As the Arctic warms, evidence shows that warmer summers substantially affect ecosystem functioning, including carbon cycling. Yet, the most rapid warming occurs in winter, with more frequent mid-winter warming events raising air temperatures above 0 °C. During these warm spells, rain-on-snow events often generate extensive, long-lasting ice layers (icing) at the snow-ground interface, potentially altering tundra ecosystems beyond the duration of the events. We quantified the effects of winter icing, alone and combined with summer warming, on tundra CO2-fluxes and litter decomposition potential with the Tea Bag Index. Over consecutive years, we manipulated winter icing and summer warming in a full-factorial experiment in mesic tundra in high-Arctic Svalbard. Icing was induced by a single watering each winter to form a basal ice layer persisting until spring snowmelt, while open-top chambers increased summer temperatures. Icing and warming each shifted the ecosystem from a near-neutral summer carbon balance (–0.1 µmol m−2 s−1) to a net source, with their combination causing the largest change, increasing CO2 emissions by 2.1 µmol m−2 s−1. Decomposition responses were smaller in magnitude, yet detectable: icing reduced summer and winter litter mass loss (3–9

We present a detailed and quality-controlled chronological database for past glacier, relative sea level, and paleoclimate changes in Iceland during the Late Quaternary ( similar to 60 ka ): ICEland-1. The curated database includes 1744 data points and metadata from 442 sites located in the marine and terrestrial realm, with dates derived from radiocarbon ( 14 C ), tephrochronology, and terrestrial cosmogenic nuclides (TCN). Each date's reliability has been assessed using a three-tier ranking system, following explicitly defined criteria modified from other recent ice sheet chronology databases. This filtering approach reveals significant spatiotemporal gaps in our understanding of Late Quaternary ice sheet, relative sea level, and paleoclimate chronology in and around Iceland. We highlight several key avenues for future research that can help minimize existing spatiotemporal uncertainties and biases in the empirical data. The implementation of ICEland-1 for local ice sheet model calibrations and data-model comparisons will improve our understanding of past and future changes of ice sheets in the North Atlantic and Antarctica. The curated database is openly available at 10.5281/zenodo.19376039 (Harning et al., 2026).

On recently deglaciated terrain, soil instability can be a physical barrier limiting seedling establishment. Here we used a space-for-time approach to study the role of biological soil crust (biocrust) as soil surface stabilizer and facilitator of ecological succession along a glacier forefield chronosequence at the retreating Conejeras glacier in the Tropical Andes. We used the point-intercept method to estimate surface cover of plants, biocrust, bare ground and rocks, as well as soil surface roughness; and a field soil aggregate kit to estimate soil stability. As hypothesized, following a bare-ground stage near the edge of the glacier, the successional trajectory involved the development of a biocrust belt, including bryophytes and lichens, followed by an increasing vascular plant cover. The development of biocrust was accompanied by higher soil surface roughness and soil stability, which likely increased seed entrapment and seedling establishment. Our results suggest that the development of cyanobacterial-dominated biocrust at the forefield of the Conejeras glacier may favor the establishment of plants with large seeds, such as graminoids from the Festuca genus. Overall, our findings highlight the key role of biocrusts in the ecological dynamics that follow glacier melt in the Tropical Andes.

Land degradation due to unsustainable land use is of major concern worldwide and recovery is often slow. A potential mechanism behind slow recovery of degraded ecosystems is the retarding impacts of allelopathic plant species on the establishment of species that might facilitate the recovery process. However, the strength of the retarding impact may depend on soil type. In this study, we investigated the potential role of an abundant, evergreen and allelopathic dwarf shrub, Empetrum nigrum , in trapping tundra ecosystems in a degraded state in Iceland after centuries of unsustainable land use. We first ran a series of bioassays to assess the potential allelopathic legacy effects of the Empetrum ‐associated volcanic soils (Andosol and Vitrisol) on seed germination and root elongation of the common grass species Festuca richardsonii in comparison with Empetrum ‐associated non‐volcanic Histosol and Podzol soil types. Then we assessed the Empetrum leaf–soil interactions for all soil types using leaves from a degraded site in Iceland. We found no potential allelopathic legacy effects of Empetrum ‐associated volcanic soils, whereas the non‐volcanic soils negatively impacted Festuca root elongation. Empetrum leaves alone affected both seed germination and root elongation. These effects were strongly alleviated by the volcanic soils, but not by the non‐volcanic soils. We conclude that abundant allelopathic plant species may significantly contribute to trapping tundra ecosystems in a degraded state, but the strength of this trapping mechanism depends on the soil environment.

