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a Department of Ecology and Environmental Science, Umeå University, SE 901 87 Umeå, Sweden.
b Old Tjikko Photo Art & Science, Handöl 544, SE 837 71, Duved, Sweden.
*Corresponding author: E-mail: leif.kullman@umu.se
Chapter 2
Print ISBN: 978-81-971580-2-5, eBook ISBN: 978-81-971580-9-4
A One Hundred-Year-Study of Climate
Change and the Upper Range of Tree
Growth (Terminus arboreus) on MT.
Getryggen in the Swedish Scandes –
Updated Change in an Historical
Perspective
Leif Kullman a* and Lisa Öberg b
DOI: 10.9734/bpi/eieges/v9/11730F
Peer-Review History:
This chapter was reviewed by following the Advanced Open Peer Review policy. This chapter was thoroughly checked to
prevent plagiarism. As per editorial policy, a minimum of two peer-reviewers reviewed the manuscript. After review and
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comments of the editor(s), etc. are available here: https://peerreviewarchive.com/review-history/11730F
ABSTRACT
In the context of global climate change, positional treeline change since the early
20th century and up to 2023 was assessed along two elevational transects on
Mt. Getryggen in the southern Swedish Scandes. Baseline data within permanent
line transects, initially representing the year 1915, i.e. right at the onset of the
present warming phase, were compared to later intermittent records up to 2023.
These were complemented by repeat photography of individual trees. Concerned
species were the regional treeline dominants; mountain birch (Betula pubescens
ssp. czerepanovii), Norway spruce (Picea abies), Scots pine (Pinus sylvestris)
and Grey alder (Alnus incana). Treelines of these species responded with
different degrees of upshifts, although with substantial inter-site variability,
related to topoclimatic conditions. Betula displayed the largest advance, by 215
m over the entire past 100 years. This maximum magnitude of change complies
with data from widely different parts of the Swedish Scandes. Such a common
performance indicates that regionally recorded summer and winter warming by
1.4 and 1.9°C, respectively, is the ultimate cause. In a long-term historical
perspective, recorded by local megafossil tree remnants, most congenial
conditions for birch and pine growth at high elevations prevailed around 10 500-
9400 cal. yr BP, when the local treeline of Betula and Pinus reached 1355 and
1250 m a.s.l., respectively. The former elevation coincides with the upper limit of
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Vaccinium myrtillus and the upper range of the low-alpine belt. With the
exception of Pinus, recent treeline upshifts were mainly accomplished by
phenotypic responses of millennial-old krummholz specimens (environmentally
dwarfed modes), prevailing as relicts above the treeline by the early 20th century.
Only occasionally, has treeline advance by Betula and Picea originated from
seed regeneration during the past century. These circumstances may set the
limit for further rapid and extensive advance of the last-mentioned species, where
and when the pool of high-altitude old-established krummholz specimens
becomes depleted. The prospect of further advance of seed disseminated Pinus,
is judged to be higher, according to recent trends
Keywords: Climate change; treeline dynamics; regeneration modes; Betula
pubescens ssp. czerepanovii; Picea abies; Pinus sylvestris; Alnus
incana; Swedish Scandes.
1. INTRODUCTION
Impacts and consequences of climate change and variability on the living
landscape and its various compartments have become a common scientific and
landscape management concern in recent days. Climate models disseminate a
prospective view of ubiquitous and calamitous climate warming over the rest of
the present century [1]. In particular, subalpine and alpine ecosystems, prevailing
at their edge of climatic tolerance, are supposed to display first and
straightforward responses to altered climatic conditions. Alpine treelines and
treeline ecotones are complex ecological zones, primarily considered to depend
on altitudinally decreasing summer and winter air and soil temperatures,
Accordingly, they stand out as primary monitoring targets, and in this context [2-
11]. They have proven an ability to display clearly interpretable indications of
changed growth conditions and pending landscape transformations in response
to altered climatic conditions [12-22]. However, a close association with altered
climatic conditions has been disputed by certain authors, stressing herbivory
[e.g., 23,24], an option questioned by Kullman [25]. Thus, further inquiry into this
issue seems motivated.
The treeline, a central concept in this study, is the limit for successful survival of
the tree form at high elevations. Henceforth, the treeline is narrowly defined for
each species as the maximum elevation (m a.s.l.) of individuals with stems at
least 2 m tall [cf. 26-29]. Elevational treeline dynamics integrates climate-
vegetation interactions on scales of decades to centuries and longer. Compared
to other delimitations of the forest/alpine tundra transition, this treeline definition
provides the most clear-cut expression of the climatically-conditioned upper limit
of a certain tree species, apt for adequate comparisons in space and time [30,20,
7,31,32].
Treeline studies in different parts of the world display large inter-site variations
with respect to degree of upslope treeline shifts during the past 100 years of
climate warming [33,34,15,35-44]. Spatially disparate upshifts relate to
topoclimatic and ecological constraints in combination with ground cover
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[34,5,6,45-56]. A general problem within dynamic treeline science is that many
recent studies suffer from unprecise treeline definitions [cf. 57,55,7], which
impairs adequate inter-site and temporal comparisons. Moreover, recent treeline
shifts may locally result from abandoned land use, contemporaneous with climate
change, which further complicates cause attribution of past and modelling of
future performance.
