Low soil temperature inhibits the stimulatory effect of elevated [CO2] on height and biomass accumulation of white birch seedlings grown under three non‐limiting phosphorus conditions

White birch (Betula papyrifera Marsh.) seedlings were exposed to ambient or doubled ambient carbon dioxide concentration (CO₂]), three soil temperatures (T_soil) (low, intermediate, high), and three phosphorus (P) regimes (low, medium, high) in environment‐controlled greenhouses. Height (H), root‐collar diameter (RCD), biomass, and leaf phosphorus concentration (leaf P) were determined four months after initiation of treatments. The low T_soil reduced H, RCD, shoot biomass, root biomass and total seedling biomass whereas the high‐P level and the CO₂] elevation increased all the growth and biomass parameters. Elevated CO₂] significantly reduced leaf P. There were significant two‐factor interactions suggesting that the effect of elevated CO₂] on (1) H, total biomass, biomass of plant components, and leaf P was dependent on T_soil, (2) total biomass was contingent on P regime. For instance, the positive response of H and total biomass to elevated CO₂] was limited to seedlings raised under the intermediate and high T_soil, respectively. In addition, CO₂] elevation increased total biomass only at the high‐P regime but not at the low‐ or medium‐P level where the effect of CO₂] was statistically insignificant. No significant main effect of treatment or interaction was observed for root to shoot biomass ratio.