First and second-year aboveground growth and productivity of two Populus hybrids grown at ambient and elevated CO2

Two hybrid poplar (Populus) clones (the fast-growing clone Beaupre (P. trichocarpa Torr. and Gray x P. deltoides Bartr. ex Marsh.) and the slow-growing clone Robusta (P. deltoides Bartr. ex Marsh. x P. nigra L.)) were grown from hardwood cuttings for one or two growing seasons (1993-1994) in either ambient or elevated (= ambient +350 micromole mol-1) CO2 in open-top chambers at the University of Antwerpen. Both clones responded positively to the elevated CO2 treatment with increased stem volume and aboveground biomass production; however, the clones exhibited different response strategies to the elevated CO2 treatment, and the responses varied with cutting age and duration of exposure. Clone Beaupre responded to the elevated CO2 treatment with increases in leaf area and leaf area index during both the first and second growing seasons, but little increase in height growth. Clone Robusta exhibited increased height growth, leaf biomass and total leaf nitrogen content in response to elevated CO2, but no increase in leaf area index. The elevated CO2 treatment increased the total number of branches and total branch biomass in both clones during both growing seasons. At the end of the first growing season, woody stem biomass of the fast- and slow-growing clones was increased by 38 and 55%, respectively. At the end of the second growing season, stem volume was increased by 43% in clone Beaupre and by 58% in clone Robusta. The increase in stem volume was a result of the stimulation of both height and diameter growth in the Slow-growing clone, whereas only height growth was stimulated in the fast-growing clone. In the fall of the first growing season, the average date of bud set in clone Robusta was advanced by 4 days in the elevated CO2 treatment; there were no other significant effects of the elevated CO2 treatment on bud set. The elevated CO2 treatment enhanced leaf C/N ratios in both clones in both years.