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Abstract In the Arctic, winter soil temperatures exert strong control over mean annual soil temperature and winter CO₂emissions. In tundra ecosystems there is evidence that plant canopy influences on snow accumulation alter winter soil temperatures. By comparison, there has been relatively little research examining the impacts of heterogeneity in boreal forest cover on soil temperatures. Using seven years of data from six sites in northeastern Siberia that vary in stem density we show that snow-depth and forest canopy cover exert equally strong control on cumulative soil freezing degrees days (FDD_soil). Together snow depth and canopy cover explain approximately 75% of the variance in linear models of FDD_soiland freezingn-factors (n_f; calculated as the quotient of FDD_soiland FDD_air), across sites and years. Including variables related to air temperature, or antecedent soil temperatures does not substantially improve models. The observed increase in FDD_soilwith canopy cover suggests that canopy interception of snow or thermal conduction through trees may be important for winter soil temperature dynamics in forested ecosystems underlain by continuous permafrost. Our results imply that changes in Siberian larch forest cover that arise from climate warming or fire regime changes may have important impacts on winter soil temperature dynamics. 

Abstract Larix cajanderiforests, which occupy vast regions of Siberia, grow atop and protect carbon‐rich permafrost. Regeneration of these forests has important implications for long‐term feedbacks into the climate system and their regeneration is strongest following stand‐replacing fires. The goal of this project was to assess sources of regeneration limitation inL. cajanderiforests in northeastern Siberia. We focused on (1) regeneration potential of stands varying in tree density and (2) analyzing seedling establishment patterns in relationship to microsite conditions (safe sites) in the landscape. Seed sources were assessed through cone counts and stand surveys in the summers of 2017 and 2018 in 17 matureL. cajanderistands.L. cajanderirecruitment patterns in relationship to safe site availability were assessed in 15 areas, spanning approximately 800 km²along the northern portion of the Kolyma River (69.5477° N, 161.3641° E). Density of trees in a stand was negatively related to the number of cones that the average tree produced and stands of moderate density produced more cones per area than either high‐ or low‐density stands.L. cajanderiseedling establishment was facilitated by safe sites in the landscape. We discovered strong evidence that safe sites are considerably more important for seedling establishment in lowland sites than upland areas. The biological explanation for this pattern is presently unknown; however, we hypothesize this pattern is driven by persistently wet (marshy) soils in some lowland sites as a limiter of seedling establishment. Overall, these data suggest the potential for complex linkages between forest density, propagule availability, fire, safe sight colonization, and seedling establishment that may regulate long‐term dynamics in the understudiedL. cajanderiforests of the Siberian Arctic. 

Climate change is intensifying the fire regime across Siberia, with the potential to alter carbon combustion and post‐fire carbon re‐accumulation trajectories. Few field‐based estimates of fire severity (e.g., carbon combustion and tree mortality) exist in Siberian larch forests (Larixspp.), which limits our ability to project how an intensified fire regime will affect regional and global climate feedbacks. Here, we present field‐based estimates of fire‐induced tree mortality and carbon loss in eastern Siberian larch forests. Our results suggest that fires in this region result in high tree mortality (means of 83% and 76% at Arctic and subarctic sites, respectively). In both absolute and relative terms, aboveground carbon loss following fire is higher in Siberian larch forests than in North America, but belowground carbon loss is considerably lower. This suggests fundamental differences in wildfire behavior and carbon dynamics between dominant vegetation types across the boreal biome. 

Abstract Permafrost is increasingly vulnerable to thaw and collapse because of Arctic climate warming and wildfire activity. Arctic permafrost holds one third of global soil carbon (C) and large nitrogen (N) pools. A majority of permafrost organic matter is in the Russian Yedoma Domain. Soils in this remote region have high mineral soil C and N concentrations and massive, patterned ice wedges susceptible to degradation after disturbance. Yet, how Yedoma C and N pools will respond to the interaction of climate warming, wildfire, and permafrost thaw remains unknown. Here, we examined fire and permafrost thaw impacts in the Yedoma Domain of far northeast Siberia forests burned in 2001. We measured C and N pools, soil characteristics, and foliar chemistry and productivity. We found burning reduced soil organic layer depth, promoted active layer deepening, and initiated ground subsidence. Active layer permafrost thaw resulted in a 50% reduction in soil C pools in the top 125 cm, supported by evidence of increased decomposition from soil C isotope signatures and declining C:N. Burning and subsidence similarly diminished total soil N pools 50%, labile N pools 75%, and foliar N. Foliar N isotope signatures became more depleted after disturbance, suggesting greater reliance on mycorrhizal uptake and/or NO₃⁻. Collectively, permafrost thaw mobilized soil organic matter, reducing soil C storage, N pools, and overall nutrient capital. Permafrost collapse is not only a significant atmospheric C source but N cycle restrictions could further diminish long‐term C sequestration potential which balances permafrost C loss as the ecosystem recovers from disturbance. 

