Further Details on Holocene Treeline, Glacier/Ice Patch and Climate History in Swedish Lapland

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DOI: 10.20431/2454-8685.0304008
Cite this publication
Abstract
The present paper reports results from an extensive project aiming at improved understanding of postglacial subalpine/alpine vegetation, treeline, glacier and climate history in the Scandes of northern Sweden. The main methodology is analyses of mega fossil tree remnants, i.e. trunks, roots and cones, recently exposed at the fringe of receding glaciers and snow/ice patches. This approach has a spatial resolution and accuracy, which exceeds any other option for tree cover reconstruction in high-altitude mountain landscapes. The main focus was on the forefields of the glacier Tärnaglaciären in southern Swedish Lapland (1470-1245 m a.s.l.). Altogether seven megafossils were found and radio-carbon dated (4 Betula, 2 Pinus and 1 Picea). Betula and Pinus range in age between 9435 and 6665 cal. yr BP. The most remarkable discovery was a cone of Pice aabies, contained in an outwash peat cake, dating 11 200 cal. yr BP. The peat cake also contained common boreal ground cover vascular plant species and bryophytes. All recovered tree specimens originate from exceptionally high elevations, about 600-700 m atop of modern treeline positions. This implies, corrected for land uplift, summer temperatures, at least 3.6 °C higher than present-day standards. The current results, in combination with those from other Swedish glaciers, contribute to a new view on the early postglacial landscape and climate in high-altitude Swedish Scandes
International Journal of Research in Geography (IJRG)
Volume 3, Issue 4, 2017, PP 61-69
ISSN 2454-8685 (Online)
http://dx.doi.org/10.20431/2454-8685.0304008
www.arcjournals.org
International Journal of Research in Geography (IJRG) Page | 61
Further Details on Holocene Treeline, Glacier/Ice Patch and Climate
History in Swedish Lapland
Leif Kullman1
1Department of Ecology and Environmental Science, SE 90187 Umeå, Sweden
Abstract: The present paper reports results from an extensive project aiming at improved understanding of
postglacial subalpine/alpine vegetation, treeline, glacier and climate history in the Scandes of northern Sweden.
The main methodology is analyses of mega fossil tree remnants, i.e. trunks, roots and cones, recently exposed at
the fringe of receding glaciers and snow/ice patches. This approach has a spatial resolution and accuracy,
which exceeds any other option for tree cover reconstruction in high-altitude mountain landscapes. The main
focus was on the forefields of the glacier Tärnaglaciären in southern Swedish Lapland (1470-1245 m a.s.l.).
Altogether seven megafossils were found and radio-carbon dated (4 Betula, 2 Pinus and 1 Picea). Betula and
Pinus range in age between 9435 and 6665 cal. yr BP. The most remarkable discovery was a cone of Pice
aabies, contained in an outwash peat cake, dating 11 200 cal. yr BP. The peat cake also contained common
boreal ground cover vascular plant species and bryophytes. All recovered tree specimens originate from
exceptionally high elevations, about 600-700 m atop of modern treeline positions. This implies, corrected for
land uplift, summer temperatures, at least 3.6 °C higher than present-day standards. The current results, in
combination with those from other Swedish glaciers, contribute to a new view on the early postglacial
landscape and climate in high-altitude Swedish Scandes
Keywords: Treeline, glacier, megafossils, climate change, Holocene, Swedish Scandes, Betula pubescensssp.
czerepanovii, Pinus sylvestris, Piceaabies.
1. INTRODUCTION
Recent glacier/ice patch recession in association with post-Little Ice Age climate recovery of the past
100 years or so has exposed a plethora of previously ice-entombed megafossil tree remains in many
parts of the world (Nicolussi & Patzelt 2000; Hormes et al. 2001; Schlűchter & Jörin 2004;Koch et al.
2007, 2014; Grosjean et al. 2007; Benedicht et al. 2008; Wiles et al. 2008;Scapozza et al.