It may be a truism that the past has an influence on the present and future
landscape structure [cf. 58]. Therefore, recent treeline dynamics can be properly
understood only with a longer retrospective and observational view on the
concerned ecosystem dynamics [cf. 59]. In the Scandes, modern treeline change
has to be evaluated in perspective of a common and predominantly climate-
driven retraction towards lower elevations throughout most of the Holocene, with
an elevational nadir right before the onset of the modern episode of climate
warming in the early 20th century [60,14,25].
Effects of climate warming over the past century, subsequent to the dire, dark
and cool centuries of the Little Ice Age, roughly between 1300 and 1850 AD
[61,62] offer a unique opportunity to improve our understanding of the
relationship between landscape-scale high-elevation tree growth and thermal
conditions. Despite predictions of pending climate warming and extensive tree-
and forest encroachment on the alpine tundra [63-66], there is little factual
evidence of ongoing forest advance on a broad general landscape scale [67,68].
This discrepancy between models and the real world-performance may be a
consequence of imperfect understanding of climate-treeline relationships, giving
rise to over-simplified models, not subjected to proper validation tests. These
aspects constitute the background and rationale of the present study, which
mainly focuses on factual elevational dynamics during the past 100 years of the
dominating tree species in the southern Swedish Scandes, i.e., mountain birch
(Betula pubescens ssp. czerepanovii), Norway spruce (Picea abies), Scots pine
(Pinus sylvestris) and Grey alder (Alnus incana). The results are reviewed in
perspective of local and regional Holocene treeline history, based on robust
megafossil data.
This paper is an updated and extended version of a previously published paper
[69].
2. STUDY AREA
2.1 Location, Climate, Physiogeography and Land Use
The present study focuses on treeline performance on Mt. Getryggen in the
southern Swedish Scandes (63° 11´ N; 12° 19´ E). The mountain peak reaches
1382 m a.s.l., while surrounding valley bottoms linger about 700-750 m a.s.l. The
bedrock consists of amphibolite and gneisses and is covered with glaciofluvial
deposits and peat. Predominant soils in the treeline ecotone are thin spodosols.
The study site is ideal for monitoring treeline movements as an even topography
and adequate soils provide possibilities for upslope tree advancement over broad
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mountain slopes. Particularly important is the fact that this mountain has been in
the focus of different generations of treeline researchers since the early 20th
century [34,70,71]. Thus, modern multispecies treeline dynamics may be
evaluated against a proper baseline record, a truly unique circumstance.
Fig. 1. Location map, showing the position of Mt. Getryggen (arrow) in the
Handölan Valley, northern Sweden
With respect to the climate character, the concerned mountain is within a
transition zone between sub-oceanic and sub-continental influences [72]. The
nearest official meteorological station (Storlien/Visjövalen, 642 m a.s.l., 20 km to
the northwest) accounts for climate data, representing the so-called normal
period 1991-2020. Mean temperatures for January, July and the year are -5.4,
12.3 and 2.0 °C. For the same period of time, annual precipitation averages 827
mm, of which 45 % falls as snow (Swedish Meteorological and Hydrological
Institute).
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2.2 Modern Climate Change (Instrumental and Proxy Records)
Like the situation in many other northern regions worldwide, the climate in this
part of the Scandes has improved during the past 100 years, as expressed by
records from Storlien/Visjövalen meteorological station (see above), with data
from the early 20th century. For the period June-August and December-
February, linear trends, of + 1.4 and 1.9 °C, respectively, were recorded (Figs. 2,
3). Summer and winter warming were most prominent during the first four
decades of the 20th century, with a peak in the late 1930s. Thereafter, the
temperatures have prevailed at a relatively high level, although with a dip 1940-
1990 and a large variability on the scale of years and short periods of years.
Precipitation has increased by c. 15 % since the early 20th century [5,6,73].
Fig. 2. Annual records (1901-2023) of June-August temperature at
Storlien/Visjövalen meteorological station. Source: Swedish Meteorological
and Hydrological Institute
Fig. 3. Annual records (1901-2023) of December-February temperature at
Storlien/Visjövalen meteorological station. Source: Swedish Meteorological
and Hydrological Institute
Proxy records of centennial climate change are provided by shrinking glaciers,
earlier melt-out of alpine/subalpine snow patches and vanishing or reduced
permafrost (Fig. 4). In addition, snowmelt and birch leafing and defoliation in the
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subalpine forest belt nowadays takes place about two weeks earlier and two-
three weeks later, respectively, than in the early 20th century and [7,29,74,75].
As a consequence of earlier and more complete snow-melt, the ground in alpine
and subalpine areas in general is found to have dried out at earlier dates during
the past century, with cascading ecological effects [76,77,78].
Fig. 4. The history of the glacier Storsylglaciären (Jämtland) over the past
100 years. The area is estimated to have diminished by 40 % over the
period. Photos: Left. Fredrik Enqvist 1908. Mid. 2001-09-04. Right. 2021-09-
15.
2.3 Plant Cover
The study area provides a representative view of the subalpine biogeographic
organization of the Swedish Scandes. A discrete subalpine belt with almost
monospecific predominance of mountain birch (Betula pubescens ssp.
czerepanovii) prevails between c. 700 and 900 m a.s.l. On the mountain here
concerned, the upper distribution of closed birch forest stops at a broad front
along a topographic discontinuity in the form of “knee”, 840 m a.s.l., representing
transition from concavity to a virtually flat terrace (Fig. 5). Solitary tree birches,
minor groves and a few narrow wedges extend substantially higher, benefitting
from local wind shelter. Large and late-thawing snow patches at high elevations
provide more or less continuous supply of meltwater throughout the summer,
which supports the well-developed birch forest belt below, also embracing
extensive mires [cf. 69].
