DEVELOPMENT OF PALSA MIRES ON THE NORTHERN EUROPEAN CONTINENT IN RELATION TO HOLOCENE CLIMATIC AND ENVIRONMENTAL CHANGES

PIRITA O.OKSANEN

Faculty of Science,Department of Biology,

University of Oulu

OULU 2005

PIRITA O. OKSANEN

DEVELOPMENT OF PALSA MIRES ON THE NORTHERN EUROPEAN CONTINENT IN RELATION TO HOLOCENE CLIMATIC AND ENVIRONMENTAL CHANGES

Academic Dissertation to be presented with the assent ofthe Faculty of Science, University of Oulu, for publicdiscussion in Keckmaninsali (Auditorium HU106),Linnanmaa, on November 19th, 2005, at 12 noon

OULUN YLIOPISTO, OULU 2005

Copyright © 2005University of Oulu, 2005

Supervised byProfessor Peter KuhryProfessor Jari Oksanen

Reviewed byProfessor Rauno RuuhijärviProfessor Karl-Dag Vorren

ISBN 951-42-7888-7 (nid.)ISBN 951-42-7889-5 (PDF) http://herkules.oulu.fi/isbn9514278895/

ISSN 0355-3191 http://herkules.oulu.fi/issn03553191/

OULU UNIVERSITY PRESSOULU 2005

Oksanen, Pirita O., Development of palsa mires on the northern European continentin relation to Holocene climatic and environmental changes Faculty of Science, Department of Biology, University of Oulu, P.O.Box 3000, FIN-90014University of Oulu, Finland 2005Oulu, Finland

AbstractThis thesis deals with the Holocene development of palsa mires in continental Europe, especiallypermafrost dynamics and its consequences on vegetation succession and peat accumulation. Peatdeposits of four permafrost mires in boreal and subarctic northeastern European Russia and innorthern oroboreal Finland have been studied using plant macrofossil analysis, (AMS) radiocarbondating, dry bulk density and carbon content measurements. In addition, preliminary results areavailable from another palsa mire in northeastern European Russia. Modern vegetation has beeninvestigated to support the interpretation of fossil plant assemblages. Earlier literature on vegetation,stratigraphy and dating of permafrost mires in Europe has been reviewed.

The vegetation of palsa mires in general is well known. As a rule, palsas are dry ombrotrophichabitats, surrounded by wet flarks of variable trophic levels. There is a lack of information aboutvegetation in different small-scale habitats within palsa mires, which would have been useful whenstudying the permafrost-vegetation relationship. Although no functional indicator species ofpermafrost have been found, permafrost dynamics in peat stratigraphy can often be detected with highdegree of probability based on changes in vegetation. Some plant assemblages and vegetationsuccessions are typical on permafrost, while many species rarely grow on or near to permafrost.Relatively sudden changes between dry and wet mire environments and continuously dynamicconditions are good signs of permafrost impact. Also gradual changes towards drier conditions maybe caused by permafrost; in these cases the timing of first permafrost aggradation is more difficult toascertain and can usually be pronounced only in terms of maximum and minimum ages. Changes inpeat accumulation rates and even hiatuses in stratigraphy are additional tools to support theinterpretation on permafrost history at the studied sites.

Dry organic matter and carbon accumulation rates for different developmental stages arecalculated for the five studied mires. From earlier studies this information is not available.Accumulation rates in the permafrost environment are very variable: from zero or negative rates inold palsas to as high as 100 gC/m2yr in incipient palsas. On moist plateau palsas, permafrost flarksand in unstable permafrost conditions, accumulation continues at low to moderate rates. Thermokarstprocesses result in decomposition of former peat deposits with important consequences for theecosystem carbon balance, especially in plateau palsa mires.

Radiocarbon datings are available from 27 permafrost mires in continental Europe; only 5 of theseare situated in Russia. Many of the published dates cannot be considered reliable as dating permafrostaggradation. Based on limited material, permafrost started to develop at latest about 3000 BP in miresof northern Russia and 2500 BP in Fennoscandia. Older permafrost formation is suggested for a fewsites, but the evidence is insufficient to confirm this interpretation. The oldest preserved palsas are ca.2500–2000 14C years old. Most of the modern palsas are less than 600 14C years old. Permafrostaggradation follows the major climate development in the Holocene, with formation being mostactive during the coldest stages.

Global warming is expected to greatly affect the Arctic in the near future, which would implysignificant changes in ecosystem functioning and carbon balance of permafrost mires. This studycontributes to the understanding of the possible impacts of climate change on these ecosystems usingpaleoecological techniques.

Keywords: peat accumulation, permafrost, plant macrofossil, radiocarbon dating,stratigraphy, vegetation

Acknowledgements

I wish to express my gratitude to my supervisor Prof. Peter Kuhry (Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden; formerly at the Arctic Centre, University of Lapland, Rovaniemi, Finland) for guidance during fieldwork, paleoecological analyses and writing. My second supervisor, Prof. Jari Oksanen (Department of Biology, University of Oulu, Finland) is especially acknowledged for his help with statistical methods. Thanks are also extended to Prof. Emer. Rauno Ruuhijärvi (Department of Biology, University of Helsinki, Finland) for his constructive comments on my licentiate thesis, which was largely an earlier version of this thesis. The field trips in Russia were organised by the Institute of Biology, Komi Science Centre, Syktyvkar. The Kevo Research Station and its staff provided services during the fieldwork in Finnish Lapland. For assistance and company during the field trips, I am grateful to Rimma Alekseeva, Viktor Alekseev, Seppo Töllikkö, Seija Kultti, Ankku Lohila, Timo Turrek, Nanka Karstkarel, Tarmo Virtanen, Tauno Luosujärvi and others. Terttu Lempiäinen (University of Turku), Risto Virtanen (Botanical Museum, University of Oulu) and Minna Väliranta (Department of geology, University of Helsinki) helped with the identification of plants and subfossil plant remains. The radiocarbon dating was accomplished in the R.J. van de Graaff Laboratory, Utrecht University and in the Dating Laboratory of the University of Helsinki. The work for this thesis was mainly carried out at the Arctic Centre, Rovaniemi. Many thanks to its staff for providing a sociable and inspiring working atmosphere. I am also indebted to my present employer, the Department of Botany of Trinity College (Dublin, Ireland), for allowing me to work on the completion of this thesis. The Forest Research Institute (Rovaniemi) kindly offered the laboratories for peat sample preparation. During different stages, this work has been financially supported by the Arctic Centre of the University of Lapland; the FIGARE Programme of the Academy of Finland (ARCTICA project); the Arctic Study School ARKTIS, University of Lapland (Rovaniemi); the EnviroNet Graduate School in Environmental Issues at the University of Oulu; the Cultural Fond of Lapland; the Environment and Climate Programme of the European Commission (TUNDRA project; contract nr. ENV4-CT97-0522; Climatology and Natural Hazards); the Konkordia Union of Finland; Faculty of Science, University of Oulu; and, the Maj and Tor Nessling Foundation, Helsinki.

Pirita Oksanen Dublin, July 2005

List of original articles

This thesis is based on the following original papers:

I Oksanen PO, Kuhry P and Alekseeva RN (2001) Holocene development of the Rogovaya River peat plateau, East-European Russian Arctic. The Holocene 11(1): 25–40.

II Oksanen PO, Kuhry P and Alekseeva RN (2003) Holocene development and permafrost history of the Usinsk mire, northeast European Russia. Géographie Physique et Quaternaire 57(2–3): 169-187.

III Kultti S, Oksanen PO and Väliranta M (2004) Holocene treeline, permafrost and climate dynamics in the Nenets Region, East-European Russian Arctic. Canadian Journal of Earth Sciences 41: 1–18.

IV Oksanen PO (2006) Holocene development of the Vaisjeäggi palsa mire, Finnish Lapland. Boreas 1, in press.

V Oksanen PO (2005) Vegetation, stratigraphy and Holocene permafrost dynamics in palsa mires of northern continental Europe: a review of published data. Manuscript.

VI Oksanen PO and Kuhry P (2003). Permafrost induced changes in the hydrology and ecology of mires. In: Järvet A and Lode E (eds) Ecohydrological processes in northern wetlands. Selected papers of an international conference and educational workshop, Tallinn, Estonia 2003, 92–98.

Contents

Abstracts Acknowledgements List of original articles Contents 1 Introduction ................................................................................................................... 13 2 Aims .............................................................................................................................. 18 3 Study areas..................................................................................................................... 19 4 Materials and methods................................................................................................... 21

4.1 Modern vegetation and permafrost conditions .......................................................21 4.2 Peat stratigraphy, dating and accumulation ............................................................21

5 Results and discussion................................................................................................... 23 5.1 Palsa mire vegetation..............................................................................................23 5.2 Peat stratigraphy .....................................................................................................33 5.3 Peat and carbon accumulation ................................................................................35 5.4 Identifying and dating permafrost induced changes in mires

using plant macrofossil analysis ............................................................................37 5.5 Holocene permafrost history of European palsa mires ...........................................39

6 Conclusions ................................................................................................................... 43 References Original articles

1 Introduction

Mires play an important role in carbon, water and biogeochemical cycles and as wildlife habitats. The total area occupied by peatlands has been estimated at approximately 4 million km2, or ca. 3% of the total land surface on Earth. Of the total peatland area, 38% is found in the former USSR, 28% in Canada and 6% in Scandinavia (Maltby & Proctor 1996). Mires also provide a valuable historical archive of local (and regional) environmental changes. They deposit peat, which mainly consists of partly decomposed plant remains. Using plant macrofossil analysis, past vegetation at a mire site can be reconstructed and inferences made regarding hydrological and ecosystem changes.

