Réf. Valsecchi & Tinner 2010 - A

Référence bibliographique complète

VALSECCHI, V. and TINNER, W. 2010. Vegetation responses to climatic variability in the Swiss Southern Alps during the Misox event at the early–mid Holocene transition. Journal of Quaternary Science, Vol. 25-8, 1248–1258. [PDF]

Abstract: Sedimentary pollen, charcoal and plant macrofossil analyses with high resolution and precision suggest a strong shift in vegetation composition during the early to mid-Holocene transition in the upper mountain belt. At Piano mire (1439m above sea level (a.s.l.), Ticino, Switzerland) forests were dominated by Abies alba during the early Holocene (prior to ca. 8000 cal. a BP). Abrupt collapses of A. alba at ca. 7800–7400 cal. a BP enabled the expansion of the light-demanding pioneer Betula. Afterwards A. alba populations regained their previous abundance in the forests. Within the dating uncertainties of our record we assume that a unique combination of wet and cold years between 8400 and 7500 cal. a BP led to repeated lethal disadvantages for Abies. Our record of Abies oscillations is in good biostratigraphic agreement with the record that has been used to define the Misox cold event (Pian di Signano, 1540ma.s.l.), which has been previously correlated with the 8200 cal. a BP event. Given the age estimates of the Abies collapses in our well-dated record, our results suggest that additional efforts are needed to understand the linkage between the Misox and the 8200 cal. a BP event. They imply a high sensitivity of mountain vegetation far below the tree line (~800m) to Holocene climatic changes of about 2°C in annual air temperature.

Mots-clés

Pollen - Plant macrofossils - 8200cal. a BP event - Misox - Alps

 

Organismes / Contact

Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

 

(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

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Swiss Alps

Val Maggia, Ticino (northwest of Lago Maggiore)

Piano mire

 

1439m a.s.l.

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

The Holocene is a period with relatively stable climatic conditions if compared to glacial–interglacial variability. However, Holocene climatic changes of minor amplitude are documented in several palaeoclimatic records (Mayewski et al., 2004; Wanner et al., 2008). Climatic forcing can operate at low (millennial), intermediate (centennial) and/or high (decadal to annual) scales. Holocene climatic changes may have been triggered by external forcing (e.g. orbital parameters, solar activity) or by internal forcing (e.g. weakening of the North Atlantic thermohaline circulation, volcanic eruptions) or result from changes in internal short-term variability mode changes (e.g. North Atlantic Oscillation, El Ninõ; Bradley, 2003). Many studies show that climatic forcing at different scales plays an important role in shaping Holocene environmental variability.

During the Holocene, climatic variability in and around the Alps probably reached its maximum at ca. 8400–7400 cal. a BP (Wick and Tinner, 1997; Haas et al., 1998; von Grafenstein et al., 1998; Heiri et al., 2004; Spötl et al., 2010), when climate shifted from the early Holocene dry continental to the mid Holocene moist oceanic mode (Tinner and Lotter, 2001, 2006). This reorganisation of the European climatic system corresponds to the Boreal/Atlantic transition in the early palaeoecological European literature (e.g. Firbas, 1949/1952) and thus has been a topic of discussion for more than 50 years. During this transitional period annual air temperature experienced strong oscillations (reaching about 2°C in magnitude; von Grafenstein et al., 1998) and the precipitation regime was significantly altered (e.g. Magny et al., 2003; Spötl et al., 2010). In the Swiss Southern Alps cold phases were inferred from the evidence of three strong minima of Abies alba pollen percentages at Pian di Signano by Zoller (1960) that defined the term ’Misox oscillations’. Misox (or with the Italian place name Mesolcina) is the valley where repeated collapses of Abies were observed (Zoller, 1960). The Abies oscillations defining the Misox event (sometimes erroneously called ’Miser oscillation’; see, for example, Alley and Ágústsdóttir, 2005) were radiocarbon-dated to ca. 7500–6500 14C a BP (Zoller, 1960). Almost 40 years later the event was correlated with the cold event CE-3 comprising tree line and glacier oscillations in Central Europe (Haas et al., 1998) and with the 8200 cal. a BP event in the Greenland ice cores (Wick and Tinner, 1997).

