Photosynthesis of coppicing poplar clones in a free-air CO2 enrichment (FACE) experiment in a short-rotation forest

Photosynthetic capacity was assessed in coppices of three poplar clones (Populus alba L. genotype 2AS11, P. × euramericana (Dode) Guinier genotype I-214 and P. nigra L. genotype Jean Pourtet) growing in the POPFACE/EUROFACE free-air CO2 enrichment experiment in central Italy. Plants were grown either at an elevated CO2 concentration of 550 μmol mol–1 or in control conditions for 3 years and were then harvested and allowed to coppice. Plants were either fertilised with the addition of liquid fertiliser at a level of 212 kg N ha–1 year–1 or unfertilised after harvesting. No evidence was found of changes in the maximum Rubisco carboxylation rate (VCmax) and thus there was no photosynthetic downregulation caused by the FACE treatment in either P. × euramericana or P. nigra, but there was a marginally significant reduction in VCmax of fertilised P. alba (P<0.09). Carbon assimilation rates were significantly higher in FACE plants than control plants. Maximum carbon assimilation rate was stimulated by an average of 32.8% in these clones, with individual stimulation values of 27.6% for P. alba, 32.1% for P. × euramericana and 49.5% for P. nigra. No significant interactions between the FACE and fertilisation treatments were found for any of the photosynthetic variables measured. The day respiration rate in leaves of P.�×�euramericana was significantly increased by FACE treatment, but it was unaffected in the other clones. This work shows that photosynthesis remains stimulated at elevated CO2 concentration in these plants following harvesting, although to a lesser extent than seen normally, which may be related to a reduction in sink strength.