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Journal Article•DOI•

A century of tree line changes in sub-Arctic Sweden shows local and regional variability and only a minor influence of 20th century climate warming

Rik Van Bogaert1, Kristof Haneca1, Jan Hoogesteger2, Christer Jonasson3, Christer Jonasson4, Morgan De Dapper1, Terry V. Callaghan5, Terry V. Callaghan4 •
Ghent University1, University of Helsinki2, Uppsala University3, Royal Swedish Academy of Sciences4, University of Sheffield5
01 May 2011-Journal of Biogeography (Blackwell Publishing Ltd)-Vol. 38, Iss: 5, pp 907-921
TL;DR: In this article, the authors used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics in a sub-Arctic model area at different temporal and spatial scales.
Abstract: Aim Models project that climate warming will cause the tree line to move to higher elevations in alpine areas and more northerly latitudes in Arctic environments. We aimed to document changes or stability of the tree line in a sub-Arctic model area at different temporal and spatial scales, and particularly to clarify the ambiguity that currently exists about tree line dynamics and their causes. Location The study was conducted in the Tornetrask area in northern Sweden where climate warmed by 2.5 degrees C between 1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine tree line. Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics. Results Since 1912, only four out of eight tree line sites had advanced: on average the tree line had shifted 24 m upslope (+0.2 m year-1 assuming linear shifts). Maximum tree line advance was +145 m (+1.5 m year-1 in elevation and +2.7 m year-1 in actual distance), whereas maximum retreat was 120 m downslope. Counter-intuitively, tree line advance was most pronounced during the cooler late 1960s and 1970s. Tree establishment and tree line advance were significantly correlated with periods of low reindeer (Rangifer tarandus) population numbers. A decreased anthropozoogenic impact since the early 20th century was found to be the main factor shaping the current tree line ecotone and its dynamics. In addition, episodic disturbances by moth outbreaks and geomorphological processes resulted in descent and long-term stability of the tree line position, respectively. Main conclusions In contrast to what is generally stated in the literature, this study shows that in a period of climate warming, disturbance may not only determine when tree line advance will occur but if tree line advance will occur at all. In the case of non-climatic climax tree lines, such as those in our study area, both climate-driven model projections of future tree line positions and the use of the tree line position for bioclimatic monitoring should be used with caution.

Summary (1 min read)

Jump to: [A century of tree line changes in sub-Arctic Sweden shows local and regional] – [Tables] – [Figure legends] and [Figures]

A century of tree line changes in sub-Arctic Sweden shows local and regional

  • Twentieth century tree line changes in Swedish sub-Arctic Abstract 1 Models project that climate warming will cause the treeline to move to higher 2 elevations in alpine areas and more northerly latitudes in Arctic environments, also known as Running head.
  • The lack of 446 recent tree (>2 m) establishment and the browsing scars documented in the tree rings 447 indicated that, in addition to moth herbivory, reindeer browsing is still a controlling 448 factor at these sites (Fig. 5, Table 4).

Tables

  • Previous field studies on observed treeline shifts and their presumed causes in the Torneträsk area of sub-Arctic Sweden.
  • Browsing damage was classified visually and for five sites also by dendrochronological analysis (the values listed in brackets).
  • So as not to bias the results, Mount Nuolja (site S3), for which the two treeline sites were not randomly selected, was not included in the calculation of the mean elevational shift of the treeline in the Torneträsk area.
  • Pearson correlation coefficients and R2-values (the proportion of explained variance in documented treeline shifts) obtained by forward selection of the different variables are listed at the bottom of the table.

Figure legends

  • The Torneträsk study area in sub-Arctic Sweden.
  • The locations of the historical transects and photo points that have been revisited to study changes in the tree line ecotone are indicated.
  • The statistically-determined 30%-tree cover isoline is plotted in yellow.
  • Upper photo on the left: E. Persson, bottom left: B. Mesch; upper and bottom right: R. Van Bogaert.
  • Fig. 8. Tree (>2 m) establishment at the Torneträsk tree line versus summer (June- August) temperature and reindeer population numbers for the period 1800-2000.

