Contrasting climate and land use driven

437

Contrasting climate- and land-use-driven tree
encroachment patterns of subarctic tundra in
northern Norway and the Kola Peninsula1
Sigrun Aune, Annika Hofgaard, and Lars So¨derstro¨m

Abstract: High-latitude regions are experiencing substantial climate change, and the forest–tundra transition is assumed to
sensitively track these changes through advancing treeline and increased tundra encroachment. However, herbivores may
influence these responses. The present study addresses, through analyses of age structures, growth characteristics, and climate correspondence, how mountain birch (Betula pubescens Ehrh. ssp. czerepanovii (Orlova) Ha¨met-Ahti) treelines and
sapling cohorts beyond the treeline have responded to the last decade’s warming in six North European subarctic areas
with different climate and grazing characters. The results show different response patterns among areas representing advancing, stationary, and possibly retreating treelines. Recruitment was abundant over the last decades in all areas except
one, with predominantly arctic conditions, where both tree and sapling cohorts were old. Areas with high annual precipitation show advancing birch populations characterized by young individuals and partly overlapping tree and sapling age distributions. Areas in reindeer herding districts show stationary or retreating birch populations characterized by
nonoverlapping age distributions and low sapling survival. Recruitment patterns beyond the treeline generally corresponded
with non-growing-season climate variables, mainly precipitation, indicating the importance of a protecting snow cover.
The results highlight the important interplay between abiotic and biotic control over tundra encroachment and treeline dynamics and the importance of multisite studies when addressing responses to warming.
Re´sume´ : Les re´gions situe´es a` des latitudes e´leve´es subissent d’importants changements climatiques. On assume que la
zone de transition entre la foreˆt et la toundra re´agit de fac¸on perceptible a` ces changements et que cette re´action se traduit
par la progression de la limite des arbres et l’augmentation de l’empie`tement sur la toundra. Cependant, les herbivores
` partir de l’analyse des structures d’aˆge, des caracte´ristiques de la croissance et de la
pourraient influencer ces re´ponses. A

concordance avec le climat, cette e´tude examine la fac¸on dont le bouleau pubescent (Betula pubescens Ehrh. ssp. czerepanovii (Orlova) Ha¨met-Ahti) pre´sent a` la limite des arbres et les cohortes de gaules situe´es au-dela` de la limite des arbres
ont re´agi au re´chauffement de la dernie`re de´cennie dans six re´gions subarctiques du nord de l’Europe dont les caracte´ristiques diffe`rent du point de vue du climat et de la paissance. Les re´sultats re´ve`lent diffe´rents patrons de re´ponse qui correspondent a` des situations ou` la limite des arbres progresse, demeure stationnaire ou possiblement re´gresse selon la re´gion.
Le recrutement a e´te´ abondant au cours des dernie`res de´cennies dans toutes les re´gions, a` l’exception d’une re´gion caracte´rise´e par des conditions essentiellement arctiques ou` les cohortes d’arbres et de gaules e´taient vieilles. Dans les re´gions ou`
la pre´cipitation annuelle est e´leve´e, les populations de bouleau progressent; elles sont caracte´rise´es par de jeunes individus
et des distributions d’aˆge des arbres et des gaules qui se chevauchent partiellement. Dans les re´gions ou` il y a des districts
d’e´levage de rennes, les populations de bouleau sont stationnaires ou en re´gression et sont caracte´rise´es par des distributions d’aˆge qui ne se chevauchent pas et une faible survie des gaules. Les patrons de recrutement au-dela` de la limite des
arbres correspondaient ge´ne´ralement aux variables climatiques de la saison dormante, principalement la pre´cipitation; ce
qui illustre l’importance d’un couvert nival protecteur. Les re´sultats font ressortir d’importantes interactions entre les facteurs abiotiques et biotiques qui controˆlent l’empie`tement sur la toundra et la dynamique de la limite des arbres ainsi que
l’importance d’e´tudier plusieurs re´gions lorsqu’on s’inte´resse aux re´ponses qu’engendre le re´chauffement climatique.
[Traduit par la Re´daction]