Observations

 

Modélisations

 

Hypothèses

 

 

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

Bibliographic review

 

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

Reconstitutions

Vegetation and fire dynamics inferred from pollen, plant macrofossils and microcharcoal

PinusBetula forests, 11 300–9500 cal. a BP

The basal pollen zone is characterised by Pinus sylvestris type (35–45%), Betula (20–40%) and Pinus cembra (around 15%). Scattered Larix and Juniperus pollen were also found. Nonarboreal pollen types have a low percentage from ~20% to ~5% and are represented by Artemisia, Ranunculus acris type, Rumex acetosa/acetosella type and Apiaceae. The local presence of Pinus sylvestris (P. mugo is not present in the study area), Larix decidua and Juniperus are documented by the presence of stomata and by Pinus sylvestris and Larix decidua needles (found in the radiocarbon samples, data not shown). The palaeobotanical record thus suggests the presence of Pinus sylvestrisP. cembraBetula forests around the former lake, with a scattered presence of Larix decidua and Juniperus. The low pollen values of Quercus, Tilia, Ulmus and Corylus are best explained by long-distance transport from lower altitudes, but their local presence cannot be excluded. Similar early Holocene vegetation compositions had been found at a site located in the mountain belt in the Ticino Valley (Zoller, 1960; Zoller and Kleiber, 1971; ller, 1972). Our site was then a shallow lake (gyttja, peaty gyttja, with Pediastrum boryanum var. boryanum and var. longicorne, P. angulosum, which are algae that live in small peaty lakes). Pollen concentration is quite high and microscopic charcoal concentration and influx follow the same pattern, suggesting that regional fires were intense. Pollen concentrations decrease around 9300 cal. a BP and charcoal influx markedly declines around 9500 cal. a BP, almost in correspondence with a change in the sediment from peat gyttja to peat.

Abies alba–Betula forests, 9500–7800 cal. a BP

Around 9500 cal. a BP, Abies pollen increased markedly. Pollen percentages of Pinus sylvestris type and Betula show a gentle decrease, whereas the values of upland herbs are stable. Abies alba and Larix decidua plant macrofossils are present from 9100 cal. a BP (PIA-M 1), attesting to the local presence of these taxa. Our data point to the presence of a new type of forest, in which rather drought-sensitive and shade-tolerant Abies alba was dominant, whereas Betula, a pioneer light-demanding species, became less important. Nevertheless, forests were still rather open, as shown by the local presence of Larix and Juniperus. Given the lack of macrofossils of Pinus sylvestris and P. cembra it is assumed that most pollen of these species was coming from extra-local sources. The change in the vegetation goes along with a change in the aquatic ecosystem: the beginning of the pollen zone (LPAZ PIA-2) delimits the beginning of the conversion of the lake into mire. This is shown by the sediment change towards sedge–Sphagnum peat, marked by a pronounced increase of the testate amoebae species Amphitrema flavum (a sphagnum-bog species; Beyens and Meisterfeld, 2001) and Assulina, both indicating oligotrophic conditions (van Geel, 1978). Charcoal concentration and influx remain rather stable and lower than in the previous zone, suggesting low regional fire activity when Abies alba dominated the vegetation.

Rapid and repeated reciprocal replacements of Abies alba and Betula forests, ca. 7800–7400 cal. a BP

