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Modelling past and future peatland carbon dynamics across the pan‐Arctic
Abstract
The majority of northern peatlands were initiated during the Holocene. Owing to their mass imbalance, they have sequestered huge amounts of carbon in terrestrial ecosystems. However, although recent syntheses have filled some knowledge gaps, the extent and remoteness of many peatlands pose challenges to developing reliable regional carbon accumulation estimates from observations. In this work, we employed an individual‐ and patch‐based dynamic global vegetation model (LPJ–GUESS) with peatland and permafrost functionality to quantify long‐term carbon accumulation rates in northern peatlands and to assess the effects of historical and projected future climate change on peatland carbon balance. We combined published datasets of peat basal age to form an up‐to‐date peat inception surface for the pan‐Arctic region which we then used to constrain the model. We divided our analysis into two parts, with a focus both on the carbon accumulation changes detected within the observed peatland boundary and at pan‐Arctic scale under two contrasting warming scenarios (RCP8.5 and RCP 2.6). We found that peatlands continue to act as carbon sinks under both warming scenarios, but their sink capacity will be substantially reduced under the high warming (RCP8.5) scenario after 2050. Areas where peat production was initially hampered by permafrost and low productivity were found to accumulate more carbon because of the initial warming and moisture‐rich environment due to permafrost thaw, higher precipitation and elevated CO2 levels. On the other hand, we project that areas which will experience reduced precipitation rates and those without permafrost will lose more carbon in the near future, particularly peatlands located in the European region and between 45‐55 °N latitude. Overall, we found that rapid global warming could reduce the carbon sink capacity of the northern peatlands in the coming decades.
... Based on the climate space theory, it is possible that amplified climate warming may turn, or is turning, Arctic tundra into an even more peatland-rich landscape, with widespread peat patches that may serve as the initial stage of new peatland formation. Although some studies highlight that climate warming may result in the loss of suitable climate space and the shrink of C sinks for peatlands in relatively lower latitudes (Fewster et al., 2022;Gallego-Sala et al., 2018), recent coupled model simulations with peatland nodules do support the potential future trajectory of poleward peatland migration (Chaudhary et al., 2020;Müller & Joos, 2021;Qiu et al., 2020). However, we do not fully understand how this migration may occur, and more specifically, the connections in ecosystem functions and processes between shallow peat patches and mature peatlands due to the paucity of records that can reveal their genesis and evolution histories. ...
... Finally, this study also underscores the urgent need to incorporate and improve ecosystem processes related to peat formation and accumulation into Earth system models to accurately predict the future trajectory of C balance over Arctic tundra or northern peatlands . Although several recent studies have made important progress toward this goal (Chaudhary et al., 2020;Müller & Joos, 2021;Qiu et al., 2020;Stocker et al., 2014;Zhao & Zhuang, 2023), not all model predictions incorporate interactions between peatlands and nonpeatlands, dynamic vegetation changes, and peatland area changes, which we find to be important in simulating the possible transformation from tundra soils to mature peatlands. For example, future predictions using a peatland-specific dynamic global vegetation model (Chaudhary et al., 2022) or a statistical climate-space model (Gallego-Sala et al., 2018) did not have peatland C accumulation on NSA because it is not within the current peatland area. ...
Recent amplified climate warming in the Arctic has caused profound changes in terrestrial ecosystems, with the potential for strong feedback on climate change. Arctic tundra landscapes have developed patchy and thin organic soil (peat) layers at the surface that may continue to grow into mature peatlands and become a larger carbon sink under future warming. Here we use paleoecological analyses of multiple soil and peat cores collected from the North Slope of Alaska to document and understand the formation and development histories of tundra peat patches and permafrost peatlands. We find a consistent peat development sequence for peat patches, first from mineral soils to sedge peat during the Little Ice Age, and then to Sphagnum peat during the recent warming with high carbon accumulation rates. These findings suggest that climate cooling is likely critical for the initial peat buildup on tundra soils due to reduced decomposition, whereas climate warming triggers the regime shift into an increased abundance of Sphagnum mosses that are likely central to enhancing their carbon sink capacity. Additionally, peat patches become similar to permafrost peatlands in the vicinity in terms of ecosystem processes and carbon dynamics, and therefore may have developed the same ecohydrological feedback system to maintain their long‐term stability. This study implies that the potential future expansion of peat patches into peatlands may strongly alter the carbon balance of Arctic tundra, supporting the new United Nations Environment Programme's report that calls for incorporating widespread shallow peat into understanding the peatland–carbon–climate nexus.
