Imaging Spectroscopy of Forest Ecosystems: Perspectives for the Use of Space-borne Hyperspectral Earth Observation Systems
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Abstract
The emerging challenges in preserving and managing forest ecosystems are multiscale in terms of space and time, and therefore require spatially and temporally contiguous information sources. Imaging spectroscopy has the potential to contribute information that cannot be raised by other Earth Observation Systems. In particular, the spectral capacity to monitor the distributions of chemical traits, such as canopy foliar nitrogen distribution, and to track changes in water content or the percentage water in plants, has already opened novel pathways toward assessing the global variability of ecosystem functions and services. However, there is an ongoing debate on how to best extract this type of information from the spectral measurements. Empirical approaches have demonstrated their efficiency in a multitude of local studies, but are criticized with respect to poor generalization capacities. Alternative strategies, such as the use of physically based models of leaf or canopy reflectance, or hybrid approaches, have the potential advantage to be more widely applicable. This paper attempts to assess achievements and shortcomings of these strategies and finds that the often-cited disadvantages of using empirical approaches are becoming less pronounced in the light of recent research results. While retrievals based on physically based models on leaf/needle level are close to laboratory quality, results on canopy level available to date still have considerable deficits. Owing to improved instrumental designs, better data calibration, new approaches for compensating canopy effects, and the use of increasingly efficient methods for establishing data-driven models, the scope of empirical approaches has considerably widened and they have been successfully applied to large areas. The future availability of regularly acquired hyperspectral imagery from Earth orbits will substantially contribute to their generalizability.
Keywords
Imaging spectroscopy Forest ecosystems Biochemical traits Retrieval strategies1 Introduction
Forest ecosystems provide numerous environmental goods and services, such as conservation of biological diversity and climatic control, and, of course, timber and non-timber products (FAO 2010). However, forests and forested ecosystems face numerous risks under increasing pressure due to global warming (Birdsey and Pan 2011; Peng et al. 2011; Swann et al. 2018) and expanding human populations and economies (Hansen et al. 2008). Deforestation associated with conversion of forests to agricultural land and development, legal and illegal logging, drought stress, biotic stress, and increasing wildfire frequency, extent and severity are some of the most important processes which affect forested landscapes (Anderegg et al. 2013a, b; Bond et al. 2010; Ciais et al. 2005).
The European Environmental Agency expects forests in Europe to be among the ecosystems most affected by climate change (EEA 2012). Examples of recent die-offs have been reported from southern and central parts of Europe (e.g., Adams et al. 2017; Bréda 2003). It is expected that forests will become increasingly affected by drought, either during exceptional events or under a long-term drift toward more arid conditions (Ma et al. 2012). Extreme climatic events, such as the heat waves and dry spells experienced during the European summers 2003 and 2018, are expected to occur at increased frequencies (Beniston 2004; Fink et al. 2004). Diffuse tree mortality within forest stands, frequently reported after such extreme events, may be exacerbated by soil properties (mainly local water storage capacity) or decreases in genetic diversity among and within tree species (Lindner et al. 2010; Rennenberg et al. 2006). Due to the direct impact of heat waves and dry spells, forest ecosystems are also becoming more susceptible to pests and diseases. The combined effects bear the risk of a reduction of important ecosystem services, such as carbon sequestration, water retention, soil protection, preserving biodiversity and, last but not least, biomass and timber production (Lindner et al. 2010; Schelhaas et al. 2003; Seidl et al. 2011).
In order to keep track of these effects, monitoring of forest ecosystems is a necessity, both in situ and through remote sensing. While in situ terrestrial forest monitoring usually employs surveys at plot level, logistically it is not feasible to make measurements everywhere that data are needed. Remote sensing offers the potential to sample with reasonable accuracy in inaccessible locations (Asner et al. 2015) while also providing wall-to-wall information at multiple temporal and spatial scales. The emerging challenges in forest management, intricately linked to society’s needs to preserve multiple forest values and continue to benefit from all services of forest ecosystems, are multiscale in terms of space and time and therefore need spatially and temporally contiguous information sources. Moreover, since sustainable forest management aims at reconciling economic interests with ecologic concerns, remote sensing must serve both, economically oriented assessments and management requirements, as well as studies of ecological processes and functions (Franklin 2001).
Lausch et al. (2018, 2016, and 2017) summarize this wide scope under the term “forest health,” whose proper assessment requires utilitarian as well as ecosystem indicators to be implemented. In their review papers, they provide a detailed summary of indicator methods and strategies to assess forest health. These include various types of Earth Observation systems.
Applications of remote sensing contributing to sustainable forest management are generally presented in four categories that include classification of forest type (i.e., tree species), the assessment of forest structure and available resources (e.g., timber volume, height, age, crown closure), modeling forest ecosystem processes, and forest change detection. For each category, measurable indicators are needed to quantify the effects of management activities and natural phenomena on the sustainability of forest ecosystems on large spatial scales. Some of these indicators can be derived best by imaging spectroscopy data.
We are currently on the brink of a new age of space-borne imaging spectrometers. While the first space-borne imaging spectrometers, CHRIS–Proba and Hyperion, were not able to create a sustainable impact on operational forest remote sensing, the next generation is expected to more reliably deliver a higher data quality. Recently, the imaging spectrometer DESIS (Eckardt et al. 2015) has been installed in the International Space Station. In the near future, several space-borne imaging spectrometers, such as PRISMA (Labate et al. 2009), SHALOM (Feingersh and Ben Dor 2015), HISUI (Matsunaga et al. 2017), EnMAP (Guanter et al. 2015), HYPXIM (Carrere et al. 2013), Surface Biology and Geology (SBG, formerly known as HyspIRI) (Lee et al. 2015), or the recently announced Earth Surface Mineral Dust Source Investigation (EMIT) will be launched. In the Copernicus program, the hyperspectral mission CHIME is in consideration (Nieke and Rast 2018). During the preparation programs to these sensors, especially EnMAP and SBG, large-scale airborne campaigns, have been carried out. So now a few large-scale data sets are available, including some at multiple points in time. Given the unprecedented range of future remote sensing systems in space, an essential question is which of the spectral traits can be better characterized with high spectral, medium spatial resolution instruments. The first instruments will cover only limited areas and are thus expected to be used primarily in scientific contexts. Products developed with scientific instruments may be of general interest for forest practitioners once operational wall-to-wall space-borne imaging spectroscopy becomes available with SBG and CHIME. In the following, we will first highlight which role imaging spectroscopy (IS) data is likely to play in three key application fields of forestry and then outline the scope of this study.
1.1 Tree Species Composition
Spatially explicit information on tree species composition of managed and natural forests is relevant for biomass or timber volume and quality estimation, habitat quality assessment, and biodiversity characterization. It thus provides valuable information for nature conservationists as well as for forest managers and is frequently required over large spatial extents. Assuming the same spatial resolution, hyperspectral imaging certainly has the potential to provide higher classification accuracies for forest species mapping than multispectral data (e.g., Asner and Martin 2009; Fassnacht et al. 2014; Foster and Townsend 2004), especially when implemented at local scales. As well, the range of advanced spectral classifiers has been substantially expanded so that it is now possible to make the best use of the available data (e.g., Ghamisi et al. 2017). However, a high degree of species diversity does not necessarily lead to an equally high diversity of traits (Lausch et al. 2016), and large area species mapping is rather boosted by phenological dynamics observed by operational multi- or super-spectral observation systems (Somers and Asner 2013; Stoffels et al. 2015). Considering this and the limited data availability, it is not surprising that only few examples for using hyperspectral data for successful tree species classifications over large and diverse geographic areas have been presented (Fassnacht et al. 2016; Sommer et al. 2016). On the one hand, this might change as soon as space-borne imaging spectrometers will regularly cover large areas. On the other hand, the spatial resolution of proposed sensors may not be sufficient for species identification, especially if species in mixed stands are to be mapped, thus requiring mapping of ecological communities (e.g., Foster and Townsend 2004).
