Objective  Poplar leaves and fine roots play a key role in the carbon and nutrient cycle (such as nitrogen and phosphorus) in the forest ecosystem. However, the response of fine roots to ozone (O₃) stress has been poorly studied, and the difference in responses of leaves and fine roots to O₃ is still unclear. This study thus aimed to clarify the difference in responses to O₃ dose of leaves and fine roots, two rapidly decomposing organs, and to provide reference for understanding the response mechanism of poplar to O₃ pollution from the perspective of aboveground and belowground feedback.

  Method  Five O₃ concentration levels were set with the open-top chambers to explore whether the functional traits of poplar leaves and fine roots respond differently to O₃ dose.

  Result  There was significant hormesis on the response of tannin content of leaves and P content of fine roots to O₃ stress, which showed a positive effect before toxicological O₃ threshold and a negative effect after toxicological O₃ threshold. Leaf saturated photosynthetic rate, fine root biomass and soluble sugar content of leaves and fine roots showed a toxicological O₃ threshold, but not significant hormesis. Water use efficiency and the biomass of leaves, stems, coarse roots and total roots decreased linearly with O₃ dose increasing. The ratio of fine roots to leaves in the content of C, N, soluble sugar, lignin, and lignin∶N did not change with O₃ dose increasing. The ratio of fine roots to leaves in the content of P, starch, and TNC increased with the O₃ dose increasing, while tannin content first declined and then rose with O₃ dose increasing (O₃ threshold value of 37.25 μmol/mol·h).

  Conclusion  These results indicate that the significant differences in the responses of leaves and fine roots to O₃ pollution will result in marked changes in the relative belowground roles of these two litter sources within poplar plantations. O₃ stress could induce more P and non-structural carbohydrates to fine roots for storage relative to leaves, which might be a coping strategy for poplar exposed to O₃ stress.