Mathilde Le Moullec

Empirical in-situ measurements of ecosystem carbon dioxide respiration ( R _eco ) in high-latitude ecosystems remain limited, yet they are essential for understanding how tundra carbon cycling responds to climate warming across different environmental contexts and for reducing uncertainties in upscaled carbon budgets and carbon–climate feedbacks. Here, we present the TundraFlux Database, which to date is the most comprehensive synthesis of tundra R _eco responses to experimental warming. The database compiles over 24 000 daily-aggregated in-situ R _eco measurements from control and warmed plots with open-top chambers at 64 Arctic and alpine tundra sites across 12 countries. By coupling R _eco measurements with extensive metadata on climate, vegetation, and soil characteristics, the TundraFlux Database enables the integration of field-scale ecological processes into large-scale models, offering new opportunities to refine global carbon budgets and test predictions of tundra ecosystem responses to warming. Open access to the TundraFlux Database will empower the research community to better quantify and predict how warming alters carbon cycling in Arctic and alpine tundra ecosystems. The data can be accessed on Zenodo (https://doi.org/10.5281/zenodo.17976235, Schwieger, 2026a).

Aims: 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 CO ₂ -fluxes and litter decomposition potential with the Tea Bag Index. Methods: 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. Results: 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 CO ₂ 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% difference relative to unmanipulated tundra), whereas warming had weaker effects but offset icing impacts, resulting in decomposition comparable to unmanipulated tundra. Conclusion: Winter icing and summer warming jointly influence tundra CO ₂ -fluxes and decomposition, pointing to enhanced carbon losses. While elevated winter temperatures are known to directly affect Arctic ecosystem functioning, their indirect effects via rain-on-snow-induced icing may be similarly important.

Empirical in-situ measurements of ecosystem carbon dioxide respiration (R_eco) in high-latitude ecosystems remain limited, yet they are essential for understanding how tundra carbon cycling responds to climate warming across different environmental contexts and for reducing uncertainties in upscaled carbon budgets and carbon–climate feedbacks. Here, we present the TundraFlux Database, which to date is the most comprehensive synthesis of tundra R_eco responses to experimental warming. The database compiles over 24,000 daily-aggregated in-situ R_eco measurements from control and plots warmed with open-top chambers at 64 Arctic and alpine tundra sites across 12 countries. By coupling R_eco measurements with extensive metadata on climate, vegetation, and soil characteristics, the TundraFlux Database enables the integration of field-scale ecological processes into large-scale models, offering new opportunities to refine global carbon budgets and test predictions of tundra ecosystem responses to warming. Open access to the TundraFlux Database empowers the research community to better quantify and predict how warming alters carbon cycling in Arctic and alpine tundra ecosystems.

The Arctic is warming rapidly and much faster in winter than in summer. Warm spells in winter lead to more frequent extreme rain‐on‐snow events that alter snowpack conditions and can encapsulate plants in ‘basal ice’ (‘icing’) for months. Yet, how icing affects plant communities, especially over multiple winters and under warmer summers, remains largely unstudied. We investigated winter icing and summer warming effects on vascular plants' productivity, reproduction and phenology in mesic dwarf shrub heath, an important reindeer habitat in high Arctic Svalbard, where winter temperatures have been rising particularly fast. In a full‐factorial field experiment, rain‐on‐snow and resultant icing were simulated in five consecutive winters, and each followed by experimentally increased summer temperatures. Vascular plant responses at the community level, with particular attention to the dominant dwarf shrub Salix polaris , were assessed throughout each subsequent growing season. Icing alone increased community‐level primary productivity, but only late in the growing season and reduced inflorescence production. Accordingly, S. polaris showed delayed early leaf phenophases, but accelerated subsequent development, resulting in smaller, thinner leaves. This compensatory growth response apparently occurred at the cost of delayed seed maturation. The phenological delay was associated with icing‐induced delays in spring soil warm‐up, possibly favouring resource allocation to primary productivity over reproduction. Experimental summer warming (on average 0.8°C) largely counteracted the effects of icing, enhancing community productivity throughout the growing season, offsetting S. polaris leaf size reductions and turning around its delayed phenophases, including seed dispersal. Effect sizes of icing and warming combined could be larger than those under warming alone. Yet, summer warming did not negate the reduction in community inflorescence production caused by icing. Synthesis. Extreme rain‐on‐snow events encapsulating plants in ice can influence high Arctic plant communities in mesic habitats to similar extents as—the better‐studied—summer warming. Nevertheless, the absence of magnified icing effects over the years indicates community resistance to icing, particularly under warmer summers, which contrasts with earlier documented ice‐induced die‐offs in communities dominated by evergreen shrubs. As warm spells during winter become the rule rather than exception, we call for similar experiments in coordinated circumpolar monitoring programmes across the tundra biome.

As the colder regions of the planet warm, species are moving northward and upward from the boreal forest to the tundra biome, a process that has been referred to as borealization. Here, we examine the diverse uses of the term borealization and propose the concept of ‘tundra borealization’ for terrestrial environments to specifically describe shifts and increases in abundance of boreal species into the tundra. We summarize the evidence to date for borealization of plant and animal communities in tundra ecosystems and the different approaches that can be used to quantify borealization. We discuss how land-use change is interacting with climate change, leading to species and community reorganization in colder biomes, and the consequences of borealization for food webs, ecosystem functions and northern livelihoods. Our review brings together the different definitions and lines of evidence for tundra borealization in terrestrial ecosystems to emphasize this important ecological process and rapidly evolving area of research.