Pollen and macrofossil data suggest fast vegetational changes with mutual replacements of Abies alba (and to lesser degree Larix) and Betula during this period. At 7800 cal. a BP (±120), percentage values of A. alba and Pinus sylvestris type start to decline, while Betula strongly increases, reaching values of up to 80%. The plant macrofossil diagram shows a similar pattern, with Abies alba needles first decreasing (7900–7700 cal. a BP) and then disappearing (7700–7400 cal. a BP). These palaeobotanical data suggest a sudden collapse of the Abies alba stands that had previously dominated for almost 2 ka. Pinus sylvestris also declined in the area and it is hypothesised that the species was growing on dry/shallow soils (e.g. rock ridges) or at higher altitudes, where it is also abundant today. The pollen data suggest that Betula stands replaced the former Abies forests around the mire. It was not possible to identify whether the species was Betula pendula or B. pubescens. A local expansion on the mire of B. pubescens seems unlikely since no macroremains were found despite the high pollen percentages of Betula. Instead we assume that B. pendula expanded on the slopes around the bog. Indeed, today Betula pendula, a light-demanding species, is present in the Piano area. In correspondence with the decline of Abies (and P. sylvestris type) pollen percentages ca. 7450 a huge (and unusual) peak of Fraxinus excelsior pollen (reaching 40%) suggests the expansion of the species on the Piano mire area. This pattern is mirrored to a lesser degree by pollen representatives of the vegetation at lower altitude (e.g. Quercus deciduous, Tilia and Ulmus). Microscopic charcoal peaks at ca. 7720, 7580 and 7450 cal. A BP (during the minima of Abies) suggest that regional fire activity increased during Abies (and Pinus sylvestris type) collapses. However, cross-correlation between charcoal and Abies (and Betula) for the period 7800–7400 cal. a BP show no statistically significant correlation, though negative correlations at lag 0 almost reach the significance level for Abies, indicating that these taxa were not influenced by the fire activities. The major changes in the pollen percentage spectra are roughly mirrored in pollen concentration and influx. Nevertheless, the occurrence of three distinct collapses of pollen percentage of Abies is not easily visible in the influx values. Indeed, Abies alba influx decreased around 8000 cal. A BP, reaching a minimum that lasted until 7500 cal. a BP. The last Abies pollen percentage decline (as well as those of Pinus sylvestris type) is accompanied by a strong increase in total influx. Indeed, Sphagnum reaches very low values for both spores and macrofossils, possibly due to a lowering of the water table. This may have affected peat accumulation rates around 7400 cal. a BP. On the other hand, pollen percentages and influx agree well for replacing taxa (e.g. Betula).

Mixed Abies alba forest from 7400 cal. a BP onwards

Around 7400 cal. a BP Abies and Pinus sylvestris type pollen regain their previous importance, whereas Betula pollen decreases. At the same time Picea pollen reaches its empirical limit. Pollen results are corroborated by the reappearance of plant macrofossils of Abies alba at ca. 7400 cal. a BP. These data suggest the local reestablishment of Abies alba-dominated forests after 7400 cal. A BP. The local presence of Picea abies in these forests after 7400 cal. a BP is unambiguously documented by needles (after 7200 cal. a BP). As in the previous zone, a marked increase of Poaceae accompanies the recovery of the Abies-dominated forests. The authors assume that this mirrors a local expansion of Poaceae on the mire, perhaps at the cost of Sphagnum mosses (conversion from a bog to a fen). This marked transition in local wetland vegetation is also recorded in plant macrofossils. The peat younger than 7200 cal. a BP is richer in herbaceous components and Sphagnum stems completely disappear from the macrofossil record (PIA-M 3). After a slight decrease of microscopic charcoal influx, it increases slightly around ca. 7300 cal. a BP, suggesting a slight increase in regional fire activity.


Discussion

Early to mid Holocene vegetational responses to millennial-scale climatic trends in the Alps

Variation of orbital forcing resulted in a maximum of summer insolation and a minimum in winter insolation in the Northern Hemisphere during the early Holocene. Seasonality was greater than today during the interval 12 000–6000, with a maximum around 10 000 cal. a BP in the Northern Hemisphere (Kutzbach and Webb, 1993). Insolation variation affected regional climate dynamics, resulting in warmer summers and colder winters and less moisture availability (Kutzbach and Webb, 1993). These extreme climatic conditions most probably inhibited the expansion of Abies alba at Piano during the early Holocene. The expansion of Abies alba occurred around 9000 cal. a BP, probably when climate became more favourable, i.e. soil and air moisture increased (Tinner et al., 1999) and regional fire activity decreased. Similarly, unsuitable (orbitally forced) climatic conditions have been used to explain the almost complete lack of mesophilous forests in central Europe during the early Holocene (e.g. Rudolph, 1928; Welten, 1944; Firbas, 1949/1952; Tinner and Lotter, 2001).

Vegetation response to centennial-scale climatic change in the Alps

Superimposed on orbitally induced climatic changes, centennial to decadal climate variability might have influenced ecosystems and vegetation. The climatic variability related to changes in solar irradiance, ocean forcing and volcanic eruptions might have pushed ecosystems over critical thresholds due to abrupt changes in the climate system (Alley et al., 2003). Such sudden changes in the climate are likely to be harmful for long-lived ecosystems such as mature forests (Williams et al., 2002). At the study site abrupt shifts in the vegetation started ca. 7800 cal. a BP, with collapse of the main tree Abies alba. At that time anthropogenic influences on vegetation was still weak (Mesolithic or Mesolithic/Neolithic transition). There is no evidence of pollen-inferred cultural indicators (e.g. Plantago lanceolata type, Cerealia type or Urtica; Behre, 1981) in our record. Hence it is assumed that vegetation changes were not primarily caused by human activities. Similarly, cross-correlation analyses at Piano suggest that fire disturbance (natural or anthropogenic) was not the primary trigger of the Abies alba collapses.

