Growth and ecophysiological responses of black spruce seedlings to elevated CO₂ under varied water and nutrient additions

Two controlled-environment studies examined growth and ecophysiological responses of black spruce (Piceamariana (Mill.) B.S.P.) seedlings to elevated CO 2 under varied water and nutrient additions. Growth analyses were conducted followed by measurements of gas exchange, xylem pressure potential and foliar N concentrations. Growth under elevated CO 2 (700 ppm) increased final seedling dry weights by 20–48% compared with seedling growth under ambient CO 2 (350 ppm). Percent increases in seedling dry weight were greater under drought versus well-watered conditions and higher versus lower nutrient additions. Seedlings grown under elevated CO 2 displayed higher water use efficiency than seedlings grown under ambient CO 2 . This was apparent based upon instantaneous gas exchange as well as xylem potential pressure measurements. Elevated CO 2 -induced stimulation of relative growth rate was greatest shortly after seedling emergence and decreased with increased seedling size. Acclimation of net photosynthesis was observed and was reversible. Analyses using allometric principles indicate net photosynthetic acclimation resulted from: (i) growth-induced nutrient dilution; (ii) a decrease in foliar N levels not owing to dilution; and (iii) a decrease in net photosynthetic activity.