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Received: 06.11.2009 Received in revision: 07.12.2009 Accepted: 07.01.2010 Published: 02.03.2010
One Century of Treeline Change and Stability
- Experiences from the Swedish Scandes
Leif Kullman
Department of Ecology and Environmental Science, Umeå University, 901 87 Umeå, Sweden
leif.kullman@emg.umu.se
Abstract
This paper elaborates and visualizes processes recorded in a recent regional and multi-site study of elevational
treeline dynamics during the period 1915 to 2007 in the Swedish Scandes. The purpose is to give a concrete face of
the landscape transformation which is associated with the recorded treeline shifts. The main focus is on stand-level
structure of past and present treelines and the advance zones, where climate change elicited responses by Betula pu-
bescens ssp. czerepanovii, Picea abies and Pinus sylvestris. All species shifted their treelines upslope by a maximum
of c. 200 m in elevation. Most sites, however, manifested changes of smaller magnitudes. This relates to topoclima-
tic constraints which decouple treeline performance from the macroclimate. The general character of sites which
support large and small treeline shifts, respectively, are outlined. The spacing, age structure, growth rates of the tree
advance zones are accounted for each of the concerned species. In temporal and spatial detail, the different tree spe-
cies responded individualistically according to their specific ecologies. Current spread of young seedlings and sap-
lings to increasingly higher elevations in the alpine tundra is particularly highlighted as it may represent the forefront
of future treeline advance. It is argued that the current evolution of the treeline ecotone represents a fundamental,
although not necessarily entirely unique, reversal of the long-term (Holocene) trend of neoglacial treeline descent.
Keywords:
treeline, tree species line, dynamics, stand structure, climate change, historical perspective
L. Kullman
One Century of Treeline Change and Stability
- Experiences from the Swedish Scandes
Landscape Online 17, 1-31. DOI:10.3097/LO.201017
Landscape Online
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One Century of Treeline Change ... 17 / 2010
Introduction
A conspicuous landscape ecological consequence of
climate change is manifested by elevational shift and
structural change of the boundary zone between forest
and alpine tundra, represented here by the “treeline”
(Kullman 1979, 1997, 1998; Grace et al. 2002; Holtmei-
er 2003; Holtmeier & Broll 2007; Payette 2007). Such a
course of landscape transformation interacts with plant
and animal life, geomorphological dynamics, hydrology,
and biogeochemical cycles and constitutes a pertinent
and prime focus for projective landscape ecological re-
search (Holtmeier 2003; Butler et al. 2009).
Studies at different spatial scales have empirically con-
firmed different degrees and characters of treeline res-
ponses to climate warming and variability (all seasons)
over the past century (Hustich 1958; Aas 1969; Kull-
man 1979; Meshinev et al. 2000; Juntunen et al. 2002;
Shiyatov 2003; Esper & Schweingruber 2004; Mazepa
2005; Kharuk et al. 2006, 2009; Danby & Hik 2007;
Devi et al. 2008, Harsch et al. 2009). More inertial
performance has been displayed and contemplated in
other studies (Masek 2001; Lloyd & Fastie 2002; Kör-
ner 2003; Dalen & Hofgaard 2005; Rössler et al. 2008).
A recent multi-site regional study of elevational treeline
change in the southern Swedish Scandes has evidenced
various degrees of treeline rise during the period 1915
to 2007 (Kullman & Öberg 2009), when summer and
winter temperatures rose oscillatory by 1.0 - 1.4 °C, fol-
lowing upon the “Little Ice Age” cool period ranging
about 1300-1850 (Grove 2004). After a first distinct
warming peak in the 1930s, temperatures declined mar-
ginally for some decades. Around 1988, temperatures
lifted again and re-stabilized at virtually the same level
as in the 1930s. The present paper seeks to provide a
concrete and visual “face” of the “dry” numbers and
statistics behind the landscape transformation, which
emerges from the above-mentioned study by Kullman
& Öberg (2009). This endeavour also integrates earlier
case studies performed in the same region by the pre-
sent author and puts it all in its proper historical (Holo-
cene) context. The detailed methodology and statistics
are given in the last-mentioned paper, which is briefly
summarized in the following section.
Revisitations (2005-2007) were made at well-identified
and circumscribed sites (elevational transects), with
baseline data from 1915 and 1975 (Smith 1920, Kull-
man 1979, 1981a, 1986a). The pattern of sites, as estab-
lished by Smith (1920), is based on the elevation of the
treeline at each 2-5 km kilometer along the mountain
valleys in the study region. A main result of the most
recent survey was that the treeline of Betula pubescens
ssp. czerepanovii (mountain birch), Picea abies (Norway
spruce) and Pinus sylvestris (Scots pine) had ascended
in elevation by a common maximum of c. 200 m. This
was very close to the theoretical prediction, based on
a lapse rate of 0.6 °C temperature change per 100 m
altitude. At finer spatial and temporal scales, however,
the treeline responses were more variable and species-
specific, with averages (1915-2007) between 70 and 90
m (Table 1).
Table 1. Species-specific magnitudes of altitudinal treeline shifts during different periods of time.
Source: Kullman & Öberg (2009).
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One Century of Treeline Change ... 17 / 2010
It is clear that the sensitivity to climate change was
substantially modulated and constrained by local to-
poclimatic conditions. Prior to 1975, birch and spruce
advanced more rapidly than pine. Thereafter, pine has
taken the lead and appears in the long run relatively
most suited to benefit from a warmer and drier cli-
mate.
Study region
The present study mainly concerns the southern Swe-
dish Scandes, (63°25 to 61°05´N; 12°03´to 13°11´E)
and provides results from a monitoring program en-
compassing more than 200 sites with baseline data
from the early 20th century (Fig. 1).
The treeline is defined as the elevation (m a.s.l.) of the
uppermost individual of each species, with a minimum
height of 2 m at a specific location. In general, the
mountain birch forms a subalpine forest belt and the
highest treeline towards the alpine tundra. Spruce and
pine treelines are located about 50 and 100 m below,
respectively. Typically, high above the treeline, there is
the tree species line, which is the highest elevation of
the most advanced outposts for each species, irrespec-
tive of size.
In some parts of the study region, the upper coni-
ferous forest and the lower mountain birch belt have
been subjected to low-density grazing by livestock and
some cutting for fire and construction wood during 1-2
centuries prior to the 1940s, after which these practices
have virtually ceased. As evident from numerous inter-
views with older local residents and ground surveys,
these activities were most intensive somewhat below
the treeline ecotone. Hereabouts, the structure and
composition of the virgin vegetation landscape have
been slightly altered in some areas (Kullman 1979).
Prior analyses have evidenced that the new and high-
er treeline positions, attained during the past century,
are not conditioned by responses to past and present
human land use and associated land abandonment
(Kjällgren & Kullman 1998; Moen & Lyngstad 2003;
Virtanen et al. 2003). This contention is supported by
accounts of botanists and geographers working in the
study area during the later part of the 19th century and
first half of the 20th century (Kellgren 1891, Smith
1920; Kilander 1955). These explicitly state that there
was no discernible impact of humans or livestock on
the treeline position. Strictly locally, extensive use of
Figure 1: Map showing the study region in Mid-Central
Sweden.
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One Century of Treeline Change ... 17 / 2010
natural resources in the upper subalpine belt over the
past 2000 years or so is claimed, although ambiguously
(cf. Holmgren & Tjus 1996), to have lowered the forest
limit more or less permanently (Karlsson et al. 2007). In
this respect, the current study region is fundamentally
different from many mountain ranges around the glo-
be, where human land use has been much more intense
and with a longer history into the past (e.g. Aas & Faar-
lund 2000; Motta & Nola 2001; Gehrig-Fasel et al. 2007;
Rössler et al. 2008; Brynn 2008). Grazing and tramp-
ling by reindeer are ubiquitous, chronic and integrated
disturbances to alpine and subalpine vegetation, with a
history spanning many millennia of mutual adaptation
(Nordhagen 1928; Cairns & Moen 2004; Eriksson et al.
