Three Populus genotypes (P. alba L. (Clone 2AS-11), P. nigra L. (Clone Jean Pourtet) and P. × euramericana (Clone I-214)) growing in a managed, high-density forest plantation were exposed to free-air CO2 enrichment (FACE) at CO2 concentrations expected to occur in the future (550 ppm). Leaf area index (optical LAI), measured with a fish-eye-type plant canopy analyzer, was not significantly affected by FACE after canopy closure in the second growing season. However, when stands of similar size were compared, optical LAI and number of main stem leaves were reduced by FACE. Allometric relationships, which were established to scale-up leaf area to the stand level (allometric LAI), did not differ between the FACE and control plots. Allometric LAI increased in response to FACE, as a result of increased tree dimensions and increased individual leaf size. We postulate that, although FACE increased allometric LAI, FACE had no effect on optical LAI after canopy closure, because FACE caused increased shading and competition resulting in enhanced leaf fall or leaf turnover. Specific leaf area (SLA) was unaffected by FACE. Allometric relationships and relative responses to atmospheric CO2 enrichment were genotype-dependent.