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Journal ArticleDOI

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

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the authors used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics in a sub-Arctic model area at different temporal and spatial scales.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 Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics.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)

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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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 ArticleDOI
TL;DR: It is demonstrated that browsing interacts with temperature to determine the growth of mountain birch at the upper distribution limit and that the variation in growth linked to sheep density was greater than that linked to the 50-year range of interannual temperatures.
Abstract: Summary 1. A warming climate has been linked to shifts in plant distribution and growth. The relationship between climate and growth is used to infer past climate conditions within dendrochronological studies. However, browsing may interact with climate to determine growth, yet the impact of large herbivores on tree-ring growth series is largely unknown. 2. Here, we disentangle the interactions between climate and herbivory in determining plant growth at the upper distribution limit of mountain birch (Betula pubescens ssp. czerepanovii). 3. Stem discs of 206 birch were sampled within an altitudinal tree line ecotone in southern Norway, after 9 years of experimental browsing at three sheep densities (unbrowsed, low and high with 0, 25 and 80 sheep km−2). Annual radial growth was measured using digital microscopy and related to summer temperatures, altitude and sheep density. 4. Radial growth was negatively related to altitude and related to summer temperature in a nonlinear fashion, increasing from low temperatures and saturating and decreasing at high temperatures. However, the variation between browsing treatments overrode the influence of interannual temperature in determining growth. Increasing sheep densities limited radial growth and interacted with both temperature and altitude. The temperature of peak radial growth increased at higher densities of sheep, and growth was less limited by altitude at low sheep density. 5. This demonstrates that browsing interacts with temperature to determine the growth of mountain birch at the upper distribution limit and that the variation in growth linked to sheep density was greater than that linked to the 50-year range of interannual temperatures. The ability of browsing to affect radial growth has important implications for the use of dendrochronology to infer past climatic conditions, where samples are taken from regions that have undergone past land-use change.

47 citations


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

  • ...Defoliation caused by the outbreaks of herbivorous moths influences tree-ring growth (Hoogesteger & Karlsson 1992; Karlsson et al. 2004; Van Bogaert et al. 2009)....

    [...]

  • ...The dominance of herbivory over climate in determining birch growth is consistent with the importance of herbivory in determining tree line shifts (Speed et al. 2010; Van Bogaert et al. 2011)....

    [...]

Journal ArticleDOI
TL;DR: In this paper, a meta-analysis of treeline shift rates at 143 sites from 38 published studies was conducted at the hemispheric scale and for two geographic subsets of subarctic and temperate regions (south of 60°N).
Abstract: AIM: To better understand how climate change drives altitudinal treeline dynamics at large spatial scales. LOCATION: Northern Hemisphere. TIME PERIOD: 1901–2018. MAJOR TAXA STUDIED: Tree species that constitute alpine treelines. METHODS: We conducted a meta‐analysis of annual treeline shift rates at 143 sites from 38 published studies. For each site, we calculated current change rates in annual or seasonal temperatures, precipitation and drought (standardized precipitation evapotranspiration index). We then evaluated relationships between treeline shift rates, and site characteristics and climate using standard and partial correlation analyses, as well as generalized linear models. These analyses were conducted at the hemispheric scale and for two geographic subsets of subarctic (north of 60° N) and temperate regions (south of 60° N). RESULTS: Treelines ascended at 88.8%, remained stable at 10.5% and descended at 0.7% of the sites. The mean hemispheric shift rate (0.354 m/year) was half of what would be expected from climate warming alone. Treeline shifts were significantly faster in subarctic than temperate regions. The precipitation effect was more important than temperature for predicting treeline shift rate. In the subarctic regions, autumn precipitation mostly determined treeline shift rates. In the temperate region, warmer temperatures and higher autumn precipitation accelerated shift rates, whereas wetter springs reduced them. Autumn precipitation and winter mean minimum temperature best explained the treeline shift rates across the Northern Hemisphere. MAIN CONCLUSIONS: A combination of thermal and hydrological factors drives treeline shift rates across the Northern Hemisphere, with precipitation assuming an important modifying role of the general temperature‐driven treeline ascent. Regional treeline shift rates, therefore, co‐depend on drying and wetting trends, which should be considered in future estimates of global change impacts on alpine ecosystems.