Figures

  • Historical transects and photo points that have been revisited to study changes in the tree line ecotone are indicated.
  • The statistically-determined 30% tree-cover isoline is plotted in yellow.
  • Upper photo: E. Persson, lower photo: S. Johnsson.
  • Upper photo on the left: E. Persson, bottom left: B. Mesch; upper and bottom right: R. Van Bogaert. relation to disturbance (b) and summer temperature (c) for the period 1964-2006.
  • August) temperature and reindeer population numbers for the period 1800-2000.

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This item is the archived peer-reviewed author-version of:
A century of tree line changes in sub-Arctic Sweden shows local and regional variability and only a
minor influence of 20th century climate warming
Van Bogaert, R.; Haneca, K.; Hoogesteger, J.; Jonasson, C.; De Dapper, M.; Callaghan, T.V.
In: Journal of Biogeography, 38 (5), 907-921, 2011.
doi: 10.1111/j.1365-2699.2010.02453.x
To refer to or to cite this work, please use the citation to the published version:
Van Bogaert, R.; Haneca, K.; Hoogesteger, J.; Jonasson, C.; De Dapper, M.; Callaghan, T.V.
(2011). A century of tree line changes in sub-Arctic Sweden shows local and regional
variability and only a minor influence of 20th century climate warming. Journal of
Biogeography 38 (5), 907-921. doi: 10.1111/j.1365-2699.2010.02453.x
1
A century of tree line changes in sub-Arctic Sweden shows local and regional
variability and only a minor influence of 20
th
century climate warming
Rik Van Bogaert
1,2*
, Kristof Haneca
3
, Jan Hoogesteger
4
, Christer Jonasson
5,6
, Morgan
De Dapper
2
and Terry V Callaghan
5,7
1
Flanders Research Foundation (FWO); Egmontstraat 5, B-1000 Brussels, Belgium
2
Department of Geography, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium
3
Flemish Heritage Institute, Koning Albert II-laan 19 bus 5, B-1210 Brussels, Belgium,
formerly Laboratory of Wood Technology, Ghent University Coupure Links 653, 9000
Ghent, Belgium
4
Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki,
Finland
5
Abisko Scientific Research Station, Royal Swedish Academy of Sciences, SE-98107
Abisko, Sweden
6
Department of Physical Geography, Uppsala University, S-75122 Uppsala, Sweden
7
Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield
S10 2TN, UK
*Corresponding author: Address: Yzerhand 85, B-9120 Beveren, Belgium.
rikvanbogaert@gmail.com
Running head: Twentieth century tree line changes in Swedish sub-Arctic
2
Abstract 1
Models project that climate warming will cause the treeline to move to higher 2
elevations in alpine areas and more northerly latitudes in Arctic environments. We 3
aimed to document changes or stability of the treeline in a sub-Arctic model area at 4
different temporal and spatial scales, and particularly to clarify the ambiguity that 5
currently exists about treeline dynamics and their causes. The study was conducted in 6
the Torneträsk area in northern Sweden where climate warmed by 2.5 ˚C between 7
1913 and 2006. Mountain birch (Betula pubescens ssp. czerepanovii) sets the alpine 8
treeline. We used repeat photography, dendrochronological analysis, field 9
observations along elevational transects and historical documents to study treeline 10
dynamics. Since 1912, only four out of eight treeline sites had advanced: on average 11
the treeline had shifted 24 m upslope (+0.2 m year
-1
assuming linear shifts). 12
Maximum treeline advance was +145 m (+1.5 m year
-1
in elevation and +2.7 m year
-1
13
in actual distance), whereas maximum retreat was 120 m downslope. Counter-14
intuitively, treeline advance was most pronounced during the cooler late 1960s and 15
1970s. Tree establishment and treeline advance were significantly correlated with 16
periods of low reindeer (Rangifer tarandus) population numbers. A decreased 17
anthropozoogenic impact since the early 20
th
century was found to be the main factor 18
shaping the current treeline ecotone and its dynamics. In addition, episodic 19
disturbances by moth outbreaks and geomorphological processes resulted in descent 20
and long-term stability of the treeline position, respectively. In contrast to what is 21
generally stated in the literature, this study shows that in a period of climate warming, 22
disturbance may not only determine when treeline advance will occur but if treeline 23
advance will occur at all. In the case of non-climatic climax treelines, such as those in 24
our study area, both climate-driven model projections of future treeline positions and 25
3
the use of the treeline position for bioclimatic monitoring should be used with caution.26
27
28
Key words: climate warming, dendrochronology, herbivory, human impact, mountain 29
birch, reindeer, sub-Arctic, Sweden, tree line, tree line causes 30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
4
Introduction 47
Mean annual temperatures have risen globally over the past century, with the most
48
pronounced and rapid changes at high elevations and latitudes (ACIA, 2005). As the
49
location of elevational and polar treelines is mainly caused by heat deficiency, in the
50
Northern Hemisphere climate warming is expected to cause treelines to advance to
51
higher elevations and more northerly latitudes (Harsch et al., 2009). Indeed, modern
52
evidence for such relocations exists and these have been explicitly or implicitly
53
related to recent climate warming (Shiyatov et al., 2007; Kullman and Öberg, 2009).
54
However, in many circumpolar and high-elevational areas the position of the treeline
55
has not changed (Masek, 2001; Holtmeier et al., 2003; Payette, 2007; Van Bogaert et
56
al., 2007) or has even retreated (Vlassova, 2002; Dalen and Hofgaard, 2005; Kullman,
57
2005; Cherosov et al., 2010).
58
Treeline heterogeneity increases from global to regional, to landscape and to local
59
scales of analysis (Callaghan et al., 2002). Moreover, the factors controlling the
60
position and structure of the treeline are highly scale-dependent and vary from place
61
to place (Sveinbjörnsson et al., 2002). Individual trees and the forest system may
62
respond differently to change; warming may increase tree growth, while at the same
63
time seedling survival may be reduced because of water stress brought about by
64
greater evapotranspiration and drying of the uppermost soil. Furthermore, the time-
65
scale of a study influences outcomes because it determines the processes and
66
responses that can be studied. There are short-term responses (defined as a year or
67
less and reflected in individual tree growth), medium-term responses (some years to a
68
few decades and reflected in changing survival rates of seedlings and altered tree
69
physiognomy) and long-term responses (several decades to centuries and reflected in
70
a general treeline advance or retreat) (Holtmeier and Broll, 2005).
71
Citations
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Journal Article•DOI•
Plant co-existence patterns and High-Arctic vegetation composition in three common plant communities in north-east Greenland

