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Treeline dynamics in short and long term perspectives: observational and historical evidence from the southern Swedish Scandes
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics. (Växtekologi)
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Against the background of past, recent and future climate change, the present thesis addresses elevational shifts of alpine treelines in the Swedish Scandes. By definition, treeline refers to the elevation (m a.s.l.) at a specific site of the upper trees of a specific tree species, at least 2 m tall.

Based on historical records, the first part of the thesis reports and analyzes the magnitude of treeline displacements for the main trees species (Betula pubescens ssp. czerepanovii, Picea abies and Pinus sylvestris) since the early 20th century. The study covered a large and heterogeneous region and more than 100 sites. Concurrent with temperature rise by c. 1.4 °C over the past century, maximum treeline advances of all species amount to about 200 m. That is virtually what should be predicted from the recorded temperature change over the same period of time. Thus, it appears that under ideal conditions, treelines respond in close equilibrium with air temperature evolution. However, over most parts of the landscape, conditions are not that ideal and treeline upshifts have therefore been much smaller. The main reason for that discrepancy was found to be topoclimatic constraints, i.e. the combined action of geomorphology, wind, snow distribution, soil depth, etc., which over large parts of the alpine landscape preclude treelines to reach their potential thermal limit.

Recorded treeline advance by maximum 200 m or so over the past century emerges as a truly anomalous event in late Holocene vegetation history.

The second part of the thesis is focused more on long-term changes of treelines and one specific and prevalent mechanism of treeline change. The first part of the thesis revealed that for Picea and Betula, treeline shift was accomplished largely by phenotypic transformation of old-established stunted and prostrate individuals (krummholz) growing high above the treeline. In obvious response to climate warming over the past century, such individuals have transformed into erect tree form, whereby the treeline (as defined here) has risen. As a means for deeper understanding of this mode of positional treeline change, extant clonal spruces, growing around the treeline, were radiocarbon dated from megafossil remains preserved in the soil underneath their canopies. It turned out that Picea abies in particular may attain almost eternal life due to its capability for vegetative reproduction and phenotypic plasticity. Some living clones were in fact inferred to have existed already 9500 years ago, and have thus persisted at the same spot throughout almost the entire Holocene. This contrasts with other tree species, which have left no living relicts from the early Holocene, when they actually grew equally high as the spruce. Thereafter they retracted by more than 300 m in elevation supporting that also on that temporal scale, treelines are highly responsive to climate change.

The early appearance of Picea in the Scandes, suggests that Picea “hibernated” the last glacial phase much closer to Scandinavia than earlier thought. It has also immigrated to northern Sweden much earlier than the old-established wisdom.

The experiences gained in this thesis should constitute essential components of any model striving to the project landscape ecological consequences of possible future climate shifts.

Place, publisher, year, edition, pages
Sundsvall: Department of Natural Sciences , 2010.
Series
Mid Sweden University licentiate thesis, ISSN 1652-8948 ; 51
Keywords [en]
Betula pubescens ssp. czerepanovii, Picea abies, Pinus sylvestris, climate change, monitoring, treeline advance, clones, megafossils, immigration, Holocene, cryptic refugia, Swedish Scandes
National Category
Ecology
Identifiers
URN: urn:nbn:se:miun:diva-12670ISBN: 978-91-86694-09-8 (print)OAI: oai:DiVA.org:miun-12670DiVA, id: diva2:377381
Presentation
2010-12-10, Q221, Campus, Östersund, 13:00 (Swedish)
Opponent
Supervisors
Available from: 2010-12-15 Created: 2010-12-14 Last updated: 2013-02-18Bibliographically approved
List of papers
1. Post-Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective.
Open this publication in new window or tab >>Post-Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective.
2009 (English)In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 97, no 3, p. 415-429Article, review/survey (Refereed) Published
Abstract [en]

