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TitleMeasurement of forest structure with hemispherical photography
AuthorLeblanc, S GORCID logo; Fournier, R A
SourceHemispherical photography for forest science; Theory, methods, applications; by Fournier, R A (ed.); Hall, R (ed.); 2017 p. 53-83, 3
Alt SeriesEarth Sciences Sector, Contribution Series 20110002
PublisherSpringer Netherlands
Mediapaper; on-line; digital
File formatpdf
Subjectsremote sensing; photography; Beer's law
Illustrationsschematic diagrams; formulae; graphs
ProgramPeople Support
Released2017 05 14
AbstractThis chapter presents the theoretical concepts necessary to link optical field sensor data with forest structural attributes. These concepts are important for all the other chapters of this book referring to measurement of forest structure. Forest stand architecture is relatively complex and spatially variable, and its measurements have been done from many approaches: from traditional forest measurements in ground plots (sample plots from the forest inventory), to statistical representation of forest attributes with the use of allometric relationships, and towards the use of optical sensors. Thus, a short description of the forest parameters routinely measured will be followed by the most common allometric approaches which will also be followed by specific measurements that optical field instruments provide. The emphasis will be placed on the optical sensors measuring light transmission through forest canopy using hemispherical view, and more specifically on the use of hemispherical photographs.
The Beer's law is a theoretical relationship used to calculate the transmission of light in a turbid medium. With increasing path through the canopy, the reduction of light transmission depends greatly on the amount and spatial distribution of forest elements. Forest canopy structural attributes can therefore be extracted from this mathematical formulation, in particular from the use of the canopy gaps as seen from a hemispherical sensor or photograph. However, Beer's law assumes a turbid medium or random distribution of obstructing elements of negligible size, which is not the case in forest canopies. Therefore the original Beer¿s law was adapted for its application in forest. The main adjustments account for including significant complex structural aspects of forest environments, namely the apparent projection of conifer shoots, the relative contribution of wood and foliage structures to light obstruction, foliage transmittance and reflectance and the clumping of foliage components at the shoot, the branch, the tree and the canopy levels. The inversion of Beer's law is often used to extract many of these structural parameters. Finally, scaling issues are introduced from single trees to stand and from stand to area mapping with satellite sensing.

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