Vegetation and carbon dynamics of high-latitude peatlands in a changing climate : from early Holocene to recent past

The high-latitudes are warming at more than twice the rate of the global average. Warming and the consequent changes in hydrology affect peatland functioning, especially through changes in vegetation and carbon dynamics. The majority of the world’s peatlands are found in the Northern Hemisphere, forming a globally significant carbon storage and they are in constant interaction with the atmosphere through carbon uptake and release. The global importance of peatlands is widely recognised; however, the role played by high-latitude peatlands in changing climates is still unclear. It is not thoroughly understood how warmer future climates and hydrological changes will affect peatland vegetation and carbon processes. These uncertainties result from the complexity of peatlands and from the manifold future trajectories that are affected by different forcing factors from climate to local conditions. In this dissertation, I aim to increase our knowledge of high-latitude peatland vegetation and carbon dynamics under changing climatic conditions. My approach is palaeoecological, because I use peat records as an archive to reconstruct the response of high-latitude peatlands to known changes in climate. Peatlands function as important archives, since under anoxic and acidic conditions, peat-forming plant remains are well preserved. By identifying these plant remains, we can reconstruct past vegetation compositions. The various peat-forming plants have their own ecological niche, since they prefer and require specific hydrological or nutritional conditions and thus are good indicators for past hydrological changes and conditions. For this dissertation, I collected, in total, 47 peat records from eastern Canada, northern Sweden and Finland, the Kola Peninsula and the northeast of European Russia. I investigated how peatland habitats, carbon accumulation and cycling of our study sites have changed in response to changes in climate. For this, I used plant macrofossils, peat geochemical measurements and dating methods. In addition, I statistically inspected the change in vegetation compositions over time and used a model of carbon accumulation that considered peat decay aspects to determine whether carbon accumulation has been higher or lower than what we would otherwise predict, based on carbon accumulation models. My results show that during recent centuries, the vegetation compositions of the microhabitats examined have mostly changed from typical wet sedge fen vegetation to Sphagnum moss-dominated intermediate surfaces and dry moss- and dwarf shrub-dominated surfaces. During recent decades, these vegetation compositions have remained rather stable, with no major changes in vegetation. However, the spatiotemporal variation within and between the study sites was prominent, and thus to detect any large-scale signals from the data it was essential to use multiple samples and sampling points. Based on my data, it was plausible to consider that if high-latitude peatland vegetation changes from sedge fen vegetation to more hummocky vegetation types, high-latitude peatland carbon accumulation and sink capacity may remain significant or even increase. To better predict the role of peatlands under changing climates, it is crucial that peatland vegetation responses, carbon dynamics and their linkages with climate are more thoroughly understood. My data also support the prevailing understanding that peatlands are important carbon sinks and storages and thus preserving ecosystems that form a nature-based solution to the problem of mitigating the effects of climate warming is highly important.Pohjoinen pallonpuolisko lämpenee yli kaksi kertaa nopeammin kuin maapallo keskimäärin. Lämpeneminen ja sen aiheuttamat muutokset hydrologisessa kierrossa vaikuttavat soiden toimintaan, etenkin kasvillisuuteen ja hiilen kiertoon. Suurin osa maapallon soista sijaitsee pohjoisilla alueilla ja ne ovat maailmanlaajuisesti merkittävin hiilen varasto ja jatkuvasti vuorovaikutuksessa ilmakehän kanssa hiilen sitoutumisen ja vapautumisen kautta. Soiden maailmanlaajuinen merkitys on laajalti tunnustettu, mutta on kuitenkin edelleen epäselvää, mikä niiden rooli on muuttuvassa ilmastossa ja miten lämpeneminen muuttaa kasvillisuutta ja hiilen kiertoa. Syynä tähän on suoekosysteemien monimutkaiset ja toisiinsa liittyvät yhteydet, jotka ohjaavat niiden toimintaa, ilmastosta paikallisiin olosuhteisiin. Tässä työssä pyrin lisäämään tietoa pohjoisten soiden kasvillisuuden ja hiilen kierron vasteesta muuttuviin ilmasto-olosuhteisiin. Lähestymistapani on paleoekologinen eli käytän turvekerrostumia arkistona, jonka pohjalta pyrin rekonstruoimaan ja ymmärtämään soiden vastetta muuttuviin ilmasto-oloihin menemällä ajassa taaksepäin. Suot ovat tunnustetusti hyviä ja tärkeitä arkistoja, sillä hapettomissa, happamissa olosuhteissa esimerkiksi turvetta muodostavien kasvien jäänteet säilyvät hyvin. Tunnistamalla nämä kasvien jäänteet saamme käsityksen suolla aiemmin vallinneesta kasviyhteisöstä. Eri suokasveilla on oma ekolokeronsa eli ne suosivat ja vaativat tietynlaisia kosteus- tai ravinteisuusoloja ja siksi ne toimivat hyvinä indikaattoreina kosteusolojen muutoksista. Olemme keränneet yhteensä 47 turvenäytettä Itä-Kanadasta, Ruotsin ja Suomen pohjoisosista, Kuolan niemimaalta sekä Euroopan puolisen Venäjän koillisosasta tätä väitöskirjaa varten. Väitöskirjassani selvitin miten tutkimusalueiden soiden kasvillisuus, hiilen kertyminen ja kierto ovat muuttuneet ajassa ja linkitin nämä muutokset tunnettuihin muutoksiin ilmastossa. Tätä varten tutkin turpeen kasvimakrofossiileja, geokemiallisia ominaisuuksia sekä ajoitin turvekerroksia. Lisäksi tarkastelin tilastollisesti kasviyhteisöjen muutosta ajassa ja mallinsin hiilen kertymistä turpeen maatumisprosessi huomioiden, jotta saimme käsityksen siitä vaikuttaako hiilen kertyminen olleen suurempaa vai pienempää kuin mallien perusteella voisi olettaa. Aineistoni osoittaa, että viimeisten satojen vuosien aikana lähes poikkeuksetta soiden pienmuotojen kasvillisuus on muuttunut pohjoisten soiden tyypillisestä märästä saravaltaisesta kasvillisuudesta kohti rahkasammalvaltaista välipintojen ja kuivien pintojen sammaleista ja varpuista kasvillisuutta. Viime vuosikymmenien aikana näytekohtiemme kasvillisuudessa ei ole tapahtunut suuria muutoksia. Kasvillisuuden muutos heijastuu myös hiilen kertymiseen, joka osoittaa ensin pienenevää signaalia, mutta suurenee ilmaston lämmetessä pienen jääkauden jälkeen. Näytekohtainen ja soiden sisäinen vaihtelu kasvillisuuden ja hiilen kertymisen osalta on kuitenkin huomattavaa ja siksi tarvitsemme paljon näytteitä laajempien maantieteellisten signaalien havaitsemiseksi ja todentamiseksi. Tuloksieni perusteella on mahdollista, että kasvillisuuden muuttuessa kosteasta saravaltaisesta kohti kuivempaa mätäshabitaattia, pohjoisten soiden hiilen varastokapasiteetti voi jopa kasvaa tai ainakin pysyä merkittävänä. Kasvillisuuden ja hiilen kierron muutoksien parempi ymmärtäminen on tärkeää, jotta voimme luoda parempia ennusteita soiden roolista ja tulevaisuudesta ilmaston muuttuessa. Tutkimukseni myös tukevat vallitsevaa ajatusta siitä, että suot ovat merkittävä hiilen varasto ja sitä kautta, niiden säilyttäminen on merkittävä luontopohjainen keino hidastaa ilmaston lämpenemistä

