Tree taxa immigration to the eastern Baltic region ... - Springer Link

Veget Hist Archaeobot (2014) 23:207–216 DOI 10.1007/s00334-014-0442-6

ORIGINAL ARTICLE

Tree taxa immigration to the eastern Baltic region, southeastern sector of Scandinavian glaciation during the Late-glacial period (14,500–11,700 cal. B.P.) Leeli Amon • Siim Veski • Ju¨ri Vassiljev

Received: 17 November 2013 / Accepted: 5 February 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract The eastern Baltic region is situated in the southeastern part of the area which was covered by the last Scandinavian glaciation. Four well-dated sediment profiles from sites distributed along a *330-km north–south transect were analysed for their macrofossil contents. The immigration of tree taxa during the Late-glacial (LG) period, which was the time of environmental change from tundra to woodland in previously glaciated areas, can be determined from these data. The pioneer vegetation in the study area was treeless dwarf shrub tundra with various dominant taxa. The so-called Allerød hemispheric warming permitted the Post-glacial immigration of trees into the southern part of the eastern Baltic region; however, these most probably disappeared during the following cold period, the Younger Dryas/GS-1. The local presence of Betula sect. Albae, Pinus sylvestris, Populus tremula and Picea abies during the LG period in the southern part of the region was confirmed. The northern part of the area presumably remained treeless for the entire LG period. Therefore, until the beginning of the Holocene, the tree line in the eastern Baltic region did not reach beyond 58°N. Keywords Late-glacial vegetation Macrofossil Immigration Tree line Eastern Baltic

Communicated by A. E. Bjune. L. Amon (&) S. Veski J. Vassiljev Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia e-mail: [email protected]

Introduction The presence or absence of particular tree taxa and tree line fluctuations after the last glacial maximum (LGM) of Europe have been scrutinised in several studies (Willis and van Andel 2004; Birks and Willis 2008; Binney et al. 2009; Va¨liranta et al. 2010). The tree line itself is a major ecoclimatic feature that is sensitive to environmental changes and it reflects the interaction of climate, physiography, species ecology and physiology (Prentice et al. 1992). The presence of a tree species is an indicator of past climate because a particular taxon may require a certain minimal temperature for growth and reproduction. Large-scale climate changes have been affecting and will continue to affect the position of the boreal tree line; the movement of the tree line during the Post-glacial period has been recorded in palaeoecological studies, and tree line fluctuation continues today (Sturm et al. 2001; Petit et al. 2008). The presence of trees is often inferred from the results of pollen analysis. Increasingly precise minimum pollen accumulation rates that are used to identify the presence of the forest limit have been calculated (Seppa¨ and Hicks 2006); however, the relationship between pollen and past vegetation is not straightforward. Relying only on pollen would lead to a misinterpretation of past vegetation trends (Birks 2003, 2008; Birks and Birks 2000). Therefore, plant macrofossil records may have an advantage over the much more frequently used pollen data. Plant macroremains primarily reflect local-scale vegetation and thus the vegetation directly around the sedimentary basin, while pollen analysis represents a regional-scale vegetation signal. Due to problems with long distance transported pollen during the Late-glacial (LG) period, plant macrofossils provide a more realistic reconstruction and confirmation of the actual presence of plants. Recent experimental data from the

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Veget Hist Archaeobot (2014) 23:207–216

Table 1 The characteristics of the study sites Lake

Udriku ja¨rv

Prossa ja¨rv

Nakri ja¨rv

Coordinates

25°550 50.1600 E

26°340 4000 E

26°160 23.5200 E

0

00

0

00

58°38 57 N

57°53 42.18 N

56°450 3000 N

Altitude (m.a.s.l.)

95

61

48.5

96

Area (ha)

23.7

24.2

0.9

18.8

Depth (m)

4.8

4.2

3.2

4

Landscape

NW slope of the Pandivere upland; hummocky landscape

Inter-drumlin depression; surrounded by 1,200 km2 drumlin field

Slightly undulating glacial topography; adjacent to local end moraine

Glacial lowland; flat topography with elevations of 96–101 m a.s.l.

