Background/Question/Methods Atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations are rising, which may have opposing effects on tree C balance and allocation to fine roots.  More information is needed on interactive CO₂ and O₃ effects on roots, particularly fine-root life span, a critical demographic parameter and determinant of soil C and N pools and cycling rates.  We conducted a study in which ponderosa pine (Pinus ponderosa) seedlings were exposed to two levels of CO₂ and O­₃ in sun-lit controlled-environment terracosms for three years.  Minirhizotrons were used to monitor individual fine roots in three soil horizons every 28 days.  Proportional hazards regression was used to analyze effects of CO₂, O₃, diameter, depth, and season of root initiation on fine-root survivorship. 

Results/Conclusions More fine roots were produced in the elevated CO₂ treatment than in ambient CO₂.  Median life spans varied from 140-448 days depending on the season of root initiation.  Elevated CO₂, increasing root diameter, and increasing root depth all significantly increased fine-root survivorship and median life span.  Life span was slightly, but not significantly, lower in elevated O₃, and increased O₃ did not reduce the effect of elevated CO₂.  These results indicate the potential for elevated CO₂ to increase the number of fine roots and their residence time in the soil, which is also affected by root diameter, root depth, and phenology.