New data on vegetation of two localities in the southern part of the typical tundra subzone in the Gydan Peninsula

The syntaxonomic diversity of vegetation of two localities in the southern part of the typical tundra subzone in the Gydansky Peninsula includes seven associations, three subassociations and eight variants. New associations (Tanaceto bipinnati–Salicetum polaris Khitun ass. nov. hoc loco, Calliergono cordifolii–Salicetum lanatae Khitun ass. nov. hoc loco), subassociations (Hierochloo alpinae–Hylocomietum splendentis asahinetosumchrysantae Khitun subass. nov. hoc loco, Carici rariflorae–Sphagnetum baltici sphagnetosum steerei Khitun subass. nov. hoc loco) and eight variants are described in this region for the first time. Although in the northern part of the typical tundra subzone they occupy zonal positions, dwarf shrub communities with abundant herbs (Luzula tundricolae–Hylocomietum splendentis Telyatnikov et al. 2019) were not found in the studied locations. Tussock tundra (Sphagno–Eriophoretum vaginati typicum) occupies flat or gently sloping hills, and we consider it as the predominant zonal vegetation. The position of the boundary between the southern and the typical tundra subzones is corrected. According to our observations, it lies further north (up to 100 km on the eastern part of the peninsula) than it was shown on previous zoning schemes. Comparison of the syntaxonomic diversity in the southern and northern parts of the typical tundra showed little similarity, only two associations are common (Hierochloo alpinae–Hylocomietum splendentis Telyatnikov et al. 2019 and Tripleurospermo hookerii–Poetum alpigenae Czerosov, Sleptsova et Mironova 2005). We explain this by differences in local lithology: loams were predominant in the northern part and sands dominated in the southern part.

Geomorphology and InSAR-Tracked Surface Displacements in an Ice-Rich Yedoma Landscape

Ice-ridge Yedoma terrain is susceptible to vertical surface displacements by thaw and refreeze of ground ice, and geomorphological processes of mass wasting, erosion and sedimentation. Here we explore the relation between a 3 year data set of InSAR measurements of vertical surface displacements during the thaw season, and geomorphological features in an area in the Indigirka Lowlands, Northeast Siberia. The geomorphology is presented in a geomorphological map, based on interpretation of high resolution visible spectrum satellite imagery, field surveys and available data from paleo-environmental research. The main landforms comprise overlapping drained thaw lake basins and lakes, erosion remnants of Late Pleistocene Yedoma deposits, and a floodplain of a high-sinuosity anastomosing river with ancient river terrace remnants. The spatial distribution of drained thaw lake basins and Yedoma erosion remnants in the study area and its surroundings is influenced by neotectonic movements. The 3 years of InSAR measurement include 2 years of high snowfall and extreme river flooding (2017–2018) and 1 year of modest snowfall, early spring and warm summer (2019). The magnitude of surface displacements varies among the years, and show considerable spatial variation. Distinct spatial clusters of displacement trajectories can be discerned, which relate to geomorphological processes and ground ice conditions. Strong subsidence occurred in particular in 2019. In the wet year of 2017, marked heave occurred at Yedoma plateau surfaces, likely by ice accumulation at the top of the permafrost driven by excess precipitation. The spatial variability of surface displacements is high. This is explored by statistical analysis, and is attributed to the interaction of various processes. Next to ground ice volume change, also sedimentation (peat, colluvial deposition) and shrinkage or swelling of soils with changing water content may have contributed. Tussock tundra areas covered by the extreme 2017 and 2018 spring floods show high subsidence rates and an increase of midsummer thaw depths. We hypothesize that increased flood heights along Siberian lowland rivers potentially induce deeper thaw and subsidence on floodplain margins, and also lowers the drainage thresholds of thaw lakes. Both mechanisms tend to increase floodplain area. This may increase CH4 emission from floodplains, but also may enhance carbon storage in floodplain sedimentary environments.