Permafrost in continental Europe is found in areas where the annual mean temperature is below zero and more extensively in areas with annual mean temperatures below ca. −2 °C (Fig. 1). When present in mires, it is a factor of major importance affecting microtopography, hydrology, vegetation and carbon balance. In the northern taiga of Europe, permafrost has a local appearance and is restricted mainly to mires, where it forms high palsas, 1–7 m high frozen mounds emerging from the otherwise unfrozen mire. Peat is a good thermal insulator, which is demonstrated also south of the permafrost area, where seasonal frost is often found in mires until late summer. Precipitation and wind conditions contribute to the distribution of palsas too. In areas of thin snow cover, due to limited precipitation or high winds, frost penetrates deeper into the ground during the winter, and it thaws slower during the summer if the surface peat remains dry. In the forest-tundra and southern tundra, in the zone of discontinuous permafrost, permafrost is found largely in mires and also partly in mineral soils. Contiguous, flat palsas are called peat plateaus or plateau palsas. In this thesis, accordingly to the Nordic tradition, the more descriptive term plateau palsa is used instead of peat plateau in order to avoid confusion with non-permafrost plateau bogs, although peat plateau is used in the included papers published in international journals. Plateau palsas become more usual and larger towards the north. In the region of continuous permafrost the entire ground, except taliks under water bodies, is permanently frozen. There, mires are mainly thin-peated polygonal mires, rarely found in the European North. Pounus are usually roundish, small (< 1 m high), thin-peated perennially frozen hummocks, found in palsa mire margins.

14

Fig. 1. Study sites, approximate mean annual temperature isotherms and distribution of permafrost. Climate data compiled from Alalammi (1987), Nauchno-prikladnoi spravochnik po klimatu SSSR (1989), Climatological statistics of Finland 1961–1990 (1991), Alexandersson and Eggertsson Karlström (2001), Aune (1993), Swedish Meteorological and Hydrological Institute (http://www.smhi.se), Norwegian Meteorological Institute (http://met.no) and Russian HydroMetCenter (http://meteo.infospace.ru).

Palsas as a conspicuous phenomenon have been the object of keen scientific interest for over a hundred years (e.g., Kihlman 1890, Fries & Bergström 1910, Auer 1924, Sumgin 1934). In spite of the vast literature dedicated to palsas, little is known of their age and the effect of permafrost on peat accumulation. More information is available on the effect of permafrost on mire ecology. The most extensive palsa mire vegetation study has been conducted by Vorren (1979a). Reviews often state as a fact that palsas started to form after 4000–3000 BP, when the climate became colder (e.g., Seppälä 1988, Korhola & Tolonen 1996). This might be true, but the assumption is based on paleoclimate data derived from pollen analyses (Auer 1927, Lundqvist 1951, P’yavchenko 1955, Ruuhijärvi 1962), and not on actual palsa research. Also, references are made (e.g., Moore 1984) to Salmi´s old palsa formation dates of 8000–7000 BP. Salmi (1968, 1972) did use radiocarbon dating, but either ignored it in his interpretation or did not consider peat stratigraphy, so his palsa datings are also based exclusively on pollen inferred changes in climate. Ruuhijärvi (in Oeschger & Riesen 1965) published the first 14C datings from European palsas, and Sonesson (1968, 1970a, 1970b) presents useful information on plant macrofossil composition and age of palsa peat layers, although the emphasis of these studies was on dating regional forest development. Vorren (1972, 1979b) and Vorren and Vorren (1976) were the first to use radiocarbon dating and plant macrofossil analysis specifically to date the latest palsa stage, but they do not discuss the peat profiles

500 km

Arkhangelsk

Russia

White Sea

Murmansk

Rovaniemi

Kiruna

Troms

Finlan

dSwed

en

Pechora

Urals

Barents Sea

permafrost region or its southern limit

mountains >500 masl

Arctic circle

II

IIV III

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0

0

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-3

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Sites:I Rogovaya River (Paper I)II Usinsk (Paper II)III Khosedayu (Paper III)IV Vaisjeäggi (Paper IV)X Shar’yu (Oksanen 2002)

O

Vorkuta

Norwa

y

60 -2

0

15

below that. Later datings from Scandinavia lack detailed plant composition of the peat profiles (Göttlich et al. 1983, Zuidhoff & Kolstrup 2000, Seppälä 2003). Beside the publications in this thesis, one other macrofossil study with datings is available from northeastern European Russia (Väliranta et al. 2003). There are no publications on dry matter or carbon accumulation during different phases of palsa mire development in Europe prior to the papers included in this study.

The global store of carbon in peatlands is estimated at 329–528 Pg, i.e., 20% or more of the total soil carbon pool (Immirzi et al. 1992). Northern peatlands accumulate carbon at a net rate of 23–29 gC/m2yr (Gorham 1991, Turunen & Tolonen 1996). On the other hand, global peatlands release carbon in the form of methane at a rate of 110 Tg/yr, i.e., about 6–10% of the methane emissions of the world; 34% of this is released from northern peatlands (Martikainen 1996). An estimate for the carbon pool of West-Siberian and European peatlands in Russia is 198 Pg, with a net carbon accumulation rate of 46 Tg/yr (Botch et al. 1995), and a carbon release through methane emissions of 14 Tg/yr (Rozanov 1995). The carbon pool of peatlands in Finland is calculated at 3.5 Pg, and the mean carbon accumulation rate for Finland and Sweden is 18 gC/m2yr (Lappalainen 1996, Fredriksson 1996). Permafrost mires are estimated to represent about 22% of the total carbon pool in mires of the former Soviet Union (Botch et al. 1995). There is a large storage of carbon in permanently frozen peat. Permafrost mires produce methane too, but amounts are significantly smaller than in wetlands of warmer areas.

Increases of 1–3.5 °C are predicted in global mean temperatures during the next century (Houghton et al. 1996). In Scandinavia, an average rise of 2.5 °C (± 1.5 °C) is expected by 2050 with a ca. 10% increase in rainfall. For northern Russia, an even higher temperature rise is predicted (Mattson & Rummukainen 1998). It should be noted that predicted temperature increases are even greater in more recent GCM runs using the newest IPCC (SRES) greenhouse gas concentrations and aerosol levels (Cubash et al. 2001, Johns et al. 2001). Climate warming by only a few degrees would thaw permafrost in large areas. This would have significant consequences on the ecosystem functioning, especially at the southern limit of permafrost distribution, where permafrost is already near its thawing point (Williams & Michael 1989).

The influence of climatic warming upon mires is a complex issue, and therefore it is difficult to predict its effects on mire ecology and carbon balance. For assessing the importance of anticipated future global warming, it is valuable to know how palsa mires have reacted to climatic changes in the past. Rising temperatures favour both the production of biomass and decomposition (Moore et al. 1998). Temperature changes affect the mires also indirectly through water level changes. The effect of climate on carbon cycles in peatlands and their feedback effect on climate are recognised as key areas of current research (Maltby & Proctor 1996). Knowledge about the rates of carbon accumulation and their changes through time is an essential step in this research. Wet tundra ecosystems in Alaska are reported to have changed from a net carbon sink to a net source during the last decades, because of increased soil respiration under warmer and drier conditions (Oechel et al. 1995). Similar results were obtained in the East-European tundra (Heikkinen et al. 2004). On the other hand, thawing of permafrost in the mineral ground may cause paludification and new peat accumulation; this could increase the accumulation of carbon by 0.03 Pg/yr in northern mires. Collapsing palsas due to a warmer climate and increased rainfall would add to the amount of wet surface, which

16

would lead to increased methane releases. It has been estimated that the thawing of permafrost could increase the CH4 release from northern mires by 10 Tg/yr (Martikainen 1996).

This thesis consists of six papers; four of them are based on original field studies by our research groups (Arctic Centre of the University of Lapland, Komi Science Centre of the Russian Academy of Science and Department of Geology of the University of Helsinki), one is a review of published data and one is a short conference proceeding. One paper (I) was included also in the licentiate thesis by Oksanen (2002), and the review (V) is based on two manuscripts of the licentiate thesis. Three papers discuss the development of palsa mires in subarctic and northern boreal European Russia (I,-III) and one the development of a palsa mire in Finnish Lapland (IV), on the base of plant macrofossil analysis, radiocarbon dating and carbon content measurements. The conference proceeding (VI) focuses on hydrology and carbon balance of permafrost mires. The review (V) deals with the vegetation and stratigraphy of European palsa mires, and dating of permafrost aggradation and subsequent permafrost dynamics.

Fig. 2. Plateau palsas in the Rogovaya River, Russian forest-tundra. Photo P. Kuhry.

17 Fi

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2 Aims

The aim of this thesis is to study Holocene permafrost dynamics in palsa mires in the European continent. Specific questions include:

1. The relationship in mires between vegetation and permafrost, possible indicator species of permafrost, typical plant communities and successions on permafrost, and species absent on and near permafrost landforms

2. The possibility and difficulty of recognising permafrost induced changes in peat stratigraphies using plant macrofossil analysis

3. The changes in mire hydrology, vegetation and peat/carbon accumulation due to permafrost aggradation and degradation

4. The timing of permafrost aggradation and degradation in European palsa mires 5. The general climate development in the region during the Holocene epoch and the

sensitivity of permafrost to small changes in climate

Fig. 4. Plateau palsa in the Usinsk mire, Russian taiga, with the field team. Note small pines growing on the edges of the plateau. Photo P. Oksanen.