2007). Since treeline rise of birch, spruce and pine does
not discriminate between areas with and without reinde-
er (Kullman 2004b, 2005b), there is no reason to invoke
reindeer impact as principal drivers in this respect. This
is not to say that reindeer have no structural and com-
positional impact on subalpine/alpine vegetation (cf.
Olofsson et al. 2009). However, the treeline position per
se is not determined by reindeer action.
General accounts of historical land use in the subalpi-
ne/alpine regions of the Swedish Scandes are provided
by several authors (e.g. Emanuelsson 1987; Ericsson
2001; Kullman 2005a; Linkowski & Lennartsson 2006;
Ljungdahl 2007; Öberg 2009). Further details concer-
ning the study area and its climatic, biogeographic, au-
tecological and paleoecological context are outlined by
Kullman (2005a), Bergman et al.( 2005), Kullman &
Öberg (2009).
Results
Below, the species-specific modes and circumstances
of treeline (in a broad sense) change or stability are
outlined and related to the topoclimatic heterogeneity
which characterizes the mountain landscape. Based on
Kullman & Öberg (2009), the focus is on three speci-
fic aspects, (1) the treeline proper, (2) the tree species
line, (3) the advance zone, i.e. the interval between the
treeline in 1915 and the new and higher treeline (if any)
prevailing in 2007. Unreferenced statements refer to the
last-mentioned paper.
Mountain birch
In its present continuous form, the subalpine birch belt
is usually comprehended as a consequence of a cold oce-
anic/suboceanic macroclimate with a deep and late-lying
snow cover. Historically, it owes its existence to neogla-
cial summer cooling and increase of snow cover over
the past 5000-6000 years, when this forest type expan-
ded in the wake of altitudinally receding and increasingly
fragmented pine-dominated subalpine forests (Kullman
1995a, 2003, 2004a; Bergman et al. 2005).
In most temporal and spatial scales, the performance of
the birch belt is closely related to seasonal snow cover
dynamics. Specifically, a thick winter snow pack and
a steady supply of melt water from alpine snow fields
throughout the summer are prerequisite for the “health”
and continued existence of the subalpine birch belt (Hä-
met-Ahti 1963; Kullman 1981a; Aas & Faarlund 2000).
Sustained life of individual mountain birches also de-
pends on recurrent physical disturbances, which kill
the main stems in each clone. If that does not hap-
pen before these, by age, have lost the ability to resp-
rout, the individual birch will die. This mechanism
contributes to the often sparse birch stands and tree-
less subalpine heaths, prevailing in flat areas where the
potential for frequent snow breakage is relatively low.
Thus, frequent disturbances, mediated e.g. by snow or
insect outbreaks, are essential for the long-term conti-
nuity and vitality of the mountain birch forest (Kull-
man 1981a; Miles 1978; Haukioja & Koricheva 2000).
Modes and magnitudes of treeline rise
Experiences from extensive aging of individual stems by
boring, in combination with historical treeline positional
records, make it quite easy to pin point the elevation of
the early 20th century treeline with a fairly high degree
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One Century of Treeline Change ... 17 / 2010
of accuracy (Kullman 1979). The position is marked by
the uppermost large and gnarled trees, often with fissu-
red bark with dark lichens (Fig. 2). In cases where the
treeline has raised during the past century, trees growing
higher upslope are distinctly slender, with smooth bark
and with less epiphytic lichens (Fig. 3).
pulse of the 20th century, i.e. the 1920s-1930s (Kull-
man 1979).
Particularly during the past few decades, a second wave
of modest elevational birch advancement has been ac-
complished also by establishment and rapid juvenile
height growth of newly seed-established individuals,
so-called genotypic treeline rise. That course of change
is consistent with a dramatic rise in birch seed viability
over the past 20 years (Kullman 2007a).
As indicated above, the extent of treeline rise during the
past century and up to the present was highly depen-
dent on the topographical relief, which governs local
climates and associated ecological conditions, such as
wind exposure, snow cover and soil depth. The largest
upshifts, i.e. around 200 m occurred in long, sweeping,
concave and well-watered slopes, with a rugged me-
sotopography (boulders, crevices and ledges) offering
wind shelter and enduring supply of soil moisture from
local snow accumulations higher upslope (Fig. 5, 6).
Quite frequently the newly raised treeline is situated in
steep and rocky terrain that is virtually inaccessible to
Figure 2: Old birch tree, representing the treeline position
in 1915. The position of the raised treeline, 1010 m a.s.l., is
indicated by the arrow. Mt. Mettjeburretjakke 885 m a.s.l. 6
September 2007.
Figure 3: Young and rapidly growing birch, constituting the
recently elevated treeline, 1010 m a.s.l., at the site depicted in
Figure 2.
To a large extent, treeline rise over the past century
has been achieved by accelerated height growth of
old-established individuals, implying a transformation
from krummholz, i.e stunted low-growing individuals,
to erect arborescent modes. This option is due to an
eminent capability for vegetative regeneration by ad-
ventitious shoots from individual stem bases and old
root stocks (Kallio & Mäkinen 1978), which also ma-
kes birch relatively resilient to physical disturbance, e.g.
logging and browsing (Hustich 1958; Holtmeier 1974).
Even tiny and slender birch trees quite often display
disproportionally stout basal trunks, which may yield
150-500 years of age (Kullman 1993a, 2005b) (Fig. 4).
Given this ability of long life, provided by vegetative
regeneration, it is easily comprehended that some of
these birches are residuals from past millennia, when
higher temperatures and treelines than today prevailed
(Kullman 2003; Kullman & Kjällgren 2006). This me-
chanism for treeline rise has been termed “phenotypic”
(Kullman 2005a) and contributed substantially to the
relatively swift upslope response to the first warming
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One Century of Treeline Change ... 17 / 2010
reindeer and humans (Fig. 6).This implies that birch trees
cannot have been excluded here by prior reindeer grazing
or any kind of human activity. This is one line of arguing
that treeline rise is a natural, climatically forced process.
Over most of the mountain landscape, where a more
simple topography prevails, treeline ascent was to some
degree decoupled from the regional climate and less ex-
tensive than the theoretical prediction and locally even
non-existing. Therefore, the average treeline rise over the
entire study region was much smaller than 200 m. A value
around 70 m is more representative of the modal situa-
tion (Kullman & Öberg 2009). It is an intriguing questi-
on whether sites with no or insignificant treeline advance
have remained analogously inertial during earlier parts of
the Holocene.
In slopes with sub-optimal rise, birch expansion has, as a
rule, been halted where wind-exposure abruptly exceeds a
certain threshold in the terrain and snow deflation beco-
mes aggravated. This constraint may become acute in as-
sociation with some topographic discontinuity, i.e. a more
or less abrupt transition from concavity to wind-exposed
convexity (Fig. 7. In fact, wind appears as one of the most
important factors which sets and forms the local treeline
and its structure (Holtmeier 2003; Seppälä 2004). Notably,
certain paleoecological data from the Norwegian Scandes
suggest that strong winds have become an increasingly re-
stricting agent throughout the Holocene (Paus 2010).
During the period after 1975, the pace of birch treeline
rise has declined relative to the preceding 60 years and
relative to spruce and pine. This is particularly evident
on south-facing slopes and in those parts of the study
area with the relatively most dry and continental climate
regime. In these settings, treeline birches are frequently
losing vitality, which manifests as drying and dying of
individual stems and branches and lack of height incre-
ment (Fig. 8). Obviously, these phenomena relate to in-
creased drought as late-lying snow patches have tended
to disappear earlier during the summers of the past 15-
20 years (Kullman 2007a,b).
Figure 4: Left. A birch tree clone with no stems higher than 2 m in the early-20th century, when this site was
above the local treeline. Right . Disproportionally stout lower trunks display 150-200 tree rings. Mt. Falkstolen,
925 m a.s.l. 14 September 2009.
Figure 5: Concave slope morphology offers optimal condi-
tions for treeline rise in near-equilibrium with regional evoluti-
on of the thermal climate. This landscape was entirely treeless
in the early-20th century. Subsequently, the treeline has shif-
ted 130 m upslope. Mt. Lillskarven, 1020 m a.s.l. 16 July 2006.
