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

Background/Question/Methods Meta-analyses reveal that fast growing species have a greater relative growth response to elevated CO₂ than slow growing species. It is unknown whether this is a direct response, or if inter-specific differences in growth are simply correlated with other physiological or morphological differences among species that affect the growth response to CO₂. Here we determine if intra-specific variation in the growth response to elevated CO₂ in Picea glauca is also related to variation in growth rate, or if it is related to variation in other genetic traits. Relative growth rate (RGR) of 29 genotypes grown at ambient (370 µL L^-1) or elevated CO₂ (740 µL L^-1) was measured and the relationship between growth at ambient versus elevated CO₂ examined using linear regression analysis. 

Results/Conclusions Although there was a strong correlation between RGR740 and RGR370, we found no evidence that genotypes that grew fast at ambient CO₂ had a greater relative growth response to CO₂ than genotypes that grew more slowly (i.e. the slope of the linear relationship between RGR740 and RGR370 was not significantly different from one). Variation in 12 physiological and morphological traits describing differences in allocation, canopy structure, stomatal function and photosynthesis were also determined for this set of 29 genotypes. Multiple correlation analysis revealed that four of these traits, all of them related to differences in canopy structure were significantly associated with variation in RGR740. Collectively, these four traits explained 74% of the variation in RGR740, the single most important trait being the slope of the allometric relationship between plant height and stem diameter. Given that the pattern found at the intra-specific level differed from that previously reported at the inter-specific level, our results suggest differences in relative growth rate per se do not affect the growth response to CO₂. Rather, the CO2 growth response is determined by traits that may or may not be correlated with relative growth rate.