Response of permafrost peatland hydrology and carbon dynamics to warm and cold climate phases during the last centuries

Peer reviewe

Recent Changes in Peatland Testate Amoeba Functional Traits and Hydrology Within a Replicated Site Network in Northwestern Quebec, Canada

Northern peatlands, which are highly heterogeneous ecosystems, are a globally important carbon (C) store. Understanding the drivers and predicting the future trajectory of the peatland C store requires upscaling from cores and sites to regions and continents, alongside a detailed understanding of the mechanisms governing their C sequestration. Studies incorporating replication are therefore important to quantify how peatland heterogeneity may affect upscaling from local-scale dynamics to models. In addition, we need to better understand the processes driving observed variability, but the interplay between plants, microbes and C cycling in peatlands remains poorly understood. One approach to address both issues is to examine replicated microbiological functional traits within a multi-proxy framework to provide an ecosystem-level perspective on ecological and biogeochemical processes. Peatland testate amoebae are a functionally important group of protists that are well suited to such an approach. Analysing testate amoeba functional traits provides an opportunity to examine processes that may affect key peatland ecosystem services, such as C sequestration. Here, we compared four key testate amoeba functional traits (mixotrophy, biovolume, aperture size and aperture position) to C accumulation, hydrological and vegetation changes in 12 post-Little Ice Age peat records. Samples were collected from high-boreal and low-subarctic regions in northwestern Quebec, Canada in an experimental design that includes internal and external replication at both site and regional scales. Our results showed that correspondence between C accumulation, hydrology and testate amoeba functional traits varied, but recent changes in mixotrophy and aperture size, which may affect peatland C sequestration potential and microbial food web structure, respectively, showed tentative links to recent C accumulation increases. Vegetation, especiallySphagnumabundance was important in promoting mixotrophy and small aperture size in testate amoeba communities. Future impacts of climate change on peatland vegetation will further influence the functional role of testate amoebae on C sequestration through changing mixotrophic testate amoeba abundance.Peer reviewe

Newly initiated carbon stock, organic soil accumulation patterns and main driving factors in the High Arctic Svalbard, Norway

