The ongoing lateral expansion of peatlands in Finland.

Peatlands are the most dense terrestrial carbon stock and since the last glacial epoch northern peatlands have accumulated between 400 and 1000 Gt of carbon. Although the horizontal development history of the peatlands during the Holocene has been previously researched, these studies have overlooked the current peatland margins. This has led to a long-standing view that the lateral expansion of the peatlands has halted or significantly slowed down. However, no concentrated effort focusing on the recent development of the peatland margins has been conducted. To fulfil this knowledge gap, we studied the development of peatland margins in five Finnish peatlands. In addition, we studied the effect of peatland subsoil characteristics and past forest fires on the peatland expansion. We sampled 15 transects with a total of 47 peat cores utilizing 14 C radiocarbon dating on the basal layers of these peat cores. Our results show that the Northern peatlands are still expanding with four of our study sites having recent, post-1950's basal ages in the peatland margins. In addition, the rate of peatland lateral expansion has increased during the last 1500 years in our study sites, challenging the current knowledge of the recent peatland expansion dynamics. We recorded lateral expansion rates of 0.1-6.4 cm/year from the sites studied. The rate of lateral expansion was restricted by local characteristics, especially the steepness of subsoil (p = .0108). Forest fires likely played an important role as the trigger for lateral expansion in southern study sites with large number of charcoal found at the basal layer of the peat cores. Depending on the scope of this recent lateral expansion across the vast northern peatlands, the effect on the carbon balance could be significant and should be taken into account when estimating the development of carbon pools in these crucial ecosystems.

Consistent centennial-scale change in European sub-Arctic peatland vegetation toward Sphagnum dominance-Implications for carbon sink capacity.

Climate warming is leading to permafrost thaw in northern peatlands, and current predictions suggest that thawing will drive greater surface wetness and an increase in methane emissions. Hydrology largely drives peatland vegetation composition, which is a key element in peatland functioning and thus in carbon dynamics. These processes are expected to change. Peatland carbon accumulation is determined by the balance between plant production and peat decomposition. But both processes are expected to accelerate in northern peatlands due to warming, leading to uncertainty in future peatland carbon budgets. Here, we compile a dataset of vegetation changes and apparent carbon accumulation data reconstructed from 33 peat cores collected from 16 sub-arctic peatlands in Fennoscandia and European Russia. The data cover the past two millennia that has undergone prominent changes in climate and a notable increase in annual temperatures toward present times. We show a pattern where European sub-Arctic peatland microhabitats have undergone a habitat change where currently drier habitats dominated by Sphagnum mosses replaced wetter sedge-dominated vegetation and these new habitats have remained relatively stable over the recent decades. Our results suggest an alternative future pathway where sub-arctic peatlands may at least partly sustain dry vegetation and enhance the carbon sink capacity of northern peatlands.

Identifying main uncertainties in estimating past and present radiative forcing of peatlands.

Reconstructions of past climate impact, that is, radiative forcing (RF), of peatland carbon (C) dynamics show that immediately after peatland initiation the climate warming effect of CH4 emissions exceeds the cooling effect of CO2 uptake, but thereafter the net effect of most peatlands will move toward cooling, when RF switches from positive to negative. Reconstructing peatland C dynamics necessarily involves uncertainties related to basic assumptions on past CO2  flux, CH4 emission and peatland expansion. We investigated the effect of these uncertainties on the RF of three peatlands, using either apparent C accumulation rates, net C balance (NCB) or NCB plus C loss during fires as basis for CO2 uptake estimate; applying a plausible range for CH4 emission; and assuming linearly interpolated expansion between basal dates or comparatively early or late expansion. When we factored that some C would only be stored temporarily (NCB and NCB+fire), the estimated past cooling effect of CO2 uptake increased, but the present-day RF was affected little. Altering the assumptions behind the reconstructed CO2  flux or expansion patterns caused the RF to peak earlier and advanced the switch from positive to negative RF by several thousand years. Compared with NCB, including fires had only small additional effect on RF lasting less than 1000 year. The largest uncertainty in reconstructing peatland RF was associated with CH4 emissions. As shown by the consistently positive RF modelled for one site, and in some cases for the other two, peatlands with high CH4 emissions and low C accumulation rates may have remained climate warming agents since their initiation. Although uncertainties in present-day RF were mainly due to the assumed CH4 emission rates, the uncertainty in lateral expansion still had a significant effect on the present-day RF, highlighting the importance to consider uncertainties in the past peatland C balance in RF reconstructions.