Fig. 5. The south-facing slope of Mt. Getryggen. Continuous birch forest
stops beneath the distinct terrace around 840 m a.s.l., while small groves
and solitary trees (not visible here) extend to much higher elevations.
Photo: 2010-09-10
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Scattered specimens of subordinate tree species prevail in the birch forest -
Sorbus aucuparia, Picea abies, Pinus sylvestris, Populus tremula and Alnus
incana. Except for Sorbus, which as a tree extends virtually as high as Betula, all
other tree species have their respective treelines in the lower reaches of the birch
belt.
The ground cover vegetation in the birch forest and up to the top of the mountain
is since long ago affected by summer grazing and trampling by reindeer
(Rangifer tarandus), and activities associated with reindeer pastoralism. Since
the early Holocene, these impacts constitute an integral part of this environment
in the Scandes [79,80]. In the Sami tradition, the study mountain is referred to as
the “Reindeer Mountain” (Sw. Renfjället). Past human usage is indicated e.g., by
the finding of an arrow head, which recently melted out from a late-laying snow
patch, and probably it was lost in connection with reindeer hunting about 1000
years ago [81]. A Sami dwelling site prevailed until the early 20th century close to
the valley floor on the southeast-facing flank of the mountain. Associated
activities caused some local thinning of the birch forest and possibly logging of
outlier pine trees, although leaving the birch treeline ecotone virtually unaffected,
as seen at the present day. At the present-day, birch forest reclaims ground,
caused by these activities.
Since the early 20th century, reindeer numbers have increased steadily in this
region [29], which has to be taken into account in connection with evaluation of
recent vegetation dynamics in these mountains [75]. Aside of Sami utilization,
human disturbance to the plant cover in the treeline zone is negligible in this
region [56,82] although Mt. Getryggen is currently frequently used by tourists,
both summer and winter.
2.4 Holocene Arboreal History
The course of present-day vegetation and treeline evolution integrate modern
climate change and variability with vegetation structures and patterns reflecting
climate and environmental change prevailing during earlier epochs of the
Holocene. Therefore, a short historical overview may be motivated, as a basis for
interpretation of the obtained results in proper perspective.
For long, vegetation history has been synonymous with pollen analysis.
However, that approach has been proven to provide an imperfect and incomplete
view of Holocene treeline elevation, composition and structure [83-88]. A more
qualitatively accurate view in these respects is provided by analyses of
megafossil tree remains, preserved in peats and under glacial ice [60,85,89].
Accordingly, all of our principal treeline trees were present in northern and
western Scandinavia on early deglaciated nunataks already during the Late-
Glacial, high above (500-700 m) the current (early 21st century) treeline positions
[88]. The relatively highest elevations of tree growth were obtained 11 500-9000
cal. yr BP, ultimately forced by higher- than-present orbitally driven summer solar
irradiance and accordingly summer temperatures, about 3 °C, warmer than
during the first decades of the present century [15,85]. Examples of high-
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elevation tree growth during this period are provided by megafossils of Pinus and
Betula (Figs. 6,7). The most striking aspect of warmer climate in the distant part
of the Holocene is provided by uncovered megafossil remnants of tree birch,
exposed by minor erosion close to the peak of the mountain, 1355 m a.s.l. This is
310 m higher than the treeline of the present day and 525 m higher than the
position by the early 20th century [cf. 39]. This site coincides with the upper
range of soil podzolization and Vaccinium myrtillus. Tentatively, the upper limit of
the last-mentioned species may be used as a proxy for the maximum elevation of
tree growth during the warmer-than-present early Holocene.
As evident from robust megafossil records, these conditions also enabled the
growth of warm-demanding broad-leafed trees (9500-7000 cal. yr BP), such as
Quercus robur, Corylus avellana (Fig. 8), Ulmus glabra, Alnus glutinosa and Tilia
cordata, at relatively high subalpine elevations, currently dominated by cool-
adapted boreal species, foremost mountain birch, spruce and pine [84,90].
methodological challenge in high-alpine vegetation history [cf. 85], which impairs
projections of landscape evolution in a possibly warmer future.
Fig. 6. Megafossil remains of a pine tree, recently exposed by wind erosion.
The position is 1250 m a.s.l., which is 510 m higher than the present-day
pine treeline in this slope. Radiocarbon dating yielded 10 520 cal. yr BP.
Photo: 2003-09-01. Source: [91].
Apparently, the early Holocene high-elevation forests contained a richer tree flora
than today. In particular Alnus incana constituted a prominent member of the the
early subalpine ecosystems. Picea abies, for long considered as a late-Holocene
immigrant to the Scandes [e.g., 92], prevailed at high elevations much earlier on
Mt. Getryggen than previously assumed (Fig. 9), as sustained also at other
localities in this region [69,88]. Subsequently, gradual cooling was initiated and
maintained throughout much of the Holocene, which forced regression of the
dominating pine treeline ecotone and the warmth-demanding trees. As a further
consequence of this Neoglacial environmental change, the present-day character
of the treeline ecotone gradually evolved. The result was the current zonation
pattern, characterized with a distinct upper subalpine birch forest belt and
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montane spruce/pine forests below. Apparently, the subalpine birch forest belt
was initiated gradually in the wake of a distinct cooling episode around 8200 cal.
yr BP [60]. This course of climate and associated regressive tree cover evolution
reached a Holocene nadir by the late 19th century, culminating with the Little Ice
Age of the past 700 years or so.
