Measuring Leaf Water Content with Dual-Wavelength Intensity Data from Terrestrial Laser Scanners

Samuli Junttila,Antero Kukko,Xinlian Liang,Harri Kaartinen,Hannu Hyyppä,Mikko Vastaranta,Juha Hyyppä,Markus Holopainen,Sanna Kaasalainen

doi:10.3390/rs9010008

Abstract

Decreased leaf moisture content, typically measured as equivalent water thickness (EWT), is an early signal of tree stress caused by drought, disease, or pest insects. We investigated the use of two terrestrial laser scanners (TLSs) employing different wavelengths for improving the understanding how EWT can be retrieved in a laboratory setting. Two wavelengths were examined for normalizing the effects of varying leaf structure and geometry on the measured intensity. The relationship between laser intensity features, using red (690 nm) and shortwave infrared (1550 nm) wavelengths, and the EWT of individual leaves or groups of needles were determined with and without intensity corrections. To account for wrinkles and curvatures of the leaves and needles, a model describing the relationship between incidence angle and backscattered intensity was applied. Additionally, a reflectance model describing both diffuse and specular reflectance was employed to remove the fraction of specular reflectance from backscattered intensity. A strong correlation (, RMSE = 0.004 g/cm2) was found between a normalized ratio of the two wavelengths and the measured EWT of samples. The applied intensity correction methods did not significantly improve the results of the study. The backscattered intensity responded to changes in EWT but more investigations are needed to test the suitability of TLSs to retrieve EWT in a forest environment.

Highlights

Climate change is increasing global mean surface temperature, affecting precipitation patterns and the frequency and duration of drought periods globally [1]
A wide range of forestry applications has been developed over two decades using the three-dimensional information, but relatively little focus has been put on the fifth dimension because the calibration of intensity data has been problematic [5,6,7]
Leaf-level measurements were conducted to test whether laser-based spectral indices calculated from the intensity measurements could improve the ability to estimate the equivalent water thickness (EWT) of leaves compared to using only intensity from 1550 nm laser

Summary

Introduction

Climate change is increasing global mean surface temperature, affecting precipitation patterns and the frequency and duration of drought periods globally [1]. Laser scanners can measure an object in five dimensions using a light detection and ranging technique whereby laser light is emitted and the reflected light is received by the detector [4]. These five dimensions include three spatial dimensions (x, y, and z-coordinates), time as the fourth dimension, and the intensity of the reflected light as the fifth dimension. Since reflectance information has been widely utilized in optical remote sensing techniques and proven useful for many applications, more focus should be put on investigating the potential of intensity information from laser scanners

Methods

Results

Discussion

Conclusion