Réf. Nicolussi & al. 2005 - A

Référence bibliographique complète

NICOLUSSI, K., KAUFMANN, M., PATZELT, G., VAN DER PLICHT, J., THURNER, A. 2005. Holocene tree-line variability in the Kauner Valley, Central Eastern Alps, indicated by dendrochronological analysis of living trees and subfossil logs. Vegetation History and Archaeobotany, 14, 221–234. [PDF]

Abstract: The altitude of the Alpine tree-line has often been used as proxy for the climatic conditions in the Holocene epoch. The usual approach for establishing a record for this proxy is the analysis of pollen and macro remains. We analysed living trees and subfossil logs from the timberline ecotone in the innermost Kauner valley in the Central Eastern Alps in order to assemble a Holocene dendrochronological tree-line record. Data sets comprising age and height of living Stone Pines (Pinus cembra L.) were collected at one site. Sections of 170 subfossil Stone Pine logs from five other sites were dendrochronologically analysed and dated. Besides using dendrochronological analyses, radiocarbon dating served as a means of obtaining the age of some logs. For most of the samples we could provide dendrochronological dates (1-year dating precision, back to 5125 B.C.) or wiggle matched dates (between approx. 7100 and 5040 B.C., dating precision with 95% probability: ±7 years). In the first half of the 19th century the tree-line was located at about 2180 m a.s.l. in the innermost Kauner valley. After approximately A.D. 1860 the altitude of the upper limit of the occurrence of Pinus cembra individuals (tree-species-line) and, being closely linked, also that of the tree-line both rose. The current tree-line (trees >2 m) is located at 2245 m a.s.l. due to climatic conditions around 1980. Additionally we observed saplings up to a present (A.D. 2000) tree-species-line at approx. 2370 m a.s.l. The dendrochronologically analysed subfossil logs found at up to 2410 m a.s.l. date from within the last 9000 years (between approx. 7100 B.C. and A.D. 1700). In the space of the last 4000 years the dendrochronological tree-line record is not continuous, probably due to human impact. Tree-line positions similar to or slightly above the 1980 tree-line are established for the time periods approx. 1000 to 640 B.C. and A.D. 1 to 330 respectively. For the time period between approx. 7100 and 2100 B.C. the dendrochronologically analysed logs show nearly continuous evidence of a tree-line above the 1980s limit. Very high elevation of the tree-line, between 120 and 165 m above the 1980s level (2245 m a.s.l.) and even higher than the A.D. 2000 tree-species-line (2370 m a.s.l.), are recorded for the periods 7090–6570, 6040–5850, 5720–5620, 5500–4370 B.C., approx. 3510–3350 B.C. and 2790–2590 B.C. Additionally, a tree-line which was located at least 50 m above the 1980s limit can be shown for the periods 6700–5430, 4920–3350 and 3280–2110 B.C. The dendrochronological record from the Kauner valley, showing high and very high tree-line positions between approx. 7100 and 2100 B.C. with only two gaps (around 6490 B.C. and from 3350 to 3280 B.C.), suggests that summer temperatures as observed in the late 20th century were at the normal or the lower limit of the temperature range which can be assumed for long periods of the early and middle Holocene epoch.

Mots-clés

Holocene - Alps - Tree line - Pinus cembra - Dendrochronology

 

Organismes / Contact

• Institute of Geography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria e-mail: Kurt.Nicolussi@uibk.ac.at
• Center for Isotope Research, Groningen University, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Faculty of Archaeology, Leiden University, PO Box 9515, 2300 RA Leiden, The Netherlands

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

Temperature, precipitations

Tree vegetation (treeline)

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Austrian Alps

Ötztal Alps

Inner Kauner valley

 

2170-2325 m a.s.l.

Approx. The last 9000 years

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

Since growth of Pinus cembra at tree-line sites is mainly controlled by summer temperatures (Tranquillini 1979) we can roughly estimate minimum temperatures for the lifespan of the subfossil logs found up to approx. 165 m above the 1980 tree-line in the innermost Kauner valley. Using a temperature lapse rate of about –0.6 to –0.7◦C/100 m we assess summer temperatures for those time periods as being at least 1.0◦C higher than the values measured between about A.D. 1920 (the approx. germination of the living Stone Pine trees at the 1980 tree-line) and 1980. Summer temperatures for the late 20th century observed at high altitude stations were about 0.7◦C (Böhm et al. 2001) above the 1920/1980 mean and at that value are still below the estimated minimum values for very high tree-line position (TLP) periods within the Holocene.

