Modeling Radiative Transfer through Forest Canopies: Implications for Canopy Photosynthesis and Remote Sensing

Nilson, Tiit; Ross, Juhan

doi:10.1007/978-94-011-5446-8_2

Cited by 31 publications

(27 citation statements)

References 29 publications

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“…Finally, to test the model, simulated shoot scattering phase functions should be compared to empirical measurements. To our knowledge such model-based comparison has not been made, although some investigations have involved measurements of the scattering phase function of shoots (Nilson & Ross, 1997;Ross, Meinander, & Sulev, 1994).…”

Section: Discussionmentioning

confidence: 99%

“…A possible way to overcome this problem is to use the shoot as the basic structural element in radiative transfer models for conifers (Nilson & Ross, 1997). The canopy structure is then described in terms of the spatial and angular distribution of shoots, and the geometrical and spectral properties of leaves are replaced with those of shoots.…”

Section: Introductionmentioning

confidence: 99%

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A method to account for shoot scale clumping in coniferous canopy reflectance models

Smolander¹,

Stenberg²

2003

Remote Sensing of Environment

125

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The three-dimensional structure of a coniferous shoot gives rise to multiple scattering of light between the needles of the shoot, causing the shoot spectral reflectance to differ from that of a flat leaf. Forest reflectance models based on the radiative transfer equation handle shoot level clumping by correcting the radiation attenuation coefficient with a clumping index. The clumping index causes a reduction in the interception of radiation by the canopy at a fixed leaf area index (LAI). In this study, we show how within-shoot multiple scattering is related to shoot scale clumping and derive a similar, but wavelength dependent, correction to the scattering coefficient. The results provide a method for integrating shoot structure into current radiative transfer equation based forest reflectance models. The method was applied to explore the effect of shoot scale clumping on canopy spectral reflectance using simple model canopies with a homogeneous higher level structure. The clumping of needles into shoots caused a wavelength dependent reduction in canopy reflectance, as compared to that of a leaf canopy with similar interception. This is proposed to be one reason why coniferous and broad-leaved canopies occupy different regions in the spectral space and exhibit different dependency of spectral vegetation indices on LAI. D

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

A method to account for shoot scale clumping in coniferous canopy reflectance models

Smolander¹,

Stenberg²

2003

Remote Sensing of Environment

125

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“…The number of reported measurements of the spectral properties of coniferous shoots is small. Shoot scattering phase functions were measured for the first time in Estonia, for Scots pine, in the 1990s [66,67]. The direction of illumination (relative to shoot axis) was varied, and the scattered radiation in a single plane was measured.…”

Section: Spectral and Directional Optical Properties Of Coniferous Shmentioning

confidence: 99%

Spectral Properties of Coniferous Forests: A Review of In Situ and Laboratory Measurements

Rautiainen

Lukeš²,

Homolová³

et al. 2018

Remote Sensing

107

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Abstract:Coniferous species are present in almost all major vegetation biomes on Earth, though they are the most abundant in the northern hemisphere, where they form the northern tree and forest lines close to the Arctic Circle. Monitoring coniferous forests with satellite and airborne remote sensing is active, due to the forests' great ecological and economic importance. We review the current understanding of spectral behavior of different components forming coniferous forests. We look at the spatial, directional, and seasonal variations in needle, shoot, woody element, and understory spectra in coniferous forests, based on measurements. Through selected case studies, we also demonstrate how coniferous canopy spectra vary at different spatial scales, and in different viewing angles and seasons. Finally, we provide a synthesis of gaps in the current knowledge on spectra of elements forming coniferous forests that could also serve as a recommendation for planning scientific efforts in the future.

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“…We hypothesize that this is caused by needle wax [40]. In the visible wavelengths where leaf pigments are highly absorbing, leaf scattering is dominated by specular reflectance at the surface [41].…”

Section: Discussionmentioning

confidence: 99%

“…2017, 9, 1005 15 of 17 Figure 12. Ratio of Scots pine canopy TOC reflectance close to hot spot ( = 19°) to that at dark spot ( = 76°) and nadir ( = 55°) scaled by a factor of three (for different sunlit fraction intervals, dashed lines), plotted together with the ratio of Scots pine needle reflectance to transmittance measured in situ [40] (solid line).…”

Section: Discussionmentioning

confidence: 99%

Spectral Similarity and PRI Variations for a Boreal Forest Stand Using Multi-angular Airborne Imagery

2017

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Abstract:The photochemical reflectance index (PRI) is a proxy for light use efficiency (LUE), and is used in remote sensing to measure plant stress and photosynthetic downregulation in plant canopies. It is known to depend on local light conditions within a canopy indicating non-photosynthetic quenching of incident radiation. Additionally, when measured from a distance, canopy PRI depends on shadow fraction-the fraction of shaded foliage in the instantaneous field of view of the sensor-due to observation geometry. Our aim is to quantify the extent to which sunlit fraction alone can describe variations in PRI so that it would be possible to correct for its variation and identify other possible factors affecting the PRI-sunlit fraction relationship. We used a high spatial and spectral resolution Aisa Eagle airborne imaging spectrometer above a boreal Scots pine site in Finland (Hyytiälä forest research station, 61 • 50 N, 24 • 17 E), with the sensor looking in nadir and tilted (off-nadir) directions. The spectral resolution of the data was 4.6 nm, and the spatial resolution was 0.6 m. We compared the PRI for three different scatter angles (β = 19 • , 55 • and 76 • , defined as the angle between sensor and solar directions) at the forest stand level, and observed a small (0.006) but statistically significant (p < 0.01) difference in stand PRI. We found that stand mean PRI was not a direct function of sunlit fraction. However, for each scatter angle separately, we found a clear non-linear relationship between PRI and sunlit fraction. The relationship was systematic and had a similar shape for all of the scatter angles. As the PRI-sunlit fraction curves for the different scatter angles were shifted with respect to each other, no universal curve could be found causing the observed independence of canopy PRI from the average sunlit fraction of each view direction. We found the shifts of the curves to be related to a leaf structural effect on canopy scattering: the ratio of needle spectral reflectance to transmittance. We demonstrate that modeling PRI-sunlit fraction relationships using high spatial resolution imaging spectroscopy data is suitable and needed in order to quantify PRI variations over forest canopies.

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Modeling Radiative Transfer through Forest Canopies: Implications for Canopy Photosynthesis and Remote Sensing

Cited by 31 publications

References 29 publications

A method to account for shoot scale clumping in coniferous canopy reflectance models

A method to account for shoot scale clumping in coniferous canopy reflectance models

Spectral Properties of Coniferous Forests: A Review of In Situ and Laboratory Measurements

Spectral Similarity and PRI Variations for a Boreal Forest Stand Using Multi-angular Airborne Imagery

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