1. Using experimental atmospheric CO2 enrichment, we tested for tree growth stimulation at the high-elevation treeline, where there is overwhelming evidence that low temperature inhibits growth despite an adequate carbon supply. We exposed Larix decidua (European larch) and Pinus mugo ssp. uncinata (mountain pine) to 9 years of free-air CO2 enrichment (FACE) in an in situ experiment at treeline in the Swiss Alps (2180 m a.s.l.).
2. Accounting for pre-treatment vigour of individual trees, tree ring increments throughout the experimental period were larger in Larix growing under elevated CO2 but not in Pinus. The magnitude of the CO2 response in Larix ring width varied over time, with a significant stimulation occurring in treatment years 3–7 (marginal in year 6).
3. After 9 years of treatment, leaf canopy cover, stem basal area and total new shoot production were overall greater in Larix trees growing under elevated CO2, whereas Pinus showed no such cumulative growth response. The Larix ring width response in years 3–7 could have caused the cumulative CO2 effect on tree size even if no further stimulation occurred, so it remains unclear if responsiveness was sustained over the longer term.
4. Larix ring width was stimulated more by elevated CO2 in years with relatively high spring temperatures and an early snowmelt date, suggesting that temperatures were less limiting in these years and greater benefit was gained from extra carbon assimilated under elevated CO2. The magnitude of CO2 stimulation was also larger after relatively high temperatures and high solar radiation in the preceding growing season, perhaps reflecting gains due to larger carbon reserves.
5. Synthesis. Contrasting above-ground growth responses of two treeline tree species to elevated CO2 concentrations suggest that Larix will have a competitive advantage over less responsive species, such as co-occurring Pinus, under future CO2 concentrations. Stimulation of Larix growth might be especially pronounced in a future warmer climate.