Réf. Reasoner & Tinner 2008 - E

Référence bibliographique complète

REASONER, M., TINNER, W. 2008. Holocene Treeline Fluctuations. In Gornitz V. (Ed.) Encyclopedia of Paleoclimatology and Ancient Environments, Springer, Dordrecht: 442-446. [PDF]

Abstract:

Mots-clés

Holocene treeline dynamicsDendroclimatology - Holocene Climates - Hypsithermal - Macrofossils - Palynology - Pollen Analysis - The 8,200-Year b.p. Event

 

Organismes / Contact

 

(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

World / Alps

- European Alps

- Scandinavia
- North America

 

 

 

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

Holocene treeline dynamics

Since reconstructions of Holocene treeline fluctuations reflect past local dynamics, comparisons among the sites are needed to assess larger-scale patterns that could be related to climatic changes. Comparisons among pollen and macrofossil stratigraphies in the Swiss and Italian Alps allowed the detection of phases of warm (dry) and cool (wet) phases (Wick and Tinner, 1997) that are in agreement with lake-level fluctuations in the alpine forelands (CE-l to CE-8, Haas et al., 1998) and glacier oscillations. The analysis of pollen and macrofossils from lake and mire deposits along altitudinal transects permits the reconstruction of treeline fluctuations during the Holocene (Tinner and Theurillat, 2003). These reconstructions show that the positions of timberline and treeline fluctuated within a band of 100-180 m during the Holocene and that treeline was at its uppermost limit (ca. 180 m higher than today) during the period 10,000-6,000 cal. yBP. Severe diebacks of treeline vegetation occurred during cold (humid) periods (CE-l to CE-8 as well as during the Little Ice Age). A severe treeline decline lasted from 8,200 to 7,500 cal. yBP (CE-3), with two minima at 8,000 and 7,600 cal. yr B.P. (sites Gouille Rion and Lengi Egga). After 6,000-5,000 cal. yBP, timberline and treeline progressively declined by about 180 m and 300-400 m, respectively. The regression of densely forested areas (timberline) in the Alps during the past 5,000 years was primarily caused by human impact, whereas the course of treeline gives a more realistic estimation of the climatic influence. Assuming constant lapse rates of 0.7° C / 100 m, it is possible to estimate the range of Holocene temperature oscillations in the Alps to 0.8-1.2° C between 10,500 and 4,000 cal. yBP, when average (summer) temperatures were about 0.8-1.2° C higher than today. After 4,000 cal. yBP, summer temperatures successively declined to values comparable to those of the twentieth century.

The general course of Holocene treeline fluctuations in the Alps is similar to that reconstructed for Scandinavian sites. Megafossil, macrofossil, and pollen studies suggest that treeline was at its uppermost limit between 9,500 (-9,000) and 6,500 (-6,000) cal. yBP in Norway and Sweden, about 200-400 m above today's treeline position (Kullman, 1995; Dahl and Nesje, 1996; Barnekow, 1999; Barnett et al., 2001). At some study sites, treelines declined temporarily between 8,300 and 8,000 cal. yBP (Dahl and Nesje, 1996; Barnekow, 1999; Barnett et al., 2001). Using today's lapse rates, it has been estimated that temperatures were 1.3-2.0° C higher than today during the period of highest treeline positions.

A number of pollen profiles from western North America provide multi-millennial year records of alpine timberline fluctuations. These records were derived primarily from sediments deposited in high-elevation lakes and bogs, and, in most cases, limited dating control of the sedimentary records restricts their application to the reconstruction of only the general trends in treeline position throughout the Holocene. A more detailed picture of timberline fluctuations during the last 1,000 years has emerged from the study of tree rings taken from living trees or dead snags situated near the alpine timberline in the North American Cordillera. Most pollen records indicate that the alpine timberline was higher than present during the early to mid-Holocene and a general and widespread decline was underway by ca. 6,000-5,000 cal. yBP (Hebda, 1995). Pollen-based estimates of Holocene timberline position have been corroborated by several complementary proxies for estimating past environmental and climatic conditions. Taken together, these data suggest that mean growing season temperature in western North America was on the order of 1-4° C warmer than present during the early-mid Holocene.

