Validating canopy clumping retrieval methods using hemispherical photography in a simulated Eucalypt forest

Woodgate, W, Armston, J, Disney, M, Suárez Barranco, L, Jones, S, Hill, M, Wilkes, P and Soto-Berelov, M 2017, 'Validating canopy clumping retrieval methods using hemispherical photography in a simulated Eucalypt forest', Agricultural and Forest Meteorology, vol. 247, pp. 181-193.

Document type: Journal Article
Collection: Journal Articles

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Title Validating canopy clumping retrieval methods using hemispherical photography in a simulated Eucalypt forest
Author(s) Woodgate, W
Armston, J
Disney, M
Suárez Barranco, L
Jones, S
Hill, M
Wilkes, P
Soto-Berelov, M
Year 2017
Journal name Agricultural and Forest Meteorology
Volume number 247
Start page 181
End page 193
Total pages 13
Publisher Elsevier BV
Abstract The so-called clumping factor (Ω) quantifies deviation from a random 3D distribution of material in a vegetation canopy and therefore characterises the spatial distribution of gaps within a canopy. Ω is essential to convert effective Plant or Leaf Area Index into actual LAI or PAI, which has previously been shown to have a significant impact on biophysical parameter retrieval using optical remote sensing techniques in forests, woodlands, and savannas. Here, a simulation framework was applied to assess the performance of existing in situ clumping retrieval methods in a 3D virtual forest canopy, which has a high degree of architectural realism. The virtual canopy was reconstructed using empirical data from a Box Ironbark Eucalypt forest in Eastern Australia. Hemispherical photography (HP) was assessed due to its ubiquity for indirect LAI and structure retrieval. Angular clumping retrieval method performance was evaluated using a range of structural configurations based on varying stem distribution and LAI. The CLX clumping retrieval method (Leblanc et al., 2005) with a segment size of 15° was the best performing clumping method, matching the reference values to within 0.05 Ω on average near zenith. Clumping error increased linearly with zenith angle to > 0.3 Ω (equivalent to a 30% PAI error) at 75° for all structural configurations. At larger zenith angles, PAI errors were found to be around 2530% on average when derived from the 5560° zenith angle. Therefore, careful consideration of zenith angle range utilised from HP is recommended. We suggest that plot or site clumping factors should be accompanied by the zenith angle used to derive them from gap size and gap size distribution methods. Furthermore, larger errors and biases were found for HPs captured within 1 m of unrepresentative large tree stems, so these situations should be avoided in practice if possible.
Subject Earth Sciences not elsewhere classified
Keyword(s) 3D modelling
Hemispherical photography
Leaf Area Index
Woody-to-total area
DOI - identifier 10.1016/j.agrformet.2017.07.027
Copyright notice © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
ISSN 0168-1923
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