Link zu WSL Hauptseite Swiss Federal Institute for Forest, Snow and Landscape Research WSL
Duration: 2001 - 2014

Alpine treelines in a CO2-rich and warm world

Delivery of CO2
Delivery of CO2.
Photo: Frank Hagedorn.
Treeline ecosystems enriched with CO2
Treeline ecosystems are enriched with CO2 using the FACE approach.
Photo: Frank Hagedorn.

We are experimentally increasing atmospheric CO2 concentrations (+200 ppm) and soil temperatures (+4K) at the alpine treeline and study the response of plant growth and soil processes. First results indicate that elevated CO2 affects the C cycling rates rather than the C pools in plants and soils. Warming has increased the rate of soil organic matter decomposition and turned the ecosystem into a CO2 source.


The strong increase in atmospheric CO2 is changing ecosystems either directly through the CO2-effects on plant growth, or indirectly through its impact on temperatures. It is likely that high altitude soils will be particularly sensitive to the ongoing atmospheric and climatic changes. The temperature sensitivities of most biogeochemical processes are greater at a low temperature range. Since alpine and montane soils contain large pools of labile C, they play an important role in the response of the overall ecosystem’s C balance to the changing environment and in feedbacks at the ecosystem level.


With our studies, we aim to

  • identify how and why tree and dwarf shrub growth and physiology change in response to increasing temperatures and CO2 concentration.
  • estimate how elevated CO2 and warming affect the competition between trees, dwarf shrubs, and grasses, and therefore the vegetation structure and composition.
  • determine if elevated CO2 and increased soil temperature alters plant sensitivity to frost events during the growing season
  • quantify the response of C fluxes (soil respiration, DOC leaching, accumulation in different SOM pools, aboveground biomass) and nutrient status.
  • study how composition of the soil microbial community responds to elevated CO2 and warming.
  • elucidate whether the new plant-derived rapidly cycling soil C fraction or the older slower cycling soil C fraction responds more sensitive to climatic warming.
  • estimate if warming alters the partitioning of recent assimilates between plants and soils.
  • study the transfer of carbon and nutrients between plants and mycorrhiza.
  • test for the effect of atmospheric and climate change on seedling emergence and survival.

In our project, we are experimentally increasing atmospheric CO2 concentrations (+200 ppm) and temperatures (+4K) at the alpine treeline at the research site of Stillberg, Davos. Larch and mountain pine trees that were planted slightly above treeline in the course of an afforestation experiment in 1975 have been exposed (along with the natural understorey layer of dwarf shrubs, forbs, and grasses) to elevated CO2 concentrations since 2001 using the FACE approach (n=20; Hättenschwiler et al., 2002). The added CO2 originates from fossil fuel burning and is depleted in 13C as compared to normal air (-30‰ vs. -8‰). This allows us to trace the isotopic signal through the plant and soil system. For the warming experiment, we laid out 26 m of heating cables in spirals on the ground surface in the 1.1 m2-plots. Soil and the air around the dominating dwarf shrubs and grasses have been heated by 4K since 2007.

  • Dawes MA, Hagedorn F, Handa IT, Streit K, Ekblad A, Rixen C, Körner C, Hättenschwiler C (2013) An alpine treelines in a CO2-rich world: synthesis of a nine year free air CO2 enrichment study. Oecologia 171, 623-637.
  • Dawes, MA, Hättenschwiler S, Bebi P, Hagedorn F, Handa IT, Körner C, Rixen C (2011). Species-specific tree growth responses to nine years of CO2 enrichment at the alpine treeline. Journal of Ecology 99: 383–394.
  • Dawes MA, Hagedorn F, Zumbrunn T, Handa IT, Hättenschwiler S, Wipf S, Rixen C (2011). Growth and community response of alpine dwarf shrubs to in situ CO2 enrichment and soil warming. New Phytologist 191: 806-818
  • Dieleman W, Vicca, S., Dijkstra F, Hagedorn F, Hovenden M, Larsen K, Morgan J, Volder A, Beier C, Dukes J, King J, Leuzinger S, Linder S, Oren R, Tingey D, Hoosbeek M, Lup Y, Janssens I (2012) Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature. Global Change Biology 18, 2681-2693. doi: 10.1111/j.1365-2486.2012.02745.x
  • Hagedorn F, Hiltbrunner D, Streit K, Ekblad A, Lindahl B, Miltner A, Frey B, Handa IT, Hättenschwiler S (2013) Nine years of CO2 enrichment at the Alpine treeline stimulates soil respiration but does not alter soil microbial communities. Soil Biology and Biochemistry 57, 390-400.
  • Hagedorn F, Martin M, Rixen C, Rusch S, Bebi P, Zürcher A, Siegwolf RTW, Wipf S, Escape C, Roy J, Hättenschwiler S (2010) Short-term responses of ecosystem carbon fluxes to experimental warming at the Swiss alpine treeline. Biogeochemistry 97, 7-19
  • Hagedorn F, van Hees PAW, Handa IT, Hättenschwiler S (2008) Elevated atmospheric CO2 fuels leaching of old dissolved organic matter at the alpine treeline. Global Biogeochemical Cycles 22. GB2004, doi:10.1029/2007GB003026
  • Hagedorn F, Machwitz M (2007) Controls on dissolved organic matter leaching from forest litter grown under elevated atmospheric CO2. Soil Biology and Biochemistry 39, 1759-1769
  • Handa, I.T., Hagedorn, F., und S. Hättenschwiler 2008. No stimulation in root production in response to four years of in situ CO2 enrichment at the Swiss treeline. Functional Ecology 22, 348-358.
  • Martin M, Gavazov K, Körner C, Hättenschwiler S, Rixen C (2010) Reduced early growing season freezing resistance in alpine treeline plants under elevated atmospheric CO2. Global Change Biology 16, 1057-1070.
  • Müller M, Alewell C, Hagedorn F (2009) Effective retention of litter-derived dissolved organic carbon in organic layers. Soil Biology and Biochemistry 41, 1066-1074.
  • Rixen C, Dawes MA, Wipf S, Hagedorn F (2012) Evidence of enhanced freezing damage in treeline plants during six years of CO2 enrichment and soil warming. Oikos. Doi:10.1111/j.1600-0706.2011.20031.x
  • Streit K, Rinne KT, Hagedorn F, Dawes MA, Saurer M, Hoch G, Werner RA, Buchmann N, Siegwolf RTW (2013) Tracing fresh assimilates through Larix deciduas exposed to elevated CO2 and soil warming at the alpine treeline using compound-specific stable isotope analysis. New Phytologist 197, 838-849.
Links and downloads

Experimental station Stillberg

Project description of University of Basel (PDF, 840KB)

The treeline (PDF in German, 481KB)

Keywords alpine soils, elevated CO2, climate change , carbon, nutrients, DOC, respiration, soil chemistry, stable isotopes, temperature, organic layers, plant ecology, dendroecology, treeline, tree growth