JOURNAL OF QUATERNARY SCIENCE (2009) 24(7) 802–815
Copyright ß 2009 John Wiley & Sons, Ltd.
Published online 20 February 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1254
Rapid Lateglacial tree population dynamics and
ecosystem changes in the eastern Baltic region
MAIJA HEIKKILÄ,1,2* SONIA L. FONTANA3 and HEIKKI SEPPÄ1
1
Department of Geology, University of Helsinki, Finland
2
Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario, Canada
3
Department of Geography, University of Liverpool, Liverpool, UK
Heikkilä, M., Fontana, S. L. and Seppä, H. 2009. Rapid Lateglacial tree population dynamics and ecosystem changes in the eastern Baltic region. J. Quaternary Sci.,
Vol. 24 pp. 802–815. ISSN 0267-8179.
Received 9 October 2008; Revised 1 December 2008; Accepted 4 December 2008
ABSTRACT: A growing body of evidence implies that the concept of ’treeless tundra’ in eastern and
northern Europe fails to explain the rapidity of Lateglacial and postglacial tree population dynamics of
the region, yet the knowledge of the geographic locations and shifting of tree populations is
fragmentary. Pollen, stomata and plant macrofossil stratigraphies from Lake Kurjanovas in the poorly
studied eastern Baltic region provide improved knowledge of ranges of north-eastern European trees
during the Lateglacial and subsequent plant population responses to the abrupt climatic changes of the
Lateglacial/Holocene transition. The results prove the Lateglacial presence of tree populations (Betula,
Pinus and Picea) in the eastern Baltic region. Particularly relevant is the stomatal and plant macrofossil
evidence showing the local presence of reproductive Picea populations during the Younger Dryas
stadial at 12 900–11 700 cal. a BP, occurring along with Dryas octopetala and arctic herbs, indicating
semi-open vegetation. The spread of Pinus–Betula forest at ca. 14 400 cal. a BP, the rise of Picea at ca.
12 800 cal. a BP and the re-establishment of Pinus–Betula forest at ca. 11 700 cal. a BP within a span of
centuries further suggest strikingly rapid, climate-driven ecosystem changes rather than gradual plant
succession on a newly deglaciated land. Copyright # 2009 John Wiley & Sons, Ltd.
KEYWORDS: rapid climate change; vegetation dynamics; north European trees; Lateglacial; north-eastern Europe.
Introduction
The transition from the Last Glacial to the Holocene
interglacial was characterised by abrupt and significant
environmental changes in northern Europe. During the Last
Glacial Maximum (LGM) ca. 21 000–18 000 cal. a BP the southeastern sector of the Scandinavian Ice Sheet (SIS) extended
south to central Poland, Lithuania and northern Belarus
(Fig. 1(A)) and continuous permafrost was present to 458 N
in Germany and Poland (Huijzer and Vandenberghe, 1998).
The climate on the continent south of the SIS was dry and cold,
reconstructed and modelled annual mean temperatures being
10–248C lower than modern temperatures (Tarasov et al., 1999;
Wu et al., 2007; Kim et al., 2008).
Deglaciation in the region started when the temperature rose,
but the ice margin retreat was punctuated by oscillations in
climate. Substantial warming occurred during the Bølling/
Allerød interstadial (B/A, Greenland Interstadial-1) ca. 14 700–
12 900 cal. a BP (Rasmussen et al., 2006) and the SIS margin
retreated across Estonia and southern Sweden. During the
Younger Dryas stadial (YD, Greenland Stadial-1) ca. 12 900–
* Correspondence to: M. Heikkilä, Department of Geology, PO Box 64, FI-00014,
University of Helsinki, Finland.
E-mail: [email protected]
11 700 cal. a BP (Rasmussen et al., 2006) the ice sheet margin
was positioned over southern Scandinavia (Fig. 1(A)). The YD is
probably the most well-studied abrupt climate event in the late
Quaternary, as clear cooling is pronounced in glacial, marine
and terrestrial records, centred in the North Atlantic region
(Overpeck and Cole, 2006). The mean annual temperatures
during the YD lowered 5–158C (Severinghaus et al., 1998;
Alley, 2000) and the seasonality was clearly strengthened
(Denton et al., 2005; Broecker, 2006). Permafrost affected the
land and soils as far south as 548 N and evidence for
periglacial phenomena is found to 508 N (Isarin, 1997). At the
end of the YD mean July temperatures rose 4–108C (Birks and
Ammann, 2000; Renssen and Isarin, 2001; Wohlfarth et al.,
2004).
Fossil evidence of plant communities at the Lateglacial/
Holocene transition shows distinct changes in tandem with the
climatic changes. Pollen and plant macrofossil diagrams depict
unique combinations of tundra and steppe plants colonising the
deglaciated landscape of northern Europe following the LGM
(Birks, 1986). The majority of the European temperate and
boreal trees survived the Last Glacial cold phases in the
favourable mid-altitude pockets of the mountain ranges in the
Balkan, Italian and Iberian Peninsulas (Bennett et al., 1991;
Hewitt, 1999; Tzedakis et al., 2002; Carrion et al., 2003). The
nature of the glacial and Lateglacial vegetation, including
the migration patterns and pathways of the trees and their
LATEGLACIAL TREE POPULATION DYNAMICS IN THE EASTERN BALTIC REGION
803
Figure 1 Maps showing the location of Lake Kurjanovas and other sites mentioned in the text. (A) Location of Lake Kurjanovas in northern Europe.
LGM ice extent (Svendsen et al., 2004) is shown in white and Younger Dryas ice extent (Donner, 1995) by a dotted line. (B) Three of the ice marginal
moraines in the Baltic region that are discussed in the text (Raukas et al., 1995): A, the Pomeranian moraine; B, the Middle Lithuanian moraine; and C,
the North Lithuanian (Luga, Haanja) moraine. Lakes (*) and moraine sediment sections (~) mentioned in the text: 1, Lake Kurjanovas; 2, Lake Tamula;
3, the Kurenurme section; 4, Lake Kirikumäe in the Haanja Heights; 5, the Raunis section; 6, Lake Kasučiai;
7, Lake Okono. See references in the text
ˇ
possible glacial ranges, are poorly known, especially in eastern
Europe. A general view has been that the areas north and east of
the southern mountain ranges were treeless, dominated by
plants tolerant of harsh climatic conditions. Tree pollen has
been found from the glacial deposits, but it has been considered
to be reworked from previous interglacial layers or distantly
transported. Recently, however, it has been suggested that
sparse local tree populations also existed in central and eastern
Europe during glacial times (Stewart and Lister, 2001; Willis
and van Andel, 2004; Feurdean et al., 2007). Plant community
response to Lateglacial climatic change is known to have been
very rapid (Birks and Ammann, 2000; Post, 2003; Birks and
Birks, 2008); hence potential small outlier populations could
have played an important role in Lateglacial and postglacial
plant population dynamics.
