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. x euramericana and, in parallel, in vivo maximum capacity for carboxylation (V_c,max) and maximum rates of electron transport (J_max) were determined by gas exchange measurement. • Light saturated (A_sat) and daily integrated (A′) photosynthesis increased at elevated [CO₂] in all species. Elevated [CO₂] decreased V_c,max and J_max for P. nigra and J_max for P. x 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 A _sat in P. nigra and P. x euramericana due to large decreases in V_c,max and J_max. • A 50% increase in [CO ₂] under these open-air field conditions resulted in a large and sustained increase in A_sat. 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].