How forests will respond to rising [CO 2 ] in the long term is uncertain, most studies having involved juvenile trees in chambers prior to canopy closure. Poplar free‐air CO 2 enrichment (Viterbo, Italy) is one of the first experiments to grow a forest from planting through canopy closure to coppice, entirely under open‐air conditions using free‐air CO 2 enrichment technology. Three Populus species: P . alba , P . nigra and P...