Modern vegetation

Mixed conifer forest

Arable land

Mixed conifer forest, fen

Birch woodland

Study region The topography of Estonia and Latvia has largely been shaped by Pleistocene glaciations, particularly the last, Weichselian, glaciation. The investigated sites were selected to form a 330 km long north–south directional transect over the eastern Baltic region to determine vegetation development in both tundra and probably wooded areas during the LG period (Table 1, Fig. 1a). An important palaeogeographical feature of the deglaciated region

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00

27°080 4.9200 E

59°22 17 N

current tree line in Norway has demonstrated that the pollen record was unable to detect the position of a tree line, but plant macrofossils captured the changes well (Birks and Bjune 2010). In Canada and Alaska, all the pollen-based biomisation experiments have over-predicted woodland cover (Binney et al. 2011). Although it has many advantages, the weaknesses of the plant macrofossil record include uneven transport, taphonomy and scarcity of plant remains. The plant macrofossil record also contains much fewer available data, which may inhibit the drawing of conclusions over large regions. During the LGM, the Scandinavian ice-sheet covered large areas of northern and eastern Europe, including the eastern Baltic region. Several recent articles have discussed the deglaciation history of the region (Zelcˇs and Markots 2004; Kalm 2006, 2012; Saarse et al. 2012) and vegetation development in the LG (Stancikaite et al. 2009; Heikkila¨ et al. 2009; Ozola et al. 2010; Amon et al. 2012; Koff and Terasmaa 2011; Veski et al. 2012). The present article discusses tree migration during the LG period (14,500–11,700 cal. B.P.). The interpretation consists of a synthesis of three previously published studies (Amon and Saarse 2010; Amon et al. 2012; Veski et al. 2012) and a new plant macrofossil record from Prossa ja¨rv, a carefully selected lake site near the probable LG treeline.

0

Lielais Sve¯tin ¸ u ezers

was the formation of ice lakes which hampered the migration of terrestrial plants (Fig. 1b). The present lake study sites are situated in northern Estonia (Udriku ja¨rv), eastern Estonia (Prossa ja¨rv), southern Estonia (Nakri ja¨rv) and eastern Latvia (Lielais Sve¯tin ¸ u ezers), and they are of various sizes (Table 1). The study area lies in the recent hemiboreal vegetation zone, which is within the boreo-nemoral ecotone and located between two phytogeographical units, the boreal and nemoral forest zones. The zonal vegetation type is boreo-nemoral coniferous forest, which consists primarily of Picea abies (Norway spruce) forest with Betula pendula (silver birch), Populus tremula (aspen) and Pinus sylvestris (Scots pine), and, to a lesser extent, Tilia cordata (smallleaved lime), Fraxinus excelsior (ash), Ulmus glabra (wych elm) and Quercus robur (oak) in the tree layer (Laasimer 1965; Paal 1998).

Materials and methods Sediment cores from the study sites were recovered from the deepest and most representative parts of the lake basins where the sediment thickness was the greatest. The cores were recovered using both 7.5 and 10 cm diameter and 1 m long Russian type peat samplers. The sediment lithostratigraphy was described and photographed in the field. The sediments were carefully packed into 1 m plastic semitubes, wrapped in polyethylene film, labelled and transported to the laboratory for further analyses (Table 2). One exception to the above process was Prossa ja¨rv. Here, cores from more littoral areas of the lake were used instead (K-2009; Kihno et al. 2011) to obtain additional material for the plant macrofossil analysis. Prossa was analysed after the other three cores; the plant macrofossil record from Prossa is important due to its central location on the studied transect.


