Birger Ulf Hansen

Trace gas exchange in a high‐Arctic valley: 3. Integrating and scaling CO2 fluxes from canopy to landscape using flux data, footprint modeling, and remote sensing

Within the framework of the European Land Arctic Physical Processes project and as part of the Danish Research Councils Polar Program, a study on trace gas exchange in a high-arctic ecosystem was conducted in NE Greenland, May-August 1997. On the basis of carbon dioxide flux measurements from three dominant surface types, this paper reports on the upscaling of such measurements from canopy to landscape level. Over a three-week period starting in mid-July, the different surfaces revealed large differences in the CO2 flux with uptake rates ranging from 0.7 g C m(-2) d(-1) over the dwarf shrub heath to 3.0 g C m(2) d(-1) over denser parts of the fen, while willow snowbed revealed intermediate uptake rates. The carbon dioxide exchange could be simulated by a CO2 model, combining photosynthesis and soil respiration routines, for which the parametrization depended on the vegetation type. Results from the simulation were supported by a sensitivity analysis based on a three-dimensional footprint model where it was shown that the CO2 uptake was strongly related to the measured leaf area index. The CO2 model was used to calculate the spatial distribution in Net Ecosystem Exchange (NEE) on the basis of Landsat satellite data acquired at the peak of the growing season and stratified according to vegetation type. It was found that there was a reasonable agreement between the satellite-based flux estimate (-0.77 g C m(-2) d(-1)) and the CO2 flux found by areal weighting of the eddy correlation measurements (-0.88 g C m(-2) d(-1)) for Me specific study day. Finally, the summer season NEE was calculated for the whole Zackenberg Valley bottom. In June, there was a valley-wide carbon loss of 8.4+/-2.6 g C m(-2) month(-1), whereas the valley system accumulated 18.8+/-6.7 g C m(-2) season(-1) during the growing season (July-August). (Less)

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.

Soil and plant community characteristics and dynamics at Zackenberg

Arctic soils hold large amounts of nutrients in the weatherable minerals and the soil organic matter, which slowly decompose. The decomposition processes release nutrients to the plant-available nutrient pool as well as greenhouse gases to the atmosphere. Changes in climatic conditions, for example, changes in the distribution of snow, water balance and the length of the growing season, are likely to affect the complex interactions between plants, abiotic and biotic soil processes as well as the composition of soil micro- and macro-fauna and thereby the overall decomposition rates. These interactions, in turn, will influence soil-plant functioning and vegetation composition in the short as well as in the long term. In this chapter, we report on soils and. plant communities and their distribution patterns in the valley Zackenbergdalen and focus on the detailed investigations within five dominating plant communities. These five communities are located along an ecological gradient in the landscape and are closely related to differences in water availability. They are therefore indirectly formed as a result of the distribution of landforms, redistribution of snow and drainage conditions. Each of the plant communities is closely related to specific nutrient levels and degree of soil development including soil element accumulation and translocation, for example, organic carbon. Results presented here show that different parts of the landscape have responded quite differently to the same overall climate changes the last 10 years and thus, most likely in the future too. Fens represent the wettest sites holding large reactive buried carbon stocks. A warmer climate will cause a permafrost degradation, which most likely will result in anoxic decomposition and increasing methane emissions. However, the net gas emissions at fen sites are sensitive to long-term changes in the water table level. Indeed, increasing maximum active layer depth at fen sites has been recorded together with a decreasing water level at Zackenberg. This is in line with the first signs of increasing extension of grasslands at the expense of fens. In contrast, the most exposed and dry areas have less soil carbon, and decomposition processes are periodically water limited. Here, an increase in air temperatures may increase active layer depth more than at fen sites, but water availability will be critical in determining nutrient cycling and plant production. Field manipulation experiments of increasing temperature, water supply and nutrient addition show that soil-plant interactions are sensitive to these variables. However, additional plant-specific investigations are needed before net effects of climate changes on different landscape and plant communities can be integrated in a landscape context and used to assess the net ecosystem effect of future climate scenarios.

Trace gas exchange in a high-arctic valley. 2. Landscape CH4 fluxes measured and modeled using eddy correlation data.

Eddy correlation measurements of methane exchange were conducted during a period of 43 days covering the summer season in high-arctic, NE Greenland. Measurements were carried out over a fen area and showed fluxes ranging from no exchange in the early part of the campaign to 120 mg m(-2) d(-1) during midsummer. The emission showed a clear variation related to soil temperatures and water table level in the late part of the summer, whereas the thickness of the active soil layer was a main controlling parameter in the thaw period, in the early part of the season. A model to assess methane emission dependency on physical parameters is found to give a realistic estimate for the seasonal variations in flux. The proportion of C returned to the atmosphere as CH4 relative to the total C cycling was around 2%, which is similar to ratios often reported in literature. On the basis of these findings, an estimate is given for the total summer season emission of CH4, in which a simple model including both physical parameters and net primary production (NPP) is adapted to reproduce CH4 exchange rates for periods when no measurements were undertaken. In this way the total emission of CH4 during the period June 1 to September 1 1997, is found to equal 3.7 +/- 0.57 g m(-2), which is a relatively high rate given the harsh climate in which the measurements were made. (Less)

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.