Perhaps the most direct and unequivocal evidence for past high stands of alpine timberline in western North America is radiocarbon-dated macrofossils that have been preserved in alpine environments. In some cases these data are derived from abundant conifer needles preserved in lacustrine sediments at sites that are currently above natural treeline (Reasoner and Hickman, 1989) and in others they are obtained from logs (megafossils) preserved in alpine bogs or soil profiles (Kearney and Luckman, 1983; Carrara, et aI., 1991). Similarly, fossil wood dating to the early Holocene has been recovered from retreating termini of a number of glaciers in western Canada (Luckman, 1988), which indicates that early Holocene forests occupied areas now covered by ice. The recovery of wood ranging in age from ca. 10,000-5,000 cal. yBP from sites situated 100-200 m above the present timberline clearly indicates that subalpine forests reached at least this elevation during the early Holocene. Reasoner et al. (2001) compiled early-mid Holocene macrofossil information from across south-western Canada and plotted this information relative to modem timberline. The results indicate two clear periods of higher-than-present timberline during the early Holocene: an early phase from the onset of the Holocene to approximately 8,200 cal. yBP (7,500 14C yBP) and a later phase between 7,300 and 5,700 cal. yBP (6,400 and 5,000 14C yBP). These data are in general agreement with European records that indicate the timberline was higher than present during the early-mid Holocene and suggest the 8.2 event may have also impacted alpine treelines in western North America. Additional high-resolution records for the North American Cordillera are necessary to resolve this issue.

Several small-scale fluctuations of the alpine timberline have been documented during the last millennium, primarily from dendrochronological analyses of living trees near alpine timberline and in situ dead snags. For example, the abrupt synchronous termination of 92 tree-ring records related to the 1690's "cold snap" (Luckman, 1994) clearly indicates that significant changes to the position of the alpine timberline have occurred on decadal timescales and that these changes have been driven by natural variability of the regional climate.

Linking the past to the future

It is clear that the position and composition of the timberline ecotone has been sensitive to Holocene climate change. The millennial-scale trends generally reflect gradual decline in Northern Hemisphere insolation from approximately 11% higher-than-present in the early Holocene. Superimposed on this long-term trend are higher-frequency fluctuations related to changes in oceanic circulation, volcanic activity, and solar irradiance or a combination of these factors. It is also clear that anthropogenic climate forcing over the next 100 years is likely to rival or exceed the warmest conditions of the Holocene. Concerns about global warming involve adjustments, collapses, migrations, or extinctions of boreal and alpine life. Surprisingly, relatively few studies have addressed past responses of ecosystems such as treeline communities to climatic change. One of the reasons for avoiding this topic is that accurate studies require independent climatic proxies and very high temporal resolution (10-20 years/sample). To assess how treelines could respond to global change, high resolution studies including macrofossil analysis are urgently needed.

Observations

 

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

Timberline and treeline

Treelines are climatically sensitive transitional zones between closed forests and alpine, polar, or dry grassland communities. In most cases, temperature and precipitation thresholds determine whether a site is treeless or not (Amo and Hammerly, 1993; Komer, 1999). However, many other factors, such as vegetation type, soil type, snow cover, topography, and wind can locally co-determine the boundary of tree growth. Independently from the biotic and abiotic causes that form this conspicuous vegetation limit, the spatial position of treeline depends on the definition of "tree." The most commonly used criterion to define "tree" at treeline is a minimum height between 8 and 2 m (Amo and Hammerly, 1993; Komer, 1999). Some studies prefer to distinguish between timberline and treeline (e.g., using the height of trees as a diagnostic criterion: 8m for timberline and 2 m for treeline). The width of the transitional zone separating closed forests from treeless plant communities is not uniform: polar treelines and drought-caused treelines can form very broad transition zones such as parklands with widely spaced trees (Amo and Hammerly, 1993). Conversely, mountain treelines have rather narrow transitional zones (i.e., 100-200 m of vertical extent). It has been suggested that this range overlaps with the altitudinal span between timberline and treeline as defined above (8 vs. 2 m of tree height, Tinner and Theurillat, 2003).

The climatic sensitivity of treelines has led to much effort on the reconstruction of past treeline positions in order to quantify Holocene climatic changes. The significance of the method for paleoclimatology lies mainly in two factors: (a) Treeline studies have local to regional spatial resolutions and therefore provide valuable information about climatic changes of mountainous or polar sites, and (b) If combined with independent environmental proxies (e.g., oxygen isotopes as climatic proxy), treeline studies can give insights into biosphere responses to rapid environmental changes.

Reconstructing treeline fluctuations

The reconstruction of treeline fluctuations can be performed using different approaches. Analysis of pollen, stomata, macrofossils, and megafossils (including tree rings) are the most important methods. In addition, charcoal, soil, and phytolith analysis may be applied to determine maximum treeline positions. The methods have different spatial resolutions. Since pollen is easily lifted, transported, and deposited by winds over long distances, pollen records have rather low spatial resolutions. Thus, it is difficult to reconstruct treeline fluctuations by using the pollen approach alone. Macrofossil and megafossil analyses are considered to be more reliable tools for reconstruction of timberline fluctuations (Birks, 2001; Tinner and Theurillat, 2003) since they have higher taxonomic and spatial resolutions (meters to decameters) than pollen studies (decametre to kilometers). A disadvantage of macrofossil and megafossils is that, in contrast to pollen, they are not ubiquitous in Holocene deposits. Producing treeline reconstructions requires records from today's treelines that reach back to the late glacial. Two preconditions are essential: (a) the chronology of the treeline records must be fixed to an absolute scale with sufficient 14C-dates of terrestrial fossils, and (b) the study sites should be situated near to the present-day treeline position.