Observations

 

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

[See below]

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

Results

The results for the present changes in the timberline ecotone in the innermost Kauner valley are based on the analysis of 412 living and 13 dry dead Stone Pines at site 1. The life span of the trees and saplings analysed by altitude of the growth stand is shown. The gap between 2250 and 2285 m a.s.l. reflects the difference in altitude between the upper limit of plot 1 and the lower limit of plot 2. Trees more than 200 years old and more than 12 m tall can only be found in the lowest part of plot 1. Here the authors were also able to sample dry dead trees. These trees, having multi-centennial life spans, do not show the same date of death, moreover the dates of the last tree-rings measured spread over the period A.D. 1717 to 1871. The tree-ring series of these mature trees show some tree-ring width minima of multi-year duration. The last of these growth depressions occurred around A.D. 1815 and is in accordance with the well-known climatic deterioration in the same decade (Fliri 1996). After A.D. 1820 the tree-ring series do not show any similar growth decline.

These improved climatic conditions after about A.D. 1860 allowed the establishment of new Stone Pines above the early 19th century tree-line at site 1. Living trees found (heights today of up to 10 m) started growing between A.D. 1870 and 1890 up to an altitude of 2210 m a.s.l. and thus about 30 m above the early 19th century tree-line (approx. 2180 m). Another Stone Pine (sample code KM-1, height: 4.2m) germinated around A.D. 1880 at an altitude of 2245 m a.s.l. but died during the relatively cold 1970s. Within the same decade a small and shrub-like Stone Pine (height <0.6 m, 2320 m a.s.l.) died after a lifetime of approx. 50 years indicating that the upper limit of the “Krummholz” belt rose above 2300 m a.s.l. in approx. A.D. 1920.

Living Stone Pines, which started to grow in around A.D. 1920, can be found at altitudes of up to 2245 m a.s.l. at site 1. These trees are now up to 4.8 m tall and have established a new tree-line (trees >2 m, Ellenberg 1996) at 2245 m a.s.l. Beside these living Stone Pines, the standing tree KM-1 (2245 m a.s.l.), which died in the late 1970s, also proves that the altitude of 2245 m a.s.l. was the limit of growth of mature trees in the late 20th century. Climatic conditions did not allow the establishment and survival of Stone Pine trees (>2 m) at higher altitudes at site 1. The decade of A.D. 1970/80 was the coldest period of the second half of the 20th century (Böhm et al. 2001) and the treeline at 2245 m a.s.l. was in equilibrium with the climatic conditions of about A.D.1980. The increase in altitude of the A.D. 1980 tree-line above the early 19th century tree-line (2180 m a.s.l.) is about 65 m at site 1.

Since about A.D. 1980, intensive germination of Pinus cembra L. can be observed at site 1. Small individuals (<0.4 m) grow up to 45 m above the upper limit of plot 2 (2325 m a.s.l.). Through these Stone Pines, which often show shrub-like growth forms, a current (A.D. 2000) tree species limit (upper limit of the “Krummholz” belt, Körner 1998) can be defined as about 2370 m a.s.l. The germination phase in the late 20th century is synchronous with an observed temperature increase in the same time period (Böhm et al. 2001). From the growth forms and height of growth observed (further details will be discussed in a separate paper) and assuming that no significant decrease in temperature will occur in the near future, we expect the establishment of a new tree-line at about 2300 to 2320ma.s.l. at site 1 which would be in equilibrium with the climatic conditions since about A.D. 1980.

From the innermost Kauner valley 170 tree-ring series from subfossil logs of the Holocene timberline ecotone are available. These series spread over a time period between approx. 7100 B.C. and A.D. 1700. The subfossil samples of the last approx. 7000 years are, with a few exceptions, dated dendrochronologically. 14C-dates were also available for some of the logs from the middle and late Holocene. Most of the tree-ring series from sections older than approx. 7000 years are synchronized with a floating chronology, dated by wiggle matching (Nicolussi et al. 2004) and hence highly accurate wiggle matching results are available for these samples also. The temporal distribution of dated tree-ring series, obtained from the subfossil logs is shown together with the altitude of the growth position. The different levels of dating (dendrochronological date, wiggle matching date, single 14Cdate) are indicated by different shading.

