Mikkel P. Tamstorf

Ecological Dynamics Across the Arctic Associated with Recent Climate Change

Assessing the Arctic The Arctic is experiencing some of the most rapid climate change currently under way across the globe, but consequent ecological responses have not been widely reported. At the close of the Fourth International Polar Year, Post et al. (p. 1355) review observations on ecological impacts in this sensitive region. The widespread changes occurring in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering. At the close of the Fourth International Polar Year, we take stock of the ecological consequences of recent climate change in the Arctic, focusing on effects at population, community, and ecosystem scales. Despite the buffering effect of landscape heterogeneity, Arctic ecosystems and the trophic relationships that structure them have been severely perturbed. These rapid changes may be a bellwether of changes to come at lower latitudes and have the potential to affect ecosystem services related to natural resources, food production, climate regulation, and cultural integrity. We highlight areas of ecological research that deserve priority as the Arctic continues to warm.

Large tundra methane burst during onset of freezing

Terrestrial wetland emissions are the largest single source of the greenhouse gas methane. Northern high-latitude wetlands contribute significantly to the overall methane emissions from wetlands, but the relative source distribution between tropical and high-latitude wetlands remains uncertain. As a result, not all the observed spatial and seasonal patterns of atmospheric methane concentrations can be satisfactorily explained, particularly for high northern latitudes. For example, a late-autumn shoulder is consistently observed in the seasonal cycles of atmospheric methane at high-latitude sites, but the sources responsible for these increased methane concentrations remain uncertain. Here we report a data set that extends hourly methane flux measurements from a high Arctic setting into the late autumn and early winter, during the onset of soil freezing. We find that emissions fall to a low steady level after the growing season but then increase significantly during the freeze-in period. The integral of emissions during the freeze-in period is approximately equal to the amount of methane emitted during the entire summer season. Three-dimensional atmospheric chemistry and transport model simulations of global atmospheric methane concentrations indicate that the observed early winter emission burst improves the agreement between the simulated seasonal cycle and atmospheric data from latitudes north of 60° N. Our findings suggest that permafrost-associated freeze-in bursts of methane emissions from tundra regions could be an important and so far unrecognized component of the seasonal distribution of methane emissions from high latitudes.

Present-Day Climate at Zackenberg

Publisher Summary This chapter outlines the most prominent parameters of climate at Zackenberg and focuses on the short-term spatiotemporal variations of these parameters within the valley Zackenbergdalen and along the east coast of Greenland. The individual climatological parameters demonstrate large spatiotemporal variations. The greatest variations occur in winter when the differentiated influence of the solar energy is low or equal to zero, but this is connected to the fact that in the cold winter period, the cyclonic activity is more intensive and frequent than in the warmer summer period. In addition, the temperature contrast between the arctic air and the advected air from the mid-latitudes is highest during this period. In turn, the effect of the underlying surface is not large because snow and sea ice cover almost the entire arctic area. In the warm summer period, the solar radiation is the most important climatological element, and it causes the greatest heterogeneity of the meteorological elements in all spatial scales: micro-, macro-, and topo-climatic. The albedo of the underlying surface that is significantly differentiated increases the influence of solar radiation in the radiation balance. However, because of the attenuated influence of the atmospheric and oceanic circulations and the large areas of the Arctic Ocean and adjacent seas not covered by sea ice, the climatic spatiotemporal differences are lesser in summer than in winter.

The uncertain climate footprint of wetlands under human pressure

Significance Wetlands are unique ecosystems because they are in general sinks for carbon dioxide and sources of methane. Their climate footprint therefore depends on the relative sign and magnitude of the land–atmosphere exchange of these two major greenhouse gases. This work presents a synthesis of simultaneous measurements of carbon dioxide and methane fluxes to assess the radiative forcing of natural wetlands converted to agricultural or forested land. The net climate impact of wetlands is strongly dependent on whether they are natural or managed. Here we show that the conversion of natural wetlands produces a significant increase of the atmospheric radiative forcing. The findings suggest that management plans for these complex ecosystems should carefully account for the potential biogeochemical effects on climate. Significant climate risks are associated with a positive carbon–temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the “cost” of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse–response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange.

Effects of snow cover on the timing and success of reproduction in high-Arctic pink-footed geese Anser brachyrhynchus

During four breeding seasons, 2003–2006, we studied the relationship between snow cover and nesting performance in pink-footed geese (Anser brachyrhynchus) in a key breeding site on Svalbard. Snow cover in late May, i.e., at the time of egg laying of geese, was derived from MODIS satellite images. Snow cover had a profound cascading effect on reproductive output via the number of nesting pairs and timing of nesting, which affected nest success, while there was only a tendency for a negative effect on clutch size. Hence, we estimated a five-fold difference in the number of young produced (to post-hatching) between years with little snow and years with high snow cover. The results from the study area correlated with whole-population productivity estimates recorded in autumn. Thus, snow cover derived from MODIS satellite images appears to provide a useful indicator of the breeding conditions in the Arctic.