1.2 Structural Traits
2D or 3D structural spectral traits depending on leaf arrangement and geometry, tree height or area, density, size, the shape of forest patches as well as fragmentation, complexity, and homogeneity are important characteristics of forests that strongly influence the spectral response of forest canopies. Moreover, forest canopy height, forest extent, and the vertical and horizontal vegetation structure of forests are important proxies for assessing standing timber volume (Nink et al. 2015). There is evidence that multi-resolution multispectral imagery can be used to estimate forest structure based on texture (Wolter et al. 2009) which likely also applies to hyperspectral data, but not with the accuracy and physical realism of active remote sensing data. Given the physical limitations of addressing structure and biomass information over closed canopies with mono-directional, passive optical systems, it is not surprising that hyperspectral data alone will contribute little that goes beyond the information provided by active remote sensing such as airborne laser scanning and synthetic aperture radar (Swatantran et al. 2011).
1.3 Detection of Stress and Biochemical Traits
Many studies emphasize the importance of hyperspectral data for characterizing the physiological state of canopies, for example those under stress (e.g., Abdullah et al. 2018; Asner et al. 2004; Dotzler et al. 2015; Lausch et al. 2016; Paz-Kagan et al. 2018). As such, one of the most important application domains of hyperspectral imaging is the observation of biochemical traits. Alterations to biochemical traits such as cellulose, protein, and lignin, changes in the composition and configuration of photosynthetically active pigments in leaves (chlorophyll, xanthophyll, carotenoids) are highly relevant indicators for climate- and site-specific stress conditions, disturbances or resource limitations (Asner et al. 2015; Féret et al. 2017; Singh et al. 2015), although their detection may be hampered by leaf structural changes such as rolling (Chávez et al. 2013). Accurate retrieval of foliar nitrogen, for example, could be used to detect nutrient deficiencies, but also enhance ecosystem process models that describe ecosystem functioning, since nitrogen is an important input parameter of these models. Foliar N is a primary regulator of physiological processes (Wang et al. 2017) and an important indicator of photosynthetic and growth rates (Townsend et al. 2003) since it reflects the investment in photosynthetic enzymes, structure, and light-harvesting complexes of a plant (Singh et al. 2015). Most foliar N (70–80%) is stored in proteins, and less in chlorophyll (Hank et al. 2019; Huber et al. 2008; Schlerf et al. 2010).
The spectral capacity to monitor the distributions of biochemical traits, such as canopy foliar N distribution (Knyazikhin et al. 2013; Schlerf et al. 2010; Serbin et al. 2014; Singh et al. 2015; Townsend et al. 2003), and changes in the water content (Asner et al. 2016; Paz-Kagan et al. 2018), has already revealed the potential to assess indicators for ecosystem functioning on extended spatial dimensions. Pigment-related trait variations also support the observation of shifts in the photosynthesis system (Gamon et al. 1992; Garbulsky et al. 2011; Peñuelas et al. 1995; Suárez et al. 2008), although unveiling the causal relationships behind diurnal, seasonal, and spatial variations of photosynthesis will require instruments with high repetition rates and exceptionally high spectral resolution, capable of tracking changes in the solar-induced chlorophyll fluorescence (Drusch et al. 2017; Rascher et al. 2015).
1.4 Scope of the Paper
To make the best use of space-borne imaging spectroscopy data, robust retrieval algorithms that work over large geographic and temporal scales, are needed. Ideally, these should also output uncertainties for all values retrieved. While in the past many imaging spectroscopy measurements were singular studies, in the age of operational space-borne sensors, operational algorithms are required. The way to estimate spectral traits and their variation is intricately linked to the fundamental debate about empirical and physically based retrieval strategies, discussed in the following section.
Since neither tree species classification (with present technologies and data) nor the determination of structural traits is substantially improved by imaging spectroscopy, we will focus on chemical traits. A main scope will be on the comparison of empirical, physically based, and hybrid modeling on the leaf and canopy scale and the question whether empirical approaches can also be generalized to an extent that they become applicable to a wider range of conditions. There are a number of reasons to focus on biochemical traits and the generalizability of retrieval approaches: the availability of radiometrically well-calibrated data and the level-2 processing capacities have constantly improved (Frantz et al. 2016; Masek et al. 2006; Storch et al. 2013; Thompson et al. 2018, 2019; Vermote et al. 2016). In addition, the hyperspectral observation capacities have been increasing during the past years and will further increase with the advent of hyperspectral satellites. Last, but not least, the efficiency of algorithms for building adequate models has been boosted by a range of powerful machine learning algorithms (Fassnacht et al. 2014; Hultquist et al. 2014; Paz-Kagan et al. 2018; Waske et al. 2012).
Here, we provide a review of the literature, with some points illustrated with measurements recently conducted by the authors. The second section provides a comparison of empirical versus physically based modeling; Sect. 3 deals with studies on the estimation of biochemical traits on the leaf and canopy scale, with Sect. 4 providing discussion and conclusions.
2 Empirical Versus Physically Based Modeling
2.1 Empirical Modeling
Principal workflows of empirical and physically based approaches for estimating biochemical leaf traits on leaf and canopy level
Empirical models, both parametric and nonparametric, aim at building a statistical relationship between measured traits and an experimental reflectance database. However, these empirical relationships are considered limited in transferability and robustness because of their dependency on the particular conditions under which they were established (field data collection strategy, species diversity, instrumental characteristics, date of acquisition, phenological state of the vegetation, atmospheric, and lighting conditions) (Féret et al. 2017; Li and Wang 2011; Verrelst et al. 2015). Establishing empirical relationship builds on regression algorithms (e.g., multivariate linear regression, principal component regression, partial least-squares regression), machine learning techniques (support vector machines, import vector machines, artificial neural networks, deep learning networks, random forests, or Gaussian processes regression), or unmixing approaches, such as multiple endmember spectral mixture analysis (Roberts et al. 1998) or iterative spectral mixture analysis (Rogge et al. 2006).
In recent years, partial least-squares regression (PLSR) has emerged as the most popular parametric empirical method that can be used to estimate variables of interest even if the explaining variables (spectral reflectance) are redundant and collinear (Atzberger et al. 2010; Wold et al. 2001). The popularity of PLSR is related to its long history (Wold et al. 1984), ease of implementation, relative robustness to new data, and operational computational speed relative to deep learning methods. PLSR generates comparable results to machine learning methods such as Gaussian processes (Wang et al. 2019), but uncertainties need to be assessed through permutation rather than direct calculation (Singh et al. 2015). Asner et al. (2011) and Martin et al. (2018a), for example, used PLSR to measure a wide range of canopy chemicals using field and laser-guided airborne spectroscopy. On the machine learning side (nonparametric empirical models), deep learning networks are increasing in popularity due to improved computational capacities, but other machine learning approaches are also successfully employed. For example, Hultquist et al. (2014) assessed the post-fire burn severity with airborne MASTER data and found that random forest, support vector regression, and Gaussian process regression outperformed conventional multiple regression by 48%, 29%, and 27%, respectively.