Samples from altitudes below c. 2200 m spread over the last c. 8000 years. Most of them are from peat bogs (sites 3 and 4) at these elevations. A few sections from logs were collected on the forefield of the Gepatschferner (site 2). A concentration of tree-ring data appears between about 3200 and 1700 B.C. Older samples are relatively rare. This can be explained by the collecting strategy: Only logs less than approx. 0.7 m below the surface were usually detected during sampling. This is proven by the oldest section (KOA-10, approx. 6300 B.C.) from an altitude below 2200 m a.s.l., which was found 1.3 m below the bog surface in a trench.

Samples younger than approx. 1700 B.C. are also quite rare. In contrast to the mid-Holocene period replication of subfossil logs shows repeated gaps for the last nearly four millennia. Neither collecting strategy nor possible climatic impact can explain this lack of samples completely. However, other studies suggest that this relatively low number of samples from the late Holocene is a result of the human impact which has been demonstrated at different sites in the Ötztal Alps. This influence on the vegetation has intensified since the Bronze Age (Patzelt 1996; Bortenschlager 2000). Grazing animals and burning of the forests caused deforestation within the timberline ecotone of the Central Alps. This is also supported by the observation of charcoal layers in peat bogs during sampling activities in the Kauner valley. Pollen analytical investigations and the analysis of charcoal remains also show that grazing activities started in the mid-Holocene epoch in the innermost Kauner valley (Krapf 2001). As a result, the gaps in the sample replication since the early Bronze Age must be mainly seen as an indication of human activities in this area, which were seemingly very intensive between approx. 1500 and 800 B.C.

However, evidence for climatically favourable conditions during the last 4000 years is given by logs found on the forefield of the glacier Gepatschferner. These samples indicate both possible high tree-line position (TLPs) and glacier extents smaller than the observed extent of approx. A.D. 1930 (Nicolussi and Patzelt 2001). The logs from the Gepatschferner reveal favourable climatic conditions, which caused glacier retreat and allowed tree germination at high altitudes during different periods of the late Holocene, e.g. during the late Bronze Age (approx. 1000 to 640 B.C.) and the Roman Period (approx. A.D. 1 to 330). Also the 14C-date of a small piece of wood (sample code: KG-1, VRI-1882, 2760±50 BP, 2 σ range: 1010–800 cal B.C.) with only 12 counted tree-rings found at site 6 at an altitude of about 2415 m a.s.l. indicates both a high position of the tree-species-line (the upper limit of the occurrence of Pinus cembra individuals) and a high possible position of the associated tree-line within the former period.

At site 5 (code GDM, 2295 m a.s.l.) 44 samples were collected and dated. This site is directly connected to plot 2 of site 1. Site 5 is located above the 1980-treeline and the former presence of mature Stone Pines at this location indicates climatic conditions during the lifetime of these trees better than those of the mid-20th century. Currently living Stone Pine individuals with heights up to 1.1 m (as of summer 2002) and of an age of up to 40 years can be found just below the altitude of the lake. The subfossil samples from site 5 spread over a time period from approx. 6700 to 2100 B.C. So far no younger samples from this site have been found. The dated logs are distributed over three time periods which are divided by two major gaps at approx. 5430–4920 B.C. and around 3300 B.C. The first period with proven growth of mature trees at site 2 dates back to approx. 6700 to 5430 B.C. The tree-ring data set shows two small gaps in this period which emerge around 6490 B.C. (15 years) and 6015 B.C. (10 years). Most of the samples found at site 5 (GDM) are from two mid-Holocene periods (4920–3350 B.C. and 3280–2110 B.C.). Particularly high numbers of trees within these periods can be deduced from the accumulation of samples during the intervals approx. 4700 to 4200 B.C. and 3250 to 2700 B.C. The gap of about 70 years at approx. 3300 B.C. interrupts an almost 3000 year long phase of evidence for a high TLP from site 5.

Finds of subfossil logs at site 6 (Krummgampen, 2365–2410 m a.s.l.) are located at the same altitude as, or above the tree-species-line of A.D. 2000 observed at site 1. Evidence for the former presence of trees up to 2410 m a.s.l. is mainly for the first part of the mid-Holocene epoch, from c. 5500 to 4370 B.C. Only few samples are older (up to approx. 7100 B.C.) or younger (around 3450 B.C. and 2650 B.C.). The dendrochronological analysis of these samples shows that tree growth at such altitudes was possible, sometimes over a period of more than 350 years (e.g. tree KG-17, 2365 m a.s.l., approx. 5400 to 5050 cal B.C.), in the early and mid Holocene. Tree KG-7 grew at least 298 years (4674–4377 B.C.) at an altitude of 2400 m a.s.l. which indicates a multi-centennial duration of favourable growth conditions.