The Arctic is warming faster than anywhere else on Earth, placing tundra ecosystems at the forefront of global climate change. Plant biomass is a fundamental ecosystem attribute that is sensitive to changes in climate, closely tied to ecological function, and crucial for constraining ecosystem carbon dynamics. However, the amount, functional composition, and distribution of plant biomass are only coarsely quantified across the Arctic. Therefore, we developed the first moderate resolution (30 m) maps of live aboveground plant biomass (g m− 2) and woody plant dominance (%) for the Arctic tundra biome, including the mountainous Oro Arctic. We modeled biomass for the year 2020 using a new synthesis dataset of field biomass harvest measurements, Landsat satellite seasonal synthetic composites, ancillary geospatial data, and machine learning models. Additionally, we quantified pixel-wise uncertainty in biomass predictions using Monte Carlo simulations and validated the models using a robust, spatially blocked and nested cross-validation procedure. Observed plant and woody plant biomass values ranged from 0 to ~6000 g m− 2 (mean ≈350 g m− 2), while predicted values ranged from 0 to ~4000 g m− 2 (mean ≈275 g m− 2), resulting in model validation root-mean-squared-error (RMSE) ≈400 g m− 2 and R2 ≈ 0.6. Our maps not only capture large-scale patterns of plant biomass and woody plant dominance across the Arctic that are linked to climatic variation (e.g., thawing degree days), but also illustrate how fine-scale patterns are shaped by local surface hydrology, topography, and past disturbance. By providing data on plant biomass across Arctic tundra ecosystems at the highest resolution to date, our maps can significantly advance research and inform decision-making on topics ranging from Arctic vegetation monitoring and wildlife conservation to carbon accounting and land surface modeling 

Abstract Circum-boreal and -tundra systems are crucial carbon pools that are experiencing amplified warming and are at risk of increasing wildfire activity. Changes in wildfire activity have broad implications for vegetation dynamics, underlying permafrost soils, and ultimately, carbon cycling. However, understanding wildfire effects on biophysical processes across eastern Siberian taiga and tundra remains challenging because of the lack of an easily accessible annual fire perimeter database and underestimation of area burned by MODIS satellite imagery. To better understand wildfire dynamics over the last 20 years in this region, we mapped area burned, generated a fire perimeter database, and characterized fire regimes across eight ecozones spanning 7.8 million km 2 of eastern Siberian taiga and tundra from ∼61–72.5° N and 100° E–176° W using long-term satellite data from Landsat, processed via Google Earth Engine. We generated composite images for the annual growing season (May–September), which allowed mitigation of missing data from snow-cover, cloud-cover, and the Landsat 7 scan line error. We used annual composites to calculate the difference Normalized Burn Ratio (dNBR) for each year. The annual dNBR images were converted to binary burned or unburned imagery that was used to vectorize fire perimeters. We mapped 22 091 fires burning 152 million hectares (Mha) over 20 years. Although 2003 was the largest fire year on record, 2020 was an exceptional fire year for four of the northeastern ecozones resulting in substantial increases in fire activity above the Arctic Circle. Increases in fire extent, severity, and frequency with continued climate warming will impact vegetation and permafrost dynamics with increased likelihood of irreversible permafrost thaw that leads to increased carbon release and/or conversion of forest to shrublands. 

Abstract In post‐fire Siberian larch forests, where tree density can vary within a burn perimeter, shrubs constitute a substantial portion of the vegetation canopy. Leaf area index (LAI), defined as the one‐sided total green leaf area per unit ground surface area, is useful for characterizing variation in plant canopies. We estimated LAI with allometry for trees and tall shrubs (>0.5 and <1.5 m) across 26 sites with varying tree stem density (0.05–3.3 stems/m²) and canopy cover (4.6%–76.9%) in a uniformly‐aged mature Siberian larch forest that regenerated following a fire ∼75 years ago. We investigated relationships between tree density, tree LAI, and tall shrub LAI, and between LAI and satellite observations of Normalized Difference and Enhanced Vegetation Indices (NDVI and EVI). Across the density gradient, tree LAI increases with increasing tree density, while tall shrub LAI decreases, exhibiting no patterns in combined tree‐shrub LAI. We also found significant positive relationships between tall shrub LAI and NDVI/EVI from PlanetScope and Landsat imagery. These findings suggest that tall shrubs compensate for lower tree LAI in tree canopy gaps, forming a canopy with contiguous combined tree‐shrub LAI across the density gradient. Our findings suggest that NDVI and EVI are more sensitive to variation in tall shrub canopies than variation in tree canopies or combined tree‐shrub canopies in these ecosystems. The results improve our understanding of the relationships between forest density and tree and shrub leaf area and have implications for interpreting spatial variability in LAI, NDVI, and EVI in Siberian boreal forests.