Climate change is increasing global mean surface temperature, affecting precipitation patterns and the frequency and duration of drought periods globally [1
]. Environmental changes result in alterations in forest ecosystems that are potentially harmful to the native fauna and can cause tree dieback (e.g., the spread of non-native tree pests and pathogens) and changes in the susceptibility of host plants [2
]. The collection of forest health data has traditionally been based on visual subjective estimation of tree canopies, which is time-consuming and prone to error and bias; thus, new methods for mapping and monitoring forest health are needed to provide objective and efficient estimation of forest health, improve the prediction of global climate-induced tree mortality, and provide tools for the strategic mitigation of damage.
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
, and z
-coordinates), time as the fourth dimension, and the intensity of the reflected light as the fifth dimension. Two types of methods can be used to calculate the distance the target: (1) time of flight technique when the light source is pulse-based, or (2) phase modulation technique when continuous wave light is used. The elementary difference between these techniques is that pulse-based techniques are able to record multiple range measurements from a single pulse, whereas continuous wave techniques provide a single range measurement. 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
]. 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. The recent technical development of multispectral laser scanners supports this as a wider array of wavelengths can be put to use [4
A number of studies have investigated the use of intensity information from single wavelength laser scanners to impute single-tree attributes, classify species, and map understory trees from airborne laser scanning data [10
]. Terrestrial laser scanner (TLS) data with intensity information have been used to study the estimation of leaf area distribution, measurement of tree stem diameters, and discrimination of marls and limestones using mainly 1535 nm wavelength [13
]. However, it has been found that the laser intensity measurement is affected by the incidence angle between the scanner and the target surface, and the distance to the target complicates the utilization of intensity data [16
]. Other issues include the edge effect, when the laser beam hits the target with only a part of the laser footprint reducing the return intensity [17
]. The edge effect is likely to be pronounced when phase modulation techniques are used for range measurement as the background is taken into account, producing noisy ghost points [18
], whereas pulsed laser scanners can detect the returned light energy from the first object that the laser beam hits. The calibration of laser intensity data is necessary because of the aforementioned effects for obtaining more accurate results in radiometric measurements [19
The calibration of intensity measurements from TLS data has recently been investigated in a search for correction methods regarding the range and incidence angle effects [20
]. These studies have shown that the range effect is strongly dependent on the instrument used; thus, reference measurements using an external target are required. The effect of incidence angle on recorded intensity has been found to be influenced by the reflectance and surface properties (i.e., the surface roughness or grain size) of the target [20
]. If the target has a surface that is characterized by perfectly diffuse reflection (i.e., light is reflected with constant intensity in all directions), then the backscatter intensity is proportional to the incidence angle, following the Lambert’s cosine law. However, usually natural surface reflection is a combination of specular (i.e., mirror reflection) and diffuse reflection, thus the determination of the fraction of either specular or diffuse reflection is required. A surface characterized by a more specular reflection is likely to be more affected by the incidence angle as more light is mirrored to the detector, as was found in the visible light domain [22
]. Kaasalainen et al. [20
] proposed an empirical model combining the Lommel–Seeliger function and Lambert’s cosine law to correct for the effect of incidence angle on TLS intensity data. Höfle [23
] investigated radiometric correction methods for individual maize plant detection from TLS data, and showed that the amplitude of variation was significantly reduced for homogeneous areas using his method.
Vegetation water content, which is typically measured as equivalent water thickness (EWT) in the remote sensing literature, is an important indicator of plant physiological status [24
]. EWT is a measure of the weight of water per unit of leaf area, usually given in g/cm2
. Decreased EWT has been identified as an early signal of forest drought stress [25
] and infestation of forest insect pests (e.g., the mountain pine beetle) [26
]. Early detection of tree stress is vital in reducing the spread of insect pests and the mitigation of damage since tree stress increases the susceptibility of the host plant and infected trees should be removed to minimize attacks on living trees [27
]. Since a detailed description of the relationship between leaf water content and vegetation is beyond the scope of this paper, a more detailed review of the relationship between vegetation status and water content and the detection of EWT using reflectance in the optical domain can be found in Ceccatto et al. [28
The mapping of EWT has recently been studied at leaf level using a single wavelength full-waveform TLS operating at 1550 nm, resulting in a significant correlation between backscattered intensity and EWT (R2
= 0.74), with eight broadleaf species exhibiting a variety of leaf surface types [29
]. However, the achieved correlation was a result of a number of intensity correction methods including the removal of the specular reflection component, and correction of incidence angle. The removal of the specular reflection component requires a priori information about the reflectance model parameters, complicating the use of such methods in a heterogeneous forest environment. Zhu et al. [30
] also investigated the estimation of canopy leaf water content in a recent publication showing that EWT can be predicted with a coefficient of determination of 0.66 after incidence angle correction. While the proposed method has potential for mapping EWT in broadleaf species, the applicability of the method for conifer species remains a research topic for the future.
