Canopy height affects the allocation of photosynthetic carbon and nitrogen in two deciduous tree species under elevated CO.

Down-regulation of leaf N and Rubisco under elevated CO (eCO) are accompanied by increased non-structural carbohydrates (NSC) due to the sink-source imbalance. Here, to investigate whether the canopy position affects the down-regulation of Rubisco, we measured leaf N, NSC and N allocation in two species with different heights at maturity [Fraxinus rhynchophylla (6.8 ± 0.3 m) and Sorbus alnifolia (3.6 ± 0.2 m)] from 2017 to 2019. Since 2009, both species were grown at three different CO concentrations in open-top chambers: ambient CO (400 ppm; aCO); ambient CO × 1.4 (560 ppm; eCO1.4); and ambient CO × 1.8 (720 ppm; eCO1.8). Leaf N per unit mass (N) decreased under eCO, except under eCO1.8 in S. alnifolia and coincided with increased NSC. NSC increased under eCO in F. rhynchophylla, but the increment of NSC was greater in the upper canopy of S. alnifolia. Conversely, Rubisco content per unit area was reduced under eCO in S. alnifolia and there was no interaction between CO and canopy position. In contrast, the reduction of Rubisco content per unit area was greater in the upper canopy of F. rhynchophylla, with a significant interaction between CO and canopy position. Rubisco was negatively correlated with NSC only in the upper canopy of F. rhynchophylla, and at the same NSC, Rubisco was lower under eCO than under aCO. Contrary to Rubisco, chlorophyll increased under eCO in both species, although there was no interaction between CO and canopy position. Finally, photosynthetic N content (Rubisco + chlorophyll + PSII) was reduced and consistent with down-regulation of Rubisco. Therefore, the observed N reduction under eCO was associated with dilution due to NSC accumulation. Moreover, down-regulation of Rubisco under eCO was more sensitive to NSC accumulation in the upper canopy. Our findings emphasize the need for the modification of the canopy level model in the context of climate change.