Ozone at AspenFACE: Effects on plant productivity and ecosystem carbon storage

Background/Question/Methods

Concentrations of tropospheric ozone (O₃) and atmospheric carbon dioxide (CO₂) are both predicted to increase in coming decades. Although increases in CO₂ are hypothesized to enhance forest productivity and soil carbon (C) storage, O₃ is phyto-toxic to many species and genotypes and is thought to reduce soil C storage.  In order to examine the effects of these gases on forest productivity and soil C storage, here we present the results of a comprehensive forest harvest in three young northern temperate forest communities exposed to elevated O₃ and/or elevated CO₂ for 12 years.

Results/Conclusions

As factorial effects, total forest biomass was increased 64% by elevated CO₂ (P = 0.008) and decreased 23% by elevated O₃ (P = 0.077). For both CO₂ and O₃, these effects were larger aboveground (CO₂: +76% and P = 0.007, O₃: -27% and P = 0.058) than belowground (CO₂: +33% and P = 0.022, O₃: -9% and P = 0.342).  No significant interactions between CO₂ and O₃ or between these gases and community types were observed in our analysis of these forest biomass pools. Although the changes in aboveground biomass matched hypotheses for the effects of CO₂ and O₃, our results suggest that models cannot simply extrapolate the changes in aboveground biomass to belowground biomass pools. For soil C, we sampled the top 50 cm of soil in 10 cm increments. As factorial effects, CO₂ and O₃ reduced soil C storage in the top 50 cm by 7% and 5%, respectively, but these effects were not statistically significant (P > 0.35). However, elevated O₃ significantly reduced soil C storage in the top 10 cm of soil (-11%, P = 0.042), while elevated CO₂ significantly reduced soil C storage in the 10-20 cm increment (-18%, P = 0.034). For elevated O₃, the observed changes in soil C pools are consistent with simple conceptual models which predict that reduced plant productivity will translate to lower soil C storage. However, changes in productivity under elevated CO₂ do not appear to have the same effect. For elevated CO₂, other research suggests that the increased acquisition of soil N to support greater forest productivity may be responsible for the lack of soil C storage.