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 ([CO 2 ]) (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 [CO 2 ] 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 [CO 2 ] effects on biomass allocation or [CO 2 ]×T soil ×moisture interactions on biomass production/allocation.