Introduction
Over the past half century, temperatures in arctic and subarctic regions have increased 1–2 8C, but with large regional
variations (Arctic Council and the International Arctic Sci-

ence Committee (IASC) 2005; Intergovernmental Panel on
Climate Change (IPCC) 2007). This temperature increase is
predicted to continue during the twenty-first century, with
best estimates ranging between 1.8 8C and 4.0 8C, depending on the model used (IPCC 2007). A general agreement


Received 11 February 2010. Accepted 24 March 2010. Published on the NRC Research Press Web site at cjfr.nrc.ca on 3 February 2011.
S. Aune2 and L. So¨derstro¨m. Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
A. Hofgaard.3 Norwegian Institute for Nature Research, NO-7485 Trondheim, Norway.
1This

article is a contribution to the series Tree recruitment, growth, and distribution at the circumpolar forest–tundra transition.
address: Norwegian Institute for Agricultural and Environmental Research, NO-8860 Tjøtta, Norway.
3Corresponding author (e-mail: annika.hofgaard@nina.no).
2Present

Can. J. For. Res. 41: 437–449 (2011)

doi:10.1139/X10-086

Published by NRC Research Press

438

among scenarios has caused a growing interest in how predicted climate changes might affect the circumpolar forest–
tundra boundary (Callaghan et al. 2002; Arctic Council and

the IASC 2005; Harsch et al. 2009). The boundary, through
its circumpolar distribution, has importance at regional to
global scales for its contribution to land–atmosphere interactions, C sequestration, and biodiversity (Callaghan et al.
2002; Goodale et al. 2002; Arctic Council and the IASC
2005). One of the most important vegetation shifts in the
arctic (observed and predicted) is the expansion of trees and
shrubs (Sturm et al. 2001; Forbes et al. 2009; Kullman and
¨ berg 2009). A region-wide tree cover change would reO
structure the present tundra and its current services (Chapin
et al. 2005; Post and Pedersen 2008).
One distinct component of the forest–tundra boundary is
the treeline (i.e., latitudinal and (or) altitudinal limit for
tree-sized growth; Holtmeier 2003). This ‘‘line’’ is generally
considered a temperature-restricted frontier beyond which
tree seedlings may establish but do not reach tree size because of limitations to photosynthesis and growth rate and
the destruction of leading shoots (Grace et al. 2002; Sveinbjo¨rnsson et al. 2002; Holtmeier 2003). Thus, the treeline location is widely regarded as being sensitive to climate
warming, and advances in response to climate change are
frequently reported (e.g., Caccianiga and Payette 2006;
Wang et al. 2006; Devi et al. 2008). The current treeline
ecotone may respond to increased temperature by densification of the current scattered tree layer (Danby and Hik 2007;

Batllori and Gutierrez 2008) and by advance of the existing
¨ berg 2009). An advance is initially
treeline (Kullman and O
associated with a change in height growth of previously established individuals, but further relocation is dependent on
new establishment beyond the current treeline (Kullman
2002; Caccianiga and Payette 2006; Hofgaard et al. 2009).
Successful establishment of new advanced individuals depends on production, dispersal, and germination of seeds
and long-term survival of the emerging seedlings into sapling and tree stages. Although temperature is an important
factor structuring the treeline ecotone, any change is controlled by interplay of both abiotic and biotic factors, including precipitation, snow cover and duration, topography, and
herbivory (Cairns and Moen 2004; Holtmeier and Broll
2005; Sturm et al. 2005). Increased abundance of trees and
tree saplings in response to warming will enhance snowtrapping during winter and thus further promote tree growth
and establishment (Dalen and Hofgaard 2005; Sturm et al.
2005). However, herbivore activity may inhibit warminginitiated responses (Post and Pedersen 2008; Olofsson et
al. 2009; Hofgaard et al. 2010).
The forest–tundra boundary in Scandinavia and the Kola
Peninsula is dominated by mountain birch (Betula pubescens
Ehrh. ssp. czerepanovii (Orlova) Ha¨met-Ahti), and there is
evidence of advancing treelines during the 20th century. Establishment of new birch individuals peaked in the early to
mid-20th century in response to climate warming and led to

