Réf. Wick & Tinner 1997 - A

Référence bibliographique complète

WICK, L., TINNER, W. 1997. Vegetation Changes and Timberline Fluctuations in the Central Alps as Indicators of Holocene Climatic Oscillations. Arctic and Alpine Research, Vol. 29, No. 4, 445–458. [PDF]

Abstract: Pollen and plant-macrofossil data are presented for two lakes near the timberline in the Italian (Lago Basso, 2250 m) and Swiss Central Alps (Gouillé Rion, 2343 m). The reforestation at both sites started at 9700-9500 BP with Pinus cembra, Larix decidua, and Betula. The timberline reached its highest elevation between 8700 and 5000 BP and retreated after 5000 BP, due to a mid-Holocene climatic change and increasing human impact since about 3500 BP (Bronze Age). The expansion of Picea abies at Lago Basso between ca. 7500 and 6200 BP was probably favored by cold phases accompanied by increased oceanicity, whereas in the area of Gouillé Rion, where spruce expanded rather late (between 4500 and 3500 BP), human influence equally might have been important. The mass expansion of Alnus viridis between ca. 5000 and 3500 BP probably can be related to both climatic change and human activity at timberline. During the early and middle Holocene a series of timberline fluctuations is recorded as declines in pollen and macrofossil concentrations of the major tree species, and as increases in nonarboreal pollen in the pollen percentage diagram of Gouillé Rion. Most of the periods of low timberline can be correlated by radiocarbon dating with climatic changes in the Alps as indicated by glacier advances in combination with palynological records, solifluction, and dendroclimatical data. Lago Basso and Gouillé Rion are the only sites in the Alps showing complete palaeobotanical records of cold phases between 10,000 and 2000 BP with very good time control. The altitudinal range of the Holocene treeline fluctuations caused by climate most likely was not more than 100 to 150 m. A possible correlation of a cold period at ca. 7500-6500 BP (Misox oscillation) in the Alps is made with paleoecological data from North America and Scandinavia and a climatic signal in the GRIP ice core from central Greenland 8200 yr ago (ca. 7400 yr uncal. BP).

Mots-clés

 

 

Organismes / Contact

Institute of Geobotany, Section Paleoecology, University of Bern, Altenbergrain 21, CH-3013 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

 

 

 

 

 

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

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

 

 

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

Reconstitutions

Results

VEGETATION HISTORY

Detailed descriptions of the Late Glacial and Holocene vegetation history at the two sites are given in Wick (1991, 1994a, 1994b) for Lago Basso and Tinner (1994) and Tinner et al. (1996) for Gouillé Rion. The following brief overview of the vegetation development and major vegetation changes is based on the results of pollen and macrofossil analysis. Ages are presented as uncalibrated radiocarbon years BP.

Lago Basso

Late Glacial (ca. 12,000-10,000 BP): Sparse pioneer vegetation dominated by Gramineae, Caryophyllaceae, Saxifraga, Chenopodiaceae, Artemisia, Thalictrum, Rumex, and Apiaceae grew in the area during the Late Glacial.

10,000-9400 BP: The beginning of the Holocene is indicated by a sediment change from silt to fine-detritus gyttja. The pioneer vegetation, including Dryas octopetala and Salix dwarf shrubs, became more dense. According to macrofossil analysis the first trees of Betula pubescens and Pinus cembra arrived near the lake at ca. 9700 BP, along with Rhododendron.

9400-8700 BP: An initial phase with very open stands of Betula pubescens, Pinus cembra, and scattered Larix trees preceded the development of the subalpine forest for several hundreds of years. Dwarf shrubs such as Juniperus nana (can be identified by needle shape and by distribution of stomata), Rhododendron, and Vaccinium played a major role during this period, whereas pioneer herbs decreased.

8700-8200 BP: The mass expansion of Larix and a rapid decrease in Juniperus nana is recorded at around 8700 BP A Larix-Pinus cembra forest with Betula pubescens covered the area until 8200 BP, when Larix lost its predominance in the subalpine forest. The species-rich understory consisted mainly of tall herbs (e.g. Peucedanum ostruthium and other Apiaceae) and Ericaceae (Vaccinium and Rhododendron).