In order to investigate the Lateglacial plant communities and
their response to the major climatic changes in the relatively
unexplored south-eastern sector of the SIS, a sediment core was
obtained from Lake Kurjanovas in eastern Latvia. Here we
present pollen, stomata and plant macrofossil analyses of this
core. The objectives of the paper are (1) to investigate the
nature of the vegetation prior to the YD, including potential
occurrence of tree taxa in the region, (2) to probe the response
of plant communities to rapid environmental changes at
the onset and the end of the YD, and (3) to contribute to
understanding of the glacial distribution history and Lateglacial
and postglacial migration patterns and pathways of north
European trees.
Study area
Study site description
Our study site, Lake Kurjanovas (568 310 N, 278 590 E; 111 m
a.s.l.), is located in south-eastern Latvia, near the borders of
Russia and Belarus (Fig. 1). The basin lies on the lowland area
east of the Latgale highlands. Lake Kurjanovas is a relatively
small (1.6 km2) and shallow (maximum depth 5.8 m) lake,
Copyright ß 2009 John Wiley & Sons, Ltd.
surrounded by lush, broad-leaved and coniferous forest patches
in addition to open fields and pastures. Mean annual
temperature at the nearest meteorological station in Daugavpils
(558 520 N, 268 370 E; 122 m a.s.l.) is þ5.58C with seasonal
variability from 6.78C in January to þ16.88C in July. The
landscape in temperate Europe is largely agricultural and the
largest patches of forest remain in the northern and eastern parts
of the continent. The most common deciduous species are
birches (Betula), alders (Alnus) and aspen (Populus tremula),
but thermophilous trees are also common, such as elms
(Ulmus), pedunculate oak (Quercus robur), ash (Fraxinus
excelsior), small-leaved lime (Tilia cordata) and hazel (Corylus
avellana).
Study area topography and deglaciation
dynamics
The relief of Latvia is slightly undulating, with moderate
absolute and relative variations in elevation. The average
elevation is 87 m (Zelcs and Markots, 2004). The highest
elevations, around 300 m, occur in the bedrock highlands of
Vidzeme, Latgale and Alu
¯ksne, where the glacigenic overburden can be up to 160 m thick. The topography results mostly
from the glacigenic processes of the Pleistocene glaciations.
During the last (Weichselian) glaciation Lake Kurjanovas was
located in the peripheral zone of the SIS. The ice marginal
moraine marking the LGM extent of the SIS (Fig. 1(A)) lies
100 km south-east of the study site. The SIS started to retreat
north-west from its maximum position in Lithuania and Belarus
ca. 20 000–18 000 cal. a BP (Velichko and Faustova, 1986;
Raukas et al., 1995; Rinterknecht et al., 2006). The retreat was
marked by stagnation and readvance, leaving behind several
successive ice marginal moraines before the SIS margin
reached its YD position in southern Scandinavia (Fig. 1(A)).
Pomeranian, Middle Lithuanian and North Lithuanian moraines were deposited in the aforementioned order at ca. 18 000,
ca. 17 000 and ca. 15 500 cal. a BP (Raukas et al., 1995; Ehlers
et al., 2004; Zelcs and Markots, 2004).
J. Quaternary Sci., Vol. 24(7) 802–815 (2009)
DOI: 10.1002/jqs
804
JOURNAL OF QUATERNARY SCIENCE
Lake Kurjanovas is situated between the Middle and the North
Lithuanian (Luga, Haanja, Linkuva) moraines (Fig. 1(B)). The
North Lithuanian moraine has conventionally been used as
the marker of the last short ice stagnation before the final
deglaciation of Latvia. The determination is based on the
radiocarbon ages from the organic-bearing sediments of
the Raunis beds in north-central Latvia (Fig. 1(B)), which centre
around 13 000 14C a BP (ca. 15 500 cal. a BP), i.e. just before the
start of the B/A interstadial (Punning et al., 1968; Stelle et al.,
1975; Ceriņa and Kalniņa, 2000; Stelle and Savvaitov, 2002;
Zelcs and Markots, 2004). Additional support for the B/A age of
the final deglaciation of Latvia comes from the Estonian
chronostratigraphy. The Kurenurme section on the proximal
side of the Haanja moraine (Fig. 1(B)) has been dated by optically
stimulated luminescence (OSL) to ca. 14 500 cal. a BP (Kalm,
2006), which is consistent with radiocarbon dates ca. 14 500–
15 000 cal. a BP of the intermorainic plant remains from the
same section (Liiva et al., 1966). Correlation of the clay varve
magnetostratigraphy from the bottom sediments of Lake Tamula
(Fig. 1(B)), about 20 km west of the Kurenurme section, yielded
an age of 14 675 cal. a BP for deglaciation of the lake catchment
(Sandgren et al., 1997; Kalm, 2006). Thus the age of the
North Lithuanian moraine in Latvia and Estonia lies between
ca. 14 500 and 15 500 cal. a BP. As the Middle Lithuanian
moraine has an age of ca. 17 000 cal. a BP, the final deglaciation
of Lake Kurjanovas took place well before the B/A warming.
The nine boulder exposure ages calculated from
9
Be/10Be ratios, however, resulted in considerably younger
ages than mentioned above. For the Middle Lithuanian and the
North Lithuanian moraine the mean ages were 13 600 300
and 13 100 300 a, respectively (Rinterknecht et al., 2006).
Thus the Middle and the North Lithuanian moraines would
have been deposited during the B/A, suggesting a stagnation of
the ice sheet during the period generally regarded as the turning
point for substantial warming. These implications are contradictory to the established palaeobiotic and palaeogeographical
setting of northern Europe during the B/A (Iversen, 1954;
Björck, 1995; Houmark-Nielsen et al., 2006).
