Comparison of leaf angle distribution functions: Effects on extinction coefficient and fraction of sunlit foliage

Wang, W. M.; Li, Z. L.; Su, Hongbo

doi:10.1016/j.agrformet.2006.12.003

Cited by 178 publications

(106 citation statements)

References 35 publications

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“…As demonstrated by Wang et al (2007), natural leaf angle distributions can be fitted reasonably well by the two-parameter beta distribution (Goel and Strebel, 1984). The beta distribution has also been adopted in recent research on the photographic method for measuring LAD (e.g., Pisek et al, 2011Pisek et al, , 2013.…”

Section: Fitting the Leaf Inclination Angle Distributionmentioning

confidence: 89%

Photographic measurement of leaf angles in field crops

Zou

Mõttus

Tammeorg

et al. 2014

Agricultural and Forest Meteorology

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a b s t r a c tLeaf angle distribution (LAD) is one of the most important parameters used to describe the structure of horizontally homogeneous vegetation canopies such as field crops. LAD affects how incident photosynthetically active radiation is distributed on plant leaves, thus directly affecting plant productivity. Knowledge of LAD is also required for retrieval of other important biophysical variables from measurements of canopy radiation transmittance or spectral reflectance. Unfortunately, its determination is laborious and measured data is rarely available. In this study, we applied a recently developed method for determination of the inclination angles using leveled digital photography to the leaves of six cool-temperate crops: faba bean, narrow-leafed lupin, turnip rape, wheat, barley, and oat. The method, previously applied only to small and flat leaves of broadleaved trees and bushes, was extended to be applicable to the narrow and curved leaves of cereals. A reasonable match was found between the leaf angles determined by photographic measurements and the mean leaf tilt angles (MTA) measured using a LAI-2000 plant canopy analyzer for five out of the six species (R 2 = 0.92). The error caused by assuming a spherical LAD, when calculating LAI from canopy transmittance measurements, varied between 0 and 1.5 LAI units, depending on species. Finally, we analyzed the correlation between photographically determined species-specific LADs and airborne imaging spectroscopy data acquired for the same species in a similar growth stage. The highest correlation between spectral reflectance factor and leaf mean tilt angle was found at a wavelength of 748 nm (R = 0.80). The high correlation between MTA and this red edge waveband can be useful for MTA determination from imaging spectroscopy.

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Section: Fitting the Leaf Inclination Angle Distributionmentioning

confidence: 89%

Photographic measurement of leaf angles in field crops

Zou

Mõttus

Tammeorg

et al. 2014

Agricultural and Forest Meteorology

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“…At the same time, scientists have incorporated three dimensional canopy elements [27,28] to assess light interception inside a canopy either by using cubic units that were either filled or empty with leaf area [5], or by using field measurements and Monte Carlo simulations of canopy gap fraction and LAI to examine the interaction of beam radiation with clumped forest canopies [25]. However, the contribution of explicit geometric canopy models compared to different multilayer canopy simplifications in calculating irradiance absorption and therefore surface radiation balance has not yet been thoroughly assessed and quantified.…”

Section: Introductionmentioning

confidence: 99%

Assessing the Impact of Canopy Structure Simplification in Common Multilayer Models on Irradiance Absorption Estimates of Measured and Virtually Created Fagus sylvatica (L.) Stands

