Why do genotypes of Picea glauca differ in their growth response to elevated CO₂?

Meta-analyses reveal that fast-growing species have a greater growth response to elevated CO(2) than slow-growing species. It is unknown whether this is a direct response or whether inter-specific differences in growth are simply correlated with other physiological or morphological differences among species that affect the growth response to CO(2). Here we use intra-specific variation in Picea glauca to examine the mechanistic basis for this relationship. Relative growth rate (RGR) of 29 genotypes grown at ambient (370 µl l(-1)) or elevated (740 µl 1(-1)) CO(2) was measured. Physiological and morphological traits describing differences in allocation, canopy structure, stomatal function and photosynthesis were determined. Most variation in RGR (74%) was explained by traits associated with canopy structure. Although there was a strong correlation between RGR(740) and RGR(370), we found no evidence that genotypes that grew fast at ambient CO(2) had a greater relative growth response to CO(2). Given that the pattern found at the intra-specific level differed from that reported at the inter-specific level, our results suggest that RGR per se does not affect the growth response to CO(2). Rather, the CO(2) growth response is determined by traits that may or may not be correlated with RGR.