Réf. Tinner & Theurillat 2003 - A

Référence bibliographique complète

TINNER, W., THEURILLAT, J.-P. 2003. Uppermost Limit, Extent, and Fluctuations of the Timberline and Treeline Ecocline in the Swiss Central Alps during the Past 11,500 Years. Arctic, Antarctic and Alpine Research, 35, 158–169.

Abstract: Pollen and macrofossils were analyzed at two sites above today’s treeline (or tree limit) in the Swiss Central Alps (Gouillé Loéré, 2503 m a.s.l., and Lengi Egga, 2557 m a.s.l.) to test two contrasting hypotheses about the natural formation of timberline (the upper limit of closed forest) in the Alps. Our results revealed that Pinus cembraLarix decidua forests near timberline were rather closed between 9000 and 2500 B.C. (9600–4000 14C yr BP), when timberline fluctuations occurred within a belt 100–150 m above today’s tree limit. The treeline ecocline above timberline was characterized by the mixed occurrence of tree, shrub, dwarf-shrub, and herbaceous species, but it did not encompass more than 100–150 altitudinal meters. The uppermost limit reached by timberline and treeline during the Holocene was ca. 2420 and 2530 m, respectively, i.e., about 120 to 180m higher than today. Between 3500 and 2500 B.C. (4700–4000 14C yr BP) timberline progressively sank by about 300 m, while treeline was lowered only ca. 100 m. This change led to an enlargement of the treeline-ecocline belt (by ca. 300 m) after 2500 B.C. (4000 14C yr BP). Above the treeline ecocline, natural meadows dominated by dwarf shrubs (e.g., Salix herbacea) and herbaceous species (e.g., Helianthemum, Taraxacum, Potentilla, Leontodon t., Cerastium alpinum t., Cirsium spinosissimum, Silene exscapa t., and Saxifraga stellaris) have been present since at least 11,000 cal yr ago. In these meadows tree and tall shrub species (> 0.5 m) never played a major role. These results support the conventional hypothesis of a narrow ecocline with rather sharp upper timberline and treeline boundaries and imply that today’s treeless alpine communities in the Alps are close to a natural stage. Pollen (percentages and influx), stomata, and charcoal data may be useful for determining whether or not a site was treeless. Nevertheless, a reliable and detailed record of past local vegetation near and above timberline is best achieved through the inclusion of macrofossil analysis.

Mots-clés

 

 

Organismes / Contact

• Institut für Pflanzenwissenschaften, Abteilung Paläoökologie, Universität Bern, Altenbergrain 21, CH-3013 Bern, Switzerland. Willy.Tinner@ips.unibe.ch
• Centre Alpien de Phytogéographie, Fondation J.-M. Aubert, CH-1938 Champex, Switzerland, and Laboratoire de Biogéographie, Département de Botanique et de Biologie végétale, Université de Genève, 1 ch. de l’Impératrice, CH-1292 Chambésy, 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

 

Vegetation (forest treeline)

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

Swiss Central Alps

Valais

• Simplon
Gouillé Rion
Gouillé Loéré
Lengi Egga

 

2017 m a.s.l.
2343 m a.s.l.
2503 m a.s.l.
2557 m a.s.l.

The Holocene

 

(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

The data presented in this article support the conventional hypothesis of a more or less sharp Holocene upper forest boundary (timberline) at most 100–150 m higher than today’s potential timberline (2420 vs. 2300). The treeline ecocline had an extension of about 100–150 m. The treeline upper limit was at most 150–200 m higher than today’s tree limit (2530 vs. 2350) between 9000 and 2500 B.C. (9600–4000 14C yr BP). During this period timberline fluctuations occurred between the two sites Gouillé Loéré and Gouillé Rion. However, these conclusions are based on just four subalpine and alpine macrofossil profiles. (…) [See detailed results in the study]

ALPINE VEGETATION RESPONSE TO HOLOCENE CLIMATIC

OSCILLATIONS

Pollen-percentage fluctuations (sum of tree pollen, Pinus cembra, Betula) at Lengi Egga and Gouillé Rion coincide with Holocene glacier and lake level oscillations in the Alps (Tinner and Ammann, 2001) and are best explained by several conspicuous Holocene tree diebacks in the subalpine forests of the region. At Lengi Egga pollen oscillations occurred at ca. 9100 B.C. (minimum of Betula pollen), 7200 B.C., 5850 B.C., 3800 B.C., 1600 B.C., 400 B.C., A.D. 1400, and A.D. 1800 (minima of tree pollen…). Surprisingly, despite the registration of these events by subalpine forests of the central Alps, these climatic coolings did not clearly affect the local alpine vegetation at Lengi Egga. It is likely that the meadows were not severely disturbed because presently most alpine species are far below their altitudinal limits at Lengi Egga, so that local vegetation appears well buffered against climatic coolings. The paradox that climatic reversals were revealed by plant species not growing at the site emphasizes again the relevance of regional pollen transport in the Alps across different vegetation belts.