2010;Nicolussi&Schlűchter 2012; Lee 2012). These ancient remnants, derive from subglacial
preservation sites and are currently exposed at the margin of basins presently occupied by glacier ice
and perennial snow. Theyoffer a unique opportunity to improve our understanding about past treeline
positions and associated plant cover characteristics and thereby indirectly provide clues to ancient
climates.This archive, also containing numerous human and cultural artefacts, has been widely
recognized and exploited by archaeologists (e.g. Nesje et al. 2011; Lee & Benedicht 2012; Reckin
2013), but in Scandinavia surprisingly little payed attention to by palaeoecologists.
In the Swedish Scandes, however, important findings of megafossil trees under above-mentioned
circumstances, high above current treelines, have been reported and discussed in some studies
(Kullman 2004; Öberg& Kullman 2011a; Kullman &Öberg 2013, 2015; Kullman 2017a). These
results have proven accuracy in time, space and species composition, going far beyond the resolution
of pollen analysis and other microfossil approaches (cf. Kullman 2017a) and lining up with inferences
originating from DNA-methodologies (Parducci et al. 2012; Parducci&Tollefsrud 2016). Late-glacial
and early-Holocene presence of boreal tree species are evidenced (megafossils) in this way for
restricted sites, situated much higher than current treeline elevations. Hereabouts peat deposits on the
open alpine tundra are rare and shallow, with little ability to preserve trees and other macroscopic
plant remains from ancient times. Therefore, little has been known about the highest treeline positions
and associated plant cover structure during the earliest part of the Holocene. The most promising
archives for that purpose are found in glacier cirques and nivation hollows, which were ice free before
Further Details on Holocene Treeline, Glacier/Ice Patch and Climate History in Swedish Lapland
International Journal of Research in Geography (IJRG) Page | 62
and became ice covered in accord with the mid-Holocene neoglacial cooling. In many cases, it is quite
obvious that the mega fossil tree remnants have been washed out by subglacial meltwater streams
from primary growing sites higher upslope. Some exceptional outlying records of 500-700m support
an even higher elevational origin as a more general pattern. The possible generality of this supposition
needs to be further elucidated in perspective of its implications for Holocene vegetation history and
paleoclimate (Öberg & Kullman 2011a,b; Kullman &Öberg 2013, 2015). With this background, the
present study reports efforts to sustain and further approach the uppermost limit of mega fossil trees
within an area previously well researched with respects to mega fossil tree remnants (debris wood)
occurring at glacier forefields at relatively modest levels above the present-day treelines (Öberg &
Kullman 2011a; Kullman & Öberg 2013, 2015).
For logistic reasons it has been judged impractical and dangerous (slippery bedrock, collapsing glacier
fronts and moving rock slabs) to investigate these higher potential source areas in search for mega
fossils. However, during the early autumn of 2017, the present author made a tentative approach, the
results of which are reported here.
2. STUDY AREA
The study was carried out within the central Swedish Scandes, in the southern part of the province
Lapland (Fig. 1). Focus is here on the forefield of the glacier “Tärnaglaciären”, which is located to the
Norra Storfjället massif (65° 51´N; 15° 16´E), with some peaks reaching above 1600 m a.s.l. and
valley floors at 700-800 m a.s.l. The glacier is contained within a cirque facing SE (Fig. 2). Currently
the glacier area is estimated to c. 0.2 km2, with an upper and lower margin at 1470 and 1245 m a.s.l.,
respectively. By the late 19th and early 20th century, the glacier was mapped by Gavelin(1897, 1910),
who estimated its area to 0.5 km2. Thus, the glacier has lost more than 50% of its area during the past
100 years or so, and the lower front has withdrawn by approx. 175 m in elevation. Figure 3 depicts the
maximum extent of the glacier by the late 19th century, manifested in the form of an incomplete
moraine bow in an outwash lake below the glacier, 1070 m a.s.l. (Fig. 3) (cf. Gavelin 1910; Lindgren
& Strömgren 2001).Substantial frontal retreat and thinning have taken place since the late 1990s and
up to the present day (Fig. 2)
On the slope below the glacier, a large snow/ice patch extends down to the outwash lake (Fig. 2). The
size of this patch varies on an annual basis, depending on prevailing weather conditions. Prior to the
present study, most megafossil recoveries have been made in association with melt water streams
close to the lower fringe of this patch.