3 Study areas

For the purpose of this study the European palsa area is divided into four regions: Northern Pechora (1), Mezen’ Coast - Kanin Peninsula (2), Kola Peninsula (3) and Scandinavia (4). The Northern Pechora region covers lowland areas of taiga, forest-tundra and tundra, extending from the Urals in the east to the Timan Ridge in the west. Permafrost is widespread. The second region includes lowland areas west of the Timan Ridge to the southern coast of the Mezen’ Bay. It is mainly northern taiga, with tundra found in the northern Kanin Peninsula and on the White Sea coast. The position of permafrost in the area is unclear, but at least in some parts of the Kanin Peninsula palsa mires are common. The palsa mires of the Kola Peninsula and Scandinavia are mainly located in upland areas. Scandinavia is here understood to include Sweden, Norway and Finland. Most palsa mires are situated within the oroboreal birch belt. Scandinavia is the best studied of these regions.

Fig. 5. “Classical” mature palsa mound in Shar’yu, Russian taiga. A low palsa and embryonal palsa formation in a flark are located in the foreground. Photo P. Oksanen.

20

The field studies took place in regions 1 and 4 (papers I–IV; Fig. 1), during seven summers 1995–96 and 1998–2002. Two of the field sites, the Rogovaya River (paper I, see also Fig. 2) and Khosedayu (paper III, Fig. 3), are situated at the arctic treeline, in the zone of discontinuous permafrost in subarctic East-European Russia. Rogovaya is a plateau palsa mire with plateau palsas divided by narrow gullies and larger thermokarst ponds. In Khosedayu, a small valley mire is mainly occupied by palsas that could be called plateau palsas due to their flat surface but are much smaller than in Rogovaya. The Usinsk (paper II, Fig. 4) and Shar’yu (Oksanen 2002, Fig. 5) mires are located in the northern taiga of East-European Russia, at the southern limit of sporadic permafrost. Vaisjeäggi (paper IV, Fig. 6) is situated in the Scandinavian oroboreal birch belt, in the sporadic permafrost zone. Usinsk and Shar’yu are aapa mires with small plateau palsa (Usinsk) or high palsa (both) areas restricted to the shores of some lakes. In Vaisjeäggi, high palsas are very common over most parts of the mire. Additional sites are discussed based on literature (paper V).

Fig. 6. String palsa in Vaisjeäggi, Finnish oroboreal birch belt. Photo P. Oksanen.

4 Materials and methods

4.1 Modern vegetation and permafrost conditions

In the field, vegetation descriptions were made on 1 m2 relevés, randomly selected within different vegetation types on the investigated mires (papers I–IV). Species were recorded as cover percentages for separate ground, field and shrub layers. The relationship of the studied community to permafrost was registered. pH, with a test paper, and permafrost and water tables were measured. The purpose of the vegetation relevés was to study modern analogues for plant macrofossil assemblages in peat deposits. Taxa, which are difficult or impossible to recognise as macrofossils, e.g., lichen or liverwort species, were usually not considered. A literature review on the present knowledge of vegetation of European palsa mires was compiled (paper V). A total of 40 publications were used for the review; only a few of them include all plant and lichen species and detailed site descriptions in relation to permafrost. Cluster analysis was used to organise the large data set and to distinguish between different vegetation units.

4.2 Peat stratigraphy, dating and accumulation

Peat samples were taken from already naturally exposed edges of palsas that were further excavated and cleaned, and, in unfrozen peat, by coring with a Russian peat sampler (papers I–IV). Peat stratigraphies were described in the field and profiles were sampled at 1–15 cm intervals. Samples were packed in plastic bags for transportation. In the laboratory, the peat samples were analysed for plant macrofossils, decomposition, bulk density, and organic and carbon contents (papers I–IV). Important stratigraphic boundaries were dated by the radiocarbon method (mainly AMS, but also some conventional datings). The former plant communities and surface conditions were reconstructed based on plant macrofossil analysis. All the encountered plant macrofossils were identified to as low a taxonomic level as possible and their relative abundances were assessed. Additionally, some groups of aquatic animals were used to support interpretations. The survey of present vegetation cover was used as modern analogue data in the reconstruction of past environments. Dating, bulk density and organic and carbon

22

content values were used for calculating peat, dry organic and carbon accumulation during different periods of mire development (papers I–IV). A review of 42 publications is used to assess what is known about the stratigraphy and botanical composition of peat in palsa mires and about the age of permafrost aggradation (paper V). Detailed plant macrofossil studies of palsas are few, but numerous radiocarbon datings are available, especially from Scandinavia. However, not many of these dates are reliable for dating permafrost aggradation.

5 Results and discussion

5.1 Palsa mire vegetation

In my studies, 53 vegetation relevés are presented from five mires (papers I–IV, Oksanen 2002). High palsas are dry and ombrotrophic; Dicranum elongatum, Polytrichum strictum, Pleurozium schreberi, lichens and bare peat form the ground layer. Two types can be separated according to the dominant vegetation: lichen and moss type. Some Sphagnum fuscum may be present. Hepatics are more frequent in the Finnish site (paper IV), where also Pohlia nutans and Tetraphis pellucida are encountered. Dicranum fuscescens is accounted for in Khosedayu (paper III). The field layer consists of dwarf-shrubs, especially Empetrum nigrum (ssp. hermafroditum) and, in Pechora (papers I–III), Ledum palustre. Arctostaphylos alpina is found in Khosedayu (paper III) and Eriophorum russeolum in Shar’yu (Oksanen 2002). Dry plateau palsas do not differ from the lichen-type high palsas. In moist plateau palsas, the ground layer is formed predominantly by S. fuscum, sometimes replaced by S. capillifolium (paper I, II). In addition to the same dwarf-shrubs as on high palsas, some Vaccinium microcarpum is found and, in Rogovaya (Fig. 7, paper I), also Carex globularis. Slopes of palsas can be mainly bare of vegetation when collapsing, resemble moss type palsas (with Carex) or show a full succession series towards wetter conditions like in Khosedayu (paper III). Embryonal palsas were encountered in Khosedayu (1998) and Shar’yu (1999). Smaller non-permafrost hummocks have similar ground layer vegetation to Sphagnum type plateau palsas or moss type high palsas, but in their field layer are found in addition C. rariflora, Drosera anglica, Chamaedaphne calyculata and larger quantities of V. microcarpum.

Some palsa mire flarks are recognised as collapse scars in the field by their form and location. Their vegetation does not differ from that of other flarks near to palsas, or the sample is too small to recognise any patterns. Sphagnum lindbergii, S. annulatum (= jensenii), S. riparium, Drepanocladus aduncus or Cladopodiella fluitans dominate the ground layer of flarks. Different Cyperaceae form the field layer: Eriophorum angustifolium (Vaisjeäggi, paper IV), E. russeolum (Shar’yu), E. vaginatum or Carex rostrata (Usinsk, paper II) and C. rariflora or C. aquatilis (Khosedayu, paper III). In the forest-tundra palsa mire, Khosedayu, where palsas are located nearer to each other than in the other two palsa sites, there are rather dry and narrow gullies between palsas.

24

S russowii, S. angustifolium and Polytrichum commune grow in these sites. In Vaisjeäggi (paper IV), species like S. teres, S. warnstorfii, Rhizomnium pseudopunctatum and Paludella squarrosa are found only further away from palsas, in a shrubby fen. In Khosedayu (paper III), S. riparium grows only in pools nearer to the mire edge and in infilling ponds. In the forest-tundra plateau palsa mire, Rogovaya River (Fig. 7, paper I), there are two types of flarks slightly below the plateau level: with or without permafrost in the organic layer. S. balticum is common in both types. Typical taxa in permafrost flarks include S. annulatum (S. jensenii), Hepaticopsida and C. rotundata, while in non-permafrost flarks S. obtusum, S. fimbriatum, C. rariflora and E. russeolum are found instead. S. riparium and C. aquatilis are encountered in non-permafrost fens at the thermokarst level only.

25

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e on

pal

sas (

Hep

atic

opsid

a, o

ther

Sph

agna

and

fiel

d la

yer v

eget

atio

n ot

her t

han

dwar

f-shr

ubs)

. The

tota

l per

cent

ages

ar

e ab

ove

100

beca

use

field

and

gro

und

laye

r co

vera

ges o

verl

ap. I

n ad

ditio

n, fo

r ea

ch ty

pe a

nd re

gion

are

giv

en th

e pe

rcen

tage

of c

ases

wi

th to

p of

the

peat

stra

tigra

phy

cons

istin

g of

dwa

rf-sh

rub/

root

let p

eat (

with

min

or a

mou

nt o

f mos

ses)

, Pol

ytric

hum

-Dic

ranu

m p

eat (

with

so

me

dwar

f-shr

ubs)

, Sph

agnu

m fu

scum

/cap

illifo

lium

pea

t (wi

th so

me

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ubs)

and

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t not

dep

osite

d by

a p

alsa

. st

udie

d hi

gh a

nd

plat

eau

pals

as (n

umbe

r of

vege

tatio

n de

scrip

tions

)

bare

pea

t do

min

ates

%ba

re p

eat

aver

age

liche

ns

dom

inat

e %

lic

hens

av

erag

e Po

lytr

./ D

icra

num

do

min

ate

%

Poly

tr./

Dic

ranu

m

aver

age

dwar

f-

shru

bs

dom

inat

e %

dwar

f-

shru

bs

aver

age

S. fu

scum

/ S.

cap

ill.

dom

inat

e %

high

pal

sa F

enno

scan

dia

(30)

13

1.

3 56

4.

1 13

2.

1 80

4.

4 0

high

pal

sa N

EE R

ussi

a (1

2)

17

1.7

33

3.2

33

2.2

58

3.5

8

plat

eau

pals

a Fe

nnos

cand

ia (4

) 0

1.0

0 3.

3 25

3.

5 10

0 5.

0

0 pl

atea

u pa

lsa

NEE

Rus

sia

(11)

9

1.8

27

3.1

18

2.0

100

4.

