred in the basal than in the top layer but there was no difference in basal layer seed density between exclosed and grazed plots. This study shows that geese have strong effects on floral abundance and conse-quently on the seed bank. We argue that goose grazing in these systems influences the potential for recovery after a dis-turbance event and thus the long-term plant species diversity and dynamics. Résumé : Les études conduites dans l'Arctique Canadien révèlent des effets considérables suite à l'augmentation du brou-tage par les oies sur la structure de la végétation et les conditions du sol, mais on sait peu de choses sur le rôle du brou-tage par les oies dans l'Arctique Européen. Les auteurs ont cherché à savoir comment les oies peuvent affecter le recrutement des plantes via leurs effets sur la production des graines et la banque de graines du sol, dans le Svalbard en Haut Arctique. Le broutage expérimental par des bernaches captives diminue la densité des fleurs, aussi bien par le brou-tage normal qu'à haute intensité. Les oies montrent une nette préférence pour les tiges reproductives plutôt que les tiges végétatives. Des échantillons de sols récoltés à l'intérieur et à l'extérieur d'exclos âgés de 7 ans, dans une région intensi-vement broutée par les oies, révèlent des effets significatifs sur la banque des graines du sol, aptes à germer. Dans les en-clos, la densité des graines viables des couches superficielles du sol est six fois plus élevée que dans les parcelles broutées. On retrouve des densités plus faibles de graines viables dans la couche basale que dans la couche supérieure, mais à ce niveau il n'y a pas de différence entre la densité des graines des parcelles excloses et broutées. Cette étude mon-tre que les oies exercent un effet considérable sur l'abondance des fleurs et conséquemment sur la banque de graines dans le sol. Les auteurs suggèrent que dans ces systèmes, le broutage par les oies influence le potentiel de recouvrement après un événement perturbant et ainsi la diversité des espèces à long terme et leur dynamique.
Canadian Journal of Botany 03/2011; · 1.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In tundra areas where the growing season is short, any delay in the start of summer may have a considerable effect on plant development, growth and reproductive success. Climate models suggest long-term changes in winter precipitation in the Arctic, which may lead to deeper snow cover and a resultant delay in date of snow melt. In this paper, we investigated the role of snow depth and melt out date on the phenological development and reproductive success of vascular plants in Adventdalen, Svalbard (78° 10'N, 16° 06'E). Effects of natural variations in snow accumulation were demonstrated using two vegetation types (snow depth: meadow 21 cm, heath 32 cm), and fences were used to experimentally increase snow depth by over 1m. Phenological delay was greatest directly after snowmelt in the earlier phenological phases, and had the largest effect on the early development of those species which normally green-up early (i.e. Dryas, Papaver, Salix, Saxifraga). Compressed growing seasons and length of the reproductive period led to a reduced reproductive success in some of the study species. There were fewer flowers, fewer plots with dispersing seeds, and lower germination rates. This can have consequences for plant establishment and community composition in the long-term.
[Show abstract][Hide abstract] ABSTRACT: Sexual reproduction is crucial for plant populations to track and adapt to climate change, but it is uncertain to what degree arctic vascular plants reproduce by seed. Several studies on arctic species show low germination. To re-examine seed germination and evaluate factors limiting sexual reproduction, seeds of 6–22 arctic species were germinated in five different, increasingly more realistic, conditions. Thirteen out of 15 species that were tested in an earlier study in Svalbard, Norway, germinated better in our study. Compared with perceived optimal conditions in a growth chamber, average germination per species was 6%–52% lower in five out of six species germinating at a colder temperature in soil, 36%–64% lower when germinating outdoors in soil, 49%–91% lower when germinating in a moss covered moraine, and 55%–91% lower when germinating in open soil on a moraine. Germination outdoors was below 5% in 10 out of 13 species and not correlated to germination in perceived optimal conditions. The high germination compared with earlier studies suggests that climate warming has already increased seed viability. However, caution should be taken when evaluating species-recruitment potential based on laboratory studies, as germination in the field was limited by species-specific responses to low temperatures, moisture, predation, and safe-site availability.