Widespread recent ecosystem state shifts in high-latitude peatlands of northeastern Canada and implications for carbon sequestration.

Northern peatlands are a major component of the global carbon (C) cycle. Widespread climate-driven ecohydrological changes in these ecosystems can have major consequences on their C sequestration function. Here, we synthesize plant macrofossil data from 33 surficial peat cores from different ecoclimatic regions, with high-resolution chronologies. The main objectives were to document recent ecosystem state shifts and explore their impact on C sequestration in high-latitude undisturbed peatlands of northeastern Canada. Our synthesis shows widespread recent ecosystem shifts in peatlands, such as transitions from oligotrophic fens to bogs and Sphagnum expansion, coinciding with climate warming which has also influenced C accumulation during the last ~100 years. The rapid shifts towards drier bog communities and an expansion of Sphagnum sect. Acutifolia after 1980 CE were most pronounced in the northern subarctic sites and are concurrent with summer warming in northeastern Canada. These results provide further evidence of a northward migration of Sphagnum-dominated peatlands in North America in response to climate change. The results also highlight differences in the timing of ecosystem shifts among peatlands and regions, reflecting internal peatland dynamics and varying responses of vegetation communities. Our study suggests that the recent rapid climate-driven shifts from oligotrophic fen to drier bog communities have promoted plant productivity and thus peat C accumulation. We highlight the importance of considering recent ecohydrological trajectories when modelling the potential contribution of peatlands to climate change. Our study suggests that, contrary to expectations, peat C sequestration could be promoted in high-latitude non-permafrost peatlands where wet sedge fens may transition to drier Sphagnum bog communities due to warmer and longer growing seasons.

Widespread global peatland establishment and persistence over the last 130,000 y.

Glacial-interglacial variations in CO2 and methane in polar ice cores have been attributed, in part, to changes in global wetland extent, but the wetland distribution before the Last Glacial Maximum (LGM, 21 ka to 18 ka) remains virtually unknown. We present a study of global peatland extent and carbon (C) stocks through the last glacial cycle (130 ka to present) using a newly compiled database of 1,063 detailed stratigraphic records of peat deposits buried by mineral sediments, as well as a global peatland model. Quantitative agreement between modeling and observations shows extensive peat accumulation before the LGM in northern latitudes (>40°N), particularly during warmer periods including the last interglacial (130 ka to 116 ka, MIS 5e) and the interstadial (57 ka to 29 ka, MIS 3). During cooling periods of glacial advance and permafrost formation, the burial of northern peatlands by glaciers and mineral sediments decreased active peatland extent, thickness, and modeled C stocks by 70 to 90% from warmer times. Tropical peatland extent and C stocks show little temporal variation throughout the study period. While the increased burial of northern peats was correlated with cooling periods, the burial of tropical peat was predominately driven by changes in sea level and regional hydrology. Peat burial by mineral sediments represents a mechanism for long-term terrestrial C storage in the Earth system. These results show that northern peatlands accumulate significant C stocks during warmer times, indicating their potential for C sequestration during the warming Anthropocene.

Decreased carbon accumulation feedback driven by climate-induced drying of two southern boreal bogs over recent centuries.

