The quickly rising atmospheric carbon dioxide (CO2)-levels, justify the need to explore all carbon (C) sequestration possibilities that might mitigate the current CO2 increase. Here, we report the likely impact of future increases in atmospheric CO2 on woody biomass production of three poplar species (Populus alba L. clone 2AS-11, Populus nigra L. clone Jean Pourtet and Populus euramericana clone I-214). Trees were growing in a high-density coppice plantation during the second rotation (i.e., regrowth after coppice; 2002-2004; POPFACE/EUROFACE). Six plots were studied, half of which were continuously fumigated with CO2 (FACE; free air carbon dioxide enrichment of 550 ppm). Half of each plot was fertilized to study the interaction between CO2 and nutrient fertilization. At the end of the second rotation, selective above-and belowground harvests were performed to estimate the productivity of this bio-energy plantation. Fertilization did not affect growth of the poplar trees, which was likely because of the high rates of fertilization during the previous agricultural land use. In contrast, elevated CO2 enhanced biomass production by up to 29%, and this stimulation did not differ between above-and belowground parts. The increased initial stump size resulting from elevated CO2 during the first rotation (1999-2001) could not solely explain the observed final biomass increase. The larger leaf area index after canopy closure and the absence of any major photosynthetic acclimation after 6 years of fumigation caused the sustained CO2-induced biomass increase after coppice. These results suggest that, under future CO2 concentrations, managed poplar coppice systems may exhibit higher potential for C sequestration and, thus, help mitigate climate change when used as a source of C-neutral energy.

The goal of this study was to investigate whether increased nitrogen use efficiency found in Populus nigra L. (Jean Pourtet) under elevated CO2 would correlate with changes in the production of carbon-based secondary compounds (CBSCs). Using Free-Air CO2 Enrichment (FACE) technology, a poplar plantation was exposed to either ambient (about 370 μmol mol-1 CO2) or elevated (about 550 μmol mol-1 CO2) [CO2] for 5 years. After three growing seasons, the plantation was coppiced and half of the experimental plots were fertilized with nitrogen. CBSCs, total nitrogen and lignin-bound nitrogen were quantified in secondary sprouts in seasons of active growth and dormancy during 2 years after coppicing. Neither elevated CO2 nor nitrogen fertilisation alone or in combination influenced lignin concentrations in wood. Soluble phenolics and soluble proteins in wood decreased slightly in response to elevated CO2. Higher nitrogen supply stimulated formation of CBSCs and increased protein concentrations. Lignin-bound nitrogen in wood ranged from 0.37-1.01 mg N g-1 dry mass accounting for 17-26% of total nitrogen in wood, thus, forming a sizeable nitrogen fraction resistant to chemical degradation. The concentration of this nitrogen fraction was significantly decreased by elevated CO2, increased in response to nitrogen fertilisation and showed a significant CO2 × fertilisation interaction. Seasonal changes markedly affected the internal nitrogen pools. Soluble proteins in wood were 52-143% higher in the dormant than in the growth season. Positive correlations existed between the biosynthesis of proteins and CBSCs. The limited responses to elevated CO2 and nitrogen fertilisation indicate that growth and defence are well orchestrated in P. nigra and that changes in the balance of both resources-nitrogen and C-have only marginal effects on wood chemistry.