... Cryohydrogeological models are numerical models with the added capability of modeling subsurface freeze-thaw dynamics. Several modeling efforts have simulated permafrost processes [13][14][15][16]. Modeling studies have assessed the sensitivity of the active layer depth to near-surface hydraulic properties [12,17]. ...
Modeling peatland hydraulic processes in cold regions requires defining near-surface hydraulic parameters. The current study aims to determine the soil freezing and water characteristic curve parameters for organic soils from peatland-dominated permafrost mires. The three research objectives are as follows: (i) Setting up an in situ soil freezing characteristic curve experiment by installing sensors for measuring volumetric water content and temperature in Storflaket mire, Abisko region, Sweden; (ii) Conducting laboratory evaporation experiments and inverse numerical modeling to determine soil water characteristic curve parameters and comparing three soil water characteristic curve models to the laboratory data; (iii) Deriving a relationship between soil freezing and water characteristic curves and optimizing this equation with sensor data from (i). A long-lasting in situ volumetric water content station has been successfully set up in sub-Arctic Sweden. The soil water characteristic curve experiments showed that bimodality also exists for the investigated peat soils. The optimization results of the bimodal relationship showed excellent agreement with the soil freezing cycle measurements. To the best of our knowledge, this is one of the first studies to establish and test bimodality for frozen peat soils. The estimated hydraulic parameters could be used to better simulate permafrost dynamics in peat soils.
... Although the rates of primary production in peatlands exceed those of decomposition, allowing for peatlands to act as net C sinks, anthropogenic warming and disturbance are projected to increase peatland greenhouse gas emissions and possibly turn these ecosystems into a net source of C [4][5][6][7][8][9]; this climate feedback remains to be observationally detected in unmanipulated environments. OM decomposition and C loss are projected to accelerate due to more frequent and/or prolonged droughts with climate change, as oxygen entering drying soils gives rise to the enzymes needed to decompose peat [10][11][12][13]. Warming also slows the growth and production of Sphagnum mosses [14], possibly as they become shaded by the growth of shrubs [15]. ...
We employed two compelling and distinct methods, Fourier Transform Infrared Spectroscopy (FTIR) and Ramped Pyrolysis Oxidation (Ramped PyrOx), to examine the quality of organic matter (OM) stored in four peatlands located along a latitudinal gradient (Tropical (4˚N), Subtropical (27˚N), Boreal (48˚N), and Polar (68˚N)). FTIR was used to quantify the relative abundance of carbohydrates, a relatively labile compound class, and aromatics, which are more recalcitrant, in a sample set of four peat cores. These samples were then prepared using Ramped PyrOx, a second, independent method of determining OM quality that mimics the natural diagenetic maturation of OM that would take place over long timescales. Previous large-scale studies using FTIR to evaluate OM quality have observed that it generally increases with increasing latitude (more carbohydrates, less aromatics). Here, we demonstrate that the Ramped PyrOx approach both validates and complements the FTIR approach. The data stemming from each Ramped PyrOx preparation was input to a model that generates an estimated probability density function of the activation energy (E) required to break the C bonds in the sample. We separated these functions into three fractions (“low E,” “medium E,” and “high E”) to create Ramped PyrOx variables that could be quantitatively compared to the compound class abundance data from FTIR. In assessing the agreement between the two methods, we found three significant relationships between Ramped PyrOx and FTIR variables. Low E fractions and carbohydrate content were positively correlated (R² = 0.51) while low E fractions were negatively correlated with aromatic content (R² = 0.58). Medium E fractions were found to be positively correlated with aromatics (R² = 0.69).
... Several recent studies have focused on process-based modelling of high latitude peatland C accumulation, decomposition rate and climatic influence in the past and future (Belyea and Malmer, 2004;Chaudhary et al., 2020;Gorham et al., 2012;Qiu et al., 2020;Yu, 2011;Zhang et al., 2018). However, uncertainties remain in the magnitude, extent, and inter-annual variability of climate-carbon feedback, as is the role of multiple biogeochemical processes. ...
