What was done
Four Sitka spruce genotypes were collected (two from 53.2° N latitude and two from 41.3° N latitude) and grown in open-top chambers near Edinburgh, UK (55.5° N latitude), for three growing seasons at atmospheric CO2 concentrations of 350 and 700 ppm to study the genotypic growth responses of spruce to elevated CO2.

What was learned
Saplings grown in elevated CO2 were significantly taller than those grown in ambient CO2 throughout the entire experiment.  However, it was not until the end of the third growing season that all four CO2-enriched genotypes exhibited significantly larger dry weights than those of ambiently-grown genotypes.  Thus, these data suggest that when experimenting with long-lived perennial species --like trees-- it is critical to maintain differential CO2 treatments for multiple seasons (the longer, the better) in order to accurately determine their growth responses to elevated CO2.

With respect to genotype, the more southerly clones were significantly taller than the more northerly ones, regardless of treatment CO2 concentration.  At ambient CO2, there were no significant differences in dry mass between genotypes.  However, at elevated CO2, the taller southerly clones produced about 20% more dry mass than did the shorter northerly genotypes, as often is the case with inherently fast-growing species.  Averaged across all genotypes, growth in elevated CO2 increased sapling dry mass by approximately 42% relative to that produced by ambiently-grown genotypes.

What it means
As the CO2 content of the air continues to rise, it is likely that Sitka spruce will significantly increase its biomass, regardless of genotype.  This is of great economic importance in Europe, where this species is grown as a major harvestable forest crop.  Because the southerly clones were a little more responsive to atmospheric CO2 enrichment than the northerly genotypes, forest managers who selectively reforest commercially harvested lands with southerly clones can likely expect greater timber returns in the future, relative to projected yields from northerly clones, as the atmospheric CO2 concentration continues to rise.