Laser scanners utilizing two or more wavelengths simultaneously could potentially help in overcoming the influence of incidence angle and partial hits to leaves if the wavelengths are similarly influenced by these factors [17
]. A normalized ratio of two wavelengths should then be insensitive to the effect of incidence angle since the effect is partially cancelled out [31
]. This also enables the use of spectral ratios where optimally one wavelength is affected by the variable of interest and the other is located at a stable region of the spectrum. A small number of studies have investigated the utilization of multispectral laser scanners and reflectance indices calculated from the intensity data in estimation of vegetation biochemical parameters and tree health [31
]. Gaulton et al. [31
] used a dual-wavelength TLS (1064 nm & 1550 nm, for more details on the instrument [34
]) for estimating EWT in a laboratory setting, resulting in a high correlation (R2
= 0.80) between the normalized ratio of the two wavelengths and the measured EWT of leaf samples from herbaceous species, but the number of samples and species in this study was very small and there is a need for further investigations, especially using tree species. Eitel et al. [33
] examined the ability of a dual-wavelength laser scanner (532 nm & 658 nm) in providing information about foliar nitrogen content, and Nevalainen et al. [35
] proposed a non-destructive method for the estimation of chlorophyll in tree canopies with a hyperspectral laser scanner utilizing eight wavelengths, both concluding that spectral indices calculated from these wavelengths were able to improve the estimation of the investigated biochemical parameters.
This paper tests the feasibility of using multisensor multispectral laser scanning (690 nm & 1550 nm) for the estimation of leaf-level EWT of five different tree species in combination with laser intensity correction methods in a laboratory setting. The rationale for using laser indices calculated from two wavelengths is the expected insensitivity to confounding factors that could affect the results. So far, the use of multispectral TLS data for retrieval of leaf EWT has been studied only with a very limited number of species (no tree species) and samples, emphasizing the need for further investigation using a larger dataset—also with multisensory data including intensity correction methods. The study has only investigated broadleaved species, whereas here conifer species, which comprise a majority of boreal forest biomass, are also investigated. The complex nature of conifer leaves sets more challenges to the processing of intensity data when a single laser measurement is produced from several leaves instead of one as the width of the needles are smaller than the laser footprint. Here, different methods for correcting multispectral intensity data due to the influence of incidence angle and between-species variation in reflective properties are also considered in combination with spectral indices calculated from the intensity data in a novel manner. The developed method could improve ground-based measurement of canopy EWT, which could be beneficial as a calibration and validation tool for airborne and space borne remote-sensing techniques, allowing more accurate estimation of EWT on larger scales. The objectives of this study are: (1) to investigate the correlation between EWT- and TLS-measured laser intensity using dual-wavelength data and calculated spectral ratios; (2) to examine the influence of incidence angle correction to account for wrinkles and curvatures of the leaves in the results; (3) to examine if the removal of specular backscatter intensity will provide a more general model for the relationship of EWT and laser intensity with this set of species, and (4) to examine the effect of leaf dry mass on backscattered intensity.
Calibrated intensity at 1550 nm and EWT were found to be closely linked in this study, supporting the modeling results from the PROSPECT-5 model. Gaulton et al. [31
] showed somewhat similar results (R2
= 0.65), with only five leaf samples employing a laser wavelength at 1545 nm. Zhu et al. [29
] also presented a significant correlation (R2
= 0.74) between laser intensity at 1550 nm and EWT with a wide range of leaf structures, but there was wide variability in the correlation between the species.