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In more oceanic regions, the birch trees have usually
become more vital, with higher individual stems and
denser canopies relative to the mid-1970s (Fig. 9). As a
consequence, the stands now appear denser.
ple snow cover, in some cases even close to receding
fronts of high-alpine glaciers (Fig. 11).
Likely, much of these 500 m represents a recent ad-
vance of the tree species line (Kullman 2007a), which
has reached well into the mid-alpine belt. In the ear-
ly- and mid-20th century, birch saplings and low shrubs
(krummholz) rarely existed more than 100 m above the
treeline (Smith 1920; Kilander 1955). As indicated abo-
ve, some of these individuals constituted the basis for
subsequent treeline rise. The present-day upshifts of
the tree species line to extremely high relative elevations
is a widespread phenomenon, reported also from the
northernmost Scandes (Karlsson 1973; Sundqvist et al.
2008).
The common and swift expansion of the tree species
line indicates that there is little reason to take dispersal
limitation into account when projecting future elevatio-
nal progression of birch vegetation. Findings of birch
seeds on snow patches, 600 m above the treeline (Smith
1920) further substantiates this contention. This upshift
has taken place despite intensive reindeer grazing (Fig.
12). At several sites, solitary birches, 1-1.5 m high, now
occur some tens of meters above the treeline, indicative
of potential further rise in a near future.
Figure 6: View of the uppermost section of the same type
of slope as shown in Figure 5. A particularly complex geo-
morphology offers ideal preconditions for substantial tree-
line advance (190 m). Mt. Lillstendaslfjället, 1070 m a.s.l. 18
September 2006.
Figure 7: Over the past 100 years, the treeline has shifted
65 m upslope in this landscape. This implies that the upper
half of the forest band did not exist in the early-20th centu-
ry. Further treeline rise is virtually prohibited by the abrupt
transition to wind-exposed and snow-poor terrain right at
the current treeline position. Mt. Hårdeggen. 5 August. 2006.
As a rule in the entire study region, the treeline bir-
ches, which were assessed in 2007 and compared with
photographs from the mid-1970s (Kullman & Öberg
2009) have produced virtually no seed-based offspring
in their nearest vicinity during the past 30 years. More
than lack of viable seed (see above), this circumstance
reflects the strong patchiness of specific microscale
conditions for the success (“safe sites”) and sustained
life of the mountain birch (Fig. 10).
A notable aspect of the treeline rise after 1975 is that
some new treeline markers seem to be of hybridogeneous
origin, with morphological leaf characteristics of both
Betula pubescens ssp. czerepanovii and Betula nana.
Higher tree species line
At several localities, the tree species line is located
more than 500 m above the treeline (Kullman 2004 a,b,
2007a,b). Young birch seedlings (10-20 years old) have
become sparsely established in sheltered sites with am-
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The recent advance of the tree species line is paral-
leled by increased alpine plant species richness, inclu-
ding upshifts and increasing abundance of several
boreal plant species, e.g. Epilobium angustifolium,
Cornus suecica, Linnaea borealis, Melampyrum syl-
vaticum, Trientalis europaea, Juniperus communis,
Anthyllis vulneraria, Maianthemum bifolium, Oxalis
acetosella (Kullman 2007a,b). In addition, the relative
proportions of dwarf shrub heaths and alpine grass-
lands have increased in the low alpine zone throug-
hout the past century (Kullman 2002, 2004c, 2007a).
Structural patterns in the advance zone
This zone, which was entirely treeless alpine tundra
shortly prior to 1915, is now sparsely and patchily popu-
lated by birch trees. Closer inspection reveals that some
peat and humus hummocks contain subfossil wood rem-
nants, indicating prior existence of sparse birch stands,
most which seem to have succumbed in response to
the cold conditions prevailing during the Little Ice Age
(Kullman 2005c). Analogous retrogressional processes
are described from northern Finland (Holtmeier & Broll
2006) and south-central Norway (Paus 2010).
Nearest above the treeline markers of the early-20th
century, uniformly younger stems have transformed the
prior alpine tundra to a narrow band of birch stands with
variable density (Fig. 13). Predominantly, the ground
cover within these stands is characterized by Vaccinium
myrtillus heaths. Only rarely have more extensive birch
wood emerged above the former treeline and virtually
never has the advancement of closed forest reached the
same elevation as the new and raised treeline. Thus, af-
ter a century of oscillatory climate warming, there is no
evidence of any major elevational expansion of birch
forest into previous alpine tundra (Kullman 1990) (Fig.
14). The relatively small shift of the upper range of
closed forest, as evidenced also in parts of the Norwe-
gian Scandes with insignificant prior land use (Moen &
Lyngstad 2003; Rössler et al. 2008), suggests that climate
influence on this parameter is subdued and indirect (cf.
Enquist 1933), mediated e.g. by mires, boulder screes,
naked rocks and semi-perennial snow fields, which pre-
cludes the evolution of continuous forest but allows the
existence of single trees and small clusters. This was rea-
lized already by Smith (1920), who found that the “forest
limit”, in contrast to the treeline, varied substantially in
elevation over short distances in areas with virtually the
same climate. Possibly, the large separation of advanced
treelines and forest limits is also a matter of time, as fo-
rest expansion and infilling has to rely more on dispersal
and successful establishment than treeline advance. This
is suggested e.g. from the fact that abrupt treelines and
minor separation of treeline and forest line are charac-
teristic of situations where the altitudinal distribution of
arboreal vegetation remained stable.
Figure 8: Left. The treeline of birch by the mid-1970s (1115 m a.s.l.), when it had advanced by 160 m since 1915. Right. Ty-
pically for the situation in continental areas after 1975, stem mortality is unbalanced and there is virtually no individual size
increment. Mt. Brattriet. 22 August 2006.
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In general, the advance zone has the character of an
intricate patchwork of widely contrasting habitats,
shaped by small-scale toposequences (0.5-10 m) bet-
ween windswept crests and leeward depressions. As
a rule, open spaces predominate, and increase in size
with elevation. Solitary trees or small groves consistent-
ly occupy minor swales and upper leesides within the
microrelief with ample climatic and soil conditions.
Typically, the ground cover in the birch spots is dwarf-
shrub heath, dominated by Vaccinium myrtillus and
Empetrum hermaphroditum. Scattered specimens of
snow-bed plants suggest that prior to dwarf-shrub esta-
blishment and tree emergence, these sites experienced
later snowmelt than today. On most convex surfaces,
strong wind erosion, in conjunction with reindeer gra-
zing and trampling, has removed humus and finer mi-
neral soil, which impedes seed-based birch regenerati-
on (cf. Holtmeier 2003; Holtmeier et al. 2003; Kullman
2005c; Anschlag 2008) (Fig. 15). The pattern outlined
above is consistent with the well-established fact that
reproductive success and radial growth of mountain
birch are critically dependent on adequate snow cover,
soil moisture and associated nutrient conditions (Kull-
man 1993a; Kirchhefer 1996; Karlsson & Weih 2001).
The critical role of soil water availability, as mediated
by wind-conditioned snow drifting is stated by Vajda
et al. (2006).
This topographic and edaphic dependence implies that
the recorded arboreal adjustments to the new and war-
mer climate have enhanced a spatial mosaic of trees
and alpine tundra, which is unique for each slope. The
pattern varies between diffuse dilution zones with mo-
saics of single trees, ribbons, “fingers” along furrows
Figure 9. Characteristically, in areas with relatively more oceanic climate, the canopy of most treeline birches has increased
in size and density since the mid-1970s. Mt. Hamrafjället, 985 m a.s.l..
Figure 10: Young birch tree growing in a minor depression
with more ample snow and moisture conditions than in
the surrounding wind-exposed heath. Mt. Tjalmetjallentj-
akke, 890 m a.s.l. 29 August, 2006.
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and tree “islands” in shallow depressions (Kullman 1979).
