Are changes in high alpine flora paralleling climate change simultaneously with the retreat of glaciers and shorter length of snow cover? Are cold-adapted alpine plants disappearing while species from lower altitudes are advancing into the mountains? To explore these questions, we have mapped the flora of 150 mountain summits and mountain passes in southeastern Switzerland over the past two summers and compared this data with historical floristic inventories from the same mountains.
There are now more vascular plant species growing on our mountain tops
The results show: In comparison with data from botanists over a hundred years ago, there are distinctly more plant species that grow on our peaks today. Also, plant species have clearly advanced to higher elevations.
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| Fig.1: Today (blue area), summit flora species occur approximately 82 meters higher than previously (red area). |
Piz Linard (3410 m) reflects these impressive results: In 1835, Oswald Heer, “summit florist” and former professor in Zürich, documented only one species on top of the mountain. On average, scientists have repeated the floristic survey on the summit every 20 years, and in almost every instance, found new plant species. In the last 20 years, this trend has further accelerated, so that today 16 species occur on the summit. Among the recent colonizers are some plant species that previously grew at elevations 200 m lower than today.
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Fig. 2: Significantly more species grow on the summit of Piz Linard today than in the middle of the 19th century. | |
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Fig. 3: List of species from the summit of Piz Linard from 1835 as noted in the field notebook of Oswald Heer. Source: "Zentralbibliothek Zürich, Nachl. O. Heer 63, S. 55" |
Most plant species have spread since the time of their historic records. About 250 species have been found on an average of three additional peaks. Sixty eight new species have been recorded as summit flora species while 19 species are no longer found on summits. Additionally, 49 species have become rarer.
Common species and species from lower elevations are increasing on summits
Many plant species which previously grew on numerous mountain peaks have since become more common (see Table 1). Also, some species which were not previously on our peaks are now quite numerous (Table 2). The three most common new species on our summits are typical of subalpine grassland vegetation (see Table 2).
| Table 1: The three species which have colonized the most summits within the past century. Number indicates number of mountains where the species occurred and trend shows increase or decrease of number of mountains. | ||
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Fig. 4: Today, the most commonly observed species on mountain peaks has also colonized the most peaks: The Alpine Bluegrass Poa alpina. |
| Table 2: Three plant species of subalpine grasslands which now grow on mountain peaks. Number indicates number of mountains where the species occurred and trend shows increase or decrease of number of mountains. | ||
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Fig. 5: Arnica, typical of subalpine grasslands, colonizes more and more mountain summits. |
Significantly more ferns than before
The group of plants known as ferns has increased dramatically over the last century on summits. Fern species were previously found on 16 summits, while today ferns can be found on 68 peaks. Ferns disperse through the transportation of their microscopic spores by wind. It is likely that fern spores arrived earlier on the summit, but could not survive or germinate due to a colder climate.
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Fig. 6: In comparison with other fern species, the Common Moonwort (Bothrychium lunaria) was the most successful colonizer of new summits (23). This species was previously found on only 3 peaks. |
Species-rich peaks are not bigger in area, but located at lower elevations
The investigated peaks have varying species richness. In the uppermost 10 m of the summit, we have observed between one species (Mt. Washington Bluegrass, Poa laxa, on Piz Chalchagn, 3154 m) and 118 species (Chörbsch Horn, 2654 m). Principally, species richness decreases with increased elevation above sea level despite the surface area of the summit. It was also observed that the flora of adjacent peaks is not more similar than the flora of peaks separated by more distance.