Because of the usually high number of inter-correlated spectral channels, it is beneficial for most empirical modeling approaches to reduce the number of spectral features (feature reduction), either by selecting a subset of important bands (feature selection) or by transforming original bands into new features with maximal correlation to the dependent variable of interest (feature extraction) (Fassnacht et al. 2014; Held et al. 2015). Main feature extraction strategies include spectral indices (Féret et al. 2011), wavelets (Cheng et al. 2014; Wang et al. 2017), or linear transformations such as principal components or minimum noise fractions (Green et al. 1998), and singular value decomposition (Rogge et al. 2014). In feature selection approaches, only significant bands are used before establishing a regression model. Popular techniques include stepwise multiple regression (Schlerf et al. 2010), competitive adaptive reweighted sampling (CARS) (Vohland et al. 2016), and band selection based on variable importance in the projection (VIP) score (Abdullah et al. 2018; Heim et al. 2015). Empirical models may also be improved by spectral pre-treatment techniques like first and second derivative, transformation from reflectance to absorption, z-transformation, or Savitzky–Golay smoothing (Buddenbaum and Steffens 2012; Serbin et al. 2014).
2.2 Physically Based Modeling
Physically based modeling approaches aim at overcoming the limitations in transferability and robustness by simulating the radiative transfer between light source, canopy, and sensor (Feret et al. 2008). Leaf and canopy reflectance models predict the directional spectral reflectance based on radiation principles and a mathematical description of absorption and scattering processes in leaves, needles, or complete canopies. Foliar or canopy traits are then retrieved from spectral measurements by inverting the models (Combal et al. 2002; Jacquemoud and Baret 1990; Jacquemoud et al. 2009; Koetz et al. 2004; Schlerf and Atzberger 2006; Wang et al. 2018).
The physically based approach involves numerous strategies of varying complexity. There are models for leaf or needle optical properties, 1-D models for homogeneous canopies, and 3-D models for heterogeneous stands. Among the models of leaf optical properties (Miller et al. 2005) the by far the most widely used representative is PROSPECT. While early versions of PROSPECT only considered Chlorophyll a and b (Jacquemoud et al. 2000; Jacquemoud and Baret 1990; Jacquemoud et al. 1996), PROSPECT-4 and -5 added carotenoid content (Feret et al. 2008), and PROSPECT-D added anthocyanins (Féret et al. 2017). The PROSPECT model family has been designed for broadleaf species, but can also be recalibrated for needles (Malenovský et al. 2006a). A popular alternative for needles is LIBERTY (Dawson et al. 1998).
The most simplistic model to up-scale leaf measurements to canopy scale is the Lillesaeter model (Lillesaeter 1982). The model calculates canopy reflectance as ρ/(1–τ2), where ρ is leaf reflectance and τ is leaf transmittance. Leaf-scale reflectance models are often combined with canopy models to account for effects like leaf inclination, multiple scattering of several leaf layers, background effects, and illumination and observation geometry. The SAIL (Scattering by Arbitrarily Inclined Leaves) model (Verhoef 1984) is commonly combined with PROSPECT to simulate reflectance of homogeneous canopies (Berger et al. 2018; Jacquemoud et al. 2009). Since forest canopies tend to be more complicated, further model extensions like GeoSail (Huemmrich 2001), PROFLAIR (Omari et al. 2013), and InFoRM (Atzberger 2000; Schlerf and Atzberger 2006) add realism related to discontinuous canopies, varying tree geometry and density and multiple scattering. More advanced forest reflectance models such as FRT (Kuusk et al. 2014), FLIGHT (Barton and North 2001; North 1996), and 3D ray tracing models like DART (Gastellu-Etchegorry et al. 2004) permit even more differentiated and detailed parametrization of canopy architectures.
The inversion of these models is based either on numerical algorithms, or on searching through lookup tables (LUT) which have been created by using the model in forward mode. Owing to the structural complexity of forest canopies with their diverse architectures, the inversion problem is complex and becomes frequently ill-posed with increasingly complicated models, i.e., different combinations of input parameters may produce almost identical spectra in the forward mode (Combal et al. 2002).
A typical strategy (e.g., Bicheron and Leroy 1999) is to fix some the model parameters to “reasonable” values, and to determine the remaining (free) parameters by minimizing a merit function, i.e., a function of difference between observed and modeled reflectance.
The LUT-based approach identifies the best-fitting spectrum from the collective of pre-computed solutions; the corresponding parameters represent the inversion result. The LUT has to be created just once, and only the relatively simple task of finding the best match has to be repeated for each spectrum. This can be much faster, especially for large data sets (e.g., a hyperspectral image) and computationally complex models. But again, if a large number of free parameters need to be determined, LUTs tend to become too large and this may as well lead to ill-posedness. Since the best-fitting spectrum does not always yield the best set of parameters, a range of similarly good fitting spectra can be selected and their mean or median accepted as the inversion result; their standard deviation then provides a measure of uncertainty (Koetz et al. 2007; Locherer et al. 2015).
Figure 1 illustrates the alternative workflows of estimating biochemical traits on leaf and canopy level via empirical or physically based methods. The desired estimations of leaf biochemistry can be obtained directly from the measured canopy reflectance through inversion of a physically based model. If empirical models on canopy levels are to be used, measured leaf biochemistry values are needed that have to be up-scaled to canopy level. It may also be advisable to transform the spectral data using approaches like double ratios or directional area scattering factor (DASF, Knyazikhin et al. 2013, see Sect. 4). A different pathway to estimating leaf biochemistry on canopy level can be taken through measurements of leaf reflectance. Since reflectance measurements are faster and cheaper than chemical analyses they can be used to create a large ensemble of leaf biochemical values that can be up-scaled to canopy level.
3 Identifying Biochemical Traits on Leaf and Canopy Level
Spectroscopic studies on the leaf level serve a number of purposes: first, they can supplement or even replace laboratory analyses and help creating reference data sets for studies on canopy level. Second, they provide a basis to understand interactions between plants and radiation without confounding effects of the canopy structure. Generally, identifying biochemical traits is much more straightforward on leaf level than on canopy level (Ustin et al. 2009), although the structural contributions at the canopy level can reinforce empirical relationships at the expense of physical realism (Townsend et al. 2013). Third, results on leaf level can be up-scaled to canopy level (Gara et al. 2018; Malenovský et al. in review).