8200-6300 BP: A Pinus cembra-Larix forest with Rhododendron and Vaccinium grew around the lake for about 2000 yr. Betula continuously decreased and disappeared from the area at ca. 7000 BP. At ca. 8200 BP, simultaneously with the decrease in Larix, great parts of the tall-herb vegetation were replaced by meadow plants (e.g. Ligusticum mutellina, Bupleurum). At about 7500 BP Picea abies started to spread in the lower parts of the subalpine forest. Several changes in macrofossil and pollen concentrations of Pi- nus cembra and Larix during this period indicate timberline fluc- tuations (for more detailed description and discussion, see be- low).

6300-4500 BP: Decreases in pollen and macrofossils of Pinus cembra and Larix point to opening of the subalpine forest. The first needles of Picea indicating local presence of spruce are recorded at around 6000 BP, shortly before Alnus viridis started to expand at timberline. These events coincided with the beginning of Neolithic human impact at timberline, which is shown by increasing numbers of charcoal particles in the macrofossil record.

4500-3000 BP: This period is characterized by a mass expansion of Alnus viridis and continuous decreases in Pinus cembra, Picea, and Larix until ca. 3200 BP, when macrofossil and stomata analysis show that the last trees disappeared from the Lago Basso area. High frequencies of Loiseleuria procumbens seeds indicate open patches exposed to wind.

3000-0 BP: During the last 3000 yr the timberline was located below Lago Basso. The sparse finds of Larix needles can be attributed to regional transport by wind. Charcoal and indicators of pasture (e.g. Juniperus nana, Plantago alpina, Rumex, Cichoriaceae) suggest strong human impact since the Bronze Age (Wick, 1994b). Alnus viridis, strongly affected by grazing animals (mainly by goats), was able to recover during periods of low grazing pressure (e.g. Migration period).

Gouillé Rion

Late Glacial (ca. 12,000-10,000 BP): During the Late Glacial pioneer vegetation dominated by Gramineae, Chenopodiaceae, Artemisia, Thalictrum, Rumex, Apiaceae, and dwarf willows grew in the area of Gouillé Rion.

10,000-9200 BP: Alpine pioneer vegetation with Salix dwarf shrubs dominated at the beginning of the Holocene. As recorded for Lago Basso, the local vegetation cover became more closed and more diverse around 9700 BP, when the first trees arrived near the lake. Unlike at Lago Basso, reforestation at Gouillé Rion started with Juniperus nana and Larix.

9200-8000 BP: Pinus cembra arrived near the site at ca. 9200 BP, and a Pinus cembra-Larix-Betula forest with a species-rich tall-herb vegetation grew in the area.

8000-4700 BP: After 8000 BP Larix and Betula declined, and Pinus cembra became the dominant tree species near the forest limit. The optimum period for Pinus cembra probably was around 6000 BP (maximum macrofossil concentrations), when it also reached the maximum Holocene elevation at ca. 2400-2450 m a.s.l., as shown by charcoal from soil profiles (Tinner et al., 1996). Changes in macrofossil concentrations as well as in pollen percentages and concentrations indicate timberline fluctuations (see below).

4700-3600 BP: Pollen and macrofossil records show that timberline started to move down at ca. 4700 BP. Tall herbs retreated, while mead- ow plants indicating anthropogenic influence (e.g. Cichoriaceae, Plantago alpina) increased. Picea abies and Alnus viridis started to expand around 4000 BP. The last macrofossils indicating local presence of Pinus cembra are dated to 3600 BRP.

3600-0 BP: Alnus viridis and Juniperus nana shrubs dominated around the lake, although single trees of Larix may still have been present. Macroremains of dwarf willows and Loiseleuria point to open vegetation after 2500 BP, and changes in the composition of the herb vegetation indicate that the subalpine/alpine area was used for pasture (Tinner et al., 1996). As at Lago Basso, the last 1700 yr are characterized by strong human impact.

TIMBERLINE FLUCTUATIONS RECORDED IN THE POLLEN- AND MACROFOSSIL DIAGRAMS

The richness of terrestrial plant macrofossils in both lakes, as well as their similar position near the Holocene timberline, make it possible to compare the results of both macrofossil and pollen analysis from the two lakes in terms of timberline fluctuations caused by climatic changes in the Alps. Because the plant macrofossils represent the local vegetation, decreases in tree macrofossils are presumed to document openings of the upper subalpine forest or declines of the forest limit from a level above the lake to a level below the lake. (In the macrofossil and pollen diagrams these periods of minimum values are indicated by SPL-1 to SPL-12 and Rion-1 to Rion-7). The pollen-influx estimates show more or less the same tendencies as the pollen concentrations, but because of dating problems and sediment changes ignored by interpolation of the dates the results partly are not reliable. Therefore the macrofossil concentrations are compared with pollen concentrations.