Discussion

Dendrochronological sequences for subfossil logs from the innermost Kauner valley spread over the last approx. 9000 years. During the late Holocene (after about 1700 B.C.) the distribution of tree-ring series shows repeated gaps and evidence of tree-line positions above the 1980 tree-line is rare due to human impact on the timberline ecotone in this part of the Eastern Alps. However, favourable climatic conditions for tree growth at locations of relatively high elevation are demonstrated for the first centuries of the first millennium B.C. (approx. 1000 to 640) and A.D. (approx. 1 to 330), respectively. For the period from approx. 7100 B.C. to 2100 B.C. dendrochronologically analysed logs show evidence of tree growth above the 1980 tree-line with only two gaps. Very high altitudes of the tree-line, approx. as high or higher than the tree-species-line of A.D. 2000, are recorded for the periods 7090–6570, 6040–5850, 5720–5620, 5500–4370, approx. 3510–3350 and 2790–2590 B.C. TLPs which correspond to an expected level for the climatic conditions of the late 20th century (2300–2320 m a.s.l.) can be shown for the time periods 6700–6500, 6490–6020, 6010–5430, 4920–3350 and 3280–2110 B.C. In the middle Holocene epoch our dendrochronological tree-line record only shows one gap around 3300 B.C. During this gap no sample has been found at lower sites in the Kauner valley to date.

There are a few similar reports of finds of logs above the current tree-line in the Alps. Furrer and Holzhauser (1984) mention six 14C-dates carried out on wood remains from locations above the “actual potential timberline” in the Swiss Alps. The results date between 4355±80 and 3515±80 B.P. (approx. 3000 to 1850 B.C.). Hence, these samples are partly contemporaneous with the accumulation of subfossil logs between 3280 and 2110 B.C. at site 5 (code GDM, 2295 m a.s.l.). Pott et al. (1995) 14C-dated a few Stone Pine samples with up to nearly 200 tree-rings found at 2365 m a.s.l. in the Fimber valley, Eastern Alps, to within the period 6860±75 to 6140±75 B.P. (approx. 5700 to 5100 B.C.). The dates of these finds are comparable to those of the sections found at site 6 (code KG) in the Kauner valley. Staffler and Feichter (1999) report the dates of small pieces of wood found at altitudes between 2380 and 2520ma.s.l. in the valley of Langtaufers which is located just south of the Kauner valley. The calibrated dates again show mid-Holocene ages between approx. 5900 and 4100 B.C.

Our dendrochronological tree-line record shows that Stone Pines grew nearly continuously at high and very high altitudes between approx. 7100 B.C. and 2100 B.C. in the innermost Kauner valley. Also in the time window around 8.2 ky B.P. tree growth was possible at site 5 (GDM, 2295 m a.s.l.). Evidence is given by two trees. The Stone Pine GDM-1 (approx. 6480 to 6150 B.C.) shows reduced width growth from c. 6230 B.C. until the end of the tree-ring series. Within the lifetime of this tree the Stone Pine GDM-21 started to grow a couple of years before 6170 B.C. Also pollen analytical evidence from site 6 (KG, pollen profile Krummgampen, 2395 m; Krapf 2001) shows a reduced amount of tree pollen at approx. 6200 B.C. indicating a climatic deterioration.

The pollen profile established at site 6 by Krapf (2001) additionally shows minima of arboreal pollen around 5300, 4400, 3300 and 1500 B.C. These minima are contemporaneous with reductions in the tree-line altitude (e.g. 4400 B.C.) or missing evidence for a high TLP (e.g. 3300 B.C.) indicated by the dendrochronological tree-line record. On the other hand, analysis of pollen and macro remains in sediment cores from other highly elevated sites in the Alps (e.g. Wick and Tinner 1997; Tinner and Ammann 2001) suggest long time periods with high TLPs in the early and middle Holocene. Both of the longest optima, approx. 7000–6200 and 5000 to 4200 B.C. (Tinner and Ammann 2001), are concurrent with evidence for high TLPs from the dendrochronological tree-line record. The altitudinal difference between the present (1980/90) and the Holocene maximum TLPs in the Swiss Alps also shows high agreement with our study. Tinner and Theurillat (2003) report a TLP approx. 180 m higher which is of the same order as the value of 165 m found in the Kauner valley.