Substantial warming during the past 100 years represents a fundamental break
in the Holocene course of climate and biogeographic evolution in the study
region [39,91,93]. Recent treeline rise of Betula and Pinus have locally reached
levels higher than any time during the past 5000-7000 years [15,60]. In fact,
temperature reconstructions from the southern Swedish Scandes indicate a long-
term cooling trend over much of the past 800-1000 years prior to the recent warm
phase [25,94]. Thus, present-day climate and biotic change, as focused in this
paper, take start from a natural bottom level, representing “ice age-near” climatic
conditions and climatically depauperized subalpine and alpine ecosystems.
Fig. 7. Erosion scar (1355 m a.s.l.) close to the peak of Mt. Getryggen,
which has exposed wood remnants of a birch stem, dated 9370 cal. yr BP.
Photo: 2012-09-12.
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Fig. 8. Nut shells of Corylus avellana, recovered in a sloping fen on the
south facing slope of Mt. Getryggen, 740 m a.s.l., which is about 300 m
lower than the current treeline of birch. The result of radiocarbon-dating
was 9300 cal. yr BP. Source: Kullman [90].
Fig. 9. This specimen of Picea abies, 770 m a.s.l. attests to the long-term
existence of spruce on the east-facing slope of Mt. Getryggen. Wood
remnants buried in the soil indicate that this spruce existed 5650 cal. yr BP.
Tree size was attained well after 1915, thus it was part of the general
treeline rise process during the past 100 years. The spruce is a “natural
monument”, protected by law (Sw. naturminne) and is named Old Pompe,
to the memory of a legendary dog. Photo: 2011-08-31.
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3. METHODS
3.1 Positional Treeline Shift
Treeline shift was elucidated along broad elevational belt transects, running
upslope and downslope from the birch treeline positions given by Smith (1920)
and resurveyed by Kullman (1979) and Kullman & Öberg (2009). A recent
treeline survey of two of the mountain slopes was carried out in 2023, i.e. east
and south-facing aspects, SITE 1-2, respectively. Each transect embraces about
1 km in width. At the most recent survey, trees above the treeline position by the
mid-1970s were tallied. Some specimens had been photographed at earlier
occasions, which enabled rephoto analysis and clear-cut evidence of the overall
character of change or stability.
The earliest measurements of altitudes (m a.s.l.) were made with an aneroid
barometer (System Paulin), while the most recent assessments are based on a
GPS navigator (Garmin 60CS).
The accuracy of Smith´s treeline records, with virtually the same definition as the
one used in this study, was assessed and confirmed by boring birches at 2 m
above ground level, i.e., the minimum treeline definition [34]. Growth rings were
counted in the laboratory, using a stereomicroscope.
3.2 Radiocarbon Dating
This study contains a few radiocarbon datings of megafossil tree remnants,
representing early Holocene treelines. These have been performed by Beta
Analytic Inc., Miami, USA. In the running text and figure captions the dates are
converted to calendar years before present (cal. yr BP, with “present” = AD
1950), based on IntCal13 [95]. For the sake of simplicity, they are cited as”
intercept values”.
3.3 Germinability Studies
Annual records of birch germinability originate from seed collected in a stand 775
m a.s.l. on the south-facing slope of Mt. Getryggen. Percentage germinated
seeds was tested in the laboratory [cf. 96].
4. RESULTS
4.1 SITE 1. East-Facing Slope of Mt. Getryggen
Birch treeline: Smith [71] measured the treeline of mountain birch to be at 810
m a.s.l., a figure which was confirmed as reasonable, by boring old-looking trees
2 m above the ground level around this specific elevation (Fig. 10) [34]. By the
mid-1970s, the treeline had reached 920 m a.s.l. (Fig. 11) [34] and in 2010 it was
positioned at 935 m a.s.l. (Fig. 12). The most recent assessment was in 2023,
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when 945 m a.s.l. was attained (Fig. 13). Thus, over the past 100 years, the
treeline has shifted 135 m upslope. This continual course of improved growth
conditions is manifested by increased ramification and foliation of birches
growing in the treeline advance zone (Figs. 11-13).
Fig. 10. This birch, growing at 810 m a.s.l. represents the treeline around
1915, as evident from dendroecological analysis and early 20th century
field records [71]. Photo: 2003-07-20.
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Fig. 11. In the mid-1970s, a solitary birch tree, 920 m a.s.l., with a recently
broken top marked the treeline. The same tree has recovered substantially
up to the present day. Photos: Upper left.1978-09-15. Upper right. 2010-06-
28. Lower left. 2019-08-15. Lower right. 2023-08-05.
Fig. 12. By 2010, the treeline had advanced further 15 m in altitude. Up to
the present day, that position has become consolidated by sustained
growth of this multi-stemmed individual 935 m a.s.l. Photos from left to the
right: 2010-06-28, 2019-08-15, 2023-08-05.
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Fig. 13. A new and still higher treeline position is represented by this pair
of photos. 945 m a.s.l., which displays sustained increment of the canopy.
An extensive and multi-stemmed stool of stems from the same root
indicate that this is a decidedly old-established specimen. Photos: Left.
2017-08-15. Right. 2023-08-05.
Treeline rise relative to the position prevailing in 1915, seems to have been
accomplished predominantly by phenotypic plasticity of old-established
polycormic specimens, i.e. transformation from prostrate to erect tree form.
Judging from the presence/absence of stools with multiple stems, recently
established specimens are rare in the zone where the treeline has advanced
during the past hundred years.