Strong human impact combined with deforestations (Tinner et al., 1996; Wick, 1991, 1994b) make it impossible to trace climatic changes during the last 2000 yr; therefore we discuss only the time period between 10,000 and 2000 yr BP

According to the macrofossil record at Lago Basso, Larix and Pinus cembra reacted simultaneously and concordantly to environmental changes, and retreats in both species generally were accompanied by decreases in Vaccinium and Rhododendron, and in the younger Holocene also by Picea abies. Minimum values of stomata- and pollen concentrations correspond to the minima in macrofossils and thus confirm the interpretation as forest openings, but regular finds of single conifer-needles and bud scales during the less favorable periods might suggest that the treeline did not decline below 2250 m a.s.l. until about 3200 BP. The fluctuations described for macrofossils and pollen concentrations are not visible in the pollen percentages. The ratio of nonarboreal pollen (NAP, including Gramineae and up- land herbs) and arboreal pollen (AP), which often is used as a criterion for forest openings, is very stable throughout the first 5000 yr of the Holocene. Slight changes, though, can be observed in the percentage curves of Pinus cembra and Picea abies; declines of Pinus cembra obviously made it possible for Picea to expand gradually in the subalpine forest.

At Gouillé Rion, where Pinus cembra was the dominant tree species during the early and middle Holocene, its decreases were partly compensated by increases in Larix. However, since needle production of Larix is very high, decreases in Pinus cembra suggest forest openings, confirmed by distinct increases in pollen percentages of Gramineae and herbs. The correspondence between the curves of pollen- and macrofossil concentrations in the Gouill6 Rion diagrams is not so good as for Lago Basso, but at Lago Basso the fluctuations of the macro- fossil concentrations hardly is reflected in the AP/NAP ratio; possible causes might be differencies in pollen-source areas, forest density near the lakes, etc. At both sites the pollen concentrations indicate fluctuations of the forest limit in periods, when there are no or only very few macrofossils available: before the reforestation in the Preboreal (Rion-1 and SPL-1) and after the decline of the forest limit below the sites in the younger Holocene.

Decreases in Pinus cembra and Betula pollen concentrations at both sites, accompanied by a remarkable NAP peak at Gouillé Rion, suggest a decline of the forest limit in the Preboreal (ca. 9500 BP). Also, the timberline fluctuations of the younger Holocene mainly are recorded in pollen concentrations. At both sites decreasing tree macrofossils indicate a general decline of the forest limit after about 6000-5000 BP, probably caused by both climatic change and human impact since the Neolithic. The beginning of Neolithic human activity near Lago Basso, documented by charcoal particles in the macrofossil record and indicators of pasture (Wick, 1991), is dated to 5500-6000 BP, whereas the first indicators of pasture at Gouillé Rion occur at about 4700 BP (Tinner et al., 1996). Since about 3000-3500 BP (Bronze Age: increases in NAP) there is evidence of strong human impact in both areas. Nevertheless, between 3500 and 2500 BP two timberline fluctuations, which we consider to be due to climatic deterioration rather than to human impact, are shown by decreases in tree pollen concentrations at both sites, and by decreases in needles of Larix and Juniperus nana at Gouillé Rion.

An overview of all the timberline fluctuations recorded at Gouillé Rion and Lago Basso shows a good correspondence in time, i.e. the major fluctuations occur more or less simultaneously at both sites. This synchroneity makes it possible to understand the timberline fluctuations not only as local events, but as regional processes controlled by climate. Differencies between Gouillé Rion and Lago Basso in the younger Holocene are likely to be explained by uncertainties in dating and rapid changes in sediment accumulation rates at Lago Basso. The great number of fluctuations at Lago Basso during the early and middle Holocene is due to the high resolution in time (ca. 25-30 yr) obtained by fine sampling of the sediment core.

