Interactive effects of CO2 and soil water treatments on growth and biomass allocation in pines and spruces

Growth, components of growth, and biomass allocation were quantified for eight species in two commercially important genera, Pinus and Picea, grown in a 2 x 2 factorial of atmospheric CO2 and soil moisture stress. Four of the pines and three of the spruces are native to eastern North America; a fourth spruce, Norway spruce (NS: P. abies), is native to Europe but is used for reforestation in northeastern North America. Height, basal diameter (BD), and total biomass response of pines were often more than two times greater than that of spruces under elevated CO2 (eCO(2)). A significant species x CO2 interaction for total biomass was a result of species' differential response to eCO(2): Pinus rigida had the greatest biomass stimulation (59%), followed by P. resinosa (39%), P. strobus (26%), and P. banksiana (19%). Among spruces, Picea glauca showed the greatest response (30%), and P. mariana the least response under eCO(2) (5%). Overall, soil moisture stress reduced total productivity by 12%. Most pines did have greater growth under moisture stress, and NS and BS grew well. Percent needle mass was lower under eCO(2), but this was not due directly to eCO(2) but to ontological changes. Controlling for size, pines had 20% greater needle biomass than spruces, while having a negative relationship to total biomass. A comparison of total biomass under eCO(2) in relation to aCO(2) by species showed that overall, the greater the species mass, the greater the mass gain under eCO(2), and the greater the mass loss under drought conditions. In addition, our results for spruces lend strong support to the theory that late-successional species have greater growth response under eCO(2) than early to mid-successional species. A diverse portfolio of tree species for artificial reforestation would help forest management adapt to the many uncertainties over future environments and markets, but our results on responses in spruces and pines to eCO(2) and soil moisture stress support a shift toward increased use of pines in forest management and artificial reforestation.