Reflectance measurements of broadleaves are ideally acquired using an integrating sphere (Asner et al. 2011; Jay et al. 2016; Kokaly et al. 2009), although measurements with a leaf clip are fast and popular. Needle reflectance is a little harder to determine in a reliable way. Malenovský et al. (2006a), Yáñez-Rausell et al. (2014a, b), and Hovi et al. (2017) have presented studies on how to measure needle reflectance with integrating spheres. Alternative ways to determine needle reflectance include measuring needle stacks or mats (Dawson et al. 1998; Schlerf et al. 2010).
a, b Specific absorption coefficients of main absorbing leaf constituents according to the PROSPECT-D leaf optical model. Inset in subfigure b shows weak water absorption bands near 975 and 1200 nm. Note that the absorption coefficients for brown pigments are given in arbitrary units. c Absorption coefficients of protein and lignin + cellulose according to Wang et al. (2015)
Both empirical models and the inversion of physically based reflectance models for analyzing biochemical traits on the level of leaves and needles have already reached quite a high level of accuracy, in particular for pigments, water content, leaf mass per area, but not for cellulose, lignin, and protein in fresh leaves (Gitelson et al. 2006; Jay et al. 2016; Li et al. 2018; Malenovský et al. 2006a; Wang et al. 2015, 2018).
Asner et al. (2012, 2017) propose seven leaf traits that are functionally relevant to forest diversity and mostly uncorrelated (Asner et al. 2017). Foliar nitrogen (N), water, and LMA, which play important roles in photosynthesis and primary production, have been characterized remotely in various studies (Ustin et al. 2004). Polyphenols act as foliar defense compounds (Kokaly et al. 2009), while lignin is an important leaf structural compound that is recalcitrant to decomposition. Foliar phosphorus (P) and calcium (Ca) are related to species community turnover in tropical forests and have also been mapped by IS (Asner and Martin 2016; Chadwick and Asner 2016), although relationships with spectra appear correlative and unrelated to currently known absorption features in the VSWIR. As considering the full range of traits is beyond the scope of this paper, we will focus on foliar pigment, water, and nitrogen content.
On the level of tree canopies, either the integrated content of a biochemical trait per ground area can be measured (e.g., canopy water content, CWC, or canopy chlorophyll content, CCC), or the respective content per leaf area (e.g., EWT or leaf chlorophyll content, LCC). These are related by the leaf area index (LAI), i.e., \( {\text{LCC}} \cdot {\text{LAI}} = {\text{CCC}} \) and \( {\text{EWT}} \cdot {\text{LAI}} = {\text{CWC}} \) (Hank et al. 2019). Where LAI is not estimated, estimates generally refer to traits at the top of the canopy (Singh et al. 2015); note also that some traits vary with canopy position (especially LMA, but not so much N), so that any calibration or validation of an integrated canopy estimate must take into consideration within-canopy variation. The retrieval of biochemical traits of leaves is also substantially compromised through structural properties of the crown layer and the ground reflectance that lead to nonlinear signal mixing through multiple scattering (Ewald et al. 2018). Various approaches for compensating these effects have been presented. These include inversion of physically based reflectance models and approaches like the normalization of spectra through a spectral index correlated with leaf area (double-ratio approach, Colombo et al. 2008) or by physically based normalization factors such as the directional area scattering factor (DASF, Knyazikhin et al. 2013). The selection of illuminated crown pixels is a valid strategy to be applied to either approach, provided the spatial resolution is sufficiently detailed (Asner et al. 2015; Dotzler et al. 2015).
Dotzler et al. (2015) demonstrated the importance of canopy-related effects when investigating the potential of EnMAP and Sentinel-2 to detect early drought stress effects in deciduous forests. Differences between stands on different soil moisture regimes that were statistically significant in moisture stress index (MSI), normalized difference water index (NDWI), and chlorophyll index (CI) disappeared after these indices were normalized to leaf level by using the double-ratio approach of Colombo et al. (2008). The comparison with data from airborne laser scanning suggested that the moisture and pigment differences detected on canopy level were actually due to variations in crown density and volume. The detection of drought-related stress symptoms on leaf level therefore requires that canopy effects be addressed and removed, in particular when spectral information from spectral regions with high transmission and scattering characteristics is used (especially near infrared).
On canopy level, the retrievals of biochemical and structural traits through inversion of physically based models are so far less successful than on leaf level. While most geometric-optical canopy reflectance models [e.g., InFoRM (Atzberger 2000; Schlerf and Atzberger 2006)] are surprisingly good in reproducing measured canopy reflectance as long as important canopy properties (leaf area index, leaf angle distribution, etc.) are known (forward mode), their performance tends to break down in the inversion (where these parameters must be retrieved simultaneously with the biochemical traits).
We summarize accuracies of cited studies in tables. Each study reports the accuracies in a different way and has been conducted under different conditions, so these studies should not be compared with each other directly. Since the coefficient of determination (R2) is the only accuracy measure reported in nearly all studies, we give R2 values of all studies cited although they are quite dependent on the range of values considered. If the root-mean-square error (RMSE) or the RMSE relative to the mean value or the range of values is also reported, that can be found in the respective table. From a statistical standpoint, achievement of a low RMSE and seemingly poor R2 is common when the data range is limited (e.g., often seen with the prediction of carbon concentration, Singh et al. 2015). The resulting maps may exhibit reasonable uncertainty based on RMSE, but the models will be unlikely to be extensible.
3.1 Pigment Content Retrieval
Accuracies and further information of pigment estimation studies
| Study | Pigment | Unit | Range | Accuracy | Species | Method | ||||
|---|---|---|---|---|---|---|---|---|---|---|
| Min | Max | Mean | R 2 | RMSE | RMSE% | |||||
| Leaf level | ||||||||||
| Gitelson and Merzlyak (1997) | Chlorophyll a + b | µg/cm2 | 0.27 | 62.9 | ≥ 0.9 | ≤ 5.3a | 8 Different species | Indices | ||
| Gitelson et al. (2003) | Chlorophyll a + b | µmol/cm2 | 1 | 832 | ≥ 0.95 | ≤ 75 | 8 Different species | Indices | ||
| Gitelson et al. (2006) | Chlorophyll a + b | mg/m2 | 83–408 | ≥ 0.91 | ≤ 51 | < 25 | 6 Different species | Indices | ||
| Gitelson et al. (2006) | Carotenoids | mg/m2 | 16 | 166 | 50–96 | ≥ 0.70 | ≤ 17.2 | ≤ 19 | 3 Different species | Indices |
| Gitelson et al. (2006) | Anthocyanins | mg/m2 | > 0.93 | 2 Different species | Indices | |||||
| Fassnacht et al. (2015) | Carotenoids | mg/m2 | 15.96 | 137.2 | 63.38 | 0.88 | 8.12 | 3 Tree species | Index fusion | |
| Asner et al. (2011) | Chlorophyll a | mg/g | 0.83 | 15.99 | 4.78b | 0.83 | 6.2 | Many tropical tree species | PLSR on transm | |
| Asner et al. (2011) | Chlorophyll b | mg/g | 0.29 | 5.83 | 1.78b | 0.82 | 6.2 | Many tropical tree species | PLSR on transm | |
| Asner et al. (2011) | Carotenoids | mg/g | 0.35 | 4.32 | 1.41b | 0.79 | 8.1 | Many tropical tree species | PLSR on transm | |
