Experiment FindingsPDF Available

Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains warmer-than-present conditions and a patchily treed and rich mountainscape

Authors:
International Journal of Research in Geography (IJRG)
Volume 7, Issue 1, 2021, PP 01-08
ISSN 2454-8685 (Online)
http://dx.doi.org/10.20431/2454-8685.0701001
www.arcjournals.org
International Journal of Research in Geography (IJRG) Page| 1
Early Holocene presence of beaver (Castor fiber L.) in the
Scandes sustains warmer-than-present conditions and a patchily
treed and rich mountainscape.
Leif Kullman*
Department of Ecology and Environmental Science, Umeå University, SE 901 87 Umeå, Sweden.
1. INTRODUCTION
Post-Little Ice Age climate warming and associated glacier and ice patch shrinkage have opened a
new window to past high-mountain vegetational and faunal composition as well as human utilization
of the ancient mountainscape (Schlüchter & Jörin 2004; Benedict et al. 2008; Nesje et al. 2011;
Reckin 2013; Koch et al. 2014; Kullman & Öberg, 2020a). In the Swedish Scandes, summers (J.J.A.)
have warmed by 1.7 °C during the period 1901-2020 (Kullman & Öberg 2020b). This course of
change has caused glaciers and perennial ice patches to disintegrate (Lundqvist 1969; Holmlund et
al.1996; Lindgren & Strömgren 2001), which has released ancient woody material, so-called
megafossils. These remnants represent former tree stands, later on extirpated and entombed by snow
and ice for many millennia. By the current warming phase of the past 100 years or so, megafossil tree
remnants (logs, brances, roots and cones) are being exposed by the late summer at the forefields of
many Swedish glaciers and perennial ice patches, as reported and discussed previously (Öberg &
Kullman 2011; Kullman & Öberg 2013, 2015). From these studies it appears that current sites of
glaciers and ice patches, 400-700 elvational meters atop of current treelines, harboured early
Holocene stands of many of today´s boreal tree- and ground-cover species prevailing in northern
Scandinvia. In addition, one regionally extirpated species, viz. Larix sibirica, was found under these
circumstances. Moreover, the last-mentioned studies evidenced early Holocene presence of Picea
abies (Kullman 2018), previously for long considered as a more recent immigrant from the east.
Here, distinct morphs of megafossil tree remains are reported and discussed. Focus is specifically of
ancient logs with characteristic signs of being gnawed by beaver (Castor fiber L.). These records add
to articulate the climatic and vegetational implications of the existing megafossil narrative from the
same sites as focused in previous studies of postglacial tree histories and climate change (Öberg &
Kullman 2011; Kullman & Öberg 2013, 2015; 2020a,b; Kullman 2017).
2. STUDY AREAS
This study concerns two areas in the Scandes of northern Sweden (Fig.1). No. 1 is the glacier
Kårsaglaciären in northern Lapland and No. 2 comprises the glaciers Tärnaglaciären and
Abstract: Post-Little Ice Age climate recovery of the past 100 years or so has evoked significant changes of
the high-mountain landscape. For example, alpine treelines have advanced and glaciers/ice patches have
shrinked. By the last-mentioned process, megafossil tree remains of different boreal species have been
exposed for the first time since their burial by growing snow and ice millennia ago. Obviously, the concerned
remains represent tree exclaves in ice-empty glacier cirques. In these settings, finds of megafossil of Pinus
sylvestris, with signs of beeing gnawed by beaver (Castor fiber L.) are reported from diffferent sites in
northern Sweden. They age 9500-9300 cal. yr BP and are located 500-400 m atop of present-day treelines.
The detailed character of these ”forest” outposts have been largely unknown, althogh macrofossils indicate a
forest floor of present-day boreal affinity. Presence of beaver, an obligate forest-dweller, further enhances the
inference of a genuine forest character with a high biodiversity of these tree ouliers. Early Holocene presence
of beaver in the Scandes raises questions about postglacial immigration routes.
Keywords: Megafossil trees, beaver, early Holocene, wood outposts, palaeoclimate biodiversity, Swedish
Scandes
*Corresponding Author: Leif Kullman, Department of Ecology and Environmental Science, Umeå
University, SE 901 87 Umeå, Sweden.
Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains warmer-than-present
conditions and a patchily treed and rich mountainscape.
International Journal of Research in Geography (IJRG) Page| 2
Murtserglaciären in southern Lapland. Detailed accounts of their settings and postglacial and modern
arboreal histories are provided by last-mentioned six references.
