Abstract:
Measurements of the forest-atmosphere exchange of carbon isotopes can be used to partition the net total carbon exchange (measured by standard eddy covariance) into its photosynthetic and respiratory components. This partitioning requires an estimate of the isotopic signature of canopy-scale photosynthesis, which has been obtained to date by assuming that the canopy behaves like a single ‘big leaf’. This assumption neglects the heterogeneity of the canopy both vertically and with respect to leaf angles. Leaves at various heights in the canopy and at various angles to the sun experience different sunlight, temperature, and wind. Our research goal was to quantify the relevant characteristics of a range of leaves in the canopy sufficiently to constrain a multi-leaf isotopic partitioning analysis, and to use that analysis to assess errors associated with the usual big leaf approach. Carbon dioxide and water vapor exchange was measured in leaves of the site-dominant deciduous species Quercus rubra and Acer rubrum using a Li-Cor 6400 portable photosynthesis system. In order to characterize canopy heterogeneity, we carried out spot measurements, CO2 response curves, and light response curves at the top (fully sunlit), middle, and bottom (shaded) part of the canopy. A walk-up tower and bucket lift allowed us to take intact gas exchange measurements from terminal leaves of trees in the surrounding area. A multi-leaf isotopic partitioning analysis more accurately models photosynthesis in the canopy and can be used to assess the errors associated with the big leaf assumption.