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Oriol Grau1, Josep M. Ninot1, Aaron Pérez-Haase1, Terry V. Callaghan2•
University of Barcelona1, Royal Swedish Academy of Sciences2
25 Sep 2014-Polar Research
TL;DR: This study provides novel background knowledge of plant co-existence patterns at patch scale and of the structure of contrasting Arctic plant communities, which will help to better assess the potential effects of varying abiotic stress regimes on Arctic vegetation.
Abstract: Arctic regions are expected to experience substantial changes in climate in the coming decades. In order to predict potential changes of Arctic vegetation, it is important to understand the distinct role of life forms of plants and of individual species in relation to plant co-existence patterns. Our aim is to investigate if three common Arctic plant patch types dominated by contrasting life forms (by the dwarf shrubs Salix arctica or Dryas octopetala × intermedia or by mosses) are related (a) to the co-existence of vascular plants and species richness at patch scale and (b) to the floristic composition in three distinct plant communities ( Salix snowbed, Dryas heath and fell-field) associated with contrasting abiotic regimes. The study was conducted at Zackenberg, in north-east Greenland. Dryas patches showed a clear negative effect on small-scale plant richness and co-existence in the fell-field. Salix and moss patches showed a similar pattern in all the plant communities, although the number of individuals growing in Salix patches was lower than in moss patches. Salix and mosses in the fell-fields hosted a high number of species in spite of the much less vegetated aspect of this harsh, upper zone. The floristic composition varied between plant communities, but it did not change substantially between patch types within each community. This study provides novel background knowledge of plant co-existence patterns at patch scale and of the structure of contrasting Arctic plant communities, which will help to better assess the potential effects of varying abiotic stress regimes on Arctic vegetation. To access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools). Keywords: Abiotic stress; Arctic vegetation; co-existence patterns; plant community; species richness. (Published: 25 September 2014) Citation: Polar Research 2014, 33 , 19235, http://dx.doi.org/10.3402/polar.v33.19235

4 citations

Cites background from "A century of tree line changes in s..."

  • ...…the consequences of these effects at a larger scale have led to an increasing importance of understanding climate ecosystem interactions in northern latitudes, particularly as not all ecosystems have responded as expected to climate change (Callaghan, Tweedie et al. 2011; Van Bogaert et al. 2011)....