| Abdullah et al. (2018) | Chlorophyll a + b | mg/g | 0.4 | 1.1 | 0.64 | 0.24c | Healthy Spruce Needles | PLSR | ||
| Abdullah et al. (2018) | Chlorophyll a + b | mg/g | 0.2 | 1 | 0.55 | 0.75c | Infested Spruce Needles | PLSR | ||
| Malenovský et al. (2006a) | Chlorophyll a + b | µg/cm2 | 25.57 | 85.1 | 8.05 | Norway spruce | Modified PROSPECT-3 inv. | |||
| Féret et al. (2011) | Chlorophyll a + b | µg/cm2 | 0.3 | 106.72 | 32.81 | 5.38d | Wide range of species | PLSR | ||
| Féret et al. (2011) | Carotenoids | µg/cm2 | 0.04 | 25.3 | 8.51 | 1.9d | Wide range of species | PLSR | ||
| Buddenbaum et al. (2011) | Chlorophyll a + b | µg/cm2 | 28.73 | 82.35 | 57.75 | 0.7764 | 15.4592 | 26.77 | European Beech | PROSPECT-3 inversion |
| Buddenbaum et al. (2011) | Chlorophyll a + b | µg/cm2 | 48.13 | 102.11 | 78.87 | 0.5351 | 8.951 | 11.35 | Oak | PROSPECT-3 inversion |
| Buddenbaum et al. (2012) | Chlorophyll a + b | µg/cm2 | 7 | 29 | 0.73 | 20.9 | European Beech | PROSPECT-5b inversion | ||
| Schlerf et al. (2010) | Chlorophyll a + b | mg/g | 1.8 | 4.94 | 3.21 | 0.83 | 0.26 | Norway spruce | SMR, CM | |
| Canopy level | ||||||||||
| Malenovský et al. (2006b) | Chlorophyll a + b | µg/cm2 | 28 | 94 | 0.72 | 9.53 | Norway spruce | ANMB605-725 index | ||
| Schlerf et al. (2010) | Chlorophyll a + b | mg/g | 2.71 | 3.62 | 3.21 | 0.8 | 0.13 | Norway spruce | SMR, CM | |
| Asner et al. (2015) | Chlorophyll a + b | mg/g | 2.65 | 8.53 | 0.58 ± 0.05 | 1.5 ± 0.71 | 28.14 | Many tropical tree species | PLSR | |
| Asner et al. (2015) | Carotenoids | mg/g | 0.69 | 1.84 | 0.49 ± 0.06 | 0.25 ± 0.08 | 21.2 | Many tropical tree species | PLSR | |
| Martin et al. (2018a) | Chlorophyll a + b | mg/g | 2 | 10.3 | 5.6 | 0.21 | 1.68 | 29.8 | Many tropical tree species | PLSR |
| Buddenbaum et al. (2015) | Chlorophyll a + b | µg/cm2 | 15 | 40 | 0.325 | 4.059 | 16.85 | European Beech | PLSR | |
| Omari et al. (2013) | Chlorophyll a + b | µg/cm2 | 39 | 56 | 0.2553 | 4.46 | Trembling aspen, balsam poplar, jack pine | PROFLAIR inversion | ||
Gitelson and Merzlyak (1997) and Gitelson et al. (2003) showed that simple spectral indices can be used for empirical pigment estimations. Gitelson et al. (2006) further proposed spectral three-band indices for estimating chlorophyll, carotenoids, and anthocyanin contents from leaf reflectance. Fassnacht et al. (2015) introduced a method for merging vegetation indices for carotenoid estimation over a large range of values. Carotenoid contents of the test data set ranged from 16 to 137 mg/m2 and the merged index could explain 88% of the variance. These index-based pigment estimations reach quite high accuracies and have been applied over a wide range of vegetation types (e.g., Beamish et al. 2018; Fawcett et al. 2018; Hernández-Clemente et al. 2012; Sonobe and Wang 2018). Alternatively, Asner et al. (2011) used full-spectrum empirical regression models (PLSR) to estimate numerous foliar traits of many tropical tree species on leaf level using reflectance and transmittance spectra. The highest accuracies for pigments were achieved using transmittance in the full spectral range of 400–2500 nm with R2 values of 0.83, 0.82, and 0.79 for chlorophyll a, chlorophyll b, and carotenoids, respectively. Feilhauer et al. (2015) conducted a multi-method ensemble selection of spectral bands and estimated chlorophyll concentrations of several sets of leaf measurements with large variations in R2 values. The highest accuracies were achieved using an optimized PLSR. Abdullah et al. (2018) used PLSR to estimate chlorophyll content of spruce needles from the reflectance in the 400 to 790 nm range to detect bark beetle attacks. In healthy needles, chlorophyll content was estimated with R2 = 0.64, in infested needles with R2 = 0.55.
High to very high leaf-level estimation accuracies are also achieved by inverting the PROSPECT model or by using it for hybrid approaches. Le Maire et al. (2004) used PROSPECT simulated spectra to derive universal leaf chlorophyll content spectral indices. In order to test the applicability of PROSPECT for spruce needles, Malenovský et al. (2006a) applied a constrained inversion of PROSPECT-3.01 and retrieved chlorophyll content with an RMSE of 8.05 µg/cm2. Féret et al. (2011) compared the performance of spectral indices, PLSR, and inversion of PROSPECT-5 for the estimation of chlorophylls and carotenoids on synthetic and real leaf reflectance and transmittance data. They found that the inversion approach is only reliable when both leaf reflectance and transmittance data are available, in which case, the accuracies were higher than using regression. Using PROSPECT-3 inversion, Buddenbaum et al. (2011) explained 0.78 and 0.54 of the variance of SPAD chlorophyll measurements of beech and oak leaves, respectively, that were collected by tree climbers from the crowns of old and young trees. In a greenhouse experiment, Buddenbaum et al. (2012) yielded R2 = 0.73 for leaf chlorophyll content of potted young beech trees by inverting PROSPECT-5B based on spectra measured with a leaf clip. Dechant et al. (2017) also measured reflectance spectra with a leaf clip and inverted PROSPECT-5B to determine chlorophyll, carotenoid, and water content of leaves instead of chemical analyses, assuming they achieve laboratory level precision.
PROSPECT-D inversion results versus laboratory measurements of chlorophyll a + b (left) and carotenoids (right)
On canopy level, the results from published studies are inconsistent and in some cases conflicting. Several studies report reasonably high estimation accuracies of foliar pigment content on canopy level (Malenovský et al. 2006b; Schlerf et al. 2010), while others report low accuracies. Malenovský et al. (2006b) reached R2 = 0.72 for canopy chlorophyll content estimation in a Norway spruce stand with a high variability of chlorophyll contents (27.5-–93.5 µg/cm2) using their ANMB605–725 index and only R2 = 0.17 using the established TCARI/OSAVI index that was originally designed for agricultural crops. Schlerf et al. (2010) used stepwise multiple regression for chlorophyll estimation on stand level in Norway spruce stands with chlorophyll concentrations of 2.7 to 3.6 mg/g dry matter and reached cross-validated R2 values between 0.46 and 0.9, depending on spectral transformation before the regression step. Asner et al. (2015) used airborne high-fidelity visible-to-shortwave infrared spectroscopy to estimate multiple foliar chemicals in tropical forests with PLSR to establish chemical fingerprints of numerous tree species. Independent field validation gave a mean R2 of 0.58 for chlorophyll estimation for several tropical tree species with chlorophyll concentrations of 2.65–8.53 mg/g and an R2 of 0.49 for carotenoid estimation in a range of 0.69–1.84 mg/g. Martin et al. (2018a) evaluated the methods of Asner et al. (2015) on data and field measurements of a wide array of canopies in Bornean forests of Sabah, Malaysia. Despite the strong chlorophyll absorption, the retrieval accuracy of chlorophyll concentration was among the lowest of all variables with R2 values of 0.43, 0.17, and 0.21, respectively, for model calibration, validation, and test for a range of 2–10.3 mg/g. Buddenbaum et al. (2015) used field-level IS and PLSR to estimate leaf chlorophyll content of beech seedlings with chlorophyll contents of 15 to 40 µg/cm2. The R2 only reached 0.325, but an interesting finding was that the regression coefficients were significantly different from zero only in the chlorophyll-dominated region of the spectrum, which showed that PLSR was able to determine important bands and ignore the bands outside chlorophyll absorption features.