Below the treeline, substantial ingrowth of birch has taken place in topographical
concavities, where previously too much late-lying snow precluded any tree
growth. This phenomenon is exemplified for a site on the lower east-facing slope
of Mt. Getryggen (Figs. 14,15). An age structure study revealed that by the early
20th century, a sparse cover of low-growing birches prevailed at the bottom of
this depression [67]. A peak of instatement of new stems occurred in the 1930s.
Intermittent observations showed that until the late 1980s, these birches
staggered as low growing shrubs (< 0.5 m tall). Subsequently and up to the
present day, stem density and height have increased substantially, without
perceivable stem mortality. By 2023 the height of the stand was 5.1±1.5 m, to be
compared with 0.12±0.03 m in 1980. A more detailed account of the evolution of
birch growth at this site is given by Kullman [67].
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Fig. 14. In the early 1980s, this topographic depression, 795 m a.s.l., in the
birch forest, well below the treeline was totally devoid of trees, although
prostrate specimens lingered in the ground cover. Large masses of late-
lying snow restricted their growth to trees. Up to the present day, the snow
cover has tended to melt increasingly early, and as a consequence, birch
vegetation has proliferated, with trees more than 5 m tall. Photos: Left.
1980-09-07. Mid. 2017-08-07. Right. 2023-09-12.
Fig. 15. The same view as depicted in Fig. 14, showing its character as a
snow-accumulating site. The new birch population has benefitted from
snow retraction at the lower margin of the huge snow drift. The birch stand
at the opposite and relatively snow-poor side of the depression is virtually
unchanged or has somewhat declined in density over the same period of
time. Photo: 2016-05-02.
Spruce treeline: Spruce occurs infrequently as clonal groups in the birch forest
belt on this slope. Radiocarbon dates of subfossil wood preserved underneath
the clones indicates that presence of spruce is here an ancient feature, with its
roots in the mid- Holocene or earlier ([96,97]. Dendroecological analysis
indicated that by the early 20th century the treeline was positioned 730 m a.s.l. in
the form of a multi-millennial old specimen (Fig. 16) [98]. Present-day treeline is
at 830 m a.s.l. (Fig. 17). Thus, during the past 100 years, the treeline has
advanced by 100 m, as a consequence of increased height increment of ancient
krummholz individuals, existing prior to the 20th century. Seed regeneration has
been extremely sparse during this period of time, and has not contributed to
treeline rise, but see Fig. 18.
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Pine treeline: During the past 100 years, the treeline of pine has risen from 700
to 780 m a.s.l., i.e. 80 m in elevation, as manifested by tree-sized pines, younger
than 100 years, growing at various positions above the early-20th century
treeline (Figs. 19-22). In one case an upshifted pine of this category has
produced offspring during the early 21st century (Fig. 20). Elevational pine
expansion, entirely dependent of seed-generated trees, is largely confined to
open habitats, such as mire margins and sparse birch forest. Hereabouts, young
pines have continually increased in size and vigor up to the present day.
Fig. 16. Left. The spruce treeline, 730 m a.s.l. as it was positioned in the
early 20th century and until the mid-1970s. This specimen is radiocarbon-
dated 5905 cal. yr BP [96]. The oldest living stems are more than 400 years
old. Right. During the past few decades, the vitality has perceivably
declined and some of the major stems have died. No seedlings or saplings
have appeared in the neighbourhood or within the clone. Photos: Left.
1973-06-28. Right. 2017-08-14.
Fig. 17. Time series of the new and raised treeline of spruce, 830 m a.s.l.
Stout basal trunks, the largest one > 150 years old, show that the spruce
existed at this spot as a low-growing shrub by the early 20th century. Tree
size was attained in the mid-1970s, when the highest stems measured 2.2
m. At the present day, this is still the treeline and the stems have reached a
height of 2.9 m. Photos: Left. 2010-06-28. Mid. 2019-08-15. Right. 2023-08-05
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Fig. 18. Young spruce, quite recently established by seed,755 m a.s.l. The
height has increased during the past few years. Left. 2019-09-23. Right.
2023-07-06.
Fig. 19. The pine treeline, 780 m a.s.l., that has quite recently reached its
present position. Left. 2019-09-22. Right. 2023-07-06.
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Fig. 20. Pine growing slightly below the new and raised treeline, 775 m a.s.l.
The tree became established in the mid-1950s. During the past 20 years,
the crown has grown and densified. Photos: Upper left. 2004-07-27. Upper
right. 2010-06-28. Lower left. 2019-09-23. Lower right. 2023-07-06.
Fig. 21. Pine, 770 m a.s.l., which is part of the elevational expansion
process and penetration into open spaces at increasingly higher elevations
in the birch forest belt. Left. 2012-07-31. Mid. 2016-06-30. Right. 2023-07-06.
4.2 SITE 2. South-Facing Slope of Mt. Getryggen
Birch treeline: Smith [71] assessed the treeline of mountain birch to be at 830 m
a.s.l., a value which was corrected to 835 m a.s.l. by Kullman [34], by means of
boring an old-looking and still living tree 2 m above ground level (Fig. 23), while
no such old trees were confirmed at higher elevations. This aspect was further
tested in 2017, when 21 trees, at approximately the same elevation, were bored
by the same premises. It turned out that 15 of 21 trees were older than 100 years
at the boring stem position, which means that they were 2 m or higher in 1915.
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Fig. 22. Solitary pine, 750 m a.s.l. established in the early 1940s, and
growing 50 m above the treeline of the early 20th century. Between 1991
and 2023, the height of the stem has increased from 3.4 to 6 m. During the
past 10-15 years, the pine has produced offspring within a radius of 10 m.