Boreal peatlands store vast amounts of soil organic carbon (C) owing to the imbalance between productivity and decay rates. In the recent decades, this carbon stock has been exposed to a warming climate. During the past decade alone, the Arctic has warmed by ~ 0.75 • C which is almost twice the rate of the global average. Although, a wide range of studies have assessed peatlands' C cycling, our understanding of the factors governing source / sink dynamics of peatland C stock under a warming climate remains a critical uncertainty at site, regional, and global scales. Here our focus was on answering two key questions: (1) What drives the interannual variability of carbon dioxide (CO 2) fluxes at the Bonanza Creek rich fen in Alaska, and (2) What are the internal carbon allocation patterns during the study years? We addressed these knowledge-gaps using an intermediate complexity terrestrial ecosystem model calibrated by a Bayesian model-data fusion framework at a weekly timestep with publicly available eddy covariance, satellite-based earth observation, and in-situ datasets for 2014 to 2020. We found that the greening trend (a relative increase of leaf area index ~0.12 m 2 m-2 by 2020) in the fen ecosystem is forced by a CO 2 fertilisation effect which in combination resulted in increased gross primary production (GPP). Relative to 2014, GPP increased by ~75 gC m-2 year-1 (by 2020; 95% confidence interval (CI):-41.35 gC m-2 year-1 to 213.55 gC m-2 year-1) while heterotrophic respiration stayed constant. Consistent with the observed greening, our analysis indicates that the ecosystem allocated more C to foliage (~50%) over the structural (A carbon pool consisting of branches, stems and coarse roots; ~30%) and fine root C pools (~20%).
... These highly degraded drained peatlands lose their wetland-specific vegetation diversity (Hammerich et al., 2022) and the ability to store C and nutrients (Richardson and Marshall, 1986). Degraded peatlands are responsible for around 5% of global anthropogenic carbon dioxide (CO 2 ) emissions (Chaudhary et al., 2020;IPCC, 2022). Therefore, global peatland degradation contributes to multiple environmental problems, and restoration is essential and urgent. ...
Introduction
Drainage for agricultural purposes is one of the main drivers of peatland degradation, leading to significant greenhouse gas (GHG) emissions, biodiversity loss, and soil eutrophication. Rewetting is a potential solution to restore peatlands, but it generally requires a land-use shift to paludiculture or nature areas.
Methods
This study explored whether three different water level management techniques (subsoil irrigation, furrow irrigation, and dynamic ditch water level regulation) could be implemented on dairy grasslands to yield increases in essential ecosystem services (vegetation diversity and soil biogeochemistry) without the need to change the current land use or intensity. We investigated vegetation diversity, soil biogeochemistry, and CO2 emission reduction in fourteen agricultural livestock pastures on drained peat soils in Friesland (Netherlands).
Results
Across all pastures, Shannon-Wiener diversity was below 1, and the species richness was below 5. The plant-available phosphorus (P) was consistently higher than 3 mmol L⁻¹. None of the water level management (WLM) techniques enhanced vegetation diversity or changed soil biogeochemistry despite a notable increase in water table levels. The potential for CO2 emission reduction remained small or even absent. Indicators of land-use intensity (i.e., grass harvest and fertilization intensity), however, showed a strong negative correlation with vegetation diversity. Furthermore, all sites’ total and plant-available P and nitrate exceeded the upper threshold for species-rich grassland communities.
Discussion
In conclusion, our research suggests that incomplete rewetting (i.e., higher water tables while maintaining drainage) while continuing the current land use does neither effectively mitigate GHG emissions nor benefit vegetation diversity. Therefore, we conclude that combining WLM and reducing land-use intensity is essential to limit the degradation of peat soils and restore more biodiverse vegetation.
... Climate change impacts on peatlands include hydrologic impacts and the reduction in the carbon sink capacity (Chaudhary et al. (2020)). Wattendorf et al. (2010) found that peatlands in Baden-Württemberg, Germany would have longer lasting periods of low water levels despite increases in annual precipitation. ...
