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 ].