[Show abstract][Hide abstract] ABSTRACT: Summary1. Understanding the impact of disturbance on vegetation and the resilience of plant communities to disturbance is imperative to ecological theory and environmental management. In this study predictors of community resilience to a simulated natural disturbance are investigated. Responses to disturbance are examined at the community, plant functional type and species level.2. Field experiments were set up in seven tundra plant communities, simulating disturbance based on the impact of grubbing by an increasing herbivore population of pink-footed geese (Anser brachyrhynchus). The short-term resilience of communities was assessed by comparing community dissimilarity between control plots and plots subject to three disturbance intensities based on the foraging impact of these geese. Potential for long-term recovery was evaluated across different disturbance patch sizes.3. Resilience to disturbance varied between communities; those with higher moss cover and higher soil moisture, such as wetlands and mires, were most resilient to disturbance.4. The wetter communities demonstrated greater long-term recovery potential following disturbance. In wetland communities, vegetative recovery of vascular plants and moss was greater in smaller disturbed patches and at the edges of patches.5. The response of vegetation to disturbance varied with intensity of disturbance, plant community and plant species. The use of functional type classifications only partially explained the variation in species responses to disturbance across communities, thus their use in predicting community changes was limited.6. Synthesis. The impact of disturbance is shown to be plant-community specific and related to the initial abiotic and biotic properties of the community. By showing that resilience is partly predictable, the identification of disturbance-susceptible communities is possible, which is of relevance for ecosystem management.
[Show abstract][Hide abstract] ABSTRACT: The influence of goose grazing intensity and open-topped chambers (OTCs) on near-surface quantities and qualities of soil organic carbon (SOC) was evaluated in wet and mesic ecosystems in Svalbard. This study followed up a field experiment carried out in 2003–05 (part of the project Fragility of Arctic Goose Habitat: Impacts of Land Use, Conservation and Elevated Temperatures). New measurements of soil CO2 effluxes, temperatures and water contents were regularly made from July to November 2007. SOC stocks were quantified, and the reactivity and composition measured by basal soil respiration (BSR) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Results reveal variations in soil carbon cycling, with significant seasonal trends controlled by temperature, water content and snow. Experimental warming (OTCs) increased near-surface temperatures in the growing season, resulting in significantly higher CO2 effluxes. Different grazing intensities had no significant effects on observed soil respiration, but BSR rates at the mesic site (13–23 µg CO2 g soil-C−1 h−1) were highest with moderate grazing and lowest in the absence of grazing. A limited effect of grazing on microbial respiration is consistent with a lack of significant differences in SOC quantity and quality. NMR data show that the composition of A-horizon SOC is dominated by O-N-alkyl C and alkyl C groups, and less by carboxyl C and aromatic C groups: but again no marked variation in response to grazing was evident. It can be concluded that two years after a goose grazing experiment, SOC cycling was less than the natural variation within contrasting vegetation types.
Polar Research 03/2010; 29(1):46 - 57. · 1.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent climate change predictions suggest altered patterns of winter precipitation across the Arctic. It has been suggested that the presence, timing and quantity of snow all affect microbial activity, thus influencing CO2 production in soil. In this study annual and seasonal emissions of CO2 were estimated in High-Arctic Adventdalen, Svalbard, and sub-Arctic Latnjajaure, Sweden, using a new trace gas-based method to track real-time diffusion rates through the snow. Summer measurements from snow-free soils were made using a chamber-based method. Measurements were obtained from different snow regimes in order to evaluate the effect of snow depth on winter CO2 effluxes. Total annual emissions of CO2 from the sub-Arctic site (0.662–1.487 kg CO2 m–2 yr–1) were found to be more than double the emissions from the High-Arctic site (0.369–0.591 kg CO2 m–2 yr–1). There were no significant differences in winter effluxes between snow regimes or vegetation types, indicating that spatial variability in winter soil CO2 effluxes are not directly linked to snow cover thickness or soil temperatures. Total winter emissions (0.004–0.248 kg CO2 m–2) were found to be in the lower range of those previously described in the literature. Winter emissions varied in their contribution to total annual production between 1 and 18%. Artificial snow drifts shortened the snow-free period by 2 weeks and decreased the annual CO2 emission by up to 20%. This study suggests that future shifts in vegetation zones may increase soil respiration from Arctic tundra regions.
Polar Research 03/2010; 29(1):75 - 84. · 1.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Winter respiration in snow-covered ecosystems strongly influences annual carbon cycling, underlining the importance of processes related to the timing and quantity of snow. Fences were used to increase snow depth from 30 to 150 cm, and impacts on respiration were investigated in heath and mesic meadow, two common vegetation types in Svalbard. We manually measured ecosystem respiration from July 2007 to July 2008 at a temporal resolution greater than previously achieved in the High Arctic (campaigns: summer, eight; autumn, six; winter, 17; spring, nine). Moisture contents of unfrozen soil and soil temperatures throughout the year were also recorded. The increased snow depth resulted in significantly higher winter soil temperatures and increased ecosystem respiration. A temperature–efflux model explained most of the variation of observed effluxes: meadows, 94 (controls) and 93% (fences); heaths, 84 and 77%, respectively. Snow fences increased the total non-growing season efflux from 70 to 92 (heaths) and from 68 to 125 g CO2-C m−2 (meadows). The non-growing season contributed to 56 (heaths) and 42% (meadows) of the total annual carbon respired. This proportion increased with deeper snow to 64% in both vegetation types. Summer respiration rates were unaffected by snow fences, but the total growing season respiration was lower behind fences because of the considerably delayed snowmelt. Meadows had higher summer respiration rates than heaths. In addition, non-steady state CO2 effluxes were measured as bursts lasting several days during spring soil thawing, and when ice layers were broken to carry out winter efflux measurements.