As a consequence of the strong reliance on vegetative
regeneration, these spatial tree distribution patterns are
strikingly conservative (Kullman 1991). Complete closure
of the tree cover in the advance zone rely on slowly wor-
king positive feedback loops by which initial presence of
trees and small stands facilitate further evolution of tree
cover. Accordingly, and counter to the theory advocated
by Hoch & Körner (2003), aggregation of trees, ground
shading and associated soil cooling do not seem to be the
crucial mechanism behind the treeline phenomenon in
general (cf. Malanson et al. 2009). In fact, clustering ap-
pears to be a highly beneficial mechanism for sustained
tree growth and reproduction at the treeline of mountain
birch (Fig. 16). Apparently, the reasons are multiple, inclu-
ding wind shelter, protection from frost desiccation and
herbivores, potential for seed and snow trapping (moistu-
re) and absorbing of radiation heat.
In lower parts of the advance zone, quite abundant birch
recruitment is actively taking place in lee topography, main-
ly on modestly moist soils in snow bed and meadow plant
communities. Excess of late-melting snow precluded es-
tablishment, survival and sustained growth of birch prior
to the past 20-30 years. Subsequently, infilling by new re-
cruits has become a conspicuous process, particularly in
snow-rich north-and east-facing slopes. As a consequence,
minor “embryonic” birch stands are forming in the local
recharge topography, which causes some patchy densifica-
tion by shrubs and trees (Fig. 17). This is also a characteris-
tic feature in snow glades within the upper reaches of the
subalpine birch forest, below the advance zone (Kullman
2007a,b). These processes are particularly pronounced
in the most maritime parts of the study area, but virtu-
ally non-existing in more continental settings (Kullman
2004b), where late-lying snow did rarely constrain the es-
tablishment of birch in the past. Consequently, the birch is
developing an increasingly “geriatric” age structure in the
latter region where it appears to be slowly losing ground
(Kullman 2004b). In a hypothetical case of continued
warming, this process is likely to become more ubiquitous
as late-summer soil drought would come to prevail also
in regions, which today harbour large snow patches in the
late summer. On these premises, the mountain birch fo-
rest becomes increasingly fragmented and the advance
zone remains open. Indications that such a trajectory
Figure 11: Young birch sapling, which has established at the
top of a frontal moraine close to the lower margin of the
glacier Ekorrglaciären, 1370 m a.s.l. This site is about 500 m
higher than the local treeline. 5 September 2003.
Figure 12: Despite heavy reindeer grazing, young saplings of
birch and pine have become established on this alpine peak,
300 m above the treeline. Moreover the plant species rich-
ness has increased by 156% since the mid-1990s (Kullman
2007a). Mt. Norder Tväråklumpen, 1250 m a.s.l. 9 August
2009.
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One Century of Treeline Change ... 17 / 2010
is already under way are provided by observations of
reduced vitality of well-established and mature birches
on dry, flat and convex surfaces, where drought effects
are likely to show up first (Kullman 2007b; Öberg 2008).
In contrast, birches established in more snow rich, con-
cave and steep terrain do frequently display increasing
crown density and height (long annual shoots). Quite
commonly, birch stems in these slopes, and which ma-
nifested a treeline rise in response to the warm 1930s,
have recently reached a stature when they easily break
close to the base by the weight of snow. Typically, the
broken stems are replaced by basal shoots and the conti-
nued individual survival is rarely threatened (Fig. 18). As
indicated above, this mechanism is a prerequisite for the
vigour and long-term continuity of the birch belt.
Norway spruce
As a rule, spruce is the dominant tree species in the upper
forest adjacent to the subalpine birch forest belt. This is
mainly and ultimately a consequence of a relatively more
humid and snow rich climate compared to lower eleva-
tions, where pine and spruce alternate as late-successional
dominants in the boreal forest landscape. Spruce usually
prefers mesic or moist sites with a sufficient and stable
snow cover, moving soil water and freedom of drought
during the early summer (Schmidt-Vogt 1977; Tallantire
1977). It is rarely established in pronouncedly dry sites
with a thin and fluctuating snow cover, causing severe
and prolonged seasonal ground freezing or frequent
freeze-thaw cycles. During periods of years with these
kinds of winter conditions, seed regeneration is zero and
mature treeline spruces suffer from extensive needle loss
and even stem mortality. The individual spruces usually
survive these harsh periods and recover when more
favourable conditions return (Kullman 1997, 2007a).
Modes and magnitudes of treeline rise
The spruce possesses an eminent capability to regene-
rate vegetatively (Kallio et al. 1971) and the majority
Figure 13: Typically in the advance zone, minor tree stands
and copses occur in a mosaic with alpine dwarf-shrub heath.
This sub-belt extends just some tens of meters above the
old treeline. The new treeline (arrow) has the character of
isolated outliers, in this case 140 higher than the treeline in
1915. Mt. Getryggen, 835 m a.s.l. 15 July 2009.
Figure 14: Upper. Mountain side with a fairly sharp “forest
line”, coinciding with the treeline. Photo by Harry Smith. 14
April 1914. Lower. Approximately the same view 18 April
2007. The upper forest band has become denser and the
“forest line” may be somewhat higher. Although not visible
here, the treeline (arrow) has advanced substantially and is
currently 125 m higher than in 1915. Mt. Mettjeburretjakke.
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One Century of Treeline Change ... 17 / 2010
(c. 95%) of the spruces which form the present-day
(2007) treeline are layering and multi-stemmed (10-30
stems) clones. Living stems with ages ranging between
400 and 600 years are occasionally found (Kullman
1995b, 2001, 2009a). The oldest individual spruce clo-
ne hitherto dated yielded about 9500 years (Kullman
2001, 2005b, 2009a; Öberg 2008). Thus, some treeline
spruces became established in the warmer climate du-
ring the early Holocene, when treelines in general were
much higher than today (Kullman & Kjällgren 2006).
This implies that they have persisted on the same spots
during subsequent periods both warmer and colder than
at present. Quite frequently, these spruces are bound to
the vicinity of irregularly distributed streams and wells,
which provide ample moisture and some relief from
seasonal ground frost (Kullman & Engelmark 1997;
Kjällgren 2003). As climate gradually cooled throug-
hout the Holocene, they transformed from upright
trees to prostrate krummholz and persisted in that sta-
te for long periods of time. During the past century,
most of this ancient pool of stunted individuals has
attained tree stature, a process that took its start in the
Figure 15: Birch regeneration is impeded on wind-exposed
terrain features, where plant cover, humus and fine mineral
soil is constantly removed. Quite frequently, wind erosion
and reindeer trampling exposes megafossil birch remnants
from stands which prospered here during the Medieval peri-
od and succumbed during the Little Ice Age. Mt. Prediksto-
len, 930 m a.s.l. 4 July 2008.
Figure 16: Stem clustering facilitates survival, growth and
reproduction at the treeline. In the present case, a founder
tree became established between 1910 and 1920. It has been
followed by further recruitment during subsequent decades
(Kullman 2007a). Mt. Storsnasen 970 m a.s.l. 18 July 2009.
1930s-40s (Kullman 1986a). Thus, treeline rise during
the past century has been predominantly a phenotypic
affair (Kullman 2005a, 2009a). In this way, the treeline
of spruce has shifted upslope with a maximum extent
of 220 m, since 1915 (Fig. 19).
Between the mid-1960s and late-1980s, the centenni-
al trend of temperature rise was temporarily broken
and treeline markers and trees in the advance zone re-
sponded sensitively to cooler conditions by extensive
needle loss and stem mortality. At some sites, the tree-
line displayed minor phenotypic downshifting for this
reason (Kullman 1989, 1997). During the past 20-25
years the thermal trend has turned upwards again and
the treeline has frequently advanced to somewhat higher
Figure 17: “Embryonic” birch wood rapidly forming in a
depression, where late-laying snow precluded tree growth
until the mid-2000s. Intensive reindeer grazing and tramp-
ling at this site has not prevented birch establishment. Mt.
Getryggen, 795 m a.s.l. 17 July, 2009.