A similar series of rapid population collapses and expansion of Abies was recorded at Pian di Signano (Zoller, 1960), a site located in the upper mountain vegetation (1540m a.s.l.) belt of the Southern Alps. There Abies alba was repeatedly replaced by Pinus sylvestris, whereas at our site Betula mainly replaced A. alba. Because the two sites are at similar altitudes, it is unlikely that the differences in vegetation composition were caused by temperature. Pian di Signano is located in a more continental and drier valley, where today Pinus sylvestris is still an important element of the forest. It is  assumed that in the more oceanic Piano area Pinus sylvestris (which prefers continental environments; Ellenberg, 1996) was more affected by precipitation increases than at the continental site, so that pioneer Betula took advantage of the forest collapses. The present results and non-pollen evidence of climatic change (e.g. Heiri et al., 2003; Wick et al., 2003) confirm the original interpretation of Zoller (1960) that widespread Abies population collapses in the mountain belt of the Southern Alps were probably caused by cold phases (Misox event). Indeed, Abies alba is highly sensitive to late spring frost and keen winter frost (Ellenberg, 1996). However, both sites recording the Abies declines are situated ~800m below the tree line and ~400–500m below the upper limit of Abies alba in the region. Thus Abies communities should have been well buffered against a temperature decrease of Holocene (~2°C). It is hypothesised that a unique combination of wet (e.g. Magny and Schoellammer, 1999; Spötl et al., 2010) and cold years led to repeated lethal disadvantages for Abies. Afterwards, Abies populations recovered when temperature in the Alps increased at ca. 7500 cal. a BP, as suggested by chironomid-inferred temperature (Heiri et al., 2003) and Swiss glacier retreated (Joerin et al., 2006, 2008).

Within the mutual dating uncertainties, the repeated collapses of Abies at Piano are in good chronological agreement with a long-lasting decline of Pinus cembra forests at the tree line in the nearby Swiss Central Alps (Gouillé Rion; Tinner et al., 1996; Wick and Tinner, 1997; Reasoner and Tinner, 2008), which has also been successfully simulated by using independent climatic evidence from the Northern Alps (Heiri et al., 2006). These modelling experiments showed that tree line pollen and macrofossils primarily record population dynamics and not productivity variations as an alternative explanation (Ammann et al., 2000; van der Knaap and van Leeuwen, 2003). Farther east in Austria, high-altitudinal Pinus cembra forests also declined at ca. 8400–7400 cal. a BP as a result of low temperature during the growing season (Kofler et al., 2005). Climatic evidence from the Eastern Alps (Grotta di Ernesto; Frisia et al., 2005; Spötl et al., 2010) suggests an anomalously high rainfall between ca. 8200 and 7300 cal. A BP, which may have additionally affected Pinus cembra stands in that region.

In the Alps a series of cold and wet climatic excursions had started ca. 8400 cal. a BP (e.g. Haas et al., 1998; Heiri et al., 2003, 2004; Tinner and Kaltenrieder, 2005; Spötl et al., 2010). Linkages between solar output fluctuations and Holocene climatic and environmental centennial-scale variability have been suggested by several authors (e.g. Denton and Karlén, 1973; Hu et al., 2003; Rohling and Palike, 2005). The Abies collapse at Piano shows a rather good temporal agreement with high values of residual Δ14C (indicating low solar activities) and low reconstructed total solar irradiance, which may have influenced climatic and environmental systems (Reimer et al., 2004; Steinhilber et al., 2009). Nonetheless, chronologically the onset of the A. alba collapse at Piano is at ca. 7800 cal. yr BP ±120 a, in agreement with final freshwater pulses into the Atlantic ocean (e.g. Carlson et al., 2009), perhaps suggesting that this marine event may have influenced climatic and environmental conditions in the Alps. However, this does not explain why previous (e.g. 8400–8200 cal. a BP) changes in the Atlantic ocean circulation did not affect the climate and vegetation in the Southern Alps. In addition to this, multiple volcanic eruptions could have reduced hemisphere temperature on decadal and multi-decadal timescales (Briffa et al., 1998), reinforcing the effects of meltwater pulses and declining solar activity at 8000–7500 cal. a BP.


