Ecosystem consequences of elevated atmospheric CO2 concentration at the alpine treeline

Ecosystem oriented research essentially developing in parallel to the technological advance  of Free Air CO₂ Enrichment (FACE) provides evidence for a multitude of CO₂-induced ecosystem consequences beyond potential changes in plant biomass and production. After a five-year experiment at the Swiss treeline FACE site (2180 m asl, Davos, Switzerland),  we offer a synthesis of results obtained at different levels of organization and put them explicitly in an ecosystem context. In response to elevated CO₂ concentration, we observed: (1) a sustained stimulation of photosysnthesis and no change in stomatal conductance  in either of the dominant conifer species, (2) contrasting species specific above-ground growth responses in Larix decidua, Pinus uncinata, Vaccinium myrtillus, Vaccinium uliginosum, and Empetrum hermaphroditum, (3) an increase in seed abundance of the annual hemiparasites Melampyrum sylvaticum and Melampyrum pratense; (4) changes in leaf quality, evident particularly as higher non-structural carbohydrates, in the dominant perennials; (5) reduced relative growth of herbivores in the ecosystem resulting from changes in forage quality studied on the alpine grasshopper, Miramella alpina, and the larch bud moth, Zeiraphera diniana; (6) higher frost damage of Vaccinium buds in light of a late spring frost; (7) no evidence of any changes in root production, standing crop or decomposition; (8) a trend towards increased mycorrhization of seedling root tips; (9) slightly later litter fall of Larix and Vaccinium but no change in Larix litter decomposition rates; (10) higher soil organic matter, particularly evident  in seasonal measurements of dissolved organic carbon, and higher rates of soil respiration and measured microbial activity. Collectively, these results show that the consequences of elevated CO₂ concentration extend beyond potential carbon sequestration through accelerated plant growth and can have significant impacts on ecosystem processes with feedbacks through trophic levels and with implications for carbon and nutrient cycling.