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One Century of Treeline Change ... 17 / 2010
elevations than prior to the temperature dip. For the same
basic reason, most treeline spruces have taken on a fresh
green and healthy appearance, with exceptionally long
needles and annual shoots (Fig. 19, 20). Frost desiccation
injuries, which was previously a conspicuous sight close to
the treeline, have been virtually absent over the past 10-15
years (Kullman 2007a). In fact, shoot growth has been so
remarkable, that it is not easy to comprehend it solely in
terms of rising temperature. Although a somewhat cont-
roversial issue (cf. Holtmeier & Broll 2007), “fertilization”
by rising CO
2
levels in the atmosphere may have contri-
buted to the documented new and more vigorous state
of young cold-marginal trees (cf. Bergh et al. 2003; Ains-
worth & Long 2005; Loehle 2007; Idso & Singer 2009).
Hypothetically, CO
2
enrichment may also have affected
stomatal conductance in a way that may have contribut-
ed to the reduced incidence of frost drought desiccation.
Higher tree species line
Older botanical records in this region do not indicate the
presence of a tree species line more than 100 meters or
less above the treeline (Smith 1920; DuRietz 1942; Kilan-
der 1955; Kullman 1986a). Many of these are old-estab-
lished krummholz spruces and in this mode spruce may
survive more or less indefinitely and at higher elevations
than similar morphs of mountain birch (Kihlman 1890;
Kullman 2008, 2009a). This presupposes that a complex
local topography creates wind protection and snow burial
of the canopy (Fig. 21). Saplings of spruce now occur
sparsely in the alpine tundra, up to 400-500 m above the
treeline. These specimens, with a size of 0.1-0.3 m, have
germinated during the past two decades and occasionally
they have produced viable seeds (Kullman 2002, 2007a,b)
(Fig. 22). Notably, spruce seeds have been found on late-
lying snow patches (Smith 1920), virtually as high as the
uppermost seedlings recorded in recent years. Thus, in a
hypothetical case of future climate warming, dispersal
limitation is unlikely to influence the treeline position.
Structural patterns in the advance zone
As mentioned above, the spacing of spruce individuals in
the advance zone is conservative and partly a reminis-
Figure 18. Left. Birches which have emerged above the treeline position in 1915. By the mid-1970s they had reached a size when
they were readily broken by the weight of the snow pack. Right. In recent decades, they have rapidly retained their tree size as new
stems have developed vegetatively from the bases of the downed trunks. Mt. Blåhammarkläppen 945 m a.s.l. 10 August 2006.
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One Century of Treeline Change ... 17 / 2010
cence of warmer climates in the past. Historical ob-
servations and age analyses indicate that much of the
spatial pattern of spruce in the treeline landscape was
established prior to the onset of 20th century warming
(Smith 1920; Kullman & Öberg 2009). In general, the
drought intolerance of spruce implies that the largest
densities of spruces occur in north-facing slopes, whe-
re soil drought is less likely to occur.
Despite the fact that seed viability of old-established
spruces growing in the advance zone has increased
substantially since the late 1980s (Kullman 2007a),
there is little evidence in general that significant re-
cruitment of new individuals has taken place in the
advance zone (Öberg 2008). This may relate to increa-
sing surface dryness in combination with a higher fre-
quency of freeze-thaw cycles, which seems to hamper
establishment more than growth and survival of older
well-established individuals (Sykes & Prentice 1996).
Strictly locally, however, single-stemmed individuals
have emerged from seed during the present centu-
ry (Kjällgren 2003). That is mainly on relatively low
mountain outliers, surrounded by an upper coniferous
forest rim dominated by spruce and where there is a
steady supply of propagule from all directions (cf. Ki-
lander 1955; Kullman 2004b). On some mountains of
this kind, particularly at the periphery of the mountain
chain, relatively little melt water from late-lying snow at
higher elevations exists throughout the growth period
and therefore a birch belt is insignificant or even absent
(Wistrand 1981, Kullman 2004b, 2005b). On these pre-
mises, spruce may form the upper treeline and domina-
te the forest-alpine tundra ecotone. Apparently, spruce
dominates here largely due to the snow-accumulating
ability of multi-stemmed high-elevation clones, which
provides sufficient local soil moisture. Some of these
mountains, with only a small cap of alpine tundra with
stunted birch shrubs and some krummholz spruces
about a century ago, have become more or less over-
grown by predominantly spruce trees during the past
century (Fig. 23). In most cases, this process is a resto-
ration to the situation just prior to the onset of the
Little Ice Age (Kullman 2004b, 2005d, e).
Many spruces which raised the treeline during the first
half of the 20th century suffered heavy crown dete-
rioration (needle loss) during some recent cold deca-
des (1960s-1980s), as outlined above. Most individuals
have recovered remarkably over the past 20 years by
swift emergence of new tree-sized stems from the old
Figure 19: Vigorously growing spruce which marks treeline
rise by 185 m since 1915. Prior to that, it persisted as stun-
ted krummholz for centuries or even millennia. Mt. Här-
jehågna, 1050 m a.s.l. 12 July 2007.
Figure 20: Spruce tree growing in the zone where the tree-
line advanced relative to early 20th century. During the past
2-3 decades, annual shoots have been exceptionally long.
Mt. Anåkroken, 990 m a.s.l. 1 August 2007.
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One Century of Treeline Change ... 17 / 2010
root stocks (Kullman 2007a) (Fig. 24). This is a trans-
formation which is clearly perceivable at the landscape
level (Fig. 25). However, for some individual spruces,
foliage decline is still in progress, despite the fact that
the conditions which initiated it have virtually ceased.
Reasonably, this is because they were deprived of a
major part of their photosynthetically active foliage
during the latest cool period. As a consequence, the
basic positive radial growth/temperature relationship
was uncoupled. This has deprived those trees from the
ability to benefit from rising temperatures and CO
2
–
concentrations. Therefore, they currently fail to sup-
port a large unproductive trunk structure (Kullman
1996). Thus, these spruces decline in a feedback loop,
which prevails over decades and provides an example
of the importance of site history. Only rarely has this
course of change led to the death of individuals, since
new stems regularly emerge vegetatively from old root
stocks. As a consequence of these contrasting changes,
the advance zone today is a contradictory mixture of
healthy, rapidly growing spruce clones and individuals
with different degrees of past and present crown dete-
rioration.
Scots pine
Pine is a shade-intolerant species, which is most com-
petitive and invasive in relatively dry, exposed and
snow poor subalpine habitats. During the early Ho-
locene, pine formed the highest treeline towards the
alpine tundra in most parts of the Swedish Scandes.
The treeline reached at least 500 m higher than about
a century ago, when it had descended to its postgla-
cial nadir after many millennia of oscillatory climate
cooling, culminating with the Little Ice Age. This has
been firmly evidenced by radiocarbon-dated subfossil
tree remnants preserved in peat and lake sediments
(Kullman 1995a; Kullman & Kjällgren 2006). Pollen
data yield a broadly similar view (Bergman et al. 2005).
Modes and magnitudes of treeline rise
The treeline position about a century ago stands out
quite distinctly in the landscape as the uppermost
strikingly stout, often moribund and wide-crowned
trees. Typically these are situated in sparse subalpine
birch forest where they appear as solitary trees or small
restricted groves, often in local south-facing slopes.
These trees were the last survivors at the trailing edge
of elevational pine retraction, that had proceeded, with
Figure 21: Upper. Many krummholz spruces growing in the
alpine tundra are old-established relicts from warmer epochs
in the distant past. In this mode, spruce is the hardiest tree
species in the Scandes. Radiocarbon-dated megafossils right
underneath the canopy indicate that this individual existed
already about 1300 years ago (Kullman 2001). This spruce,
which is entirely snow-covered in the winter, was described
by Kilander (1955). Its physiognomy has not changed per-
ceivably since then. Lower. In 2004, a tiny sapling was disco-
vered and tagged, about 4 m leeward of the old krummholz
spruce. The latter has produced seed filled cones at several
occasions during the past decades and is most likely the “mo-
ther” of the sapling, which had increased somewhat in size
in 2009. Mt. N. Tväråklumpen, 1090 m a.s.l. 1 August 2009.
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One Century of Treeline Change ... 17 / 2010
some short breaks, for most of the Holocene (see abo-
ve). Today, they quite often stand as “mother trees” in
the centre of clusters of younger individuals, which
have become established during and after the 1930s, in
close accord with post-Little Ice Age climate warming
(Kullman 2007c) (Fig. 26).
