Low moisture availability reduces the positive effect of increased soil temperature on biomass production of white birch (Betula papyrifera) seedlings in ambient and elevated carbon dioxide concentration

White birch (Betula papyrifera Marsh.) seedlings were grown under two carbon dioxide concentrations (CO₂]) (360 vs 720 μmol mol^?1), three soil temperatures (T_soil) (5, 15, 25°C initially, increased to 7, 17, 27°C, respectively, one month later), and three moisture regimes (low: 30–40%, intermediate: 45–55%, high: 60–70% field water capacity) for four months in environment‐controlled greenhouses. The dry mass of stem, leaves, and roots was measured after 2 and 4 months of treatment. Low T_soil decreased stem, leaf and total biomass in both measurements, however, the decrease was significantly greater in the elevated than ambient CO₂] after 4 months. Intermediate T_soil increased root biomass in both measurements. Low moisture reduced stem, leaf, root and total biomass after both 2 and 4 months of treatment. There was a significant T_soil‐moisture interactive effect on leaf, root, and total biomass after 4 months of treatment, suggesting that the magnitude of biomass enhancement in warmer T_soil was dependent on the moisture regime. For instance, the increase in total biomass from the low to high T_soil was 22, 50, and 47% under the low, intermediate and high moisture regimes, respectively. In contrast, the T_soil×moisture effect on stem biomass was significant after 2 months, but not after 4 months of treatment. High T_soil increased leaf mass ratio (LMR) after 4 months of treatment, but decreased both root mass ratio (RMR) after both 2 and 4 months, and root:shoot ratio (RSR) after 4 months of treatment. The low moisture regime decreased LMR after 2 and 4 months of treatment, but increased RSR after 4 months of treatment. There were no significant CO₂] effects on biomass allocation or CO₂]×T_soil×moisture interactions on biomass production/allocation.