Are chlorophyll concentrations and nitrogen across the vertical canopy profile affected by elevated CO2 in mature Quercus trees?
Anna Gardner, David S. Ellsworth, Jeremy Pritchard, A. R. MacKenzie
Abstract
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.