Can Leaf Water Content Be Estimated Using Multispectral Terrestrial Laser Scanning? A Case Study With Norway Spruce Seedlings

Abstract

Changing climate is increasing the amount and intensity of forest stress agents, such as drought, pest insects, and pathogens. Leaf water content, measured here in terms of equivalent water thickness (EWT), is an early indicator of tree stress that provides timely information about the health status of forests. Multispectral terrestrial laser scanning (MS-TLS) measures target geometry and reflectance simultaneously, providing spatially explicit reflectance information at several wavelengths. EWT and leaf internal structure affect leaf reflectance in the shortwave infrared region that can be used to predict EWT with MS-TLS. A second wavelength that is sensitive to leaf internal structure but not affected by EWT can be used to normalize leaf internal effects on the shortwave infrared region and improve the prediction of EWT. Here we investigated the relationship between EWT and laser intensity features using multisensor MS-TLS at 690, 905, and 1,550 nm wavelengths with both drought-treated and Endoconidiophora polonica inoculated Norway spruce seedlings to better understand how MS-TLS measurements can explain variation in EWT. In our study, a normalized ratio of two wavelengths at 905 and 1,550 nm and length of seedling explained 91% of the variation (R2) in EWT as the respective prediction accuracy for EWT was 0.003 g/cm2 in greenhouse conditions. The relation between EWT and the normalized ratio of 905 and 1,550 nm wavelengths did not seem sensitive to a decreased point density of the MS-TLS data. Based on our results, different EWTs in Norway spruce seedlings show different spectral responses when measured using MS-TLS. These results can be further used when developing EWT monitoring for improving forest health assessments.

Light detection and ranging explains diversity of plants, fungi, lichens, and bryophytes across multiple habitats and large geographic extent

“…At the wavelength used here, vegetation surface reflectivity (and thus point amplitude) relates to leaf water content (Junttila et al. ) and soil moisture (Zlinszky et al. ).…”

“…It is low for tall canopies where the light energy is distributed between a number of returns, for complex or opaque surfaces such as leaves, and for surfaces with low reflectivity. At the wavelength used here, vegetation surface reflectivity (and thus point amplitude) relates to leaf water content (Junttila et al 2018) and soil moisture (Zlinszky et al 2014).…”

Four Dimensional Mapping of Vegetation Moisture Content Using Dual-Wavelength Terrestrial Laser Scanning

“…However, no successful attempts to utilize such systems to estimate EWT at the canopy level in field campaigns have been reported in the literature, as these systems are still considered proof-of-concept. Junttila, et al [44] and Elsherif, et al [38] showed that intensity data from commercially-available NIR and SWIR TLS systems can be combined to calculate the NDI, reporting a high correlation between the NDI and EWT at the leaf level for coniferous and broadleaf species, respectively. At the canopy level, Junttila, et al [45] used an NDI of 905 and 1550 nm wavelengths with the FARO S120 and FARO X330 TLS instruments, respectively, to detect European spruce bark beetle infestation symptoms in 29 mature Norway spruce trees.…”

Terrestrial Laser Scanning Intensity Captures Diurnal Variation in Leaf Water Potential

“…A clear linkage between leaf water content, measured as equivalent water thickness (EWT), and TLS intensity has been shown in several studies (Elsherif et al, 2019a;Gaulton et al, 2013;Junttila et al, 2018Junttila et al, , 2016. Reflectance at the 1550 nm wavelength, which is often utilized in TLS sensors, increases as the leaf water content decreases (Junttila et al, 2016;Rallo et al, 2014).…”