9

27

stud

ied

high

and

pl

atea

u pa

lsas

(num

ber o

f ve

geta

tion

desc

riptio

ns)

S. fu

scum

/ S.

cap

ill.

aver

age

Hep

ati-

cops

ida

aver

age

othe

r Sp

hagn

a av

erag

e

Cyp

er./

Poac

eae

etc.

ave

rage

uppe

r pea

t nu

mbe

r of

site

s

dwar

f-sh

rub/

ro

otle

t pe

at %

Poly

tr./

Dic

ranu

m

peat

%

S. fu

scum

pe

at

%

flark

/ fen

pe

at

%

high

pal

sa F

enno

scan

dia

(30)

0.

5 0.

8

0.1

0.

4

30

37

17

23

23

high

pal

sa N

EE R

ussi

a (1

2)

0.7

0.

3

0.3

1.

7

15

27

40

13

20

plat

eau

pals

a Fe

nnos

cand

ia (4

) 1.

0

2.0

0

0 2

0 50

50

0

plat

eau

pals

a N

EE R

ussi

a (1

1)

1.5

0.

2

0 0.

5

18

28

28

47

7

26

Fig.

7. S

chem

atic

tran

sect

ove

r a

plat

eau

pals

a se

ctio

n in

the

Rog

ovay

a R

iver

, Rus

sian

fore

st-tu

ndra

.

pond

crac

k

m ou

100

m

S.ba

lticu

mS.

fim

bria

tum

S.ob

tusu

mE

.ru

sseo

lum

E.

vagi

natu

mC

.ra

rifl

ora

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dber

gii

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nula

tum

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com

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tund

ata

T.

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itosu

m

lich

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shru

bS.

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umS.

capi

llifo

lium

C.

glob

ular

isli

chen

Dic

ranu

m

S.ri

pari

umC

.aq

uati

lis

E.

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m

S.fi

mbr

iatu

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squa

rros

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.aq

uati

lisPo

tent

illa

S.ru

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iiP.

com

mun

eP.

jens

enii

1b,1

c1d

,1f

Tes

tpr

ofil

es

Prof

iles

sam

pled

; (fo

r num

bers

, see

Tab

le 1

in

Pap

er V

)

ixgani

cde

posi

t,de

pth

rapo

late

dfr

ompr

ofil

es

nera

lgr

ound

S.Sp

hagn

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Poly

tric

hum

C.

Car

exE

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riop

horu

mT

.T

rich

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rum

ter

Pote

ntil

laP.

pal

ustri

sis

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rozi

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Dre

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afro

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peat

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ark

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silt

silt

pera

ceae

Lege

nd fo

r Fig

ures

7 a

nd 8

.

S.fu

scum

1e silt

dwar

f-sh

rub

dwar

f-sh

rub

D.

Dic

ranu

mV.

Vacc

iniu

m

Dic

rane

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. cer

vicu

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Ptili

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P. c

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iner

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rop

1m

2m0

1m

2m0

Sal

Or

ext

Mi

Wa

Up

sand

Cy

1a sand

27

The review (paper V) includes a first attempt to classify the permafrost mire vegetation at the European level. Its main purpose is to give background information for macrofossil interpretation and test the reliability of the method in interpreting permafrost dynamics in mires. As a mire site classification it is yet exploratory due to the variable nature and quality of the input material and often inaccurate site descriptions. The vegetation of palsa mires and especially palsas is rather uniform over Europe and the same main characteristics observed in the four studies (papers I–IV) from different areas included in this thesis, are revealed in the review. Schematic transects over a section of plateau and high palsa mires are shown in Figs. 7 and 8. Distinctive features of vegetation on high and plateau palsas in western and eastern study regions are presented in Table 1. The clustering results for permafrost sites and the presence of non-permafrost indicator species are summarised in Table 2.

Common dwarf-shrubs on palsas (and other hummocks) are the same for the whole area. Slight regional differences can be seen: Empetrum nigrum (ssp. hermafroditum) covers more area on the Fennoscandian palsas while Ledum palustre dominates in Pechora. Some occasionally encountered species on palsas such as Vaccinium myrtillus, Calluna vulgaris and Betula pubescens ssp. czerepanovii are clearly western/maritime. Also Eriophorum vaginatum and Calamagrostis lapponica have a western distribution on palsas, possibly connected to highly deflated palsas that are typical for western Finland and Norway. Dicranum elongatum and Polytrichum strictum could be called palsa mosses, as they are consistently present on palsas. Pleurozium schreberi is clearly more usual in Pechora than elsewhere. Several unpretentious mosses, e.g., Tetraphis pellucida and Pohlia nutans, seem to be characteristic for Scandinavia and cracks on palsas. D. fuscescens or perhaps rather the form D. flexicaule (paper III) might be common on palsas in the whole region. According to Yurkovskaya (1975), it is characteristic for Russian high palsas but not for plateau palsas. Sphagnum fuscum and S. capillifolium are more usual on plateau than other palsas, as also noted by Yurkovskaya (1975).

28

Table 2. Potential of cluster analysis and indicator taxa to identify the presence or absence of permafrost. For each cluster is presented: 1) the percentage of palsa top relevés; 2) the percentage of other permafrost relevés (palsa slopes, palsa bases, young palsas, collapsing palsas, permafrost flarks and pounus); 3) the percentage of cases with species rarely encountered on palsas and having plant associations unlikely for palsas; 4) the percentage of relevés with species never encountered on palsas (excluding palsa embryos). Lichen and liverwort species are not considered.

cluster (number relevés) 405)

% relevés palsa top

% relevés other permafrost

% relevés rarely permafrost

% relevés never permafrost

total %

1 (37) 57 24 16 97 2 (24) 54 34 4 8 100 3 (18) 67 22 11 100 4 (27) 11 41 4 44 100 5 (18) 33 28 22 17 100 6 (33) 100 100 7 (9) 100 100 8 (12) 8 8 84 100 9 (24) 100 100 10 (19) 100 100 11 (15) 100 100 12 (41) 10 90 100 13 (35) 3 97 100 14 (8) 12 88 100 15 (30) 3 3 94 100 16 (17) 100 100 17 (38) 21 79 100

The cluster analysis distinguished four (five) types of palsas: eroding (bare, hepatics), lichen-dominated, Dicranum-Polytrichum -dominated and a drier and a moister Sphagnum fuscum type. Plateau palsas are not separated in their own group, but more of them fall into the Sphagnum-clusters than the drier palsa clusters. Almost no regional differences are notable in the clusters, except that more Scandinavian palsas are found in the eroding palsas –cluster. Vorren (1979a) distinguished five high palsa facies in his extensive study within one mire (Faerdesmyra, Norway, Fig. 8): palsa crack vegetation (photophobous mosses and hepatics) and Empetrum-Ledum vegetation (Dicranum or Polytrichum modification) are equivalent to eroding and moss palsas. Lichen palsas are further separated into two facies, south exposed and palsa summit types. Moister vegetation prevails on other hummocks in Faerdesmyra, being uncommon on palsas. Zuidhoff and Kolstrup (submitted) suggest a high palsa vegetation classification of four stages, based on studies in four mires covering the Swedish palsa mire region. Young palsas are most notably invaded by P. strictum and Betula nana, while on mature palsas various dwarf-shrubs, Dicranum and lichens are dominant at the expense of P. strictum. There are insufficient descriptions on young palsas to check the validity of this classification for the whole European palsa area. On one young palsa in Kola P. strictum and B. nana are present, in Pechora they are not. Also, recognising a palsa as young

29

without dating the time of permafrost aggradation is not always straightforward. High palsas with sparse vegetation can be young and low palsas old. Zuidhoff and Kolstrup (submitted) note the prevalence of bare peat and hepatics on degrading palsas. Their fourth type consists of remnant palsas, with a combination of dry and wet vegetation.

Other hummocks are usually lower than palsas and thus their vegetation includes more moisture demanding species than the palsas can support. Lichens, Empetrum nigrum, Vaccinium vitis-idaea, Dicranum elongatum and Polytrichum strictum, though still common, play less of a central role while Rubus chamaemorus, V. uliginosum, V. myrtillus, Chamaedaphne calyculata (in Pechora), Sphagnum fuscum and S. capillifolium are more important. In the cluster analysis, most hummocks are clustered together with moist palsas, because the prevailing species are the same, although non-permafrost hummocks nearly always support some species never found on palsas (Table 2). A few species within this study are found on the top parts of palsas only. These species like lichens Alectoria nigricans or Bryoria fuscescens and mosses Aulacomnium turgidum, Ceratodon purpureus, Dicranella cerviculata or Dicranoweisia crispula are occasionally reported from palsas.

Although palsas are usually ombrotrophic, exceptions towards minerotrophy occur. Thin peat-cover, young age or possibly the effect of seawater explain most of these cases. For the few cases of minerotrophic species reported on thick-peated high palsas, an occasional nutrient input (e.g., an animal), or wrong identification are possible explanations. Most of the exceptional species are encountered in mires located in the tundra or in the mountains above the treeline. These mires, although called palsa mires (or flat-mound mires in the Russian literature), differ essentially from the typical palsa mires of the forest-tundra and northern taiga. In the tundra, typical plateau palsas change into mire polygons, showing an intermediate form of low, flat permafrost mounds, usually not as extensive as proper plateau palsas. Spring floods probably reach their top in the tundra where most of the ground is permanently frozen and evaporation is very low. Also, the peat layer in tundra mires is usually shallower than in the more southern areas. In the mountains, melt waters transport minerals along the slopes, and here too, palsas are usually low and the peat layer often thin. Sometimes the permafrost hummocks in these mires should rather be called pounus than palsas.

30

Fig.

8. S

chem

atic

tran

sect

ove

r a

palsa

sect

ion

in F

aerd

esm

yra,

at t

he n

orth

ern

pine

lim

it in

Nor

way

. Sim

plifi

ed a

fter

Vor

ren

(197

9b).