The altitudinal limits of the main vegetational belts during the past 13,000 cal yr as well as the temporal and altitudinal position of timberline fluctuations are summarized [see Fig. 9 in the study]. Gouillé Rion was densely forested between 9000 and 2500 B.C. (9600–4000 14C yr BP, Tinner et al., 1996). Since closed forests (timberline) never reached Gouillé Loéré, the maximum range of timberline fluctuations can be estimated to be less than 160 m. After 2500 B.C. (4000 14C yr BP) the range of vegetational oscillations is difficult to ascertain, although several key taxa (Juniperus nana, Alnus viridis, and Picea) show that they occurred within the treeline ecocline (Wick and Tinner, 1997).

The striking expansion of the treeline ecocline after 2500 B.C. (4000 14C yr BP) was probably caused by both climate change and human disturbance. In the study region the Bronze Age began at 2200 B.C. (3850 14C yr BP, Hochuli et al., 1998). The results of Tinner et al. (1996) indicate that this transition was the onset of a new and more intensive land-use system based on alpine summer farming. However, since similar Holocene treeline trends could be found elsewhere in Europe and on other continents where anthropogenic influence was negligible (Bradley, 1999), it is likely that human impact amplified the effect of climatic changes by lowering timberline—but not treeline—by about 200 additional meters. In this sense, the Holocene treeline history in the study region roughly mirrors the general long-term climatic trend of the Northern Hemisphere and especially, if compared with today, the prevalence of warm and dry summers raising treeline during the early and middle Holocene (see summer insolation curve: Kutzbach and Webb, 1993).

The lower temporal resolution at Gouillé Loéré probably impedes a clear capture of the climatic signals, although Gouillé Rion (which recorded climatic oscillations in both the pollen and the macrofossil records) is only 800 m away. This shows the significance of well-dated high-resolution pollen analysis for reconstructing short- to middle-term vegetational oscillations.

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

The treeline ecocline is a climatically sensitive transitional zone between closed forests (timberline) and alpine communities characterized by the mixed occurrence of tree, shrub, and herbaceous species, although woody species normally prevail (for the distinction between ecocline and ecotone see). The elevation of the upper boundary reached by tree species in the Alps during the Holocene has recently become a debated topic in alpine ecology and paleoecology. Conventionally it is assumed that on rather undisturbed slopes tree species formed an upper timberline, separated from alpine vegetation by a narrow treeline ecocline about 100 (–200) m wide. According to this hypothesis, herbaceous and dwarf-shrub species are presumed to have formed natural alpine meadows above the treeline ecocline throughout the Holocene. In this alpine belt, tree and tall (>0.5 m) shrub species were uncommon and occurred only in favorable microhabitats (e.g., sheltered by warmed rocks on steep slopes). Most paleoecological studies indicate that the upper treeline never oscillated more than ±100 m from today’s position. Therefore, alpine treeless meadows in the Alps are thought to be natural and more than 10,000 cal yr old.  [see references in the study]

New charcoal-analytical studies suggest a different scenario. According to Thinon and Talon (1998), Pinus cembra occurred rather densely in the alpine belt of the southern French Alps up to 2900 m a.s.l. Carcaillet et al. (1998) proposed that tree species occurred at least 300 m above today’s tree limit in the Vanoise massif (French western Alps) and postulated that an expanded treeline ecocline (at least 300 altitudinal meters) existed with a mixture of isolated trees or tree groups (mainly Pinus cembra, the Swiss stone pine), shrubs, and herbaceous species on well-drained soils. According to Carcaillet et al. (1998), such a belt has never been described in the Alps by modern ecologists, and the authors therefore proposed that fire and grazing pressure have destroyed it, converting it into the modern treeless alpine meadows approximately 3000 yr ago. Their estimates of the maximum altitude reached by trees is at least 200 m higher than those obtained by palynology, and if compared with the time before 3000 yr BP the modern treeline appears depressed by >300 m according to anthracology (charcoal analysis), but only about 100 m according to palynology (see Carcaillet et al., 1998). Thus these authors questioned the results of palynology and emphasized that vegetative reproduction of woody species may be dominant above timberline. Their interpretation is supported by findings of charred tree remains in the southern French Alps at very high altitudes (2919 m a.s.l.), ca. 200–500m above today’s tree limit (Talon et al., 1998; Thinon and Talon, 1998).