... The Arctic is often defined as a land of tundra with climatic conditions restricting tree growth (Robinson, 2001;Walker et al., 2005); however, a big part of the areas above the Arctic Circle also contains taiga forests (Montesano et al., 2009). Vast peatland coverage storing huge carbon reserves is a characteristic feature of the arctic region (Chaudhary et al., 2020;Tarnocai, 2009;Tarnocai and Stolbovoy, 2006). Antarctica is a predominantly rocky continent more than 98% occupied by ice and characterized by irregularly developed soil or its absence (Bockheim and Haus, 2014;Pires et al., 2017). ...
Fungi are widely distributed on our planet, including in extremely harsh habitats, such as the polar regions. The extreme conditions of those habitats limit the number of organisms capable of living there, but some fungi are adapted to the polar conditions and play essential roles in nutrient cycling. However, knowledge about their diversity, distribution, and functioning is fragmented, and approaches used to study them are diverse, often yielding difficult-to-compare results. We present maps with locations of mycological studies in the Arctic and Antarctica, as well as a list of mycelial fungi found on various terrestrial substrates through cultivation on nutrient media and/or molecular methods. These fungi were identified to the species level based on morphological-cultural features or gene-sequence analysis. Analysis of the methods applied to study fungi in different substrates shows that a combination of multiple methods is optimal to study species composition. The taxonomic affiliation of the identified species to different fungal divisions is largely determined by habitat conditions and research methods. The largest number of species belongs to the divisions Ascomycota and Basidiomycota. The predominant ecological groups were saprotrophic and symbiotic fungi. The majority of 1324 discovered fungal species are known as cosmopolitan species. Approximately one-fifth of the fungi were identical between the Arctic and Antarctica, only a few species are known to be endemic to Antarctica or Arctic, and there are 1–6 identified bipolar species. Claims of endemism of polar-region fungi are relatively weakly supported.
... While difficult to simulate at larger scales, the models were frequently used to study belowground properties, such as permafrost (Burke et al., 2017, Melton et al., 2019, Shirley et al., 2022b, and soil carbon (Larson et al., 2022), methane cycling (Grant et al., 2017), and peat accumulation (Chaudhary et al., 2017, Chaudhary et al., 2020, Chadburn et al., 2022, Chaudhary et al., 2022 A c c e p t e d M a n u s c r i p t of microtopography and soil heterogeneity on vegetation , Mekonnen et al., 2021, Shirley et al., 2022b. The suite of ecosys papers demonstrates how creative questions can be asked of models to focus on different ecosystem properties and the subsequent effects on vegetation, highlighting the dynamism of Arctic vegetation in a changing climate. ...
Modeling Arctic-Boreal vegetation is a challenging but important task, since this highly dynamic ecosystem is undergoing rapid and substantial environmental change. In this work, we synthesized information on 18 dynamic vegetation models (DVMs) that can be used to project vegetation structure, composition, and function in North American Arctic-Boreal ecosystems. We reviewed the ecosystem properties and scaling assumptions these models make, reviewed their applications from the scholarly literature, and conducted a survey of expert opinion to determine which processes are important but lacking in DVMs. We then grouped the models into four categories (specific intention models, forest species models, cohort models, and carbon tracking models) using cluster analysis to highlight similarities among the models. Our application review identified 48 papers that addressed vegetation dynamics either directly (22) or indirectly (26). The expert survey results indicated a large desire for increased representation of active layer depth and permafrost in future model development. Ultimately, this paper serves as a summary of DVM development and application in Arctic-Boreal environments and can be used as a guide for potential model users, thereby prioritizing options for model development.
... Meanwhile, carbon sequestration by plants is an important carbon cycle process in peatlands [13,14]. Plants are important sources of organic carbon in the soil as they absorb CO 2 to form photosynthetic carbon from plants and then transport it to the soil [15,16]. Changes in the composition of the vegetation community alter the carbon balance of ecosystems by affecting soil-plant interactions [6]. ...