In contrast to birch and spruce, pine does not repro-
duce vegetatively and therefore the long-term postg-
lacial treeline retreat did not leave behind a pool of
well-established millennial-old krummholz individuals,
which could rapidly and opportunistically transform
into erect trees as the climate started to warm in the
early 20th century. This seems to be one important
reason behind the relatively sluggish treeline rise du-
ring the period 1915-1975 (Kullman & Öberg 2009).
Obviously, the first distinct warming pulse of the 20th
century (1920s to 1930s) was not durable enough to
accomplish genotypic treeline rise, although large sap-
ling cohorts appear to have emerged commonly in the
Fennoscandian high mountains (Arnborg 1943; Hu-
stich 1958; Kallio 1975; Kullman 1981b). When the
warming had culminated in the early-1940s, the apical
meristems of most newly seed-produced saplings came
to remain for decades in the hazardous zone just above
the snow cover, where the risk of winter desiccation
and snow/ice abrasion peaks (Hustich 1958, Kullman
1981b, Holtmeier 2003). Observational data from many
parts of Fennoscandia indicate that mortality was high
during some subsequent decades (Kallio 1975; Kull-
man 1981b; Holtmeier 2003). Nevertheless, a fraction
of these pines survived the colder period as suppressed
and deformed shrubs. Quite often they display multiple
branching at the stem bases, indicating that they have
suffered repeated dieback by frost desiccation and snow
abrasion. Some of these pines have been able to take
advantage of the resumed warming, which has prevailed
in a virtually unbroken sequence over the past 20 years.
Thereby they have frequently grown to normal upright
trees and raised the treeline (Fig. 27).
During the past few decades, earlier snowmelt and drying-
out of high-elevation soils have provided preconditions
for increased competitiveness and growth of pine to tree
size in sparse heath birch forests and locally even high
on the slopes above the birch region. Taken together,
these circumstances have acted to promote a relatively
Figure 22: Young spruce sapling growing about 400 m high-
er in elevation than the local treeline. Similar nearby indivi-
duals have produced cones with some viable seeds (Kull-
man 2002). Mt. Åreskutan, 1370 m a.s.l. 2 September 2006.
Figure 23: Low mountain at the eastern continental periphe-
ry of the mountain chain. According to old maps and recent
tree aging, this and similar mountains supported caps of
treeless alpine tundra in the early 20th century. In this case,
spruce is the main colonist. Mt. Fjällskaftet. 22 July 2008.
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One Century of Treeline Change ... 17 / 2010
significant treeline readvance and break of the long-term
retrogressive trend which gradually converted wide tracts
of the highlands of northern and western Sweden from
more or less productive and diverse forest to alpine tundra
and subalpine birch forest (Kullman & Kjällgren 2006). A
particularly striking landscape-scale phenomenon in this
context, is the tendencies for the treelines of pine to su-
persede those of birch and spruce in the most continental
parts of the study area (Kullman 2004b, 2005b) (Fig. 28).
As a consequence, a sparse pine belt is emerging on the
prior alpine tundra, i.e. a return to the early-Holocene zo-
nation pattern.
The great spatial variability with respect to the extent of
treeline rise may relate to the stand heterogeneity of the
birch forest matrix, in combination with seed shortage.
The importance of the last-mentioned aspect is sugges-
ted from the striking clustering of seedlings, saplings and
young trees to the vicinity old “mother trees” just below
the advance zone (Kullman 2007c) (Fig. 26). Also in other
northern hemisphere treeline regions, lack of seeds ap-
pears to be a more general obstacle for the evolution of
denser high-elevation pine stands (cf. Shiyatov 2003; Holt-
meier 2003). Spatial variability with respect to the magnitu-
de of treeline rise is caused also by intensive herbivory on
young pines by moose (cf. Stöcklin & Körner 1999)
Indeed it has been a remarkable experience during the past
20 years to find treeline pines of all ages and sizes practically
devoid of frost desiccation (Kullman 2007c). This contrasts
with the situation prevailing during earlier decades, when a
large proportion of the foliage used to be killed each win-
ter/spring (Kullman 1993b, 2007c). As a consequence,
treeline pines, both trees and saplings, have displayed an
unusually vigorous appearance, with freshly green and long
annual shoots and needles, clearly stressing that current cli-
matic conditions, particularly during the winter period, have
been highly conducive to pine (Öberg 2008). The role of
increasing atmospheric CO
2
concentrations needs further
consideration in this context (cf. Idso & Singer 2009).
Higher tree species line
Although in very low frequency, pine seeds are regularly
spread long distances on the snow crust. Single seedlings
and saplings have established in the alpine tundra 10-20
km from the nearest potential seed sources and 500 -700
m above the local treeline (Kullman 2007a) (Fig 29). At the
present day, the majority of these individuals seems to be
ephemeral and usually they die by frost desiccation when
they start to reach above the critical snow surface (see abo-
ve).
Scarcity of old-growth krummholz in the alpine tundra re-
lates to the fact that the pine treeline is regularly located far
below the alpine tundra and often in a matrix of den-
se and highly competitive birch or birch-spruce forest.
Figure 24: Left. Subalpine spruce which suffered almost complete defoliation (frost desiccation) during cold winters of the
1970s. 14 July 1972. Right. During the past few decades it has recovered substantially by emergence of new stems from old
root stocks. 16 July 2006.
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Under such conditions, the shade-intolerant pine rarely
survives long enough to form genuine krummholz. Whe-
re this occurs, it is mainly on dry and wind-exposed knolls
or rock outcrops with an insignificant soil cover, which
precludes the closure of the tree cover. The strict confi-
nement to windy habitats implies that these pines, often
with a dense, streamlined canopy and stout trunks, are little
externally affected by modest warming. At the lowest ele-
vations, however, some specimens of this category, which
have been monitored since the early 1970s, have recently
displayed tendencies for more upright growth. This seems
to be due to reduced pruning by winter desiccation during
the past few decades (cf. Kullman 2007c). In addition, vi-
able seeds have been produced for at least a decade and
young saplings are showing up leeside of the old krumm-
holz pines (Fig. 30).
Given that the warming trend of the past 100 years or so
will continue, treeline rise is likely to proceed since rapidly
growing pine saplings of near tree-size (1-1.9 m) are quite
abundant at several sites in a zone 0-50 m above the treeli-
ne (Öberg 2008) (Fig. 31).
The newly raised tree species line has reached an elevation,
which corresponds to the highest known postglacial treeli-
ne position, which was during the early Holocene (Lundq-
vist 1969; Kullman 2004a, Kullman & Kjällgren 2006).
Figure 25: Upper. Episodic climate cooling during the 1980s conditioned landscape-scale defoliati-
on of high-elevation spruce forests (grayish trees) in the southern Swedish Scandes. On this speci-
fic slope, the majority of spruce trees lost more than 60 % of their needles (Kullman 1989). Lower. Du-
ring the past decade a remarkable foliage recovery has changed the face of the entire landscape. Mt.
Täljstensvalen (1989 and 2009). Both images were taken in the early evening with similar light conditions.
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One Century of Treeline Change ... 17 / 2010
Structural patterns in the advance zone
Within the advance zone, there is a strict spatial pola-
rization of the tree species mixture. Preferentially, pine
inhabits the warmest, driest and earliest snow free habi-
tats. At the same elevations, mountain birch dominates
sites with more abundant snow cover and moister soils,
while spruce occupies more intermediate positions in
these respects. Reasonably, this differentiation provides
a clue also to the relative success of these species in
perspective of past and future climatic changes.
With increasing elevation, pine trees become conspi-
cuously smaller and younger, clearly indicative of a sur-
ge of upslope spread. Only rarely has pine managed
to become established in steep slopes with abundant
snow and associated dense birch forest. In these set-
Figure 26: Old-established pine (to the right), about 300 ye-
ars old and marking the treeline about a century ago. It has
produced at least three generations of offspring in its close
surroundings during the past century. Mt. Storsnasen, 670
m a.s.l. 22 July 2006.
