Spectral composition of shortwave radiation transmitted by forest canopies
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SHORT COMMUNICATION
Spectral composition of shortwave radiation transmitted by forest canopies Aarne Hovi1 · Miina Rautiainen1,2 Received: 9 December 2019 / Accepted: 18 June 2020 © The Author(s) 2020
Abstract Key message Leaf area index and species composition influence red-to-near-infrared and red-to-shortwave-infrared transmittance ratios of boreal and temperate forest canopies. In this short communication paper, we present how the spectral composition of transmitted shortwave radiation (350– 2200 nm) varies in boreal and temperate forests based on a detailed set of measurements conducted in Finland and Czechia. Our results show that within-stand variation in canopy transmittance is wavelength dependent, and is the largest for sparse forest stands. Increasing leaf area index (LAI) reduces the overall level of transmittance as well as red-to-near-infrared and red-to-shortwave-infrared transmittance ratios. Given the same LAI, these ratios are lower for broadleaved than for coniferous forests. These results demonstrate the importance of both LAI and forest type (broadleaved vs. coniferous) in determining light quality under forest canopies. Keywords Spectra · Transmission · Boreal · Temperate · Radiative transfer
Introduction The spectral composition of light a plant canopy receives, sometimes also referred to as light quality, influences both the morphology and physiology of the canopy. Thus, it is not a surprise that light quality has long been acknowledged in agriculture as a means to influence crop yields and commercial value (e.g., Jones 2018). Light quality in other vegetation types, such as forests, has been much less studied. This is an unfortunate situation; a better understanding of the spectral quality of shortwave radiation above and below forests would have a profound influence on the development and validation of forest radiative transfer models needed in, for example, characterizing forest–climate interactions or forest productivity, or in remote sensing applications as a Communicated by T. Rötzer. * Aarne Hovi [email protected] 1
Department of Built Environment, School of Engineering, Aalto University, P.O.Box 14100, 00076 Aalto, Finland
Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, P.O.Box 15100, 00076 Aalto, Finland
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tool to interpret satellite data. A better understanding of the spectral transmittance of forest canopies would also improve our interpretation of phenological processes of forest vegetation (Brelsford et al. 2019). The spectral transmittance of leaves or needles detached from trees varies by species (e.g., Hovi et al. 2017). However, the spectral transmittance of an entire forest canopy is not only influenced by leaf optical properties but also by stand structure. To date, empirical measurements of the spectral transmittance in forest canopies have focused on the visible and near-infrared spectral region between approximately 400 and 1100 nm. The studies have also been limited to single or a few stands (e.g.,
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