Réf. Walther & al. 2005 - A

Référence bibliographique complète

WALTHER, G.-R., BEßNER, S., BURGA, C. 2005. Trends in the upward shift of alpine plants. Journal of Vegetation Science, 16, 541–548.

Abstract:

Questions: The 1990s were the warmest decade since the beginning of climate measurements. Based on almost 100 years of monitoring in the Swiss Alps, we asked (1) whether the extraordinary warm climatic conditions of the 1990s are reflected in the floristic composition of Alpine summit vegetation and, if so, (2) what the magnitude and rate of species change has been over the last few decades compared to the documented increase in species richness within the first 80 years of the 20th century.

Location: Ten high mountain summits of the Bernina area in the southeastern Swiss Alps.

Methods: Resurvey of the floristic composition of the uppermost altitudinal 10 m of these summits, applying the same methodology of former two surveys (1905 and 1985) and recording the presence of all vascular plant species.

Results: Whereas the continued increase in plant species richness of high alpine summit vegetation is confirmed, our results also suggest an acceleration of the trend in the upward shift of alpine plants.

Conclusion: Vegetation change in the southeastern Swiss Alps has accelerated since 1985, consistent with a climate change explanation.

 

Mots-clés

Bernina - Climate change - Long-term monitoring - Resurvey - Switzerland - Vegetation shift

 

Organismes / Contact

• Institute of Geobotany, University of Hannover, Nienburger Str. 17, DE-30167 Hannover, Germany (walther@geobotanik.uni-hannover.de)
• Department of Geography, University of Zürich-Irchel, Winterthurerstrasse 190, CH-8057 Zürich, 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

 

Observations

Evidence for world-wide ecological responses to recent climate change (Hughes 2000; Walther et al. 2002) is supported by an ever increasing number of ecological ‘fingerprints’ of global warming (Walther et al. 2001; Root et al. 2003; Parmesan & Yohe 2003). Plants in particular, may respond to the recent period of warmer climatic conditions by either adapting their life cycles or shifting their ranges to suitable habitats (Walther 2004). Alpine areas belong to those regions expected to experience above average warming with continued global climate change (Guisan et al. 1995; Theurillat & Guisan 2001; Beniston 2003; Burga et al. 2003; see also Krajick 2004; Kullman 2004). At the same time, they are among those habitats with the longest tradition in floristic surveys and vegetation analyses (see e.g. Körner 1999; Grabherr et al. 2001; Pauli et al. 2001), and thus provide a rich source of data reaching far back into the last century (cf. Rübel 1912; Braun 1913; Schröter 1926; see also Burga et al. 2004a). The flora of high mountain summits has been assumed to be an indicator of changing climatic conditions since the end of the warming phase in the first half of the 20th century (see Braun-Blanquet 1955, 1957). In the second half of the 20th century, data of an increasing number of high mountain summits were resurveyed in the 1980s (Hofer 1992) and early 1990s (e.g. Grabherr et al. 1994). Since then, global warming has not only continued, but the trend within the 1990s has sharply increased with the warmest decade since the beginning of climate measurements (Houghton et al. 2001). More than two decades after the first resurvey by Hofer (1992), one may ask (1) whether the extraordinary warm climatic conditions of the 1990s are reflected in the floristic composition of high mountain summits and, if so, (2) what the magnitude and rate of species change would be, compared to the documented increase in species richness within the first 80 years of the 20th century.

Results

Over all the investigated summits, we found more than 90% of the species listed by Rübel (1912) and almost 90% of those from Hofer (1992). There was a strong general trend towards increasing species numbers on all but one summit (Piz Trovat) not only compared to the first survey but also between the two more recent resurveys of the 1980s and the present. Whereas the mean increase in species recorded by Hofer (1992) was 86%, species numbers recorded in 2003 were generally more than double (138%) compared to the results by Rübel (1912) and 26% higher than those reported by Hofer (1992). The mean increase in species richness in absolute values between 1905 and 1985 was higher (10.5 species) than the mean increase in the later two decades (7.3 species; P > |z| 0.176). However, if the increase in species richness is calculated on a per decade basis, the rate of change in species richness (3.7 species/decade) was significantly greater (P > |z| 0.006) in the later period compared to the Hofer resurvey (1.3 species/decade). Several species were recorded at higher altitudes than reported in the past both for the Bernina area generally and in terms of individual mountains. Based on the comparison of the uppermost occurrences of the same species on the same mountain in the past (Rübel 1912) and present, the altitudinal shift over the last century of those species occurring for the first time on a new highest summit varies from 46 m up to 532 m (mean: 27.8 ± 14.6m/decade; n = 18).

