Leaf area dynamics in a closed poplar plantation under free-air carbon dioxide enrichment

Three Populus genotypes (P. alba L. (Clone 2AS-11), P. nigra L. (Clone Jean Pourtet) and P. x euramericana (Clone I-214)) growing in a managed, high-d ensity forest plantation were exposed to free-air CO2 enrichment (FACE) at CO2 concentrations expected to occur in the future (550 ppm). Leaf area ind ex (optical LAI), measured with a fish-eye-type plant canopy analyzer, was not significantly affected by FACE after canopy closure in the second growi ng season. However, when stands of similar size were compared, optical LAI and number of main stem leaves were reduced by FACE. Allometric relationshi ps, which were established to scale-up leaf area to the stand level (allome tric LAI), did not differ between the FACE and control plots. Allometric LA I increased in response to FACE, as a result of increased tree dimensions a nd increased individual leaf size. We postulate that, although FACE increas ed 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 FAC E. Allometric relationships and relative responses to atmospheric CO2 enric hment were genotype-dependent.