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  • ...The marked effect of climate change on organisms (e.g., Post et al. 2009; Callaghan, Tweedie et al. 2011) and on ecosystem functioning (e.g., Euskirchen et al. 2006; Lara et al. 2012; Myers-Smith et al. 2011) in the Arctic and the consequences of these effects at a larger scale have led to an increasing importance of understanding climate ecosystem interactions in northern latitudes, particularly as not all ecosystems have responded as expected to climate change (Callaghan, Tweedie et al. 2011; Van Bogaert et al. 2011)....

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Point of no return – absence of returning birds in the otherwise philopatric willow warbler Phylloscopus trochilus

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Johanna Hedlund1, Frida Sjösten1, Kristaps Sokolovskis1, Sven Jakobsson1•
Stockholm University1
01 Mar 2017-Journal of Avian Biology
TL;DR: The return of individual birds to a specific area in successional years, i.e. philopatry, is a remarkable behavioural trait, but here the remarkably reversed: the complete absence of returality is reported.
Abstract: The return of individual birds to a specific area in successional years, i.e. philopatry, is a remarkable behavioural trait. Here we report on the remarkably reversed: the complete absence of retur ...

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Book Chapter•DOI•
Detection of Vegetation in Environmental Repeat Photography: A New Algorithmic Approach in Data Science

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Asim Khan1, Anwaar Ulhaq1, Anwaar Ulhaq2, Randall W. Robinson1, Mobeen Ur Rehman3 •
Victoria University, Australia1, Charles Sturt University2, Air University (United States Air Force)3
01 Apr 2020
TL;DR: An algorithm which can be used to observe the change in vegetation utilizing the images of a particular site and a novel approach towards segmentation is proposed which works on the machine learning based algorithm.
Abstract: Environment change being one of the major issues in today’s world needs special attention of the researchers. With the advancement in computer vision researchers are equipped enough to come up with algorithms accomplishing automated system for environment monitoring. This paper proposes an algorithm which can be used to observe the change in vegetation utilizing the images of a particular site. This would help the environment experts to put on their efforts in a right direction and right place to improve the environment situation. The proposed algorithm registers the image so that comparison can be carried out in an accurate manner using single framework for all the images. Registration algorithm aligns the new images with the existing images available in the record of the same particular site by performing transformation. Registration process is followed by segmentation process which segments out the vegetation region from the image. A novel approach towards segmentation is proposed which works on the machine learning based algorithm. The algorithm performs classification between vegetation patches and non-vegetation patches which equips us to perform segmentation. The proposed algorithm showed promising results with F-measure of 85.36%. The segmentation result leads us to easy going calculation of vegetation index. Which can be used to make a vegetation record regarding particular site.

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Long-term Changes in Four Plant Communities Along an Elevational Gradient in the Front Range of Colorado

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Gregory J. Sproull
01 Jan 2014
TL;DR: In this article, the authors surveyed four plant communities along an elevational gradient in the Front Range of the Colorado Rocky Mountains for long-term overstory and understory changes and found that all communities changed significantly, though in different manners.
Abstract: We surveyed four plant communities along an elevational gradient in the Front Range of the Colorado Rocky Mountains for long-term overstory and understory changes. Our results were compared to those found in 1981 and 1996. We evaluated changes in succession, elevational species migration and range expansion, community diversity, and composition. We related temporal floristic shifts to prior literature on disturbance history at each site. Over time, all communities changed significantly, though in different manners. This analysis shows that plant communities are changing in dynamic and idiosyncratic ways that correspond to individualistic distribution shifts. Moreover, we exhibit the necessity of comprehensively investigating long-term community change using multiple approaches, incorporating plant guild relationships, and concentrating efforts to further understand the interplay between climate effects and disturbance.

