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Harvard Forest REU Symposium Abstract 2009

  • Title: Leaf and Canopy-level Relationships Between Optical Properties, Photosynthetic Productivity, and Nitrogen Content
  • Author: Megan K Bartlett (Harvard University)
  • Abstract:

    Photosynthetic productivity, average foliar nitrogen content, and albedo are significantly positively correlated at the ecosystem level. This project examines the physiological mechanism inducing this ecosystem-level relationship by examining the connections between these parameters at the level of individual leaves, thereby linking physiological and morphological properties to ecosystem processes that impact global carbon, nutrient, and energy cycles. Because heightened nitrogen content enhances photosynthetic capacity, which increases the structural complexity of leaf cell organization, it was hypothesized that higher nitrogen content would induce more cellular light scattering and therefore correlate with light reflectance and transmittance. Nitrogen content, photosynthetic rate, and abaxial and adaxial reflectance and transmittance were measured for single leaves and stacks of 2, 3, 4, 6, 8, and 12 leaves (as simulations of the canopy) for abundant understory and canopy species at Harvard Forest. Abaxial and adaxial comparisons were used to determine the effect of adaxial cuticular waxes and epidermal features on optical properties. A correlation was found between leaf nitrogen content and photosynthetic rate, as hypothesized, but no association was found between nitrogen content and abaxial or adaxial reflectance or transmittance at any stack thickness. No significant differences were found between abaxial and adaxial optical properties, suggesting that in terms of light scattering they are functionally indistinguishable. The canopy-level association between nitrogen content, photosynthetic rate, and albedo therefore does not reflect leaf-level processes, and may instead be caused by nitrogen-induced changes in tree growth or structure, such as increased canopy thickness or a more reflective leaf angle.

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