Methods
Sediment coring, description and constructing
the chronology
Lake Kurjanovas was cored at the point of the greatest water
depth while still ice covered in March 2005, using a Russian
corer equipped with a barrel of 10 cm in diameter and 1 m in
length. An 8 m sequence of sediment was recovered, in
addition to a 2 m replicate sequence from the bottom
sediments. Sediment cores were correlated visually and
described in detail in the laboratory. Cores were subsampled
at 1 cm intervals and stored in the cold room at þ48C. The long
core (interval 1016–1148 cm) was used for pollen sample
preparation and the bottom replicate (interval 1015–1147 cm)
for macrofossil analysis.
Organic matter and carbonate content of the sediment were
estimated by loss on ignition (LOI). Samples of about 1–2 g of
wet sediment were analysed at 2–4 cm intervals. Water content
was estimated by oven-drying the samples for 24 h at 1058C.
Samples were heated in a muffle furnace for 5 h, first at 5008C
and then at 9508C. Following Heiri et al. (2001), results are
expressed as percentages of weight loss of the sediment in each
step in relation to the dry weight of the samples before
combustion. Sediment colours were determined using Munsell
soil colour charts.
Six samples of terrestrial and telmatic plant macrofossils were
radiocarbon dated by accelerator mass spectrometry in the
University of Uppsala Tandem Laboratory and Poznan Radiocarbon Laboratory. The resulting dates were calibrated using
the IntCal04 curve (Reimer et al., 2004) and Oxcal 4.0 program
(Bronk Ramsey, 1995, 2001) (Table 1). A Bayesian age–depth
model was created using the deposition model implemented in
Oxcal 4.0 (Bronk Ramsey, 2008), with a k parameter
(increments per units of length) of 100 m1. Sample dates that
clearly deviated from the overall age–depth relations were not
included in the model. Rejection was based on the obvious
incompatibility with the known biostratigraphic boundaries
(see below), large uncertainty (>100 a), reversal in the
stratigraphic order or a combination of these factors (Table 1).
Dating discrepancies do not attest to systematic behaviour.
However, plant tissue type has commonly been reported as a
source of dating error for various reasons. Among Lake
Kurjanovas dating results, conifer bark (1046–1047 cm) and
conifer wing-seed (1081–1082 cm) appear younger than their
context, and mostly telmatic plant material (1089–1090 cm)
older than its context, respectively (Table 1). The intrusive age
of the conifer bark could be due to material that was derived
from roots of the tree instead of the tree trunk. The intrusive age
of the conifer wing-seed, in turn, could result from high surface
area/volume ratio that makes the sample susceptible to modern
carbon contamination. In general, dating of filamentous or flat
plant tissues can result in younger ages than dating of woody
samples and seeds (Turney et al., 2000; Oswald et al., 2005). In
addition, contamination by modern carbon usually leads to
Table 1 AMS radiocarbon and calibrated dates for the Kurjanovas macrofossil samples. Calibration was performed with OxCal 4.0 program (Bronk
Ramsey, 1995, 2001), using IntCal04 calibration curve (Reimer et al., 2004). The reasons of rejecting an age are specified, residual meaning an older
age than the context, intrusive a younger age than the context and reversal a younger age than dated sample(s) above
Depth
Laboratory
number
Material
dated
1021–22
Poz-18408
1029–30
1046–47
1081–82
Poz-18410
Ua-32639
Ua-32640
Pinus bark, Pinus bud-scales,
Betula pendula/pubescens
fruits, and female catkin bract
Petiole of cf. Betula
Conifer bark
Conifer wing-seed
1089–90
Poz-18411
1115–16
Ua-32641
Plant material:
mostly telmatic
Telmatic plant leaves
Copyright ß 2009 John Wiley & Sons, Ltd.
Age
14
C a BP
2s
age range
Median age
(cal a BP)
8 340 40
9 470–9 260
9 363
8 840 50
8 035 140
9 205 275
10 159–9 705
9 896–8 551
11 090–9 675
9 928
8 910
10 422
10 600 60
12 805–12 397
12 663
10 930 75
13 034–12 815
12 892
Grounds for
rejection
Large uncertainty, reversal
Large uncertainty, intrusive: falls to the
beginning of the Holocene in the
biostratigraphy
Residual: falls to the beginning of the
Holocene in the biostratigraphy
J. Quaternary Sci., Vol. 24(7) 802–815 (2009)
DOI: 10.1002/jqs
LATEGLACIAL TREE POPULATION DYNAMICS IN THE EASTERN BALTIC REGION
more significant dating errors due to higher 14C activity of
young carbon compared to older carbon for equal amount of
contaminant. Nevertheless, incorporation of hard-water
derived old carbon in plant tissues of the telmatic vegetation
at 1089–1090 cm is a probable explanation for the residual age,
in contrast to the sample of telmatic leaves (1115–1116 cm)
deposited during the time of carbonate deposition minimum
(Fig. 3).
In addition to the selected radiocarbon dates, the ages
according to the GICC05 timescale, derived from the Greenland ice core records (Rasmussen et al., 2006), were used in the
model for the following stratigraphic points of reference: the
beginning of the B/A, the boundary between the YD and the B/
A, the end of the YD and the 8200 cal. a BP cold event. These
levels were assigned based on the typical features in pollen
stratigraphies from the region (Ilves and Mäemets, 1996;
Velichko et al., 1997; Kabailiene, 1998; Goslar et al., 1999;
Latalowa and Borówka, 2006), Pinus and Artemisia curves
being the key references. The beginning of the B/A (14 700 cal.
a BP) was assigned a stratigraphic depth of 1128 cm, marked by
the rise in the Pinus pollen curve coupled with the appearance
of Pinus macrofossils, and the fall in Artemisia and in
Chenopodiaceae curves and disappearance of Dryas leaves.
The boundary between the B/A and the YD (12 900 cal. a BP)
was assigned to the level 1112 cm, characterised by the rising
frequencies of herbaceous and shrub pollen, especially
Artemisia, Chenopodiaceae and Juniperus, and the decreasing
frequencies of Pinus pollen. The end of the YD (11 700 cal. a
BP) was placed to 1084 cm, where (tree) Betula and Pinus
curves rise, accompanied by the abundance of their macrofossils, at the same time as the decline of Artemisia,
Chenopodiaceae and Juniperus. The 8200 cal. a BP event
was assigned to the stratigraphic depth of 980 cm (not shown in
the figures), where marked decline in temperate trees,
particularly Corylus and Alnus, takes place. The abrupt decline
of thermophilous trees during this cold climate event is a
common feature in the pollen diagrams from the region (Veski
et al., 2004; Seppä et al., 2007).