Empirical studies worldwide show that warming has variable effects on plant litter decomposition, leaving the overall impact of climate change on decomposition uncertain. We conducted a meta-analysis of 109 experimental warming studies across seven continents, using natural and standardised plant material, to assess the overarching effect of warming on litter decomposition and identify potential moderating factors. We determined that at least 5.2° of warming is required for a significant increase in decomposition. Overall, warming did not have a significant effect on decomposition at a global scale. However, we found that warming reduced decomposition in warmer, low-moisture areas, while it slightly increased decomposition in colder regions, although this increase was not significant. This is particularly relevant given the past decade's global warming trend at higher latitudes where a large proportion of terrestrial carbon is stored. Future changes in vegetation towards plants with lower litter quality, which we show were likely to be more sensitive to warming, could increase carbon release and reduce the amount of organic matter building up in the soil. Our findings highlight how the interplay between warming, environmental conditions, and litter characteristics improves predictions of warming's impact on ecosystem processes, emphasising the importance of considering context-specific factors.

The caldera Lake & Ouml;skjuvatn, is at 1050 m elevation in Iceland's interior. It is a deep (217 m) dimictic lake formed after an eruption in 1875. Geothermal activity with gas and liquid inflows, down to 84 m depth, maintains a similar to 0.13 km2 permanent ice opening in winter. Remote sensing data revealed a progressively disappearing ice cover in the winter 2012. Physical and chemical conditions were explored in April 2012 (ice-free) and April 2013 (ice-cover). Measurements included CTD profiles and continuous temperature records. Meteorological observations from an automatic station show frequent southwesterly winds in the first quarter of 2012. Near-linear temperature increase with depth in April 2012 indicated effective whole lake vertical mixing. In contrast, the lake was weakly stratified in April 2013 with heat stored below 60 m depth. Moored temperature records in the winters of 2013 and 2014, revealed sustained under ice temperature, and hence a density rise in the upper 60 m, which is half the lake volume. The April 2012 concentrations of geochemical temperature indicators gave no indications of enhanced thermal activity. However, the concentrations of dissolved mineral constituents had decreased since 1975. Chloride and lithium decreased by 30 degrees lo but the geothermal indicators, silicate and sulphate, had decreased less, at similar to 15 degrees lo. The inflowing water, from local precipitation, had not changed. The estimated annual lake-air flux of carbon dioxide, 19000 tons in 2012, had substantially decreased. The unexpected winter ice loss in Lake & Ouml;skjuvatn in February-March 2012 was driven by a complex interplay of wind stress, the lake's seasonal deep water heat storage and geothermal activity. Inflow of geothermal gas at 84 m depth off the western shore enhances the vertical transport of heat to the lake's upper layer which, together with frequent southwesterly wind stress 2012, eventually induced vertical instability, whole lake turnover and a complete ice melt.

Following rapid climate change, tundra plant communities are experiencing extensive compositional shifts. A conservation concern is the potential encroachment of boreal species into the tundra ('borealisation'). Tundra borealisation has been sporadically reported, but not systematically quantified. Here, we synthesised data from across 32 study areas, spanning 1137 plots and 287 vascular plant species, resurveyed between 1981 and 2023. We (i) quantified tundra borealisation as the colonisation and increase in abundance of Boreal and Boreal-Tundra species, (ii) assessed biogeographical, climatic and local borealisation drivers and (iii) identified species contributing to borealisation and their associated traits. Half of the plots experienced borealisation, although borealisation rates were not different to random expectation. Borealisation was greater in Eurasia, closer to the treeline, at higher elevations, in warmer and wetter regions, where climate change was limited, and where initial boreal abundance was lower. Boreal coloniser species were generally short-statured, and more often shrubs and graminoids. Boreal species colonised around three times less frequently than Boreal-Tundra species. Hence, our findings indicate that tundra borealisation is mainly driven by the spread of already established boreal-low Arctic tundra species. These plant community composition changes could have cascading impacts on land-atmosphere interactions, trophic dynamics and Indigenous and local livelihoods.

