Phosphorus (P) is a common limiting nutrient element to plants and its supply and uptake by plants are strongly influenced by soil temperature. However, the interactive effects of the two factors on the physiological responses of plants to global change are poorly understood. In this study, we examined how P supply and T_soil interacted in affecting physiological responses in white birch (Betula papyrifera) to [CO₂]. We exposed seedlings to 7°C, 17°C and 27°C T_soil, 0.1479, 0.3029 and 0.5847 mM P₂O₅, and 360 and 720 μmol·mol^-1 [CO₂] for four months. We have found that both the low soil temperature and CO₂ elevation resulted in photosynthetic down regulation but the specific mechanisms of the down regulation were different between the two treatments, particularly the relative contributions of biochemical and photochemical capacity, mesophyll conductance and sink strength for carbohydrate utilization to the down regulation. Furthermore, our data suggest that morphological adjustments, such as reduced leaf size and total leaf area, were the primary form of responses in white birch to low phosphorus supply and no significant physiological acclimation to P supply was detected. Our results suggest that white birch will likely enhance water use efficiency under the projected future climate conditions with doubled carbon dioxide concentration, particularly at warmer soil temperatures. Although a trade-off between water use efficiency and nutrient use efficiency is widely accepted, our results suggest that there does not have to be a trade-off between the two, for instance, CO₂ elevation increased both use efficiencies and low soil temperature and reduced nitrogen efficiency without affecting water use efficiency under elevated CO₂.