No. 1. Kårsaglaciären (68° 18´N; 18° 20´E) is located in the east-facing slope of Mt. Kårsatjåkka
(Fig. 2A). During the past 100 years, the lower front has moved upslope from 810 to 965 m a.s.l., and
a forefield about 1 km in length has become freed of ice. By the late summer, this area is ”strewn”
with megafossils of different species, recently exposed by the retreating ice margin. These remains of
former tree growth, high above current treelines, date between 11760 and 5900 cal. yr BP, which
roughly bracket postglacial absence or much reduced size of the glacier until the late-Holocene
Neoglacial inception of the glacier. The nearest present-day treeline of pine is 520 m a.s.l. (Kullman
2015)
No. 2. Tärnaglaciären (65° 51´N; 15° 36´E) is situated in an cirque with a south-eastern aspect of Mt,
Murtsertjåkke (Fig. 2B). Withdrawal of the lower front ranges 1070 to 1240 m a.s.l., i.e. a total 170 m
in elevation during the past 100 years. Below the front, an extensive and semi-permanent snow/ice
patch extends to a proglacial meltwater pond 1070 m a.s.l. Megafossils of different species have been
recovered at the forefields and range between 11 200 and 4480 cal. yr BP
No. 3. Murtserglaciären (65° 49´N; 15°14´E) is a former small glacier in the east-facing slope of Mt.
Murtsertjåkke, ca 2 km to the south of Tärnaglaciären (Fig. 2C). Its lower margin is about1390 m
a.s.l. Today, after a century of substantial thinning and insignificant frontal retreat, it has changed
character to an ice patch. Megafossils of pine and birch are dated 9195- 5900 cal yr BP.
Figure1. Location of study areas in northern Sweden (Lapland). 1. Kårsaglaciären. 2. Tärna - and 3.
Murtserglaciären.
Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains warmer-than-present
conditions and a patchily treed and rich mountainscape.
International Journal of Research in Geography (IJRG) Page| 3
Figure 2. Overviews of the studied glacier habitats, including adjacent meltwater ponds. Megafossil
tree remains are found at these levels and even higher upslope. A. Kårsaglaciären. Photo: 2013-09-
12. B. Tärnaglaciären. Photo: 2012-08-28. C. Murtserglaciären Photo: 2012-08-28. All sites display
meltwater ponds in close association with the lower forefields, recently released from the ice.
3. RESULTS
Altogether three megafossil tree remains (Pinus sylvestris L.) with characteristic signs of being
gnawed by beaver, Castor fiber L. (general appearance and teeth furrows), have been recovered on the
forefields of receding glaciers/ice patches in Swedish Lapland (Table 1, Fig. 3). They all date to the
early Holocene, 9530-9190 cal yr BP, at sites 380-520 m higher than the present-day treelines of the
respective species, and in near association with meltwater ponds in the lower forefields. All recovered
specimens represent medium-sized trees. It is unlikely that they are preserved in situ, but more likely
that they have been dislocated from higher primary growth positions by meltwater streams or snow
avalanches.
Table1. Radiocarbon dates of subfossil pine remains with signs of beeing gnawed by beaver (Castor fiber L).
Sources: Öberg & Kullman 2011; Kullman & Öberg 2013, 2015).
Site no.
14C yr BP
Intercept cal.yr BP
Rel. elevation m
Lab. code
1
8270±60
9280
420
Beta- 250914
2
8520±70
9530
380
Beta-264394
3
8220±40
9190
520
Beta-332277
Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains warmer-than-present
conditions and a patchily treed and rich mountainscape.
International Journal of Research in Geography (IJRG) Page| 4
Figure 3. Megafossil tree remains (Pinus sylvestris) with signs of being felled by beaver (Castor
fiber). Radiocarbon dates are given here in intercept form. A & B. Site 1. 9280 cal yr. BP, 420 m
higher than the local pine treeline. C. Site 2. 9435 cal. yr BP, 380 m higher than the local pine
treeline. D. Site 3. 9190 cal. yr BP, 520 m higher than the local pine treeline. The dated log is upraised
from its original position, partly beneath an eroding moss cover.
Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains warmer-than-present
conditions and a patchily treed and rich mountainscape.
International Journal of Research in Geography (IJRG) Page| 5
Figure 4. A.”Peat ball”, outwashed from beneath the glacier cover and with a multitude of plant macro
remains (vascular plants and bryophytes), representative of present-day northern boreal ground cover
flora. A bulk sample of the peat and its content returned an age 5175 cal. yr, indicative of a glacier
smaller-than-present at that time and a species-rich forest floor. Ice patch near Tärnaglaciären, 1115
m a.s.l. Photo: 2012-09-22. B. Peat cake dated 3890 cal. yr BP, without tree remains and reasonably
shortly predating glacier inception. Tärnaglaciären, 1075 m a.s.l. Photo: 2010-08-20. Source:
Kullman & Öberg 2013, 2015.
