Elevated CO 2 and conifer roots: effects on growth, life span and turnover

Elevated CO 2 increases root growth and fine (diam. 2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO 2 changes the proportion of C allocated to root biomass, measured as either the root∶shoot ratio or the fine root∶needle ratio. Elevated CO 2 tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO 2 on conifer fine‐root life span, and there is no clear trend. Elevated CO 2 increases the absolute fine‐root turnover rates; however, the standing crop root biomass is also greater, and the effect of elevated CO 2 on relative turnover rates (turnover∶biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine‐root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO 2 can stimulate fine‐root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO 2 concentration initially increases biomass, but this stimulation declines with the response to elevated CO 2 because increases in assimilation are not matched by increases in nutrient supply.