Permafrost and Periglacial Geomorphology at Zackenberg

Publisher Summary This chapter discusses the permafrost and periglacial geomorphology at Zackenberg. Permafrost is a climatically sensitive thermal state, the top of which is particularly vulnerable to climatic changes. Therefore, monitoring of the thermal state and geomorphological activity in the active layer and top permafrost is part of the GeoBasis monitoring program. All permafrost monitoring is carried out in the valley bottom, a short distance from the Zackenberg Research Station. This has enabled the collection of a unique summer-thaw-progression data set in two Circumpolar Active Layer Monitoring (CALM) network sites since 1996. Periglacial landforms exist in the Zackenberg landscape and include ice-wedges, sorted patterns, rock glaciers, active-layer detachment slides, soli-fluction lobes and sheets, nivation hollows and associated fans, and basins together with avalanche fans. Coastal landforms along Young Sund display changes in sea ice cover. The characteristics and activity of all these landforms are important parts of the GeoBasis program, providing improved knowledge about the development of modern high-arctic periglacial landscapes. An important periglacial condition that has been monitored with high frequency in the Zackenberg lowland is the seasonal thaw progression of the active layer at the ZEROCALM-1 and ZEROCALM-2 sites. Zackenberg as an important future Greenlandic permafrost observatory is discussed in the chapter.

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.

Climatic conditions at the Mittivakkat Glacier catchment (1994–2006), Ammassalik Island, SE Greenland, and in a 109-year perspective (1898–2006)

Abstract Geografisk Tidsskrift, Danish Journal of Geography 108(1):51–72, 2008 The present-day climate in the Mittivakkat Glacier catchment (65°N), Southeast Greenland, is investigated spatiotemporally based on time series (13 years, 1994–2006) and standard synoptic climate data from the meteorological station in Tasiilaq (Ammasslik), covering 109 years (1898–2006). Within the catchment, meteorological conditions are monitored at the coast (Station Coast, 25 m a.s.l.) for the period 1998–2006 and in the glacier area (Station Nunatak 515 m a.s.l.)for 1994–2006. During this 13-year period, solar radiation shows increasing values, averaging 0.5 W m−2 y−1, at the nunatak and decreasing values, averaging 1.4 Wm−2 y−1, at the coast. The mean annual solar radiation at Station Coast is 102 Wm−2y−1, which is about 10% lower than at Station Nunatak, and is probably caused by increasing and higher percentages of dense clouds and sea fog in the coastal area. The mean annual air temperature is increasing by 0.10.°C y−1 at the nunatak and by 0.05°C y−1 at the coast, extending the thawing periods by about 50 days and 5 days, respectively. A snow-free period of 64 days is observed at the nunatak. The coastal area is highly dominated by air temperature inversion and sea breezes during spring and summer, strongly controlling the lapse rates within the catchments. The glacier area is highly dominated by katabatic fall winds, resulting in an almost total lack of calm periods. The wind speed is highest during winter, with mean average values around 6.0 m s−1, and gusts up to 35.0 m s−1. The total annual precipitation varies from 1,851 mm w.eq. y−1 at the nunatak (solid precipitation: 80%, mixed: 6%, and liquid: 14%) to 1,428 mm w.eq. y−1 at the coast (53%, 31%, and 16%), covering an average positive orographic effect for solid precipitation during winter (113 mm w.eq. 100 m−1) and a negative effect for liquid precipitation during summer (-52 mm w.eq. 100 m−1). Over the last 109 years (1898–2006) precipitation in the catchment has increased about 85 mm w.eq., covering two significant precipitation-rich periods: 1901–1914 (1,560 mm w.eq. y−1) and 1963–1978 (1,563 mm w.eq. y−1). Mean annual air temperature in the catchment has generally increased 0.2°C through the 109-year period, most significantly ∼2.7°C within the last 25 years. The warmest 10-year period since 1898 was 1938–1947, showing an annual average of -1.83°C, while 1997–2006 was the warmest 10- year period within the last 60 years, with an annual average of—2.10°C.

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.

Spectral measures and mixed models as valuable tools for investigating controls on land surface phenology in high arctic Greenland

BackgroundChanges in land surface phenology are of major importance to the understanding of the impact of recent and future climate changes in the Arctic. This paper presents an extensive study from Zackenberg Ecological Research Operations (ZERO) where snow melt, climate and growing season characteristics of six major high arctic vegetation types has been monitored during 1999 to 2005. We investigate the growth dynamics for dry, mesic and wet types using hand held measurements of far red normalised difference vegetation index (NDVI-FR) and generalized additive mixed models (GAMM).ResultsSnow melt and temperature are of major importance for the timing of the maximum growth as well as for the seasonal growth. More than 85% of the variance in timing of the maximum growth is explained by the models and similar for the seasonal growth of mesic and wet vegetation types. We find several non-linear growth responses to the environmental variables.ConclusionWe conclude that the uses of GAMMs are valuable for investigating growth dynamics in the Arctic. Contrary to several other studies in the Arctic we found a significant decreasing trend of the seasonally integrated NDVI-FR (SINDVI) in some vegetation types. This indicates that although greening might occur wide-spread in the Arctic there are variations on the local scale that might influence the regional trends on the longer term.