805
program C2, version 1.5.0 (Juggins, 2007), applying a squareroot transformation on the species data.
Plant macrofossil analysis
Plant macrofossil remains were analysed at 1–2 cm intervals.
Sample size was 8–21 cm3 of wet sediment and all samples had
a stratigraphic thickness of 1 cm. Samples were soaked
overnight in 5% NaOH and sieved through a 150 mm mesh.
Plant macrofossil remains were identified with reference to
Bertsch (1941), Bialobrzeska and Truchanowiczowna (1960),
Tomlinson (1985), Grosse-Brauckmann and Streitz (1992) and
Van Dinter and Birks (1996).
Results
Chronology
Results and calibration of the radiocarbon ages are shown in
Table 1. As explained above, several radiocarbon dates of
macrofossils were not used for the age–depth model, but
application of those considered reliable, together with the
known ages of the biostratigraphic boundaries, supported
development of a satisfactory age–depth model (Fig. 2).
However, the pre-B/A part of the chronology should be
considered with caution, as the deposition rates in a newly
formed, unstable glacial lake system may be highly variable
(Cohen, 2003). The lowermost sediments of the Lake
Kurjanovas core must be younger than the Middle Lithuanian
moraine (ca. 17 000 cal. a BP) (Fig. 1(B)), and are probably
substantially older than the age of the North Lithuanian
moraine (ca. 14 500–15 500 cal. a BP) (Fig. 1(B)). It is difficult to
determine the basal age of the sediments precisely based on the
deglaciation history as the initiation of lacustrine sedimentation
may be delayed by several hundred years (Warner et al., 1991).
If the pace of the ice recession from the Middle Lithuanian
moraine to the North Lithuanian moraine was fairly uniform
Pollen and stomatal analysis
Pollen samples were prepared with the standard methods of
KOH, HF and acetolysis, stained with safranin and mounted in
silicone oil for the microscope slide preparation. At least 700
terrestrial pollen grains from each sample were identified to the
highest possible taxonomic level, employing the nomenclature
of the northern European pollen–climate calibration set (Seppä
et al., 2004). The following taxa, which are not included in the
calibration set, were identified according to Moore et al.
(1991): Helianthemum, Athyrium distentifolium-type and
Thelypteris palustris. The remains of conifer stomata were
identified from the pollen microscope slides to species level
(Picea abies, Pinus sylvestris) when possible; identification was
based on Sweeney (2004). The pollen diagram was zoned by
optimal partitioning (Birks and Gordon, 1985) using the
program ZONE version 1.2 (S. Juggins, unpublished).
Ordination analysis was carried out on the pollen data to
facilitate interpretation of the vegetation shifts. To estimate the
linearity of the latent gradients in the data, detrended
correspondence analysis (DCA) was carried out using the
program CANOCO, version 4.0 (ter Braak and Smilauer, 1998).
The longest DCA axis gradient length was <2.0 standard
deviation units (1.54), and thus the linear ordination method
(principal component analysis, PCA) was chosen (ter Braak,
1995). PCA for the pollen data was carried out using the
Copyright ß 2009 John Wiley & Sons, Ltd.
Figure 2 Age–depth model showing medians of calibrated radiocarbon ages with 2s probability range in addition to incorporated biostratigraphic markers
J. Quaternary Sci., Vol. 24(7) 802–815 (2009)
DOI: 10.1002/jqs
806
JOURNAL OF QUATERNARY SCIENCE
and the lake formed without major delays, the basal age of the
sediments, just north of the Middle Lithuanian moraine, would
be ca. 16 000–17 000 cal. a BP. Our age–depth model gives a
basal age of ca. 16 400 cal. a BP, which agrees with this
interpretation.
Sediment stratigraphy
The Lake Kurjanovas sequence can be divided into clearly
visible sedimentary units consisting of variable amounts of
mineral and organic matter (Fig. 3). The lowermost horizon
(1148–1123 cm; ca. 16 400–13 900 cal. a BP) is composed of
sand, silt and clay, overlain by a unit of clayey organic matter
with telmatic plant leaves (1123–1111 cm; ca. 13 900–12 800
cal. a BP). The overlying horizon (1111–1079 cm; 12 800–
11 600 cal. a BP) is again mineral-rich, composed of clay,
followed by a thin layer of organic matter with Sparganium
seeds (1079–1072 cm; 11 600–11 300 cal. a BP), a carbonaterich clay unit (1972–1039 cm; 11 300–10 200 cal. a BP), and
then grading from clayey to highly organic-rich in the
uppermost sedimentary units.
The above-mentioned units of organic and mineral matter
are apparent in the LOI results as well (Fig. 3). Organic-rich
(20–60%) horizons were deposited ca. 13 800–128 00 and ca.
11 700–11 200 cal. a BP and organic matter content increases
up to 80% at ca. 9700 cal. a BP. In addition, a highly
carbonaceous (20–28%) layer was deposited ca. 11 300–
10 200 cal. a BP. These results are consistent with the
biostratigraphic boundaries: mineral-rich sediment layer at
1148–1123 cm was deposited before the B/A, organic-rich
layer at 1123–1111 cm during the B/A, mineral-rich layer at
1111–1079 cm during the YD and organic gyttja at 1079–
1072 cm in the Holocene.
Biostratigraphy and vegetation changes
The ages for the biostratigraphic boundaries used as the input to
the age–depth model (see above) are applied in the following
text to stress the climatostratigraphic stages. The stages differ
only slightly from optimal partitioning pollen assemblage zones
(PAZ), except for the lowest boundary (between KU1 and KU2
PAZ or the beginning of the B/A climatostratigraphic stage). The
optimal partitioning PAZ boundary is 500 a younger than the
interpreted biostratigraphic marker point at the start of the B/A.
This is due to the fact that pollen assemblage zonation does not
take into account the macrofossil data used in assessing the
biostratigraphic boundaries, in addition to gradual increase
of Pinus pollen proportion that complicates the interpretation
of the actual rise.
