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the authors used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics in a sub-Arctic model area at different temporal and spatial scales.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 Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics.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

Did you find this useful? Give us your feedback

Figures (9)

Content maybe subject to copyright    Report

biblio.ugent.be
The UGent Institutional Repository is the electronic archiving and dissemination platform for all
UGent research publications. Ghent University has implemented a mandate stipulating that all
academic publications of UGent researchers should be deposited and archived in this repository.
Except for items where current copyright restrictions apply, these papers are available in Open
Access.
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
More filters
Journal Article•DOI•
No treeline shift despite climate change over the last 70 years

[...]

Mirela Beloiu, Dimitris Poursanidis, Antonis Tsakirakis, Nektarios Chrysoulakis, Samuel Hoffmann, Petros Lymberakis, A. Barnias, David Kienle, Carl Beierkuhnlein 
01 Jan 2022-Forest Ecosystems
TL;DR: The treeline is characterized by a diffuse form, with trees growing on steep limestone slopes (>50°) and shallow soils, and the treeline elevation decreases with increasing distance from the coast and with aspect as mentioned in this paper .

10 citations

Journal Article•DOI•
Recent and Past Trees and Climates at the Arctic/Alpine Margin in Swedish Lapland : An Abisko Case Study Review

[...]

Leif Kullman
01 Jan 2015
TL;DR: In this article, the authors investigated the role of reindeer browsing in the evolution of treeline ecotone in the Abisko region of northern Swedish Lapland and concluded that the most likely candidate for climate change is recorded secular climate warming by 2.5 °C.
Abstract: For about a century, Abisko Scientific Research Station in northern Swedish Lapland has served as a logistic base for high-quality geoecological research in subalpine/subarctic environments. In recent years, and driven by the prospect of alleged man-made global warming, much of the scientific focus has been on dynamics of the treeline ecotone. In this context, field observations, analyses and interpretations emanating from research carried out in the Abisko region are discussed in perspective of recent observations and analyses. Local mountain birch (Betula pubescens ssp. czerepanovii) treeline rise by maximum 230 m during the past 100 years conforms quantitatively to data obtained further south in the Scandes. This broad-scale inter-regional coincidence indicates that a common operative agent has been responsible. The most likely candidate is recorded secular climate warming by 2.5 °C. This contention is further supported by age structure analysis in the birch treeline advance zone, indicating that the vegetative initiation of new trees peaked during the warm 1930s, when reindeer number were high and reached a nadir during the relatively cold 1960s and 1970s, coincident with smaller reindeer herds. These data suggest, contrary to previous hypotheses, stating that, relative to climate change, intensity of reindeer browsing has been of minor importance for birch treeline dynamics. The upper limit of closed stands of mountain birch and pine has shifted relatively insignificantly in elevational position during the predominantly warm past 100 years. Over the same period or longer, common aspen (Populus tremula) has frequently occurred as low-growing krummholz (stunted growth forms) over the entire mountain birch region. During the warm 1930s, and just like birch, rapid height increment was initiated and has continued up to the present day. Thereby, many individuals have attained tree-sized in recent decades. Accordingly, aspen (Populus tremula) has, presumably in response to climate warming, become a more conspicuous element in the mountain birch forest. The current analyses refute prior claims that aspen regeneration is accomplished by seed regeneration rather than phenotypic adjustment of old-growth creeping individuals. Picea abies and Larix sp. are recorded as new species in the Abisko area. In accordance with prior analyses in other parts of the Scandes, megafossil data show that the treelines of Scots pine (Pinus sylvestris), mountain birch (Betula pubescens ssp. czerepanowii) and grey alder (Alnus incana) peaked in the early Holocene. Based on the elevational difference between early Holocene and present treeline positions (adjusted for 100 m land uplift) it may be inferred that the summer temperatures exceeded those of the last few decades by about 3.0 °C.

9 citations

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

  • ...In recent years, treeline change and ecology have gained much scientific attention, primarily as proxy for climate change and variability as well as vegetation dynamics [22,23,27,35,38,40]....