Low temperatures and nutrient limitation have shaped Arctic plant communities, which are now affected by biome‐wise changes in both climate and nutrient cycling. Rising temperatures are favouring taller plant species with more resource‐acquisitive traits across the Arctic tundra. Simultaneously, declines in seabird populations may reduce subsidies of marine‐derived nutrients to terrestrial ecosystems, potentially favouring more resource‐conservative plant traits. It is crucial to understand the consequences of these concurrent changes in climate and marine‐derived nutrient inputs from seabirds for the functional composition and roles of Arctic plant communities. We use a 'space‐for‐time approach' to compare the functional composition of vascular plant communities across two elevational gradients in High Arctic Svalbard, one where climate is the major environmental driver and one influenced by nutrient input from a seabird colony. We assess changes in 13 traits related to plant size, leaf economics and nutrient cycling along the two gradients, and we also explore the relative contributions of species turnover and intraspecific variation to total trait variation across and between the gradients. Elevation per se had little impact on the plant functional composition. Instead, plants at the top of the seabird nutrient gradient, closest to the nesting sites, were taller and had resource‐acquisitive trait values, such as larger and thicker leaves and higher leaf nutrient contents. Enriched soil δ 15 N‰ signatures at these sites correlated with resource‐acquisitive values of leaf area, specific leaf area, leaf dry matter content, leaf phosphorous content and with enriched leaf δ 15 N‰ signatures. This variation in leaf economic traits and isotopes was largely driven by intraspecific variation at the nutrient gradient, whereas species turnover dominated at the reference gradient. Our results are consistent with marine‐derived nutrient subsidies from seabirds being a major driver of functional trait variation in Arctic vegetation. Ongoing declines in seabird populations may therefore affect terrestrial primary producer communities in the Arctic and beyond, with potentially important but unknown implications for biodiversity, consumer and decomposer communities, and ecosystem processes. Read the free Plain Language Summary for this article on the Journal blog.

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass-MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass-MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass-MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass-MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass-MAP relationship.

The caldera Lake Öskjuvatn lies in the remote Dyngjufjöll Mountains in Iceland´s interior. The lake is at 1050 m elevation, it is 11 km2 and 217 m deep. The lake developed after an eruption in 1875. From 1921 to 1926 there were volcanic eruptions in and around the lake. The lake is cold but thermal activity at has been observed at 80 m depth which generally maintains a small opening in the winter ice. There are furthermore several warm marginal springs and seeps (Ólafsson 1980). In February 2012 remote sensing data unexpectedly revealed progressively disappearing ice cover which resulted in Lake Öskjuvatn being totally ice-free by late March. This normally occurs in late June. We investigated the ice-free lake in early April 2012 and again when the lake was ice covered in April 2013.Using SeaBird Sea Cat CTD instrument and Niskin bottles for water sampling we acquired data to compare the state of the lake under ice-free and ice-covered conditions. From April 2012 to July 2014, we had a moored string of Star-Oddi recording temperature sensors from surface to 60 m depth at a location in the deepest part of the lake. We examined the lake water chemical composition for evidence of active volcanism. The differences in the temperature structure 2012 and 2013 yield signs of circulation and the moored temperature recorders illustrate seasonal variations. With this data combined we seek to explain why the lake became ice free in 2012 but was mostly covered with 80 cm thick ice at the same time the following year.Ólafsson, J. (1980). "Temperature structure and water chemistry of the caldera Lake Öskjuvatn, Iceland." Limnology and Oceanography 25: 779-788.