Photos. Left. 1991-07-08. Mid. 2017-06-10. Right. 2023-07-06.
Fig. 23. Old-growth and moribund birch, which marked the treeline by the
early 20th century (830 m a.s.l.). One large branch has been lost during the
past 20 years. Photo: Left. 2004-09-09. Mid. 2019-06-19. Right. 2023-07-02
Fig. 24. Left. Birch tree, 905 m a.s.l., which could be the tree, that
constituted the treeline position by the early 1950s, according to Kilander
(1955). This specimen, like many others alike, existed in a more low-
growing form much earlier. Right. Radiocarbon-dated wood remnants in
the upper soil and associated with living stems yielded a date of 4770 cal.
yr BP. Source: Öberg & Kullman [31]. Photos: 2017-06-14
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In 1951, Kilander [70] assessed the treeline at 905 m a.s.l. Buried wood remains,
associated with an extant tree stem (Fig. 24) at the same elevation, are dated
4770 cal. yr BP. Coring at the ground surface level yielded 221 year-rings [31].
Smith [71] noted prostrate birches 920 m a.s.l. and Kilander [70] found a 1.5 m
high thicket 920 m a.s.l. and 1 m shrub at 950 m a.s.l. Thus, paleoecological and
sub-recent data suggest that a pool of prostrate birches existed high above the
treeline prevailing by the early 20th century. Birches within this category may be
the result of phenotypic regression of prior tree-sized birches, in response to
Neoglacial cooling of the past 3000 years or so and survival in that state up to
the present day [60]. This contention is supported by sparsity of megafossil birch
remains above the present treeline during the past 3000 years [60]. Taken
together, these circumstances support the contention that modern treeline rise is
mainly accomplished by phenotypic progression of birches contained in a pool of
old-established, relictual krummholz birches growing above the treeline of the
early 20th century. Nevertheless, birch currently prolifically produces viable
seeds (Fig. 25).
Fig. 25. Annual tests of birch seed viability (1973-2023) from trees on the
south-facing slope of Mt. Getryggen, 775 m a.s.l.
Fig. 26. The treeline (930 m a.s.l.) advanced 100 altitudinal meters between
1915 and 2007. Subsequently, the former treeline marker still prevails and
further rise has occurred. Photos: Left. 1979-08-08. Right. 2017-08-08.
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Fig. 27. The new and most recent treeline position is manifested by this
small tree, 1045 m a.s.l. After 2017, one main branch has been broken,
presumably by snow pressure. Photo: Left. 2017-08-03. Right. 2023-08-03.
By the mid-1970s, the treeline had reached 930 m a.s.l. [34] (Fig. 26) and in
2007 it was still positioned at the same elevation [39]. The most recent
assessment of the treeline position was in 2023, when 1045 m a.s.l. was reached
in the form of a monocormic, slender and youngish looking specimen (Fig. 27).
In summary, over the past 100 years, the treeline has shifted 215 m upslope,
although the position of the upper limit of closed forest has remained virtually
unchanged. Although much densified by increased clonal growth of older birch
individuals (Fig. 28).
Fig. 28. Prolific regeneration by basal shots has resulted in densification of
the birch forest (775 m a.s.l.). Photo: 2017-06-17.
Conspicuously, many trees in the treeline advance zone have increased their
foliation over the past few decades (Fig. 26). Some specimens have suffered
from winter browsing by hare (Lepus timidus). Despite heavy damage to twigs
and buds, birches often survive this brutal treatment, which may affect growth
form at the mature stage (Fig. 29).
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Spruce treeline: Spruce is currently a truly rare tree in this slope, dwindling in
the dense birch forest. The majority of today´s uppermost trees existed as low-
growing krummholz by the early 20th century. At that time, the treeline was at
730 m a.s.l. [99]. Thereafter, it has advanced by means of accelerated height
increment of old-established krummholz spruces to 830 m a.s.l. (Fig. 30). This
implies treeline rise by 100 m over the past 100 years, although stability and
consolidation have prevailed since the mid-1970s. Notably, very few solitary
seed-generated tree- or sapling-spruces have become established between the
treeline of the early 20th century and the present treeline (Fig. 31).
Fig. 29. Young birch tree in the upper treeline advance zone, 965 m a.s.l. A.
In the early summer 2017, it displayed signs of heavy browsing by hare. B.
As evident later in the same summer, it had survived this treatment with a
more compact crown and foliation, which may impact its future growth
form. Photos: Upper left. 2017-06-15. Upper right. 2017-08-03. Lower left.
2019-07-25. Lower right. 2023-07-02.
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Fig. 30. Old-growth spruce, 830 m a.s.l., which has developed from a low
shrub in the early-20th century to an erect tree by the mid-1970s. During
the past few decades, the foliation has increased and tree growth seems to
be stabilized at this elevation. Photos: Upper left. 2004-07-27. Upper right.
2017-06-17. Lower left. 2019-09-19. Lower right. 2023-09-10.
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Fig. 31. Quite unusually today, young and fast-growing seed-generated
spruce trees have emerged in sparse birch forest close to the treeline, 810
m a.s.l. The main stem was recently broken by snow pressure. Photos:
Left. 2017-06-17. Mid. 2017-08-09. Right. 2023-09-12.
