Growth and allocation of Picea rubens, Picea mariana, and their hybrids under ambient and elevated CO2

Red spruce (RS; Picea rubens Sarg.) - black spruce (BS; Picea mariana (Mill.) B.S.P.) controlled crosses (100%, 75%, 50%, 25%, and 0% RS, balance BS) showed increasingly greater height with increasing proportion of BS in each successive year. Height growth of 4-year-old ambient CO2 (aCO(2)) grown trees was highly correlated with height of 22-year-old field-grown trees of the same or similar crosses. Bud flush was earliest in BS and declined linearly with increasing proportion of RS with no significant CO2 effect. Percent stem (stem + branches) mass increased under elevated CO2 (eCO(2)), a quarter of which was due to ontogeny. Conversely, percent needle mass had a significant negative relationship with increasing tree size, and there was a CO2 x tree size interaction. Shoot-to-root ratio was greatest for BS, whereas RS had among the lowest. Hybrid index (HI) 50 had the greatest root mass allocation, lowest shoot-to-root ratio, and among the greatest total mass under eCO(2). Growth efficiency increased with tree size and eCO(2) but decreased with HI. Percent total biomass stimulation under eCO(2) was lowest for BS at 6.5%, greatest for HI 50 at 20.3%, and RS had 17.5%.