Low soil temperature inhibits the stimulatory effect of elevated [CO ₂ ] 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 ([CO 2 ]), 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 [CO 2 ] elevation increased all the growth and biomass parameters. Elevated [CO 2 ] significantly reduced leaf P. There were significant two‐factor interactions suggesting that the effect of elevated [CO 2 ] 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 [CO 2 ] was limited to seedlings raised under the intermediate and high T soil , respectively. In addition, [CO 2 ] elevation increased total biomass only at the high‐P regime but not at the low‐ or medium‐P level where the effect of [CO 2 ] was statistically insignificant. No significant main effect of treatment or interaction was observed for root to shoot biomass ratio.