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Fig 7: The most species rich of the investigated peaks is Chörbsch Horn (2654 m) near Davos. 118 vascular plant species were observed in the uppermost 10 m of the summit. | |
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Fig 8: The peak with the lowest species richness is Piz Chalchagn (3154 m) above Pontresina. Only one species was found on the uppermost 10 m of the summit: Mt. Washington Bluegrass (Poa laxa). |
Species which became less frequent
While some of the summit flora species have disappeared from summits since the early records, there are far more species that have colonized peaks and increased in frequency on summits (see Table 3).
| Table 3: Three plant species which have clearly become scarcer on summits. Number indicates number of mountains where the species occurred and trend shows increase or decrease of number of mountains. | ||
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Fig. 9: Alpine Yarrow (Achillea moschata), the plant which flavors Iva Liqueur, has become scarcer on summits. |
Observations are not error-free
Observer data must be checked for reliability – similar to the measurement accuracy of scientific instruments. Therefore, independent surveys by two people were conducted on some peaks. The plant lists of independent surveys agree by 86.5% on average. Observer errors are highest for rare and small species or species that have a short blooming period.
Our observer bias lies within the range of error in comparable studies and is much smaller than the difference between the past and present changes in alpine flora. If we assume that the historical botanists had a similar level of observer error during their expeditions, then the differences in the data reflect actual changes in the flora over the past century.
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Fig. 10: Rare and small species can often be overlooked by botanists in the field. |
The only ecosystem throughout Switzerland with increasing biodiversity
The flora of mountain summits has significantly changed since the historical vegetation surveys were completed. Most plant species, especially subalpine grassland species and high alpine specialists, have become more frequent. Therefore, high alpine habitats are likely the only ecosystem in Switzerland where biodiversity is not decreasing. However, despite this trend, individual species have decreased or disappeared altogether.
Is climate change the driving force?
The nature and magnitude of the observed changes suggest that global warming may have caused these changes and therefore, responses by summit vegetation parallel changes in glaciers and snow cover. However, it is still unclear whether other factors, such as wildlife and hikers which are now much more numerous than 100 years ago, may have influenced the changes in distribution of alpine plants.
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Fig. 11: Does wildlife have an influence on the species richness of summit flora? |
Future Research
In the coming months we will examine the specific behavior of individual species or groups of species. For example, we will analyze whether plants with wind dispersed seeds can colonize more peaks than other species. Additionally, we will determine which species have colonized the most mountain peaks on which ibex (which were extinct 100 years ago) also thrive. In our analyses, we will also focus on species which have become scarcer than in the past and particularly those species which are on the Swiss Red List of Threatened Species.
International Summit Vegetation
Historically, botanists were not only climbing to mountaintops in the Swiss Alps, but also in mountainous regions in other parts of Europe. For example, it will be interesting to investigate whether alpine vegetation in Scandinavian and southern European mountainous regions have changed differently than in the Alps. In cooperation with local ecologists, we have therefore begun to extend our study to peaks in Scotland, Scandinavia and the Pyrenees.
Publications
Wipf S, Stöckli V, Herz K & Rixen C (in press) The oldest monitoring site of the Alps revisited: accelerated increase in plant species richness on Piz Linard summit since 1835. Plant Ecology & Diversity.
Veronika Stöckli, Sonja Wipf, Cajsa Nilsson, Christian Rixen. 2011. Using historical plant surveys to track biodiversity on mountain summits. Plant Ecology & Diversity Vol. 4, No. 4, 415-425
Sarah Burg. 2012. Observer bias and its causes in botanical revisitation studies on summits in the Swiss Alps. Master Degree Thesis in Biology, ETH Zürich
Kristina Herz. 2012. Changes in the spatial distribution of vascular plant species in the Southeastern Swiss Alps over the past century. Master Degree Thesis in Environmental Sciences, ETH Zürich
Magalí Matteodo. 2012. Vegetation change in the last century on Swiss summits: a plant traits analysis. Tesi di Laurea Magistrale in Biologia dell'ambiente, Università degli studi di Torino, Italy
Cajsa Nilsson. 2011. Changes in plant diversity on 108 Swiss alpine summits over the last century. Master Degree Thesis in Biology, Umea University, Sweden
Sandro Boggia. 2010. Changes in plant species composition and diversity on Swiss mountain summits over the last century. Master Degree Thesis in Biology, ETH Zürich
Team/Contact
Veronika Stöckli, project leader, v.stoeckli@slf.ch
Sonja Wipf, leader research, wipf@slf.ch
Christian Rixen, Senior Scientist, rixen@slf.ch