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Northern Treelines as Indicators of Climate and Land Use Changes - A Literature Review

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Oddvar Skre
01 Jan 2020
TL;DR: The treelines are the result of long-term processes involving adaptation to a cold climate with environmental stress, and with low soil temperatures and nutrient uptake rates as discussed by the authors, which may interfere with the treeline changes and lead to a retreat of treelines.
Abstract: The alpine and arctic treelines are results of long-term processes involving adaptation to a cold climate with environmental stress, and with low soil temperatures and nutrient uptake rates. Global warming and changed land use as a result of changes in global economy and agricultural policy may interfere with the treeline changes. The global warming is expected to enhance tree growth and seed reproduction, thereby increasing treeline altitudes and latitudes. Expansion of the forested area as a result of reduced logging and grazing would also reduce the albedo and enhance global warming in treeline areas. Feedback effects caused by increased soil temperatures and related output of greenhouse gases is expected to further increase global warming and treeline advance. On the other hand, local disturbance factors like increased risks of insect outbreaks, windthrow, grazing, anthropogenic disturbance and paludification would reduce or interfere with these changes, or even lead to a retreat of treelines. These limitations have to be taken into account when evaluating treelines as climatic indicators.

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Cites background from "A century of tree line changes in s..."

  • ...Higher seed reproduction as a result of increased summer temperatures [16] and lower grazing pressure is expected to lead to an advance of the treeline ecotone [67], but the process may take decades or even centuries [1,68]....

    [...]

References
More filters
Book•
Arctic climate impact assessment

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Carolyn Symon, Lelani Arris, Bill Heal, Assesment Intergration Team
01 Jan 2005
TL;DR: The Arctic Climate Impact Assessment (ACIA) as mentioned in this paper is an assessment of the effects of climate change on the Arctic environment and its impacts on the local communities and their livelihoods.
Abstract: 1. An introduction to the Arctic Climate Impact Assessment 2. Arctic climate: past and present 3. The changing Arctic: indigenous perspectives 4. Future climate change: modeling and scenarios for the Arctic 5. Ozone and ultraviolet radiation 6. Cryosphere and hydrology 7. Arctic tundra and polar fesert ecosystems 8. Freshwater ecosystems and Fisheries 9. Marine Systems 10. Principles of conserving the Arctic's Biodiversity 11. Management and conservation of wildlife in a changing Arctic environment 12. Hunting, herding, fishing, and gathering: indigenous peoples and renewable resource use in the Arctic 13. Fisheries and aquaculture 14. Forests, land management, and agriculture 15. Human health 16. Infrastructure: buildings, support systems, and industrial facilities 17. Climate change in the context of multiple stressors and resilience 18. Summary and synthesis of the ACIA Appendix A. Chapter authors Appendix B. Biographies Appendix C. Reviewers Appendix D. Species names Appendix E. Acronyms Appendix F. Glossary.

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Journal Article•DOI•
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01 Sep 1989-Journal of Ecology

1,311 citations

Journal Article•DOI•
Are treelines advancing? A global meta‐analysis of treeline response to climate warming

[...]

Melanie A. Harsch1, Philip E. Hulme1, Matt S. McGlone2, Richard P. Duncan1, Richard P. Duncan2 •
Lincoln University (New Zealand)1, Landcare Research2
01 Oct 2009-Ecology Letters
TL;DR: Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.
Abstract: Treelines are temperature sensitive transition zones that are expected to respond to climate warming by advancing beyond their current position. Response to climate warming over the last century, however, has been mixed, with some treelines showing evidence of recruitment at higher altitudes and/or latitudes (advance) whereas others reveal no marked change in the upper limit of tree establishment. To explore this variation, we analysed a global dataset of 166 sites for which treeline dynamics had been recorded since 1900 AD. Advance was recorded at 52% of sites with only 1% reporting treeline recession. Treelines that experienced strong winter warming were more likely to have advanced, and treelines with a diffuse form were more likely to have advanced than those with an abrupt or krummholz form. Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.

1,003 citations

"A century of tree line changes in s..." refers background in this paper

  • ...In contrast to Harsch et al. (2009) who concluded that the role of disturbance during recent climate warming is restricted to determining when tree line advance will occur, this study shows that disturbance and its after-effects may equally well determine ı́f tree line advance will occur at all....

    [...]

  • ...As the location of elevational and polar tree lines is mainly caused by heat deficiency, in the Northern Hemisphere climate warming is expected to cause tree lines to advance to higher elevations and more northerly latitudes (Harsch et al., 2009)....

    [...]

  • ...…widely held expectations of vegetation responses to warming, i.e. that Arctic tree lines will move northwards and elevational tree lines upslope (Harsch et al., 2009), this study documented highly varying tree line dynamics for the Torneträsk area in sub-Arctic Sweden during a period of…...

