Matteo Petit Bon

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.

Warming, sea-level rise, and changing herbivory patterns are affecting ecosystem processes in northern coastal soils, including carbon and nitrogen cycling, although their combined effects are poorly understood. We studied the impact of these changes in a full-factorial microcosm experiment using four distinct soil types from the Yukon-Kuskokwim Delta of western Alaska. Soil was collected from three coastal wetlands, which differ in flooding frequency and use by migratory geese, and one rarely-flooded upland tundra used by geese in late summer. Microcosms containing these soils were incubated for 16 weeks at 10 °C or 18 °C, with or without three short-term floods, and with or without goose feces. We measured soil carbon dioxide (CO ₂ ) and methane (CH ₄ ) emissions weekly and net ammonification during the experiment. While higher temperature generally increased CO ₂ and CH ₄ emissions and net ammonification, and feces addition increased CO ₂ emissions, the impact of flooding on both gas fluxes and net ammonification differed among soil types. Flooding suppressed soil CO ₂ emissions in the historically least-frequently flooded wetland and upland tundra but had no effect on the other two wetland soils. Conversely, under warmer temperatures, flooding increased CH ₄ emissions in the most-frequently flooded wetland but decreased CH ₄ emissions in the tundra soil. Flooding also decreased net ammonification in the most-frequently flooded wetland, but increased net ammonification in the tundra soil. Our results suggest that climate change-related effects, particularly flooding, interact with soil-specific conditions that may amplify or dampen carbon loss and net ammonification in coastal Arctic soils depending on landscape position.

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.

Climate change is rapidly altering high-latitude coastal wetlands through increasing temperatures, more frequent flooding, and changing herbivore abundance and distribution. Examining plant functional trait responses to these drivers provides insight into how plant communities are shaped by climate change. We used a one-year full-factorial mesocosm experiment in coastal Alaska to assess immediate responses of two size-related traits (vegetative height and leaf area) and two leaf economics traits (specific leaf area [SLA] and leaf dry matter content [LDMC]) to warming, flooding, and goose herbivory in three dominant species across two wetland communities (Lowland and Upland) differing in landscape position. We sampled different sedges ( Carex rariflora and Carex lyngbyei ) and the same deciduous dwarf-shrub ( Salix fuscescens ) in the two wetlands. Warming increased plant size for the sedge (leaf area: + 15%) and the shrub (height: + 15%; leaf area: + 19%) in the Lowland wetland only. Flooding promoted acquisitive trait values for the Lowland sedge (SLA: + 8%; LDMC: − 6%) and conservative values for the Upland shrub (SLA: − 9%; LDMC: + 11%). The shrub thus showed clear wetland-specific responses to warming and flooding. Herbivory triggered coordinated responses across all three species, consistently decreasing size-related trait values (height: − 17–35%; leaf area: − 33–50%) and promoting acquisitive trait values (SLA: + 14–40%; LDMC: − 12–23%). Despite the absence of interactions, each driver contributed significantly to trait variation, underscoring the value of multifactorial approaches. The combination of consistent (herbivory) and context-dependent (flooding and warming) responses highlights the complexity of trait responses and improves predictions of rapid phenotypic adjustments in coastal high-latitude wetlands.

Land cover data are commonly used to model the terrestrial carbon (C) sink, yet these data have wide margins of error that significantly alter estimates of global C storage. Here we demonstrate this data vulnerability in grasslands, which are critical to C cycling but whose estimated distribution has varied by >50 million km ² (3.5–42% of the Earth’s terrestrial surface). Comparing multiple high-resolution land cover products with expertly annotated grassland data from six continents, we show sources of mapping error and discuss C implications based on 2023 United Nations (UN) FAO estimates. Past misidentification arose from inconsistent definitions on grassland identity and classification flaws especially relating to woody plant cover. Correcting these errors adjusted grassland coverage to 22.8% of the terrestrial land base (30.1 million km ² ), elevating UN projections of soil C stocks to 155.02 Pg (0–30 cm depth). These findings underscore the challenges of biome mapping for ecosystem accounting and policy, when lacking field-validated remotely sensed data.

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.

Climate change is increasing coastal landscapes’ exposure to flooding, in addition to rapidly rising temperatures. These changes are critical in the Arctic where the effects of sea level rise are exacerbated by the loss of sea ice protecting coasts, subsidence as permafrost thaws, and a projected increase in storms. Such changes will likely alter the land-atmosphere gas exchange of high-latitude coastal ecosystems, but the effects of flooding with warming remain unexplored. In this work we use a field experiment to examine the interacting effects of increased tidal flooding and warming on land-atmosphere CO2 and CH4 exchange in the coastal Yukon-Kuskokwim Delta, a large sub-Arctic wetland and tundra complex in western Alaska. We inundated dammed plots to simulate two levels of future flooding: low-intensity flooding represented by one day of flooding per summer-month (June, July, and August), and high-intensity flooding represented by three-consecutive days of flooding per summer-month, crossed with a warming treatment of 1.4 °C. We found that both flooding and warming influenced greenhouse gas exchange. Low-intensity flooding reduced net CO₂ uptake by 20% (0.78 µmol m-2 s-1) regardless of temperature, and marginally increased CH₄ emissions 0.83 nmol m-2 s-1 (33%) under ambient temperature, while decreasing CH4 emissions by -1.96 nmol m-2 s-1 (40%) under warming. In contrast, high-intensity flooding restored net CO₂ uptake to control levels due to enhanced primary productivity under both temperature treatments. High-intensity flooding decreased CH4 emissions under ambient temperature by 0.76 nmol m-2 s-1 (30%), but greatly increased emissions under warming by 4.68 nmol m-2 s-1 (265%), presumably driven by increased plant-mediated CH₄ transport. These findings reveal that greenhouse gas exchange responds rapidly and non-linearly to intensifying flooding, and highlight the importance of short-term flooding dynamics and warming in shaping future carbon cycling in this Arctic coastal wetland.

Sea level rise and more frequent and larger storms will increase saltwater flooding in coastal terrestrial ecosystems, altering soil‐atmosphere CO₂ and CH₄ exchange. Understanding these impacts is particularly relevant in high‐latitude coastal soils that hold large carbon stocks but where the interaction of salinity and moisture on greenhouse gas flux remains unexplored. Here, we quantified the effects of salinity and moisture on CO₂ and CH₄ fluxes from low‐Arctic coastal soils from three landscape positions (two Wetlands and Upland Tundra) distinguished by elevation, flooding frequency, soil characteristics, and vegetation. We used a full factorial laboratory incubation experiment of three soil moisture levels (40%, 70%, or 100% saturation) and four salinity levels (freshwater, 3, 6, or 12 ppt). Salinity and soil moisture were important controls on CO₂ and CH₄ emissions across all landscape positions. In saturated soil, CO₂ emissions increased with salinity in the lower elevation landscape positions but not in the Upland Tundra soil. Saturated soil was necessary for large CH₄ emissions. CH₄ emissions were greatest with low salinity, or after 11 weeks of incubation when SO₄²⁻ was exhausted allowing for methanogenesis as the dominant mechanism of anaerobic respiration. In partially saturated soil, greater salinity suppressed CO₂ production in all soils. CH₄ fluxes were overall quite low, but increased between 3 and 6 ppt in the Tundra. In the future, a small increase in floodwater salinity may increase CO₂ production while suppressing CH₄ production; however, where water is impounded, CH₄ production could become large, particularly in the landscapes most likely to flood.