Pine treeline: A solitary and fast-growing young pine has established at a mire
margin in the lower slope, 740 m a.s.l. (Fig. 32). A large subfossil wood remnant
of a tree-sized pine, at the same elevation, with > 200 tree rings indicate that this
was the minimum treeline elevation during the Little Ice Age (Fig. 33). Thus,
virtually no treeline rise during the past 100 years is manifested here, although
pine has locally reclaimed its pre-20th century position, from which it was
depressed by Little Ice Age cooling in this region [cf. 25] or contemporaneous
cutting by the Sami population, formerly dwelling here in the neighborhood.
Establishment of pine in this slope is largely prohibited by much densified birch
forest stands around the pine treeline (Fig. 28). A few widely scattered pine
saplings are found in the lower reaches of the alpine tundra (Fig. 34).
Fig. 32. Present-day pine treeline, 740 m a.s.l., in the form of a 3.5 m high
idividual, established in the early-1980s. This pine tree increased its
folitation substantially after 2019. This is the first pine tree recorded in this
slope by the past 100 years. The site is a mire/birch forest transition.
Photos: Left. 2016-09-23. Mid. 2019-09-22. Right. 2023-07-08.
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Fig. 33. Megafossil pine remnant, 740 m a.s.l., close to the site of the
present treeline. Radiocarbon-dating yielded 465 cal. yr BP (Beta-446537).
Photo: 2017-08-08
Fig. 34. Young pine sapling, 0.6 m tall growing in the alpine tundra 895 m
a.s.l. Dead twigs indicate repeated dieback by frost desiccation, when
passing the maximum height of the snow cover. This specimen was found
dead in 2023. Photo: 2017-08-08.
Grey alder treeline: Today, Grey alder (Alnus incana) is a rare tree in the
subalpine birch belt [35,100], which contrasts with the situation prevailing in the
first part of the Holocene [cf. 60,101]. In this specific slope, clonally perpetuating
individuals occur sparsely in the subalpine birch belt, somewhat below the
uppermost birches [100]. Radiocarbon-dated wood remnants indicate that one
such multi-stemmed tree-sized and still living specimen existed already 4400 cal.
yr BP. It remained as a low-growing shrub until the late 1990s. Thereafter, it has
increased in height, to reach maximum 3.3 m in 2023 (Fig. 35). Tree-sized alders
displaying more than 100 growth rings 2 m above ground-level exist 755 m a.s.l.
Tentatively, these indicate, by comparison with the present position, that the
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treeline has advanced 135 m over the past 100 years. Young saplings are
extremely rare in the treeline ecotone, but exceptionally recorded at the fringe of
receding snow patches high above the treeline (Fig. 36).
Fig. 35. Clonally reproducing multi-stemmed shrub of Alnus incana, 890 m
a.s.l. Photo: Up to the present day, the entire stand has attained a height of
about 3 m. Photos: Left. 1998-07-22. Mid. 2017-08-03. Right. 2023-07-02.
Fig. 36. Sapling of Alnus incana, recovered in a moss carpet at the fringe of
a melting snow patch (1060 m a.s.l.), high above the present treeline.
Photo: 2004-08-02.
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5. DISCUSSION
Substantial treeline rise over the past 100 years, relates to climate warming after
the Little Ice Age, and has affected all extant tree species growing on the study
mountain. Treeline stands of these species display a striking lack of dead stems,
which is compatible with quite recent stand rejuvenation and sustained favorable
growth conditions.
Pine treeline has advanced on SITE 1, while grey alder shifted upslope on SITE
2 only. For all species, the extent of upshift varied substantially between the sites
(Table 1). This is an experience also from prior, more extensive studies
concerning all species focused in this study [39]. The reason was found to be
local topoclimatic conditions, causing treeline advances to manifest in muted
form in most parts of the landscape. Although the largest current upshifts display
the most clear-cut relationship with climate change, this dimension of upshift
appears quite infrequently in the treeline landscape. These idiosyncratic
responses could imply that even in a hypothetical case of future climate warming,
large expanses of the alpine landscape may remain un-treed, as inferred for the
warmest periods during the early Holocene, i.e., 10 500-9000 cal. yr BP [cf.
60,81]. This indicates that modelling projections, purporting extensive and
pending birch forest encroachment on the alpine tundra [e.g., 63,64] are not
supported by recent observations of recent performance [cf. 27,102].
At odds with results from other regionally extensive monitoring studies in the
Swedish Scandes over the period 2003-2012 [103], treeline progression in the
area here concerned has continued over the same period of time. This
discrepancy may relate to treeline definition incongruencies, representing
different sensitivities to climate change.
Consistent with the present study, results from previous investigations of birch at
the same locality as this study [104], displayed a distinct peak during the 1930s
in the age frequency distribution of the growth initiation of stems, representing
transition from krummholz to erect tree mode. Likewise, radial growth peaked
during this period.
Treeline rises of all species are manifested by a relatively low densities of new
trees atop of the treeline prevailing by the early 20th century. Relative to
extensive treeline rises, a common pattern for all tree species is that forest
encroachment on the alpine tundra is patchy and largely insignificant, which
represents a consistent regional pattern [68]. Recent spread of tree species
saplings into exceptionally high positions of the alpine tundra [5,6,78] is found to
have been a transient phenomenon as mortality in these cohorts has been
substantial during recent years [105]. However, in the case of Pinus an extensive
reproduction surge within the lower treeline ecotone has become manifested
during the past 10-15 years and may have a bearing on the future [75,106,107].
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Table 1. Total treeline upshift (m) 1915-2023 at the two investigated
transects on Mt. Getryggen (SITE 1-2).
