Paul J. Hanson

Peatlands store one-third of the global soil carbon (C) on only ∼3% of Earth’s land surface but risk becoming C sources to accelerate climate change due to warming-induced C loss. Elevated CO ₂ (eCO ₂ ) may mitigate this loss by boosting plant C uptake. However, how eCO ₂ and climate warming interact to influence peatland C dynamics remains unclear. Here, we investigated peatland C fate at five warming levels (+0°, +2.25°, +4.5°, +6.75°, and + 9°C), each under ambient CO ₂ (aCO ₂ ) and eCO ₂ (+500 parts per million), by model-guided synthesis of 18 measured variables for each of the 10 treatments from an experiment. Our results showed that the peatland C loss increased with warming at a rate of 62.6 g C m ⁻² year ⁻¹ °C ⁻¹ , reaching 476.5 ± 104.7 g C m ⁻² year ⁻¹ at +9°C under aCO ₂ , primarily because of increased Sphagnum mortality and accelerated decomposition. eCO ₂ reduced the loss rate to 17.3 g C m ⁻² year ⁻¹ °C ⁻¹ , nearly halving the C loss at +9°C (269.4 ± 80.5 g C m ⁻² year ⁻¹ ), mainly due to enhanced tree and shrub growth. Methane emissions increased with warming, similarly under aCO ₂ and eCO ₂ . Our findings suggest that northern peatlands likely become major C sources under climate warming, although rising atmospheric CO ₂ concentration may partially slow down the C loss. Climate-driven shifts in plant community structure, rather than peat decomposition alone, play a critical role in regulating peatland C fate and may threaten the stability of peatland ecosystem functions and services in the future.

Winter warming is altering plant exposure to cold events, making it increasingly important to understand how cold hardiness dynamics respond across the dormant season. Using the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment in northern Minnesota, we measured bud cold hardiness across four dormant seasons (2021–2025). There, a boreal peatland community was exposed to continuous active whole ecosystem warming across five levels (+0.00°C to +9.00°C) combined with atmospheric and elevated CO ₂ (+500 ppm) in open‐top chambers. Cold hardiness of two overstory ( Larix laricina and Picea mariana ) and two understory ( Chamaedaphne calyculata and Rhododendron groenlandicum ) species was evaluated semi‐regularly throughout seasons. Both understory and overstory species gained cold hardiness (acclimated) at warmer temperatures in the fall than those causing deacclimation in the spring, demonstrating different temperature responsiveness phases across the dormant season. Based on cold hardiness sensitivity, warming did not alter maximum midwinter cold hardiness (sensitivity~0°C change in cold hardiness/°C of warming) but delayed fall acclimation and, most importantly, accelerated spring deacclimation (sensitivity > 0°C). These changes resulted in increased cold damage risk in late winter and spring with warming (sensitivity > 1°C/°C) for L. laricina , C. calyculata and R. groenlandicum . Among overstory species, the deciduous L. laricina was more sensitive to warming than the evergreen P. mariana , indicating warming may disadvantage acquisitive species. For understory shrubs, reduced snow cover in the warmest enclosures that removed the insulating buffer and exposed buds to temperatures colder than in control plots associated with the high sensitivity of these species resulted in observed midwinter bud mortality with warming. Rather than uniformly changing cold damage risk, warming decreases risk of cold damage in fall and early winter for all species, but increases risk in late winter and spring for most species, suggesting species‐specific cold hardiness dynamics and snow‐driven microclimate may result in boreal forest composition changes.