4. DISCUSSION
The concerned megafossil pine remains comply with a general view of early Holocene tree growth of
different tree species at much higher elevations than today and consequently in a substantially warmer
climate. Obviously, ice-free glacier cirques offered congenial growth conditions to boreal trees and
other plants, with respect to insolation, moisture, wind shelter and propagule accumulation (Kullman
& Öberg 2020a,b). At all the investigated sites, megafossils of different tree species have been
recovered at higher positions and earlier than displayed by the beaver-gnawed stems presented in this
study. This implies that early Holocene summer temperature may have been at least 3 °C warmer than
the early 21st century (Kullman & Öberg 2013).
Pine is not the most preferred species by beaver for building or forageing. Thus, the confinement of
utilization of this species may be a sign of its preponderance in the treeline ecotone (Kullman 2013),
or just a consequence of a small and unrepresentative sample. Indeed, megafossils of more palatable
trees and shrubs are recorded in these habitats, e.g. Sorbus aucuparia, Alnus incana, Populus tremula,
Salix spp. (Kullman & Öberg 2013).
Aside of Pinus, early Holocene tree assemblages in these exclaves, 400-700 m atop of current
treelines, contained macrofossils of Betula pubescens ssp. czerepanovii, Picea abies, Larix sibirica,
Sorbus aucuparia, Alnus incana, Populus tremula. Of these, Larix and Picea have by traditional
pollen analysis been considered as non-existent at the late glacial and early Holocene treeline ecotone
in the Scandes (e.g. Huntley & Birks 1983), although invalidated by Kullman (2008, 2018; Paus et al.
2011).
Analyses of plant macrofossils contained in outwashed ”peat balls” from beneath the ice blanket at the
concerned ”beaver sites” (Fig. 4) have revealed that a forest floor of present-day mountain taiga
affinities prevailed in association with the early Holocene trees. Hereabouts glacier ice and perennial
snow/ice patches came into existence during the Neoglacial, shortly after about 4000 cal. yr BP and
thereby sealed this plant cover repository for millennia to come (Fig. 5).
Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains warmer-than-present
conditions and a patchily treed and rich mountainscape.
International Journal of Research in Geography (IJRG) Page| 6
Characteristic ground cover species in these early tree exclaves were, Vaccinium myrtillus, Vaccinium
vitis-idaea, Empetrum hermaphroditum, Betula nana, Calluna vulgaris, Arctous alpina, Salix
herbacea, Salix spp., Juniperus communis, Ledum palustre (Kullman & Öberg 2013). The presence of
beaver, an obligate forest-dweller has historically been restricted to areas with trees and abundant
shrubs and with lightly frozen water bodies during the winter (Tape et al. 2018). Thus, the current
findings of beaver action further sustain the inference of discrete and scattered high-productive
species-rich forest communities in ice-empty glacier cirques, high above present day treeline positions
(Kullman & Öberg 2020a, b). These habitats are assumed to have offered particularly congenial
growth conditions for a diverse tree- and ground cover flora, different from surrounding more poor
and trivial alpine tundra, prevailing over most of the high-mountain landscape during the early
Holocene. For more detailed inferences in these respects, see Öberg & Kullman (2011). All this
happened during the warmest phase of the early Holocene, 9000-9500 years before the present, a
period of extensive tree spreading at high elevations (Kullman 2013: Kullman & Öberg 2020).
Possibly, these early and species rich high elevation forest exclaves may have functioned as dispersal
nodes in the context of downslope spread of trees and other plant species by the early Holocene
(Kullman 2001, 2008; Väliranta et al. 2011).
Analogously, data from NW Alaska, have dated beaver presence in current tundra areas, about 8000
cal. yr BP. This was also in a warmer-than-present climate with more advanced treelines (McCulloch
& Hopkins 1966).
The surprisingly early presence of beaver is of particular interest in terms of immigration and spread
of biota following the general deglaciation of northern Scandinavia. During the early Holocene with a
higher Bothnian sea level in the east, incised bays penetrated more deeply westwards into the
Scandinavian penisula. This might have promoted water spread of some plants and animals towards
the west (cf. Kolstrup 2007; Kolstrup & Olsen 2012; Kullman in press). On the other hand, faunal
spread of vertebrates from the west is a possibility, judging from palaeorecords of e.g. Red Squirrel
(Sciurus vulgaris) at the Nowegian coast, dated 11 500-10 400 14C years before present (Larsen et al.
1987).
5. CONCLUSION
• Climate warming during the past 100 years has evoked substantial glacier/ice patch recession in the
Swedish Scandes.