209

Fig. 1 a Study site locations. Asterisk marks proposed LG tree line; b reconstruction of the Estonian and Latvian LG landscapes flooded by metachronous ice-dammed lakes (based on Guobyte and Satkunas 2011; Kvasov 1979; Vassiljev and Saarse 2013; Zelcˇs and Markots 2004). The study sites are indicated

Table 2 The analytical methods applied and the number of samples analysed from the study locations along the transect Lake 14

Prossa ja¨rv

Nakri ja¨rv

Lielais Sve¯tin ¸ u ezers 12

4 ? 2 unpubl

8

9

Plant macrofossils

31

24, in present paper

93

77

Pollen and stomata

19 unpubl

32

71

48

AMS

C dates

Udriku ja¨rv

Other palaeoecological methods applied

LOI, diatoms, MS, grain size

LOI, grain size

LOI, diatoms, MS

LOI, diatoms, MS

Thickness of sample, average size of plant macrofossil sample

5 cm;

5 cm;

2.5–5 cm;

5 cm;

198 cm3

390 cm3

100 cm3

410 cm3

Temporal resolution of the macrofossil sample (years)

Min. 27

Min. 1 (varved)

Min. 8

Min. 20

Max. 137

Max. 260

Max. 87

Max. 135

Average 78

Average 155

Average 27

Average 38

Unpublished radiocarbon dates are available in Amon (2011) LOI Loss-on-ignition, MS magnetic susceptibility

Three of the lake study sites which were analysed using multiple proxies and published previously (Udriku, Nakri and Lielais Sve¯tin ¸ u) displayed an interesting phenomenon where the northernmost site did not reveal the local presence of trees, while the sediments from the two southern sites contained tree macrofossils. Therefore, the fourth study site, Prossa, was chosen to find the LG tree migration limit in the region. While it is always possible to perform increasingly precise analysis, the distance between the sediment cores from Nakri (with tree macroremains) and Prossa (without tree macroremains) is approximately 87 km, which is satisfactory for this study.

radiocarbon laboratory, Poland. The dated material was of terrestrial origin, and debris from leaves, catkin bracts and twigs were used. The chronology of the studied sediment sequences was based on the calibration of the radiocarbon dates using the IntCal09 calibration dataset (Reimer et al. 2009) and the OxCal 4.1 program (Bronk Ramsey 2001). To combine radiocarbon dates and lithological data, a deposition model was used (Bronk Ramsey 2008). The dates and chronologies have been previously published (Amon 2011; Amon et al. 2010, 2012; Amon and Saarse 2010; Veski et al. 2012; Fig. 2).

Plant macrofossil analysis Dating Plant macrofossils, which were identified during the macrofossil analysis, were radiocarbon dated using accelerator mass spectrometry (AMS) 14C dating at the Poznan´

Conventional procedures were followed to prepare for the plant macrofossil analysis (Birks 2001). The sediment subsample volumes were determined by water displacement. The samples were wet-sieved on a 0.16 mm mesh. The

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Fig. 2 Lithostratigraphy, organic matter content (black line), and age depth–depth model for the lakes a Udriku; b Prossa; c Nakri and d Lielais Svetinu. The grey area/thick line represents the range of the

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modelled ages at 2 sigma. The black circles represent the weighted average with 1 sigma error bars. The black graphs show the probability distribution of the calibrated radiocarbon dates


211

Fig. 3 The tree and dwarf shrub macroremains found in the Prossa lake sediments (number of remains per 390 cm3). CS catkin scale, Betula sect. Albae (tree birch) consists of B. pendula and B. pubescens in this study

material that was retained on the sieves was examined using a stereo and a light microscope. Reference collections and literature were used to identify the plant macrofossils (Berggren 1969; Anderberg 1994; Cappers et al. 2006).