Before Bølling/Allerød > ca. 14 700 cal. a BP (¼ PAZ KU-1)
Figure 3 Sediment lithology and loss on ignition at 500 and 9508C,
reflecting proportion of organic matter and proportion of carbonates,
respectively
Copyright ß 2009 John Wiley & Sons, Ltd.
Pollen assemblages of the time before the B/A consist largely of
herbaceous grassland and tundra plants such as Artemisia (5–
12%), Gramineae (5–10%), Chenopodiaceae (2–5%) and
Rumex/Oxyria-type (Fig. 4). Remains of the leaves of Dryas
octopetala, a light-demanding arctic-alpine tundra plant, are
abundantly present in the macrofossil sequence (Fig. 5). Tundra
shrubs are also present: Juniperus (2–6%) and Salix (5%), in
addition to Betula pollen (25%) and macroremains (fruit, budscale), which can reflect the presence of dwarf birch (Betula
nana). The lake level was probably low, as remains of telmatic
(Equisetum up to 12%, Carex-type 5–14%, Juncus seeds
and a Phragmites fruit) and aquatic (Potamogeton subg.
potamogeton and Myriophyllym spicatum pollen) flora are
found.
The terrestrial plant communities were probably more
tundra-like than steppe-like, as Dryas octopetala is present
through the whole period, in addition to the other tundra
shrubs. The pollen of Artemisia and Chenopodiaceae can be
carried vast distances aerially in an open landscape (Liu et al.,
2008). On the other hand, their macrofossils are rarely present,
and thus it is hard to assess their local presence or absence. The
abundance of these taxa probably demonstrates windiness,
dryness and soil disturbance further south and east in the
Russian plain.
The landscape after deglaciation was certainly open, but
whether the landscape was treeless is more difficult to judge. If
trees were present, they must have been sparse. Betula pollen
(25%) and macrofossils can represent either dwarf or tree
birches. The Betula pollen proportion is probably too low to
represent local populations in an open landscape setting, as
are Pinus pollen values (15–30%). The Pinus curve rises
steadily towards the beginning of the B/A, probably reflecting
the approaching Pinus populations from the neighbouring
J. Quaternary Sci., Vol. 24(7) 802–815 (2009)
DOI: 10.1002/jqs
LATEGLACIAL TREE POPULATION DYNAMICS IN THE EASTERN BALTIC REGION
807
Figure 4 Percentage diagram of main pollen taxa (above) and selected herbaceous and aquatic taxa, Sphagnum and Pediastrum (below) against age
(cal. a BP). Pollen assemblage zones (PAZ) KU1–4 based on optimal partitioning are shown on the right-hand side
regions. Pinus macrofossils are absent, but stomata were
found in one sample. These can represent a local stand of
trees, but it is also possible that the stomata were redeposited
and washed in from the overburden in the catchment.
Redeposition is very probable in the case of the pollen grains
of thermophilic tree taxa (Alnus, Corylus, Ulmus, Tilia), which
appear clearly degraded compared to other concurrently
occurring grains.
Copyright ß 2009 John Wiley & Sons, Ltd.
Bølling/Allerød ca.14 700–12 900 cal. a BP (¼ PAZ KU1 until
14 200 cal. a BP and PAZ KU2)
A clear change in the plant communities took place in the
beginning of the B/A. The B/A seems to have been a rather lush
and moist phase in comparison with the earlier glacial
vegetation. Assemblages are characterised by the suppression
of tundra and steppe taxa and the growing dominance of Pinus
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Figure 5 Diagram of plant macrofossils (number of specimens per cm3) and stomata (number of specimens per 700 counted pollen grains) against age
(cal. a BP). Pollen assemblage zones (PAZ) KU1–4 of Fig. 3 are shown on the right-hand side
(Figs. 4 and 5). Aquatic and telmatic plants (e.g., Sphagnum,
Equisetum and Potamogeton subg. potamogeton) are reduced,
suggesting higher lake levels.
The change from an open landscape to a relatively dense
forest appears to have been fairly rapid. The prevalent role of
Pinus both in pollen (30–60%), stomatal and macrofossil
(bark, bud-scales, needle, anther) assemblages attest to its
dominance in the local and regional landscape. The rapid
decline in dry- and cold-favouring taxa (Artemisia, Chenopodiaceae, Juniperus) and the absence of the light-demanding Dryas octopetala is consistent with the development of
closed or at least semi-closed Pinus forest. However,
deducing the timing of the establishment of the relatively
dense forest is not straightforward, as Pinus pollen values rise
gradually, possibly indicating ongoing processes of forest
closure and population invasion. Coupling the pollen data
with the appearance of Pinus macrofossils and stomata
(Fig. 5) indicates that the closure of the Pinus forest took place
14 400 cal. a BP. Betula also seems to have been a common
element in the B/A forests: Betula pollen proportion is around
30% and tree Betula fruits are present in the macrofossil
assemblages.
Younger Dryas ca. 12 900–11 700 cal. a BP (¼ PAZ KU-3)
The change in vegetation was abrupt at the transition between
the B/A and the YD. Open communities occupied the land and
the Pinus–Betula woodland was reduced. Pinus values drop to
10% and remain at that level (Fig. 4). Betula, instead, rises
from 15% to 45% towards the Holocene and its fruits are
found in the latter part of the YD (Fig. 5).
The shrubs and herbaceous taxa indicate openness and
tundra-like conditions, with light-demanding Dryas octopetala
(leaves, also a few pollen grains) and Helianthemum (0.2%),
Juniperus (5–20%), Salix (1.5–4%), various fossils from the
families Caryophyllaceae (Lychnis viscaria-type and Dianthustype, Caryophyllaceae seed) and Asteraceae (Achillea-type,
Copyright ß 2009 John Wiley & Sons, Ltd.
Solidago-type) (Figs. 4 and 5). Also grasses (Gramineae 4–
10%) and steppe elements (Artemisia values 8–19%,
Chenopodiaceae 3–6%) are abundant, but it is difficult to
assess their importance in the local landscape.