The correlation of calibrated intensity at 690 nm and EWT was an unexpected outcome as the modeling results presented previously (see Figure 1
) showed no changes in this part of the spectrum due to decreased EWT. This may be due to other eco-physiological changes in the leaves resulting from the drying process, which can alter, for example, the internal structure of cells, and the chlorophyll and carotenoid content of leaves [48
]. Cell wall architecture of dry leaves has been studied microscopically, where they found the cell walls to contract during the dehydration process [49
]. There could be a relation between the contraction of cells and backscattered reflectance if this process is modifying the interaction between the laser beam and the leaf surface. For instance, if more light energy is reflected in a mirror-like behavior instead of diffuse scattering, more of that energy should reflect back to the detector. However, with our study design we were not able to analyze whether the backscattered reflectance model is significantly altered through the drying of leaves; this requires further investigation.
Drought stress has been shown to be related to a decreased chlorophyll content; thus, it may affect the reflectance at the 690 nm wavelength [50
]. The effect of chlorophyll content, carotenoid content, and leaf structural parameter on leaf reflectance at the studied wavelengths was evaluated with the PROSPECT-5 model (Figure 8
). The results show that the chlorophyll content and the leaves’ internal structures may affect the reflectance at 690 nm, whereas the carotenoid content and the EWT did not appear to change the reflectance at this wavelength, potentially explaining the high correlation between calibrated intensity at 690 nm and EWT in deciduous species. The differences between deciduous and conifer species at this wavelength could be derived from a different rate of change in internal structure due to drying, since conifers were shown to evaporate water more slowly, and the duration of the experiment was only 77 h. However, the wavelength at 690 nm is likely sub-optimal as a reference wavelength for normalizing the effect of different leaf structures because of the aforementioned factors, and also since it does not seem to be affected by LMA and other leaf structural effects that are affecting the 1550 nm band according to the modeling results.
The spectral indices showed strong correlations with EWT, which is in accordance with the results of Gaulton et al. [31
]. The correlations of the spectral indices were significantly stronger for the conifer species compared to the calibrated intensity at 1550 nm; thus, it seems that the wavelength at 690 nm is able to partially account for leaf structural differences that complicate the use of a single wavelength for estimating EWT. A reference wavelength located around 1000 nm could be better for normalizing structural effects since the modeling results show it is more sensitive to the leaf structural parameter (Figure 8
), and it is also physically closer to the 1550 nm wavelength, possibly resulting in more similar backscattering behavior. However, a comparison study of reference wavelengths should be conducted to find an optimal wavelength for normalization of leaf structural effects.
The differences in the performance of the laser intensity features in explaining EWT between conifer and deciduous species were likely due to alterations in leaf structure. The area of illumination from the laser beam was filled entirely in the case of deciduous species, as the leaves were large and flat, whereas conifer species, having needles less than 2 mm wide, could not fill the laser footprint completely as the diameter of the laser beam spot was larger than 3 mm at the target for both scanners. This resulted in different scattering geometry between the conifers and deciduous tree species. The greater performance of the spectral indices compared to single wavelength features in estimating EWT in conifer species (see Table 4
) may indicate that the use of two wavelengths could partially cancel out the effect of varying incidence angle on backscattered intensity, which is one rationale for using dual-wavelength laser scanning systems for the estimation of eco-physiological parameters. However, more investigations are needed in order to validate this assumption.
