Anja N. Kade

Plant communities and soils in cryoturbated tundra along a bioclimate gradient in the Low Arctic, Alaska

Nonsorted circles and earth hummocks are important landscape components of the arctic tundra. Here we describe the vegetation on these frost-heave features at seven study sites along a N-S-transect from the Arctic Ocean to the Arctic Foothills, Alaska. We established 117 releves in frost-heave features and surrounding tundra and classified the vegetation according to the Braun-Blanquet sorted-table method. We used Detrended Correspondence Analysis to analyze relationships between vegetation and environmental variables. We identified nine communities: Braya purpurascens-Puccinellia angustata community (dry nonsorted circles, subzone C); Dryas integrifolia-Salix arctica community (dry tundra, subzone C); Sal ic i rotundifol iae-Caricetum aquati l is ass. nov. (moist coastal tundra, subzone C); Junco biglumis-Dryadetum integrifol iae ass. nov. (moist nonsorted circles, subzone D); Dryado integrifol iae-Caricetum bigelowii Walker et al. 1994 (moist tundra, subzone D); Scorpidium scorpioides-Carex aquatilis community (wet tundra, subzone D); Cladino-Vaccinietum vit isidaeae ass. nov. (dry nonsorted circles and earth hummocks, subzone E); Sphagno-Eriophoretum vaginati Walker et al. 1994 (moist tundra, subzone E); and Anthelia juratzkanaJuncus biglumis community (wet nonsorted circles, subzone E). The DCA ordination displayed the vegetation types with respect to complex environmental gradients. The first axis of the ordination corresponds to a bioclimate/pH gradient, and the second axis corresponds to a disturbance/soil moisture gradient. Frost-heave features are dominated by lichens, whereas the adjacent tundra supports more dwarf shrubs, graminoids and mosses. Frost-heave features have greater thaw depths, more bare ground, thinner organic horizons and lower soil moisture than the surrounding tundra. The morphology of frost-heave features changes along the climatic gradient, with large, barren nonsorted circles dominating the northern sites and vegetated, less active earth hummocks dotting the southern sites. Thawing of permafrost and a possible shift in plant community composition due to global warming could lead to a decline in frost-heave features and result in the loss of landscape heterogeneity.

Experimental Alteration of Vegetation on Nonsorted Circles: Effects on Cryogenic Activity and Implications for Climate Change in The Arctic

ABSTRACT Nonsorted circles are relatively barren patterned-ground features common in most arctic tundra regions. We studied how vegetation changes on nonsorted circles might affect cryogenic processes, which is of relevance as arctic vegetation responds to climate change. Twenty-eight circles at a moist nonacidic tundra site in northern Alaska received one of four treatments: (a) vegetation removal; (b) vegetation removal and sedge transplants; (c) vegetation removal and moss transplants; or (d) no manipulation. We monitored soil-surface temperatures, thaw depth, frost heave, and soil-surface instability as indicators of cryogenic processes for three years. Vegetation removal led to 1.5 °C (22.3%) warmer summer soil-surface temperatures, 4.8 cm (6.2%) deeper mean thaw depth, 3.5 cm (26.2%) greater frost heave and a drastic increase in an index of soil instability when compared to the control. In contrast, moss additions lowered soil-surface temperatures by 2.8 °C (41.8%) in the summer, delayed freezing by almost two weeks and thawing by one week, decreased mean thaw depth by 10.3 cm (14.9%), and decreased frost heave by 6.6 cm (52.4%) when compared to the control. The sedge treatment had intermediate effects on thaw and heave. This study indicates that increases in plant cover and particularly moss cover on nonsorted circles due to a warming climate would decrease the heat flux between the atmosphere and the mineral soil and result in shallower thaw and less frost heave, leading to regional reductions in the activities of nonsorted circles.