The Misox event and its correlation with the 8200 cal. a BP event

The pioneering study of Zoller (1960) relied in total on three radiocarbon dates (two on bulk peat and one on Abies wood), two of which fall into the period of interest. The radiocarbon dates were recalibrated using the CALIB rev. 5 program (Stuiver and Reimer, 1993; Stuiver et al., 1998) and developed a depth–age model using the same technique as for the Piano record. According to this depth–age model the two marked Abies declines start at ca. 7900 ±320 cal. a BP (lasting ca. 200 a) and 7500 ±320 cal. a BP (lasting ca. 300 a). Nonetheless, the sequence starts with low Abies pollen percentages (one pollen sample at the base of the core) at ca. 8100 ±320 cal. a BP, pointing to an earlier Abies decline that is chronologically in agreement with the 8200 cal. a BP event, as previously suggested (Wick and Tinner, 1997). As shown by the tentative depth–age model, the dating of the onset of the Misox event is based on extrapolation, while the termination age of ca. 7200 cal. a BP relies on interpolation to the next radiocarbon date, together underscoring that the chronology of the Misox event is not well defined.

Considering that the Piano record has good chronological control across the Abies collapses, it seems evident that the vegetation reorganisation at Piano cannot be linked with the 8200 cal. a BP event. On the other hand, in the Alps a series of cold and wet climatic excursions had started around 8400 cal. a BP (i.e. chironomid-inferred temperature, Grotta di Ernesto and CE’s records) and lasted significantly longer than the 8200 cal. a BP event as recorded in the oxygen isotope records (e.g. Dansgaard et al., 1993; von Grafenstein et al., 1998; Thomas et al., 2007). Taken together, two alternative explanations may summarise the situation according to the present comparative study: (a) given the lack of chronological control the Misox event (as defined by Zoller, 1960) is not correlated with the 8200 cal. a BP event (see, as an example, Bennett, 2002); (b) climatic deteriorations in and around the Alps had complex and small-scale spatial patterns, which are not easily linked with the 8200 cal. a BP event. It is concluded that further investigations in the Southern Alps are needed to better establish the link between the 8200 cal. a BP event and the Misox event.


Conclusions

High-resolution pollen, charcoal and plant macrofossil analyses at Piano (1439m a.s.l., Ticino, Swiss Southern Alps) allow reconstruction of vegetation and fire dynamics during the early to mid Holocene transition in the upper mountain belt. Abies alba forests in the region were established during the early Holocene, when climatic conditions gradually became more suitable for the needs of this species. A marked vegetational change occurred ca. 7800–7400 cal. a BP, when Abies forests were replaced by pioneer Betula stands. Independent palaeoclimatic proxies suggest that Abies declined in response to cooler and wetter conditions. The decline of Abies alba at Pian di Signano (a site located 50 km from Piano), was used to define the Misox cold event by Zoller (1960). Biostratigraphically Pian di Signano and the new study site Piano are in good agreement. Although considerable uncertainty exists about the chronology of Zoller’s site, the Misox event has been correlated with the 8200 cal. a BP cooling in Greenland (e.g. Wick and Tinner, 1997). This new study points to substantial spatial and temporal variability; the initial decline of Abies alba at around 8200 cal. a BP is missing in the new record. This implies that care must be taken when correlating climatic anomalies or climatic impacts at around 8000 cal. a BP with the 8200 cal. a BP event. This study confirms the point of view of Zoller (1960) that Holocene climatic changes were strong enough to profoundly influence vegetational trajectories far below the tree line.

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

Short-term climatic changes at annual to decadal scales might result in displacement of ecotones, whereas long-term changes (at centennial to millennial scales) might have profound consequences on the structure of biotic communities and entire biomes (Webb, 1986; Delcourt and Delcourt, 1991). Wide climate changes may occur very rapidly when the climate system is forced to cross thresholds, and such abrupt changes are particularly harmful for long-lived and relatively immobile ecosystems (i.e. mature forests; Alley et al., 2003). In agreement, high-resolution and accurate studies unequivocally demonstrate fast biotic responses to abrupt Lateglacial and Holocene climatic changes (e.g., Birks and Ammann, 2000; Tinner and Lotter, 2001; Williams et al., 2002; Hu et al., 2003).

