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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. (Växtekologi)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Treelines in high-mountain regions are constrained by heat deficiency, although the working mechanisms are still not entirely understood. Observational and paleoecological studies on treeline performance may contribute to increased understanding of the treeline phenomenon in general. The present thesis addresses elevational shifts of alpine treelines in the Swedish Scandes. By various analytical tools, the studies embrace widely different temporal scales.

The concept treeline refers to the elevation (m a.s.l.) at a specific site of the upper individual tree of a certain tree species, at least 2 m tall. All the principal tree species in the Scandes are concerned, i.e. mountain birch (Betula pubescens ssp. czerepanovii), Norway spruce (Picea abies) and Scots pine (Pinus sylvestris).

Paper I deals with regional treeline dynamics at more than 100 sites over the past 100 years. Concurrent with temperature rise by c. 1.4 °C over the same period, maximum treeline advances of all species amount to about 200 m. Thus, under ideal conditions, treelines respond in close equilibrium with air temperature evolution. However, over most parts of the landscape, treeline upshifts have been much smaller than 200 m, which relates to the combined action of geomorphology, wind, snow distribution and soil depth. After 1975, the birch has lost its role as the most rapidly advancing tree species, being superseded by pine and spruce.

Paper II is a short-term (2005/2007-2010/2011) study of mountain birch treeline performance along a regional maritimity-continentality gradient. Upshift by 3.0 yr-1 in the maritime part of the gradient contrasts to retreat by 0.4 m yr-1 in the continental part. In the latter area, earlier and more complete melting of late-lying snow patches has seemingly progressed to a state when soil drought sets back the vigour of existing birches and precludes sexual regeneration and upslope advance of the treeline. In the maritime area, extensive and deep snow packs still exist above the treeline and constrain its position, although some release is taking place in the current warm climate.

Paper III explores treeline change by phenotypic transformation of old-established stunted and prostrate spruce individuals (krummholz) growing high above the treeline and is based on analyses of radiocarbon-dated megafossils, preserved in the soil underneath clonal groups of spruce. Living spruce clones, which in some cases may date back to the early Holocene (9500 cal. yr BP), suggests that spruce immigrated from “cryptic” ice age refugia much closer to Scandinavia than conventionally thought. As the krummholz form presupposes open and windy habitats, it is inferred that permanently open spots prevailed in the high-mountain landscape even during periods when treelines in general were much higher than today.

Paper IV reports radiocarbon dates of wood samples, retrieved from newly exposed glacier forefields at three main sites, located high above the modern treelines and embracing the entire Swedish Scandes. It appears that pine colonized early emerging nunataks already during the Late Glacial. Around 9600-9500 cal. yr BP a first massive wave of tree establishment, birch and pine, took place in “empty” glacier cirques. Both species grew 400-600 m above their present day treeline position and accordingly, the summer temperatures may have been 3.5 °C warmer than present (uncorrected for land uplift). During the entire interval 9600 to 4400 cal. yr BP, birch prospered 100-150 m above the uppermost pines. In response to Neoglacial cooling, treelines of both birch and pine descended until their final disappearance from the record 4400 and 5900 cal. yr BP, respectively. Thereafter, these habitats experienced increased snow accumulation and glacier inception.

Abstract [sv]

Avhandlingen belyser förskjutningar i olika tidsskalor av den alpina trädgränsens läge i de svenska Skanderna. Trädgränsen definieras som den högsta nivån (m ö.h.) för minst 2 m höga individer av en viss art i en definierad del av en fjällsluttning. Avhandlingen består av fyra separata uppsatser, publicerade i olika välrenommerade vetenskapliga tidskrifter. 

I Paper I analyseras förändringar av trädgränserna för fjällens vanligaste trädarter, fjällbjörk, gran och tall (Betula pubescens ssp. czerepanovii, Picea abies och Pinus sylvestris) mellan 1915 och 2007. Undersökningen omfattar ett 8000 km2 stort, naturgeografiskt heterogent område, med mer än 100 lokaler. De maximala trädgränsförskjutningarna för samtliga trädarter uppgår till omkring 200 m. Resultaten motsvarar de trädgränsförskjutningar som teoretiskt kan förutsägas utifrån den temperaturhöjning med cirka 1,4 °C som skett under samma tidsperiod, förutsatt ideala förhållanden. För större delen av området råder emellertid andra, icke-ideala förhållanden, varför trädgränsernas uppflyttning i allmänhet blivit avsevärt mindre än 200 m. Den främsta anledningen till detta är lokala topoklimatiska begränsningar, d.v.s. kombinerade effekter av geomorfologi, vind, snöfördelning, jorddjup, etc., som i stora delar av det starkt brutna fjällandskapet mer eller mindre effektivt hindrar träden från att nå sina potentiellt högst belägna växtplatser betingade av temperaturen. Efter 1975 har björken förlorat sin roll som arten med den snabbast expanderande trädgränsen. I stället har tall och gran avancerat med större hastighet. Det innebär att även om klimatets uppvärmning fortsätter, så kommer det subalpina björkbältet att expandera i avsevärt mindre omfattning än vad som ofta förebådats. Möjligtvis kommer det att ersättas av tall.