    [...]

  • ...As these inferences may be questioned, a complementary and particularly careful age structure analysis was performed in the belt where the treeline has advanced during the past century [27,28,57]....

    [...]

  • ...These present results contrast with those obtained through stem boring and ring count by Van Bogaert et al [27], who inferred minimum establishment of new birches at the treeline during the warm 1930s and maximum during the relatively cold 1960s and 1970s [27]....

    [...]

  • ...This circumstance further conflicts with the view advocated by, claiming that reindeer rather than climate warming are more important for birch establishment, growth and treeline rise than climate change [27]....

    [...]

  • ...For example, some prior works are founded on imprecise treeline definitions and poor up-todate field-based reconnaissance of present-day treeline positions of defined tree species [20-27]....

    [...]

Book Chapter•DOI•
Climatic drivers of tree growth and recent recruitment at the Pyrenean alpine tree Line ecotone

[...]

Enric Batllori1, Enric Batllori2, J. Julio Camarero3, Emilia Gutiérrez1•
University of Barcelona1, University of California2, Spanish National Research Council3
01 Jan 2012
TL;DR: In this paper, the alpine tree line ecotone (ATL) was found to be the main limiting factor for tree growth, reproduction and establishment in high elevation forests.
Abstract: Global climate is currently warming at an unprecedented rate with potentially profound and widespread effects on the distributions of plant species and ecological communities (IPCC 2007; Lenoir et al. 2008). Mountain ecosystems and their unique biota are particularly sensitive to such changes (Beniston 2003). In high elevation forests, climate has been considered to be the main limiting factor for tree growth, reproduction and establishment (e.g. Tranquillini 1979; Korner 1998; Ettinger et al. 2011). The upper elevational limit of forest and tree growth on mountain slopes, the alpine tree line ecotone (ATL), represents an abrupt transition in life form dominance and is one of the most prominent vegetation boundaries between ecosystems (Holtmeier 2009). On a global scale, heat deficiency remains the most likely explanation of ATL elevation irrespective of the latitude and the tree line forming species (Korner and Paulsen 2004). Hence, based on the traditional tree line paradigm, warm temperature is favourable to both tree radial growth and reproductive success, and thus the ATL may be exceptional for the potential it offers for the assessment of the impacts of anthropogenic warming on mountain forests.

9 citations

Journal Article•DOI•
No treeline advance over the last 50 years in subarctic western and central Canada and the problem of vegetation misclassification in remotely sensed data

[...]

Kevin P. Timoney, Steven D. Mamet1•
University of Saskatchewan1
02 Apr 2020-Ecoscience
TL;DR: In this paper, the authors examined whether there has been significant tree cover change over the period 1960-2010 in a 960,000 km2 subarctic study region in western and central Canada, and the degree to which Global Forest Change (GFC) tree cover data agree with other datasets.
Abstract: In this study we examined (1) whether there has been significant tree cover change over the period 1960–2010 in a 960,000 km2 subarctic study region in western and central Canada, and (2) the degree to which Global Forest Change (GFC) tree cover data agree with other datasets. We compared GFC tree cover to cover estimates from air photos (c. 1960), ground-level plot data (c. 1982–84), annotated low-level oblique photographs (c. 2005–09), and air photo footprints on the World Imagery Base Map (c. 2010). Tree cover changes since 1960 varied by physiographic and ecological regions. Afforestation was modest to non-significant depending on the region. We observed no evidence of northward tree migration. An increase in the areal extent of burned forests, mostly in areas south of the forest-tundra, was the largest change detected. We documented systematic discrepancies between our tree cover estimates and GFC data. GFC underestimates of tree cover typically occurred in areas of low tree density. Areas where GFC data overestimated tree cover were common, especially near the northern limits of trees and in areas dominated by dense or tall shrubs. Predictions of climate-driven vegetation response derived solely from remotely sensed data may not be reliable.