On the basis of pollen and macrofossil data, other authors (e.g., Lang and Tobolski, 1985; Ponel et al., 1992; Tessier et al., 1993; Tinner et al., 1996) working in mountain areas with vegetation formed by different belts showed that conventional pollen analysis (percentages) alone may be insufficient for reliable reconstructions of timberline and alpine vegetation history. Lowland vegetation covers wider areas than mountaintop vegetation, and its pollen is easily lifted by vertical air-mass movements and deposited at higher altitudes. This pollen movement obscures the local vegetation signal, especially in treeless situations, where local pollen production is low. A way to overcome this difficulty is to use macrofossils to reconstruct the local timberline and alpine vegetation history. No Holocene macrofossil study is available for the area above timberline in the Alps; therefore, a reliable test of the abovementioned hypotheses has not been possible.

In this study two new sites above today’s tree limit were analyzed to test which of the two hypotheses is applicable for the Central Swiss Alps. Aside from its ecological relevance, the main purposes of such a test are (1) to discuss whether the modern alpine belt above treeline is natural (i.e., like that before Neolithic, 5500 B.C.) and hence to estimate how it would evolve if anthropogenic pressure ceased, and (2) to assess whether previous paleoclimatic interpretations of timberline oscillations were based on correct assumptions (i.e., the past presence of an upper sharp boundary of tree species and a narrow treeline ecocline in stability with climate).

The authors also address the prevailing transport and diffusion mechanisms for macrofossils and macroscopic charcoal particles and discuss the role of fire-generated convection columns in lifting and transporting centimeter-sized plant material. In addition, we briefly address the suitability of pollen-influx methods for tracing past vegetational changes near treeline. (…)

The study sites Gouillé Loéré and Lengi Egga are situated at 2503 m a.s.l. and 2557 m a.s.l., respectively. Today’s upper limit of closed forest (timberline) near these sites is situated at about 2100 m, and the limit of open tree groups (height >8 m, potential timberline or presumed natural forest line) is at about 2300 m at Gouillé Loéré and 2200–2250 m at Lengi Egga. Single, well-developed trees .5 m grow at 2320 m (Gouillé Loéré) and 2250 m (Lengi Egga), whereas small individuals of 2 m height (tree limit or treeline). (…) The study sites are about 100–150 m and 200–250 m above today’s tree limit. (…) The alpine meadows around Gouillé Loéré and Lengi Egga consist mainly of the grass Nardus stricta and the sedge Carex curvula. (…) In the study region timberline and treeline are formed by Pinus cembra and Larix decidua. Because Gouillé Loéré and Lengi Egga are above today’s treeline, two additional sites below and at treeline (Simplon, 2017 m a.s.l.; Gouillé Rion, 2343 m a.s.l.) were also considered in order to compare alpine and subalpine sites. [see details and references in the study]

The methods used [see details in the study] are:
• Coring, sediments analysis and radiocarbon dating
• Palynology
• Plant macrofossils analysis

 

 

(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

The data presented in this article are based on just four subalpine and alpine macrofossil profiles. Although they are supported by pollen, anthracological, and soil investigations, there is an urgent need for additional macrofossil analyses of sites below, at, and above treeline. Further paleoecological methods (e.g., pedoanthracology, soil biosequences, phytoliths, paleoentomology) may also contribute to tracing former treeline positions. To exclude misinterpretations, the proof of the past local presence of tree species should not be based on the presence/absence of charred remains alone, as large organic fragments can be lifted for hundreds of meters in the convective columns of fires. Since timberline in the Alps is formed by different species as determined by climate, sites in the southern and northern Alps should be studied as well.

Références citées :

Bradley, R. S., 1999: Paleoclimatology—Reconstructing Climates of the Quaternary. San Diego, Academic Press. 613 pp.

Burga, C. A., and Perret, R., 1998: Vegetation und Klima der Schweiz seit dem jüngeren Eiszeitalter. Thun: Ott Verlag. 805 pp.