1 20 3 4

10mD

.el

onga

tum

D.

fusc

esce

nsli

chen

dwar

f-sh

rub

liche

nD

.el

onga

tum

D.

fusc

esce

nsPo

hlia

Pleu

rozi

umD

.m

ajus

D.

scop

ariu

mV

.m

yrti

llus

D.

maj

usD

icra

nella

P.st

rict

umPo

hlia

Ptil

idiu

m

D.

elon

gatu

mD

.fu

sces

cens

V.

myr

till

us

S.ba

lticu

mE

russ

eolu

mdw

arf-

shru

bli

chen

S.lin

dber

gii

S.an

nula

tum

E.

russ

eolu

m

S.lin

dber

gii

S.ba

lticu

mC

.ro

tund

ata

C.

rari

flor

a

coll

apse

scar

(mel

tpo

ol)

m

??

pals

acr

ack

low

ersl

ope

base

34a

34b

5

1 20

m

crac

kup

per

slop

e

31

Wet sites in palsa mires show much greater variability than hummocks. The most usual sedges in all kind of wet sites are Eriophorum angustifolium, Carex rostrata and C. limosa in Scandinavia, E. angustifolium in Kola and E. russeolum in Pechora. In flarks situated in the vicinity of permafrost landforms, C. rotundata is especially typical for Pechora, and E. vaginatum and Trichophorum cespitosum for Scandinavia. According to Yurkovskaya (1975), C. rariflora, C. rotundata and E. russeolum are the most common sedges in both high and plateau palsa mire flarks of Russia. They are all common in Scandinavia too. For fens outside the permafrost influence, C. aquatilis is characteristic in Pechora and C. canescens and C. magellanica in Scandinavia. In the ground layer of flarks, Sphagnum lindbergii and Calliergon stramineum are found in the whole area, S. balticum mainly in Pechora and Warnstorfia fluitans and Scorpidium scorpioides mainly in Scandinavia. Sphagnum riparium is usual in collapse scars, palsa lagg pools and other flarks in Scandinavia and the southern high palsa area of Pechora, while in plateau palsa and other forest-tundra mires it prefers to grow further away from permafrost. Yurkovskaya (1975) lists S. balticum characteristic for plateau palsa mire flarks and S. riparium and Warnstorfia fluitans for high palsa mire flarks. For fens, there are no clearly dominant species in the ground layer. Generally, the wet sites near to palsas are less trophic and possess fewer species than wet sites outside even the indirect effect of permafrost, and more so in Pechora than in Scandinavia, probably because the permafrost influence is more local in Scandinavia. In Scandinavia, also eutrophic palsa mire flarks are reported. Palsa mounds prevent free flow of surface waters and standing water tends to gather around them. The often very wet conditions, together with more acid waters derived from ombrotrophic palsa peat, might explain a poorer plant composition in the flarks near permafrost compared to other flarks (paper VI).

There are differences between the flarks of high and plateau palsa mires. In flarks between high palsas in the more southern areas, frost can remain during one or several summers, but true permafrost flarks are mainly found on plateau palsas, rarely on smaller flat palsas. The plant communities in them indicate weak minerotrophy. Mesotrophic species grow at the plateau level too, but usually only in depressions where the thawed layer reaches the mineral ground. Thawing palsas result in typical flark forms (paper VI). In high palsa mires they are roundish melt pools, often surrounded by remains of the collapsed palsa as a circle-formed string. In plateau palsa mires, part of the wet depressions on the plateau level are apparently secondary, thawed sections of the palsa, while others might be primary, never having experienced the dry palsa phase. Another characteristic thaw landform in plateau palsa mires is thermokarst ponds, formed by lateral erosion of palsas. Collapse scars and plateau palsa depressions have fewer species than other flarks or fens in palsa mires, probably due to unsettled conditions in addition to (for plateau palsa depressions) permafrost preventing the nutrient intake from lower deposits.

Cluster analysis divides wet sites into 12 clusters, which clearly form two larger groups. The same features can be observed in this division than when comparing flarks near permafrost to wet sites outside of permafrost influence: Sphagnum and Eriophorum are more important in the second group, which contains more near to permafrost flarks. This group presents generally less trophic sites with less species. In the first group, which contains evenly both types of wet sites, Carex and brown-mosses are more significant. With a more precise site description into wet sites in the immediate vicinity of permafrost

32

and those further away from it, the cluster division would probably become clearer. Saari (2001) made the same observation from two palsa mires in northernmost Finland: wet Sphagnum-Eriophorum type surrounds palsas as a zone a few metres wide and drier Carex (- brown-moss) type occupies the mire surface further away from palsas. S. lindbergii and S. riparium are the most typical mosses in the ground layer of the first type, and they, especially S. riparium, are clearly more common in the pools beside palsas than elsewhere. Saari (2001) observed that E. russeolum grows mainly in flarks near palsas, possibly most commonly in collapse scars only (Vorren 1979a). Several species found in other parts of the mires were not encountered next to palsas, e.g., C. aquatilis, Cinclidium stygium, Limprichtia revolvens and Warnstorfia procera. The most diverse vegetation is found near rivulets and in pools further away from palsas.

5.2 Peat stratigraphy

Macrofossil analyses were made from nine profiles in four mires (papers I–IV). In addition, gross-stratigraphy is available from 19 other profiles in six mires. One or several AMS radiocarbon datings are available from 20 profiles (papers I–IV, Oksanen 2002).

In the Russian forest-tundra, depths of organic deposits are measured to 60–200 cm, in the Russian taiga to 210–630 cm, and in northern Finland to 120–390 cm. In the Russian forest-tundra before ca. 6000 BP (200–70 cm), Limprichtia, Calliergon, Carex, Eleocharis, Equisetum and tree-Betula (paper I) remains prevail in the deposits. Between about 6000–3000 BP (190–30 cm), the main peat formers are Sphagnum squarrosum, S. teres, S. riparium, S. annulatum, S. balticum, Warnstorfia fluitans, Carex, Equisetum and Scheuchzeria. Remains of trees are rare. After ca. 3000 BP (< 70 cm), the dominant taxa are Sphagnum riparium, S. lindbergii, S. balticum, S. angustifolium, S. warnstorfii, S. fuscum, Polytrichum, Eriophorum and B. nana. Ledum is especially abundant after ca. 2000 BP (< 20 cm; papers I, III). Rapid and repeated changes between drier and wetter conditions are recorded from plateau palsa profiles in Rogovaya (paper I); a probable hiatus is suggested for the high (flat) palsa stratigraphy at Khosedayu (paper III). Thermokarst depressions have recent Sphagnum sect. Cuspidata mats floating above water. The bottom deposits are of mixed origin, as shown by reversed datings from Khosedayu (site 2b in Fig. 9).

In the Russian taiga (paper II, Oksanen 2002) Najas cf. flexilis and Zannichellia have been recovered at ca. 9550 BP. Their nearest modern location in freshwater habitats of Russia is 800 km to the south. The findings support the interpretation that climate was warmer during the early Holocene than at present. Before about 3500 BP (530–70 cm), the deposits are composed of Scorpidium, Warnstorfia, Limprichtia, Carex, Equisetum, Scheuchzeria, Menyanthes and dwarf-Betula. Tree remains seem to be absent from the beginning of the records. Later on (< 160 cm), Sphagnum balticum, S. magellanicum, S. annulatum, S. fuscum, S. capillifolium, Pleurozium and different dwarf-shrubs become dominant (paper I, site 4a). The upper layers of the non-permafrost hummock profiles of Usinsk (paper I, site 4e) lack the striped stratigraphy of the plateau palsa profiles and have remains of Pinus and Chamaedaphne, mainly absent from the palsas.

33

Fig.

9. S

elec

ted

peat

str

atig

raph

ies

from

Eur

opea

n pa

lsa m

ires

. Exa

mpl

es f

rom

pap

ers

I–IV

(si

tes

1b, 2

b, 4

a, 4

e, 4

8b, 4

8c),

Sone

sson

196

8,

1970

a, 1

970b

(site

42)

, Vor

ren

1972

(site

34b

), Sa

lmi 1

972

(site

27a

) and

Oks

anen

200

2 (s

ite 3

a).

2

7a

Mark

kin

a-a

apa

pals

a

7800

8460

4150

7200

B

SB

At2

At1

Bo

4a

U

sin

sk

pla

teau p

als

a

315

2775

3580

511

0

8520

11350

baltic

um

ma

ge

llan

icu

m

Pohlia

ca

pill

ifo

lium

fuscum

fuscum

34b

Faerd

esm

yra

flark

SA

SB

At2

At1 Bo

4

8c

Va

isje

äg

gi

co

llap

se

sca

r

1b

Rogovaya

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teau p

als

a

48b

Vais

jeäggi

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ing p

als

a 645

6090

fuscu

m

fuscu

m

wa

rnsto

rfii

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rpid

ium

fuscu

m

wa

rnsto

rfii

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ium

N

N

2490

T3

T4

6460

42

A

bis

ko

pla

teau p

als

a

2

b K

hosedayu

therm

okars

t pond

annula

tum

riparium

riparium

Palu

della

Palu

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Squarr

osa

1450

2240

3120

6390

7640

9250

1410

160

6780

NNN

L L

Er

Sh

Sh

Sh

NN Eq

Eq

L

WW W W

Er

Mn

N Eq

W

S Eq

rootlet peat

Menyanth

es

wood

Eriophoru

m

Sphagnum

bro

wn-m

oss

Care

x

gyttja

and d

y

silt

and c

lay

sand a

nd till

Equis

etu

m

Bry

idae

Dw

arf

-shru

b

Eq

B N L Mn

Er

Dic

ranum

and

Poly

tric

hum

dark

layer

Scheuchzeria

Sh

Polle

n z

ones

Subatlantic

Subbore

al

Atlantic

Bore

al

Betu

laperiod =

late

SB

, S

A

Pin

us

period =

At, e

arly S

B

0

30

60

90

12

0

15

0

18

0

21

0

24

0

27

0

30

0

33

0

36

0

39

0

42

0

45

0

4e

U

sin

sk

low

hum

mock

3150

9550

fuscum

4430

lindberg

ii

3

aS

har’yu

pals

a

190

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ata

Acutifo

lia

N Mn

wate

rWS

Salix

SA

SB

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T4

T3

In northern Finland (paper IV), the peat consists of Sphagnum teres, Paludella, Calliergon, Drepanocladus s.l., Carex, Equisetum and Potentilla until ca. 2500 BP or later (390–30 cm). After 2500–650 BP (< 70 cm), the peat in palsas has remains of Polytrichum, Dicranum, Pleurozium, Empetrum, Vaccinium and Betula nana. In recent flarks (< 60–30 cm; no datings) S. lindbergii, S. riparium, Warnstorfia, Sarmentypnum and Eriophorum prevail (paper IV, site 48b).

