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Mountain and tundra plants are particularly severely affected by climate warming. Researchers at WSL are studying how the vegetation is changing.
Temperatures below freezing with icy north-east winds and snow – such conditions are challenging for plants that want to brave them. Unlike many animal species, they cannot escape the winter or seek protection from it somewhere. Only those that are frost-resistant and well-adapted to the cold climate can survive.
The temperature therefore largely determines where particular plant species become established, as well as how they grow and reproduce. Due to climate change, however, temperatures are increasingly on the rise. What does this imply for the vegetation composition – especially in mountain regions, where the warming taking place is twice the worldwide average? How are the cultivated plants and forest trees at our latitude reacting to the higher temperatures?
More frost damage despite climate warming
Researchers at WSL and SLF are pursuing these questions in several different projects. For example, one group, led by Yann Vitasse and Martine Rebetez from WSL and the University of Neuchâtel, has been investigating, with the support of FOEN, the Swiss Federal Office for the Environment, whether late frosts are becoming less frequent in Switzerland with climate change and, correspondingly, whether the risk of frost damage is declining. To this end they analysed, together with Christian Rixen from SLF and researchers from Agroscope Conthey, long-term data from automatic weather stations. The research team also evaluated thousands of observations made by laypeople about the leaf-out dates of beech and spruce, as well as the flowering time of apple and cherry trees. The results indicate something unexpected: although the growing season for vegetation is starting earlier, the last late frost at higher elevations is generally not occurring earlier in the year despite climate warming. This means that the risk of young leaves and flowers above 800 m a.s.l. being exposed to a spring frost is increasing – and it could increase further in the future. These tree species are likely, in the long term, to be affected by a late frost more frequently than they are today. The findings, however, also show that it would not necessarily be a good idea to promote varieties of fruit and forest tree better adapted to an ever-warmer summer climate as they often start their growing season earlier in the year and are thus particularly susceptible to frost.
Summit climbers are on the increase
In another project, SLF is investigating how climate warming is affecting plant composition on mountain summits. Christian Rixen and Sonja Wipf cooperated with researchers from all over Europe on mapping the plants growing on different summits in the Alps, the Pyrenees, the Carpathians, as well as in the Scottish and Scandinavian mountains. They compared their surveys each time with up to one-hundred-year-old data from the same site. Their results show not only that the number of species on mountain peaks throughout Europe has increased, but also that this increase is taking place at an ever faster pace. The reason for this is that the climate is warming increasingly rapidly.
The higher temperatures enable species from lower lying meadows to expand their distribution range upwards. Another effect of climate warming is that many plant species already growing on the summits are also becoming more widespread. But not all plants are benefiting: several high-mountain specialists have decreased or even completely disappeared. Christian: “It remains to be seen whether the species from lower elevations that are competitively stronger in the long term will displace the specialists high up completely.”
Flowering seasons are becoming more similar
The SLF team are not only monitoring these climatically determined changes in the Alpine vegetation on the basis of historical and recent vegetation surveys. Christian and his research colleagues have also maintained the Swiss site of the International Tundra Experiment ITEX in Val Bercla in the Grisons since 2009. In this large project, research teams are performing long-term experiments in Arctic, Antarctic and alpine habitats at over 40 sites worldwide. By generating higher temperatures in warming chambers – similar to open-top greenhouses – they investigate how the simulated climate change affects the vegetation.
Christian and his team are not, however, just responsible for their own site, but are also involved in the worldwide evaluation of ITEX data. In their latest publication, they analysed long-term data on nearly 50 tundra plants from 18 sites ranging from Alaska, Spitzbergen to the Faroe Islands. The average summer temperatures at these sites range from 2.8 °C to 11.9 °C, and thus replicate a climate gradient. At all these experimental sites, researchers studied, among other things, when the plants developed leaves during the year, when they started to flower or when the leaves began to fall. This showed that all the species studied reacted more strongly to the higher summer temperatures at the colder sites than at the warmer sites, and that the dates when the leaves and flowers opened shifted to earlier in the year more in the North than further South. “We assume that the flowering season of species from northern regions is increasingly aligning itself with those from southern regions,” says Christian. If the sites are not located far apart, insects and wind can transfer pollen from flowers further South to the North, and vice versa. This then intensifies the gene exchange between these regions.
These results, like the studies of summit flora, show that the vegetation in cold regions has already altered with climate change – and will continue to change, even though the direction in which the species composition will shift is not always clear. (Christine Huovinen, Diagonal 1/18)