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 (V_Cmax) 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 V_Cmax 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.