both densification of the birch forest (Kullman 2001) and a
peak in establishment of the trees making up the current
treeline (Dalen and Hofgaard 2005). The early 20th century
warm period was followed by a colder period, 1950s to
1980, characterized by a decline in recruitment and increased mortality of saplings (Kullman 2001). Since the late

Can. J. For. Res. Vol. 41, 2011
˚ nderdalen (WCo), Dividalen
Fig. 1. Location of the study areas A
(WIn), Olderfjord (CCo), and Porsangmoen (CIn) in northern Norway and Kanentiavr (ECo) and Tuliok (EIn) in the Kola Peninsula,
Russia. Forested areas are shaded and tundra areas are shown in
white.

1980s, annual temperature has increased by about 1.5 8C but
with different effects on treeline position and recruitment
between geographical regions (Moen et al. 2008; Hofgaard
¨ berg 2009).
et al. 2009; Kullman and O
In this study, we use age structure and recruitment pattern
of both the current treeline markers (individual treeline

trees) and tree saplings beyond the treeline in northern Norway and the Kola Peninsula, northwestern Russia, to analyse
geographical variation in forest–tundra ecotone responses to
recent climate change. Three climatic regions located along
the Atlantic Ocean – Arctic Ocean air mass gradient, with
decreasing Atlantic air mass and increasing Arctic air mass
impacts from west to east (northwestern Norway to the middle of the Kola Peninsula), were selected for the study. In
addition to age structure and recruitment pattern data, we
used data on tree, sapling, and site characteristics to address
the following questions. Do recruitment periods of trees and
saplings at and beyond the treeline differ along the Atlantic–
Arctic air mass gradient? Is the generally expected climatedriven tundra encroachment supported by current treeline locations and sapling cohort patterns beyond treeline? Are current sapling cohorts showing a climate–establishment
relationship, and if so, does it vary between climate regions?

Methods
Study areas and climate
In each region along the Atlantic–Arctic air mass gradient, one coastal (Co) and one inland (In) area were used.
Two regions are located in northern Norway and one in
northwestern Russia, and these are henceforward called
western (W), central (C), and eastern (E), respectively
˚ nderdalen (WCo), is sit(Fig. 1). The western coastal area, A

uated on the island Senja in Troms County and is characterized by a rather rugged mountain terrain with surrounding
mountain peaks up to 800 m above sea level (a.s.l.), whereas
the terrain in the western inland area, Dividalen (WIn), is
slightly more gentle but with mountain peaks reaching up to
1500 m a.s.l. The central areas Olderfjord (CCo) and PorPublished by NRC Research Press

EIn
Tuliok
67842’N
33846’E
Ukspor
20 km, S
–3.7
–12.2
9.0
1066
ECo
Kanentiavr
68850’N
34844’E

Murmansk
70 km, W
0.0
–11.2
12.6
471
CIn
Porsangmoen
69855’N
25813’E
Karasjok
52 km, S
–2.4
–17.1
13.1
366
Note: The normal period for meteorological station Ukspor is 1880–1980 (Anonymous 1988).