Figure 27: Left. The pine treeline has advanced by 190 m in elevation since the early 20th century at this locality. This pine
germinated in an exposed snow poor dwarf-shrub heath by the late 1930s. Right. Multiple branching at the trunk base de-
monstrates that this pine suffered repeated dieback during the sapling state. Mt. Solberget, 690 m a.s.l. 25 September 2009.
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One Century of Treeline Change ... 17 / 2010
tings, competition with birch represents a stronger con-
straint to pine abundance and distribution than more
direct climate impacts (cf. Aas & Faarlund 2000).
The advance zone is the result of episodic infilling with
single trees (70-30 years old). These are sparsely scatte-
red in heath birch forests, on outliers of alpine tundra
and on mires, with maximum density in south-facing
slopes (Kullman 1981b, 1986b). Establishment has pre-
dominantly occurred in plant communities dominated
by low-growing Empetrum hermaphroditum and Betu-
la nana (Kullman 1981b, 2004b).
Following the first warming peak of the 20th century, the
density of the subalpine birch forests increased swiftly.
This may have contributed to the relatively slow initial
advance of pine (Kullman 1976, 1981b), since a dense
birch forest acts as a filter reducing the upslope stream
of pine seeds (cf. Holtmeier 1974). During the past few
decades, however, when tendencies for soil drought
have been enhanced, minor recession of dense subal-
pine birch forest is discernible, particularly in the most
continental regions of the southern Swedish Scandes
and as a consequence, solitary pines have successfully
spread and established in these settings (Fig. 32). This
current trend of pine intrusion into the pure subalpine
birch forest also involves expansion deeper into moun-
tain valleys where monospecific birch forests previously
prevailed. For example in the Handölan Valley, scattered
pine trees can now be found about 8 km further towards
the head of the valley than a century ago. This process
includes an elevational rise by 65 m (Fig. 33).
Today, most pine trees in the advance zone, including
the uppermost treeline pines (see above), look strikin-
gly healthy. They have been growing rapidly (0.2-0.4 m/
year) during the past two decades, virtually unchecked
by frost desiccation injuries (Kullman 2007c). Several of
these specimens have reached reproductive maturity and
have produced some offspring in their vicinity (Fig. 32).
Just like spruce, a minor fraction of the pine trees in the
advance zone are sparsely foliated and some continually
lose foliage as a legacy of the cold decades prior to the
late 1980s.
Pine stands close to and somewhat below the advance
zone show signs of selective logging of living and dead
trees by local residents in the early-20th century and ear-
lier. However, in only a few percent of all investigated
localities have such indications been found within the
advance zone (Kullman 1981b). Traces of forest fires
Figure 28: A sparse belt of pine is evolving atop of the sub-
alpine birch forest belt. Mt. Barfredhågna. 14 July 2003.
Figure 29: A few years old pine sapling which has germi-
nated in an exposed glacier forefield, about 700 m higher
than the local treeline. Mt. Storsola, 1370 m a.s.l. 5 Sep-
tember 2003.
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One Century of Treeline Change ... 17 / 2010
in the form of charred fallen trunks and living trees
with fire scars, mostly prior to the 20th century, are
quite common up to a position somewhat below the
treeline prevailing in the early-20th century. At higher
elevations, fire traces are highly exceptional and mostly
confined to the most continental part of the study area
(Kullman 1981b).
Discussion
As demonstrated by Kullman & Öberg (2009), and
further exposed in the present paper, climate warming
during the past century has evoked regional treeline
rise of all studied species in this part of the Swedish
Scandes. Rates and magnitudes were site-specific and
treelines advanced more than closed canopy forest.
The treeline, as narrowly defined here, is paralleled by
analogous changes of other biogeographic and biodi-
versity patterns in Scandinavian high mountain regions
(Kullman 2007a,b). In other words, treeline change in
pristine areas is a robust “bellwether” and indicator of
climatically changed plant growth conditions in gene-
ral.
Despite substantial climate warming and upshift of the
respective treelines, during the past century, the high-
mountain landscape remains largely unforested. The
most conspicuous changes, perceivable at the landsca-
pe level, concern the elevations around the treeline po-
sitions existing about a century ago. In these settings,
substantial stand densification has occurred and prior
alpine vegetation and flora affinities have become less
prominent (Kullman 1986b, 2005b,c).
The large upshifts of the tree species line for all species
here concerned suggest, in contrast to Malanson et al.
(2009), that the critical life stages for treeline formation
are not seed and seedling stages, dispersal or establish-
ment, but rather the tolerance of the early mature pha-
se to ambient air temperature (cf. Grace et al. 2002). If
not, alpine krummholz would not exist and most sap-
lings growing in the alpine tundra could be expected
to attain tree-size. That notion is further stressed by
the fact that even mature treeline trees suffered from
severe dieback and even mortality during some cold
Figure 30: Left. Pine krummholz, more than 400 years old, growing in an open and wind-exposed spot in the lower subalpi-
ne birch belt. During the last decade, a slight tendency for more upright growth can be discerned. Right. At the same time,
offspring has been produced leeward of the old and stunted pine. Mt. Storsnasen, 650 m a.s.l. 15 August 2009.
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One Century of Treeline Change ... 17 / 2010
decades after the mid-20th century, at the same time
as nearby small saplings could be virtually unaffected
(Kullman 1997).
Mountain birch and spruce responded more swiftly
than pine to the first warming pulse of the 20th centu-
ry (1920s-1930s). These differences highlight species-
specific regeneration modes, in response to the same
course of climate evolution. In the case of birch and
spruce, the ability of vegetative regeneration from old-
age and resource-rich root stocks conditioned a gene-
ral and opportunistic in situ transformation of old-
established krummholz individuals to erect tree forms.
Birch was the only species whose treeline upshift and
increased prominence in the advance zone relied both
on sexual and asexual regeneration. Obviously, this
provides both stability and potential for spread, i.e. a
valuable asset in a fluctuating neoglacial climate. Pine,
which virtually lacks the ability of vegetative regene-
ration, responded exclusively with seed-based regene-
ration (genotypic treeline change) and therefore more
sluggishly. In contrast to birch and spruce, it had to go
through all the early life stages before the tree-forming
process could take its start. When the establishment
phase was accomplished, the warming peak had alrea-
dy passed and most pines were locked in the hazardous
sapling phase for decades to come, if they survived at
all. Those who did, however, were well established and
could quite rapidly develop into upright tree size over
the period of climate warming that took its start in the
late 1980s.
During some colder decades after the 1940s, treelines
of all species were stabilized and locally even margi-
nally retreating (Kullman 1997). Resumed warming
over the past 20 years or so has caused treelines to start
rising again. However, the pattern is now quite diffe-
rent, both inter- and intra-specifically, from that prevai-
ling during the earlier expansion phase. The previously
so opportunistic mountain birch has lost its role as the
leading “mountain climber” in the most continental
areas, where vigour of treeline birches has frequently
declined since the mid-1970s The reason behind ap-
pears to be quite complex, and it may be inferred that
increasing soil drought is a prominent component in
this respect. This link is suggested from the fact that
birch decline is strictly confined to discharge topogra-
phy, where early drought responses are likely to mani-
fest earliest (Kullman 2007b; Kullman & Öberg 2009).
In addition, in some parts of the landscape, the treeline
has reached such a high elevation that wind exposure
Figure 31: Young and near tree-sized pine growing about
30 m above the local treeline. Mt. Städjan, 975 m a.s. l.,
14 July 2007.
Figure 32: A solitary pine, which has become established in
the birch forest belt. Obviously, it has benefited from local
drought-induced birch forest regression. A sparse cohort of
small saplings (not visible) has become established within a ra-
dius of about 10 m. Mt. Getryggen 750 m a.s.l. 12 August 2009.
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One Century of Treeline Change ... 17 / 2010
and associated factors prevent tree growth at higher
elevations more or less irrespective of current and fu-
ture thermal evolution (cf. Gamache & Payette 2005).