Discussion

TIMBERLINE FLUCTUATIONS AND CLIMATIC CHANGES IN THE ALPS

The pollen and macrofossil diagrams from Lago Basso and Gouillé Rion show several minor timberline fluctuations during the Holocene and a general retreat of the forest limit after ca. 5000 BP The latter was accompanied by a decline in Pinus cembra and the expansion of Picea abies (at Gouillé Rion) and Alnus viridis, indicating a change to cooler and more oceanic climate. Evidence of a mid-Holocene climate deterioration comes from other high-altitudinal sites in the Alps (e.g. Lang and Tobolski, 1985; Kiittel, 1990a; Burga, 1991; Oeggl and Wahlmüller, 1994). Climatic changes for this period are also recorded by glacier advances and retreats of the forest limit in Northern Europe (e.g. Birks, 1990; Nesje and Dahl, 1993) and North America (e.g. Luckman and Kearney, 1986; Carrara et al., 1991). In Valle San Giacomo and in the adjacent north-south valleys of southern (Insubrian) Switzerland, the increase in oceanicity resulted in a reduction of Pinus cembra to a few scattered stands (see Zoller, 1960; Burga, 1980), whereas in the Valais it remained a dominant tree in the subalpine forest.

Since ca. 3500 BP (Bronze Age) human impact on the timberline increased, but climatic signals were still strong enough to be recorded in the lake sediments, e.g. a cold phase between 3000 and 2500 BP, which also is documented by glacier advances in the Swiss and Austrian Alps (Zoller et al., 1966; Zoller, 1977a, 1977b; Patzelt, 1977) and by changes in density of subfossil wood (Bircher, 1982; Renner, 1982).

For the period between 10,000 and 5000 BP, four timberline depressions at Gouillé Rion and Lago Basso can be correlated with cold phases recorded in the Swiss and Austrian Alps. The earliest one, the Preboreal oscillation (Schlaten oscillation), was found not only at high altitude but also in the lowlands north and south of the Alps (Behre, 1978; Schneider and Tobolski, 1985; Lotter et al., 1992). However, because of a 14C plateau in the early Holocene (Becker and Kromer, 1986) it is not certain whether these events are synchronous. In the Austrian Alps glacier fluctuations during the Boreal (Venediger oscillation, ca. 8700-8000 BP), are reflected in NAP peaks in the pollen record from nearby peat deposits (Bortenschlager and Patzelt, 1969; Patzelt and Bortenschlager, 1973; Bortenschlager, 1984). The tripartite character of this climate oscillation (Patzelt, 1977) is nicely shown in the macrofossil concentrations of Larix and Pinus cembra at Lago Basso (SPL-2 to SPL-4). The younger part of the SPL-4 fluctuation (ca. 8000-7600 BP) might be local and due to Mesolithic human impact. Radiocarbon dates from nearby archaeological sites clustering at 6360-6760 cal. BC (7650-7900 uncal. BP) and rather high charcoal concentrations at Lago Basso suggest that the Mesolithic hunters started to visit the area before the forest was able to recover at the end of the last Venediger cold phase (Fedele and Wick, 1996). Possibly the hunters kept the surroundings of the lake open by removing young trees. However, it cannot be excluded that there was a minor climatic oscillation during this period: in the eastern Swiss Alps close to Spliigen Pass an NAP peak in the pollen record at 7750 BP was interpreted as a cold phase (Oberhalbstein oscillation; Heitz, 1975).

The end of the Venediger oscillation is also the beginning of the Holocene climate optimum between 8000 and 5000 BP, indicated in the Alps by the maximum elevation of the treeline (Lang, 1993; Oeggl and Wahlmiiller, 1994; Pott et al., 1995). During this time the climate was not stable, however; several climatic changes caused temporary retreats of the timberline (e.g. Zoller, 1960; Zoller and Kleiber, 1971; Heitz, 1975; Gamper and Suter, 1982). A rather strong climate deterioration recorded in the southern Swiss Alps, the Misox oscillation (7500- 6500 BP; Zoller, 1960), corresponds to the local phases SPL-5/ SPL-6 and Rion-3. There is no clear evidence for the Misox oscillation in the Austrian Alps, although the beginning of the Frosnitz oscillation (ca. 6600-6000 BP; Patzelt, 1977) overlaps with the last Misox cold phase. A brief timberline depression at Lago Basso (SPL-7), lasting only about 150 yr, is the only evidence for the Frosnitz cold phase in the Swiss Alps (Gamper and Suter, 1982). The phase SPL-8 (ca. 5950-5650 BP) coincides with a short period of reduced wood density, indicating lower temperatures at 5745-5695 BP The reaction of the vegetation, mainly recorded in a decrease in Larix and an increase in NAP, was rather weak and accompanied by an increase in charcoal. Thus the beginning of Neolithic human activity in this time period has to be considered.