47 citations

Journal ArticleDOI
TL;DR: Cudlin, Pavel et al. as discussed by the authors used 16 paginas and 3 pagineas to describe the authors' work in their book as discussed by the authors, which is a compana suplemento de 3paginas.
Abstract: Cudlin, Pavel et. al.- 16 paginas.- Ilustraciones.- Se acompana suplemento de 3 paginas.- © The autors 2017. Open Access under Creative Commons by Attribution Licence. Use, distribution and reproduction are unrestricted. Authors and original publication must be credited.

43 citations


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

  • ...2009), only a few published studies have included quantitative data about treeline shifts (Devi et al. 2008, Kullman & Öberg 2009, Diaz-Varela et al. 2010, Van Bogaert et al. 2011)....

    [...]

  • ...Glob Change Biol 21: 2005−2021 De Frenne P, Rodríguez-Sánchez F, De Schrijver A, Coomes DA, Hermy M, Vangansbeke P, Verheyen K (2015) Light accelerates plant responses to warming....

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  • ...Since 680 AD: intensive HADCM3_A2, Rocky sites; extreme Velchev (1997), fires, deforestation and grazing; CGGM2, hydrothermal conditions; Grunewald & since 1962: traditional mng ΔT2050 = 1.7°C, competition of shrubs and tree Scheithauer (2011), has changed; last decades tourism ΔT2080 = 2....

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  • ...Since 680 AD: intensive HADCM3_A2, Rocky sites; extreme Velchev (1997), fires, deforestation and grazing; CGGM2, hydrothermal conditions; Grunewald & since 1962: traditional mng ΔT2050 = 1....

    [...]

  • ...J For Sci 56: 505−517 Van Bogaert R, Haneca K, Hoogesteger J, Jonasson C, de Dapper M, Callaghan TV (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....

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Journal ArticleDOI
TL;DR: In this article, the northern range margins of tree distributions were examined by comparing differences in the 95th percentile locations of seedlings to adults by 0.5 longitudinal bands over 5-years and by levels of disturbance (i.e., canopy gap formation).

42 citations


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

  • ...). responses across elevation as opposed to latitude (Walther et al., 2005; Beckage et al., 2008; Holzinger et al., 2008; Kelly and Goulden, 2008; Lenoir et al., 2008; Harsch et al., 2009; Lenoir et al., 2009; Crimmins et al., 2011; Feeley et al., 2011; Van Bogaert et al., 2011)....

    [...]

  • ...Emerging work by Van Bogaert et al. (2011) suggests a similar finding that disturbance may be a stronger controlling factor on tree ranges than recent climatic trends....

    [...]

Journal ArticleDOI
TL;DR: The arctic forest-tundra ecotone (FTE) as discussed by the authors links species communities of the boreal forest with those of the arctic tundra, which is expected to respond swiftly to climate change with a profound reduction of tundras as the dominating scenario.
Abstract: The arctic forest–tundra ecotone (FTE), which links species communities of the boreal forest with those of the arctic tundra, is expected to respond swiftly to climate change with a profound reduction of tundra as the dominating scenario. With its circumarctic expanse and up to several hundred kilometres in width, the zone occupies a large part of the vegetated surface at high latitudes. Relocation and structural changes of the ecotone vegetation will affect not only plant but also animal and other biological diversity. A large number of arctic species are dependent on the FTE in terms of food and habitat during parts of their life cycle or annual migration. In the ‘Arctic Species Trend Index’, developed to provide trends in arctic vertebrates, more than half of the species and data are from the FTE. However, in assessments of arctic biodiversity, only the northernmost tundra-dominated areas of the ecotone are included. This is unfortunate and somewhat problematic since the treed part that serves as a sou...

40 citations


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

  • ...Although these rates are not supported by empirical data (Lloyd 2005; Van Bogaert et al. 2011; Hofgaard et al., forthcoming), tundra habitats and biodiversity are being affected by local, regional and global pressures including tree and shrub encroachment....

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References
More filters
Book
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.

1,775 citations

Journal ArticleDOI
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 ArticleDOI
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 ArticleDOI
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....

    [...]