CCo
Olderfjord

70827’N
24847’E
Banak
45 km, S
0.6
–10.0
12.7
345
WIn
Dividalen
68851’N
19842’E
Dividalen
9 km, S
0.8
–9.4
12.8
282
WCo
˚ nderdalen

A
69812’N
17819’E
Tromsø
81 km, NE
2.5
–4.4
11.8
1031
Name
Latitude
Longitude
Meteorological station
Distance, direction from study area
Annual temperature (8C)
January temperature (8C)
July temperature (8C)
Annual precipitation (mm)

Sampling design

In each of the six areas, one gentle north-facing mountain
slope was chosen for age determination and site characteristics of treeline trees and tree recruitment above and (or) beyond the treeline. The sampling was conducted during
summer in 2007 and 2008. In each area, 20 trees from the
local treeline (defined as the most advanced trees with a
minimum height of 2 m) were selected (Table 2; Fig. 3).
The minimum distance horizontally between selected trees
was set to 10 m. These treeline trees were cored at the base

Study area

sangmoen (CIn), Finnmark County, are situated in a landscape characterized by rounded mountains. Mountain peaks
in these areas reach 400–500 m a.s.l. and 800–1000 m a.s.l.,
respectively. The eastern inland area, Tuliok (EIn), is located in the Khibiny Mountains, which is the largest massif
on the Kola Peninsula, with several peaks reaching 900–
1100 m a.s.l. Whereas these five areas are located at the
transition from forest to subarctic alpine tundra, the coastal
easternmost area (Kanentiavr; ECo) is situated at the transition from forest to arctic tundra in a landscape characterized
by fairly flat terrain with scattered mires, lakes, and small
hills. All six areas are used as foraging domains by a number of herbivores such as moose, hare, grouse, various rodents, and insects. In addition, the areas in the central
region and western inland area belong to the main reindeer
herding districts of northern Norway used for summer grazing by large semidomestic reindeer herds.
According to Moen (1999), climate conditions are characterized as slightly oceanic in WCo, indifferent in CCo, ECo,
and EIn, and slightly continental in WIn and CIn. The western coastal area, which is located close to the Atlantic
Ocean, is characterized by high annual precipitation and
rather high mean annual temperature (Table 1). Mean annual
temperature and mean temperature during the coldest month,
January, decrease from the west to east and from the coast
to inland areas. The mean temperature for the warmest
month, July, shows no west–east gradient and only a weak
coast–inland gradient for western and central areas (warmer
inland). In the eastern region, the coastal area shows a notably higher mean July temperature and lower annual precipitation than the inland area (Table 1).
Annual temperature in the regions has increased by ca. 1–
2 8C since the late 1980s, with emphasis on winter temperature (Fig. 2a). Annual precipitation shows an overall weak
increasing trend during the same time period, except in the
western coastal area because of summer precipitation decrease in that area (Fig. 2b).
Mountain birch is the dominating tree layer species in the
treeline ecotone in all areas. This species is a favoured food
source, and birch leaves, buds, and twigs constitute an important part of the diet for many animal species. Birch has
an annual seed production with germability up to 60% at its
distribution limit across the studied region (A. Hofgaard, unpublished data). The shrub layer in the western coastal area
is dominated by dwarf birch (Betula nana L.); dwarf birch
and willows (Salix spp.) dominate in the other five areas.
The field layer in the western inland area is dominated by
sedges and low herbs, but in the other five areas, deciduous
and evergreen dwarf shrubs dominate together with low
herbs.

439
Table 1. Mean temperature and precipitation (normal period 1961–1990) for individual study areas represented by local meteorological stations (Norwegian Meteorological Institute,
http://www.met.no; Anonymous 2008).

Aune et al.

Published by NRC Research Press

440

Can. J. For. Res. Vol. 41, 2011

Fig. 2. Ten-year running mean for (a) summer, annual, and winter temperatures and (b) total summer, annual, and winter precipitation.
Tromsø, Dividalen, Banak, and Karasjok meteorological stations represent study areas WCo, WIn, CCo, and CIn, respectively. Murmansk
represents both ECo and EIn. Details for meteorological stations are given in Table 1. Data are from the Norwegian Meteorological Institute
(http://www.met.no) and Anonymous (2008).

