The long-term effects of CO2 enrichment on fine root productivity, mortality, and survivorship in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA

Fine root dynamics play an important role in the cycling of carbon belowground. Previous studies have indicated that CO2 enrichment results in increased root productivity, mortality and relative turnover; however, our understanding of the duration and long-term trends of this effect are limited. Non-destructive minirhizotron observation tubes were used to measure effects of elevated CO2 on root dynamics and survivorship in a fire dominated scrub-oak ecosystem. Open-top chambers were exposed to elevated atmospheric CO2 for 10 years at Kennedy Space Center, Florida. In this study, initial fine root dynamics from an earlier published study from this experiment (Dilustro et al., 2002) were compared to our findings 5 years later. Significant increases in root productivity, mortality, and turnover due to CO2 enrichment were no longer present after 9 years of treatment. However, the vertical variation in these parameters suggests the upper 50 cm of the soil are the most dynamic. A greater proportion of the fine roots were deeper in the soil profile later in the study, but no CO2 effect was observed. Survivorship analysis suggested the smallest fine roots (i.e. \u3c 0.1 mm in diameter and \u3c 0.25 mm in length) were most susceptible to mortality. In addition, increased root persistence was correlated with greater soil depth, suggesting that a nutrient and water limited scrub-oak ecosystem at root closure or carrying capacity produces larger, longer-lived fine roots at greater depths. Mean root diameter increased in the upper and lower portions of the soil profile. Seasonal cohort analysis implied that roots appearing in the spring and summer typically had the highest risk of mortality in the fall, although environmental factors influencing this relationship are not clear. The results from this study indicated that CO2 enrichment is no longer driving changes in fine root dynamics, but rather root closure in the upper portions of the soil profile seem to be the strongest influence. Fine roots comprise nearly 25% of the total plant biomass in the scrub-oak ecosystem and their turnover and persistence is an important pathway for carbon inputs into the soil. In order to develop accurate predictive models of the impacts of increasing anthropogenic CO2 on carbon cycling, it is imperative to examine long-term fine root dynamics rather than just shorter observations that could result in misleading conclusions regarding ecosystem responses. (C) 2010 Elsevier B.V. All rights reserved