The dendrochronological tree-line record presented for the late Holocene partially shows similar limitations for the last millennia as pollen analytical records of tree-line variability. Human impact on forests of the Alpine timberline ecotone is recognised repeatedly by different authors (e.g. Wick and Tinner 1997; Bortenschlager 2000). An assessment of the cause of a reduction in altitude of the tree-line, caused by either human activities or natural climatic variability, is often impossible. These problems might be solved by analysis of material from specific sites, e.g. from glacier forefields, where human activities have always been limited or absent. The logs from the forefield of the glacier Gepatschferner (site 2) also indicate periods of high TLP in these last four millennia (approx. 1850–1550 B.C., 1000–640 B.C. and A.D. 1–330).

Holocene oscillations of Alpine glaciers and the variability of the Alpine tree-line are both mainly forced by summer temperature variations in a given region (Patzelt and Bortenschlager 1973). Regional differences in tree-line altitudes and glacier distribution, e.g. between the Central and the Outer parts of the Alps, can be better explained by the differing amounts of precipitation and the duration of sunshine in these areas, both influencing the length of the vegetation period and ablation period, respectively. The observed TLP rise in the Kauner valley of approx. 65 m between the 19th century and 1980 is somewhat lower than the increase of the mean equilibrium line altitude (ELA) for Austrian glaciers of about 90 m (Patzelt and Aellen 1990). An additional rise of the ELA since 1980 is causing a marked retreat of Alpine glaciers (Paul et al. 2004). Under the precondition of stable climatic forcing of glacier oscillations during the Holocene, the indicated high TLPs in the early and middle Holocene suggest concurrent ELAs which were at least 150 m above those prevailing in about A.D. 1980. According to Patzelt and Aellen (1990) such a rise of the mean ELA implies a retreat of the glacier area in Austria of about 70% compared to the extent in the 1980s. Evidence for a sizeable retreat of glaciers in the Alps is mainly reported for the Holocene epoch before approx. 1700 B.C. (Nicolussi and Patzelt 2000; Hormes et al. 2001; Schlüchter and Jörin 2004). Extended radiocarbon dating of samples found on Swiss glacier forefields indicating smaller ice extents than today, shows the greatest number of dates for a period of about 850 years from c. 5400 to 4550 B.C. (Schlüchter and Jörin 2004). This is predominantly consistent with the continuous extremely high TLPs in the innermost Kauner valley shown for the period after 5500 B.C. Our dendrochronological tree-line record additionally indicates ongoing high summer temperatures until approx. 4370 B.C. Evidence for TLPs at or above 2365 m a.s.l. at approx. 3400 B.C. and from 2790 to 2590 B.C. are also contemporaneous with large reductions in glacier extents in the Swiss Alps (Hormes et al. 2001; Schlüchter and Jörin 2004).

Usually a high degree of similarity in the changes of these two Alpine climate proxies, glaciers and tree-line, can be shown. Differences suggest changed forcing of the glaciers and probably higher amounts of winter precipitation (snow) in some periods of the Holocene epoch. Such changes in the forcing of glacier oscillations were shown for Scandinavian glaciers by Dahl and Nesje (1996). We may interpret the advances of Alpine glaciers reported for the mid 6th millennium B.C. (Suter 1981; Nicolussi and Patzelt 2001), which are partly contemporary with high TLPs (Nicolussi et al. in press), as evidence of increased winter precipitation. Patzelt (1987) also assumes increased precipitation in the 6th millennium B.C. based on accumulation rates of alluvial fans in the Inn valley.

Since growth of Pinus cembra at tree-line sites is mainly controlled by summer temperatures (Tranquillini 1979) we can roughly estimate minimum temperatures for the lifespan of the subfossil logs found up to approx. 165 m above the 1980 tree-line in the innermost Kauner valley. Using a temperature lapse rate of about –0.6 to –0.7◦C/100 m we assess summer temperatures for those time periods as being at least 1.0◦C higher than the values measured between about A.D. 1920 (the approx. germination of the living Stone Pine trees at the 1980 tree-line) and 1980. Summer temperatures for the late 20th century observed at high altitude stations were about 0.7◦C (Böhm et al. 2001) above the 1920/1980 mean and at that value are still below the estimated minimum values for very high TLP periods within the Holocene.