The Piora oscillation, a bipartite cold period at 5300-4400 BP recorded in the Swiss Alps by pollen analysis, glacier advances, solifluction, and dendroclimatology (Zoller, 1960; Zoller et al., 1966; Bircher, 1982; Gamper and Suter, 1982; Renner, 1982), corresponds to the Rotmoos oscillation in the Austrian Alps (Patzelt, 1972, 1977; Patzelt and Bortenschlager, 1973). The local phases Rion-4 and SPL-9 coincide with Piora I / Rotmoos I (ca. 5300-5000 BP), the first of the two cold phases, but there is no analog to Piora II/Rotmoos II (ca. 4400-4700 BP) at Lago Basso, unless the zone SPL-10 is dated too young, according to the dating problems mentioned above.

The altitudinal range of natural Holocene treeline fluctuations is not yet known. Estimates of their maximum range from 100-150 m (Patzelt, 1975; Lang, 1993) to 300 m (Burga, 1991). The results from Gouillé Rion and Lago Basso may give additional information. Biosequences in soil profiles and Pinus cembra charcoal fragments above Gouillé Rion (2343 m) reveal a maximum Holocene treeline at 2400-2450 m (Tinner et al., 1996). The present-day treeline in Val d'Hérémence is at ca. 2320-2350 m. In the Lago Basso area (2250 m) the palynological record from nearby Lago Grande at 2303 m suggests that single trees might have grown near the lake (indicated by sparse finds of conifer stomata), but the forest limit was lower than 2300 m (Wick, 1994b). The recent treeline in the area is at ca. 2220-2250 m.

The continuous records of tree macrofossils at both sites show that the treeline was at or higher than the lakes during the cold periods of the early and middle Holocene. Thus we conclude that the Holocene treeline fluctuations caused by climatic changes did not exceed 100 or maximum 150 m. This conclusion is consistent with the estimate made by Patzelt (1975) and Lang (1993).

THE MISOX OSCILLATION: A CLIMATE EVENT OF THE NORTHERN HEMISPHERE?

The pollen diagram from Pian di Signano (1540 m a.s.1.), situated about 20 km southwest of Lago Basso in the Swiss Misox valley, shows several strong fluctuations of Abies alba and Pinus between 7500 and 6500 BP, which were interpreted as depressions of the Abies belt caused by the "Misoxer Kalt-phasen" (Zoller, 1960). This Misox cold phase is represented at Gouillé Rion by Rion-3 (ca. 7500-6700 BP) and at Lago Basso by SPL-5 and SPL-6 (7400-6450 BP), with an intermediate warm phase at ca. 7000-6800 BP. In the dendroclimatological record, which is based on radiocarbon-dated "floating chronologies," the warm phase is dated to 7080-7140 BP (Bircher, 1982; Renner, 1982). The Misox oscillation was also recorded at a site on the Swiss Plateau near Zürich by a strong decline in the frequency of Najas seeds (Haas, 1996). Zoller et al. (1966), referring to publications of glacier advances in North America (e.g. Flint, 1963), suggested that the climatic change of the Misox oscillation was not restricted to the Alps but might have influenced other areas as well. According to Flint (1963) the so-called "Cochrane readvance" in Ontario occurred not long before 6800 BP More recent paleoecological investigations (pollen, plant macrofossils, and oxygen isotopes) in the southern Canadian Cordillera revealed two periods of high timberline dated to 8800-7500 BP and 7200-5200 BP, respectively, separated by a short interval at 7400-7300 BP, when timberline approached modem levels (Luckman and Kearney, 1986). Clague and Mathewes (1989) collected conifer fragments and charcoal at or above modem timberline in the southern Canadian Cordillera. The radiocarbon ages ranging from 9100 to 4400 BP were separated into two groups by a 1000-yr period suggesting lower timberline from 7600 to 6600 BP However, Clague and Mathewes (1989) did not exclude the possibility that this gap in their record could equally be due to the small size of their data set. Carrara et al. (1991), who carried out similar studies on wood fragments in southwestern Colorado, attributed the small number of radiocarbon ages between 7800 and 6700 BP to their small data set also.

