Seasonal fine‐root biomass development of sour orange trees grown in atmospheres of ambient and elevated CO2 concentration

Sour orange trees have been grown from the seedling stage out‐of‐doors at Phoenix, Arizona, USA, in open‐top enclosures with clear plastic walls for 3.5 years. For the last 3 years of this period, half of the trees have been continuously exposed to air enriched with CO2 to 300 μmol mol−1 above the ambient concentration. At 2‐month intervals over the last 12 months, we have determined the fine‐root biomass in the top 0.4 m of the soil profile beneath the trees. Results from both treatments define a single relationship between fine‐root biomass and trunk cross‐sectional area. The data also show the CO2‐enriched trees to have approximately 2.3 times more fine‐root biomass in this soil layer than the trees grown in ambient air.