Information is gathered in the review (paper V) from 103 peat profiles from permafrost mires over the whole study region. A complete plant macrofossil analysis, except in the studies discussed above, is published in only one additional study (Väliranta et al. 2003). From about half of the profiles, some plant species are identified. The data gives a rather good overview of typical palsa profiles; for different flark types more profiles would be needed. Examples of different palsa and wet site profiles are provided in Fig. 9. The most usual peat types of high and plateau palsa deposits in western and eastern study regions are summarised in Table 1.

In previous palsa studies, only the uppermost xerophilous layers are usually connected to palsa development (e.g., Vorren & Vorren 1976, Vorren 1979b), or it is assumed that palsas do not deposit peat at all (Ǻhman 1977). The European peat stratigraphies show that different peat types are deposited under different permafrost conditions in mires. There are two main types of upper palsa peat: Dicranum (elongatum) - Polytrichum (strictum) - dwarf-shrub (-lichen) peat (1–60 cm) (sites 3a and 48b in Fig. 9) and Sphagnum fuscum peat with dwarf-shrubs and/or Dicranum-Polytrichum (site 42). These types correspond to the main vegetation types of dry and moist palsas. Subtypes dominated by moss, dwarf-shrub or Sphagnum can be distinguished. As in modern vegetation, Polytrichum is more commonly found in peat in Pechora than in Scandinavia, but S. fuscum is not distinctly more common in plateau palsa profiles. Wet flark/fen peat is found below the dry palsa crust, either directly (site 48b) or below intermediate strata (3a), the latter showing gradual drying or repeated changes of wetter and drier conditions. Seppälä (1988) notes that a thin layer of xerophilous Sphagnum peat further capped by “Bryales”-Carex fen peat is common between the lower fen peat and the uppermost palsa peat in Finnish palsa stratigraphies. Layers identified as previous palsa deposits are occasionally encountered lower in the profiles, within wet deposits (sites 1b and 42 in Fig. 9). Sometimes the peat deposited by a palsa is completely lacking from the top of palsa stratigraphy; fen peat on top of palsas proves that erosion has taken place on these palsas (site 27a). Taxa that do not usually grow on palsas are found in the deposits of shallow-peated palsas.

The most usual peat type in surface layers of flarks or below palsa layers is Sphagnum sect. Cuspidata (S. lindbergii) - Eriophorum peat (site 34b in Fig. 9). Further down in the deposits, Carex - brown-moss peat is more common. Deeper in the deposits several peat types that do not commonly have modern corresponding vegetation types in palsa mires are encountered, like Carex-wood (of trees), Sphagnum-wood (of trees) and Sphagnum-Scheuchzeria peat. Also Equisetum and Menyanthes are more usual in peat than in modern vegetation. The effect of nearby permafrost can be observed in cores taken from the edge of flarks adjacent to palsas, where dwarf-shrub, Aulacomnium or other non-fen moss peat is found in the upper 10 cm layer.

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5.3 Peat and carbon accumulation

The carbon content is measured from eight profiles in four mires (papers I–IV). It varies from 48.5% to 52.6%, the mean being 51.0%. Botch and co-authors (1995) report a significantly higher carbon content of 57.0% for Russian palsa mires. The (apparent) mean long-term carbon accumulation rates are calculated from five mires. The values are based on calibrated radiocarbon years (van der Plicht 1998, Stuiver & Reimer 1993) and are weighted for depth. The average rates for three areas and for all the studied mires are presented in Fig. 10.

In the two sites of the Russian forest-tundra the rates are 13 gC/m2yr for Rogovaya (paper I, two profiles) and 21 gC/m2yr for Khosedayu (paper III, one profile), in average 17 gC/m2yr. In the Russian taiga are revealed values of 22 gC/m2yr for the Usinsk mire (paper II, two profiles) and 35 gC/m2yr for Shar’yu (Oksanen 2002, one profile), in average 28 gC/m2yr. In Vaisjeäggi, Scandinavian oroboreal birch belt, the long-term rate is 16 gC/m2yr (paper IV, four profiles). The average values are within the normal range of the equivalent zones: 16 gC/m2yr is reported for palsa mires of the former Soviet Union (Botch et al. 1995), 17 gC/m2yr for the northern Finnish mires (Turunen 2003), 17 gC/m2yr for the mires of central and northern Sweden (Fredriksson 1996), 26 gC/m2yr for the mires of Finland (Turunen & Tolonen 1996) and 30 gC/m2yr for the mires of the former Soviet Union (Botch et al. 1995). The rates generally decrease towards the north, but local variation between mires and sites can be great.

In old dry palsas the accumulation is very low or has ceased, with values of ca. 7 gC/m2yr for old (about 2000 BP and older) stable palsa stages from subarctic Russia (papers I, III) and 9 gC/m2yr for a ca. 2500 BP old palsa stage from northern Finland (paper IV). Wind abrasion and block erosion can result in negative accumulation rates. The surface peat of eroded palsas has been dated to 1300–3900 BP (Salmi 1968, 1972, Vorren 1972). The lowest values obtained from Russia point to ceased accumulation as in Rogovaya (4 g C/m2yr for the last about 1500 BP, paper I) or to erosion as in Khosedayu (7 g C/m2yr since 4600 BP), where the peat stratigraphy suggests that some layers are lacking (paper III). A low value, 10 gC/m2yr, is also obtained for a full palsa - collapse scar - palsa cycle (1490 BP) in northern boreal Russia (Oksanen 2002). From northwestern Canada, values are reported for mature high palsas, plateau palsas and polygonal plateau palsas in the order of 4, 13 and 8 gC/m2yr for the last 1200 yrs, respectively (Robinson & Moore 1999). In the dynamical conditions of partial thaw and re-establishing permafrost prevailing in moister plateau palsas, accumulation rates can remain relatively high: 18–19 gC/m2yr in Usinsk (paper II) between 2800 and 300 BP and in Rogovaya between 1900 and 1500 BP. Very high accumulation rates are reported for young palsas: 102 gC/m2yr (190 BP) in Shar’yu (Oksanen 2002), 93 g C/m2yr for a short interval of incipient permafrost at Rogovaya River (paper I), 73 gC/m2yr (last ca. 650 BP) in Vaisjeäggi (paper IV), 49 gC/m2/yr (315 BP) for the latest collapse scar - plateau palsa cycle in Usinsk (paper II) and 36–45 gC/m2yr (last 800 years) for ombrotrophic, apparently palsa, peat in northern Sweden (Malmer & Wallén 1996). These rates are very high compared to other accumulation values obtained from northern mires. However, the datings used are mostly AMS datings and appear logical. Peat compaction cannot affect the values measured as dry matter accumulation over time. Moreover, there are too many

36

high values from a large area to be explained by an occasional mistake, and because the high values are always connected with incipient permafrost, systematic error in measurements can be ruled out. Redeposition of older material could be a factor in some cases, but in others macrofossil evidence suggests undisturbed development. In any case, even if the rates were too high, they provide a very good further evidence of the presence of permafrost, because no other northern mire environment is known to produce such high accumulation values. The mean accumulation rates during the palsa stages are higher in most studied locations than the long-term average, but possibly short-term incipient permafrost phases are over-represented.

Rather low rates have been found for permafrost flarks and collapse scars. A rate of 10 g C/m2yr is calculated from Rogovaya both in a plateau palsa flark stage between 2200 and 1500 BP and in a collapse scar between 3100 and 2200 BP (paper I). From Canada values of 15 C/m2yr for collapse scars are reported over the last 1200 yrs (Robinson & Moore 1999). These results support field observations that collapse scars are quickly invaded by flark species with renewed peat accumulation, whereas peat calving into thermokarst ponds obviously results in negative accumulation (paper VI). From thermokarst depressions, most likely replacing areas formerly occupied by 1–2 m thick peat deposits, almost all deposited material is decomposed and at present only thin gyttja deposits are found at their bottoms (papers I, III). In permafrost-free fens the rates vary between 8 and 79 gC/m2/yr, the lowest being from a pool and the highest from treed fens/incipient mires (papers I–IV). The only available value for a non-permafrost hummock is 13 C/m2yr for the last 3150 BP from Usinsk (paper II).