As the regulator of water and nutrient changes in the active layer after permafrost degradation, root signaling substances affect the plant–soil carbon allocation mechanism under climate warming, which is a key issue in the carbon source/sink balance in permafrost regions. To explore how plant root signaling substances regulate carbon allocation in plants and soils under permafrost degradation, the changes in carbon allocation and root signaling substances in the plants and soils of peatland in different permafrost regions at the time of labeling were studied by in situ 13C labeling experiments. The results showed that the fixed 13C of Larix gemlini, Carex schumidtii, and Sphagnum leaves after photosynthesis was affected by permafrost degradation. In regions with more continuous permafrost, the trend of the L. gemlini distribution to underground 13C is more stable. Environmental stress had little effect on the 13C accumulation of Vaccinium uliginosum. Nonstructural carbohydrates, osmotic regulatory substances, hormones, and anaerobic metabolites were the main root signaling substances that regulate plant growth in the peatlands of the three permafrost regions. The allocation of carbon to the soil is more susceptible to the indirect and direct effects of climate and environmental changes, and tree roots are more susceptible to environmental changes than other plants in isolated patches of permafrost regions. The physical properties of the soil are affected by climate change, and the allocation of carbon is regulated by hormones and osmotic regulators while resisting anoxia in the sporadic regions of permafrost. Carbon allocation in discontinuous permafrost areas is mainly regulated by root substances, which are easily affected by the physical and chemical properties of the soil. In general, the community composition of peatlands in permafrost areas is highly susceptible to environmental changes in the soil, and the allocation of carbon from the plant to the soil is affected by the degradation of the permafrost.
Carbon accumulation in most northern peatlands is generally positively correlated with temperature under natural climate change. In the subtropical monsoon region of China, the climate differs from that of most northern peatlands, where a significant number of peatlands have developed in mountainous areas. However, it remains unclear how the carbon dynamics of these subtropical peatlands respond to climate change. Here, we reconstructed the net carbon fluxes of a typical mountainous peatland in Tianmu Mountain, eastern China, over the past millennium. Climate records in the subtropical monsoon zone indicate fluctuating and declining temperatures alongside increasing humidity over the past thousand years. Drought and higher winter temperatures have facilitated the terrestrialization of waterlogged depressions and triggered the peatland formation in this region. The net carbon accumulation in the peatland has generally shown a downward trend due to the progressively decreasing winter temperature and increasing humidity. When winter temperatures decrease, the growing season for vegetation is shortened, resulting in less litter production and reduced carbon accumulation. Increased humidity leads to greater surface waterlogging and prolonged flooding of surface vegetation, which hampers vegetation growth, reduces litter production, and consequently lowers carbon accumulation. Despite the decline in carbon accumulation over the last millennium, the peatland’s net carbon balance remains in a 'carbon sink' state. This suggests that the risk of carbon release from the peatland carbon pool under natural climate change conditions is not substantial in the subtropical monsoon area.
Global changes in climate and land use are occurring at an unprecedented rate, often triggering drastic shifts in plant communities. This study aims to reconstruct the changes that occurred over 35 years in the plant communities of temperate bogs subjected to indirect human-induced disturbances. In 2015–17, we resurveyed the vascular flora of 76 plots located in 16 bogs of southern Québec (Canada) first sampled in 1982. We evaluated changes in species richness, frequency of occurrence and abundance, while considering species shade-tolerance and preferential habitat. We calculated beta diversity as between-site similarities in composition, and evaluated differences between the two surveys using tests for homogeneity in multivariate dispersion. We found a significant increase in species richness and beta diversity over the last 35 years associated with major species turnovers, indicating a biotic differentiation of the Sphagnum-bog plant communities. These changes were mostly associated with an increase in the abundance and frequency of shade-tolerant and facultative species, suggesting a global phenomenon of woody encroachment. Because the observed changes occurred in a few decades on sites free of in situ human disturbances, we suggest that they were likely induced by the synergic effect of the agricultural drainage occurring in the surrounding mineral soils, climate warming, and nitrogen atmospheric depositions. We also believe that further changes are to be expected, as the triggering factors persist. Finally, our results highlight the need for increased bog conservation or restauration efforts. Indeed, a rise in beta diversity due to the introduction of nearby terrestrial species could induce biotic homogenization of the bog flora with that of surrounding habitats and ultimately impoverish the regional species pool.
