Evaluation of practical schemes for correcting multiple scattering effects on gap fraction measurements by plant canopy analyzer LAI-2000/2200: implications for hemispherical photography and terrestrial laser scanners
Abstract
Correct estimates of gap fraction are essential for quantifying canopy architectural variables, such as leaf area and clumping indices, which modify land-atmosphere interactions. The plant canopy analyzer (PCA) LAI-2000/2200 has been in use for the non-destructive determination of leaf area index (LAI), and other structural attributes of vegetative canopies for over 25 years (Li-Cor, 1989). The instrument is considered the world standard for indirect LAI measurements (Li-Cor, 2014). The PCA uses the gap fraction technique for calculation of LAI and mean inclination angle of foliage. However, gap fraction measurements from optical sensors are contaminated by radiation that is scattered by plant elements and ground surfaces. Three proposed alternative schemes for correcting multiple scattering effects on gap fraction measurements by PCA are evaluated: 1) Leblanc and Chen, 2001. Agric. For. Meteorol. 110, 125-139; 2) Kobayashi et al.., 2013. Agric. For. Meteorol. 174-175, 170-183; and 3) Kuusk, 2016. Agric. For. Meteorol. 221, 242-247. Collectively these methods provide suggestions for the correction of LAI estimates from measurements under wide range of illumination conditions. The PCA measurements were taken at two different RAMI (RAdiation transfer Model Intercomparison) stands in Järvselja, Estonia. All three schemes agreed reasonably well in the sparse and homogeneous RAMI Scots pine stand; the effective LAI would be underestimated by up to 13% if the corrections are not implemented. The proposed corrections varied from 7% (method by Kobayashi et al., 2013) up to 23% (method by Leblanc and Chen, 2001) in denser and heterogeneous RAMI birch stand. The role of scattered sky radiation in the estimation of gap fraction in vegetation canopies using passive optical techniques should be clearly accounted for even in case of perfectly overcast sky. The PCA measurements are considered a reference in developing methods of the estimation of gap fraction from hemispherical photos (HP) and terrestrial laser scanners (TLS). If left uncorrected, the systematic errors of PCA are carried over into the other techniques as well. The results presented in this study provide a partial explanation of the frequently noted discrepancy in gap fraction estimates between TLS and the reference PCA/HP measurements.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.B43A0545P
- Keywords:
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- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCESDE: 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCESDE: 0476 Plant ecology;
- BIOGEOSCIENCESDE: 0480 Remote sensing;
- BIOGEOSCIENCES