Réf. Theurillat & Guisan 2001 - A

Référence bibliographique complète
THEURILLAT J-P., GUISAN A. Potential impact of climate change on vegetation in the European Alps: A review. Climatic change, 2001, Vol. 50, p. 77-109.

Abstract: Based on conclusions drawn from general climatic impact assessment in mountain regions, the review synthesizes results relevant to the European Alps published mainly from 1994 onward in the fields of population genetics, ecophysiology, phenology, phytogeography, modeling, paleoecology and vegetation dynamics. Other important factors of global change interacting synergistically with climatic factors are also mentioned, such as atmospheric CO2 concentration, eutrophication, ozone or changes in land-use. Topics addressed are general species distribution and
populations (persistence, acclimation, genetic variability, dispersal, fragmentation, plant/animal interaction, species richness, conservation), potential response of vegetation (ecotonal shift – area, physiography – changes in the composition, structural changes), phenology, growth and productivity, and landscape. In conclusion, the European Alps appear to have a natural inertia and thus to tolerate an increase of 1–2 K of mean air temperature as far as plant species and ecosystems are concerned in general. However, the impact of land-use is very likely to negate this buffer in many areas. For a change of the order of 3 K or more, profound changes may be expected.

Mots-clés
Climate change, Alps, vegetation, species distribution, populations, phenology, productivity.

Organismes / Contact
Centre Alpien de Phytogéographie (CAP), CH-1938 Champex, Switzerland.
Université de Genève, Centre de Botanique, CP60, CH-1292 Chambésy. jean-paul.theurillat@cjb.ville-ge.ch
Swiss Center for Faunal Cartography (CSCF), Terreaux 14, CH-2000, Neuchâtel, Switzerland. antoine.guisan@cscf.unine.ch

(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    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Europe Alps        

(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
Impact on general species distribution
Evidence gleaned from past climate changes tends to indicate that species are more likely to respond by migration rather than by adapting genetically (Huntley, 1991). However, there is also a body of evidences that at particular places (high elevation zones which remained snow- and ice-free above the ice-sheet even at the glacial maximum) cold resistant high elevation species (orophytes, i.e., high mountain plants) have survived probably uninterruptedly in situ since the Late Tertiary. According to Scharfetter (1938), during the warmest interglacial periods, forests climbed higher towards the summits of low mountains (1800-2300 m), thereby reducing many high elevation orophyte populations. Palynological and macro-fossil studies show that the forest limit did not climb more than 100-300 m during the warmest periods of the Boreal and Atlantic periods of the Holocene (the Atlantic, 6000-5000 BP, was the warmest period of the Holocene).
Observations
Impact on general species distribution
The colonization over the last 60 years of the subalpine-alpine ecocline by Arolla pine ( Pinus cembra ) at 2400-2500 m in the western Piemonte (Italy) is attributed by Motta and Masarin (1998) to recent warming. Similarly, Norway spruce ( Picea abies ) has colonized the subalpine-alpine ecocline (1850-1950 m) in Kärnten (Austria) for some 90 years, and in particular the last 60, according to Stützer (1999), who predicts an additional elevation of 50 m in the near future.

Impact on populations
Locally or regionally, warming in coming decades may weaken dominant species through severe defoliation due to pest outbreaks, and may alter their potential to respond to climate change. This is what happened to the web-spinning sawfly (Cephalcia arvenis Panzer) of the Norway spruce (Picea abies) in the Italian Prealps (Marchisio et al., 1994): Warmer and drier conditions occurred over several years, thus improving the quality of food (higher sugar level in the needles) while reducing the insect's mortality and speeding up its development.

Impact on vegetation
The subalpine-alpine ecocline represents a temperature-related boundary whose inertia compensates both positive and negative variations of climate, preventing a linear variation of the forest-limit (see Körner, 1998, 1999). For instance, no change was observed in the subalpine belt in the contact zone between Scots pine (Pinus sylvestris) and Arolla pine (P. cembra) despite an increase of 0.8 K of the mean summer temperature over 30 years (Hättenschwiler and Körner, 1995; Körner, 1995). Palynological and macro-fossil studies show that the forest limit did not climb more than 100-300 m during the warmest periods of the Boreal and Atlantic periods of the Holocene (the Atlantic, 6000-5000 BP, was the warmest period of the Holocene). Therefore, an increase of 1–2 K in mean annual temperature may not shift the present forest limit upwards by much more than 100-200 m.
Modélisations
Impact on general species distribution
With the help of fine-scale, local modelling at the nival belt, Gottfried et al. (1999) predict that some nival species will lose area and be more restricted with an increase of 1–2 K. On average, most alpine and nival species could tolerate the direct and indirect (e.g., competitive exclusion) effects of an increase of 1–2 K (Körner, 1995; Theurillat, 1995), but not a much greater change (3–4 K; Theurillat, 1995; Theurillat et al., 1998).