A physically based model inversion approach for chlorophyll retrieval from Hyperion data is reported in Omari et al. (2013). They inverted the PROFLAIR model (Omari et al. 2009; White et al. 2001) and yielded a low RMSE but only R2 = 0.26 for LCC in a quite limited range of 40–55 µg/cm2. They ascribed the low R2 value to heterogeneity within the site, uncertainties in preprocessing, the sampling scheme, and the temporal shift between sampling and satellite data acquisition. Due to large LAI variations, CCC also had a larger range and was estimated with R2 = 0.60 after successful LAI retrieval.
3.2 Water Content
Accuracies and further details of water content estimation studies
| Study | Unit | Range | Accuracy | Species | Method | ||||
|---|---|---|---|---|---|---|---|---|---|
| Min | Max | Mean | R 2 | RMSE | RMSE% | ||||
| Leaf level | |||||||||
| Riaño et al. (2005) | EWT (cm) | 0.001 | 0.05 | 0.94 | LOPEX data set | PROSPECT inversion | |||
| Malenovský et al. (2006a) | EWT (cm) | 0.0419 | 0.0701 | 0.0006 | 55 | Norway spruce | PROSPECT-3.01.S inversion | ||
| Buddenbaum et al. (2011) | EWT (cm) | 0.00263 | 0.0173 | 0.75 | Beech and Oak | PROSPECT-3 inversion | |||
| Buddenbaum et al. (2012) | EWT (cm) | 0.00125 | 0.005 | 0.004 | 0.87 | 11.3 | Beech | PROSPECT-5b inversion | |
| Colombo et al. (2008) | EWT (cm) | 0.0091 | 0.0154 | 0.0122 | 0.65 | Poplar | PROSPECT inversion | ||
| Cheng et al. (2011) | LWCD (%) | 32.31 | 418.2 | 143.6 | 0.71 | 26.04 | Lianas and trees | Continuous wavelet analysis | |
| Cheng et al. (2011) | LWCF (%) | 24.42 | 80.7 | 57.23 | 0.75 | 4.34 | Lianas and trees | Continuous wavelet analysis | |
| Fang et al. (2017) | EWT (cm) | 0.0037 | 0.0525 | 0.0115 | 0.95 | 3.63 | LOPEX data set | Spectral index (SWI Double SAC) | |
| Fang et al. (2017) | EWT (cm) | 0.0038 | 0.0255 | 0.0136 | 0.8 | 7.9 | PANAMA data set | Spectral index (SWI Double SAC) | |
| Canopy level | |||||||||
| Riaño et al. (2005) | FMC (%) | 0 | 200 | 0.81 | Gall oak, rosemary, Rock Rose | PROSPECT–Lillesaeter inversion | |||
| Colombo et al. (2008) | mean leaf EWT (cm) | 0.0117 | 0.0136 | 0.57 | 2.264 | Poplar | Double ratio index MSI/SR | ||
| Colombo et al. (2008) | EWTcanopy (g m−2) | 80 | 400 | 0.818 | Poplar | PROSPECT–SAILH inversion | |||
| Buddenbaum et al. (2015) | EWT (cm) | 0.001 | 0.008 | 0.7979 | 0.0007 | 9.951 | European Beech | PLSR | |
| Buddenbaum et al. (2012) | EWT (cm) | 0.00125 | 0.005 | 0.004 | 0.88 | European Beech | PLSR | ||
Physically based model retrievals of EWT usually yield high accuracies on leaf level. Riaño et al. (2005) estimated EWT and FMC by inversion of radiative transfer models to quantify fire danger. When applied to the LOPEX (Hosgood et al. 1994) leaf data set, EWT and FMC were estimated with R2 = 0.94 and 0.89, respectively, by inverting PROSPECT. Malenovský et al. (2006a) used unconstrained and constrained inversions of both PROSPECT-3.01 and an adapted version called PROSPECT-3.01.S to retrieve EWT of spruce needles from VNIR spectra. While the unconstrained inversions yielded quite low accuracies, the constrained inversion results were very close to the measured values with RMSE = 0.0006 (R2 not reported). Buddenbaum et al. (2011) inverted PROSPECT-3 to estimate EWT of beech and oak leaves with R2 = 0.75, and Buddenbaum et al. (2012) achieved R2 = 0.87 with an inversion of PROSPECT-5B for beech leaves. The inversion results were used as a plausibility check to verify the reference measurements of EWT and the leaf spectra.
Similarly, Colombo et al. (2008) tested several spectral indices and regression approaches to estimate EWT on leaf level. Reduced major axis regression exploiting the 1200 nm water absorption feature performed best among the index-based approaches and explained 0.61 of variance, while the inversion of PROSPECT produced a coefficient of determination of 0.65. Cheng et al. (2011) estimate leaf water concentration relative to fresh (LWCF) and dry (LWCD) leaf mass using continuous wavelet analysis (CWA) and several spectral indices. While the spectral indices failed at estimating LWC with R2 values around 0.1, LWCF and LWCD were estimated with R2 = 0.68 and 0.75, respectively, using a combination of six wavelet features. Fang et al. (2017) tested spectral similarity water indices on two freely available leaf data sets (LOPEX93 an PANAMA) and on PROSPECT simulations and explained up to 0.95 and 0.80 of the variance of LOPEX93 and PANAMA EWT, respectively.
Drying experiment of oak leaves. a Reflectance spectra colored by EWT, b EWT estimation through PROSPECT-D inversion, c EWT estimation using the best ratio index, d EWT estimation using PLSR, data set split into training and validation sample
In contrast to the other variables discussed here, canopy water content can be derived from IS data during atmospheric correction when absorption bands at 980 and 1160 nm of gaseous, liquid, and solid water are fit to the spectrum (Green et al. 2006; Thompson et al. 2019). Asner et al. (2016) used this approach and characterized the progressive forest canopy water loss during the 2012–2015 drought in California (see also Martin et al. 2018b; Paz-Kagan et al. 2018). Paz-Kagan et al. (2018) mapped the canopy water content of giant Sequoia trees using a random forest machine learning approach. Their results show that IS can contribute to large-scale mapping of environmental influences like drought on forest ecosystems.