Polar Research 03/2010; 29(1):58 - 74. · 1.62 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The winter CO2 efflux from subnivean environments is an important component of annual C budgets in Arctic ecosystems and consequently makes prediction and estimations of winter processes as well as incorporations of these processes into existing models important. Several methods have been used for estimating winter CO2 effluxes involving different assumptions about the snowpack, all aiming to quantify CO2 production. Here, four different methods are compared and discussed: (1) measurements with a chamber on the snow surface, Fsnow, (2) chamber measurements directly on the soil, Fsoil, after snow removal, (3) diffusion measurements, F2-point, within the snowpack, and (4) a trace gas technique, FSF6, with multiple gas sampling within the snowpack. According to measurements collected from shallow and deep snow cover in High Arctic Svalbard and subarctic Sweden during the winter of 2007–2008, the four methods differ by up to two orders of magnitude in their estimates of total winter emissions. The highest mean winter CO2 effluxes, 7.7–216.8 mg CO2 m−2 h−1, were observed using Fsoil and the lowest values, 0.8–12.6 mg CO2 m−2 h−1, using FSF6. The Fsnow and F2-point methods were both within the lower range, 2.1–15.1 and 6.8–11.2 mg CO2 m−2 h−1, respectively. These differences result not only from using contrasting methods but also from the differences in the assumptions within the methods when quantifying CO2 production and effluxes to the atmosphere. Because snow can act as a barrier to CO2, Fsoil is assumed to measure soil production, whereas FSF6, Fsnow, and F2-point are considered better approaches for quantifying exchange processes between the soil, snow, and the atmosphere. This study indicates that estimates of winter CO2 emissions may vary more as a result of the method used than as a result of the actual variation in soil CO2 production or release. This is a major concern, especially when CO2 efflux data are used in climate models or in carbon budget calculations, thus highlighting the need for further development and validation of accurate and appropriate techniques.
Global Biogeochemical Cycles 01/2010; 24(3). · 4.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Winter CO2 efflux from subnivean environments is an important component of annual C budgets in arctic ecosystems and consequently makes prediction and estimations of winter processes as well as incorporations of these processes into existing models important. Several methods have been used for estimating winter CO2 production, by using different snow pack assumptions. Here, measurements from three commonly used methods and one novel trace gas method used during the winter 2007-2008 are compared and discussed: (1) measurements with chamber on snow surface, Fsnow, (2) chamber measurements directly on the soil, Fsoil, after snow removal, (3) diffusion measurements, F2-point, within the snow pack, and (4) a novel trace gas technique, FSF6, with multiple gas sampling within the snow pack. According to measurements in shallow and deep snow cover in High-arctic Svalbard and Sub-arctic Sweden total winter emissions from the trace gas technique, 0.004-0.248 kg CO2 m-2, were found to be in the lower range of those previously described in the literature, however, results from all four methods differ by up to two orders of magnitude. Highest mean winter CO2 effluxes were observed using Fsoil, 7.7-216.8 mg CO2 m-2 h-1, and lowest values using FSF6, 0.8-12.6 mg CO2 m-2 h-1. Fsnow and F2-point were both within the lower range, 2.1-15.1 mg CO2 m-2 h-1 and 6.8-11.2 mg CO2 m-2 h-1, respectively. Differences are considered a result of contrasting methods but also that the assumptions within the methods are not equivalent when quantifying CO2 production and effluxes to the atmosphere. As snow can act as a barrier for CO2, Fsoil is assumed to measure soil production whereas FSF6, Fsnow and F2-point are considered better approaches for quantifying exchange processes between the soil, snow, and the atmosphere. This study indicates that estimation of winter CO2 emissions might vary more due to the method used than due to the actual variation in soil CO2 production or release. This is of major concern, especially when CO2 efflux data is used in climate models or in carbon budget calculations and highlights the need for further development and validation of techniques.
[Show abstract][Hide abstract] ABSTRACT: Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.