Dry peat or carbon accumulation values calculated for different zones of peat profiles on European palsa mires are available only in four papers included in this study (papers I–IV). In the licentiate thesis of Oksanen (2002) results are presented for peat accumulation in the whole palsa area, calculated as vertical peat increment. This approach is problematic, however, due to uneven density of peat, especially in palsa mires, where peat deposited under dry palsa conditions usually has higher bulk density. Furthermore, the variable amount of ice in permafrost landforms should be taken into account. The results show that the long-time mean for all Fennoscandian sites is 0.35 mm/yr (n = 19) and for all Pechora sites 0.53 mm/yr (n = 10). The mean accumulation in palsa peat of Fennoscandia is 0.23 mm/yr (n = 20) and of Pechora 0.45 mm/yr (n = 13). In the uppermost layers of Fennoscandian flarks, the rate is low as well: 0.20 mm/yr (n = 7). This value is not available from Pechora. These numbers indicate that organic accumulation in palsa mires has decreased towards recent times, and apparently more in Scandinavia than in Pechora. The organic accumulation values from the five now studied mires do not entirely confirm this result (Fig. 10), but there are far too few complete profiles available to reach a final conclusion.

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Fig. 10. Average carbon accumulation in the 5 studied mires of the Northern Pechora taiga (5 profiles), the Northern Pechora forest-tundra (3 profiles), northern Finland (4 profiles), and in the whole region (thicker dashed line).

5.4 Identifying and dating permafrost induced changes in mires using plant macrofossil analysis

This topic is most thoroughly dealt with in paper V, although discussed from different viewpoints in all of the papers. Due to lack of clear indicator species, an absolute certainty of the presence of permafrost cannot be achieved. Negative indicator species help to ascertain periods when a site could not have been a palsa or otherwise perennially frozen. Based on plant macrofossil assemblages, changes in accumulation rates and accurate 14C dating, permafrost aggradation or degradation can in most cases be reliably dated. The relatively good separation of permafrost sites by the cluster analysis, in spite of the diverse and partly questionable material used, supports the conclusion that a permafrost effect is often, though not always, distinguishable in peat stratigraphy. The cluster analysis best recognises the wet-dry gradient of the sites, thus joining most palsas with other hummocks. However, the negative indicator species nearly always separate the non-permafrost sites from the palsa summits within the clusters (Table 2). The other permafrost cases remain more problematic to identify. The method reviewed and developed in this thesis is often not applicable for tundra or sloping mountain mires, different from the typical palsa mires, because of the thin peat layer, low mounds and continuous permafrost or water supply along slopes that affect the trophic and hydrologic conditions so that minerotrophic and moisture demanding species are found even on permafrost mounds.

A classic mound-shaped palsa lifted from a flark (Fries 1913, Lundqvist 1951, Ruuhijärvi 1960) is relatively safe to date, assuming that the peat profile is intact. The change from flark to palsa peat is abrupt; new type of vegetation occupies the site in a few years (Seppälä 1982). Other processes causing drying in mires are usually more gradual. Dicranum elongatum, Polytrichum strictum, lichens and dwarf-shrubs are the

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typical taxa found in the xerophilous peat deposited by palsas. However, wind erosion is usual and former palsa deposits can be completely absent in the peat record. If in doubt that the palsa layer is not intact, it would be good to date both the uppermost flark and the lowermost palsa peat layer (Seppälä 2003). A few hundred years long gaps in between may be usual (Seppälä 2005). In gentle collapse, when a palsa sinks back to the flark, the palsa peat may preserve and is followed by collapse scar peat in the stratigraphy. Usual species involved are Sphagnum riparium, S. lindbergii and Eriophorum spp. At the initial stage, the palsa development appears often to be unstable; in the scale of years to hundreds of years (no high-resolution dating available), a permafrost surface can be uplifted from the mire, become invaded by Polytrichum or Dicranum, be thawed and uplifted again. Deposits where e.g., S. warnstorfii or S. magellanicum peat is interlayered with dark well-decomposed bands, can refer to similar unsettled conditions (4a in Fig. 9). Low hummock species are apparently able to reappear several times, but are gradually replaced by species that better tolerate dryness. From European mires a few probable previous palsa layers are detected deeper in the deposits. The changes in accumulation rates further help in the identification of palsa development, very low rates possibly pointing to mature palsas and very high rates to initial stages of permafrost.

In plateau palsas (flat palsas), where the permafrost often aggrades in Sphagnum peat (see Zoltai & Pollett 1983) and S. fuscum and S. capillifolium continue growing, it is more difficult to trace the initiation of permafrost. The lower limit of S. fuscum and S. capillifolium provides a safe maximum age for permafrost aggradation. The Sphagnum peat deposited by a plateau palsa usually includes moister and drier phases, shown as darker and lighter stripes in stratigraphy absent from the non-permafrost hummock peat in palsa mires (papers I, II; compare sites 4a and 4e in Fig. 9). Degradation of a plateau palsa by calving into a thermokarst pond is usually not traceable in peat deposits; mixed, highly decomposed peat layers and/or abnormally high accumulation rate might be indications in some cases (site 2b). Internal (partial) collapse of a plateau palsa results in wet phases alternating with the dry and moist palsa phases (paper II site 4a, Couillard & Payette 1985). Alternating layers of S. fuscum, Dicranum-shrub and S. lindbergii or Cyperaceae peat are typical for upper layers of palsas. Under the uppermost palsa layers, even in the area of high palsas, short peat series that show gradual change are so commonly found that this development can be assumed to be caused by permafrost. Field observations do not verify that embryonal palsas would support Sphagna. Possibly these layers point to more extensive permafrost surfaces at the time of their formation.

Dating of permafrost flarks is problematic, because the same common species grow in different kind of flarks in palsa mires. In plateau palsa mires oligotrophic Sphagnum lindbergii or S. balticum flarks are commonly found on palsas only. The position in stratigraphy and other possible signs of permafrost help distinguishing these deposits as permafrost flarks (paper I, see also Zoltai 1995). A permafrost effect can also be suspected when encountering species growing on the slopes of palsas, e.g., S. russowii, S. angustifolium or Polytrichum commune.

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5.5 Holocene permafrost history of European palsa mires

The permafrost dates discussed here give a preliminary insight into permafrost dynamics of continental European mires. Too few reliable datings of permafrost dynamics in peat deposits are currently available – many of the ones used are conventional 14C or even pollen datings and do not date directly the phenomenon under interest, but are interpolated from a couple of actual datings. Dating the layer crucial for the event of interest is especially important in palsas where significant changes in the accumulation rates and hiatus strata can be expected. Further, due to the lack of macrofossil analyses, the nature of the deposit can be easily misinterpreted. If erosion has occurred at the site, it is not possible to tell more than a maximum/minimum age for a palsa. More research is needed to study the stratigraphy of palsa mires located in the tundra zone in order to find out if it is possible to identify permafrost effect in these mires. The earlier hypotheses on the age of palsas (e.g., Seppälä 1988, Bolikhovskaya et al. 1988, Korhola & Tolonen 1996), though in general terms appropriate, are not based on critical scrutiny of the palsa datings, but are mainly derived from climate reconstructions.

During the early and middle Holocene the climate was relatively warm in the whole of northern Europe. The climate was at least as warm as today by the onset of the Holocene in the Pechora area (paper III, Kultti et al. 2003). Between about 9000 BP and 5000 BP, the summer temperatures in the East-European Russia have been at least 2 C, possibly even 7 °C higher, than at present (papers I, III, MacDonald et al. 2000, Kultti et al. 2003). In Fennoscandia, between ca. 7000-5000 BP the mean temperatures were higher by at least 2–3 °C compared to present (Kremenetski & Patyk-Kara 1997, Eronen et al. 1999a, Shemesh et al. 2000, Kultti et al. submitted). In the mid-Holocene, climate was drier than present in northern Scandinavia and the Kola Peninsula (Eronen et al. 1999a, Snyder et al. 2000, Korhola et al. 2002, Seppä et al. 2002); from different parts of the Pechora region the past climate has been interpreted as either moist (Andreev & Klimanov 2000) or dry (Tarasov et al. 1999). Peat deposits from the beginning of records (ca. 11 000–8200 BP) through the middle Holocene indicate permafrost-free environment in mires (papers I–V).

According to this study, permafrost aggradation matches the major climate features, with about 70% of traced permafrost aggradation taking place during the suggested coldest periods of the Holocene. A gradual cooling started around 5000–4500 BP in the whole region (II, Khotinskyi 1984, Kremenetski & Patyk-Kara 1997, Eronen et al. 1999a, Andreev & Klimanov 2000), although at ca. 4000 BP the forest was still more abundant than today in the modern treeline regions (papers I, II, Eronen 1979, MacDonald et al. 2000) The cooler climate was usually combined with increased precipitation (Kremenetski & Patyk-Kara 1997, Eronen et al. 1999a). Since the climate was nearly as cold as today, permafrost formation probably started at some locations during this period (4800–3500 BP, papers II, III, V, Väliranta et al. 2003), but the actual evidence from peat deposits is still uncertain. Most studied peat records do not show major environmental change before 3500–3000 BP (papers I–V) and, if permafrost existed, its distribution was probably much more limited than today.

A further cooling is detected beginning from ca. 3000 BP (papers I, III, Eronen et al. 1999b). In the Pechora region the whole period of 3000–1000 BP was possibly colder

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than today (paper III). Especially cold conditions are recorded both in Pechora and Fennoscandia between ca. 2500–2000 BP (papers I, III, Eronen et al. 1999b, Shemesh et al. 2001, Kultti et al. submitted). Eronen and co-authors (1999b) record an increase in humidity at about 2500 BP. From the modern forest-tundra of Russia the oldest reliable initial permafrost ages are ca. 3100 BP (paper I), from the modern northernmost taiga zone of Russia ca. 2900 BP (paper II) and from Scandinavia ca. 2500 BP (papers IV, V, Vorren 1972 reinterpreted). A younger stage of permafrost aggradation in the Russian forest-tundra dates to ca. 2200–1900 BP (I). From the Russian taiga, one 14C permafrost dating falls to ca. 1500 BP (Oksanen 2002) and from Scandinavia, one to ca. 2200 BP (V, Vorren 1972 reinterpreted). Other datings, roughly interpolated or pollen dated are 3400, 3000, 2900 and 1300 BP from Pechora and 2100, 1900, 1500 and 1100 BP from Scandinavia. The interpolated dates 1900 and 1500 BP might as well be about 2300 BP, as suggested by the pollen curves (paper V, P’yavchenko 1955, Salmi 1968, 1972, Sonesson 1970b). From the Kola Peninsula, only two pollen datings are available, suggesting permafrost aggradation at 2500–2000 BP (paper V, P’yavchenko 1955). The few available datings plotted in Fig. 11 suggest that in the Pechora region at ca. 3000 BP permafrost was as widespread as it is today. From Fennoscandia, permafrost is not recorded at 3000 BP. Fig. 12 shows the situation at about 2000 BP, when permafrost seems to have been at least as extensive as today in the whole area.

