WWRC 85-39
Influence of Shoot Structure on Light Interception and Photosynthesis in Conifers
Abstract
The influence of shoot structure on net photosynthesis was evaluated
under field conditions for the central Rocky Mountain (United States)
conifers Picea engelmannii (Parry ex Engelm.), Abies lasiocarpa ([Hook]
Nutt.), and Pinus contorta (Engelm.). In all species, the greater number
of needles per unit stem length on sun shoots correlated with a smaller
silhouette leaf area to total leaf area ratio (STAR). Decreased STAR was
due primarily to greater needle inclination toward the vertical, plus some
needle mutual shading. However, photosynthesis expressed on a total
leaf area basis did not decrease in sun shoots (lower STAR) but remained
nearly constant at approximately 3 micromoles per square meter per
second over a wide range of STAR (O.1 to 0.3). Relatively low light
saturation levels of 200 to 1400 microeinsteins per square meter per
second and diffuse light to 350 microeinsteins per meter per second
maintained photosynthetic flux densities in inclined and/or shaded needles
at levels comparable to those in unshaded needles oriented perpendicular
to the solar beam. As a result, net CO2 uptake per unit stem length
increased as much as 2-fold in sun shoots (low STAR) in direct proportion
to increasing needle density.
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