Species/Site
SITE 1
SITE 2
Betula
135
215
Picea
100
90
Pinus
80
no value
Alnus
no value
135
The most extensive treeline advance was displayed at SITE 2 by birch; 215 m.
This implies a rate of about 22 m per decade, which is double the rate as an
average for different taxonomic groups worldwide [54,108]. Seen in the
perspective of robust inter-regional secular treeline chronologies [60], recent
treeline advance with this magnitude is a geographically recurrent pattern.
Analogous high values have been obtained for birch and other tree species in
different regions from south to north along six degrees latitude along the entire
Swedish Scandes [14,29,31,39,109]. This common response pattern, at odds
with other studies [e.g., 46], indicates that secular climate change is the ultimate
driver of treeline change. This contention is further strengthened by the fact that
recorded treeline rise by more than 200 m corresponds quite well with recorded
secular warming (1915-2023) by 1.4 °C and a temperature lapse rate of 0.6 °C
per 100 m altitude [110]. Moreover, the present results provide little support [cf.
14] of generalizations that treeline birch population dynamics is primarily driven
by reindeer grazing as purported by Van Bogaert et al. [24]. In fact, reindeer
numbers have consistently increased over the past 100 years [15].
Birch treeline rise by maximum 215 m implies, in perspective of the regional
Holocene birch treeline history [60], that recent advance has reached an
elevation unprecedented during at least the past 5600 years. With respect to
Pinus in the southern Swedish Scandes, the corresponding figure is about 7000
years ago [15,29]. In view of the current discourse concerning future climate
change, these early dates of equally or higher-than-present treeline, could be
interpreted in terms that current temperatures are unprecedented during the past
6000-7000 years. However, such an opportune interpretation may be premature,
since the new trees above the old treeline are quite few and generally of small
stature, as shown in this study. This implies that they may not leave any
discernible imprint on the megafossil record in the future. Accordingly, similar
episodes may have happened unrecorded in the past, which would make the
recent upshift less unusual in an historical perspective. For example, substantial
local treeline upshifts have been recorded elsewhere in the southern Swedish
Scandes about 2000 and 1000 year before today [14,25,60].
As evident from SITE 2, birch treeline rise appears to have been accomplished to
some minor extent by rapid height growth of single-stemmed, slender and
monocormic trees. Such a growth mode is strongly indicative of quite recent
seed-regeneration [cf. 69,111], which seems reasonable in the perspective of
enhanced seed viability recorded in recent decades (Fig. 25). This contrasts with
predominant phenotypic responses of old-growth polycormic krummholz
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specimens previously responsible for treeline shift during most of the past 100
years [cf. 7,31],
The strong reliance of vegetative regeneration of old-established specimens in
connection with recent treeline displacements of Betula and Picea, could imply a
certain degree of vegetation stability. This quality combines with an ability to
respond with rapid stem height increments, when (if?) future climate warming
takes place [112,113]. Further extensive upshifts of these treelines seems
restricted as the existing pool of krummholz specimens above the present
treeline has become depleted, when many individuals have already attained tree
size [39,104]. In the case of Pinus, the prospects of future advance and
reorganization of the treeline ecotone seem more likely [75,106,107].
6. CONCLUSION
1. Elevational treeline rise (1915-2023) of boreal tree species was assessed
along two line transects in the southern Swedish Scandes.
2. Concerned species were, Betula pubescens ssp. czerepanovii, Picea abies,
Pinus sylvestris and Alnus incana.
3. Maximum change was 215 altitudinal meters, which neatly complies with
summer and winter temperature rise by 1.4 and 1.9 °C, respectively, over
the paste 100 years. This course of change is reflected along the entire
Swedish Scandes.
4. Betula, Picea and Alnus responded mainly by phenotypical transformation of
old-established krummholz individuals to upright tree form.
5. Treeline rise of Pinus was entirely dependent on establishment of new
individuals at increasingly high positions. Pinus is the only species that
displays an ongoing trend of treeline rise up to the present day.
6. Continuation of recent climate and treeline trends may reinstate the treeline
ecotone back to its position, species composition and structure, prevailing
during close to the Holocene thermal optimum. Such a putative change may
be monitored in the future, using the present records as a baseline.
ACKNOWLEDGEMENT
Two anonymous reviewers checked the manuscript.
COMPETING INTERESTS
Authors have declared that no competing interests exist.
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Biography of author(s)
Leif Kullman
Department of Ecology and Environmental Science, Umeå University, SE 901 87 Umeå, Sweden.
He was born in 1948. He is a professor emeritus in physical geography and plant ecology at Umeå
University, Sweden. His research area mainly focuses on climate-related landscape change on different
temporal scales in high-mountain regions. His achievements have concerned treeline responses to
present-century climate warming. In addition, his research provides a fundamentally new view and
paradigm shift of postglacial tree performance during late-glacial and early postglacial time.
Lisa Öberg
Old Tjikko Photo Art & Science, Handöl 544, SE 837 71, Duved, Sweden.
She was born in 1958. She has a PhD in biology with a special focus on climate-related treeline
dynamics in the Scandes. Furthermore, her work has included radiocarbon dating of uniquely ancient
spruces at the tree line. At present, she works as a freelance nature photographer and scientific author.
___________________________________________________________________________________
© Copyright (2024): Author(s). The licensee is the publisher (B P International).
DISCLAIMER
This chapter is an extended version of the article published by the same author(s) in the following journal.
International Journal of Research in Geography. 4(3): 10-35, 2018.
Available: http://dx.doi.org/10.20431/2454-8685.0403002
Peer-Review History:
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