Riaño et al. (2005) inverted the PROSPECT–Lillesaeter model to estimate EWT and FMC on canopy level with R2 = 0.75 for a range of 0–0.05 g/cm2. FMC estimations improved from R2 = 0.62 to 0.81 when dry matter content from dried leaf samples measured in a laboratory were taken into account. Koetz et al. (2004) used radiative transfer modeling for forest fire fuel estimation. A GeoSail (Huemmrich 2001) inversion yielded low RMSE for EWT retrieval, but also a very low R2. The estimation of CWC showed a better result, but no R2 is reported. Koetz et al. (2007) inverted the GeoSail model on synthetic data and could retrieve EWT values between 0.025 and 0.065 g/cm2 with R2 = 0.92.
Colombo et al. (2008) used Multispectral Infrared and Visible Imaging Spectrometer (MIVIS) data acquired over poplar plantations in Italy for landscape level estimations of EWT. A double-ratio index (MSI/SR) was used and yielded a cross-validated R2 of 0.57 for the value range of 0.0116–0.0136 g/cm2. Normalizing the moisture stress index (MSI) with the LAI-dependent simple ratio (SR) reduces the influence of LAI on the EWT estimation. An inversion of the PROSPECT + SAILH canopy reflectance model resulted in a relative RMSE of 27% (R2 = 0.818) for EWTcanopy, while the mean leaf EWT was only estimated with R2 = 0.048. This can be explained by a very small range of leaf EWT values, while LAI ranged from 0.5 to 3.5.
White et al. (2007) calculated moisture indices from Hyperion data to detect mountain pine beetle red attack damage. The moisture stress index (MSI) had highest correlation to red attack fraction. They suggest that space-borne hyperspectral imagery may be able to detect pine beetle damages at lower levels of infestation than Landsat imagery. Zhang et al. (2012) used the physically based coupled canopy-leaf radiative transfer model PROSAIL2 to derive the fraction of photosynthetically active radiation absorbed by chlorophyll and the leaf water content of a coniferous forest canopy from Hyperion data. Estimation accuracies are not provided, but the authors stress that their products provide unique information and that using the model inversion strategy, a Markov Chain Monte Carlo (MCMC) method, the globally optimal solution is found.
Buddenbaum et al. (2015, 2012) used field IS to monitor drought-stressed beech seedlings grown in pots in two different years, showing that after water supply was stopped, soil moisture declined first, followed by leaf water content and leaf chlorophyll content (Fig. 6). The drought stress is obvious in the spectra for most plants after some weeks of water deprivation (Fig. 6, red curves). For 2011 data, a PLSR in the visible/near-infrared range yielded R2 = 0.88 for EWT values from 0.001 to 0.005 g/cm2 (Buddenbaum et al. 2012); for 2012 data, a PLSR in the shortwave infrared range yielded R2 = 0.80 for a range of 0.005–0.009 g/cm2 (Buddenbaum et al. 2015).
3.3 Leaf Nitrogen Concentration
Spectra of drought-stressed beech seedlings over the course of 2 weeks
(from Buddenbaum et al. 2015)
Accuracies and further details of nitrogen concentration estimation studies
| Study | Unit | Range | Accuracy | Species | Method | ||||
|---|---|---|---|---|---|---|---|---|---|
| Min | Max | Mean | R 2 | RMSE | RMSE% | ||||
| Leaf level | |||||||||
| Schlerf et al. (2010) | % dry | 0.88 | 1.46 | 0.59 | 0.073 | 6.1 | Norway Spruce | FD-Stepw | |
| Serbin et al. (2014) | % | 0.3 | 6.4 | 0.97 | 0.13 | 4 | Northern temperate and boreal tree species | PLSR | |
| Wang et al. (2015) | g Protein/cm2 | 0.000292 | 0.002 | 0.001 | 0.47 | 0.000275 | 0.17a | LOPEX data | PROSPECT-5N inversion (fresh leaves) |
| Wang et al. (2015) | g Protein/cm2 | 0.000292 | 0.002 | 0.001 | 0.66 | 0.000202 | 0.12a | LOPEX data | SMLR |
| Dechant et al. (2017) | g/m2 | 0.52 | 3.16 | 1.53 | 0.8808 | 0.1757 | 11.5 | Mixed | PLSR |
| Wang et al. (2018) | g/m2 | 1.43 | 3.68 | 2.78 | 0.51 | 0.851 | 17 | Mixed | Prospect-5/Inform inversion |
| Canopy level | |||||||||
| Townsend et al. (2003) | % | 1.53 | 4.24 | 0.978 | 0.25 | Mixed forest | PLSR Hyperion | ||
| Townsend et al. (2003) | % | 1.53 | 4.24 | 0.849 | Mixed forest | PLSR AVIRIS | |||
| Coops et al. (2003) | % | 0.77 | 1.58 | 1.22 | 0.62 | 13 | Eucalypt | PLSR Hyperion | |
| Smith et al. (2003) | % | 1 | 2.48 | 0.79 | 0.19 | Mixed forest | PLSR AVIRIS | ||
| Smith et al. (2003) | % | 1 | 2.48 | 0.6 | 0.25 | Mixed forest | PLSR Hyperion | ||
| Huang et al. (2004) | mg/g | 9.8 | 17.8 | 14.35 | 0.8532 | Eucalypt | Modified PLSR, HySpex | ||
| Martin et al. (2008) | % | 0.7 | 2.4 | 0.83 | 0.19b | Mixed forest | PLSR AVIRIS | ||
| Martin et al. (2008) | % | 0.7 | 2.43 | 0.82 | 0.25b | Mixed forest | PLSR Hyperion | ||
| Ollinger et al. (2008) | % | 0.75 | 2.24 | 0.79 | Mixed | Univariate regression | |||
| Schlerf et al. (2010) | % dry | 0.88 | 1.46 | 0.57 | 0.055 | 4.6 | Norway spruce | FD-Stepw | |
| Knyazikhin et al. (2013) | % | 0.75 | 2.24 | 0.607 | Mixed | CSC (DASF)-Univariate | |||
| Singh et al. (2015) | % | 0.96 | 3.31 | 2.2 | 0.84 | 0.23 | 10.5 | Northern temperate and boreal tree species | PLSR |
| Asner et al. (2015) | % | 1.28 | 3.33 | 0.48 | 0.31 | 15.19 | Mixed tropical | PLSR (cal values) | |
| Wang et al. (2017) | % | 1.45 | 3.29 | 1.69? | 0.65 | 0.33 | 19 | Mixed | CSC (DASF)-wavelets |
| Wang et al. (2018) | g/m2 | 1.43 | 3.68 | 2.78 | 0.64 | 1.9 | 18 | Mixed | Prospect-5/Inform inversion |
| % | 0.6 | 4.46 | 1.79 | 0.54 | 0.43 | 24.4 | Mixed tropical | PLSR (model test) | |
Wang et al. (2015) extended the physically based leaf reflectance model PROSPECT-5 by adding protein and cellulose + lignin absorption (see Fig. 2c). They were able to invert the model for fresh and dry leaves with moderate to good accuracies for fresh leaves (R2 = 0.70 for cellulose + lignin and R2 = 0.47 for protein) and dry leaves (R2 = 0.79 for cellulose + lignin and R2 = 0.57 for protein). A stepwise multiple linear regression approach gave slightly better results (R2 = 0.83 and 0.66 for cellulose + lignin and protein in fresh leaves, respectively), but may have limited transferability.