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

The analysis of tree-line fluctuations is a classical approach to investigating the variability of the climate during the Holocene. For many years palynological research on material found in peat bogs or lakes within the timberline ecotone has been the methodological procedure for such studies (for the Western and Southern Alps: Welten 1952; Zoller et al. 1966; Carcaillet and Thinon 1996; Burga et al. 2001; Tinner and Theurillat 2003). For the middle part of the Eastern Alps Sigmar Bortenschlager and his group at the Institute of Botany, University of Innsbruck, have been carrying out palynological analysis since the late 1960s. His work and that of his colleagues (Bortenschlager 1970; Patzelt and Bortenschlager 1973; Bortenschlager et al. 1998; Bortenschlager 2000) has provided new results and insights on the variability of the Alpine tree-line. The pollen profiles established in the Alps have been used to reconstruct the vegetation history, e.g. in the context of glacier fluctuations (Zoller et al. 1966; Patzelt and Bortenschlager 1973) and of the climate (Bortenschlager et al. 1998; Burga and Perret 1998) during the Holocene. Besides analysing the pollen content of material from locations within the Holocene timberline ecotone, macro remains, e.g. needles, have also been used to investigate tree-line oscillations (Oeggl and Wahlmüller 1994; Wick and Tinner 1997) and to prove that at least single individuals were growing at a specific site and altitude.

Logs are a type of “mega” macro remains, including branches and roots as well as other remains of the tree itself. These wood remains provide additional potential for studying the Holocene tree-line variability. The number of tree-rings revealed in such finds allows the establishment of a precise minimum time period of tree growth at a specific site. The dating of pollen profiles and macro remains usually relies on radiocarbon dating. Remains of wood, e.g. logs, are also a perfect material for 14C-analysis for age estimation. Moreover, the measurement of the tree-rings of such wood remains, especially tree trunks, offers the possibility of dendrochronological dating.

Tree-ring analysis of logs from sites in the Alpine timberline ecotone has the potential to establish a record of Holocene tree-line variability with high temporal resolution and accuracy. The limitations of this approach lie mainly in the site conditions, because the remains of the trees must be well preserved. Hence Alpine areas with favourable conditions, such as lakes and the widespread peat bogs, are needed for such investigations. The full potential of tree-ring analysis of logs from tree-line sites can only be exploited if there is a possibility of dendrochronological dating. Recently the establishment of new multi-millennial Alpine tree-ring chronologies (Nicolussi and Schießling 2001; Nicolussi et al. 2004) has enabled such dating. An absolutely dated and continuous tree-ring width chronology has been established back to 5125 B.C. based on wood samples from high elevation sites (>about 2000ma.s.l.) in the Central Eastern Alps. Further chronologies are available for the early Holocene that are dated by wiggle matching of several 14C-dates of synchronised samples. The synchronisation of tree-ring series with chronologies dated by wiggle matching allows improvement of chronological delimitation as compared with single 14C-dates.

The analysis of subfossil logs and other wood remains from sites in the Holocene timberline ecotone in the Alps has been limited. Determination of the age of this kind of material (Furrer and Holzhauser 1984; Staffler and Feichter 1999) is usually based on single 14C-dates, these studies rarely giving numbers of tree-rings of the samples investigated. However, a number of 14C-dates of subfossil logs from locations within and above the current tree-line ecotone have been reported from Scandinavia (Dahl and Nesje 1996; Karl´en 1999). These samples have also been used to reconstruct Holocene climate variability in this region (Dahl and Nesje 1996).

Here, the authors present results on the variability of the Alpine tree-line during the Holocene from the Kauner valley in the Central Eastern Alps. In the innermost part of this valley, current tree-line changes have been investigated at one site and subfossil logs have been sampled at five additional sites in order to explore the past history of the tree-line in this area. Stone Pine (Pinus cembra L.), a coniferous species, is the dominant tree species in the tree-line ecotone in the Kauner valley (Schiechtl and Stern 1975).

The inner section of the Kauner valley runs S-N, is located slightly to the north of the main Alpine ridge and is dominated by vast glaciers. This mountain area is part of the Ötztal Alps. The highest peaks in the innermost Kauner valley reach 3355 m (Glockturm) to 3510 m a.s.l. (Weißsee Spitze). Actual precipitation in this area is 1130 mm/a at about 2000 m a.s.l.