High latitude organic soils form a significant carbon storage and deposition of these soils is largely driven by climate. Svalbard, Norway, has experienced millennial-scale climate variations and in general organic soil processes have benefitted from warm and humid climate phases while cool late Holocene has been unfavourable. In addition to direct effect of cool climate, the advancing glaciers have restricted the vegetation growth, thus soil accumulation. Since the early 1900’s climate has been warming at unprecedented rate, assumingly promoting organic soil establishment. Here we present results of multiple organic soil profiles collected from Svalbard. The profiles have robust chronologies accompanied by soil property analyses, carbon stock estimations and testate amoeba data as a proxy for soil moisture. Our results reveal relatively recent initiation of organic soils across the Isfjorden area. The initiation processes could be linked to glacier retreat, and improvement of growing conditions and soil stabilization. Carbon stock analyses suggested that our sites are hot spots for organic matter accumulation. Testate amoebae data suggested drying of soil surfaces, but the reason remained unresolved. If continued, such a process may lead to carbon release. Our data suggest that detailed palaeoecological data from the Arctic is needed to depict the on-going processes and to estimate future trajectories.Peer reviewe

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 similar to 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.Peer reviewe

Paleoecological assessment of cladoceran community dynamics in two subarctic peatlands

Crustacean community structure and dynamics are very well studied in lakes, rivers and oceanic systems but wetlands, where moisture conditions fluctuate, have not received equal attention in research. For example, cladoceran communities in peatland systems in the subarctic region have not been fully investigated. We used paleolimnological and paleoecological methods to study plant and cladoceran assemblages and the community dynamics in two subarctic peatlands, which differ in their hydrological characteristics. At the first site, Iitto, river floods introduce planktonic species to fen pools and the steep topography of the catchment induces rapid but relatively short flooding periods. Fluctuating environmental conditions result in a high amount of cladoceran resting stages in the samples. At the other site, Kaamanen, the cladoceran assemblage goes through clear directional changes, which could be attributed to changes in fen hydrology and ultimately to climatic changes during the past two millennia.Peer reviewe

Can tropical wetland sediments form an additional carbon feedback element in the future?

Non peer reviewe

Recent peat and carbon accumulation following the Little Ice Age in northwestern Quebec, Canada

Peatland ecosystems are important carbon sinks, but also release carbon back to the atmosphere as carbon dioxide and methane. Peatlands therefore play an essential role in the global carbon cycle. However, the response of high-latitude peatlands to ongoing climate change is still not fully understood. In this study, we used plant macrofossils and peat property analyses as proxies to document changes in vegetation and peat and carbon accumulation after the Little Ice Age. Results from 12 peat monoliths collected in high-boreal and low-subarctic regions in northwestern Quebec, Canada, suggest high carbon accumulation rates for the recent past (post AD 1970s). Successional changes in plant assemblages were asynchronous within the cores in the southernmost region, but more consistent in the northern region. Average apparent recent carbon accumulation rates varied between 50.7 and 149.1 g C m(-2) yr(-1) with the northernmost study region showing higher values. The variation in vegetation records and peat properties found within samples taken from the same sites and amongst cores taken from different regions highlights the need to investigate multiple records from each peatland, but also from different peatlands within one region.Peer reviewe

Spatially varying peatland initiation, Holocene development, carbon accumulation patterns and radiative forcing within a subarctic fen

High latitude peatlands act as globally important carbon (C) sinks and are in constant interaction with the atmosphere. Their C storage formed during the Holocene. In the course of time, the aggregate effect of the C fluxes on radiative forcing (RF) typically changes from warming to cooling, but the timing of this shift varies among different peatlands. Here we investigated Holocene peatland development, including vegetation history, vertical peat growth and the lateral expansion of a patterned subarctic fen in northern Finland by means of multiple sampling points. We modelled the Holocene RF by combining knowledge on past vegetation communities based on plant macrofossil stratigraphies and present in situ C flux measurements. The peatland initiated at ca. 9500 calibrated years Before Present (cal yr BP), and its lateral expansion was greatest between ca. 9000 and 7000 cal yr BP. After the early expansion, vertical peat growth proceeded very differently in different parts of the peatland, regulated by internal and external factors. The pronounced surface microtopography, with high strings and wet (larks, started to form only after ca. 1000 cal yr BP. C accumulation within the peatland recorded a high degree of spatial variability throughout its history, including the recent past. We applied two flux scenarios with different interpretation of the initial peatland development phases to estimate the RF induced by C fluxes of the fen. After ca. 4000 cal yr BP, at the latest, the peatland RF has been negative (cooling), mainly driven by C uptake and biomass production, while methane emissions had a lesser role in the total RF. Interestingly, these scenarios suggest that the greatest cooling effect took place around ca. 1000 cal yr BP, after which the surface microtopography established. The study demonstrated that despite the high spatial heterogeneity and idiosyncratic behaviour of the peatland, the RF of the studied fen followed the general development pattern of more southern peatlands. Holocene climate variations and warm phases did not seem to induce any distinctive and consistent peatland-scale patterns in C accumulation, whereas our data suggests that the changes in vegetation related to autogenic succession were reflected in the C accumulation patterns and RF more clearly. (C) 2020 Elsevier Ltd. All rights reserved.Peer reviewe

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), C-14, Pb-210 and Cs-137 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 (R-2 = .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 (R-2 = .4207; p <.001). Our study implies that if effective precipitation decreases in the future, the carbon uptake capacity of boreal bogs may be threatened.Peer reviewe