During the Holocene, climatic variability in and around the Alps probably reached its maximum at ca. 8400–7400 cal. a BP (Wick and Tinner, 1997; Haas et al., 1998; von Grafenstein et al., 1998; Heiri et al., 2004; Spo¨ tl et al., 2010), when climate shifted from the early Holocene dry continental to the mid Holocene moist oceanic mode (Tinner and Lotter, 2001, 2006). This reorganisation of the European climatic system corresponds to the Boreal/Atlantic transition in the early palaeoecological European literature (e.g. Firbas, 1949/1952) and thus has been a topic of discussion for more than 50 years. During this transitional period annual air temperature experienced strong oscillations (reaching about 2°C in magnitude; von Grafenstein et al., 1998) and the precipitation regime was significantly altered (e.g. Magny et al., 2003; Spötl et al., 2010). In the Swiss Southern Alps cold phases were inferred from the evidence of three strong minima of Abies alba pollen percentages at Pian di Signano by Zoller (1960) that defined the term ’Misox oscillations’. Misox (or with the Italian place name Mesolcina) is the valley where repeated collapses of Abies were observed (Zoller, 1960). The Abies oscillations defining the Misox event (sometimes erroneously called ’Miser oscillation’; see, for example, Alley and Ágústsdóttir, 2005) were radiocarbon-dated to ca. 7500–6500 14C a BP (Zoller, 1960). Almost 40 years later the event was correlated with the cold event CE-3 comprising tree line and glacier oscillations in Central Europe (Haas et al., 1998) and with the 8200 cal. a BP event in the Greenland ice cores (Wick and Tinner, 1997).

The main aim of this study is to refine the reconstruction of Holocene vegetation dynamics in the upper mountain belt of the Swiss Southern Alps. A new site is analysed to check whether the Abies collapses, which define the Misox event, can be reproduced in space and time. The new sediment record was analysed for the Piano mire (1439m above sea level (a.s.l.), Ticino, Switzerland), about 50km from Zoller’s (1960) site at a similar altitude. A special attention is given to the Misox event period, and to gain higher spatial resolution of the vegetation shift the authors combine pollen and macrofossil analyses (Birks, 2001). In addition, sedimentary microscopic charcoal is used to infer regional fire activity (Whitlock and Larsen, 2001; Conedera et al., 2009). (…)

The Piano mire is located at the transition between the mountain belt, which is dominated by Fagus sylvatica and Abies alba, and the subalpine belt, with Picea abies and Larix decidua forests. The mire is dominated by Molinia coerulea, whereas Carex sp. and Sphagnum sp. are less abundant. The slopes surrounding the mire are covered by mixed forests of A. alba (with Fagus sylvatica and Picea abies). At higher altitudes (ca. 1600m a.s.l.), F. sylvatica disappears and the forest is composed of A. alba, P. abies, L. decidua and Betula pendula. The timber line in the area is situated at ~1800m a.s.l., while the tree line reaches about 2200m a.s.l. (Landolt, 1992).

Two parallel cores (PIA 1 and PIA 2) 360 and 375cm long were retrieved from the centre of the mire with a Streif-modified Livingstone piston corer of 4.8 cm diameter (Merkt and Streif, 1970). After having compared the lithostratigraphy of the two cores, the core section from 160 to 352cm of PIA 1 and from 352 to 375cm of PIA 2 were used for this study. The upper part of the core (0–160 cm) is shown elsewhere (Valsecchi et al., 2010).

The chronology is based on nine accelerator mass spectrometry (AMS) radiocarbon dates carried out at Poznan Radiocarbon Laboratory. (…) Samples of 1 cm3 were used for pollen analysis, with Lycopodiumtablets added for estimation of pollen concentration (Stockmarr, 1971).  (…) Cross-correlations between microscopic charcoal influx (X) and pollen percentages (Y) was used to analyse the influence of fires on vegetation succession. (…) [See details in the study]

 

(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

 

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Ammann B, Birks HJB, Brooks SJ, Eicher U, von Grafenstein U, Hofmann W, Lemdahl G, Schwander J, Tobolski K, Wick L. 2000. Quantification of biotic responses to rapid climatic changes around the Younger Dryas: a synthesis. Palaeogeography, Palaeoclimatology, Palaeoecology 159: 313–347.

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