Alpine treelines in a CO2-rich and warm world

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.

Aims

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.

Methods

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.