The bedrock is of Cambro-Silurian origin, mostly mica schists. Quaternary deposits embrace
glacifluvial accumulations, till and peat. A weakly suboceanic climate characterizes the area. The
nearest meteorological station is Hemavan, 475 m a.s.l., situated in the Uman River Valley, c. 10 km
southwest of the study site. The mean temperature forJune-August and the year are 10.1 and -0.4°C,
respectively. Annual precipitation is 680 mm.
Currently, mountain birch (Betula pubescens ssp. czerepanovii) constitutes the upper treeline in this
area, 790 m a.s. l. (Fig. 4). The nearest treeline of Norway spruce (Pice aabies) and Scots pine (Pinus
sylvestris) are at 710 and 690 m a.s.l., respectively. During the past 100 years, the treelines of those
species have advanced by a maximum of more than 200 m (Kullman &Öberg 2009), which appears to
have taken them to a position uniquely high for the past 7000 years or so (Kullman 2017b).
Overviews of the structure and dynamics of the treeline ecotone in the Scandes are provided by
Kullman (2010) and Wielgolaski et al. (2017).
Fig1. Map showing the location of the study area (●) in northern Sweden.
Further Details on Holocene Treeline, Glacier/Ice Patch and Climate History in Swedish Lapland
International Journal of Research in Geography (IJRG) Page | 63
Fig 2. The glacier Tärnaglaciären, the snow/ice patch and outwash lake below (1070 m a.s.l.).
A. Prospect from southeast, 1999-09-06 (Photo: F. Lindgren & M. Strömgren). B. Virtually the same view
2017-09-01. The glacier has perceivably thinned since 1999 "Photo: 2017-09-01".
Fig3. The outwash lake below the glacier, 1070 m a.s.l. By the late 19th century, the lower glacier front was
located at the morainic ridges, protruding above the water surface (Gavelin 1910).
Fig4. The current treeline of Betulapubescens ssp. czerepanovii, 790 m a.s.l., right to the east and down slope
of Tärnaglaciären (arrow). The solitary birch copse is located at the bank of the main melt water stream from
the glacier "Photo: 2017-09-01".
Further Details on Holocene Treeline, Glacier/Ice Patch and Climate History in Swedish Lapland
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3. METHODS
During the autumn of 2017 the fore fields adjacent to the lower and lateral margins of the glacier
Tärnaglaciären were thoroughly scrutinized for the presence of outwashed megafossils and other
identifiable plant remains. Recovered specimens were wrapped in aluminium foil and stored frozen
until delivery to the dating laboratory. Species identification was unambiguous in all cases, based on
bark fragments, cone and leaf characteristics. All recovered woody remnants were sampled and
altitudes were determined by a GPS navigator (Garmin 60CS), calibrated against distinct points on the
topographical map. Reported altitudes are rounded off to the nearest 5 m. The nomenclature of
vascular plants follows Öberg et al. (2017).
Radiocarbon dating of recovered specimens has been performed by Beta Analytic Inc., Miami, USA.
All original radiocarbon dates and time-scales in running text and figures are converted to calendar
years before present (cal. yr BP), with “present” = AD 1950, based on IntCal13 (Reimer et al. 2013)
and for the sake of simplicity, they are cited as “intercept values”. Outwash peat cakes and their
contained macrofossils (e.g. cones, leaves and bryophytes) were dated indirectly on the basis of 2 cm
thick bulk peat slices.
4. RESULTS
This study adds seven new dates of mega fossil tree remnants (4 Betula, 2 Pinus, 1 Picea) to a
previous sample of 21 specimens from the same glacier (12 Betula, 9 Pinus) (Kullman &Öberg
2015). Individual dates are given in Table 1 and the samples are depicted in Figures 5-7. They range
in elevation between 1410 and 1275 m a.s.l., which is about 600 and 700 m higher than the nearest
present-day treelines of these species. The ages all represent the early Holocene, c. 11 200 to 6700
before present.
Table 1. Radiocarbon dates of recovered megafossils. Relative elevation refers to the difference in altitude
between the sampling site and the nearest present-day treeline of the concerned species.
