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- Van Bogaert, Rik, et al.
(author)
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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
- 2011
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In: Journal of Biogeography. - : Wiley. - 0305-0270 .- 1365-2699. ; 38:5, s. 907-921
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Journal article (peer-reviewed)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 Methods We used repeat photography, dendrochronological analysis, field observations along elevational transects and historical documents to study tree line dynamics.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.
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| 2. |
- Van Bogaert, Rik, et al.
(author)
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Competitive interaction between aspen and birch moderated by invertebrate and vertebrate herbivores and climate warming
- 2009
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In: Plant Ecology and Diversity. - : Informa UK Limited. - 1755-0874 .- 1755-1668. ; 2:3, s. 221-U4
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Journal article (peer-reviewed)abstract
- Background: Considerable changes in vegetation structure and distribution are predicted in high latitude ecosystems as a result of amplified climate change. However, some documented plant community changes do not follow model predictions. Aim: We compared the growth of and the responses to climate variation by the thermophilic aspen (Populus tremula) and its sub-Arctic competitor mountain birch (Betula pubescens ssp. czerepanovii) over the past 100 years. Methods: Repeat photography, high-resolution vegetation transects, dendro-ecological analysis, and local climate record archives were used to study changes in vitality and distributional range of the two tree species in response to climate variability. Results: Aspen grew 45% faster and had seven times higher recruitment numbers than birch. However, no aspen stand expansion was observed, most likely because of browsing by moose (Alces alces). Birch, on the other hand, suffered from cyclic outbreaks of the autumnal moth (Epirrita autumnata). One-hundred-year-old birch trees experienced on average 9.0 years of reduced growth due to moth herbivory compared to 1.4 years for aspen. Moreover, these moth outbreaks on birch stimulated recruitment of aspen in birch stands. Conclusions: As the sub-Arctic continues to become warmer, the dynamics between aspen and birch in forest ecosystems will likely depend on the number of vertebrate browsers relative to the number of aspen recruits, while major moth outbreaks on birch may facilitate the spread of aspen by reducing competition. Our results suggest that alternating episodes of apparent species range stabilities (homeostasis) and abrupt non-linear shifts may characterise species migration patterns in this ecosystem.
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| 3. |
- Van Bogaert, Rik, et al.
(author)
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Range Expansion of Thermophilic Aspen (Populus tremula L.) in the Swedish Subarctic
- 2010
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In: Arctic, Antarctic and Alpine research. - : Informa UK Limited. - 1523-0430 .- 1938-4246. ; 42:3, s. 362-375
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Journal article (peer-reviewed)abstract
- In subarctic Sweden, recent decadal colonization and expansion of aspen (Populus tremula L.) were recorded. Over the past 100 years, aspen became c. 16 times more abundant, mainly as a result of increased sexual regeneration. Moreover, aspen now reach tree-size (>2 m) at the alpine treeline, an ecotone that has been dominated by mountain birch (Betula pubescens ssp. czerepanovii) for at least the past 4000 years. We found that sexual regeneration in aspen probably occurred seven times or more within the last century. Whereas sexual regeneration occurred during moist years following a year with an exceptionally high June July temperature, asexual regeneration was favored by warm and dry summers. Disturbance to the birch forest by cyclic moth population outbreaks was critical in aspen establishment in the subalpine area. At the treeline, aspen colonization was less determined by these moth outbreaks, and was mainly restricted by summer temperature. If summer warming persists, aspen spread may continue in subarctic Sweden, particularly at the treeline. However, changing disturbance regimes, future herbivore population dynamics and the responses of aspen's competitors birch and pine to a changing climate may result in different outcomes.
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