Elevated CO2 and O3 effects on fine-root survivorship in ponderosa pine mesocosms

Atmospheric carbon dioxide (CO2) and ozone (O-3) 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 O-3 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 O-3 in sun-lit controlled-environment mesocosms for 3 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, O-3, diameter, depth, and season of root initiation on fine-root survivorship. More fine roots were produced in the elevated CO2 treatment than in ambient CO2. 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 O-3, and increased O-3 did not reduce the effect of elevated CO2. Median life spans varied from 140 to 448 days depending on the season of root initiation. 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.