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Arctic tundra is facing unprecedented warming, resulting in shifts in the vegetation, thaw regimes, and potentially in the ecosystem–atmosphere exchange of carbon (C). However, the estimates of regional carbon dioxide (CO 2) and methane (CH 4) budgets are highly uncertain. We measured CO 2 and CH 4 fluxes, vegetation composition and leaf area index (LAI), thaw depth, and soil wetness in Tiksi (71 ∘ N, 128 ∘ E), a heterogeneous site located within the prostrate dwarf-shrub tundra zone in northeastern Siberia. Using the closed chamber method, we determined the net ecosystem exchange (NEE) of CO 2 , ecosystem respiration in the dark (ER), ecosystem gross photosynthesis (Pg), and CH 4 flux during the growing season. We applied a previously developed high-spatial-resolution land cover map over an area of 35.8 km 2 for spatial extrapolation. Among the land cover types varying from barren to dwarf-shrub tundra and tundra wetlands, the NEE and Pg at the photosynthetically active photon flux density of 800 µ mol m -2 h -1 (NEE 800 and Pg 800) were greatest in the graminoid-dominated habitats, i.e., streamside meadow and fens, with NEE 800 and Pg 800 of up to - 21 (uptake) and 28 mmol m -2 h -1 , respectively. Vascular LAI was a robust predictor of both NEE 800 and Pg 800 and, on a landscape scale, the fens were disproportionately important for the summertime CO 2 sequestration. Dry tundra, including the dwarf-shrub and lichen tundra, had smaller CO 2 exchange rates. The fens were the largest source of CH 4 , while the dry mineral soil tundra consumed atmospheric CH 4 , which on a landscape scale amounted to - 9 % of the total CH 4 balance during the growing season. The largest seasonal mean CH 4 consumption rate of 0.02 mmol m -2 h -1 occurred in sand- and stone-covered barren areas. The high consumption rate agrees with the estimate based on the eddy covariance measurements at the same site. We acknowledge the uncertainty involved in spatial extrapolations due to a small number of replicates per land cover type. This study highlights the need to distinguish different land cover types including the dry tundra habitats to account for their different CO 2 and CH 4 flux patterns, especially the consumption of atmospheric CH 4 , when estimating tundra C exchange on a larger spatial scale. [ABSTRACT FROM AUTHOR]

This observational study compares seasonal variations of surface fluxes (turbulent, radiative, and soil heat) and other ancillary atmospheric/surface/permafrost data based on in-situ measurements made at terrestrial research observatories located near the coast of the Arctic Ocean. Hourly-averaged multiyear data sets collected at Eureka (Nunavut, Canada) and Tiksi (East Siberia, Russia) are analyzed in more detail to elucidate similarities and differences in the seasonal cycles at these two Arctic stations, which are situated at significantly different latitudes (80.0°N and 71.6°N, respectively). While significant gross similarities exist in the annual cycles of various meteorological parameters and fluxes, the differences in latitude, local topography, cloud cover, snowfall, and soil characteristics produce noticeable differences in fluxes and in the structures of the atmospheric boundary layer and upper soil temperature profiles. An important factor is that even though higher latitude sites (in this case Eureka) generally receive less annual incoming solar radiation but more total daily incoming solar radiation throughout the summer months than lower latitude sites (in this case Tiksi). This leads to a counter-intuitive state where the average active layer (or thaw line) is deeper and the topsoil temperature in midsummer are higher in Eureka which is located almost 10° north of Tiksi. The study further highlights the differences in the seasonal and latitudinal variations of the incoming shortwave and net radiation as well as the moderating cloudiness effects that lead to temporal and spatial differences in the structure of the atmospheric boundary layer and the uppermost ground layer. Specifically the warm season (Arctic summer) is shorter and mid-summer amplitude of the surface fluxes near solar noon is generally less in Eureka than in Tiksi. During the dark Polar night and cold seasons (Arctic winter) when the ground is covered with snow and air temperatures are sufficiently below freezing, the near-surface environment is generally stably stratified and the hourly averaged turbulent fluxes are quite small and irregular with on average small downward sensible heat fluxes and upward latent heat and carbon dioxide fluxes. The magnitude of the turbulent fluxes increases rapidly when surface snow disappears and the air temperatures rise above freezing during spring melt and eventually reaches a summer maximum. Throughout the summer months strong upward sensible and latent heat fluxes and downward carbon dioxide (uptake by the surface) are typically observed indicating persistent unstable (convective) stratification. Due to the combined effects of day length and solar zenith angle, the convective boundary layer forms in the High Arctic (e.g., in Eureka) and can reach long-lived quasi-stationary states in summer. During late summer and early autumn all turbulent fluxes rapidly decrease in magnitude when the air temperature decreases and falls below freezing. Unlike Eureka, a pronounced zero-curtain effect consisting of a sustained surface temperature hiatus at the freezing point is observed in Tiksi during fall due to wetter and/or water saturated soils. [ABSTRACT FROM AUTHOR]