Upscaling of CO2 fluxes from heterogeneous tundra plant communities in Arctic Alaska

Received 1 May 2012; revised 20 August 2012; accepted 29 September 2012; published 17 November 2012. [1] We characterized the tundra vegetation at three eddy covariance towers located along a toposequence in northern Alaska and studied seasonal variations in plot-level CO2 fluxes among the dominant vegetation types with chambers during the summer and with the gradient-diffusion technique during the winter. We performed footprint analyses to determine the source areas contributing to the tower fluxes and scaled plot-level to eddy-covariance CO2 data based on the proportion of vegetation types occurring within the footprints. At peak growing season, both gross ecosystem exchange and ecosystem respiration were greater in moist acidic tussock tundra and wet sedge tundra than in dry heath tundra. This resulted in relatively similar values of net ecosystem exchange as measured by chambers in July in tussock tundra across all topographic positions and wet sedge tundra (2.4 to 4.2 mmol CO2/m 2 /s) but low values in dry heath tundra (0.4 mmol CO2/m 2 /s). Winter respiration was highest for tussock tundra in December, but there were no significant differences among vegetation types in February and April. Net and gross ecosystem exchange scaled up from summer chamber measurements compared well to tower data (r 2 = 0.84 and r 2 = 0.78, respectively), especially on level terrain, whereas plot-level CO2-flux measurements in the winter did not agree well with tower data. This is one of few studies to compare plot-level and tower fluxes during both summer and winter and to demonstrate successful upscaling of carbon exchange in Arctic tundra systems under certain conditions.

Interannual and Seasonal Patterns of Carbon Dioxide, Water, and Energy Fluxes From Ecotonal and Thermokarst‐Impacted Ecosystems on Carbon‐Rich Permafrost Soils in Northeastern Siberia

Eastern Siberia Russia is currently experiencing a distinct and unprecedented rate of warming. This change is particularly important given the large amounts of carbon stored in the yedoma permafrost soils that become vulnerable to thaw and release under warming. Data from this region pertaining to year-round carbon, water, and energy fluxes are scarce, particularly in sensitive ecotonal ecosystems near latitudinal treeline, as well as those already impacted by permafrost thaw. Here, we investigated the interannual and seasonal carbon dioxide, water, and energy dynamics at an ecotonal forested site and a disturbed thermokarst-impacted site. The ecotonal site was approximately neutral in terms of CO2 uptake/release, while the disturbed site was either a source or neutral. Our data suggest that high rates of plant productivity during the growing season at the disturbed site may, in part, counter-balance higher rates of respiration during the cold season compared to the ecotonal site. We also found that the ecotonal site was sensitive to the timing of the freeze-up of the soil active layer in fall, releasing more CO2 when freeze-up occurred later. Both sites showed a negative water balance, although the ecotonal site appeared more sensitive to dry conditions. Water use efficiency at the ecotonal site was lower during warmer summers. Overall, these Siberian measurements indicate ecosystem sensitivity to warmer conditions during the fall and to drier conditions during the growing season, and provide a better understanding of ecosystem response to climate in a part of the circumpolar Arctic where current knowledge is weakest.

The Effect of Vegetation on Simulated Differential Ice Accumulation in Non-Sorted Circles Ecosystems

Changes in the Arctic landscape due to climate warming are evident in greening of the tundra, formation of permafrost thaw lakes and erosion of ice wedge polygons. The changes are caused by atmospheric as well as soil and permafrost warming trends. Freezing and thawing drive many landscape features in the Arctic; cryoturbation being the major force that moves the soil. The interaction between vegetation development and cryoturbation is the objective of our research. In particular we simulate the effects of vegetation on the freezing rate in soils in three-dimensions in order to model non-sorted circle ecosystems that are prevalent in the Arctic. Differential insulation at the soil surface causes the soil to freeze irregularly and determines the strength and location of cryoturbation, caused by ice lens formation. Redistribution of water causes ice accumulation where the soil freezes first, while the thickness of the vegetation and its organic layer slows the freezing process, which dries the soil below vegetated patches. For our research we combined two simulation models, 1) the WIT3D model that simulates the hydrology in the active layer during freezing, and 2) the ArcVeg model that simulates vegetation succession in the tundra. We have analyzed vegetation types along a bioclimatic gradient in the arctic tundra and determined an insulation factor for these vegetation types. The insulation values were used in the combined models and the results show that warming conditions simulated with bare ground, as initial condition, would form a vegetation pattern that is more uniform and would develop fewer non-sorted circles. However when an existing pattern is simulated with a warmer climate as the initial condition, then the pattern does not disintegrate but the vegetation dominates and makes the non-sorted circles smaller. Over a longer time frame the vegetation can eventually dominate the non-sorted circles completely.