GROWTH AND ECOPHYSIOLOGICAL RESPONSES OF BLACK SPRUCE SEEDLINGS TO ELEVATED CO2 UNDER VARIED WATER AND NUTRIENT ADDITIONS

Two controlled-environment studies examined growth and ecophysiologica l responses of black spruce (Picea mariana (Mill.) B.S.P.) seedlings t o elevated CO2 under varied water and nutrient additions. Growth analy ses were conducted followed by measurements of gas exchange, xylem pre ssure potential and foliar N concentrations. Growth under elevated CO2 (700 ppm) increased final seedling dry weights by 20-48% compared wit h seedling growth under ambient CO2 (350 ppm). Percent increases in se edling dry weight were greater under drought versus well-watered condi tions and higher versus lower nutrient additions. Seedlings grown unde r elevated CO2 displayed higher water use efficiency than seedlings gr own under ambient CO2. 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 aft er seedling emergence and decreased with increased seedling size. Accl imation of net photosynthesis was observed and was reversible. Analyse s using allometric principles indicate net photosynthetic acclimation resulted from: (i) growth-induced nutrient dilution; (ii) a decrease i n foliar N levels not owing to dilution; and (iii) a decrease in net p hotosynthetic activity.