Burga, C. A., and Perret, R., 2001: Monitoring of eastern and southern Swiss alpine timberline ecotones. In Burga, C. A., and Kratochwil, A. (eds.), Biomonitoring: General and Applied Aspects on Regional and Global Scales: Tasks for Vegetation Science. Dordrecht: Kluwer, 179–194.

Carcaillet, C., Talon, B. and Barbero, M., 1998: Pinus cembra et incendies pendant l’Holocène, 300 m au-dessus de la limite actuelle des arbres dans le massif de la Vanoise (Alpes du nord-ouest). Écologie, 29: 277–282.

Carcaillet, C., and Talon, B., 2001: Soil carbon sequestration by Holocene fires inferred from soil charcoal in the dry French Alps. Arctic, Antarctic, and Alpine Research, 33: 282–288.

Carnelli, A. L., Theurillat, J.-P., Thinon, M., Vadi, G. and Talon, B., submitted: Determination of the past uppermost treeline limit in the Central European Alps (Switzerland) based on soil and charcoal analysis. The Holocene.

Hochuli, S., Niffeler, U., and Rychner, V., 1998: Die Schweiz vom Paläolithikum bis zum frühen Mittelalter—Bronzezeit. Basel: Verlag Schweizerische Gesellschaft für Ur- und Frühgeschichte. 423 pp.

Kutzbach, J. E., and Webb, T. III, 1993: Conceptual Basis for Understanding Late-Quaternary Climates. In Wright, H. E., Jr., Kutzbach, J. E., Webb, T., III, Ruddiman, W. F., Street-Perrott, F. A. and Bartlein, P. J. (eds.), Global Climates since the Last Glacial Maximum. Minneapolis: University of Minnesota Press, 5–11.

Lang, G., and Tobolski, K., 1985: Hobschensee—Late-Glacial and Holocene environment of a lake near the timberline. In Lang, G. (ed.), Swiss Lake and Mire Environments during the last 15000 years. Dissertationes Botanicae, 87. Vaduz: J. Cramer, 209–228.

Ponel, P., de Beaulieu, J.-L., and Tobolski, K., 1992: Holocene palaeoenvironments at the timberline in the Taillefer Massif, French Alps: a study of pollen, plant macrofossils and fossil insects. The Holocene, 2: 117–130.

Talon, B., Carcaillet, C., and Thinon, M., 1998: Études pédoanthracologiques des variations de la limite supérieure des arbres au cours de l’Holocène dans les Alpes Françaises. Géographie physique et Quaternaire, 52: 195–208.

Tessier, L., de Beaulieu, J.-L., Couteaux, M., Edouard, J.-L., Ponel, P., Rolando, C., Thinon, M., Thomas, A. and Tobolski, K., 1993: Holocene palaeoenvironments at the timberline in the French Alps—a multidisciplinary approach. Boreas, 22: 244–254.

Theurillat, J.-P., Felber, F., Geissler, P., Gobat, J.-M., Fierz, M., Fischlin, A., Ku¨ pfer, P., Schlu¨ ssel, A., Velluti, C., Zhao, G.-F. and Williams, J., 1998: Sensitivity of plant and soil ecosystems of the Alps to climate change. In Cebon, P., Dahinden, P., Davies, H. C., Imboden, D., and Jaeger, C. C. (eds.), Views from the Alps. Cambridge, Mass.: MIT Press, 225–308.

Thinon, M., and Talon, B., 1998: Ampleur de l’anthropisation des  étages supérieurs dans les Alpes du sud: données pédoanthracologiques. Ecologie, 29: 323–328.

Tinner, W., and Ammann, B., 2001: Timberline paleoecology in the Alps. PAGES News, 9: 9–11.

Tinner, W., Ammann, B., and Germann, P., 1996: Treeline fluctuations recorded for 12,500 years by soil profiles, pollen, and plant macrofossils in the central Swiss Alps. Arctic and Alpine Research, 28: 131–147.

Tinner, W., Conedera, M., Ammann, B., Gäggeler, H. W., Gedye, S., Jones, R., and Sägesser, B., 1998: Pollen and charcoal in lake sediments compared with historically documented forest fires in southern Switzerland since AD 1920. The Holocene, 8: 31–42.

Wick, L., and Tinner, W., 1997: Vegetation changes and timberline fluctuations in the Central Alps as indicator of Holocene climatic oscillations. Arctic and Alpine Research, 29: 445–458.