Seasonal Variation in Gross Ecosystem Production, Plant Biomass, and Carbon and Nitrogen Pools in Five High Arctic Vegetation Types

Abstract The Arctic is extremely vulnerable to projected climate change, and global warming may result in major community reorganizations. The aim of this study was a thorough investigation of plant biomass production throughout an entire growing season in five different high arctic vegetation types: Cassiope, Dryas, and Salix heath, grassland, and fen. The main focus was on the gross ecosystem production (GEP), and the biotic and abiotic factors which may influence GEP. Photosynthesis, aboveground biomass, and carbon, nitrogen, and chlorophyll content were measured weekly during nine weeks. There were large differences in seasonal growth and production within and among vegetation types. Mosses contributed considerably to the total C and N pool in grassland, fen, and Salix heath. Fen, which had the highest pool of leaf N, leaf chlorophyll, and moss N, was the most productive vegetation type in terms of GEP, despite the lowest total biomass. Across vegetation types, leaf biomass, leaf N, and moss N pool size influenced GEP. Within most vegetation types GEP correlated with leaf N, in correspondence with the notion that N may limit plant production in many high arctic ecosystems. The timing of the peaks in C and N pools in leaves did not coincide with that in the mosses and in woody tissues. This emphasizes the importance of sampling throughout the growing season, when using field data from the Arctic to estimate plant biomasses and modeling C and N fluxes and pool sizes.

Trends in CO2 exchange in a high Arctic tundra heath, 2000–2010

We have measured the land-atmosphere CO2 exchange using the eddy covariance technique in a high Arctic tundra heath in northeast Greenland (Zackenberg). On the basis of 11 years of measurements (2000-2010), it was found that snow cover dynamics was important for the CO2 exchange. The start of CO2 uptake period correlated significantly with timing of snowmelt. Furthermore, for years with deep and long-lasting snowpacks, the following springs showed increased CO2 emission rates. In the first part of the study period, there was an increase of approximately 8 g C m(-2) yr(-1) in both accumulated gross primary production (GPP) and CO2 sink strength during summer. However, in the last few years, there were no significant changes in GPP, whereas ecosystem respiration (R-eco) increased (8.5 g C m(-2) yr(-1)) and ecosystem CO2 sink strength weakened (-4.1 g C m(-2) yr(-1)). It was found that temperature and temperature-related variables (maximum thaw depth and growing degree days) controlled the interannual variation in CO2 exchange. However, while R-eco showed a steady increase with temperature (5.8 g C m(-2) degrees C-1), the initial increase in GPP with temperature leveled off at the high end of observed temperature range. This suggests that future increases in temperature will weaken the ecosystem CO2 sink strength or even turn it into a CO2 source, depending on possible changes in vegetation structure and functioning as a response to a changing climate. If this trend is applicable also to other Arctic ecosystems, it will have implications for our current understanding of Arctic ecosystems dynamics. (Less)

Storage, Landscape Distribution, and Burial History of Soil Organic Matter in Contrasting Areas of Continuous Permafrost

Abstract This study describes and compares soil organic matter (SOM) quantity and characteristics in two areas of continuous permafrost, a mountainous region in NE Greenland (Zackenberg study site) and a lowland region in NE Siberia (Cherskiy and Shalaurovo study sites). Our assessments are based on stratified-random landscape-level inventories of soil profiles down to 1 m depth, with physico-chemical, elemental, and radiocarbon-dating analyses. The estimated mean soil organic carbon (SOC) storage in the upper meter of soils in the NE Greenland site is 8.3 ± 1.8 kg C m-2 compared to 20.3 ± 2.2 kg C m-2 and 30.0 ± 2.0 kg C m-2 in the NE Siberian sites (95% confidence intervals). The lower SOC storage in the High Arctic site in NE Greenland can be largely explained by the fact that 59% of the study area is located at higher elevation with mostly barren ground and thus very low SOC contents. In addition, SOC-rich fens and bogs occupy a much smaller proportion of the landscape in NE Greenland (∼3%) than in NE Siberia (∼20%). The contribution of deeper buried C-enriched material in the mineral soil horizons to the total SOC storage is lower in the NE Greenland site (∼13%) compared to the NE Siberian sites (∼24%–30%). Buried SOM seems generally more decomposed in NE Greenland than in NE Siberia, which we relate to different burial mechanisms prevailing in these regions.

Inter-Annual Variability and Controls of Plant Phenology and Productivity at Zackenberg

Publisher Summary This chapter discusses vegetation-type dynamics and species-specific reproduction in the high-arctic valley Zackenbergdalen and evaluates the plant responses to predicted climate changes. The chapter presents results from monitoring and experimental work carried out at Zackenberg Research Station in Northeast Greenland. Most of the studied species developed more flowers in years following a warmer growing season, and the results also indicate that the plants generally develop flowers and seeds faster in a warmer environment. The analyses from Zackenberg indicate that shrubs are more likely to take advantage of the predicted climatic changes with cloudier summers and increased variability in snowmelt with respect to the initiation of flowering than other plant types; that is, they are more likely to complete the development of seeds within a season when snowmelt is early. The land surface phenology and growth dynamics of six major vegetation types evaluated for Zackenbergdalen for the period 1999–2005 are presented in the chapter.

Snow and Snow-Cover in Central Northeast Greenland

Publisher Summary This chapter discusses snow and snow cover in central Northeast Greenland. In most high-arctic regions, like Zackenberg in Northeast Greenland, virtually all vegetated areas are snow covered most of the year because of the presence of vegetation. This leaves only a short time window in which the surface is free of snow, where photosynthetic activity can take place and where herbivores have easy access to food at the surface. The largest snow accumulation occurs on the valley sides on slopes with a southerly orientation, whereas on the valley floor, the accumulation is more uniform and snowdrifts are more stochastically distributed. Snowdrifting is generally more intense in snow-rich than in snow-poor years. This leads to the formation of snowdrifts that are larger in the snow-rich than in the snow-poor years. Thus, when snow amounts increase, the melting season is prolonged more in the areas with large snow accumulation. The chapter provides graphical representation based on the data from Zackenberg and depicts the annual bio-climatic variation in a typical high-arctic ecosystem. The chapter emphasizes the short snow-free summer period for the flora and fauna; for example, vegetative activity and breeding conditions for shorebirds.