    [...]

  • ...A global study by Harsch et al. (2009) showed that only 52% of all 166 global tree line sites had advanced over the past 100 years despite documented amplified climate warming at high-elevation areas and northern latitudes (ACIA, 2005)....

    [...]

Journal Article•DOI•
Sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change at landscape and local scales

[...]

Friedrich-Karl Holtmeier1, Gabriele Broll2•
University of Münster1, University of Vechta2
01 Sep 2005-Global Ecology and Biogeography
TL;DR: The sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change are increasingly discussed in terms of climate change, often forgetting that climate is only one aspect of environmental variation as mentioned in this paper.
Abstract: The sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change are increasingly discussed in terms of climate change, often forgetting that climate is only one aspect of environmental variation. As treeline heterogeneity increases from global to regional and smaller scales, assessment of treeline sensitivity at the landscape and local scales requires a more complex approach than at the global scale. The time scale (short-, medium-, long-term) also plays an important role when considering treeline sensitivity. The sensitivity of the treeline to a changing environment varies among different types of treeline. Treelines controlled mainly by orographic influences are not very susceptible to the effects of warming climates. Greatest sensitivity can be expected in anthropogenic treelines after the cessation of human activity. However, tree invasion into former forested areas above the anthropogenic forest limit is controlled by site conditions, and in particular, by microclimates and soils. Apart from changes in tree physiognomy, the spontaneous advance of young growth of forest-forming tree species into present treeless areas within the treeline ecotone and beyond the tree limit is considered to be the best indicator of treeline sensitivity to environmental change. The sensitivity of climatic treelines to climate warming varies both in the local and regional topographical conditions. Furthermore, treeline history and its after-effects also play an important role. The sensitivity of treelines to changes in given factors (e.g. winter snow pack, soil moisture, temperature, evaporation, etc.) may vary among areas with differing climatic characteristics. In general, forest will not advance in a closed front but will follow sites that became more favourable to tree establishment under the changed climatic conditions.

518 citations

Journal Article•DOI•
Variability, contingency and rapid change in recent subarctic alpine tree line dynamics

[...]

Ryan K. Danby1, David S. Hik1•
University of Alberta1
01 Mar 2007-Journal of Ecology
TL;DR: In this article, the authors examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada and found that tree line elevation and stand density increased significantly during the early to mid 20th century.
Abstract: Summary 1 Boundaries between forest and tundra ecosystems, tree lines, are expected to advance in altitude and latitude in response to climate warming. However, varied responses to 20th century warming suggest that in addition to temperature, tree line dynamics are mediated by species-specific traits and environmental conditions at landscape and local scales. 2 We examined recent tree line dynamics at six topographically different, but climatically similar, sites in south-west Yukon, Canada. Dendroecological techniques were used to reconstruct changes in density of the dominant tree species, white spruce (Picea glauca), and to construct static age distributions of willow (Salix spp.), one of two dominant shrub genera. Data were analysed to identify periods and rates of establishment and mortality and to relate these to past climate. 3 Tree line elevation and stand density increased significantly during the early to mid 20th century. However, this change was not uniform across sites. Spruce advanced rapidly on south-facing slopes and tree line rose 65–85 m in elevation. Tree line did not advance on north-facing slopes, but stand density increased 40–65%. Differences observed between aspects were due primarily to the differential presence of permafrost. Additional variability among sites was related to slope and vegetation type. Results were less conclusive for willow, but evidence for an advance was found at two sites. 4 Increases in stand density were strongly correlated with summer temperatures. The period of rapid change coincided with a 30-year period of above average temperatures, beginning in 1920. The highest correlations were obtained using a forward average of 30–50 years, supporting the hypothesis that tree line dynamics are controlled more by conditions influencing recruitment than by establishment alone. 5 The changes observed at several sites are suggestive of a threshold response and challenge the notion that tree lines respond gradually to climate warming. Overall, the results provide further evidence to support the idea that the pattern and timing of change is contingent on local, landscape, and regional-scale factors, as well as species’ biology.

334 citations

"A century of tree line changes in s..." refers result in this paper

  • ...In contrast to other studies (Danby & Hik, 2007; Kullman & Öberg, 2009), slope aspect and inclination were not correlated with elevational shifts of the tree line ecotone....

    [...]

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