Impact on populations
With a warmer and drier climate, local species' richness may increase, as shown by Kienast et al. (1997) for Swiss forests using a GIS-based static comparative model at a resolution of 1 km. Identically, in modeling regional plant species throughout Switzerland, Wohlgemuth (1998) comes to the conclusion that regional richness is likely to increase with warming, especially in mountainous areas. Regionally and locally, physiography (e.g., slope) will be the first factor in determining the distribution of many widespread species.

Impact on vegetation
According to the ForClim dynamic gap model (e.g. Bugmann, 1999; Fischlin and Gyalistras, 1997), some subalpine forests, such as the Arolla pine-larch forest (Pinus cembra, Larix decidua) in continental parts of Switzerland, appear to be very sensitive to climate change, showing unexpected new trees combination under climate change, and can even experience a catastrophic change through competition. According to static modeling, it is expected that beech-dominated forests (Fagus sylvatica) would be replaced by oak-hornbeam forests (Quercus robur, Q. petraea, Carpinus betulus) in the colline-submontane belt in the northern Alps; in the southern Alps, changes are less likely to occur due to mitigation of the temperature increase by an increase in precipitation.

Interestingly, dynamic models predict an increase of silver fir in the northern Alps, from colline to low subalpine belt (Bugmann, 1999). In a less humid climate, the present Mediterranean-type vegetation in the warmest areas of the lowest elevations of the southern border of the Alps may very likely expand, particularly on limestone. In the dry, continental part of the Alps, dynamic modeling predicts that the colline downy oak forest (Quercus pubescens) may be severely affected by drought. However, these results should be accepted with reservation, as models, in particular dynamic models, do not include parameters of potential adaptation or acclimation nor parameters such as forest management coppice, pest outbreaks, selective pressure or dispersal or species sensitivity to fire.
Hypothèses
There are three basic ways in which mountain plants may respond to a climatic change: persistence in the modified climate, migration to more suitable climates or extinction. Three types of persistence are possible: gradual genetic adaptation of populations, phenotypic plasticity (i.e., individual variation in the properties produced by a given genotype – genetic character – in conjunction with the environment) or ecological buffering (edaphic climax, i.e., where it is the soil processes that essentially control the final stage of plant succession, as opposed to climatic climax, where climate is the main determinant of the final stage). However, other influences, such as fragmentation or plant/animal interaction, must be taken into account in conjunction with climate change, as they can greatly affect plant species' populations in the near future.

Impact on general species distribution
Many isolated orophytes now living in such refugia as the peaks of low mountains in the Alps would also be threatened, because it would be almost impossible for them to migrate higher (to the present nival belt), either because they are unable to move there rapidly enough, or because the nival zone is absent (Grabherr et al., 1994, 1995; Gottfried et al., 1994). They include some endemics, i.e., plants indigenous to and restricted to a particular geographic region.
Overall, species having a great potential for adaptative responses through genetic diversity, phenotypic plasticity, high abundance, or significant dispersal capacities are least at risk of extinction (Holt, 1990).

Impact on populations
In general, moderate warming would be advantageous for late flowering species, which could benefit from a longer growing season for seed maturation. However, this does not hold for early flowering species, which would simply experience an earlier start to the growing season without further benefit.

At present, warming increasingly isolates high elevation populations. Although climate change can rapidly provide new ecological conditions, it is very unlikely, due to dispersal barriers, that different populations from low elevations could rapidly occupy the potentially available new territories at higher elevations, nor make contact in order to promote hybridization and new polyploids.

Fragmentation of population is of particular importance for endemics and orophytes. For these species, a marked fragmentation of their populations is to be expected, as a result of a decrease of the alpine and nival belts' surface area and due to an increase in steep slopes. If they cannot persist or adapt, species showing a disjointed (north-south, east-west), or fragmented distribution may see their range become even more fragmented, with local disappearances. Some categories of plants would appear to be more vulnerable. For instance, isolated arctic relict species living only in a restricted range of habitats, e.g., which are pioneers in wet habitats may very well disappear since these habitats are very rare and many of them have already been destroyed by the implantation of artificial lakes for hydro-electric plants. Alpine endemics restricted to tops of low mountains (i.e., those lacking nival belts, mainly in the eastern and lower external Alps) or those distributed over a limited area because of pedological and/or lithological barriers (e.g., massive limestone) are likely to be severely endangered by extinction.

Even though largely distributed species or species living at lower elevations than orophytes, which are supposed to be able to move upward, may not risk disappearance through climate change, they may nevertheless face fragmentation of their populations, because of land use, which can lead to reduced fecundity and offspring performance, as Kéry et al. (2000) showed for Primula veris and Gentiana lutea , two common species of nutrient-poor calcareous grassland.