Recent degradation of interior Alaska permafrost mapped with ground surveys, geophysics, deep drilling, and repeat airborne lidar

Permafrost underlies one-quarter of the Northern Hemisphere but is at increasing risk of thaw from climate warming. Recent studies across the Arctic have identified areas of rapid permafrost degradation from both top-down and lateral thaw. Of particular concern is thawing syngenetic “yedoma” permafrost which is ice-rich and has a high carbon content. This type of permafrost is common in the region around Fairbanks, Alaska, and across central Alaska expanding westward to the Seward Peninsula. A major knowledge gap is relating belowground measurements of seasonal thaw, permafrost characteristics, and residual thaw layer development with aboveground ecotype properties and thermokarst expansion that can readily quantify vegetation cover and track surface elevation changes over time. This study was conducted from 2013 to 2020 along four 400 to 500 m long transects near Fairbanks, Alaska. Repeat active layer depths, near-surface permafrost temperature measurements, electrical resistivity tomography (ERT), deep (> 5 m) boreholes, and repeat airborne light detection and ranging (lidar) were used to measure top-down permafrost thaw and map thermokarst development at the sites. Our study confirms previous work using ERT to map surface thawed zones; however, our deep boreholes confirm the boundaries between frozen and thawed zones that are needed to model top-down, lateral, and bottom-up thaw. At disturbed sites seasonal thaw increased up to 25 % between mid-August and early October and suggests measurements to evaluate active layer depth must be made as late in the fall season as possible because the projected increase in the summer season of just a few weeks could lead to significant additional thaw. At our sites, tussock tundra and spruce forest are associated with the lowest mean annual near-surface permafrost temperatures while mixed-forest ecotypes are the warmest and exhibit the highest degree of recent temperature warming and thaw degradation. Thermokarst features, residual thaw layers, and taliks have been identified at all sites. Our measurements, when combined with longer-term records from yedoma across the 500 000 km2 area of central Alaska, show widespread near-surface permafrost thaw since 2010. Projecting our thaw depth increases, by ecotype, across the yedoma domain, we calculate a first-order estimate that 0.44 Pg of organic carbon in permafrost soil has thawed over the past 7 years, which, for perspective, is an amount of carbon nearly equal to the yearly CO2 emissions of Australia. Since the yedoma permafrost and the variety of ecotypes at our sites represent much of the Arctic and subarctic land cover, this study shows remote sensing measurements, top-down and bottom-up thermal modeling, and ground-based surveys can be used predictively to identify areas of the highest risk for permafrost thaw from projected future climate warming.

Increased rainfall stimulates permafrost thaw across a variety of Interior Alaskan boreal ecosystems

Earth’s high latitudes are projected to experience warmer and wetter summers in the future but ramifications for soil thermal processes and permafrost thaw are poorly understood. Here we present 2750 end of summer thaw depths representing a range of vegetation characteristics in Interior Alaska measured over a 5-year period. This included the top and third wettest summers in the 91-year record and three summers with precipitation close to mean historical values. Increased rainfall led to deeper thaw across all sites with an increase of 0.7 ± 0.1 cm of thaw per cm of additional rain. Disturbed and wetland sites were the most vulnerable to rain-induced thaw with ~1 cm of surface thaw per additional 1 cm of rain. Permafrost in tussock tundra, mixed forest, and conifer forest was less sensitive to rain-induced thaw. A simple energy budget model yields seasonal thaw values smaller than the linear regression of our measurements but provides a first-order estimate of the role of rain-driven sensible heat fluxes in high-latitude terrestrial permafrost. This study demonstrates substantial permafrost thaw from the projected increasing summer precipitation across most of the Arctic region.

Grazing enhances carbon cycling, but reduces methane emission in the Siberian Pleistocene Park tundra site

Large herbivore grazing has been shown to substantially alter tundra soil and vegetation properties as well as carbon fluxes, yet observational evidence to quantify the impact of herbivore introduction into Arctic permafrost ecosystems remains sparse. In this study we investigated growing season CO2 and CH4 fluxes with flux chambers on a former wet tussock tundra inside Pleistocene Park, a landscape experiment in Northeast Siberia with a 22 year history of grazing. Reference data for an undisturbed system were collected on a nearby ungrazed tussock tundra. Linked to a reduction in soil moisture, topsoil temperatures at the grazed site reacted one order of magnitude faster to changes in air temperatures compared to the ungrazed site and were significantly higher, while the difference strongly decreased with depth. Overall, both GPP (gross primary productivity, i.e. CO2 uptake by photosynthesis) and Reco (ecosystem respiration, i.e. CO2 release from the ecosystem) were significantly higher at the grazed site with notable variations across plots at each site. The increases in CO2 component fluxes largely compensated each other, leaving NEE (net ecosystem exchange) similar across grazed and ungrazed sites for the observation period. Soil moisture and CH4 fluxes at the grazed site decreased over the observation period, while in contrast the constantly water-logged soils at the ungrazed site kept CH4 fluxes at significantly higher levels. Our results indicate that grazing of large herbivores promotes topsoil warming and drying, effectively accelerating CO2 turnover while decreasing methane emissions. Our experiment did not include autumn and winter fluxes, and thus no inferences can be made for the annual NEE and CH4 budgets at tundra ecosystems.