et al. 2009

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Abstract:Multilayer canopy representations are the most common structural stand representations due to their simplicity. Implementation of recent advances in technology has allowed scientists to simulate geometrically explicit forest canopies. The effect of simplified representations of tree architecture (i.e., multilayer representations) of four Fagus sylvatica (L.) stands, each with different LAI, on the light absorption estimates was assessed in comparison with explicit 3D geometrical stands. The absorbed photosynthetic radiation at stand level was calculated. Subsequently, each geometrically explicit 3D stand was compared with three multilayer models representing horizontal, uniform, and planophile leaf angle distributions. The 3D stands were created either by in situ measured trees or by modelled trees generated with the AMAP plant growth software. The Physically Based Ray Tracer (PBRT) algorithm was used to simulate the irradiance absorbance of the detailed 3D architecture stands, while for the three multilayer representations, the probability of light interception was simulated by applying the Beer-Lambert's law. The irradiance inside the canopies was characterized as direct, diffuse and scattered irradiance. The irradiance absorbance of the stands was computed during OPEN ACCESSRemote Sens. 2009, 1 1010 eight angular sun configurations ranging from 10° (near nadir) up to 80° sun zenith angles. Furthermore, a leaf stratification (the number and angular distribution of leaves per LAI layer inside a canopy) analysis between the 3D stands and the multilayer representations was performed, indicating the amount of irradiance each leaf is absorbing along with the percentage of sunny and shadow leaves inside the canopy. The results reveal that a multilayer representation of a stand, using a multilayer modelling approach, greatly overestimated the absorbed irradiance in an open canopy, while it provided a better approximation in the case of a closed canopy. Moreover, the actual stratification of leaves differed significantly between a multilayer representation and a 3D architecture canopy of the same LAI. The deviations in irradiance absorbance were caused by canopy structure, clumping and positioning of leaves. Although it was found that the use of canopy simplifications for modelling purposes in closed canopies is demonstrated as a valid option, special care should be taken when considering forest stands irradiance simulation for sparse canopies and particularly on higher sun zenith angles where the surrounding trees strongly affect the absorbed irradiance and results can highly deviate from the multilayer assumptions.

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“…The two-parameter Beta-distribution has been evaluated to be the most accurate to describe the leaf inclination distribution function (θ ) [44,45]:…”

Section: Leaf Angle Distribution and Leaf Projection Functionmentioning

confidence: 99%

Extracting Leaf Area Index by Sunlit Foliage Component from Downward-Looking Digital Photography under Clear-Sky Conditions

Zeng

Liu

et al. 2015

Remote Sensing

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Abstract:The development of near-surface remote sensing requires the accurate extraction of leaf area index (LAI) from networked digital cameras under all illumination conditions. The widely used directional gap fraction model is more suitable for overcast conditions due to the difficulty to discriminate the shaded foliage from the shadowed parts of images acquired on sunny days. In this study, a new LAI extraction method by the sunlit foliage component from downward-looking digital photography under clear-sky conditions is proposed. In this method, the sunlit foliage component was extracted by an automated image classification algorithm named LAB2, the clumping index was estimated by a path length distribution-based method, the LAD and G function were quantified by leveled digital images and, eventually, the LAI was obtained by introducing a geometric-optical (GO) model which can quantify the sunlit foliage proportion. The proposed method was OPEN ACCESSRemote Sens. 2015, 7 13411 evaluated at the YJP site, Canada, by the 3D realistic structural scene constructed based on the field measurements. Results suggest that the LAB2 algorithm makes it possible for the automated image processing and the accurate sunlit foliage extraction with the minimum overall accuracy of 91.4%. The widely-used finite-length method tends to underestimate the clumping index, while the path length distribution-based method can reduce the relative error (RE) from 7.8% to 6.6%. Using the directional gap fraction model under sunny conditions can lead to an underestimation of LAI by (1.61; 55.9%), which was significantly outside the accuracy requirement (0.5; 20%) by the Global Climate Observation System (GCOS). The proposed LAI extraction method has an RMSE of 0.35 and an RE of 11.4% under sunny conditions, which can meet the accuracy requirement of the GCOS. This method relaxes the required diffuse illumination conditions for the digital photography, and can be applied to extract LAI from downward-looking webcam images, which is expected for the regional to continental scale monitoring of vegetation dynamics and validation of satellite remote sensing products.

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Comparison of leaf angle distribution functions: Effects on extinction coefficient and fraction of sunlit foliage

Cited by 178 publications

References 35 publications

Photographic measurement of leaf angles in field crops

Photographic measurement of leaf angles in field crops

Assessing the Impact of Canopy Structure Simplification in Common Multilayer Models on Irradiance Absorption Estimates of Measured and Virtually Created Fagus sylvatica (L.) Stands

Extracting Leaf Area Index by Sunlit Foliage Component from Downward-Looking Digital Photography under Clear-Sky Conditions

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