We present a first analysis of data (June 1998 to December 2000) from the long-term eddy covariance site established in a Pinus sylvestris stand near Zotino in central Siberia as part of the EUROSIBERIAN CARBONFLUX project. As well as examining seasonal patterns in net ecosystem exchange (N E), daily, seasonal and annual estimates of the canopy photosynthesis (or gross primary productivity, G P) were obtained using N E and ecosystem respiration measurements. Although the forest was a small (but significant) source of CO 2 throughout the snow season (typically mid-October to early May) there was a rapid commencement of photosynthetic capacity shortly following the commencement of above-zero air temperatures in spring: in 1999 the forest went from a quiescent state to significant photosynthetic activity in only a few days. Nevertheless, canopy photosynthetic capacity was observed to continue to increase slowly throughout the summer months for both 1999 and 2000, reaching a maximum capacity in early August. During September there was a marked decline in canopy photosynthesis which was only partially attributable to less favourable environmental conditions. This suggests a reduction in canopy photosynthetic capacity in autumn, perhaps associated with the cold hardening process. For individual time periods the canopy photosynthetic rate was mostly dependent upon incoming photon irradiance. However, reductions in both canopy conductance and overall photosynthetic rate in response to high canopy-to-air vapour differences were clearly evident on hot dry days. The relationship between canopy conductance and photosynthesis was examined using Cowan's notion of optimality in which stomata serve to maximise the marginal evaporative cost of plant carbon gain. The associated Lagrangian multiplier (λ) was surprisingly constant throughout the growing season. Somewhat remarkably, however, its value was markedly different between years, being 416 mol mol −1 in 1999 but 815 mol mol −1 in 2000. Overall the forest was a substantial sink for CO 2 in both 1999 and 2000: around 13 mol C m −2 a −1. Data from this experiment, when combined with estimates of net primary productivity from biomass sampling suggest that about 20% of this sink was associated with increasing plant biomass and about 80% with an increase in the litter and soil organic carbon pools. This high implied rate of carbon accumulation in the litter soil organic matter pool seems unsustainable in the long term and is hard to explain on the basis of current knowledge.

Abstract. In recent years, the pan-Arctic region has experienced increasingly extreme fire seasons. Fires in the northern high latitudes are driven by current and future climate change, lightning, fuel conditions, and human activity. In this context, conceptualizing and parameterizing current and future Arctic fire regimes will be important for fire and land management as well as understanding current and predicting future fire emissions. The objectives of this review were driven by policy questions identified by the Arctic Monitoring and Assessment Programme (AMAP) Working Group and posed to its Expert Group on Short-Lived Climate Forcers. This review synthesizes current understanding of the changing Arctic and boreal fire regimes, particularly as fire activity and its response to future climate change in the pan-Arctic have consequences for Arctic Council states aiming to mitigate and adapt to climate change in the north. The conclusions from our synthesis are the following. (1) Current and future Arctic fires, and the adjacent boreal region, are driven by natural (i.e. lightning) and human-caused ignition sources, including fires caused by timber and energy extraction, prescribed burning for landscape management, and tourism activities. Little is published in the scientific literature about cultural burning by Indigenous populations across the pan-Arctic, and questions remain on the source of ignitions above 70∘ N in Arctic Russia. (2) Climate change is expected to make Arctic fires more likely by increasing the likelihood of extreme fire weather, increased lightning activity, and drier vegetative and ground fuel conditions. (3) To some extent, shifting agricultural land use and forest transitions from forest–steppe to steppe, tundra to taiga, and coniferous to deciduous in a warmer climate may increase and decrease open biomass burning, depending on land use in addition to climate-driven biome shifts. However, at the country and landscape scales, these relationships are not well established. (4) Current black carbon and PM2.5 emissions from wildfires above 50 and 65∘ N are larger than emissions from the anthropogenic sectors of residential combustion, transportation, and flaring. Wildfire emissions have increased from 2010 to 2020, particularly above 60∘ N, with 56 % of black carbon emissions above 65∘ N in 2020 attributed to open biomass burning – indicating how extreme the 2020 wildfire season was and how severe future Arctic wildfire seasons can potentially be. (5) What works in the boreal zones to prevent and fight wildfires may not work in the Arctic. Fire management will need to adapt to a changing climate, economic development, the Indigenous and local communities, and fragile northern ecosystems, including permafrost and peatlands. (6) Factors contributing to the uncertainty of predicting and quantifying future Arctic fire regimes include underestimation of Arctic fires by satellite systems, lack of agreement between Earth observations and official statistics, and still needed refinements of location, conditions, and previous fire return intervals on peat and permafrost landscapes. This review highlights that much research is needed in order to understand the local and regional impacts of the changing Arctic fire regime on emissions and the global climate, ecosystems, and pan-Arctic communities.

Using light aircraft and at intervals of approximately 14 days, vertical profiles of temperature, humidity, CO 2 concentration and 13 C/ 12 C and 18 O/ 16 O ratio, as well as concentrations of CH 4 , CO, H 2 and N 2 O, from about 80 to 3000 m above ground level have been determined for the atmosphere above a flux measurement tower located near the village of Zotino in central Siberia (60 • 45 N, 89 • 23 E). As well as being determined from flask measurements (typically at heights of 100, 500, 1000, 1500, 2000, 2500 and 3000 m) continuous CO 2 concentration profiles at 1 Hz have also been obtained using an infrared gas analyser. This measurement program is ongoing and has been in existence since July 1998. Data to November 2000 are presented and show a seasonal cycle for CO 2 concentration of about 25 µmol mol −1 within the atmospheric boundary layer (ABL) and about 15 µmol mol −1 in the free troposphere. Marked seasonal cycles in the isotopic compositions of CO 2 are also observed, with that of oxygen-18 in CO 2 being unusual: always being depleted in the ABL with respect to the free troposphere above. This is irrespective of whether the CO 2 concentration is higher or lower in the free troposphere. We interpret this as indicating a net negative discrimination being associated with the net terrestrial carbon exchange, irrespective of whether photosynthesis or respiration dominates the net carbon flux in this region. During winter flights, large fluctuations in CO 2 concentration with height are often observed both within and above the stable ABL. Usually (but not always) these variations in CO 2 concentrations are associated with more or less stoichiometrically constant variations in CO and CH 4 concentrations. We interpret this as reflecting the frequent transport of polluted air from Europe with very little vertical mixing having occurred, despite the large horizontal distances traversed. This notion is supported by back-trajectory analyses. Vertical profiles of CO 2 concentration with supplementary flask measurements allow more information on the structure and composition of an air mass to be obtained than is the case for flask measurements or for ground-based measurements only. In particular, our data question the notion that there is usually anything like "well mixed background air" in the mid-to-high northern latitudes during the winter months.

However, in all three years examined, evaporation exceeded precipitation totals by 20-40 mm in at least one calendar month during summer. During the growing season, daily averaged surface conductances varied between 0.15 and 0.20 mol m−2 s−1 (3-4.5 mm s−1) in dry or cool months and 0.30-0.35 mol m−2 s−1 (6.5-8 mm s−1) in moist and warm months. Despite a negative hydrological balance during midsummer, there was little evidence for reduced canopy conductances in response to soil water deficits. This may have been the consequence of roots accessing water from within or just above a perched water table, located at about 2 m depth.

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