Woody biomass production during second rotation of a bioenergy Populus plantation increases in a future high CO2 world.

The quickly rising atmospheric carbon dioxide (CO₂)levels, justify the need to explore all carbon (C) sequestration possibilities that might mitigate the current CO₂increase. Here, we report the likely impact of future increases in atmospheric CO₂on woody biomass production of three poplar species (Populus albaL. clone 2AS11,Populus nigraL. clone Jean Pourtet andPopuluseuramericanaclone I214). Trees were growing in a highdensity coppice plantation during the second rotation (i.e., regrowth after coppice; 20022004; POPFACE/EUROFACE). Six plots were studied, half of which were continuously fumigated with CO₂(FACE; free air carbon dioxide enrichment of 550ppm). Half of each plot was fertilized to study the interaction between CO₂and nutrient fertilization. At the end of the second rotation, selective above and belowground harvests were performed to estimate the productivity of this bioenergy 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 CO₂enhanced 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 CO₂during the first rotation (19992001) 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 CO₂induced biomass increaseafter coppice. These results suggest that, under future CO₂concentrations, managed poplar coppice systems may exhibit higher potential for C sequestration and, thus, help mitigate climate change when used as a source of Cneutral energy.