Results The chronology (Fig. 2) and reconstruction of past vegetation was based on four study sites; the results and interpretation from three of the study sites (Udriku, Nakri and Lielais Sve¯tin ¸ u) have been previously published (Amon and Saarse 2010; Amon et al. 2012; Veski et al. 2012). The new plant macrofossil record from the Prossa sediments contains remains of various dwarf shrubs and herbs; however, no tree macrofossils were discovered in the samples from the LG period (Fig. 3). Furthermore, one sample was randomly chosen from the early Holocene (*10,800 cal. B.P.), which contained both pine and tree birch remains.

these areas contained different dominant taxa. In the south, the main dwarf shrub was Betula nana. In the middle of the study region, Dryas octopetala was dominant, while in the north, cold-tolerant Salix cf. polaris (polar willow) was the main shrub. The climatic and environmental conditions improved during the Bølling-Allerød warming. At the southernmost study site (Lielais Sve¯tin ¸ u), the changes were reflected by trees that consecutively immigrated and formed a mixed conifer woodland by *13,000 cal. B.P. (Fig. 4). The Younger Dryas (GS-1) (Lowe et al. 2008) cooling then caused environmental conditions unsuitable for tree growth, and there were no local signs of the presence of trees over the eastern Baltic region at this time. Locations north of 58°N (Prossa and Udriku) did not contain tree macroremains or a notable rise in the tree pollen accumulation rate during the LG period. Therefore, it can be assumed that treeless tundra conditions prevailed until the beginning of the Holocene. At the end of the GS-1 cooling, Picea and Betula returned to eastern Latvia (Lielais Sve¯tin ¸ u); however, at the three northern study sites, the trees re-appeared at the beginning of the Holocene.

The eastern Baltic region: LG vegetation fluctuations between tundra and woodland Betula sect. Albae (tree-birch) The palaeovegetation records from the lakes Udriku (Amon and Saarse 2010), Prossa (in this study), Nakri (Amon et al. 2012) and Lielais Sve¯tin ¸ u (Veski et al. 2012) confirm that the initial LG terrestrial plant community in all four studied locations was treeless tundra. However,

The first tree that was recorded in the eastern Baltic region during the LG period was tree birch, in the group Betula sect. Albae, consisting of B. pendula and B. pubescens. Birches surrounded the lakeshore of Lielais Sve¯tin ¸ u in

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Fig. 4 The introduction of trees into the study locations. The curve represents the sum of the shrub remains (Betula nana, Salix polaris and Dryas octopetala) in 1 cm3 sediment; the number on the x-axis indicates the maximum number of shrub macrofossils found in 1 cm3. The shrub data are plotted against the tree macroremains data, age (cal. year B.P.), latitude and the LG and climatic events (Lowe et al. 2008)

eastern Latvia at *13500 cal. B.P. (Fig. 4). The sediment contained undifferentiated tree birch remains and B. pendula catkin scales. The trees advanced rapidly northwards, and the first tree birch macrofossils found in the sediment obtained from Nakri were dated to *13350 cal. B.P. (Amon et al. 2012). However, the LG lake sediments from Prossa and Udriku did not contain any tree birch macroremains.

The presence of Picea around Lielais Sve¯tin ¸ u was confirmed first by the presence of stomata and increasing pollen concentrations at 12400 cal. B.P. and then Picea macrofossils at 12000 cal. B.P.; the presence of Picea continued into the Holocene (Veski et al. 2012). The three northern study sites did not contain any traces of local Picea.

Pinus sylvestris (pine)

Populus tremula (aspen)

The presence of pine during the Allerød period (GI-1a) in the eastern Baltic region was confirmed around Lielais Sve¯tin ¸ u in Latvia and Nakri (Fig. 4). The first stomata of Pinus from Lielais Sve¯tin ¸ u were dated to 13450 cal. B.P., while other macrofossils were younger, as bark was dated to 13300–12700 cal. B.P. and needles to 13100–12700 cal. B.P. (Veski et al. 2012). This occurrence is several thousand years earlier than suggested in previous studies (Cheddadi et al. 2006; Binney et al. 2009). In Nakri, the local presence of Pinus is suggested by the finding of a single stoma (13300 cal. B.P.) and is accompanied by a rise in the Pinus pollen accumulation rate (Amon et al. 2012). If these results are sufficient, it can be concluded that during the LG period, Pinus also reached 57°N. Pinus macrofossils disappear abruptly from Lielais Sve¯tin ¸ u record in the GS-1 cold period (Fig. 4), suggesting that Pinus then rapidly decreased or disappeared from this region.

