An alpine treeline in a carbon dioxide-rich world: synthesis of a nine-year free-air carbon dioxide enrichment study
We evaluated the impacts of elevated CO2 in a treeline ecosystem in the Swiss Alps in a 9-year free-air CO2 enrichment (FACE) study. We present new data and synthesize plant and soil results from the entire experimental period. Light-saturated photosynthesis (A max) of ca. 35-year-old Larix decidua and Pinus uncinata was stimulated by elevated CO2 throughout the experiment. Slight down-regulation of photosynthesis in Pinus was consistent with starch accumulation in needle tissue. Above-ground growth responses differed between tree species, with a 33 % mean annual stimulation in Larix but no response in Pinus. Species-specific CO2 responses also occurred for abundant dwarf shrub species in the understorey, where Vaccinium myrtillus showed a sustained shoot growth enhancement (+11 %) that was not apparent for Vaccinium gaultherioides or Empetrum hermaphroditum. Below ground, CO2 enrichment did not stimulate fine root or mycorrhizal mycelium growth, but increased CO2 effluxes from the soil (+24 %) indicated that enhanced C assimilation was partially offset by greater respiratory losses. The dissolved organic C (DOC) concentration in soil solutions was consistently higher under elevated CO2 (+14 %), suggesting accelerated soil organic matter turnover. CO2 enrichment hardly affected the C–N balance in plants and soil, with unaltered soil total or mineral N concentrations and little impact on plant leaf N concentration or the stable N isotope ratio. Sustained differences in plant species growth responses suggest future shifts in species composition with atmospheric change. Consistently increased C fixation, soil respiration and DOC production over 9 years of CO2 enrichment provide clear evidence for accelerated C cycling with no apparent consequences on the N cycle in this treeline ecosystem.
KeywordsCarbon cycling Dwarf shrub Global change Nitrogen Treeline conifer
We thank many colleagues at the SLF, WSL, Paul-Scherrer Institute and University of Basel for their assistance with field and lab measurements and technical support. We are especially grateful to E. Amstutz, L. Egli, G. Grun, A. Studer, S. Wipf and P. Bebi for helping to ensure successful operation of the FACE system. Major funding sources included: the Swiss National Science Foundation 2001–2005 (grant 31-061428.00 to S. H.) and 2007–2009 (grant 315200-116861 to C. R.); an ANR-biodiversité grant to S. H. 2006–2008; and the Velux foundation 2007–2009 (grant 371 to F. H.). Additional financial support was provided by the CCES-ETH project MOUNTLAND, Swiss State Secretariat for Education and Research (COST Action 639, projects C07.0032 and C07.0033), WSL, University of Basel Botanical Institute, Swiss Federal Office for the Environment and the Fonds québecois de recherche sur la nature et les technologies (scholarship to I. T. H.).
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