Northern boreal peatlands are important ecosystems in modulating global biogeochemical cycles, yet their biological communities and related carbon dynamics are highly sensitive to changes in climate. Despite this, the strength and recent direction of these feedbacks are still unclear. The response of boreal peatlands to climate warming has received relatively little attention compared with other northern peatland types, despite forming a large northern hemisphere-wide ecosystem. Here, we studied the response of two ombrotrophic boreal peatlands to climate variability over the last c. 200 years for which local meteorological data are available. We used remains from plants and testate amoebae to study historical changes in peatland biological communities. These data were supplemented by peat property (bulk density, carbon and nitrogen content), 14 C, 210 Pb and 137 Cs analyses and were used to infer changes in peatland hydrology and carbon dynamics. In total, six peat cores, three per study site, were studied that represent different microhabitats: low hummock (LH), high lawn and low lawn. The data show a consistent drying trend over recent centuries, represented mainly as a change from wet habitat Sphagnum spp. to dry habitat S. fuscum. Summer temperature and precipitation appeared to be important drivers shaping peatland community and surface moisture conditions. Data from the driest microhabitat studied, LH, revealed a clear and strong negative linear correlation (R2  = .5031; p < .001) between carbon accumulation rate and peat surface moisture conditions: under dry conditions, less carbon was accumulated. This suggests that at the dry end of the moisture gradient, availability of water regulates carbon accumulation. It can be further linked to the decreased abundance of mixotrophic testate amoebae under drier conditions (R2  = .4207; p < .001). Our study implies that if effective precipitation decreases in the future, the carbon uptake capacity of boreal bogs may be threatened.

The extent and meaning of hybridization and introgression between Siberian spruce (Picea obovata) and Norway spruce (Picea abies): cryptic refugia as stepping stones to the west?

Boreal species were repeatedly exposed to ice ages and went through cycles of contraction and expansion while sister species alternated periods of contact and isolation. The resulting genetic structure is consequently complex, and demographic inferences are intrinsically challenging. The range of Norway spruce (Picea abies) and Siberian spruce (Picea obovata) covers most of northern Eurasia; yet their geographical limits and histories remain poorly understood. To delineate the hybrid zone between the two species and reconstruct their joint demographic history, we analysed variation at nuclear SSR and mitochondrial DNA in 102 and 88 populations, respectively. The dynamics of the hybrid zone was analysed with approximate Bayesian computation (ABC) followed by posterior predictive structure plot reconstruction and the presence of barriers across the range tested with estimated effective migration surfaces. To estimate the divergence time between the two species, nuclear sequences from two well-separated populations of each species were analysed with ABC. Two main barriers divide the range of the two species: one corresponds to the hybrid zone between them, and the other separates the southern and northern domains of Norway spruce. The hybrid zone is centred on the Urals, but the genetic impact of Siberian spruce extends further west. The joint distribution of mitochondrial and nuclear variation indicates an introgression of mitochondrial DNA from Norway spruce into Siberian spruce. Overall, our data reveal a demographic history where the two species interacted frequently and where migrants originating from the Urals and the West Siberian Plain recolonized northern Russia and Scandinavia using scattered refugial populations of Norway spruce as stepping stones towards the west.

Norway spruce postglacial recolonization of Fennoscandia.

Contrasting theories exist regarding how Norway spruce (Picea abies) recolonized Fennoscandia after the last glaciation and both early Holocene establishments from western microrefugia and late Holocene colonization from the east have been postulated. Here, we show that Norway spruce was present in southern Fennoscandia as early as 14.7 ± 0.1 cal. kyr BP and that the millennia-old clonal spruce trees present today in central Sweden likely arrived with an early Holocene migration from the east. Our findings are based on ancient sedimentary DNA from multiple European sites (N = 15) combined with nuclear and mitochondrial DNA analysis of ancient clonal (N = 135) and contemporary spruce forest trees (N = 129) from central Sweden. Our other findings imply that Norway spruce was present shortly after deglaciation at the margins of the Scandinavian Ice Sheet, and support previously disputed finds of pollen in southern Sweden claiming spruce establishment during the Lateglacial.

Recent climate change has driven divergent hydrological shifts in high-latitude peatlands.

High-latitude peatlands are changing rapidly in response to climate change, including permafrost thaw. Here, we reconstruct hydrological conditions since the seventeenth century using testate amoeba data from 103 high-latitude peat archives. We show that 54% of the peatlands have been drying and 32% have been wetting over this period, illustrating the complex ecohydrological dynamics of high latitude peatlands and their highly uncertain responses to a warming climate.