Evidence for a climatic change at 8200 calendar yr BP (corresponding to ca. 7450 uncal. BP) lasting about 200-250 yr is given by oxygen isotopes and methane concentrations from the GRIP ice core from central Greenland (Dansgaard et al., 1993; Blunier et al., 1995): the methane concentrations show an abrupt fall and a subsequent rise, which can be paralleled with a similar feature in the isotope record. Blunier et al. (1995) concluded that the climate event at 8200 calendar yr BP affected a significant part of at least the Northern Hemisphere. In Scandinavia, Dahl and Nesje (1996) correlate a decrease in mean summer temperatures after ca. 8300 cal. BP, reconstructed from glacier equilibrium-line altitudes and a tree-limit depression, with the minimum in oxygen isotopes in Greenland. Considering the low time resolution in the pollen and plant-macrofossil records, as well as errors caused by interpolation of radiocarbon dates, the climatic signal in the GRIP ice core is synchronous with the beginning of the Misox oscillation (ca. 7500 BP; Zoller, 1960) and with the declines of the forest limit at Gouillé Rion (Rion-3, ca. 7550 BP) and Lago Basso (SPL-5, ca. 7400 BP).

THE EXPANSION OF PICEA ABIES AND ALNUS VIRIDIS: A CONSEQUENCE OF CLIMATIC CHANGES OR HUMAN IMPACT?

The most striking Holocene vegetation changes in the subalpine forests of the Swiss and adjacent Italian Alps are undoubtedly due to the expansion of Picea abies and Alnus viridis during the middle and younger Holocene. According to pollen and macrofossil records, the glacial refuges of Picea were in the southeastern Alps, from where the tree started to migrate west at the beginning of the Holocene (Huntley and Birks, 1983; Lang, 1994). Climatic changes and/or human activity obviously favored the mass expansion of spruce in the Swiss Alps (Markgraf, 1970; Tallantire, 1973; Welten, 1982). In the Central Alps of Valais, where Picea expanded rather late (5000 BP or later; see Gouillé Rion) and often several thousand years after its immigration, Markgraf (1970) connected the expansion with human activity. However, Tallantire (1973), with regard to the paleobotanical records from the Eastern Alps, and Kuittel (1990a) suggest that the expansion of Picea was mainly controlled by climate.

As shown in the pollen and macrofossil diagrams from Lago Basso, in the southern Alps Picea expanded up to the forest limit before the beginning of Neolithic human impact. Spruce might have profited from higher oceanicity during the cold phases between 7400 and 6200 BP (SPL-5 to SPL-7: Misox oscillation s.1.). In the eastern Swiss Alps, the Picea expansion can also be attributed to climatic influence (Heitz, 1975; Wegmuiller, 1976). In the area of Gouillé Rion, Picea immigrated at ca. 5500 BP and expanded between 4500 and 3500 BP, most likely favored by the climatic deterioration around 5000 and 3000 BP (Rion-5/Piora II/Rotmoos I and Rion-6/Lbbben). However, human influence has to be taken in account: anthropogenic indicators in the pollen record (Tinner et al., 1996) point to increasing human impact in the lowlands and at timberline at ca. 3500 BP

The early Holocene presence of Alnus viridis in the eastern Alps, shown by pollen and macrofossils (e.g. Oeggl, 1994) as well as pollen values higher than 1% since the late Preboreal at Lago Basso, suggest that the glacial refuges of Alnus viridis were situated in the eastern/southeastern Alps. The green alder arrived and expanded at Gouillé Rion at the same time as Picea. A simultaneous expansion of Picea and Alnus viridis between 4500 and 3500 BP is also recorded from other sites in the Swiss Central Alps (e.g. Welten, 1982; Lang and Tobolski, 1985; Kittel, 1990a, 1990b).

Since Alnus viridis prefers cool and rather oceanic climatic conditions, the mid-Holocene climatic change is likely to be one of the main reasons for its mass expansion at timberline. But most probably Alnus viridis, a pioneer on raw and disturbed soils, was favored by soil erosion due to forest opening and grazing animals since the Bronze Age. Archaeologists refer the beginning of transhumance in Valais to the Neolithic/Bronze Age transition at ca. 2400 cal. BC or 4000 uncal. BP (Bezinge and Curdy, 1994). Soil conditions unfavorable to Alnus viridis might have been one of the reasons why at Lago Basso Alnus viridis did not profit from the climate oscillations between 7500 and 6000 BP and expand simultaneously with Picea. The first noticeable increase of Alnus viridis at Lago Basso is recorded for the period after the beginning of Neolithic human activity indicated by charcoal at ca. 6000 BP (SPL-8). The pollen diagram from nearby Lago Grande (2303 m) shows a short period of high percentage of Gramineae immediately before the mass expansion of Alnus viridis at ca. 5000 BP, which was accompanied by the first indicators of pasture (Wick, 1994b).

