Elevated CO2 and conifer roots: effects on growth, life span and turnover

Elevated CO2 increases root growth and fine (diam. less than or equal to 2 mm) root growth across a range of species and experimental conditions. Howe ver, there is no clear evidence that elevated CO2 changes the proportion of C allocated to root biomass, measured as either the root : shoot ratio or the fine root : needle ratio. Elevated CO2 tends to increase mycorrhizal in fection, colonization and the amount of extramatrical hyphae, supporting th eir key role in aiding the plant to more intensively exploit soil resources , providing a route for increased C sequestration. Only two studies have de termined the effects of elevated CO2 on conifer fine-root life span, and th ere is no clear trend. Elevated CO2 increases the absolute fine-root turnov er rates; however, the standing crop root biomass is also greater, and the effect of elevated CO2 on relative turnover rates (turnover : biomass) rang es from an increase to a decrease. At the ecosystem level these changes cou ld lead to increased C storage in roots. Increased fine-root production cou pled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Althoug h CO2 Can stimulate fine-root growth, it is not known if this stimulation p ersists over time. Modeling studies suggest that a doubling of the atmosphe ric CO2 concentration initially increases biomass, but this stimulation dec lines with the response to elevated CO2 because increases in assimilation a re not matched by increases in nutrient supply.