Paper II behandlar björkens trädgränsdynamik under perioden 2005/2007-2010/2011 längs en regional klimatgradient med avseende på maritimitet/kontientalitet. Trädgränsen har under den aktuella perioden avancerat 3,0 m/år i den maritimt präglade delen av gradienten, vilket kontrasterar signifikant mot en sänkning med 0,4 m/år i området med mer kontinentalt klimat. Skillnaderna diskuteras i termer av klimatförändringens varierande effekter på snötäckets utbredning och varaktighet och dess inverkan på markfuktigheten. En allt tidigare total utsmältning av snölegorna i de kontinentala områdena har av allt att döma resulterat i vattenbrist under sommaren. Torka medför reducerad vitalitet för existerande björkar och förhindrar både sexuell förökning och uppflyttning av trädgränsen. I de maritima delarna kvarligger alltjämt mycket snö under en stor del av sommaren. Trädgränsens position har därför kunnat bibehållas eller flyttas upp. Vissa omständigheter tyder på att trädgränsens stigning i högre grad har varit baserad på fröföryngring efter 1975, jämfört med perioden 1915-1975.

Utgångspunkten för Paper III är erfarenheter från Paper I, som visar att trädgränsens uppflyttning för gran och björk huvudsakligen är resultatet av ökad höjdtillväxt av äldre, i vissa fall flertusenåriga, mer eller mindre buskformiga individer (krummholz), som vuxit på nivåer långt ovanför trädgränsen. Som ett svar på de senaste hundra årens varmare klimat har dessa antagit trädform, varigenom trädgränsen höjts. För en fördjupad förståelse av den här mekanismen har megafossil, d.v.s. grova vedrester bevarade i marken under gamla grankloner i trädgränsekotonen, 14C-daterats. Resultaten tyder på att granar i exponerad fjällmiljö kan uppnå i det närmaste ”evigt” liv genom sin förmåga till vegetativ förökning och möjligheten att växla mellan busk- och trädform i takt med klimatets växlingar. Vissa nu levande kloner existerade av allt att döma redan för 9500 år sedan. Den nu dokumenterat tidiga förekomsten av gran, bekräftar den på senare tid allt tydligare bilden av granen som en tidig invandrare till fjällkedjan. Möjligtvis har granen ”övervintrat” den senaste istiden närmare Skandinavien än vad som till helt nyligen varit den gängse uppfattningen.

Paper IV behandlar en för Skandinavien ny metod för historisk trädgränsrekonstruktion. I uppsatsen analyseras 14C-dateringar av totalt 78 större veddelar (megafossil) som nyligen exponerats i anslutning till smältande glaciäris och ”perenna” snölegor i tre huvudområden, Helags-Sylarna, Tärna och Abisko, högt ovanför dagens trädgräns. Det framkommer att tall (Pinus sylvestris) koloniserade tidigt framsmälta nunatakker redan under senglacial tid. För omkring 9600-9500 år sedan inträffade en första massiv våg av björk- och talletablering i isfria glaciärnischer. Båda arterna växte 400-600 m ovanför sina nuvarande trädgränspositioner, i ett klimat som kan ha varit 3,5 °C varmare än idag.

Under intervallet 9600 till 4400 BP uppträdde björken i ett 100-150 m brett bälte ovanför de översta tallarna. Som ett svar på klimatets successiva avkylning under senare delen av Holocen sänktes både björkens och tallens trädgränser i de aktuella miljöerna, till dess de för 4400 respektive 5900 år sedan helt försvann från lokaler där glaciärer och perenna snöfält började bildas. De analyserade trädresterna, som länge bevarats av glaciäris och perenn snö representerar en period med ett klimat långt varmare än under det senaste århundradet. Med denna analogi från det förflutna kan det därför antas att i en framtid där sommartemperaturerna rent hypotetiskt är 3,5 °C högre än i nutiden, skulle trädgränserna lokalt kunna flyttas upp med ungefär 600 m.

Place, publisher, year, edition, pages
Sundsvall: Kopieringen Mid Sweden University , 2013. , 136 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 143
Keyword [en]
Betula pubescens ssp. czerepanovii, Picea abies, Pinus sylvestris, Larix sibirica, climate
Keyword [sv]
Betula pubescens ssp. czerepanovii, Picea abies, Pinus sylvestris, Larix sibirica
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:miun:diva-18458ISBN: 978-91-87103-63-6 (print)OAI: oai:DiVA.org:miun-18458DiVA: diva2:604766
Public defence
2013-03-15, Sal G1352,, Mittuniversitetet, Östersund, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2013-02-18 Created: 2013-02-12 Last updated: 2013-04-22Bibliographically 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, 415-429 p.Article, 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.

