Foliage determinants of light interception in sunny and shaded branches of Fagus sylvatica (L.)

Abstract

A 3D digitising device was used to acquire architectural information on branches from open-grown and understory saplings of Fagus sylvatica (L.). Branch and shoot images were reconstructed from the data set using an image synthesis software (freeware POV-Ray®). Image analysis enabled us to: (i) quantitatively investigate light interception at the shoot and the branch scales; (ii) to derive relevant structural parameters at both scales; (iii) to relate the observed shift with light environment on the independent effects of leaf inclination, within-shoot shading, shoot density and dispersion within the branch. Leaf display pattern, both at the branch and the shoot scale, was strongly affected by light availability. At the shoot level, reduced self-shading in shaded conditions resulted in an increased light capture as high as 32%, expressed on a leaf area basis in the vertical direction. At the branch level, shaded environment caused a substantial increase of 60%. Considering that incident irradiance varies by a factor of 20 between understory and open conditions, plasticity in branch architecture allows to reduce the discrepancy in the amount of intercepted light to a factor 12. This difference in light capture ability at the branch level was shown to result from: (i) more horizontally inclined leaves in shade; (ii) a greater branch leaf area index in sunny conditions (on average 1.60 vs. 0.95 in shaded conditions); (iii) a more regular shoot dispersion in shade (shoot dispersion index averaged 1.30 vs. 1.10 in the open). Finally, results are discussed in terms of optimal architecture for carbon gains, and the interest of the method for parameterisation of radiative transfer models is emphasised.