Snow is an important factor controlling vegetation functions in high latitudes/altitudes. However, due to the lack of reliable in -situ measurements, the effects of snow on vegetation phenology remains poorly understood. Here, we examine the effects of snow cover duration (SCD) on the start of growing season (SOS) for different vegetation types. SOS and SCD were extracted from in -situ carbon flux and albedo data, respectively, at 51 eddy covariance flux sites in the northern mid -high latitudes. The effects of SCD on SOS vary substantially among different vegetation types. For grassland, preseason SCD outperforms other factors controlling grassland SOS. However, for forests and cropland, the preseason air temperature is the dominant factor in controlling SOS. Preseason SCD mainly influences the SOS by regulating preseason air and soil temperature rather than soil moisture. The CMIP6 Earth system models (ESMs) fail to capture the effect of SCD on SOS. Thus, Random Forest (RF) models were established to predict future SOS changing trends considering the effect of SCD. For grassland and evergreen needleleaf forest, the projected SOS advance rate is slower when SCD is considered. These findings can help us better understand impacts of snow on vegetation phenology and carbon -climate feedbacks in the warming world.

Ice-wedge polygon landscapes make up a substantial part of high-latitude permafrost landscapes. The hydrological conditions shape how these landscapes store and release organic carbon. However, their coupled water-carbon dynamics are poorly understood as field measurements are sparse in smaller catchments and coupled hydrology-dissolved organic carbon (DOC) models are not tailored for these landscapes. Here we present a model that simulates the hydrology and associated DOC export of high-centered and low-centered ice-wedge polygons and apply the model to a small catchment with abundant polygon coverage along the Yukon Coast, Canada. The modeled seasonal pattern of water and carbon fluxes aligns with sparse field data. These modeled seasonal patterns indicate that early-season runoff is mostly surficial and generated by low-centered polygons and snow trapped in troughs of high-centered polygons. High-centered polygons show potential for deeper subsurface flow under future climate conditions. This suggests that high-centered polygons will be responsible for an increasing proportion of annual DOC export compared to low-centered polygons. Warming likely shifts low-centered polygons to high-centered polygons, and our model shows that this shift will cause a deepening of the active layer and a lengthening of the thawing season. This, in turn, intensifies seasonal runoff and DOC flux, mainly through its duration. Our model provides a physical hypothesis that can be used to further quantify and refine our understanding of hydrology and DOC export of arctic ice-wedge polygon terrain.

Permafrost thaw causes the seasonally thawed active layer to deepen, causing the Arctic to shift toward carbon release as soil organic matter becomes susceptible to decomposition. Ground subsidence initiated by ice loss can cause these soils to collapse abruptly, rapidly shifting soil moisture as microtopography changes and also accelerating carbon and nutrient mobilization. The uncertainty of soil moisture trajectories during thaw makes it difficult to predict the role of abrupt thaw in suppressing or exacerbating carbon losses. In this study, we investigated the role of shifting soil moisture conditions on carbon dioxide fluxes during a 13-year permafrost warming experiment that exhibited abrupt thaw. Warming deepened the active layer differentially across treatments, leading to variable rates of subsidence and formation of thermokarst depressions. In turn, differential subsidence caused a gradient of moisture conditions, with some plots becoming consistently inundated with water within thermokarst depressions and others exhibiting generally dry, but more variable soil moisture conditions outside of thermokarst depressions. Experimentally induced permafrost thaw initially drove increasing rates of growing season gross primary productivity (GPP), ecosystem respiration (R-eco), and net ecosystem exchange (NEE) (higher carbon uptake), but the formation of thermokarst depressions began to reverse this trend with a high level of spatial heterogeneity. Plots that subsided at the slowest rate stayed relatively dry and supported higher CO2 fluxes throughout the 13-year experiment, while plots that subsided very rapidly into the center of a thermokarst feature became consistently wet and experienced a rapid decline in growing season GPP, R-eco, and NEE (lower carbon uptake or carbon release). These findings indicate that Earth system models, which do not simulate subsidence and often predict drier active layer conditions, likely overestimate net growing season carbon uptake in abruptly thawing landscapes.