9 citations

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

  • ...…central and western Canada, studies to date have detected a mixture of changes such as decline in tree cover, no net change, local afforestation, or increases in stem density within existing stands (Van Bogaert et al. 2011; Brown and Johnstone 2012; Mamet and Kershaw 2012; Baltzer et al. 2014)....

    [...]

Journal Article•DOI•
Spatial patterns in soil organic matter dynamics are shaped by mycorrhizosphere interactions in a treeline forest

[...]

Nina L. Friggens1, Nina L. Friggens2, Thomas J. Aspray1, Thomas C. Parker2, Jens-Arne Subke2, Philip A. Wookey2 •
Heriot-Watt University1, University of Stirling2
01 Feb 2020-Plant and Soil
TL;DR: In this article, the authors investigated the spatial extent over which trees influence soil processes and found that individual trees exert influence substantially away from their base, creating more uniform distributions of root, mycorrhizal and bacterial activity than expected, with potential implications for spatio-temporal soil organic matter dynamics and net ecosystem C sequestration.
Abstract: In the Swedish sub-Arctic, mountain birch (Betula pubescens ssp. czerepanovii) forests mediate rapid soil C cycling relative to adjacent tundra heaths, but little is known about the role of individual trees within forests. Here we investigate the spatial extent over which trees influence soil processes. We measured respiration, soil C stocks, root and mycorrhizal productivity and fungi:bacteria ratios at fine spatial scales along 3 m transects extending radially from mountain birch trees in a sub-Arctic ecotone forest. Root and mycorrhizal productivity was quantified using in-growth techniques and fungi:bacteria ratios were determined by qPCR. Neither respiration, nor root and mycorrhizal production, varied along transects. Fungi:bacteria ratios, soil organic C stocks and standing litter declined with increasing distance from trees. As 3 m is half the average size of forest gaps, these findings suggest that forest soil environments are efficiently explored by roots and associated mycorrhizal networks of B. pubescens. Individual trees exert influence substantially away from their base, creating more uniform distributions of root, mycorrhizal and bacterial activity than expected. However, overall rates of soil C accumulation do vary with distance from trees, with potential implications for spatio-temporal soil organic matter dynamics and net ecosystem C sequestration.

9 citations

References
More filters
Book•
Arctic climate impact assessment

[...]

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.

1,775 citations

Journal Article•DOI•
Vegetation Ecology of Central Europe.

[...]

J. R. Etherington, Heinz Ellenberg, G. K. Strutt
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....

    [...]

Related Papers (5)
Are treelines advancing? A global meta‐analysis of treeline response to climate warming

[...]

01 Oct 2009-Ecology Letters
Melanie A. Harsch, Philip E. Hulme, Matt S. McGlone, Richard P. Duncan, Richard P. Duncan 
Sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change at landscape and local scales

[...]

01 Sep 2005-Global Ecology and Biogeography
Friedrich-Karl Holtmeier, Gabriele Broll
A world-wide study of high altitude treeline temperatures

[...]

01 May 2004-Journal of Biogeography
Christian Körner, Jens Paulsen
The evidence for shrub expansion in Northern Alaska and the Pan‐Arctic

[...]

01 Apr 2006-Global Change Biology
Ken D. Tape, Matthew Sturm, Charles H. Racine
Plant community responses to experimental warming across the tundra biome

[...]

31 Jan 2006-Proceedings of the National Academy of Sciences of the United States of America
Marilyn D. Walker, C. Henrik Wahren, Robert D. Hollister, Greg H. R. Henry, Lorraine E. Ahlquist, Juha M. Alatalo, M. Syndonia Bret-Harte, Monika P. Calef, Terry V. Callaghan, Amy B Carroll, Howard E. Epstein, Ingibjörg S. Jónsdóttir, Julia A. Klein, Borgthor Magnusson, Ulf Molau, Steven F. Oberbauer, Steven P. Rewa, Clare H. Robinson, Gaius R. Shaver, Katharine N. Suding, Catharine C. Thompson, Anne Tolvanen, Ørjan Totland, P. Lee Turner, Craig E. Tweedie, P. J. Webber, Philip A. Wookey