Megafossils of different tree species are exposed at the forefields of of receding glaciers and snow
patches.
They range in age between c. 11 700 and 4000 cal, yr BP, when temperatures were as most 3 °C
higher than present and treelines 500-700 m above current levels.
As particularly focused in this study, some megafossils showed sign of being gnawed by beaver
(Castor fiber L.), an obligate forest dweller. The concerned specimens range in age between 9530 and
9190 cal. yr BP.
These findings, in addition to recovered contemporary macrofossils specimens of common ground
cover plants, enhance the forest structure of the early tree enclaves, inferred by the tree megafossils.
• These ”beaver datings” are the earliest in the Swedish Scandes and raises questions about postglacial
immigration routes. This aspect is tentatively discussed.
ACKOVLEDGEMENTS
Finacial support for this study was defrayed by the Göran Gustafsson Foundation. Dr. Lisa Öberg is
thanked for competent and constructive comments on earlier versions of the manuscript.
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Citation: Leif Kullman. Early Holocene presence of beaver (Castor fiber L.) in the Scandes sustains
warmer-than-present conditions and a patchily treed and rich mountainscape International Journal of
Research in Geography. vol 7, no. 1, 2021, pp. 01-08 doi: http://dx.doi.org/10.20431/2454-8685.0701001.
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Climate warming during the past century has imposed recession of glaciers and perennial snow/ice patches along the entire Swedish Scandes. On the newly exposed forefields, subfossil wood remnants are being outwashed from beneath ice and snow bodies. In Scandinavia, this kind of detrital wood is a previously unused source of postglacial vegetation and climate history. The present study reports radiocarbon dates of a set of 78 wood samples, retrieved from three main sites, high above modern treelines and stretching along the Swedish Scandes. In accord with previous studies, pine (Pinus sylvestris) colonized early emerging nunataks already during the Late Glacial. Around 9600-9500 cal. yr BP a first massive wave of tree establishment, birch and pine, took place in "empty" glacier cirques. Both species grew 400-600 m above their present-ay treeline position and the summer temperatures may have been 3.5 oC warmer than present. In respons to Neoglacial cooling, treelines of both birch and pine descended until their final disappearance from the record 4400 and 5900 cal. yr BP, respectively. During the entire interval 9600 to 4400 cal. yr BP, birch prospered in a 100-150 broad belt above the uppermost pines. The recent emergence of tree remnants in the current habitats relates to the contemporary episode of climate warming, possibly unprecedented for several past millennia. It is inferred, by an anology with the past, that in a future scenario with summers 3.5 °warmer than present, the birch treeline may rise by 600 m or so.
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In 1945 Storglaciären located in the Kebnekaise massif, northern Sweden, was selected for a long term study of the climatic impact on glaciers and an annual mass balance programme was initiated. Since the mass balance year 1945–1946 the average annual winter precipitation has increased by 0.53 m water equivalent (w. eq.), the annual average ablation has decreased by 0.58 m w. eq., and the annual average net balance has increased from -0.80 m w. eq./year to +0.30 m w. eq./year. The decrease in ablation is caused by a decrease in summer temperature of about 1°C. In addition, frontal retreat has decreased the low altitude area of the glacier and hence contributed to this decrease in ablation. A comparison with results from mass balance studies at three additional glaciers in the Kebnekaise area show that Storglaciären is representative for the area. The mass balance of Storglaciären is also positively correlated with each of four Norwegian glaciers, although accumulation and net balance are better correlated than ablation. Correlations become less positive with increasing distance between sites. Glacier front measurements provide filtered and delayed indications of climatic changes. Small glaciers are at present in balance with the climate. However, retreating fronts of large glaciers show that they are still adjusting to the major warming of the first half of the 1900s.
Article
Data from different disciplines are integrated and used to propose an alternative hypothesis regarding the Upper Weichselian environmental developments in the central Scandinavian mountains. This hypothesis, which tries to integrate new and controversial finds and explain them in a new light, is far from proven and needs much further research. It is proposed that parts of the mountains have been glacio-isostatically more depressed than previously thought and that the glacial dynamics could have been much more pronounced than traditionally assumed. As a consequence, it is proposed that Baltic-Bothnian water crossed temporarily ice-free areas with low-altitude thresholds across the Scandinavian mountain range, sporadically allowing drainage towards the Atlantic Sea, and even that Atlantic water could temporarily have entered into Sweden from the west. Such land-water scenarios could have resulted in a water transport system that steered floating tree seeds and sprouts, transported by rivers from central Europe and/or by seawater from the Atlantic, into the mountain range. There, high isostatic rebound rates during early deglaciation phases could quickly have made coastal areas become land, allowing plants to start their life cycle almost immediately.