Locally or regionally, warming in coming decades may weaken dominant species through severe defoliation due to pest outbreaks, and may alter their potential to respond to climate change. Pine shoot beetles (Tomicus piniperda L., T. minor Htg.) and other pests may strongly affect the extended Scots pine (Pinus sylvestris) forests in the Valais in combination with other factors, in particular drought (Rigling and Cherubini, 1999). On the other hand, warming in late winter and early spring followed by late frosts may break the recurring outbreaks of the larchbud-moth (Zeiraphera diniana Gn.) in the subalpine belt in the more continental regions, and therefore modify the present equilibrium between Arolla pine (Pinus cembra) and larch (Larix decidua) (Baltensweiler, 1993).

Impact on vegetation
One widespread hypothesis is that global warming will shift – in a more or less regular pattern – the climatic ranges of species (e.g., Peters and Darling, 1985) or even whole vegetation belts (e.g., Ozenda and Borel, 1991, 1995) upward along altitudinal, thermally defined gradients. Although a shift of a whole vegetation belt is hardly likely, one can nevertheless project an initial estimate of the potential range of change that is to be expected. For Switzerland, an increase of 3.3 K in mean air temperature, corresponding to an altitudinal shift of 600 m, would reduce on average the area of the alpine vegetation belt by 63%. Interestingly, the colline and montane vegetation belts would be reduced on average by only 20%, and the subalpine vegetation belt by even less (–9%).

Given the inertia of vegetation belts, an increase of 1–2 K in mean annual temperature may not shift the present forest limit upwards by much more than 100-200 m. However, it is inconceivable that the inertia of the temperature-related forest limit, either climatic or edaphic, will withstand a 3–4 K increase, which is equal to the temperature range of an entire vegetation belt. With such an increase, the ‘kampfzone' would be very likely to invade the alpine belt, with a consequent shift of the forest limit into the low alpine belt. If a temperature increase of more than 2 K persists for several centuries, it is possible that forests could develop at even higher elevations than those observed since the last glaciation.

One of the first effects of warming will be to modify competitive relationships between plant functional types. For instance, at the lowest elevations, sclerophyllous, i.e., having tough, persistant leaves, or laurophyllous phanerophytes in the understorey may overrun the deciduous tree layer. In the subalpine belt, deciduous trees may overrun coniferous ones. And finally, in the alpine belt, chamaephytes (i.e., plants which have surviving organs lying close to the ground up to 50 cm) may overrun hemicryptophytes (i.e., plant which have surviving organs lying at the soil surface), and low shrubs may overrun chamaephytes in subalpine-alpine heaths.

As regards the dynamic change of communities, it is unlikely that every component will change simultaneously. Also, not all structural components of plant communities will be modified at the same rate.

Thermophilous heath ecosystems, with their gradually intergrading dominant synusiae along an elevation gradient, exhibit a better adaptation to warming than the mesophilous heath ecosystems with their uniform, dominant synusiae along the same gradient. The former appear better adapted to warming than the latter, which depend primarily on the duration of the snow cover period and would be unable to endure its reduction.

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
   

(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 


Références citées :

Baltensweiler,W.: 1993, ‘Why the Larchbud-Moth Cycle Collapsed in the Subalpine Larch-Cembran Pine Forests in the Year 1990 for the First Time since 1850', Oecologia (Berlin) 94 , 62–66.

Bugmann, H.: 1999, ‘Anthropogene Klimaveränderung, Sukzessionsprozesse und forstwirtschaftliche Optionen', Schweiz. Z. Forstwesen 150 , 275–287.

Fischlin, A. and Gyalistras, D.: 1997, ‘Assessing Impacts of Climatic Change on Forest in the Alps', Global Ecol. Biogeogr. Lett. 6 , 19–37.

Gottfried, M., Pauli, H., and Grabherr, G.: 1994, ‘Die Alpen im “Treibhaus”: Nachweis für das erwärmungsbedingte Höhersteigen der alpinen und nivalen Vegetation', Jahrb. Vereins Schutz Bergwelt 59 , 13–27.

Gottfried, M., Pauli, H., Reiter, K., and Grabherr G.: 1999, ‘A Fine-Scaled Predictive Model for Changes in Species Distribution Patterns of HighMountain Plants Induced by ClimateWarming', Diversity Distributions 5 , 241–251.

Grabherr, G., Gottfried, M., and Pauli, H.: 1994, ‘Climate Effects on Mountain Plants', Nature 369 , 448.

Grabherr, G., Gottfried, M., and Pauli, H.: 1995, ‘Patterns and Current Changes in Alpine Plant Diversity', in Chapin, F. S. III and Körner, C. (eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences , Springer, Heidelberg, pp. 167–181.