The viability of peatlands as terrestrial carbon sinks is unknown as peatland carbon losses could be offset, or exacerbated, by increased plant growth. The response of fine‐roots to changing conditions in these ecosystems will be critical to understanding plant access to water, acquisition of nutrients, and ecosystem structure and function. We examined the plant functional type‐specific responses of fine‐root production to warming and elevated CO ₂ treatments, and to warming treatment‐induced deepening of water‐tables in a forested boreal bog. We hypothesized that shrubs would show the most dynamic responses of fine‐root production. We used minirhizotron cameras to estimate fine‐root production, depth and peak annual standing crop from images of individually tracked roots across 7 years (2015–2021) in the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment. Fine‐root production increased with warming for shrubs 10.3, trees 2.2 and herbs 4.6%°C ⁻¹ . When water‐tables dropped to their lows of 78 cm deep, shrubs deepened their fine‐root production from 7 to 29 cm deep, and trees from 9 to 17 cm, while herbs produced fine‐roots around 31 cm deep independent of shifts in water‐tables. Fine‐root standing crop increased with warming most strongly for shrubs, even more so under elevated CO ₂ . Synthesis. We found that the fine‐roots of shrubs responded the most strongly to warming, elevated CO ₂ and lowered water‐tables. The greater responsiveness of shrub fine‐root production may partially explain increasing ‘shrubification’ at this site and in other peatlands with long‐term drainage.

Leaf phenology may influence the development of wood structure and hydraulic function across growing seasons, yet the roles of green-up, green-down, and growing season length in regulating xylem anatomy remain unclear. We quantified annual wood anatomy, leaf phenology, and growth in a whole-ecosystem experiment with 5 warming levels (up to +9 °C) and 2 CO2 levels (ambient and +500 ppm) in Picea mariana (conservative spruce) and Larix laricina (acquisitive larch). We identified a phenology-tracheid-growth spectrum, reflecting a trade-off between hydraulic safety (thicker walls, higher tracheid density, and later green-up) and fast growth (wider tracheids, delayed green-down, and longer growing seasons). In spruce, earlier green-up, longer growing seasons, and later green-down increased the latewood hydraulic diameter more than the earlywood. In larch, earlier green-up increased earlywood hydraulic diameter, while later green-down increased latewood mechanical safety via thicker walls. Larch exhibited greater phenological sensitivity to elevated CO2 in regulating wood anatomy than spruce. Warming indirectly increased spruce growth by extending the growing season, which increased the latewood hydraulic diameter and subsequently enhanced overall growth. Warming directly increased larch growth but not through enhanced earlywood hydraulic conductivity. These findings demonstrate the role of divergent phenological adjustments in hydraulic function, with implications for boreal carbon and water fluxes.

Winter warming is altering plant exposure to cold events, and its effects on seasonal cold hardiness dynamics remain poorly understood. Here we quantified the effect of whole-ecosystem warming (+0.00°C to +9.00°C) on bud cold hardiness of two overstory ( Larix laricina and Picea mariana ) and two understory ( Chamaedaphne calyculata and Rhododendron groenlandicum ) boreal peatland forest species across four dormant seasons. Warming reduced cold hardiness in fall and spring by delaying acclimation and advancing deacclimation, yet damage risk increased only in late winter and spring for three species. Warming also reduced snow cover, increasing temperature variability and cold damage for understory shrubs. Together, these results show that warming restructures rather than uniformly increasing cold hardiness risk, concentrating vulnerability at transitional seasons through species-specific dynamics and microclimate exposure.

Field-warming experiments offer insight into the response of ecosystems to rising temperatures, but cross-site comparison is needed to determine both the general tendencies of warming responses and the context dependencies of deviations from those norms. These responses are not limited to the direct effects of temperature but also their indirect effects on soil moisture, a critical factor controlling ecosystem productivity and carbon fluxes. Here we introduce SWEDDIE: the first database to characterize the whole soil profile warming response across 26 distinct soil warming experiments, encompassing forest, grassland, cropland, tundra, and wetland ecosystems. SWEDDIE is needed because prior databases and syntheses of warming effects on ecosystems were dominated by aboveground warming studies, many of which warmed soil modestly or negligibly during much of the growing season and reported only growing season averages. We demonstrate the potential of the SWEDDIE database by quantifying soil temperature and moisture changes for each experiment as a function of depth, warming methodology, ambient climate conditions, and ecosystem, as well as the relationship between soil moisture and imposed warming. Warming attenuated with depth at sites with aboveground warming only but increased with depth at sites with belowground warming only, as hypothesized. Warming led to soil drying at most sites, and drying was positively correlated with the magnitude of warming. However, the relationship between soil warming and soil drying varied by ecosystem: forest soils dried the most, while tundra soils became wetter with warming. Ambient climatic conditions also significantly influenced the relationship between experimental warming and drying, with more drying per degree of warming observed in soils with higher ambient moisture. The inconsistency of soil moisture changes with warming across ecosystems and warming methodologies demonstrates the importance of quantifying shifts in temperature and moisture in both space and time in order to overcome site-specific bias in ecosystem warming responses. The high temporal resolution and depth-resolved observations of the fundamental ecosystem properties of soil temperature and moisture in SWEDDIE v1.0.0 serve as a foundation for future experimental soil warming synthesis efforts and demonstrate the power of this actively growing community resource.