The incidence angle correction did not improve the results in this study. This is likely due to the position of the leaves as they were facing the scanner, resulting in a standard deviation of 2°–4° in mean incidence angle between the samples. Thus, the micro-topography of leaves seems to have an insignificant effect on the measured intensity since no significant improvement in the developed EWT prediction models were shown. Additionally, the limitations of the ranging device should be noted since the incidence angle was approximated from the point clouds; thus, any measuring error in ranging can cause an error in the calculation of the surface normal and the resulting incidence angle. The accuracy of the ranging measurements may not be sufficient to account for small wrinkles and curvatures of leaves, but when a wider range of leaf angles and larger surfaces are present (i.e., scanning a tree), the incidence angle correction could be able to improve the estimation of EWT, as was found in Zhu et al. [29
]. Also, the surface properties of the target affect the effect of incidence angle on the measured intensity, thus rough surfaces are less influenced by the incidence angle if the surface scattering is highly diffuse [20
]. Since a single laser footprint illuminates several needles, which are likely pointing in different directions, resulting in a complicated measuring geometry, incidence angle correction methods are more difficult to apply for conifer species. Thus, the estimation of tree crown EWT with TLS needs to be carefully investigated for single trees with several tree species representing different leaf structures. The effect of incidence angle on different vegetation indices also seems to vary between samples and wavelength combinations in TLS measurements, which calls for rigorous experiments with multiple wavelengths and a variety of samples [22
The removal of specular backscatter intensity did not improve the estimation of EWT over the range of species used in this study, which may be because of the ranging error discussed already. The estimation of model parameters was done using the incidence angle data obtained from ranging measurements, thus the parameters may have not been estimated correctly. Also, the visual method used for evaluating the parameters may have resulted in inaccuracies in the parameter estimation. Manual measurement of incidence angle (e.g., see [29
]) may be a more accurate method to obtain the variation of laser intensity caused by changes in incidence angle, allowing the parameters of the reflectance model to be more precisely estimated. However, this part of the intensity correction may be unnecessary, as our regression models were able to estimate EWT with a coefficient of determination of 0.93 without the removal of specular backscatter intensity. The accuracy of the estimation probably depends on the collection of species investigated, as varying results have been obtained using different species [29
The TLSs differed in their technical specifications: Leica HDS6100 had a larger laser footprint and beam divergence and a lower output power than the FARO X330, whereas FARO had a wider dynamic range in the intensity recording values. It is uncertain how much these differences affected the results. For example, the ability to penetrate tree canopies has been found to be affected by the output power of the laser scanner in airborne laser scanning [52
]. Dual-wavelength TLS systems with aligned laser beams at both wavelengths would be able to reduce the effect of varying optical and technical properties of the scanning mechanism, and provide a tool for studying the utilization of multi-wavelength laser scanning data. Perfectly aligned laser beams at two wavelengths could help reduce the confounding effect of the incidence angle on backscattered intensity if both of the wavelengths respond similarly to changing incidence angle, but such a system is technically very difficult to build. Also, the effect of the range on the backscattered intensity could be reduced without a complicated calibration procedure if the same terms are met as for the incidence angle. Since it is difficult to achieve the alignment of the laser beams, an averaging approach over a larger area could be more feasible, as was done here. When measuring trees it is difficult to determine if the laser returns from both laser scanners are reflected from the exact same surface since a longer wavelength is more likely to penetrate deeper into the leaves and canopies; thus, more tests are required to investigate whether the returns are spatially significantly different between two wavelengths and scanners. However, since an averaging method is used, the use of two laser scanners at different wavelengths can be a viable option for mapping EWT despite the fact that there may be some differences in the measuring geometry. Other wavelengths should be investigated as a reference, especially near-infrared wavelengths that are closer to 1550 nm and hence may be more similar in terms of tissue and canopy penetration.
LMA was shown to have a significant effect on the laser return intensity at 1550 nm when comparing leaf samples of similar EWT. The effect partially hinders the estimation of leaf EWT but the intensity variation caused by LMA was small enough to allow a high correlation between EWT and laser return intensity at 1550 nm. The lower accuracy of the regression models between laser intensity features and EWT for the conifer species could derive at least partially from the higher deviation in LMA.
Here, the effect of range on backscattered intensity was not corrected since the samples were scanned from positions with only a small variation in distance. The measurement setting in this study is very different from a forest environment when trees and canopies are scanned; thus, the correction of range effect is necessary to obtain accurate estimates of values such as EWT using TLSs in a forest environment. Kaasalainen et al. [20
] have presented correction methods for the range effect. However, it is uncertain how much varying incidence angle and range will affect the estimation of EWT at the canopy scale, and how well the correction methods will perform in a more complicated measurement environment. Future research that investigates the use of lasers at multiple wavelengths in estimation of eco-physiological parameters across different spatial scales is needed to further evaluate the suitability of multi-wavelength laser systems for conducting tree health assessments.
The investigated measurements were conducted in a laboratory where environmental variables such as temperature, humidity, wind, and illumination were fairly constant; thus, the results of this study are limited to similar conditions. In a natural environment leaves move between the consecutive scans, other parts of the canopy are observed in different angle and direction, leaf surface water may be present, and air moisture content can be high, leading to an erroneous estimation of EWT. Additionally, the leaf surface crossed by the laser beam at 690 nm and 1550 nm can be significantly different, with the 1550 nm band penetrating deeper into the canopy. All these limitations should be addressed before the investigated method can be fully applied to tree canopies in an operational context.