Elevated CO2 will not select for enhanced growth in white spruce despite genotypic variation in response

The effect of elevated atmospheric CO₂ on plant growth has been well-documented in the literature. However, few studies have quantified intra-specific genetic variation in growth response, and the potential for natural and artificial selection to act upon this variation. This study examined intra-specific variation in growth response to elevated CO₂ in 29 genotypes of white spruce (Picea glauca), a widely distributed and economically important species of the boreal forest region in North America. Trees were exposed to either ambient (370 μL L^?1) or twice-ambient CO₂ (740 μL L^?1). The opportunity for selection (i.e. the relative variation in fitness) was determined at low and high CO₂ levels with size as a measure of fitness and heritability of this variation determined. There was considerable variation among the genotypes in size and response to elevated CO₂. The increase in mass at elevated CO₂ ranged from 23% to 108% depending upon genotype. In spite of this variation, the genetic correlation between the two environments approached unity, as genotype variance was much greater than the genotype×CO₂ variance. Elevated CO₂ had no effect on heritability of the size-related traits we examined, and either had no effect on opportunity for selection, or decreased it. We conclude that selection at elevated atmospheric CO₂ is unlikely to increase mean plant size in white spruce beyond that observed for present day populations grown at elevated CO₂, despite the substantial genetic variation in CO₂ response displayed by this species.