Conclusions

The reforestation at both sites started in the Preboreal, and the forest limit reached its highest elevation between 8700 and 5000 BR After 5000 BP it retreated to more or less the present-day level. Since ca. 3200 BP (Bronze Age) the forest limit has been located below its potential elevation as a result of human impact. Timberline fluctuations during the early and middle Holocene shown in the pollen and plant-macrofossil records occurred simultaneously with cold periods recorded in the Swiss and Austrian Alps, and consequently they can be attributed to climatic changes. Macrofossil and stomata analysis provide a sharper tool for timberline reconstruction than pollen alone. At our two sites it was possible for the first time to show continuous paleobotanical records of the Holocene climatic oscillations in the Alps with a very good time control. The altitudinal range of the treeline fluctuations did not exceed 100-150 m.

A correlation of the Misox oscillation in Switzerland, timberline retreats in North America, and a climate signal obtained from the isotope and methane records in the GRIP ice core at 8200 calendar yr BP (ca. 7450 uncal. BP) is presented.

The expansion of Picea abies in the eastern and southern Alps was favored by climatic changes, i.e. Holocene cold phases accompanied by increasing oceanicity. In the Central Alps of Valais, where Picea immigrated rather late, it equally may have profited from human impact. The mass expansion of Alnus viridis can be attributed to both climatic change and human activity at timberline.

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

The recent vegetation at the subalpine/alpine transition in the Alps has been strongly influenced by man. During thousands of years various human societies settling in the mountain valleys visited areas at high altitudes for cultural, religious, economic, and other reasons. Palaeolithic and Mesolithic people used the open space at and above the timberline for hunting, and ever since the Neolithic subalpine and alpine meadows and pastures became more and more important areas for cattle breeding (Gallay, 1986; Aerni, 1991; Oeggl, 1994). As a consequence of this long-term human impact the modem timberline is located below its potential level in large parts of the Alps.

Vegetation scientists generally agree with the idea that the altitudinal range of the subalpine forest is strongly controlled by climate, especially July temperatures and the length of the growing season (Tranquillini, 1979; Arno and Hammerly, 1984; Eg- genberg, 1995). It is still discussed whether or not the natural timberline is a sharp boundary (Ellenberg, 1966) or an ecotone, i.e., a transition zone from closed forest stands to more or less isolated trees and finally to alpine grassland (Tranquillini, 1979; Holtmeier, 1985). According to Klötzli (1992), in the Alps both types of timberline occur, depending on geology, topography, snow-cover dynamics, and other factors.

The close relationship between tree growth and climatic conditions facilitates the reconstruction of past climate based on treeline fluctuations. Such treeline fluctuations have mainly been defined as decreases in arboreal pollen (AP) and increases in herb and grass pollen (nonarboreal NAP) in pollen diagrams from subalpine and alpine lakes and peat bogs (Zoller, 1960, 1977a, 1977b; Bortenschlager, 1977, 1984; Welten, 1982; Burga, 1988), and often it was possible to correlate them with glacier advances (Zoller et al., 1966; Bortenschlager and Patzelt, 1969), solifluction (Furrer, 1977), and dendroclimatological data (Bircher, 1982; Renner, 1982). However, a proper reconstruction of treeline fluctuations from pollen-percentage diagrams from high- altitude sites is difficult because of low local pollen production and high regional and extra-regional pollen input (Jochimsen, 1986; Bortenschlager, 1992). Calculations of pollen concentrations and especially accumulation rates have turned out to be useful for identifying past locations of the alpine and also the polar treelines (e.g. Hyvairinen, 1993; Vorren et al., 1993; Oeggl and Wahlmüller, 1994). In the Austrian Alps, Kral (1971) and Oeggl and Wahlmüller (1994) used changes in pollen percentages of so-called "climax" tree species (e.g. Larix, Picea, Pinus cembra) and "krummholz" species (Alnus viridis, Pinus mugo) as indicators of treeline fluctuations. The most reliable information about treeline fluctuations or about the presence and absence of trees can be obtained from combined macrofossil and pollen analysis (Lang and Tobolski, 1985; Ponel et al., 1992; Lang, 1993; Tessier et al., 1993; Oeggl, 1994; Oeggl and Wahlmüller, 1994; Tinner et al., 1996). Stomata of coniferous trees counted in pollen slides have also turned out to be good indicators of local presence of trees (Markgraf, 1969; Welten, 1982; Ammann and Wick, 1993; Wick, 1994a; Hansen, 1995). Sub-fossil wood fragments and charcoal from sites above the present-day treeline provide information about the maximum elevation of the Holocene treeline, but the data set usually is too small for the reconstruction of minor climatic changes (Luckman and Kearney, 1986; Carrara et al., 1991; Pott et al., 1995).