A striking feature is the presence of Picea both in the pollen
and macrofossil assemblages. Picea pollen values increase over
200 a from 0% to 10–20%, reaching 24% at its highest
(Fig. 4). The pollen grains were well preserved, showing no
signs of redeposition (Fig. 6(E)). Two Picea stomata (Fig. 6(C)
and (D)) and two unidentified conifer stomata were noted on
the pollen slides. A Picea needle and a conifer wing-seed
were recovered (Fig. 6(A) and (B)). Other Picea macrofossils
(a needle, a bud-scale and a wing-seed) occur at the boundary
of the YD and the early Holocene.
To indicate the local presence of Picea, the pollen threshold
of 1–5% (Huntley and Birks, 1983; Hicks, 1994; Giesecke and
Bennett, 2004; Latalowa and van der Knaap, 2006) or the rise in
the continuous pollen curve (Hafsten, 1992) has been used.
Both criteria are satisfied in the Kurjanovas data, and the
macrofossil and stomatal evidence confirm that Picea was a
significant component in the local vegetation. Yet the
proportion of herbs, grasses and shrubs shows that the
landscape was mostly open and that Picea trees formed local
stands or forest patches.
Sphagnum mosses and Pediastrum algae also appear during
the YD, and telmatic and aquatic flora are more abundant
(Juncus sp. seeds, Typha sp. seeds, Nymphaea sp. seed,
Myriophyllum spicatum-type pollen). This could indicate
subsiding lake levels, or a change in the substrate or nutrient
conditions of the lake. Particularly interesting is the presence of
Typha seeds, as at present the plant needs above 12–158C July
mean temperatures (Isarin and Bohncke, 1999; Yu, 2000).
Early Holocene >11 700 cal. a BP (¼ PAZ KU4)
In the beginning of the Holocene, the open landscape quickly
became forested. The forest closure took place rapidly, as
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LATEGLACIAL TREE POPULATION DYNAMICS IN THE EASTERN BALTIC REGION
809
Figure 6 Photos of (A) a Picea needle, ca. 12 200 cal. a BP; (B) a conifer wing-seed, ca. 11 700 cal. a BP; (C) a Picea stoma, ca. 12 100 cal. a BP; (D) a
Pinus stoma, ca. 11 500 cal. a BP; (E) a Picea pollen grain, ca. 12 200 cal. a BP
tundra shrubs and herbs such as Juniperus and Dryas octopetala
and steppe taxa (Artemisia, Chenopodiaceae) were replaced by
Pinus (25–50%) and Betula (35–65%) (Fig. 4). A variety of
macrofossils of both of these trees occur abundantly, tree Betula
fruits being remarkably plentiful (Fig. 5).
Picea pollen values fall back to the background level of <1%,
but fossil Picea bud-scales are present continuously. Picea was
probably present locally, but its abundance in the newly
formed forests of the region is more difficult to assess. Picea
pollen is underrepresented in modern boreal forests, especially
when Betula and Pinus are dominant, the relative pollen
representation being 0.3 of that of Pinus and 0.2 of Betula
(Parsons et al., 1980; Sugita et al., 1999). The pronounced
decline in the Picea curve may be a result of both the decrease
of its proportion in the regional vegetation and reduced sexual
regeneration and pollen productivity in the more oceanic
climate of the early Holocene.
Populus macrofossils are present together with Populus
pollen (Figs. 4 and 5). Populus may have been a common
element in the early Holocene Betula–Pinus forests, as well as
Sorbus aucuparia, whose pollen occurs occasionally. Temperate and broad-leaved tree taxa are also apparent in the
pollen assemblages, Corylus and Ulmus curves rising first at
ca. 11 000 cal. a BP, Alnus, Quercus and Fraxinus becoming
frequent at ca. 10 100 cal. a BP and Tilia later at ca. 9400 cal. a
BP (Fig. 4).
Pollen and fossil hair of the light-demanding Hippophaë
rhamnoides occur (Figs. 4 and 5), a common feature in the
records of the newly deglaciated terrain in northern Europe.
This implies that the landscape was not yet fully covered by
forests, as postglacial reforestation in Europe led to range
fragmentation and suppression of Hippophaë rhamnoides
populations (Bartish et al., 2006). On the other hand, Lake
Kurjanovas pollen and macrofossil evidence shows that
herbaceous taxa are greatly reduced from their Lateglacial
frequencies. Spores of various ferns are present (Dryopteristype, Dryopteris filix-mas-type, Athyrium filix-femina, Athyrium distentifolium-type, Thelypteris palustris and GymnocarCopyright ß 2009 John Wiley & Sons, Ltd.
pium dryopteris) (Fig. 4), demonstrating a change in the type of
undergrowth in the early Holocene forests.
Aquatic plants became increasingly frequent. Warmthdemanding taxa such as Nymphaea, Typha and Najas were
present instead of more cold-tolerant species like Myriophyllum. The beginning of the Holocene was commonly the time
when aquatic plants rapidly colonised lakes, but responses to
the climatological and hydroecological changes seem to have
been lake-specific (Birks, 2000; Sawada et al., 2003; Väliranta,
2006).
Discussion and conclusions
Abrupt and stepwise nature of ecosystem
changes
Lake Kurjanovas was established on recently deglaciated
terrain at 16 400 cal. a BP. The vegetation initially comprised
typical periglacial tundra plants. The open tundra community
was replaced by a dense Pinus–Betula forest during the B/A at
ca. 14 400 cal. a BP, the B/A forest by a tundra-like herb and
shrub vegetation with localised Picea populations in the YD at
ca. 12 900 cal. a BP, and finally the YD community by the early
Holocene Betula–Pinus forest at ca. 11 700 cal. a BP. The
systematic clustering of the pollen sample scores on PCA axes 1
and 2 plotted in Fig. 7 demonstrates the compositional
divergence between the time periods. Pre-B/A, B/A, YD and
early Holocene samples each have their own separated clusters
in two-dimensional space, but a directional change in time is
also observed inside each phase, with the exception of the
oscillation in the early Holocene. In fact, the early Holocene
samples from ca. 9800–9400 cal. a BP plot close to the B/A
cluster, showing the similarity of the Pinus–Betula forests
dominating at both times. The abrupt shifts in local plant
communities are also clearly apparent in the macrofossil
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JOURNAL OF QUATERNARY SCIENCE
Figure 7 PCA plot showing the sample scores on the first (PC1) and
second (PC2) axes. Samples (*) of each climatostratigraphic stage are
connected in stratigraphic order and shaded in grey (except for the YD).