Fig 5. Recovered and dated mega fossils of Betula pubescens. A. 9365 cal. yr BP. B. 9450 cal. yr BP. C. 6665
cal. yr BP. D. 8780 cal. yr BP
Altitude Relative elevation Species Lab. code Radiocarbon age
Calibrated age
Intercept Size Material
m a.s.l. m
14C yr BP BP 1 SD cal. yr BP cm
1410 700 Betula Beta-474257
83330 9447-9273 9365 45 wood
1395 685 Betula Beta-474258
8400±30 9495-9397 9450 40 wood
1380 690 Pinus Beta- 474259
8020 ±30 9010-8848 8900 37 wood
1320 630 Pinus Beta- 474254
8380±30 9779-9371 9435 19 wood
1295 585 Betula Beta-474255
5850±30 6743-6603 6665 21 wood
1275 565 Betula Beta-474252
7910±30 8791-8602 8780 18 wood
1370 630 Picea Beta-474251
9760±30 11 238-11 167 11 200 14 Cone + peat
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Fig6. Recovered and dated megafossils of Pinus sylvestris. A. 8900 cal. yr BP. B. 9435 cal. yr BP.
A cone of Picea abies, contained in a peat cake, was dated 11 200 before present at an elevation,
almost as high as the uppermost dated birches and pines (Fig. 7).The cone contained a few heavily
decayed seeds.
This is the highest position, relative to its modern treeline, ever recorded for postglacial spruce. In
addition, this peat sample showed macrofossils of the following identifiable ground cover taxa:
Empetrum hermaphroditum, Vaccinium myrtillus, Vaccinium vitis-idaea, Rhododendron tomentosum,
Hylocomium splendens, Pleurozium schreberi, Dicranum sp., Sphagnum sp. All samples of Betula
and Pinus displayed a size and form which indicated that they originated from tree-sized individuals.
In the case of Picea abies(a cone) no such inference could be made.
Fig7. A. Peat cake which contained macrofossils of ground cover species, 1370 m a.s.l. B. Cone of Piceaabies
dissected from the peat cake.
5. DISCUSSION
The present study sustains a generic pattern for the entire Swedish Scandes(cf. Öberg & Kullman
2011a; Kullman & Öberg 2013, 2015). As evident from Fig. 5A, the highest date of Betula (1410 m
a.s.l,) is obtained from an outwash stream protruding from beneath the glacier. Obviously it originates
from a primary growing site further up valley, more than 700 m above today´s treeline.
Conservatively drawing on the latter figure and a summer temperature lapse rate of 0.6 °C per 100 m
elevation (Laaksonen 1976), coulda priori mean that, summer temperatures were at least 4.2 °C
warmer than present around 9500 year before present. However, glacio-isostatic land uplift by at least
100 m since that time (Möller 1987; Påsse & Anderson 2005) implies that this figure has to be
Further Details on Holocene Treeline, Glacier/Ice Patch and Climate History in Swedish Lapland
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reduced to 3.6 °C higher than present-day levels, i.e. first decades of the 21st century. Evidently, this
was the warmth peak of the Holocene, hitherto. This inference concurs with paleoclimatic
reconstructions from Europe and Greenland (Korhola et al. 2002; Bigler et al. 2003; Paus 2013; Luoto
et al. 2014; Väliranta et al. 2015) and complies with theoretical calculations based on variations in
Earth´s orbital parameters and associated gradual change in summer insolation (Berger & Loutre
1991; Esper et al. 2012). It contrasts with common interpretations suggesting a much later thermal
optimum (Berglund et al. 1996; Seppä & Birks 2002). The latter inferences are based on pollen
analyses, which in some cases are proved to deliver less reliable vegetation history details and
temperature estimates (Paus 2013; Elven et al. 2013;Luoto et al. 2014; Kullman 2017a).
The youngest mega fossil date, 6665 cal. yr BP, suggests that the concerned glacier, like many others,
did not exist prior to that date (cf. Bakke et al. 2005). Dated peat remains indicated that neoglacial
instatement of this particular glacier ice took place after 3890 cal. yr BP (Kullman & Öberg 2013).