Arctic rivers provide an integrated signature of the changing landscape and transmit signals of change to the ocean. Here, we use a decade of particulate organic matter (POM) compositional data to deconvolute multiple allochthonous and autochthonous pan-Arctic and watershed-specific sources. Constraints from carbon-to-nitrogen ratios (C:N), delta C-13, and Delta C-14 signatures reveal a large, hitherto overlooked contribution from aquatic biomass. Separation in Delta C-14 age is enhanced by splitting soil sources into shallow and deep pools (mean +/- SD: -228 +/- 211 vs. - 492 +/- 173%) rather than traditional active layer and permafrost pools (-300 +/- 236 vs. -441 +/- 215%) that do not represent permafrost-free Arctic regions. We estimate that 39 to 60% (5 to 95% credible interval) of the annual pan-Arctic POM flux (averaging 4,391 Gg/y particulate organic carbon from 2012 to 2019) comes from aquatic biomass. The remainder is sourced from yedoma, deep soils, shallow soils, petrogenic inputs, and fresh terrestrial production. Climate change-induced warming and increasing CO2 concentrations may enhance both soil destabilization and Arctic river aquatic biomass production, increasing fluxes of POM to the ocean. Younger, autochthonous, and older soil-derived POM likely have different destinies (preferential microbial uptake and processing vs. significant sediment burial, respectively). A small (similar to 7%) increase in aquatic biomass POM flux with warming would be equivalent to a similar to 30% increase in deep soil POM flux. There is a clear need to better quantify how the balance of endmember fluxes may shift with different ramifications for different endmembers and how this will impact the Arctic system.

Non-growing season CO2 emissions from Arctic tundra remain a major uncertainty in forecasting climate change consequences of permafrost thaw. We present the first time series of soil and microbial CO2 emissions from a graminoid tundra based on year-round in situ measurements of the radiocarbon content of soil CO2 (Delta(CO2)-C-14) and of bulk soil C (Delta C-14), microbial activity, and temperature. Combining these data with land-atmosphere CO2 exchange allows estimates of the proportion and mean age of microbial CO2 emissions year-round. We observe a seasonal shift in emission sources from fresh carbon during the growing season (August Delta(CO2)-C-14 = 74 +/- 4.7 parts per thousand, 37% +/- 3.4% microbial, mean +/- se) to increasingly older soil carbon in fall and winter (March Delta(CO2)-C-14 = 22 +/- 1.3 parts per thousand, 47% +/- 8% microbial). Thus, rising soil temperatures and emissions during fall and winter are depleting aged soil carbon pools in the active layer and thawing permafrost and further accelerating climate change.

Rapid warming in alpine regions exerts important effects on carbon cycling in alpine ecosystem, which are sensitive to environmental changes. So far, little is known about the spatial and temporal variation in carbon budgets and the main influencing factors over different ecosystems. Here, we examined the monthly and annual gross primary production (GPP), net ecosystem CO2 exchange (NEE) and ecosystem respiration (ER) during 2004-2017 in four types of ecosystems (i.e., alpine meadow, steppe, forest and cropland) on the Tibetan Plateau. We explored the relationships between carbon fluxes and environmental factors. The results show that forest, cropland and alpine meadow ecosystems acted as carbon sinks, with NEE values ranging from -21.25 +/- 3.54 to -308.75 +/- 21.65 g C m-2a-1, while alpine steppe and overmature forest ecosystems serve as carbon sources (mean annual NEE: 23.12 +/- 15.88 g C m-2a-1). The temperature sensitivity values (Q10) of ER in the forest (9.39) and alpine steppe (7.47) ecosystems were greater than those in the alpine meadow ecosystems (Q10 = 4.20), indicating that the carbon emissions in the forest and alpine steppe ecosystems were more sensitive to warming. Multiple linear regression analysis indicated that the carbon fluxes (GPP, NEE, ER) of alpine steppe and alpine meadow in the permafrost regions were more sensitive to water forcing (precipitation, soil water content), while in the forest and cropland ecosystems temperature forcing (air and soil temperature) were strong predictors of all the carbon flux indices. Our results showed differential responses of carbon budgets among ecosystems, which could be considered in the future modeling of carbon cycle in alpine regions.