In this context it is interesting to note that soil drought
and strong winds are discussed as factors delaying the
early Holocene establishment of pioneer subalpine
birch forests in the southern Scandes (Paus 2010).
decades of the spruce and pine treelines, relative to
birch, is explained by milder winters and reduced frost
desiccation injury (Kullman 2007b,c), which should in
principle favour evergreen coniferous species more
than broadleaved deciduous species (cf. Kaplan & New
2006; MacDonald et al. 2007). Accordingly, over the
past few decades there has been a vague tendency for
spruce and pine treelines to approach the birch treeline
(Kullman & Öberg 2009). The highlighted importance
of winter climate conditions for treeline formation is
supported by a global meta-analysis of treeline changes
over the past century (Harsch et al. 2009).
It may be speculated that in case of continued war-
ming and earlier complete disappearance of the seaso-
nal snow cover, the birch belt will eventually become
largely replaced by conifers. A conspicuous expansion
of pine can already be gleaned in the most continental
parts of the study region. In fact, pine is the only spe-
cies, which seems to be able of extensive elevational
population progression, when climate gets drier and
more snow poor in the summer. Thereby it may ac-
complish genuine treeline rise (genotypic) and spread
into existing sparse birch stands as well as colonization
of alpine tundra.
In the relatively most maritime regions, the future tra-
jectories are less predictable. In no case, however, is the
mountain birch likely to disappear completely from the
treeline ecotone. Narrow stands may linger and even
advance upslope along particularly snow rich ravines,
in scattered snow-accumulating depressions. Possib-
ly, small isolated stands may evolve also in evacuated
glacier niches, where birch stands prevailed for some
millennia during the generally warm and dry early Ho-
locene (Kullman 2004a), when pine in general formed
the forest-alpine tundra ecotone (Kullman & Kjällgren
2006). However, steep environmental gradients within
the local topography, particularly with respect to wind
and snow cover, imply that sites available for establish-
ment of birch stands will be restricted in size in the
alpine landscape above the present treeline position.
This, in combination with extensively unfavourable
edaphic and orographic condions, precludes broadscale
upslope forest spread in a potentially warmer, although
not necessarily less windy, future. Thus, in perspective
Figure 33. Young and vigorously growing pine tree, repre-
senting 8 km upvalley and 65 m upslope spread since the ear-
ly 20th century. Mt. Laptentjakke, 770 m a.s.l. 7 July 2008.
Spruce, is less moisture-demanding than birch and its
treeline is usually located in less windy settings subs-
tantially below. That may be one reason for a relatively
high rate of treeline upshift, also after 1975. Over the
same period of time, the pine treeline has advanced
much more rapidly than both birch and spruce and re-
lative to the first expansion phase. Obviously, pine is fa-
voured by its greater tolerance of drought and the fact
that its treeline is usually situated far below the windy
alpine tundra. Concurrently, and most importantly, the
relatively large and rapid progression over the past few
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One Century of Treeline Change ... 17 / 2010
of recent experiences of high-mountain tree perfor-
mance, there is virtually nothing to suggest that in a
warmer climate, birch forest expansion will swamp and
homogenize most of the present-day alpine tundra, as
simplistically speculated (Moen et al. 2004). On the
contrary, a likely consequence will be increased habitat
heterogeneity as small birch groves and solitary trees
will break the monotony of the alpine tundra. Neither
are there any indications that the birch expansion du-
ring the past century has conditioned any competition-
induced decrease in plant species richness (Kullman
2007a, b, 2009b; Sundqvist et al. 2008). In fact, boreal
plant species, e.g. Anemone nemorosa and Chrysosple-
nium alternifolium, have invaded minor birch groves
which became established in the advance zone during
the past century (Kullman 2007a). Thus, on the balance
of existing empirical data, there is no rational ground
for fearing that future treeline rise would cause a ge-
neral impoverishment of high mountain biodiversity
(Kverndal et al. 1990; Idso & Singer 2009; Kullman
2009b). More trees and plant species are exponents of
higher primary production and also add to ecological
and geomorphic stability and resilience (Körner 2003;
Kullman 2009b). In general, the mountains have be-
come greener, more productive and biologically richer
during the past century (Kullman 2009b).
When viewing maximum pine treeline advances by c.
200 m (Kullman & Öberg 2009) within the total range
of Holocene treeline shifts, as reconstructed by mega-
fossil wood remains (Kullman 2001, Kullman & Kjäll-
gren 2006), it could be inferred that the maximal raised
treeline of pine is higher than ever during several past
millennia. Obviously, this reflects a fundamental rever-
sal of a multi-millennial (neoglacial) trend of summer
cooling and decreasing seasonality, ultimately driven
by the orbitally forced reduction in summer insolati-
on (Kullman 2004b; Kullman & Kjällgren 2006). This
phenomenon is consistent with observational data
from various parts of the world, showing e.g. that some
mountain glaciers are currently less extensive than any
time during the past 5000 years or more (Solomina et
al. 2008; Koch et al. 2006; Kullman 2004a; Bakke et al.
2008). However, in perspective of the disclosed strong
spatial variability of recent treeline responses over the
past century (Kullman & Öberg 2009), it cannot be
ruled out that some short-term episodes of high treeli-
nes have been missed by the long-term pine treeline re-
cord. For example, the Medieval period (around 1000
years ago) appears to have displayed a distinct thermal
peak and climatic conditions highly conducive to pro-
lific tree growth in the North and at high elevations
in the mountains (e.g. Hiller et al. 2001; Huldén 2001;
Shiyatov 2003; Grudd 2008; Kullman 1998, 2003,
2005d; Loehle 2007; Idso & Singer 2009). This is expli-
citly highlighted by the fact that climate warming over
the past century has not been sufficient to re-establish
entirely the stand structure that prevailed during the
Medieval period and which was disintegrated by Little
Ice Age cooling (Fig. 34). In some respects, the cur-
rent structure and composition of the treeline ecotone
still bears more influence of the Little Ice Age coo-
ling than with the current warmer phase. Accordingly,
the long-term treeline history needs further study until
this issue has reached a confidence level which allows
more definite opinions. For that purpose, paleotreeline
research should selectively focus on those sites where
the treeline proved to be most responsive and showing
the largest upshifts during the past century (Kullman
& Öberg 2009). Presumably, that would improve the
Figure 34: By 1915, this was a treeline site with only a few
tiny pine trees growing in a matrix of alpine tundra (Kull-
man 2005d). During the Medieval period, a dense stand pre-
vailed here, as indicated by radiocarbon dated megafossils
(foreground). It was gradually exterminated throughout the
Little Ice Age. Reforestation during the past century has
only succeeded in the least exposed part of the site. Mt.
Storsnasen, 670 m a.s.l. 24 June 2009.
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One Century of Treeline Change ... 17 / 2010
time resolution of the treeline history and make it an
even sharper paleoclimate proxy. Moreover, Holocene
treeline change may be largely a question of changed
(declined) seasonality, which should impose species-
specific treeline performances. For example and based
on the principle of uniformitarianism, it could be sus-
pected that the long-term pine treeline history involves
a strong link to snow cover evolution. Thus, definite
interpretations of treeline history in terms of climate
change have to await birch and spruce treeline chrono-
logies of similar quality as the one available for pine.
It is to be noted that the treeline trends outlined abo-
ve should not be uncritically extrapolated into the fu-
ture, since a deterministic model integrating natural
and human-driven climate change is still elusive (e.g.
Karlén 2008; Idso & Singer 2009). Moreover, species
interactions, herbivory (e.g. moose) and diverse feed-
back mechanisms are complex and poorly understood
(cf. Holtmeier 2003; Vajda et al. 2006). Additionally,
many climate-driven growth, reproduction and popu-
lation processes are certainly non-linear, which further
complicates projections (Holtmeier 2003; Loehle 2007;
Kullman 2007a). In this context it also needs to be
stressed that progressive treeline changes, which evol-
ved during decades (position, biomass and structure),
can become rapidly eradicated by just a few exceptio-
nally cold years in the future (cf. Kullman 1989, 1997;
Holtmeier & Broll 2007). Only time will show whether
recent treeline advancement is just a response to a na-
tural climatic caprice or the onset of an alledged new
geologic era, the Anthropocene.
Acknowledgements
This study was defrayed by the Swedish Research
Council for Environment, Agricultural Sciences and
Spatial Planning (FORMAS). I am most indebted to
Lisa Öberg and two anonymous reviewers for const-
ructive comments on the manuscript.
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