Whereas the sample size of calcareous peaks was too small for a detailed analysis across peaks, the analysis of siliceous peaks revealed an interesting pattern of floristic changes across the eight mountain peaks. The proportions of vascular plant species on summits with siliceous substrate that have increased or decreased was analysed by comparing the number of peaks for a given species in the resurvey and as reported by Rübel (1912). In both data sets, the greatest percentage of change in frequency was for species that occurred on one additional summit (+1): 36% of the species in the 1980s and 41% in 2003 were found on one more summit than reported by Rübel (1912). Whereas the position of the maximum peak remained unchanged between the two resurveys, there were obvious changes at either end of the graph. In the 1980s, more than 10% of the species had disappeared from at least one summit on which they were present in the early 1900s; in 2003, however, only two species (2.5%) were found on fewer summits than previously reported by Rübel (1912). At the same time, at the other end of the graph, there is a remarkable change in the number of species which have increased their occurrence by three or four mountain peaks. Overall, on the eight siliceous peaks, all but one species reported absent by Hofer (1992) was found on either the same or new summits, and more than 75% of the species (59 of 78 species) occurred on more mountain peaks than were recorded in either of the previous surveys.

Whereas previous studies of floristic change in European mountain vegetation have been based on a single remeasurement this study provided, for the first time, an analysis of two intervals of change. This study has not only confirmed a general species increase in high-alpine summit vegetation, but results also suggest a rapid response of alpine vegetation to conditions in the warmest decade of the 1990s and consequently, an accelerating trend in the upward shift of alpine plants. So far, the observed increase in species numbers do not entail the replacement of high alpine specialists by species from lower altitudes, but rather has led to an enrichment of the overall summit plant diversity. Future studies should also focus on the lower margins of alpine species’ distribution, where evidence for retreating species’ ranges might first be detected.

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

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

The major factor limiting altitudinal plant species richness is climate (see e.g. Grabherr et al. 1995; Körner 1999). Since climate has warmed in the periods between the surveys both globally (Houghton et al. 2001) and locally (see e.g. Beniston et al. 1997; Wanner et al. 1997; Laternser & Schneebeli 2003; Bader & Bantle 2004; Begert et al. 2005), it may also have allowed species to move upward and colonize new areas at higher altitudes. This is the most widely used explanation for the recorded plant species’ shifts on mountain peaks in the European Alps (Hofer 1992; Grabherr et al. 1994; Keller et al. 2000; Camenisch 2002; Bahn & Körner 2003; Burga et al. 2004b; cf. also Reinalter 2004) and in Scandinavia (…).

In support of these findings, the present results not only confirmed the continued increase in plant species richness of high-alpine summit vegetation, they furthermore reveal an accelerated trend in the upward species’ shift of alpine plants in the second interval of the investigation, i.e. the two decades since the mid-1980s. Although this speeding up of the altitudinal plant shift can not be conclusively linked to recent climate warming, it is synchronous to the warmest decade (i.e. the 1990s) since the beginning of climate records and consistent with model predictions (e.g. Theurillat & Guisan 2001). However, the observed increase in plant species richness of high-mountain summits does not (so far) imply the loss of high-alpine specialist species (cf. Guisan & Theurillat 2000). Contrary to the recent findings of Lesica & McCune (2004) no species with a systematic decrease in abundance were found across the investigated summits. This can either be due to the fact that the present survey took place at the upper limit of plant distribution and/or the possibility that alpine specialist species still find enough habitats to avoid competition from ascending species from lower altitudes […].

The study area was the Bernina mountains, south-eastern Swiss Alps. High mountain summits were selected from the subset of mountains whose summit flora was first surveyed in the early 1900s by Rübel (1912) and resurveyed 80 years later by Hofer (1992). Four sites were excluded of Hofer’s subset because of either the enhanced degree of difficulty of access to the sites (two cases) or the fact that the area investigated was not clearly delimited, which increases the chance of overlooking species. Hence, ten mountain summits between 2959 m a.s.l. (Piz Lagalb) and 3262 m a.s.l. (Piz Languard) were resurveyed. Eight out of these ten peaks are underlain by siliceous rocks, the other two (Piz Alv and Piz Tschüffer) have a calcareous basement. All the summits exceed the altitude of the belt with closed alpine vegetation. Hence, the vegetation cover on these sites is generally sparse with scattered occurrences of individual plants and only at few places are some isolated patches of closed vegetation found, the rest of the surface consists of bare rock or scree.

In order to ensure comparability, the authors duplicated the methodology used in the former two surveys (Rübel 1912; Hofer 1992). In general, the uppermost 10 m (altitudinal) were searched in detail and the presence of every vascular plant species recorded. The only exception was Piz Languard, where – in accordance with the previous surveys –the uppermost 30 m were included. The survey contour was determined using a Thommen pocket altimeter calibrated against the relevant topographic map during the field work. The field survey took place in July/August 2003. All the mountains were climbed at least once within the same five weeks. Wilcoxon signed-ranks tests was used for analysing means of differences between matched pairs of observations for each summit (N =10) for the two intervals 1905-1985 and 1985-2003, using both absolute values and relative changes (i.e. rate of change in species richness per decade).

The full species lists for all the investigated mountain summits are given in the study to provide a further baseline data set for future investigations.

 

(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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