Growth and phenology of mature temperate forest trees in

Are mature forest trees carbon limited at current CO2 concentrations? Will ‘mid-life’, 35 m tall deciduous trees grow faster in a CO2-enriched atmosphere? To answer these questions we exposed ca. 100-year-old temperate forest trees at the Swiss Canopy Crane site near Basel, Switzerland to a ca. 540 ppm CO2 atmosphere using web-FACE technol-ogy. Here, we report growth responses to elevated CO2 for 11 tall trees (compared with 32 controls) of five species during the initial four treatment years. Tested across all trees, there was no CO2 effect on stem basal area (BA) increment (neither when tested per year nor cumulatively for 4 years). In fact, the 4th year means were almost identical for the two groups. Stem growth data were standardized by pretreatment growth (5 years) in order to account for a priori individual differences in vigor. Although this experiment was not designed to test species specific effects, one species, the common European beech, Fagus sylvatica, showed a significant growth enhancement in the first year, which reoccurred during a centennial drought in the third year. None of the other dominant species (Quercus petraea, Carpinus betulus) showed a growth response to CO2 in any of the