Abstract
Key message In mature Q. robur, chlorophyll varied with season and canopy height, whilst eCO 2 -driven changes were consistent with M area, highlighting key factors for consideration when scaling photosynthetic processes and canopy N-use. Nitrogen-rich chlorophyll and carotenoid pigments are important in photosynthetic functioning. Photosynthetic pigments have been found to decrease with elevated CO 2 (eCO 2 ), but few such studies have been done in aged forest trees. This study aimed to assess the effects of eCO 2 (150 μmol mol −1 above ambient) and canopy position on chlorophyll content in mature Quercus robur ( Q. robur ). Over 5000 in situ chlorophyll absorbance measurements, alongside laboratory chlorophyll extractions, were collected on canopy-dominant Q. robur in the 3rd and 4th season of CO 2 fumigation of a free-air CO 2 enrichment (FACE) study in central England. Mass-based chlorophyll concentration (Chl mass , mg g −1 ) was significantly higher in the lower canopy compared to upper canopy foliage ( P < 0.05). In contrast, significantly higher chlorophyll content (Chl area , mg m −2 ) was observed in the upper canopy. ECO 2 did not affect Chl mass but Chl area significantly increased, attributable to increased leaf mass per unit area (M area , g m −2 ). We found no effect of eCO 2 on mass-based or area-based nitrogen ( N mass , mg g −1 or N area g m −2 ); however, N area significantly increased with canopy height, again attributable to M area . The parallel relationships between M area , N area and Chl area suggest the allocation of N to light harvesting is maintained with eCO 2 exposure as well as in the upper canopy, and that increased photosynthetic mass may help regulate the eCO 2 variation. An understanding of changes in the light-harvesting machinery with eCO 2 will be useful to assess canopy processes and, at larger scales, changes in biogeochemical cycles in future climate scenarios.