Considerable uncertainty exists regarding the strength, direction and relative importance of the drivers of decomposition in the tundra biome, partly due to a lack of coordinated decomposition field studies in this remote environment. Here, we analysed 3717 incubations of two uniform litter types, green and rooibos tea, buried at 330 circum-Arctic and alpine sites to quantify the effects of temperature, moisture and litter quality on decomposition. We found a surprisingly linear positive relationship between decomposition and soil temperature across all sites, counter to theory and previous model estimates. Litter mass loss was greater at wetter sites, even where soils reached almost full water saturation. However, litter quality was the strongest driver of litter mass loss across the tundra biome, explaining six times more variation in summer decomposition than soil temperature. Our results indicate that climate warming will directly increase decomposition across tundra environments. However, the indirect effects of climate change on vegetation communities, and thus plant litter inputs and quality, could have a more profound impact than direct effects on the balance of this globally important carbon store.

The Arctic is warming four times faster than the global average1 and plant communities are responding through shifts in species abundance, composition and distribution2-4. However, the direction and magnitude of local changes in plant diversity in the Arctic have not been quantified. Using a compilation of 42,234 records of 490 vascular plant species from 2,174 plots across the Arctic, here we quantified temporal changes in species richness and composition through repeat surveys between 1981 and 2022. We also identified the geographical, climatic and biotic drivers behind these changes. We found greater species richness at lower latitudes and warmer sites, but no indication that, on average, species richness had changed directionally over time. However, species turnover was widespread, with 59% of plots gaining and/or losing species. Proportions of species gains and losses were greater where temperatures had increased the most. Shrub expansion, particularly of erect shrubs, was associated with greater species losses and decreasing species richness. Despite changes in plant composition, Arctic plant communities did not become more similar to each other, suggesting no biotic homogenization so far. Overall, Arctic plant communities changed in richness and composition in different directions, with temperature and plant-plant interactions emerging as the main drivers of change. Our findings demonstrate how climate and biotic drivers can act in concert to alter plant composition, which could precede future biodiversity changes that are likely to affect ecosystem function, wildlife habitats and the livelihoods of Arctic peoples5,6.

Implementing precision fertilization to maximize crop yield while minimizing economic and environmental impacts has become critical for agriculture. Variability in biomass response to fertilization within fields, among regions, and over time creates simultaneous risks of under-yielding and overfertilization. We quantify factors determining fertilization responsiveness (i.e., biomass increases with fertilization) up to 15 years in 61 unfertilized rangelands on six continents. We demonstrate widespread multi-year variability in responsiveness, with fertilization increasing average yield by 43% but failing to improve biomass 26% of the time. All sites were responsive at least once, but only four of 61 responded in all plots and years. Modelled management scenarios highlighted that fertilizer cessation is likely to generate sizable economic savings but always reduces yield because of the difficulty in predicting when and where biomass will be unresponsive. This work reveals substantial scale-dependent variability in fertilization responsiveness globally, while clarifying the prospects and pitfalls of managing more spatially and temporally precise nutrient application.

Fractal analysis of lava flow margins can distinguish between pahoehoe and a’a’ end-member lava flow morphologies. However, the fractal dimensions of transitional lava flow morphologies have yet to be determined. This study investigates the fractal behavior of transitional lava flow morphologies emplaced during the 2022 Meradalir (Iceland) eruption. Orthomosaics and digital elevation models from seven days of the eruption were acquired, and the margins of the lava flows were traced and divided into segments based on morphology. Fractal analysis was carried out on the whole flow field and on the individual segments, and the obtained values were compared using statistical analysis. The results show that the fractal dimension of the flow field varies throughout the eruption, which may be caused by changes in the underlying topography. The fractal dimensions of slabby and spiny pahoehoe are similar, while rubbly pahoehoe yields lower values. This suggests a possible link between fractal dimension and degree of crust disruption. However, the results do not support defining expected ranges of fractal dimensions for these transitional morphologies. Fractal dimensions are low across the whole dataset ( 1.020–1.100) which may be caused by underlying topography. The steep valley walls of Meradalir valley may inhibit lava flow branching, causing lower-than-expected fractal dimensions. Topography may thus limit the utility of fractal analysis to identify lava flow morphologies using remote sensing.