The importance of northern peatlands in the global carbon cycle has been recognized, especially for long-term changes. Yet, the complex interactions between climate and peatland hydrology, carbon storage, and area dynamics make it challenging to represent these systems in land surface models. This study describes how peatlands are included as an independent sub-grid hydrological soil unit (HSU) in the ORCHIDEE-MICT land surface model. The peatland soil column in this tile is characterized by multilayered vertical water and carbon transport and peat-specific hydrological properties. The cost-efficient version of TOPMODEL and the scheme of peatland initiation and development from the DYPTOP model are implemented and adjusted to simulate spatial and temporal dynamics of peatland. The model is tested across a range of northern peatland sites and for gridded simulations over the Northern Hemisphere (>30∘ N). Simulated northern peatland area (3.9 million km2), peat carbon stock (463 Pg C), and peat depth are generally consistent with observed estimates of peatland area (3.4–4.0 million km2), peat carbon (270–540 Pg C), and data compilations of peat core depths. Our results show that both net primary production (NPP) and heterotrophic respiration (HR) of northern peatlands increased over the past century in response to CO2 and climate change. NPP increased more rapidly than HR, and thus net ecosystem production (NEP) exhibited a positive trend, contributing a cumulative carbon storage of 11.13 Pg C since 1901, most of it being realized after the 1950s.
Permafrost is a key element of the cryosphere and an essential climate variable in the Global Climate Observing System. There is no remote-sensing method available to reliably monitor the permafrost thermal state. To estimate permafrost distribution at a hemispheric scale, we employ an equilibrium state model for the temperature at the top of the permafrost (TTOP model) for the 2000–2016 period, driven by remotely-sensed land surface temperatures, down-scaled ERA-Interim climate reanalysis data, tundra wetness classes and landcover map from the ESA Landcover Climate Change Initiative (CCI) project. Subgrid variability of ground temperatures due to snow and landcover variability is represented in the model using subpixel statistics. The results are validated against borehole measurements and reviewed regionally. The accuracy of the modelled mean annual ground temperature (MAGT) at the top of the permafrost is ± 2 °C when compared to permafrost borehole data. The modelled permafrost area (MAGT < 0 °C) covers 13.9 × 106 km2 (ca. 15% of the exposed land area), which is within the range or slightly below the average of previous estimates. The sum of all pixels having isolated patches, sporadic, discontinuous or continuous permafrost (permafrost probability > 0) is around 21 × 106 km2 (22% of exposed land area), which is approximately 2 × 106 km2 less than estimated previously. Detailed comparisons at a regional scale show that the model performs well in sparsely vegetated tundra regions and mountains, but is less accurate in densely vegetated boreal spruce and larch forests.
Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as “Arctic browning.” These events can cause landscape‐scale vegetation damage and so are likely to have major impacts on ecosystem CO2 balance. However, there is little understanding of the impacts on CO2 fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO2 fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO2 fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (Reco) and soil respiration (Rsoil) was carried out in widespread sub‐Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site‐level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO2 fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO2 balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO2 fluxes. The scale of reductions in ecosystem CO2 uptake highlights the need for a process‐based understanding of Arctic browning in order to predict how vegetation and CO2 balance will respond to continuing climate change. Extreme climatic events are among the drivers of vegetation damage and decline observed across Arctic ecosystems in recent years; termed “Arctic browning.” Although these events can cause landscape‐scale vegetation damage, their impacts on ecosystem CO2 balance are little understood. Here, it is demonstrated that these events can have major impacts on CO2 balance, considerably reducing the carbon sink capacity of the ecosystem across the growing season. These impacts can be similar when associated with shoot mortality, or with shoot stress responses.
Arctic shrub expansion is occurring across large parts of the tundra biome and its potential ecological repercussions have been widely discussed. But while the term “shrub expansion” often implicitly refers to an increase in tall, deciduous species such as birch and willow, several studies have also found a strong increase in evergreen dwarf shrubs in response to warming, a fact which has received far less attention.
The effects of an evergreen dwarf shrub expansion are markedly different from the effects of an increase in taller, deciduous species. While deciduous shrubs may increase carbon (C) cycling through changes in albedo, litter input, and snow depth, the low stature of evergreen dwarf shrubs means that they are unlikely to influence snow cover. They also produce more recalcitrant litter, which reduces microbial activity. Furthermore, recent research suggests that ericoid mycorrhiza associated with evergreen shrubs may help to decelerate litter and soil organic matter turnover rates through the production of melanized hyphae that resist decomposition. Through selective browsing, herbivores may promote evergreen shrubs and facilitate C storage.