LG finds of Populus macrofossils are rather rare in Eurasia (Binney et al. 2009); however, the sediments from Lielais Sve¯tin ¸ u contained well-preserved Populus catkin scales that undeniably confirmed the local occurrence of Populus tremula between 13000 and 12700 cal. B.P. in eastern Latvia. During the GS-1 cold period, no macrofossils were found, suggesting that the climate deterioration caused Populus to disappear from the study region until *11500 cal. B.P. At Nakri in Estonia, the first Populus macrofossil was dated to *11200 cal. B.P .

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Picea abies (spruce)

Alnus sp. (alder) The local presence of Alnus during the LG period remains open to debate and further research, according to our plant


macrofossil data. None of the samples obtained from our four study locations contained Alnus macrofossils. However, recent data from Estonia has confirmed the local occurrence of Alnus due to the presence of fruits at *12400 cal. B.P. in the Solova mire in southern Estonia ¨ ntu ja¨rvi (Amon et al. 2010) and *10900 cal. B.P. in A (Laumets et al. in press). The age of the Alnus remains from Solova should be treated with caution because a sedimentary hiatus of unknown length is thought to have occurred between 13500 and 11600 cal. B.P.

Discussion The migration of trees and its controlling factors The appearance of trees and the formation of a tree line is a clear signal of succession from arctic tundra to a boreal environment. The tree line advances and fluctuations during the LGM and afterwards in Europe and northeastern European Russia have been discussed in several recent publications (Willis and van Andel 2004; Feurdean et al. 2007; Birks and Willis 2008; Binney et al. 2009; Va¨liranta et al. 2010). Trees survived the LGM in refugial areas; however, the locations of these sheltered sites remain under debate (Willis and van Andel 2004; Feurdean et al. 2007; Birks and Willis 2008; Parducci et al. 2012). The plant macrofossil evidence places the refugia not only in southern and central Europe but also near the ice front in Russia (Va¨liranta et al. 2010) and Norway (Kullman 2008). Moreover, the latest ancient DNA results discuss the presence of trees even on the Andoya refugium, *22,000 years ago (Parducci et al. 2012; Birks et al. 2012). Vegetation succession in modern newly deglaciated areas in the Arctic is highly affected by low temperatures, a short growing season, limited moisture, nutrient availability and cryoturbation of soils (Hodkinson et al. 2003). The pioneering communities are primarily affected by stochastic factors such as seed dispersal, while later on in the succession, deterministic factors such as soil properties and length of the growing period limit the taxon richness and diversity (Del Moral 2009). In the newly deglaciated eastern Baltic region, limited and fragmentary available land would have substantially influenced the pattern of vegetation migration and possibly also its speed. A mosaic of shifting local ice lakes covered large areas of the eastern Baltic region, which later drained and formed the modern topography of the study region (Fig. 1b). The water reduced the availability of dry land and the fragmentation of land most probably affected the migration paths. The cold water of the deglaciated Baltic Sea basin was most probably one of the reasons why treeless tundra persisted in northern Estonia until the Holocene. Another factor that