Empirical studies worldwide show substantial variability in plant litter decomposition responses to warming, leaving the overall impact of climate change on this process uncertain. We conducted a meta-analysis of 109 experimental warming studies across seven continents, utilizing natural and standardized plant material, to assess the overarching effect of warming on decomposition and identify potential moderating factors. Warming influences decomposition differently across macro-environmental gradients of moisture and temperature. Negative warming effects on decomposition in warmer, low-moisture areas were counterbalanced by the positive, though not significant, warming effects in colder areas, resulting in an overall non-significant effect. We determine that at least 5.2 degrees of warming is required for a significant increase in decomposition. This is particularly relevant given the past decade’s global warmth in higher latitudes, holding a significant proportion of terrestrial carbon. Low-quality plant litter was more sensitive to warming. Therefore, future vegetation changes toward low-quality, temperature-sensitive plants could increase carbon release and reduce the net supply of stored organic matter in the soil by increasing the decomposition of low-quality litter with warming. Our findings emphasize the connection between warming responses, macro-environment, and litter characteristics, refining predictions of climate change’s consequences on key ecosystem processes and its contextual dependencies. ### Competing Interest Statement The authors have declared no competing interest. The data that support the findings of this study will be made openly available on Dryad at once accepted for publication.

As magma temperature and composition drift and change, respectively, throughout an eruption, so does its rheology. These changes may span orders of magnitude in magma viscosity and result in orders of magnitude flow velocity changes, as well as transitions in eruptive style. In this study, we present a systematic high precision quantification of the rheological variations that occurred during the 2021 Fagradalsfjall Fires. In the field, we collected a suite of 22 representative samples emplaced between day 2 and 183 of the 2021 eruption. In the laboratory, we measured the melt viscosity of each sample in a concentric cylinder viscometer. Temperatures were initially raised to 1392 degrees C, and then lowered stepwise to eruptive temperatures as determined through syn - eruptive radiometric measurements. The resulting dataset is analyzed as a time series. An overall trend of viscosity decrease emerges. As the eruption progressed, melt viscosity decreased by 25%, from 40 Pa s to 30 Pa s at a constant temperature of 1200 degrees C. However, this trend is not monotonous. At least 3 positive spikes in viscosity can be identified, at day 80, 120, and 138 of the eruption. This trend tracks with geochemical variations.

Environmental changes, such as climate warming and higher herbivory pressure, are altering the carbon balance of Arctic ecosystems; yet, how these drivers modify the carbon balance among different habitats remains uncertain. This hampers our ability to predict changes in the carbon sink strength of tundra ecosystems. We investigated how spring goose grubbing and summer warming-two key environmental-change drivers in the Arctic-alter CO2 fluxes in three tundra habitats varying in soil moisture and plant-community composition. In a full-factorial experiment in high-Arctic Svalbard, we simulated grubbing and warming over two years and determined summer net ecosystem exchange (NEE) alongside its components: gross ecosystem productivity (GEP) and ecosystem respiration (ER). After two years, we found net CO2 uptake to be suppressed by both drivers depending on habitat. CO2 uptake was reduced by warming in mesic habitats, by warming and grubbing in moist habitats, and by grubbing in wet habitats. In mesic habitats, warming stimulated ER (+75%) more than GEP (+30%), leading to a 7.5-fold increase in their CO2 source strength. In moist habitats, grubbing decreased GEP and ER by ~55%, while warming increased them by ~35%, with no changes in summer-long NEE. Nevertheless, grubbing offset peak summer CO2 uptake and warming led to a twofold increase in late summer CO2 source strength. In wet habitats, grubbing reduced GEP (-40%) more than ER (-30%), weakening their CO2 sink strength by 70%. One-year CO2-flux responses were similar to two-year responses, and the effect of simulated grubbing was consistent with that of natural grubbing. CO2-flux rates were positively related to aboveground net primary productivity and temperature. Net ecosystem CO2 uptake started occurring above ~70% soil moisture content, primarily due to a decline in ER. Herein, we reveal that key environmental-change drivers-goose grubbing by decreasing GEP more than ER and warming by enhancing ER more than GEP-consistently suppress net tundra CO2 uptake, although their relative strength differs among habitats. By identifying how and where grubbing and higher temperatures alter CO2 fluxes across the heterogeneous Arctic landscape, our results have implications for predicting the tundra carbon balance under increasing numbers of geese in a warmer Arctic.