Elevational tree line change in the southern Swedish Scandes was quantified for the period 1915-2007 and for two sub-periods 1915-1975 and 1975-2007. The study focused on Betula pubescens ssp. czerepanovii, Picea abies and Pinus sylvestris at a large number of sites distributed over an 8000-km(2) area. The basic approach included revisitations of fixed sites (elevational belt transects) and measurements of tree line positions (m a.s.l.) during these three periods. Over the past century, tree lines of all species rose at 95% of the studied localities, with means of 70-90 m. All three species displayed maximum upshifts by about 200 m, which manifests a near-perfect equilibrium with instrumentally recorded air temperature change. This magnitude of response was realized only in particular topographic situations, foremost wind-sheltered and steep concave slopes. Other sites, with more wind-exposed topoclimatic conditions, experienced lesser magnitudes of upshifts. Thus, spatial elevational tree line responses to climate change are markedly heterogeneous and site-dependent. Modelling of the future evolution of the forest-alpine tundra transition has to consider this fact. Even in a hypothetical case of substantial climate warming, tree lines are unlikely to advance on a broad front and a large proportion of the alpine tundra will remain treeless. During the period 1975-2007, the tree lines of Picea and Pinus (in particular) advanced more rapidly than that of Betula towards the alpine region. These species-specific responses could signal a potential trajectory for the evolution of the ecotone in a warmer future. Thereby a situation with some resemblance with the relatively warm and dry early Holocene would emerge. Substantial tree line upshifts over the past two to three decades coincide with air and soil warming during all seasons. This implies that both summer and winter temperatures have to be included in models of climate-driven tree line performance. Synthesis. Maximum tree line rise by 200 m represents a unique trend break in the long-term Holocene tree line regression, which has been driven by average climate cooling for nearly 10 000 years. Tree line positions are well-restored to their pre-Little Ice Age positions. Recent tree line ascent is a truly anomalous event in Holocene vegetation history and possibly unprecedented for seven millennia.

Keywords
climate change
National Category
Ecology
Identifiers
urn:nbn:se:miun:diva-8820 (URN)10.1111/j.1365-2745.2009.01488.x (DOI)000265035400005 ()2-s2.0-64549134657 (Scopus ID)
Available from: 2009-04-27 Created: 2009-04-27 Last updated: 2017-12-13Bibliographically approved
2. Ancient subalpine clonal spruces (Picea abies) – sources of postglacial vegetation history in the Swedish Scandes
Open this publication in new window or tab >>Ancient subalpine clonal spruces (Picea abies) – sources of postglacial vegetation history in the Swedish Scandes
2011 (English)In: Arctic, ISSN 0004-0843, E-ISSN 1923-1245, Vol. 64, no 2, p. 183-196Article in journal (Refereed) Published
Abstract [en]

This study addresses the long-standing issue of postglacial immigration of Picea abies (Norway spruce) into Scandinavia. Methodologically, the main focus is on megafossil tree remains (wood and cones) of spruce and other species, retrieved from the treeline ecotone (Swedish Scandes), as a tool for vegetation reconstruction. Radiocarbon dating of megafossils, preserved in the soil underneath layering clonal groups of Picea abies, provide the core data. Living spruce clones, with in some cases likely continuity back to the early Holocene (9500 cal. yr BP onwards), were found at high-elevations. First postglacial arrival to the Swedish Scandes at this stage concurs with previous megafossil inferences. This is several millennia earlier than inferred from pollen data and very soon after regional deglaciation. Persistence of some individual Picea clones since the early Holocene thermal optimum and up to the present is indicative of permanently open or semi-open spots in the high-mountain landscape, also at times when treelines in general were much higher than present. Initially, Picea clones appear to have existed in a regional no-analogue vegetation matrix of widely scattered pine (Pinus sylvestris), mountain birch (Betula pubescens ssp. czerepanovii), Siberian larch (Larix sibirica) and thermophilic broadleaved deciduous species. In response to subsequent neoglacial cooling, the alpine character of the landscape has been enhanced through a lowered pine treeline and disappearance of larch and thermophiles. Spruces, which escaped fire and other calamities, endured due to their inherent phenotypic plasticity. Increasing climatic harshness throughout the Holocene conserved them as crippled krummholz, protected from winter stress by almost complete snow coverage. Appearance of Picea abies exclusively in the west, shortly after the deglaciation, could suggest that it has immigrated from “cryptic” ice age refugia much closer to Scandinavia than conventionally thought.

Keywords
Picea abies, clones, megafossils, immigration, Holocene, cryptic refugia, Swedish Scandes.
National Category
Ecology
Identifiers
urn:nbn:se:miun:diva-12666 (URN)000292226100004 ()2-s2.0-79958114279 (Scopus ID)
Available from: 2010-12-14 Created: 2010-12-14 Last updated: 2017-12-11Bibliographically approved

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Öberg, Lisa

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