The contemporary potential altitude of the tree-line in the innermost Kauner valley has been fixed at between approximately 2200 and 2300 m a.s.l. (Schiechtl and Stern 1975). Schiechtl and Stern (1975) do not include the present changes in the tree-line position (TLP) due to current climatic change in the Alpine region. Hence current tree-line development was studied in two plots at site 1, each 35 m wide and 110 m and respectively 90 m long. Plot 1 reached from 2170 to 2250 m and Plot 2 from 2285 to 2325 m a.s.l. Within the plots all Stone Pine individuals (Pinus cembra L.) taller than 8 cm were recorded. The lower boundary of plot 1 (site coded KM) was determined by the highest elevation of the presence in this area of mature individuals, as indicated by height (up to 21 m) and stem diameter (up to 0.7 m). Few saplings of the species Pinus cembra L. were growing higher than the upper limit of plot 2. A few single mature trees growing outside the plots were also studied. In addition to the living Stone Pine trees and saplings, dry dead trees found within the plots were also sampled and analysed. The date of germination and the life span of all living Stone Pines and stems analysed at site 1 were determined.

Sections of subfossil logs were collected at 5 different locations in the innermost Kauner valley. All samples found are of the species Pinus cembra L. Samples of logs found on the glacier forefield of the glacier Gepatschferner (site 2, site-code GP) have mainly been investigated to reconstruct the Holocene oscillations of this glacier (Nicolussi and Patzelt 2001). The subfossil stems were found at two different locations on the glacier forefield. For this paper only logs which were still embedded in till (n=25) at the position where they were found were taken into consideration. Hence, the altitudinal position of growth could be determined for such stems. Other Stone Pine logs found on the slopes below the moraines and therefore lacking information about their position of growth (Nicolussi and Patzelt 2001) were not used for this study. Site 3 (site code GLI) is located above the lowest part of the forefield of the Gepatschferner. It is characterised by a relatively even relief where various peat bogs exist upon slopes and in small vales. 39 logs found at altitudes between 2135 and 2160 m a.s.l. have been dated. Site 4 (site code KOA) is located on both sides of the Riffl stream. Subfossil logs have been found in peat bogs located on the lower and flat parts of the slopes in this valley. The altitudinal range of these finds (n=41) ranges from 2160 to 2190 m a.s.l. Currently only a few small living Stone Pine individuals can be observed at sites 3 and 4. This is probably due to intensive grazing activities in this part of the valley. Most of the sections from logs (n=44) at site 5 (site code GDM) originate from a small lake (2295 m a.s.l.) retained behind a late-glacial moraine. A few samples were taken from a small peat bog which developed at the lower end of this lake. Plot 2 of site 1 is on the edge of site 5, sites 1 and 5 being contiguous. Currently only small (<1.1 m in summer 2002) and young living individuals can be observed around the lake. The altitudinal range of the most elevated site where we found subfossil logs (n=21) in the Kauner valley is 2365–2410 m a.s.l. (site 6, code KG). Peat bogs have developed in small vales and on flat slopes. Currently no living trees can be observed at this site.

See ‘Materials and methods’ in the study for details

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

Conclusions

1. The analysis of present (last approx. 200 years) treeline variability shows a very low position following the mid-1810s climatic deterioration. After about 1860 the altitude of both the tree-species-line and the tree-line increased gradually by about 65 m until 1980.

2. Stone Pines (Pinus cembra L.) grew at sites above the late 20th century tree-line during long periods of the early and middle Holocene. Individual tree ages of up to 350 years indicate long time spans with favourable conditions for tree growth. Evidence for a position of the tree-line above the altitude of the 1980 tree-line is nearly continuous for the period between approx. 7100 and 2100 B.C.

3. The late Holocene (after about 2000 B.C.) contains only a few periods with tree growth above the 1980 tree-line position. However, both the effects of human impact documented by repeated gaps in the sample replication and the effect of climate variability can not be distinguished.

4. Generally the Holocene glacier record and the dendrochronological tree-line record show corresponding variability due to similar forcing. However, during some periods glacier advances are not concurrent with low tree-line positions, suggesting a modified climatic forcing of these glacier oscillations. Increased winter precipitation may be the cause of some differences in the proxy records.

5. Summer temperatures as observed after A.D. 1980 will lead to a higher position of the tree-line than at the moment. However, the current temperature values are still below the summer temperature level inferred from the subfossil logs for long time periods of the early and middle Holocene.

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