FIRST-YEAR AND 2ND-YEAR ABOVEGROUND GROWTH AND PRODUCTIVITY OF 2 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 sl ow-growing clone Robusta (P deltoides Bartr. ex Marsh. x P. nigra L.)) were grown from hardwood cuttings for one or two growing seasons (199 3-1994) in either ambient or elevated (= ambient + 350 mu mol 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 exhibi ted different response strategies to the elevated CO2 treatment, and t he responses varied with cutting age and duration of exposure. Clone B eaupre responded to the elevated CO2 treatment with increases in leaf area and leaf area index during both the first and second growing seas ons, but little increase in height growth. Clone Robusta exhibited inc reased height growth, leaf biomass and total leaf nitrogen content in response to elevated CO2, but no increase in leaf area index. The elev ated CO2 treatment increased the total number of branches and total br anch biomass in both clones during both growing seasons. At the end of the first growing season, woody stem biomass of the fast- and slow-gr owing clones was increased by 38 and 55%, respectively. At the end of the second growing season, stem volume was increased by 43% in clone B eaupre 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 sl ow-growing clone, whereas only height growth was stimulated in the fas t-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 elevate d CO2 treatment; there were no other significant effects of the elevat ed CO2 treatment on bud set. The elevated CO2 treatment enhanced leaf C/N ratios in both clones in both years.