Fig. 11. Distribution of permafrost in mires at about 3000 BP, according to the available permafrost aggradation datings.

The Medieval climatic optimum (1000–750 BP) is distinguished at least in some parts of northern European Russia (Klimenko & Klimanov 2000, Rusanova & Kuhry 2003). In Finnish Lapland, the years 1080–850 BP are seen in the tree rings as a favourable period for growth, but interrupted several times by temporary coolings (Eronen & Zetterberg

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1996). It has been reconstructed to have been 0.5–0.8 °C warmer than present (Shiyatov 1993, Kullman 1998, Hiller et al. 2001, Kultti et al. 2003, Kultti et al. submitted). Only a couple of permafrost aggradation datings fall into this period (1000 from Pechora and 770 BP from Scandinavia; paper V, P’yavchenko 1955, Vorren & Vorren 1976). Out of the investigated 38 recent palsas, 15 have formed before and survived the Medieval warm period.

A cold episode, the Little Ice Age from ca. 700 BP to 100 BP, followed the Medieval optimum (Khotinskyi 1984, Grove 2001). Grudd and co-authors (2002) report an early Little Ice Age period from 850 BP until 580 BP based on tree-ring records from northern Sweden. In Finnish Lapland the most distinguished cooling occurred between 400–250 years ago (Eronen et al. 1999b). A number of relatively reliable datings are presented for permafrost aggradation during this period: 4 datings between 600 and 160 BP for Pechora and 7 datings between 645 and 105 BP for Scandinavia (papers I–V, Vorren & Vorren 1976, Vorren 1979b, Göttlich et al. 1983, Zuidhoff & Kolstrup 2000, Oksanen 2002, Seppälä 2003). Most modern palsas have formed during the Little Ice Age (18 out of 38).

Fig. 12. Distribution of permafrost in mires at about 2000 BP, according to the available permafrost aggradation datings.

Around 150 years ago a pronounced warming is reported in Scandinavia (Eronen et al. 1999b). The last century in northern Russia has experienced a thermal maximum during the 1930’s–1940’s, a cooling by 0.6 °C during the subsequent decades (Khotinskyi 1984) and a recent warming (Chapin 1992). Some studies consider the last century as the warmest of at least the last millennium and thus warmer than the Medieval optimum (Mann et al. 1999, Crowley & Lowery 2000, Briffa 2000). P’yavchenko’s (1955) observations of large-scale permafrost collapse in northern Russia date to the late 1940’s. Nevertheless, he also refers to embryonal palsas. Palsa embryos have been discovered in

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the whole area at least in the 1950’–70’s and late 1990’s (paper V). Pounus and other novel permafrost formations have been recently reported from the interior permafrost area of Scandinavia (Seppälä 1998). Two datings, from Pechora and Scandinavia, suggest modern ages for palsas (paper III, Zuidhoff & Kolstrup 2000).

By means of peat research it is difficult to attest if the former palsa area has been larger than now, e.g., during the Little Ice Age or around 2000 BP. In the marginal permafrost regions, permafrost is very localised in mires and the preservation of its signs insecure – with good luck perhaps an earlier palsa layer could be found from the deposits of a mire outside the modern palsa area. Based on the present knowledge the only speculative indication of possible larger distribution of permafrost are the intermediate layers between fen and palsa peat in the modern high palsa mires, suggesting a plateau palsa type gradual drying instead of sudden change as expected in classical palsa formation.

Unfortunately, long-term monitoring of palsa mires is not carried out systematically. For example it is not known if the embryonal palsas observed during the last decades have matured to high palsas. Some information is available on the changes of permafrost area from the early 60’s to the late-90’s: Lindqvist (1995), Sollid & Sørbel (1998) and Zuidhoff & Kolstrup (2000) report permafrost degradation in mires of northernmost Norway (coastal lowland area), southern Norway and at the southern permafrost limit of Sweden. The authors believe that under the present climatic conditions permafrost does not form in these marginal areas. Some of the southernmost palsa mires of Finland reported by Salmi (1972) or marked on the map (Inarijärvi 1:100 000) were not found in 2001 (personal observation). Instead nearly peatless ring-shaped formations were discovered, possibly marking the place of former palsas. Salmi studied his mires in the 1960’s when Scandinavia experienced a cold climate episode (Climatological statistics… 1991), but at least one palsa probably originated from pre-Medieval times. However, Salmi’s site descriptions are not precise and possibly the exact site was not located. Luoto and Seppälä (2003) based on mapping of thermokarst ponds estimated that the former distribution of palsas was three times larger than at present in their study area in the northernmost Finland. On the other hand, in the southernmost known palsa mire of Sweden palsas have preserved from 1910 (when first studied) to present (Nihlén 2000). The latest reports on palsa mires on both sides of the White Sea are from 1920–30’s (Sumgin 1934, Leont’ev 1935); it would be interesting to know if permafrost can be found in these areas today. Frenzel (1959) indicates permafrost on the mid-run of the Pechora River, farther south than is probably possible to find it today. Fronzek and co-authors (2005) predict total disappearance of palsas from Scandinavia by the 2080s, when the mean annual temperature is expected to be 4 °C higher than now. The model does not take into account the endurance of mature palsas, but undoubtedly already 1–2 °C temperature increase will substantially reduce the permafrost area of northern Europe.

6 Conclusions

The classification of palsa mire vegetation according to landscape units (palsa top, palsa slope, flark, non-permafrost hummock and wet surface outside the palsa complexes) demonstrates that permafrost is a major contributor to the occurrence of certain vegetation associations. Furthermore, cluster analysis distinguishes relatively well (in spite of diverse source material) between different palsa mire units. Cluster analysis recognises five palsa types, albeit clustered together with other hummocks; 12 wet mire types have been differentiated. Although the same species are found on other habitats as well, some communities are very regularly found on or near to permafrost. Most notable are Dicranum elongatum – Polytrichum strictum – lichen communities on palsas and Sphagnum lindbergii or S. riparium – Eriophorum communities in flarks near to palsas, while brown-moss – Carex communities are more common in wet habitats further from palsas. Slight differences between regions can be perceived, but especially palsa vegetation is rather uniform over the whole of Europe. Some species in this study are located on palsa summits only, but due to their rareness they are not functional indicator species. Negative indicators, species never or not normally encountered on permafrost, are many. Supplementary studies on the subject are needed for detailed vegetation descriptions from palsa mires. More confident clustering results would result from complete vegetation and more precise site descriptions.

The equivalent stratigraphies to most of the modern vegetation types can be distinguished. Several peat types have no equivalences for recent mire types in palsa mires. Permafrost is also an important factor in peat accumulation rates, being responsible for both the lowest and highest recorded rates. The accumulation on old, mature palsas is very low or negative, while on incipient permafrost or in conditions of continuously unstable permafrost, the rates can be very high. The long-term average rates in permafrost mires are usually low.

This study shows that plant macrofossil analysis and radiocarbon dating can be used in detecting permafrost dynamics in peat deposits, based on typical communities, absence of species and expected changes in vegetation and peat accumulation. There can never be certainty, because of the lack of indicator species, but in many cases the permafrost dynamics can be dated with a high degree of probability. Different pathways of permafrost aggradation and degradation complicate the tracing of permafrost dynamics in

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peat deposits. Permafrost may develop in flarks causing upheaval and formation of a palsa, under Sphagnum fuscum hummocks or in flarks without (immediately) raising the mire surface. Permafrost thawing can cause peat calving, collapse within a palsa or a whole palsa sinking into a flark. The preservation of previous peat layers depends on the type of degradation. Further difficulties are provided by frequent erosion and thus hiatuses in stratigraphies. Ideally, there should be several AMS-datings from each core, due to possibly missing layers and changes in accumulation rates.

Based on available datings, permafrost aggradation in northern Pechora started no later than 3000 BP and in Fennoscandia no later than 2500 BP. First Holocene permafrost formed possibly by ca. 5000 BP in Pechora and 4000 BP in Fennoscandia, but the presented signs are too speculative as yet. Much more datings with detailed stratigraphies would be needed to solve the question of first permafrost development. The oldest preserved palsas are 2500–2000 14C-years old.

Permafrost aggradation datings generally fit well in the climate development of Europe during the Holocene. Before 5000 BP, when the climate first started to cool after the warm early and middle Holocene, there are no signs of permafrost. The climate in Europe further cooled at 3000–2500 BP and after that palsas are found over the whole study area. Possible more active periods are detected in permafrost formation around 2200–1900 BP in Pechora, 2500–1900 BP in Fennoscandia and during the Little Ice Age (700–100 BP) in the whole area. Between these periods palsa formation seems to have been rare. This picture is based on only few reliable datings, however. Western European Russia especially has remained little studied. At least some older palsas have survived over the warmer climate intervals of the late Holocene. Probably young palsas are more sensitive and new permafrost tends not to form under unfavourable climate conditions.

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