Table 2. Characteristics for sampled treeline trees in the six study areas (mean values and standard deviations per area are given for all
measured and calculated variables).
Study area
Variable
Altitude (m above sea level)
No. of trees sampled
No. of trees included in age structure:
0m
No. of trees included in age structure:
2m
Age at stem base
Age at 2 m
Years to become 2 m
Annual growth rate 0–2 m (cm/year)
Tree height (m)
Stem diameter at 0 m (cm)
Stem diameter at 1.3 m (cm)
Crown area (m2)

WCo
306
21
17

WIn
626
20
8

CCo
310
20
19

CIn
348
20
17

ECo
244
20
19

EIn
534
20
19

20

16

20

20

18

20

66.5±29.1
16.8±15.3
48.1±29.4
5.8±3.4
2.9±1.0
9.2±4.8
3.7±3.3
4.7±6.4

92.4±9.9
59.9±16.0
32.8±19.3
7.5±2.9
5.8±1.4
18.8±5.1
14.1±5.5
12.4±5.9

52.1±14.8
26.2±12.9
26.3±8.4
8.3±2.5
4.4±1.1
9.7±4.0
6.0±3.4
6.0±4.7

54.3±19.3
31.1±16.1
25.7±8.7
9.1±5.3
4.0±1.6
10.8±6.3
6.4±3.4
5.7±5.9

70.8±28.1
24.9±18.2
46.9±24.0
5.8±4.1
3.0±0.6
8.4±3.4
4.3±2.2
8.3±4.5

33.4±8.0
4.9±1.9
28.5±7.6
7.6±2.4
2.6±0.4
5.1±1.4
1.8±0.6
1.6±1.4

Published by NRC Research Press

Aune et al.

441

Fig. 3. View of the Dividalen study area (WIn), which is used as summer grazing grounds for reindeer and is characterized by old treeline
trees (mountain birch, Betula pubescens ssp. czerepanovii) and the lack of tree recruitment. (Photo: Sigrun Aune.)

(0 m) and at 2 m aboveground for age determination. Trees
too thin to be cored at the 2 m level were sampled by cutting at that height.
Above and (or) beyond the treeline, all birch saplings
( 3.5 cm) or
cut at the base for age determination. The following variables were recorded for each sampled sapling and treeline
tree: height, stem diameter at ground level and, when available, at breast height (1.3 m), crown diameter (two perpendicular measures), vitality, ground moisture conditions, land
cover type, and GPS position. The vitality was classified as
healthy, dead, and damaged by climate, herbivores, or fungi;
ground moisture classes used were dry, mesic, and moist;
and land cover types were classified as stream, stagnant
water, rocky outcrop, meadow, lee side, ridge, and snow
bed (i.e., late-laying snow locations).

Core samples were mounted on a wooden support, dried,
and brought to the lab, together with cut samples, where
their age was determined using a stereomicroscope (6–
40). To increase the contrast between late and early wood,
the samples were smoothed with a scalpel and zinc ointment
was applied. In total, 121 treeline trees and 618 saplings
were sampled and analysed. Stem rot or failure to hit the
pith precluded exact age determination of some specimens.
For samples with an estimated two–five annual rings missing (estimated by eye using the density of the rings near the
pith and the approximate distance to the pith), the estimated
number was added to the counted age. This was the case for
14.0% and 11.6% of the treeline tree age samples from the
0 m and 2 m levels, respectively, and 1.9% of the sapling
samples. Samples with larger estimated missing numbers of
rings were omitted from the age structure analysis. The total
data set was used in analyses, not including age. The number of treeline trees included in age analyses ranged from
eight to 19 (Table 2). Of the 618 sampled saplings, four,
two, three, and two samples from WCo, CIn, ECo, and EIn,
respectively, were omitted from the age analysis.
Data handling and analyses
The age data for each study area were grouped into
classes of 5 years, and height data were grouped into classes
of 10 cm. Because of difficulties in finding small individuals
(

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