Hättenschwiler, S. and Körner, C.: 1995, ‘Responses to Recent Climate Warming of Pinus sylvestris and Pinus cembra within their Montane Transition Zone in the Swiss Alps', J. Veget. Sci. 6 , 357–368.

Holt, R. D.: 1990, ‘The Microevolutionary Consequences of Climate Changes', Trends Ecol. Evol. 5 , 311–315. Huntley, B.: 1991, ‘How Plants Respond to Climate Change: Migration Rates, Individualism and the Consequences for Plant Communities’, Ann. Botany 67 (Suppl. 1), 15–22. Kéry,M., Matthies, D., and Spillmann, H.-H.: 2000, ‘Reduced Fecundity and Offspring Performance in Small Populations of the Declining Grassland Plants Primula veris and Gentiana lutea ', J. Ecol. 88 , 17–30.

Kienast, F., Wildi, O., and Brzeziecki, B.: 1997, ‘Potential Impact of Climate Change on Species Richness in Mountain Forests – an Ecological Risk Assessment', Biol. Conserv. 83 , 291–305.

Körner, C.: 1995, ‘Impact of Atmospheric Changes on Alpine Vegetation: The Ecophysiological Perspective', in Guisan, A., Holten, J. I., Spichiger, R., and Tessier, L. (eds.), Potential Ecological Impacts of Climate Change in the Alps and Fennoscandian Mountains , Conservatoire et Jardin botaniques, Genève, pp. 113–120.

Körner, C.: 1998, ‘A Re-Assessment of High Elevation Treeline Positions and their Explanation', Oecologia (Berlin) 115 , 445–459.

Körner, C.: 1999, Alpine Plant Life , Springer, Heidelberg, p. 338.

Marchisio, C., Cescatti, A., and Battisti, A.: 1994, ‘Climate, Soils and Cephalcia arvensis Outbreaks on Picea abies in the Italian Alps', Forest Ecol. Manage. 68 , 375–384.

Motta, R. and Masarin, F.: 1998, ‘Strutture e dinamiche forestali di popolamenti misti di pino cembro ( Pinus cembra L.) e larice ( Larix decidua Miller) in alta valle Varaita (Cuneo, Piemonte)', Archivio Geobotanico 2 , 123–132.

Ozenda, P. and Borel, J.-L.: 1991, Les conséquences écologiques possibles des changements climatiques dans l'Arc alpin , Rapport FUTURALP 1, Centre International pour l'Environnement Alpin (ICALPE), Chambéry, p. 49.

Ozenda, P. and Borel, J.-L.: 1995, ‘Possible Response of Mountain Vegetation to a Global Climatic Change: The Case of the Western Alps', in Guisan, A., Holten, J. I., Spichiger, R., and Tessier, L. (eds.), Potential Ecological Impacts of Climate Change in the Alps and Fennoscandian Mountains , Conservatoire et Jardin botaniques, Genève, pp. 137–144.

Peters, R. L. and Darling, J. D. S.: 1985, ‘The Greenhouse Effect and Nature Reserves: Global Warming Could Diminish Biological Diversity by Causing Extinctions among Reserve Species', Bioscience 35 , 707–717.

Rigling, A. and Cherubini, P.: 1999, ‘Wieso sterben die Waldföhren im “Telwald” bei Visp?', Schweiz. Z. Forstwesen 150 , 113–131.

Scharfetter, R.: 1938, Das Pflanzenleben der Ostalpen , Deuticke, Wien, p. 419.

Stützer, A.: 1999, ‘Im permanenten Überlebenskampf: Bäume über der Waldgrenze', Carinthia II. 109 , 353–360.

Theurillat, J.-P.: 1995, ‘Climate Change and the Alpine Flora: Some Perspectives', in Guisan, A., Holten, J. I., Spichiger, R., and Tessier, L. (eds.), Potential Ecological Impacts of Climate Change in the Alps and Fennoscandian Mountains , Conservatoire et Jardin botaniques, Genève, pp. 121– 127.

Theurillat, J.-P., Felber, F., Geissler, P., Gobat, J.-M., Fierz, M., Fischlin, A., Küpfer, P., Schlüssel, A., Velutti, C., and Zhao, G.-F.: 1998, ‘Sensitivity of Plant and Soils Ecosystems of the Alps to Climate Change', in Cebon, P., Dahinden, U., Davies, H. C., Imboden, D., and Jaeger, C. C. (eds.), ‘Views from the Alps: Regional Perspectives on Climate Change', MIT Press, Cambridge, MA, pp. 225–308.

Wohlgemuth, T.: 1998, ‘Modelling Floristic Species Richness on a Regional Scale: A Case Study in Switzerland', Biodiv. Conserv. 7 , 159–177.