Interspecific and intraspecific variation in leaf toughness of Arctic plants in relation to habitat and nutrient supply

Leaf toughness is an important functional trait that confers resistance to herbivory and mechanical damage. We sought to determine how species composition, climate, seasonality, and nutrient availability influence leaf toughness in two types of tundra in northern Alaska. We measured leaf toughness as force to punch for 11 species of Arctic plants in tussock tundra and dry heath tundra at 17 sites distributed along a latitudinal gradient. Rubus chamaemorus and the graminoids occupied opposite ends of the leaf toughness spectrum, with R. chamaemorus requiring the least force to punch, while one of the graminoids, Eriophorum vaginatum, required the most. Leaf toughness increased with mean summer temperature for E. vaginatum and Betula nana, while it declined with warmer temperatures for the other species. Toughness of mature leaves of E. vaginatum did not vary through the growing season but declined significantly after senescence. Application of N and P fertilizer in an experimental site decreased leaf toughness in three species but had no effect on four others. Leaf toughness of four out of five species in dry heath was greater than for the same species in tussock tundra, but there was no difference in community-weighted mean toughness between tussock tundra and dry heath.

Vegetation responses to 26 years of warming at Latnjajaure Field Station, northern Sweden.

Climate change is rapidly warming high latitude and high elevation regions influencing plant community composition. Changes in vegetation composition have motivated the coordination of ecological monitoring networks across the Arctic, including the International Tundra Experiment (ITEX). We have established a long-term passive warming experiment using open-top chambers, which includes five distinct plant communities (Dry Heath; Tussock Tundra; and Dry, Mesic, and Wet Meadow). We have measured changes in plant community composition based on relative abundance differences over 26 years. In addition, relative abundance changes in response to fertilization and warming treatments were analysed based on a 7-year Community-Level Interaction Program (CLIP) experiment. The communities had distinct soil moisture conditions, leading to community specific responses of the plant growth forms (deciduous shrubs, evergreen shrubs, forbs and graminoids). Warming significantly affected growth forms, but the direction of the response was not consistent across the communities. Evidence of shrub expansion was found in nearly all communities, with soil moisture determining whether it was driven by deciduous or evergreen shrubs. Graminoids increased in relative abundance in the Dry Meadow due to warming. Growth form responses to warming are likely mediated by edaphic characteristics of the communities and their interactions with climate.

Rainfall Alters Permafrost Soil Redox Conditions, but Meta-Omics Show Divergent Microbial Community Responses by Tundra Type in the Arctic

Soil anoxia is common in the annually thawed surface (‘active’) layer of permafrost soils, particularly when soils are saturated, and supports anaerobic microbial metabolism and methane (CH4) production. Rainfall contributes to soil saturation, but can also introduce oxygen, causing soil oxidation and altering anoxic conditions. We simulated a rainfall event in soil mesocosms from two dominant tundra types, tussock tundra and wet sedge tundra, to test the impacts of rainfall-induced soil oxidation on microbial communities and their metabolic capacity for anaerobic CH4 production and aerobic respiration following soil oxidation. In both types, rainfall increased total soil O2 concentration, but in tussock tundra there was a 2.5-fold greater increase in soil O2 compared to wet sedge tundra due to differences in soil drainage. Metagenomic and metatranscriptomic analyses found divergent microbial responses to rainfall between tundra types. Active microbial taxa in the tussock tundra community, including bacteria and fungi, responded to rainfall with a decline in gene expression for anaerobic metabolism and a concurrent increase in gene expression for cellular growth. In contrast, the wet sedge tundra community showed no significant changes in microbial gene expression from anaerobic metabolism, fermentation, or methanogenesis following rainfall, despite an initial increase in soil O2 concentration. These results suggest that rainfall induces soil oxidation and enhances aerobic microbial respiration in tussock tundra communities but may not accumulate or remain in wet sedge tundra soils long enough to induce a community-wide shift from anaerobic metabolism. Thus, rainfall may serve only to maintain saturated soil conditions that promote CH4 production in low-lying wet sedge tundra soils across the Arctic.