Synthesis. In this mini review, we argue that basing predictions of how shrub expansion will affect tundra ecosystems on characteristics only applicable to tall deciduous shrubs hampers our understanding of the complex feedbacks related to Arctic vegetation shifts.
The carbon sink potential of peatlands depends on the balance between carbon uptake by plants and microbial decomposition. The rates of both these processes will increase with warming but it remains unclear which will dominate the global peatland response. Here we examine the global relationship between peatland carbon accumulation rates during the last millennium and planetary-scale climate space. A positive relationship is found between carbon accumulation and cumulative photosynthetically active radiation during the growing season for mid- to high-latitude peatlands in both hemispheres. However, this relationship reverses at lower latitudes, suggesting that carbon accumulation is lower under the warmest climate regimes. Projections under RCP2.6 and RCP8.5 scenarios indicate that the present-day global sink will increase slightly until ~2100 AD but decline thereafter. Peatlands will remain a carbon sink in the future, but their response to warming switches from a negative to a positive climate feedback (decreased carbon sink with warming) at the end of the 21st century.
Peatlands represent large terrestrial carbon banks. Given that most peat accumulates in boreal regions, where low temperatures and water saturation preserve organic matter, the existence of peat in (sub)tropical regions remains enigmatic. Here we examined peat and plant chemistry across a latitudinal transect from the Arctic to the tropics. Near-surface low-latitude peat has lower carbohydrate and greater aromatic content than near-surface high-latitude peat, creating a reduced oxidation state and resulting recalcitrance. This recalcitrance allows peat to persist in the (sub)tropics despite warm temperatures. Because we observed similar declines in carbohydrate content with depth in high-latitude peat, our data explain recent field-scale deep peat warming experiments in which catotelm (deeper) peat remained stable despite temperature increases up to 9 °C. We suggest that high-latitude deep peat reservoirs may be stabilized in the face of climate change by their ultimately lower carbohydrate and higher aromatic composition, similar to tropical peats.
Extreme winter events that damage vegetation are considered an important climatic cause of arctic browning—a reversal of the greening trend of the region—and possibly reduce the carbon uptake of northern ecosystems. Confirmation of a reduction in CO2 uptake due to winter damage, however, remains elusive due to a lack of flux measurements from affected ecosystems. In this study, we report eddy covariance fluxes of CO2 from a peatland in northern Norway and show that vegetation CO2 uptake was delayed and reduced in the summer of 2014 following an extreme winter event earlier that year. Strong frost in the absence of a protective snow cover—its combined intensity unprecedented in the local climate record—caused severe dieback of the dwarf shrub species Calluna vulgaris and Empetrum nigrum. Similar vegetation damage was reported at the time along ~1000 km of coastal Norway, showing the widespread impact of this event. Our results indicate that gross primary production (GPP) exhibited a delayed response to temperature following snowmelt. From snowmelt up to the peak of summer, this reduced carbon uptake by 14 (0–24) g C m⁻² (~12% of GPP in that period)—similar to the effect of interannual variations in summer weather. Concurrently, remotely-sensed NDVI dropped to the lowest level in more than a decade. However, bulk photosynthesis was eventually stimulated by the warm and sunny summer, raising total GPP. Species other than the vulnerable shrubs were probably resilient to the extreme winter event. The warm summer also increased ecosystem respiration, which limited net carbon uptake. This study shows that damage from a single extreme winter event can have an ecosystem-wide impact on CO2 uptake, and highlights the importance of including winter-induced shrub damage in terrestrial ecosystem models to accurately predict trends in vegetation productivity and carbon sequestration in the Arctic and sub-Arctic.
Significance
Peatlands are organic-rich wetlands that have acted as globally important carbon sinks since the Last Glacial Maximum. However, the drivers of peat initiation are poorly understood. Using a catalog of radiocarbon dates combined with simulations of past climates, we demonstrate that peat initiation in the deglaciated landscapes of North America, northern Europe, and Patagonia was driven primarily by warming growing seasons rather than by any increase in effective precipitation. In Western Siberia, which was not glaciated, climatic wetting was required to convert existing ecosystems into peatlands. Our findings explain the genesis of one of the world’s most important ecosystem types and its potentially fragile, distributed carbon store, with implications for understanding potential changes in peatland distribution in response to future warming.