The present paper aims to reconstruct climatic changes in the Central Alps between 10,000 and 2000 yr BP, based on pollen and macrofossil analysis at two lakes situated within the altitudinal range of Holocene timberline fluctuations.

Paleobotanical investigations were made on the sediments of two small lakes: Lago Basso (2250 m a.s.1.) in the upper San Giacomo valley near Spligen Pass, Italy, and Gouillé Rion at 2343 m a.s.l. in the Swiss Central Alps of Valais. The bedrock geology at both sites consists of gneiss and schist of the Penninic nappes. The absence of glacier moraines and Late Glacial sediments at the base of both of the profiles indicate that no local glaciers covered the sites during the Younger Dryas. (…)

Study areas: The upper limit of the subalpine forest, which is dominated by Picea abies in the lower part and by Larix decidua near the timberline, is located at 2100 m a.s.l. in the Val d'Hérémence and at 1950 to 2000 m in the Lago Basso area. The maximum elevation of the present-day forest limit in the Spligen Pass region is 2050 to 2100 m and the treeline is located at ca. 2220 to 2250 m. Swiss stone pine (Pinus cembra) is largely absent from the Spligen Pass area today, though near Gouillé Rion it occurs as single trees up to an elevation of 2320-2350 m. In the transition zones between forest and subalpine/alpine meadows and pastures are scrubs of Alnus viridis. In the Lago Basso area many archaeological sites dating back to the Mesolithic (ca. 10,000-6500 BP) indicate human activity at timberline since the early Holocene (Fedele, 1992; Fedele and Wick, 1996). At both sites, increasing human activity since the Bronze Age (ca. 3800-3500 BP) has been recorded (Tinner, 1994; Wick, 1994b).

Methods: The sediment cores were taken with a Livingstone piston corer from the central parts of the lakes at 8 m water depth in Gouillé Rion and 80 cm in Lago Basso. For calculation of pollen percentages pollen types of plants not growing in the subalpine/ alpine area were excluded from the pollen sum, as well as spores and water plants. Lycopodium tablets (Stockmarr, 1971) were added to the sediment samples for estimation of pollen concen- trations (grains/cm3). Conifer stomata were counted on the pollen slides. Sampling resolution for macrofossil analysis was 1 cm (8 cm core diameter) for Lago Basso and 3 cm (5 cm core diameter) for Gouillé Rion. Wet sieving was done with a 0.2-mm mesh width. The macrofossil diagrams show concentrations per 25 cm3 of sediment at Gouilé Rion and per 45 cm3 at Lago Basso. For plotting of the diagrams the TILIA and TILIA*GRAPH programs (Grimm, 1992) were used. The visual zonation of the diagrams (Rion-1 to Rion-7, and SPL-1 to SPL-12) is mainly based on changes in macrofossil and pollen concentrations. The time period discussed in this paper covers about 8000 radiocarbon years (ca. 10,000-2000 BP).

Radiocarbon Dating: The time scales used for the diagrams are based on 21-AMS datings of terrestrial macrofossils for Lago Basso and 14 for Gouillé Rion. The age-depth curve for Lago Basso is smoothed by locally weighted regression, the one for Gouillé Rion is linear. The age-depth diagram of Lago Basso shows a distinct decrease in sediment accumulation rates at ca. 6000 BP, probably caused by low water level (the recent maximum water depth is ca. 80 cm) and water-level fluctuations during the later Holocene, as indicated by the sediment alternating between fine-detritus gyttja and moss peat during the last ca. 2500 yr.

 

(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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