Climate warming in the Arctic is very strong compared to other regions on Earth. Arctic winter climate and cryosphere conditions are changing towards more frequent mild spells. Precipitation is often falling as rain, followed by the formation of basal ice on frozen ground, particularly in Gulf Stream-influenced climates as in Svalbard. Such conditions encapsulate tundra plants in ice for several months, which is assumed to reduce land surface greenness due to plant damage. We investigated whether extensive basal ice (presence and thickness) and increased summer temperatures (growing degree days (GDD)) from in-situ time series impact satellite-derived land surface greenness. We measured greenness as the magnitude and timing of growing season maximum normalized difference vegetation index (NDVI). Our study covers Svalbard from 2013 to 2023, a decade with record breaking summer temperatures and many icy winters. We found lower maximum NDVI values when basal ice was present only at higher elevations (Estimated effect size: −0.0119, 95% CI: −0.0207 to −0.0031). We further found an eight-day advance in the timing of maximum NDVI (Estimated effect size: −7.56, 95% CI: −14.81 to −0.31) with basal ice presence in the region that was characterized by spatially and temporally extensive basal ice. Ice thickness, in contrast to presence, or GDD did not influence the magnitude or timing of maximum NDVI. Taken together, our findings indicate that basal ice presence could become a driver of vegetation change in the High Arctic as climatic extremes intensify, which could alter tundra greenness over larger landscapes and ultimately influence Arctic food webs.

Studying adaptation to extreme climates is essential for understanding evolutionary processes and how species evolve and persist under changing environmental conditions, such as climate warming. Here, we investigate the genomic basis of adaptations in the Svalbard reindeer (Rangifer tarandus platyrhynchus), an endemic subspecies that colonized the High Arctic approximately 7,000 years ago and developed a suite of adaptations for survival under conditions of extreme cold, changes in day length, and resource scarcity. Applying scans of selection, functional analysis of coding region variation, and characterization of copy number variation across reindeer populations from Svalbard, mainland Norway, mainland Russia, and Novaya Zemlya, our comparative genomics approach identified 150 genomic regions that are differentiated in Svalbard reindeer relative to mainland reindeer (R. tarandus). These genomic regions include genes linked to fat metabolism, energy conservation, cold tolerance, body size, fur morphology, and seasonal circadian rhythm. Our study highlights the advantages of using distinct approaches to uncover the genomic basis of adaptations and provides a path for future research into the evolution of species in similar environments.

Herbivores are an integral part of Arctic terrestrial ecosystems, driving ecosystem functioning and sustaining local livelihoods. In the context of accelerated climate warming and land use changes, understanding how herbivores contribute to the resilience of Arctic socio-ecological systems is essential to guide sound decision-making and mitigation strategies. While research on Arctic herbivory has a long tradition, recent literature syntheses highlight important geographical, taxonomic, and environmental knowledge gaps on the impacts of herbivores across the region. At the same time, climate change and limited resources impose an urgent need to prioritize research and management efforts. We conducted a horizon scan within the Arctic herbivory research community to identify emerging scientific and management priorities for the next decade. From 288 responses received from 85 participants in two online surveys and an in-person workshop, we identified 8 scientific and 8 management priorities centred on (a) understanding and integrating fundamental ecological processes across multiple scales from individual herbivore–plant interactions up to regional and decadal scale vegetation and animal population effects; (b) evaluating climate change feedbacks; and (c) developing new research methods. Our analysis provides a strategic framework for broad, inclusive, interdisciplinary collaborations to optimise terrestrial herbivory research and sustainable management practices in a rapidly changing Arctic.

Overharvest can severely reduce the abundance and distribution of a species and thereby impact its genetic diversity and threaten its future viability. Overharvest remains an ongoing issue for Arctic mammals, which due to climate change now also confront one of the fastest changing environments on Earth. The high‐arctic Svalbard reindeer (Rangifer tarandus platyrhynchus), endemic to Svalbard, experienced a harvest‐induced demographic bottleneck that occurred during the 17–20th centuries. Here, we investigate changes in genetic diversity, population structure, and gene‐specific differentiation during and after this overharvesting event. Using whole‐genome shotgun sequencing, we generated the first ancient and historical nuclear (n = 11) and mitochondrial (n = 18) genomes from Svalbard reindeer (up to 4000 BP) and integrated these data with a large collection of modern genome sequences (n = 90) to infer temporal changes. We show that hunting resulted in major genetic changes and restructuring in reindeer populations. Near‐extirpation followed by pronounced genetic drift has altered the allele frequencies of important genes contributing to diverse biological functions. Median heterozygosity was reduced by 26%, while the mitochondrial genetic diversity was reduced only to a limited extent, likely due to already low pre‐harvest diversity and a complex post‐harvest recolonization process. Such genomic erosion and genetic isolation of populations due to past anthropogenic disturbance will likely play a major role in metapopulation dynamics (i.e., extirpation, recolonization) under further climate change. Our results from a high‐arctic case study therefore emphasize the need to understand the long‐term interplay of past, current, and future stressors in wildlife conservation.