Mosses strongly affect water and heat fluxes due their high water holding capacity and the provision of insulation. A land surface model (the coupled hydrological and biogeochemical process model, CHANGE) was used to quantitatively assess the influence of moss cover on soil temperature (T-SOIL), active layer thickness (ALT), and ecosystem carbon balance. The CHANGE model was coupled with a moss process module, enabling the explicit representation of heat, water, and carbon exchange in the atmosphere-vegetation-moss-soil system. The model was applied to a tundra site in northeastern Siberia over the period of 1980-2013. The results were validated with in situ observations and indicated a high level of insulation by the moss, resulting in warmer winter and cooler summer T-SOIL and smaller ALT. The sensitivities of T-SOIL and ALT to moss coverage and thickness were examined using model experiments. An increase in moss thickness lowered the summer T-SOIL by 0.9-2.1 degrees C and reduced ALT by 9-20cm compared with a moss-free experiment. The moss-induced cooler T-SOIL in the root zone could limit the productivity of vegetation by reducing water availability to plant roots due to the presence of ice. This limitation increased with increasing moss layer thickness and coverage. The productivity of the moss itself increased with thickness, partially offsetting the reduction in vegetation productivity. Our modeling study suggests that the moss layer has a significant impact on T-SOIL, ALT, and carbon balance in the Arctic tundra and may play an important role in future Arctic warming.

Shrub expansion in tundra ecosystems may act as a positive feedback to climate warming, the strength of which depends on its spatial extent. Recent studies have shown that shrub expansion is more likely to occur in areas with high soil moisture and nutrient availability, conditions typically found in subsurface water channels known as water tracks. Water tracks are 5-15 m wide channels of subsurface water drainage in permafrost landscapes and are characterized by deeper seasonal thaw depth, warmer soil temperatures, and higher soil moisture and nutrient content relative to adjacent tundra. Consequently, enhanced vegetation productivity, and dominance by tall deciduous shrubs, are typical in water tracks. Quantifying the distribution of water tracks may inform investigations of the extent of shrub expansion and associated impacts on tundra ecosystem carbon cycling. Here, we quantify the distribution of water tracks and their contribution to growing season CO2 dynamics for a Siberian tundra landscape using satellite observations, meteorological data, and field measurements. Wefind that water tracks occupy 7.4% of the 448 km(2) study area, and account for a slightly larger proportion of growing season carbon uptake relative to surrounding tundra. For areas inside water tracks dominated by shrubs, field observations revealed higher shrub biomass and higher ecosystem respiration and gross primary productivity relative to adjacent upland tundra. Conversely, a comparison of graminoid-dominated areas in water tracks and inter-track tundra revealed that water track locations dominated by graminoids had lower shrub biomass yet increased net uptake of CO2. Our results show water tracks are an important component of this landscape. Their distribution will influence ecosystem structural and functional responses to climate, and is therefore of importance for modeling.

This chapter describes the methods and case studies of element flux measurements in the Arctic and subarctic rivers, in the Russian boreal and subarctic zone, along the gradient of permafrost-free terrain to continuous permafrost settings, developed on various lithology and vegetation coverage. The majority of existing flux measurements is based on a combination of daily discharges from Russian Hydrological Survey gauging stations with grab samples or year-round sampling of dissolved and particulate load following the chemical analysis. In this chapter, a new, geochemical-based perspective on the functioning of aquatic boreal systems is described which takes into account the role of the following factors on riverine element fluxes: (1) the specificity of lithological substrate; (2) the importance of organic and organo-mineral colloidal forms, notably during the spring flood; (3) the role of permafrost presence within the small and large watersheds; and (4) the governing role of terrestrial vegetation in element mobilization from rock substrate to the rivers. This kind of multiple approach allows a first-order prediction of element fluxes in a scenario of progressive warming in high latitudes. Two novel dimensions added to the existing knowledge on element transport from the land to the Arctic Ocean by the Russian boreal and subarctic rivers are (i) evaluation of colloidal flux of dissolved substances and low molecular weight (LMW) fraction and (ii) assessing, for the first time, the isotopic signatures of Ca, Mg, Si, and Fe in several case watersheds of various lithology and permafrost coverage. The results of this study and available data from the literature demonstrate that, while climate warming will certainly affect the wintertime element fluxes and speciation, it is unlikely to change the nature and magnitude of the main fraction of trace elements TE flux to the ocean. This fraction of the flux occurs in colloidal form during several weeks of the spring flood. At the present time, it is not strongly affected by climate change, or this influence is within the uncertainty of the flux measurements. Overall, the major changes in the chemical and isotopic nature of riverine fluxes to the Arctic Ocean from Northern Eurasia in a climate warming scenario are likely to be linked to the change in the vegetation (species, biomass and geographical extension), rather than temperature and hydrology. The increase in the depth of the active layer has an influence of second-order importance on the riverine fluxes given that the majority of continental flux to the Arctic Ocean is formed on permafrost soils, highly homogeneously over the depth profile.