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strongly drove the LG vegetation development in the study area was climate; there were notable correlations between climatic fluctuations and changes in plant communities in all four studied sites. When the abiotic factors became more favourable for vegetation development, biotic interactions such as facilitation and competition became increasingly important; however, the response may not have been linear (Malkinson and Tielbo¨rger 2010). Plant migration also depends on the size of the taxon pool and the location of the nuclei from which migration begins. In areas where the taxon pool is large and the nuclei are close, as in the modern glacier forefields in the Alps, woody plants can establish themselves in about 10 years after deglaciation (Burga 1999). Trees migrated into the Baltic region primarily from southern regions, as suggested by the older ages of the tree macroremains in Lithuania. In some cases, refugia may have been located elsewhere, for example, a refugium for spruce has been proposed to exist in western Russia (Tollefsrud et al. 2008) or the Valdai region of Russia (Wohlfarth et al. 2007), relatively close to the Baltic region. These areas may have served as the migration nuclei for the study area. In addition to migration directions, migration speeds are also difficult to discuss (Feurdean et al. 2013). Another aspect that inhibits the direct estimation of tree migration rates from plant macrofossil data is the dispersal and taphonomy of the plant remains. The dispersal ability of tree macroremains differs between taxa; however, in contrast to pollen, the tree macroremains are clearly of local origin. Light Pinus bud scales can be transported for several kilometres and Picea remains can be dispersed up to 1 km from the tree; tree-birch seeds can travel for several hundred metres (Birks and Bjune 2010). Due to transport, tree remains can be present in areas where the local community did not exist, but instead was growing some distance away, such as a few km. LG trees in the eastern Baltic region—possible migration nuclei and migration speeds Betula sect. Albae The Post-glacial expansion nuclei for tree birches in the eastern Baltic region are difficult to locate; however, tree birches were present earlier than 14300 cal. B.P. in Lithuania (Stancˇikait_e et al. 2008) and [13800 cal. B.P. in central Belarus (Novik et al. 2010). If these age estimations and our chronologies are correct, the hypothetical rate of migration of Betula from Lithuania to Latvia can have been ca. 130 m year-1 and from Latvia to southern Estonia ca. 800 m year-1, calculated from the distance between the sites and the differences in the radiocarbon age estimates. The latter rate is surprisingly high and exceeds the early

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successional tree migration rates proposed by Feurdean et al. (2013). The scarcity of tree birch remains in the Nakri sediments and their absence from the Prossa record suggest that the LGM northern limit for the presence of tree birch was in southern Estonia (*58°N) according to this study. At the beginning of the LG cold episode (GS-1 or the Younger Dryas), tree birches most probably disappeared from the study region, or they survived only in small groups that did not deposit either plant macrofossils or notable amounts of pollen into the sediment, and they then re-expanded into Latvia *12000 cal. B.P. (Fig. 4). In more northern areas, north of 57°N, tree birch re-appeared after the beginning of the Holocene (Fig. 4). Pinus sylvestris The rapid migration and development of mixed woodland in central and eastern Latvia occurred *13400–12700 cal. ¨ et al. B.P., during the GI-1e and GI-1c periods (Heikkila 2009). The general trend suggests that tree birches arrived first and formed open woods; pine arrived later and expanded at the expense of the birches, becoming the dominant woodland-forming trees throughout the GI-1c/ GI-1a periods. Pinus was present in Belarus during the pre-LGM period and spread from its refugia in western Russia during the LGM (Binney et al. 2009). Expanding southwards from the eastern Baltic region, Pinus sylvestris was present *14000 cal. B.P. in Lithuania (Stancˇikait_e et al. 2008) and [13800 cal. B.P. in Belarus (Novik et al. 2010). Pinus macrofossils, dating back to 15000–13000 cal. B.P., have been found in Belarus and east of the Gulf of Finland (cf. Binney et al. 2009). One single Pinus stoma dating back[ 16,000 cal. B.P. was found in Latvia; however, the authors are cautious about its chronology (Heikkila¨ et al. 2009). The average Pinus migration speed from Latvia to Estonia was 900 m year-1, calculated from the distance between the sites and the differences in the radiocarbon age estimates, which is the maximum Pinus migration speed estimated by Feurdean et al. (2013). Picea abies The combination of fossil pollen and Picea abies genetic data have revealed the LGM refugium area of Picea to have been in the Russian plain, and this was the basis for the northern lineage of the present Picea population in northern Europe (Giesecke and Bennett 2004; Latałowa and van der Knaap 2006; Tollefsrud et al. 2008) and even as far north as northeastern Norway (Parducci et al. 2012). Plant macrofossil data show the continuous presence of Picea in northeastern European Russia from more than