Warming and elevated atmospheric CO ₂ profoundly impact peatland ecosystems, particularly through changes in plant species composition. Plants regulate the initial input of organic compounds to peatland belowground systems, controlling the availability of electron donors and electron acceptors that fuel microbially mediated organic matter decomposition to CO ₂ and CH ₄ . However, explicit links between porewater CO ₂ and CH ₄ dynamics and plant‐derived chemical compounds remain relatively undefined. In a whole ecosystem warming experiment, we investigated how warming affects plant leaf chemical composition and species assemblages, and how the alteration of leaf‐derived organic compounds supplied to the subsurface impacts belowground CO ₂ and CH ₄ production. While earlier studies at our site found no temperature‐dependent changes in CH ₄ production pathways, our extended timeseries has revealed increased acetoclastic methanogenesis at higher temperatures in certain peat depths, correlated with elevated porewater phenolics. These changes appear driven by the observed increased plant productivity and altered vegetation inputs, which accelerate decomposition and fuel CH ₄ production through enhanced substrate availability. We observed warming‐induced changes in molecular composition both between and within plant species, suggesting that plant‐mediated controls on belowground carbon processing are more complex than previously recognized.

Extreme drought events are predicted to increase with climate change, yet their impacts on ecosystem carbon dynamics under warming and elevated carbon dioxide (eCO ₂ ) remain unclear. In a peatland experiment with five warming treatments each under ambient carbon dioxide (aCO ₂ ) and eCO ₂ (+500 parts per million), a 2-month extreme drought in 2021 reduced net ecosystem productivity by 444.0 ± 65.8 and 736.6 ± 57.8 grams of carbon per square meter at +9°C under aCO ₂ and eCO ₂ , respectively—228.6 ± 56.8% and 381.9 ± 83.4% of the reduction at +0°C under aCO ₂ . This exacerbation was driven by warming-induced water table decline, prolonged low water tables, and CO ₂ -enhanced substrate availability through increased plant carbon inputs. Findings indicate that future climate will greatly amplify carbon loss during extreme drought, reinforcing positive carbon-climate feedbacks.

The response of microbial communities that regulate belowground carbon turnover to climate change drivers in peatlands is poorly understood. Here, we leverage a whole ecosystem warming experiment to elucidate the key processes of terminal carbon decomposition and community responses to temperature rise. Our dataset of 697 metagenome-assembled genomes (MAGs) represents the microbial community from the surface (10 cm) to 2 m deep into the peat column, with only 3.7% of genomes overlapping with other well-studied peatlands. Community composition has yet to show a significant response to warming after 3 years, suggesting that metabolically diverse soil microbial communities are resistant to climate change. Surprisingly, abundant and active methanogens in the genus Candidatus Methanoflorens, Methanobacterium, and Methanoregula show the potential for both acetoclastic and hydrogenotrophic methanogenesis. Nonetheless, the predominant pathways for anaerobic carbon decomposition include sulfate/sulfite reduction, denitrification, and acetogenesis, rather than methanogenesis based on gene abundances. Multi-omics data suggest that organic matter cleavage provides terminal electron acceptors, which together with methanogen metabolic flexibility, may explain peat microbiome composition resistance to warming.