PARENT SESSION
Oral Session #53: Elevated CO₂: Communities, ecosystems, soils.
Presiding: G. Lin
Wednesday, August 7. 8:00 AM to 11:30 AM. Coconino Meeting Room, TCC.

Treeline trees in a CO₂ enriched world.

HANDA, TANYA^*,1, HTTENSCHWILER, STEPHAN¹, ¹ University of Basel, Basel, Switzerland

ABSTRACT- Carbon has been argued to limit the growth of trees at the upper altitudinal treeline. If this is the case, then stimulatory effects on tree growth may be expected as a result of rising atmospheric CO₂ concentrations. In an ongoing project, we are experimentally testing this hypothesis by the CO₂ enrichment of 30-yr-old trees ( Larix decidua and Pinus uncinata) and their understory shrubs at the alpine treeline in Davos, Switzerland (2200 m a.s.l.). We are using the new technology of Free Air CO₂ Enrichment (FACE) in which pure CO₂ is emitted under high pressure. To date, carbon dioxide concentrations have been successfully maintained close to the target value of 550 ppm. Results after the first growing season indicate that trees growing at elevated CO₂ (in contrast to ambient controls) showed a significant increase in photosynthesis, an accumulation of non-structural carbohydrates in leaf and branch tissue, and an increase in the elongation of leading and lateral shoots. Enriched trees also showed a significant decrease in stomatal conductance and in the nitrogen concentration of leaf branch and leaf litter samples. While the direction of the above responses was similar for both pine and larch trees, the magnitude of the CO₂-response varied significantly among species. Collectively, these results suggest that both species may show an improved carbon balance as atmospheric CO₂ continues to increase with likely consequences for community stability and structure of the treeline ecotone.

KEY WORDS: Treeline, Elevated CO₂, Larix decidua , Pinus uncinata