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. xeuramericana and, in parallel, in vivo maximum capacity for carboxylation (V-c,V-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,V-max and J(max) for P. nigra and J(max) 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 A(sat) in P. nigra and P. xeuramericana due to large decreases in V-c,V-max and J(max). A 50% increase in [CO2] 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].