The leaf morphological and stomatal characteristics of four paper birch (Betula papyrifera Marsh) populations, grown at four treatment conditions of carbon dioxide [CO₂] and soil water levels were investigated to determine whether future increases in atmospheric [CO₂] and water deficit affected the leaf characteristics. The populations from Cussion Lake, Little Oliver, Skimikin and Wayerton were grown for 12 weeks under ambient (360 ppm) and elevated (720 ppm) [CO₂] at both high and low water levels. The populations significantly differed in leaf area and stomatal characteristics due to the interaction effects of [CO₂], water levels and population differences. Most leaf morphological characteristics and stomatal density varied due to the effects of [CO₂] and/or populations, but not due to the effect of water levels. Although elevated [CO₂] alone barely affected stomatal area of the birch populations, simultaneous elevated [CO₂] at both water levels had stimulated stomatal characteristics within and among the populations. Overall, elevated [CO₂] reduced leaf area and increased stomatal density; and low water level resulted in smaller stomatal area, pore area and guard cell width. However, the populations responded differently to an increase in [CO₂] and water levels. All populations showed plastic responses with respect to [CO₂] and water levels either by decreasing stomatal area under low water level or by increasing stomatal density under elevated [CO₂]. Hence, integration between and within leaf characteristics had helped paper birch populations maintain balance between [CO₂] gain and water loss.