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Summit flora and climate change

 

Many mountain plants are restricted to below certain altitudes because of harsh conditions, with the summers becoming too cool or too short at higher elevations. With current global warming more and more species are shifting upwards to ever higher altitudes. Mountain summits are particularly useful to investigating these gradual shifts in alpine flora and their drivers, as many historical plant lists for summits (Fig. 4) provide an ideal basis for assessing floristic change at a particular location.

In recent years, members of an international research network led by Sonja Wipf and Christian Rixen from the WSL Institute for Snow and Avalanche Research (SLF) visited some 250 of these summits across Europe to collect data on the current composition of species found there. This allowed us to investigate how global change has affected alpine flora, and whether species adapted to low temperatures will be able to survive future milder conditions.

Average temperatures in European mountains
Fig. 1. Average temperatures in European mountain ranges have risen sharply over the past century, with an acceleration of this warming in recent decades (top line in the graphs). At the same time, there has been an ever faster increase in the number of plant species on summits (bottom line). The greater the warming between the vegetation surveys, the greater the species enrichment, forming a significant correlation (see Steinbauer et al. 2018).
 

Europe-wide shift in summit flora due to global warming

We looked at how alpine flora has changed across Europe in a large-scale study involving researchers from 11 countries. As part of this we compared new surveys of vegetation with older ones from the same summit, yielding a unique data set encompassing 302 mountain summits in all, stretching from the Pyrenees in the southwest to the Alps and on to Svalbard and Scotland in the north and the Carpathians in the east (Fig. 1), and covering a period of 145 years. The results of this study, which have just been published in leading journal Nature (Steinbauer et al. 2018), show for the first time not only that species richness on mountain summits is increasing throughout Europe but also that the rate of this increase is accelerating.

Seeking to identify the driver behind this acceleration in species enrichment, we looked at three potential influencing factors: changes in summer temperatures, changes in annual precipitation levels, and accumulated nitrogen deposition. Only the temperature had a consistent and significant effect on species richness: the greater the warming between two vegetation surveys on a summit, the greater the species enrichment. This process has gathered momentum in recent decades, in parallel with the ever increasing rate of warming. This accelerated response to climate warming, which we have established in alpine habitats for the first time had previously been known mainly from abiotic systems, such as glaciers.

 
Vegetation survey with Bavarian gentian
Fig. 2. The Bavarian gentian (Gentiana bavarica) has disappeared from several peaks in the Swiss Alps.

Are alpine species being displaced by newly arrived species?

At first glance, increased biodiversity seems like good news. In fact, though, it is not completely beneficial because many of the new species colonising Europe's summits are species from lower altitudes, which now due to rising temperatures have reached higher elevations where they could not survive previously. A lot of these are larger and more competitive than typical mountain species, which are regarded as stress-tolerant specialists adapted to surviving low temperatures and short summers. This raises the prospect that specialists tolerant to the cold could eventually be displaced by more thermophilic generalist counterparts, shifting the distribution of typical alpine species upwards to ever higher altitudes and reducing the distinctiveness of alpine flora.

However, alpine landscapes' heterogeneous microclimate could counteract the displacement of cold-tolerant species. Within the space of a few metres  temperatures can differ by several degrees Celsius, e.g. between south- and north-facing slopes. While the newly arrived species settle in small-scale particularly mild sites, species adapted to low temperatures may be able to survive for even longer in especially cool locations.

Who are the winners and losers?

The species best equipped to compete with the new arrivals will be the ones that can grow on scree, according to investigations in which we mapped the small-scale distribution of 12 plant species on 11 summits and compared them with the long-term success of the same species on summits in the Swiss Alps (Kulonen et al. 2018). The species whose numbers have increased most on Swiss summits over the past 100 years are those gravitating most to scree. Scree is not only common on summits but is also an environment where there is little competition because this substrate is too unstable, and provides insufficient levels of water and nutrients for many plants from lower altitudes to grow. Therefore, plants with the ability to thrive on scree are not displaced so quickly, even in the face of rising temperatures.

However, the situation looks less rosy for high-mountain specialists having a preference for organic soil (Fig. 3). Their numbers have generally been in decline over the past century. On stable organic soil, the competition from neighbouring species is at its greatest. Shifting upwards to higher, colder locations is problematic due to the scarcity of organic substrate on summits, with this becoming less and less common as the altitude increases.

 
Piz Linard in summer time
Fig. 3: Significantly more species grow on the summit of Piz Linard today than in the middle of the 19th century.
List of species from the summit of Piz Linard from 1835 as noted in the field notebook of Oswald Heer.
Fig. 4: 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
Prof. Carl Schröter (right) and colleagues during a botany-excursion in mountainous terrain around 1920.
Fig. 5: Since the early 19th century botanists have climbed mountains to study the summit flora. Their data allow us to analyse how the flora changed at these sites. Picture: Prof. Carl Schröter (right) and colleagues during a botany-excursion in mountainous terrain around 1920. Source: ETH-Bibliothek Zürich, Bildarchiv.

Floristic change on Piz Linard over 176 years

Piz Linard (3,410 metres; Fig. 3), the longest- and best-studied summit in Europe, provides a striking illustration of the Europe-wide acceleration of species enrichment: in 1835, Oswald Heer, the first climber to reach the peak of this mountain and a University of Zurich professor at the time, only recorded one individual plant on the summit (Fig. 4). Since then, on average, naturalists have been studying Piz Linard every 20 years. In the past 20 years, the rate of colonisation of this summit has been accelerating sharply, with no fewer than 16 species now thriving there (Wipf et al. 2013). While the early decades saw the emergence of typical high-alpine species there, none of the colonisers over the past 20 years have ever been found at such high altitudes before. This clearly shows that only the rising temperatures of recent decades have made this species enrichment possible.

Research using historical data

Historical data provide a valuable basis for monitoring long-term changes in the composition of ecosystems (Stöckli et al. 2011). Over a century ago, famous botanist Josias Braun(-Blanquet) knew that the quality of his work would have major implications for subsequent research: "To [...] establish a sound foundation for future comparisons, I examined many summits in detail. [...] Using the detailed site information, it should not be difficult to review my lists and identify any future increase or decrease in the species composition with near certainty" (Braun 1913, p. 327). Based on this and similar evidence, we can be sure of the high quality of historical summit data.

Having said that, the reliability of observation data, including our own, must be checked, in the same way as would be the case for instruments' measurement accuracy. Therefore, on some summits, two individual botanists collected the data separately from each other (Burg et al. 2015). On average, the plant lists they produced are 87% the same. Cases of observation bias mainly involved small and rare species (Fig. 6) or species that only blossom for a short period. Our level of observation bias is in line with that of similar studies and is almost three times less than the difference between the historical and current compositions of species. As a result, we can be confident that the substantial increase in species richness recorded by observers does indeed reflect the actual situation.

 
Vegetation survey on the Lenzerhorn
Fig. 6: A particularly close eye needs to be kept on small and rare species.

Project details

Project duration

2010 - 2016

Project lead

Dr. Sonja Wipf

 

Mountain Ecosystems

Alpine environment and natural hazards

Davos

 

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