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12000 cal. B.P. (Va¨liranta 2005), even in the case of very low pollen counts (Va¨liranta et al. 2006). Picea from the Younger Dryas period was also found in eastern Latvia (Heikkila¨ et al. 2009); however, no Picea macrofossils were found in the area around Burtnieki ezers, a lake in Latvia (Ozola et al. 2010). The local presence of Picea north of 58°N in the eastern Baltic region remains open to debate because there has been some evidence using pollen percentages; however, no evidence from macrofossils has been obtained (Saarse et al. 1999). Populus tremula Populus pollen is poorly preserved in sediments, so its presence or absence is difficult to estimate by pollen analysis. Fortunately, P. tremula catkin scales were found in the sediments and are relatively easy to recognise. Binney et al. (2009) admitted that the data for this taxon are scarce. The present dataset undeniably confirms the local occurrence of P. tremula between 13000 and 12700 cal. B.P., so during the LG warm period (GI-1a) in eastern Latvia. Moreover, the GS-1 cold period removed aspen from the study region until it returned *11500 cal. B.P.. At Nakri in Estonia, the first Populus macrofossil was dated to *11200 cal. B.P. Alnus sp. According to previous studies, Alnus has been present in Estonia since the Pre-boreal period (Saarse et al. 1999). In eastern Europe, Alnus macrofossils have been recorded from Belarus [13,000 cal. B.P.; however, they are more common from 11,000 cal. B.P. (Binney et al. 2009). Alnus fruits have also been found in western Lithuania dating to *11800 cal. B.P. (Stancˇikait_e et al. 2008). The possible maximum tree line limit The LG immigration of trees along the 330 km transect in the eastern Baltic region has been well dated and demonstrated, using the dataset created in this study. Because the migration of trees broadly coincided with LG climatic oscillations, it can be assumed that some of the main driving factors for tree line fluctuations were climate and temperature. Trees first arrived at the southernmost study site, Lielais Sve¯tin ¸ u (56°N), where Betula woodlands formed *13500 cal. B.P., which was followed by the development of a mixed conifer woodland during the warmer parts of the LG period (Fig. 4). The GS-1 cold period then caused the disappearance of tree macroremains from the sediment record; trees then partially returned in the second half of the GS-1 period. Nakri is the northernmost study location in the eastern Baltic region where tree macrofossils were present during


the LG period. Therefore, the LG tree line did not extend northwards of 58°N (Fig. 1a). The first trees to arrive were always tree birches. Birch macroremains of fruits and catkin scales were present in several adjacent samples. However, the small numbers of these suggest that only sparse colonies were present at the edge of the birch distribution area.

Conclusions Our tree immigration results for the eastern Baltic region during the LG period (*14,500–11,700 cal. B.P.) are based on four study sites located along a 330 km transect from eastern Latvia to northern Estonia, which is the region that was affected by the late Weichselian glaciation. The main factors controlling the Post-glacial migration of trees into the region were climate, the flooded and fragmented Postglacial landscape, the cold Baltic Sea to the north of the study region and the locations of migration nuclei. Initially, the pioneering vegetation that spread along the transect was treeless tundra. The fluctuating ambient environmental factors caused both the immigration into and then the retreat of trees from the study region. During the warmest part of the LG period, birch-pine woods with aspen grew in the southern part of the study region even though the tree line did not reach northern Estonia. During the Younger Dryas cold period, the trees retreated from the eastern Baltic region. Four tree taxa have been identified in the eastern Baltic region from their macroremains found in the LG sediments, Betula sect. Albae, earliest evidence at *13000 cal. B.P., Pinus sylvestris, earliest evidence at *13450/13,300 cal. B.P., Populus tremula, earliest evidence at *13000 cal. B.P., and Picea abies, earliest evidence at *12000 cal. B.P. The location of Alnus remains open to debate. The finds have good dating evidence, which suggests their local presence in the region earlier than previously suggested. The maximum northward tree line extension in the eastern Baltic region during the LG period most probably did not exceed 58°N. Acknowledgements The first author is grateful to Hilary H. Birks (University of Bergen) for the introduction to plant macrofossil analysis and the fascinating realm of LG vegetation that thousands of years ago covered my home country. We acknowledge the anonymous referees for their comments and suggestions on the manuscript. The study was financially supported by ETF 8552 and IUT 1-8.

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