Holocene Vegetation and Climate Dynamics in the Northwestern Swiss Alps
The treeline ecocline is very sensitive to climatic change, because trees exist at their lower limits of thermal tolerance. Therefore even small changes have marked effects on the species composition of high altitude forest communities and the elevation of treeline. With Climate Change today's treeline is expected to shift to higher elevations, but the rate and extent of change are largely unknown. In this study we combine paleoecological methods like pollen and macrofossil analysis, with dynamic landscape modeling to assess the impact of past climatic changes and human disturbance on the vegetation at the treeline.
We focus our study on three lakes at different altitudes in the Bernese Oberland and the Wallis to gain a high resolution overview of regional treeline dynamics. The highest lake in the study area is close to Col-du-Sanetsch (VS). With an altitude of 2288 m a.s.l., this lake is in an ideal position for the reconstruction of the maximum extent of Holocene treeline. The Iffigsee (BE) is located at today's potential treeline (2065 m a.s.l.) and has a long history of human presence in the area. Archeological findings point to an important mountain pass nearby, that was used by humans since the Neolithic. The last lake used in this study is Lauenensee at 1381 m a.s.l. With this lake, located in the upper montane vegetation belt, we can complete our altitudinal transect and hope to gain important insights into mountain forest dynamics in the Holocene.
We are using pollen, plant macrofossils and charcoal conserved in the lake sediment to reconstruct the local vegetation, fire and land-use history. Aquatic invertebrates (i.e. chironomids, ostracodes) and diatoms, as well as abiotic proxies like Loss-On-Ignition or magnetic susceptibility can be used to reconstruct Holocene temperatures, trophic state and lake level changes.
The reconstruction of Holocene plant occurrence, abundance and diversity gives an indication of the potential natural vegetation and is therefore valuable in the development of nature conservation strategies in montane to alpine environments. In addition, by combining these paleorecords with a dynamic landscape succession model, we can test different hypotheses explaining alpine vegetation dynamics and differentiate between anthropogenic and climatic impacts on the ecosystem. Once validated with the pollen and macrofossil record, the succession model can ultimately be used to simulate the response of the treeline ecocline to different future climate scenarios.
The study is funded by the Dr. Alfred Bretscher Fonds
Paul Henne, Erika Gobet, Stéphanie Samartin, Tiziana Pedrotta, Carole Adolf and Willy Tinner
Martin Grosjean (Oeschger), Lukas Glur (Eawag), Flavio Anselmetti (EAWAG, ETH) and Adrian Gilli (ETH Zürich)