Multi-angular reflectance spectra of small single trees

Petri R Forsström,Aarne Hovi,Giulia Ghielmetti,Michael E Schaepman,Miina Rautiainen

doi:10.1016/j.rse.2021.112302

Petri R Forsström, Aarne Hovi + Show 3 more

Open Access

https://doi.org/10.1016/j.rse.2021.112302

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Abstract

Understanding the reflectance anisotropy of forests and the underlying scattering mechanisms is needed to improve the accuracy of retrievals of fundamental forest characteristics from optical remote sensing data. In this paper, we developed a laboratory measurement set-up for a large goniometer (LAGOS) and measured multi-angular spectra (350–2500 nm) of 18 small trees, composed of three common European tree species: Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) H. Karst), sessile oak (Quercus petraea (Matt.) Liebl.). For all trees, we measured tree spectra in 47 view angles in the upper hemisphere. To our knowledge, this is the first study reporting multi-angular reflectance spectra of single trees. We also measured the reflectance and transmittance spectra of needles and leaves, as well as reflectance spectra of bark of the sample trees. We analyzed the spectro-directional characteristics of the trees, and the inter- and intraspecific variations of these characteristics. The anisotropy of trees was shown to be strongly asymmetrical and characteristic to species: while pine and spruce exhibited strong hotspot effects, oak showed a strong specular component. Our results indicate that simultaneous measurements of both spectral and directional characteristics of trees may enhance the discrimination of species and thus, support the retrieval of information of their biophysical properties.

Highlights

A wealth of satellite data is available for environmental monitoring applications
We examine the spectral charac teristics and anisotropy of trees in the principal plane and cross-plane, sometimes referred to as the orthogonal plane, and report the normal ized difference vegetation index (NDVI) (Rouse et al, 1974) at leaf and tree level
Spruce and oak scattering was notably stronger compared to pine in nadir (Fig. 5a): oak exhibited the highest DSC in blue (~450 nm) and green spectral regions (~560 nm), while spruce DSC was the highest in the red region (~660 nm)

Summary

Introduction

A wealth of satellite data is available for environmental monitoring applications. The patterns have first been analyzed through radiative transfer modeling (e.g., Li and Strahler, 1992; Roujean et al, 1992; Jacquemoud, 1993; Gerard and North, 1997; Chen and Leblanc, 1997; Rautiainen et al, 2004; Kobayashi and Iwa buchi, 2008; Kuusk et al, 2014) and later on using multi-angular remote sensing data (e.g., Lacaze and Roujean, 2001; Canisius and Chen, 2007; Rautiainen et al, 2008; Verrelst et al, 2010) These studies have demonstrated that the geometric structure of can opies has a notable impact on the reflectance anisotropy of forests. Planning data collection by either air- or spaceborne platform relies on the availability of high-quality reference data, typically collected at ground level or modeled (Schneider et al, 2014)

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