Abrupt high-latitude climate events and decoupled seasonal trends during the Eemian.

The Eemian (the Last Interglacial; ca. 129-116 thousand years ago) presents a testbed for assessing environmental responses and climate feedbacks under warmer-than-present boundary conditions. However, climate syntheses for the Eemian remain hampered by lack of data from the high-latitude land areas, masking the climate response and feedbacks in the Arctic. Here we present a high-resolution (sub-centennial) record of Eemian palaeoclimate from northern Finland, with multi-model reconstructions for July and January air temperature. In contrast with the mid-latitudes of Europe, our data show decoupled seasonal trends with falling July and rising January temperatures over the Eemian, due to orbital and oceanic forcings. This leads to an oceanic Late-Eemian climate, consistent with an earlier hypothesis of glacial inception in Europe. The interglacial is further intersected by two strong cooling and drying events. These abrupt events parallel shifts in marine proxy data, linked to disturbances in the North Atlantic oceanic circulation regime.

Warm summers during the Younger Dryas cold reversal.

The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. The sudden cooling is typically linked to an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in response to meltwater discharges from ice sheets. However, inconsistencies regarding the YD-response of European summer temperatures have cast doubt whether the concept provides a sufficient explanation. Here we present results from a high-resolution global climate simulation together with a new July temperature compilation based on plant indicator species and show that European summers remain warm during the YD. Our climate simulation provides robust physical evidence that atmospheric blocking of cold westerly winds over Fennoscandia is a key mechanism counteracting the cooling impact of an AMOC-slowdown during summer. Despite the persistence of short warm summers, the YD is dominated by a shift to a continental climate with extreme winter to spring cooling and short growing seasons.

First physical evidence for forested environment in the Arctic during MIS 3.

Old sedimentological and geochronological records can be preserved underneath the central parts of the continental ice sheets under non-erosive, cold-based subglacial conditions. Organic deposits that predate the last deglaciation are of particular value for the information held on glacial-time climate and environmental conditions. In this study, we present multiproxy data derived from a well-preserved MIS 3 interstadial (55-25 ka ago) organic layer from inside the Arctic Circle in the Finnish Lapland. Biological proxy evidence, namely coming from aquatic plant species, indicates July temperatures as high as 14.4 °C, i.e. higher than those of today for the study site. Macrofossil evidence demonstrates for the first time the presence of pines accompanied by tree birch during the MIS 3 interstadial in northern Fennoscandia. These results concur with contemporary insolation model outcomes but contradict with the previous proxy-based view of open tundra conditions during the MIS 3. The data suggest that there are highly dynamic interstadial continental ice-sheet dynamics following changes in orbital forcing. Warm climate enabled the establishment of forests on exposed landscape. Moreover, we suggest that in the light of these new data, previous MIS 3 pollen data could be re-interpreted.

Proxy comparison in ancient peat sediments: pollen, macrofossil and plant DNA.

We compared DNA, pollen and macrofossil data obtained from Weichselian interstadial (age more than 40 kyr) and Holocene (maximum age 8400 cal yr BP) peat sediments from northern Europe and used them to reconstruct contemporary floristic compositions at two sites. The majority of the samples provided plant DNA sequences of good quality with success amplification rates depending on age. DNA and sequencing analysis provided five plant taxa from the older site and nine taxa from the younger site, corresponding to 7% and 15% of the total number of taxa identified by the three proxies together. At both sites, pollen analysis detected the largest (54) and DNA the lowest (10) number of taxa, but five of the DNA taxa were not detected by pollen and macrofossils. The finding of a larger overlap between DNA and pollen than between DNA and macrofossils proxies seems to go against our previous suggestion based on lacustrine sediments that DNA originates principally from plant tissues and less from pollen. At both sites, we also detected Quercus spp. DNA, but few pollen grains were found in the record, and these are normally interpreted as long-distance dispersal. We confirm that in palaeoecological investigations, sedimentary DNA analysis is less comprehensive than classical morphological analysis, but is a complementary and important tool to obtain a more complete picture of past flora.