Available dates are too few to allow any firm conclusions as to the zonation patterns during the early
Holocene. Anyhow, Betula appears to have been the highest ascending tree species. Such a pattern
also emerges from earlier more extensive megafossil studies, although a distinct subalpine birch forest
belt, as we know it today, appears to have formed later on (Kullman 2013). In that context, it is of
some interest to note that the nearest living birches (tree-line markers), in the form of a dense and
isolated copse, are located within the main outwash stream furrow from the glacier here concerned
(Fig. 4). This pattern is compatible with an earlier inference, based onmega fossil performance, that
trees (and possibly other plant species, have in general spread downslope from primary “occurrence
sites at high elevations, e.g. empty glacier cirques (Kullman 2002).
Information from ground cover macrofossil plant species contained in a peat cake (Fig. 7A), indicate
that the recovered mega fossils grew in a matrix of dwarf shrubs and bryophytes with present-day
quite ordinary boreal forest affinities. Predominance of Sphagnum spp. could indicate that the
megafossils were preserved by peat growth prior to the final burial by glacier ice.
Recent data on early Holocene presence of Picea abies at high elevations in the Scandes comply
temporally with megafossil and some recent pollen studies from different parts of the Scandes
(Kullman 1996, 2000; Segerström & von Stedingk 2003; Öberg & Kullman 2011b; Paus et al. 2011;
Kullman & Öberg 2013, 2015).This pattern contrasts with traditional inferences from pollen data,
suggesting a mid- or late Holocene wave-like spread of spruce from the east (e.g. Moe 1970; Hafsten
1992; Huntley & Birks 1983;Giesecke 2005;Seppä et al. 2009).Recent DNA analyses in lake
sediments and emergent patterns in genetic structure of extant spruce populations support the
contention of Late-glacial andearly Holocene presence of spruce enclaves in western and northern
Scandinavia (Parducci et al. 2012). Furthermore, multimillennial old prostrate spruces, prevailing high
above the current treeline in some mountain areas, provide support to the latter option (Öberg&
Kullman 2011b; Kullman 2015).
Apparently, these peripheral spruce occurrences were confined to restricted, widespread and
particularly favorable habitats, acting as dispersal nodes during later parts of the Holocene. This
option was originally inferred from megafossil spruce data gathered along the entire Swedish Scandes
(Kullman 1996, 2001, 2008, 2017a; Kullman &Engelmark 1997), a mechanism reiterated by
Väliranta et al. (2011) on evidence from north-eastern European Russia.
Importantly, mega fossil data of the kind accounted for above, in combination with DNA analyses in
soils and lake sediments, urge pollen analysts to adopt a less conservative attitude when interpreting
trace amounts of pollen. Evidently, much of the commonly narrated pollen-based postglacial history
of subalpine/alpine regions has to be reconsidered in the light of emerging megafossil evidence. These
latter results, in combination with those, analogously derived, from other Swedish glaciers, provide a
new view on the early postglacial landscape and climate in high-altitude Swedish Scandes.
Paleoecologists are forced to reconsider standard views on Late-Glacial and early Holocene
environments in the high mountains (cf. Anderson et al. 2009; Horáček et al. 2015). This view is
strongly substantiated by the present paper.
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6. ACKNOWLEDGEMENTS
This study was defrayed by a grant from the Göran Gustafsson Foundation. Dr. Lisa Öberg is thanked
for competent and constructive comments of an earlier version of the manuscript.
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Citation: Leif Kullman. " Further Details on Holocene Treeline, Glacier/Ice Patch and Climate History
in Swedish Lapland". International Journal of Research in Geography, vol 3, no. 4 2017, pp. 61-69.
doi:http://dx.doi.org/10.20431/2454-8685.0304008.
Copyright: © 2017 Authors. This is an open-access article distributed under the terms of the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are credited.
  • ... These tree trunks uncovered from retreating glaciers are irrefutable witnesses of extended preindustrial warm periods as they grew up well above the present-day tree lines [38]. from stalagmites in the Alps [39] and tree line investigations in Lapland [40] gave similar results, just as did ice core analyses from Greenland [41] and from the Antarctica [42]. ...
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