To assess the magnitude of past climate changes, reconstructions of Holocene treeline fluctuations must be related to modem treeline positions. For estimations of past temperature changes it is assumed that today's occurrence of trees is in equilibrium with the climate of the past few decades. In regions affected by strong human impact (e.g., the Alps), single trees in remote areas may still indicate the potential altitudinal limit of forest and tree growth. Using today's lapse rates (under the assumption of Holocene stability), it is possible to convert past treeline fluctuations (m) into (summer) temperature changes (°C) (Haas et aI., 1998), although it must be considered that higher treelines in the past reflected warmer periods that were stable enough to allow subalpine forest to migrate up-slope 100-200 years of potential response lags). In an attempt to synthesize common patterns across two continents, the authors focus on a few well-documented but representative case studies from temperature-controlled mountain treelines in Europe and Northern America.

 

(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

 

Références citées :

Arno, S.F., and Hammerly, R.P., 1993. Timberline - Mountain and Arctic Forest Frontiers. Seattle: The Mountaineers, 304pp.

Barnekow, L., 1999. Holocene tree-line dynamics and inferred climatic changes in the Abisko area, northern Sweden, based on macrofossil and pollen records. Holocene, 9, 253-265.

Barnett, C., Dumayne-Peaty, L., and Matthews, J.A., 2001. Holocene climatic change and tree-line response in Leirdalen, central Jotunheimen, south central Norway. Rev. Palaeobot. Palynol., 117, 119-137.

Birks, H.H., 2001. Plant macrofossils. In Smol, J.P., Birks, H.J.B., and Last, W.M. (eds.), Tracking Environmental Change Using Lake Sediments. Dordrecht: Kluwer, 49-74.

Carrara, P.E., Trimble, D.A., and Rubin, M., 1991. Holocene treeline fluctuations in the Northern San-Juan Mountains, Colorado, USA, as indicated by radiocarbon-dated conifer wood. Arct. Alp. Res., 23, 233-246.

Dahl, S.O., and Nesje, A., 1996. A new approach to calculating Holocene winter precipitation by combining glacier equilibrium-line altitudes and pine-tree limits: A case study from HardangeIjokulen, central southern Norway. Holocene, 6, 381-398.

Haas, J.N., Richoz, I., Tinner, W., and Wick, L., 1998. Synchronous Holocene climatic oscillations recorded on the Swiss Plateau and at timberline in the Alps. Holocene, 8, 301-309.

Hebda, R.J., 1995. British Columbia vegetation and climate history with a focus on 6 ka BP. Geographie physique et Quaternaire, 49, 55-79.

Hormes, A., Muller, B.U., and Schliichter, C., 2001. The Alps with little ice: Evidence for eight Holocene phases of reduced glacier extent in the Central Swiss Alps. Holocene, 11, 255-265.

Kearney, M.S., and Luckman, B.H. 1983. Holocene timberline fluctuations in Jasper National Park, Alberta. Science, 221, 261-263.

Komer, C., 1999. Alpine Plant Life. Berlin: Springer, 338pp.

Kullman, L., 1995. Holocene Tree-Limit and Climate History from the Scandes-Mountains, Sweden. Ecology, 76, 2490-2502.

Luckman, B.H., 1988. 8000 year old wood from the Athabasca Glacier, Alberta. Can. J Earth Sci., 25, 148-151.

Luckman, B.H., 1994. Evidence for climatic conditions between ca. 900-1300 A.D. in the Southern Canadian Rockies. Clim. Change, 26, 171-182.

Maisch, M., Wipf, A., Denneler, B., Battaglia, J., and Benz, c., 1999. Die Gletscher der Schweizer Alpen. Gletscherhochstand 1850, Aktuelle Vergletscherung, Gletscherschwund-Szenarien. Zurich: vdf, 373pp.

Reasoner, M.A., and Hickman, M., 1989. Late Quaternary Environmental Change in the Lake Ohara Region, Yoho-National-Park, British Columbia. Palaeogeogr. Palaeoclimatol. Palaeoecol., 72, 291-316.

Reasoner, M.A., Davis, P.T., and Osborn, G., 2001. Evaluation of proposed early-Holocene advances of alpine glaciers in the North Cascade Range, Washington State, USA: Constraints provided by palaeoenvironmental reconstructions. Holocene, 11, 607-611.

Tinner, W., and Kaltenrieder, P., 2005. Rapid responses of high-mountain vegetation to early Holcene environmental changes in the Swiss Alps. Journal of Ecology, 93(5): 936-947.

Tinner, W., and 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. Arct., Antarct., Alp. Res., 35, 158-169.

Wick, L., and Tinner, W., 1997. Vegetation changes and timberline fluctuations in the Central Alps as indicator of Holocene climatic oscillations. Arct. Alp. Res., 29, 445-458.