Sample depths are not shown for every sample of the early Holocene
cluster due to label crowding
sequence, which shows virtually immediate transitions at the
climatostratigraphic boundaries (Figs. 5 and 8).
Comparison of the rapid ecosystem changes with the
climatostratigraphic stages illustrated in the d18O stratigraphy
from the Greenland NGRIP ice core (Fig. 8) suggests that the
ecosystem development does not reflect gradual plant succession on the deglaciated terrain (Iversen, 1958; Birks, 1986) but
is predominantly driven by rapid Lateglacial climate changes.
During the ca. 1700 a of the B/A interstadial, the inferred
presence of Pinus forest suggests that summer temperatures
were higher than 128C (Kultti et al., 2006), which is consistent
with the reconstructed and modelled July mean temperatures of
13–208C in western and central Europe (Renssen and Isarin,
2001; Peyron et al., 2005; Heiri et al., 2007). The summer
temperature decrease of 2–108C at the onset of the YD (Birks
and Ammann, 2000; Marshall et al., 2002; Peyron et al., 2005;
Larocque and Finsinger, 2008) is apparent in the decline of
Pinus–Betula forest and its replacement by predominantly open
tundra-like landscape with local presence of Picea for the
following ca. 1200 a.
How was Picea able to reproduce and advance to new
regions during the cold YD stadial, when other forest elements
were replaced by cold-adapted tundra and steppe flora? The
most common Picea species in Europe is Norway spruce, Picea
abies (L.) Karst. Picea abies is a boreal tree with more
Figure 8 Summary diagram of the environmental stages and associated ecological patterns in the study region. The records of key pollen (%) and
macrofossil (specimens per cm3) taxa, Pinus and Picea stomata (specimens per pollen sample) are shown. The NGRIP ice core d18O (with respect to
Vienna Standard Mean Ocean Water) record provides a general climate background, indicating the rapid climatic shifts during the Lateglacial (North
Greenland Ice Core Project (NGRIP) members, 2004; Rasmussen et al., 2006). The proportion of organic matter determined by LOI at 5008C shows
mostly inorganic deposition during the cold climatic stages, except for the low percentages in the early Holocene due to carbonate deposition (see
Fig. 3). Interpretations of the stages shown on the right-hand side: (1) before B/A, open tundra characterised by Dryas octopetala, a light-demanding
arctic herb; (2) B/A, Pinus–Betula forests, probably a closed or semi-closed forest as the light-demanding taxa disappear; (3) YD, semi-open tundra
characterised by the reappearance of Dryas, possibly by light-demanding steppe taxa Artemisia and Helianthemum, and by the replacement of Pinus
and Betula forest by scattered Picea stands, suggesting permafrost conditions and extremely continental climate; (4) rapid early Holocene warming
with re-establishment of the Betula–Pinus forest and disappearance of the arctic taxa
Copyright ß 2009 John Wiley & Sons, Ltd.
J. Quaternary Sci., Vol. 24(7) 802–815 (2009)
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LATEGLACIAL TREE POPULATION DYNAMICS IN THE EASTERN BALTIC REGION
continental distribution than Pinus sylvestris or the two tree
Betula species (Dahl, 1998; Gervais et al., 2002). In the present
range of Picea in Fennoscandia the mean temperature of the
coldest month is lower than 1.58C and the sum of growing
degree days is over 600 GDD (Prentice and Helmisaari, 1991;
Miller et al., 2008), which is higher than for boreal tree species
in general. Climate reconstructions of the YD indicate a drop in
summer temperatures of only 2–108C on the European
continent (see references above). Also the presence of Typha
in our record during the YD suggests that the summer
temperature decline was moderate in the region. Winters
were, however, drastically colder. The amplification of
seasonality by 10–208C compared to the B/A (Renssen et al.,
2001; Denton et al., 2005; Lie and Paasche, 2006), as well as
the permafrost conditions, favoured Picea and prevented the
growth of Pinus and Betula, which are less tolerant of cold
winters and permafrost. In addition, the high Picea pollen
values during the YD can be explained by the dryness of the YD
(Goslar et al., 1999; Magny et al., 2001), as seed and pollen
production of Picea abies is correlated with dry, warm
summers, while wet summers encourage vegetative growth
(Selås et al., 2002). The disappearance of Picea pollen in the
beginning of the Holocene and the readvance of Betula–Pinus
forest may indicate that the shift from the YD was foremost a
shift in winter temperatures and from a continental climate
during the YD to a more oceanic climate in the early Holocene
(Giesecke et al., 2008).
The rapid climate-driven vegetation changes in the Lake
Kurjanovas record are in accordance with recent arguments for
immediate to rapid response times of plant associations to
climate change (Birks and Ammann, 2000; Post, 2003), as
opposed to delayed, gradual spread (Davis and Botkin, 1985;
Payette, 2007). It has been argued that the rapid response
potential is a typical feature of ecotonal regions (Loehle, 2000;
Payette et al., 2001). This is well demonstrated in the past and
recent dynamics of the Northern Hemisphere ecotonal
ecosystems, for example the altitudinal vegetation belts in
northern Fennoscandia (Seppä et al., 2002) and the Swiss Alps
(Ammann et al., 2000; Tinner and Kaltenrieder, 2007), the
treeline in eastern and northern Canada (Williams et al., 2002;
Danby and Hik, 2007) and the outermost coast of western
Norway (Birks and Birks, 2008). Lake Kurjanovas is situated on
the flat forested terrain of the eastern Baltic region, yet the
stratigraphy suggests rapid ecosystem responses to changing
climate. This implies that during the Lateglacial the lake may
have been located ecotonally within the latitudinal vegetation
zones south of the retreating SIS.
Climate influences on multiple biotic processes and the
inherently complex character of ecosystems can create
nonlinearity and threshold-type responses (Burkett et al.,
2005; Morin et al., 2008). Vegetation shifts may not be simply
forced by rapid changes in temperature, but by more complex
climate-controlled changes in the environment. Apparently the
Lateglacial to Holocene transition near the south-eastern
margin of the receding SIS involved active landscape
restructuring, pedogenesis and permafrost fluctuations. Based
on current data these interactions are challenging to disentangle, but clearly the ecosystem changes in the vicinity of Lake
Kurjanovas were rapid and predominantly climate-driven.
