Photosynthesis and stomatal conductance responses of poplars to free‐air CO₂ enrichment (PopFACE) during the first growth cycle and immediately following coppice

• Using the Poplar Free Air CO2 Enrichement (PopFACE) facility we investigated the effects of elevated [CO2] on the diurnal and growth cycle responses of photosynthesis and conductance in three poplar species. • In situ diurnal measurements of photosynthesis were made on Populus alba, P. nigra and P. ×euramericana and, in parallel, in vivo maximum capacity for carboxylation (Vc,max) and maximum rates of electron transport (Jmax) were determined by gas exchange measurement. • Light saturated (Asat) and daily integrated (A′) photosynthesis increased at elevated [CO2] in all species. Elevated [CO2] decreased Vc,max and Jmax for P. nigra and Jmax for P.¥euramericana but had no effect on stomatal conductance in any of the species throughout the first growth cycle. During post-coppice re-growth, elevated [CO2] did not increase Asat in P. nigra and P.×euramericana due to large decreases in Vc,max and Jmax. • A 50% increase in [CO2] under these open-air field conditions resulted in a large and sustained increase in Asat. Although there were some differences between the species, these had little effect on photosynthetic rates at the growth [CO2]. Nevertheless the results show that even fast growing trees grown without rooting volume restriction in the open may still show some down-regulation of photosynthetic potential at elevated [CO2].