Keyword
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: 2016-09-26Bibliographically approved
2. Contrasting short-term performance of mountain birch (Betula pubescens ssp. czerepanovii) treeline along a latitudinal continentality-maritimity gradient in the southern Swedish Scandes
Open this publication in new window or tab >>Contrasting short-term performance of mountain birch (Betula pubescens ssp. czerepanovii) treeline along a latitudinal continentality-maritimity gradient in the southern Swedish Scandes
2012 (English)In: Fennia, ISSN 0015-0010, Vol. 190, no 1, 19-40 p.Article in journal (Refereed) Published
Abstract [en]

Positional treeline shift is a fundamental aspect and indicator of high-mountain vegetation response to climate change. This study analyses treeline performance during the period 2005/2007 -2010/2011 in the Swedish Scandes. Focus is on mountain birch (Betula pubescens ssp. czerepanovii) along a regional climatic maritimity-continentality gradient. Treeline upshift by 3.0 yr-1 in the maritime part differed significantly from retreat by 0.4 m yr-1 in the continental part of the transect. This discrepancy is discussed in terms of differential warming-induced snow cover phenology patterns and their influence on soil moisture conditions. In the continental area, earlier and more complete melting of prior relatively rare late-lying snow patches, even high above the treeline, has progressed to a state when melt water irrigation ceases. As a consequence, soil drought sets back the vigor of existing birches and precludes sexual regeneration and upslope advance of the treeline. In the maritime area, extensive and deep snow packs still exist above the treeline and constrain its position, although some release is taking place in the current warm climate. Thereby, the birch treeline expands upslope as the alpine snow patches shrink, but continue to provide sufficient melt water throughout the summer. Treeline rise appears to have been based primarily on seed regeneration over the past few decades. This is a novelty, since prior (1915-2007) treeline advance was accomplished mainly by in situ shifts in growth form of relict krummholz birches, in some cases millennial-old, prevailing above the treeline. By the snow phenology mechanism, birch can benefit from climate warming in the maritime region, which contrasts with the situation in the continental region. This discrepancy should be accounted for in projective models. In a hypothetical case of sustained warming, the subalpine birch forest belt may expand less extensively than often assumed, although advance may continue for some time in snow rich maritime areas.

Place, publisher, year, edition, pages
Helsinki: , 2012
Keyword
Betula pubescens ssp. czerepanovii; Climate warming; Regeneration; Revisitation study; Swedish Scandes; Treeline
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-18309 (URN)2-s2.0-84867440878 (Scopus ID)
Available from: 2013-01-18 Created: 2013-01-18 Last updated: 2017-12-06Bibliographically approved
3. 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, 183-196 p.Article 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.

Keyword
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
4. Recent Glacier Recession – a New Source of Postglacial Treeline and Climate History in the Swedish Scandes
Open this publication in new window or tab >>Recent Glacier Recession – a New Source of Postglacial Treeline and Climate History in the Swedish Scandes
2012 (English)In: Landscape Online, ISSN 1865-1542, E-ISSN 1865-1542, Vol. 26, 1-38 p.Article in journal (Refereed) Published
Abstract [en]

Climate warming during the past century has imposed recession of glaciers and perennial snow/ice patches along the entire Swedish Scandes. On the newly exposed forefields, subfossil wood remnants are being outwashed from beneath ice and snow bodies. In Scandinavia, this kind of detrital wood is a previously unused source of postglacial vegetation and climate history. The present study reports radiocarbon dates of a set of 78 wood samples, retrieved from three main sites, high above modern treelines and stretching along the Swedish Scandes. In accord with previous studies, pine (Pinus sylvestris) colonized early emerging nunataks already during the Late Glacial. Around 9600-9500 cal. yr BP a first massive wave of tree establishment, birch and pine, took place in "empty" glacier cirques. Both species grew 400-600 m above their present-ay treeline position and the summer temperatures may have been 3.5 °C warmer than present. In respons to Neoglacial cooling, treelines of both birch and pine descended until their final disappearance from the record 4400 and 5900 cal. yr BP, respectively. During the entire interval 9600 to 4400 cal. yr BP, birch prospered in a 100-150 broad belt above the uppermost pines. The recent emergence of tree remnants in the current habitats relates to the contemporary episode of climate warming, possibly unprecedented for several past millennia. It is inferred, by an anology with the past, that in a future scenario with summers 3.5 ° warmer than present, the birch treeline may rise by 600 m or so.

Place, publisher, year, edition, pages
Bonn: Geographisches Institut,Universität Bonn, 2012
Keyword
climate change, glacier forefields, megafossil trees, vegetation history, paleoclimate, mountain birch, pine, Holocene
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-15642 (URN)10.3097/LO.201126 (DOI)2-s2.0-84855565082 (Scopus ID)
Projects
Treeline dynamics in short and long term perspectives – observational and historical evidence from the southern Swedish Scandes
Available from: 2012-01-04 Created: 2012-01-04 Last updated: 2017-12-08Bibliographically approved

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