Eastern Baltic Lateglacial populations of tree
Betula, Pinus sylvestris and Picea abies
The prevailing concept has been that as the climate became
gradually warmer during the Lateglacial, more warmthCopyright ß 2009 John Wiley & Sons, Ltd.
811
demanding species spread over the European continent (Birks,
1986). These species are known to have moved back and forth
from their southern locations during Quaternary climatic
fluctuations (Willis and van Andel, 2004). The established
picture of the Lateglacial spread of trees in Europe from their
southern refugia in Italian, Iberian and Balkan peninsulas
(Bennett et al., 1991) has recently become complicated by
accumulating evidence based on well-dated pollen stratigraphies and plant charcoal material indicating the presence of
several tree species in the Lateglacial environments of central,
eastern and northern Europe (Giesecke and Bennett, 2004;
Willis and van Andel, 2004; Latalowa and van der Knaap,
2006; Feurdean et al., 2007; Wohlfarth et al., 2007).
In the south-eastern sector of the SIS, pollen and plant
macrofossil stratigraphies do not often extend back to the
Lateglacial, as the continental ice sheet covered most of
northern Europe. Where the Lateglacial records are available,
tree Betula and Pinus have normally been reported as the first
colonising tree taxa (Pirrus, 1969; Stancikaite et al., 2004;
Latalowa and Borówka, 2006), as in Lake Kurjanovas. There is
also evidence of the presence of Alnus and Picea (Ilves and
Mäemets, 1996; Saarse and Rajamäe, 1997; Latalowa and van
der Knaap, 2006; Stancikaite et al., 2008), although the low
Lateglacial pollen percentages of Picea, Alnus and other
temperate tree taxa are most often attributed to long-distance
transport or redeposition (Saarse et al., 1999), as pollen is
usually present sparsely and the grains are often somewhat
degraded.
Genetic analyses have recently provided new evidence of
the glacial history of European tree species. It is shown that the
most common chloroplastic DNA (cpDNA) haplotypes of
modern European Betula (B. pendula Roth, B. pubescens Ehrh.)
populations are clearly separated in north-eastern and northwestern geographical domains, and that the southern haplotypes of the Balkan, Italian and Iberian peninsulas do not occur
north of the Alps (Palmé et al., 2003, 2004; Maliouchenko
et al., 2007). A similar situation is suspected for modern Pinus
sylvestris L. populations, as the mitochondrial DNA (mtDNA)
haplotypes of this tree in the Iberian and Italian peninsulas are
restricted to northern Europe (Sinclair et al., 1999; Cheddadi
et al., 2006; Pyhäjärvi et al., 2008). There is also evidence of
full-glacial and Lateglacial core populations in the Danubian
plain in eastern central Europe (Willis and van Andel, 2004;
Cheddadi et al., 2006) and mtDNA indications of population
origins in north-eastern Europe (Sinclair et al., 1999; Pyhäjärvi
et al., 2008). Our study supports this view, as the rapidity of the
vigorous Pinus colonisation during the B/A implies a nearby
origin. It is unclear, however, how much the histories of the first
colonising tree Betula and Pinus populations in the eastern
Baltic region resemble each other. The timings of their
proliferation and decline seem to reflect similar responses to
Lateglacial and early Holocene environmental changes.
Several regions have been suggested as the potential glacial
ranges for Picea, e.g. the Alpine forelands, the Danubian plain
and the Russian plain (Terhürne-Berson, 2005). Our record
confirms the YD presence of Picea in the eastern Baltic region.
Numerous other pollen diagrams from the Baltic region and
adjacent areas show Picea pollen curves rising in the YD and
declining in the early Holocene. For example, the highresolution pollen stratigraphy from Lake Kirikumäe (Saarse and
Rajamäe, 1997) and other pollen diagrams from the Haanja
heights in south-eastern Estonia (Pirrus, 1969; Ilves and
Mäemets, 1996) show the YD peak of Picea pollen values
(Fig. 1(B)). Similar patterns are typical in Belarus, as clearly
apparent in the stratigraphy of Lake Okono in the northern part
of the country (Makhnach et al., 2004) (Fig. 1(B)). The difficulty
in assessing the Lateglacial presence of Picea arises from the
J. Quaternary Sci., Vol. 24(7) 802–815 (2009)
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JOURNAL OF QUATERNARY SCIENCE
lack of direct evidence for its existence in situ, such as stomata
or macrofossil remains. A recent study from Lake Kašučiai in
western Lithuania (Stancikaite et al., 2008) reported Picea
needles dated to the YD. Although Stancikaite et al. (2008) do
not elaborate on this feature, it is important for clarifying the
Lateglacial dynamics of Picea in the region. It appears that the
local populations of Picea survived in the Baltic region and
adjacent areas, but the more detailed patterns remain to be
untangled. The core populations for spread may have been
located east of the region, potentially in western Russia, where
Picea was commonly prevalent during the B/A instead of Pinus
(Zelikson, 1997; Velichko et al., 2002).
In general, our results are in accordance with the pattern of
persistence suggested by Bhagwat and Willis (2008), who
argued that the tree species that survived farther north in
Europe during the full-glacial were mostly small-seeded,
wind-dispersed and coniferous. The Lake Kurjanovas record
demonstrates that Pinus occurred densely in eastern Latvia
during the B/A together with Betula, which was possibly present
even earlier. Our evidence also demonstrates the presence of
Picea in Latvia during the YD. We therefore suggest that the fullglacial locations of Pinus, Betula and Picea populations were in
the regions south and south-east of Lake Kurjanovas, possibly as
far north as 55–608 N (see Fig. 1(A) for LGM ice extent).
Acknowledgements We thank Siim Veski for providing sampling
equipment and for help in the field. Charlotte Sweeney is thanked
for help in identification of Pinus and Picea stomata, and Gina Hannon
and Thomas Giesecke for help in identification of plant macrofossils.
We are grateful to Thomas Giesecke and Thomas Edwards for their
helpful comments on the manuscript, and to Hanneke Bos and an
anonymous referee for their valuable reviews. Thomas Edwards and
Bronwyn Brock kindly helped improve the English. We acknowledge
the funding provided by the Academy of Finland, project number
210879, High-Resolution Climate Reconstructions in Eastern Europe
(HOT).
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