Dechant et al. (2017) measured reflectance, leaf mass per area, nitrogen content, and traits linked to photosynthesis, maximum carboxylation capacity (Vcmax), and maximum electron transport rate (Jmax) of leaves of several tree species. They found clear relationships of the photosynthesis traits with leaf nitrogen content. The nitrogen content per leaf area was estimated via PLSR with R2 = 0.88. Studying the effects of bark beetle green attack on foliar reflectance and biochemical properties, Abdullah et al. (2018) also employed a PLSR to estimate N concentration and reached R2 = 0.76 and 0.68 for healthy and bark beetle-infested spruce needles, respectively. Schlerf et al. (2010) compared several spectral preprocessing techniques for estimating N concentration in Norway spruce (Picea abies) needles with a stepwise multiple regression approach and could explain 0.59 of the variance (cross-validated) with first derivative spectra, although the range of N in this single-species data set was quite limited (0.88 to 1.46%). Asner et al. (2011) successfully (R2 = 0.81) estimated foliar N concentration of thousands of tropical species from around the world with a PLSR using visible-to-shortwave infrared transmittance spectra, and Serbin et al. (2014) established PLS regressions for estimating seven biochemical traits (N, C, δ15N, SLA, cellulose, ADF, ADL) from ground and dried leaf samples for a large number of North American tree species. The regression models were able to explain between 60% (δ15N) and 97% (N) of the variance in the validation data set. These large samples are an important step toward the development of transferable models. Serbin et al. (2014) have published their PLSR coefficients so that they can be applied to any data set.
Estimation of leaf nitrogen content from dried and powdered samples collected in Germany (NP = Nationalpark Hunsrück-Hochwald, Ge = Gerolstein area). a Application of PLSR coefficients from Serbin et al. (2014), b local PLSR, small squares represent calibration subsample, large circles are the validation sample
Large area assessments of ecosystem productivity and health are improved tremendously by reliable estimations of foliar nitrogen content (McNeil et al. 2007a, b; Ollinger et al. 2008; Townsend et al. 2003), just as applications of fertilizers to tree plantations (Albaugh et al. 2003; Allen et al. 2005; Liechty and Fristoe 2013). Large efforts have been undertaken to collect data sets that enable transferable estimations of foliar nitrogen on canopy level.
Townsend et al. (2003) used space-borne EO-1 Hyperion and airborne AVIRIS IS data to map canopy N in Green Ridge State Forest in Maryland, USA, where leaf samples were collected in 27 ground plots. PLSR models explained 97.9% of the variation of N for Hyperion and 84.9% for AVIRIS for N concentrations between 1.2 and 4.1%. Similarly, Smith et al. (2003) explained 79% and 60% of variance of validation data in the range between 1 and 2.5% N using AVIRIS and Hyperion, respectively. Coops et al. (2003) also used Hyperion data. They mapped N concentration of Eucalypt foliage with R2 = 0.62 for a range of 0.77–1.58%N using PLSR. Huang et al. (2004) also worked in an Eucalypt forest. They estimated N concentrations from continuum-removed HyMap airborne data with stepwise regression, PLSR, and neural networks and reached R2 = 0.85 for a range of 0.98–1.78% N. Martin et al. (2008) used PLSR to estimate foliar N content on airborne (AVIRIS) and space-borne (Hyperion) hyperspectral data from 137 forested sites in North America, Costa Rica, and Australia. They were able to retrieve N concentrations in the range of 0.7–3.2% with a cross-validated R2 of 0.83 and 0.82 with AVIRIS and Hyperion, respectively, for all sites combined. Singh et al. (2015) built on the leaf-level study by Serbin et al. (2014) and established respective regression models for airborne hyperspectral data. 145 AVIRIS classic data sets from the Northern USA and data from 237 field plots were used to create transferable algorithms for retrieving foliar traits and their uncertainties. They calculated mean stand values of the leaf traits by weighting them by leaf biomass of the trees. Between 0.49 (cellulose content) and 0.88 (SLA) of the variance could be explained by the canopy-level PLSR. N concentration was estimated with R2 = 0.85 for a range of 0.96–3.31% using the average of 500 prediction models.
Ollinger et al. (2008) developed a regression model for foliar N concentration across several North American mixed forests that exploits a high positive correlation between foliar N and near-infrared (NIR) reflectance. The model explained 79 percent of the variance for a data range of 0.75–2.35% N. The regression model was applied to all forested areas in North America using MODIS satellite data. However, Knyazikhin et al. (2013) pointed out that the positive correlation between reflectance and the content of an absorbing material is counter-intuitive, contending that the observed relationship must be spurious. They showed that the NIR reflectance of closed forest canopies can be explained to a very high degree (R2 = 0.81) by the broadleaf fraction of a canopy and that the N content is also highly dependent on the broadleaf fraction (R2 = 0.89). This corresponds to the common knowledge that (a) leaves contain significantly higher amounts of N than needles and (b) that conifers appear darker than broadleaf trees. The lower reflectivity of coniferous trees is mostly due to structural effects: needle shoots have a much higher photon recollision probability due to their higher complexity compared to broadleaves. Knyazikhin et al. (2013) propose a normalizing factor, the DASF, that takes this complexity into account and can be easily computed from remote sensing data. If the reflectance of closed forest stands is divided by the DASF, the structural differences are removed from the signal. The resulting signal is termed canopy scattering coefficient (CSC). By adopting this normalization strategy, Wang et al. (2017) were able to estimate foliar N concentration of mixed forest stands in the Bavarian forest national park from airborne hyperspectral imagery using continuous wavelet analysis with a coefficient of determination of 0.65 for a range of 1.45–3.29% N. Ewald et al. (2018) also stress the influence of forest structure on the prediction of canopy N and P concentrations. They integrated predictor variables derived from airborne laser scanning to improve PLSR models (\( R_{cv}^{2} \) of N estimation was improved from 0.31 to 0.41 for a range of 14–25 g/kg N). The relatively low accuracies are attributed to the structural differences between and within broadleaved and coniferous species and the relatively low range of values.
Foliar N retrievals through physically based model inversions are still rare, partially because there are a range of spectral features associated with N bonds. When coupling the canopy reflectance model InFoRM with their revised leaf-level model PROSPECT-5N (Wang et al. 2015), the achieved accuracy in estimating foliar N concentration was quite limited (R2 = 0.46), while canopy N content was estimated with R2 = 0.64 (Wang et al. 2018). The inversion procedure consisted of creating a LUT with 200 000 entries, then iteratively applying three filters to delete improbable parameter combinations from the LUT, and lastly identifying the best-fitting spectra in several different spectral subsets. The mean parameters from the 10, 20, or 100 best-fitting spectra were taken as inversion result. In total, 48 inversion results (4 differently filtered LUTs × 4 spectral subsets × 3 numbers of best fits) were obtained.
BRF and CSC spectra of beech and spruce trees. Lower row: scatter plots of PLSR accuracies for estimating foliar nitrogen content from the respective spectra
4 Conclusions
Imaging spectroscopy of forest ecosystems has come a long way since the first analyses of foliar biochemistry by Wessman et al. (1989, 1988), and abundant literature on experiments and applications has in principle demonstrated its value for assessing forest ecosystems. It is obvious that mapping biochemical traits, especially those with less dominant absorption features like carotenoids, anthocyanins, or proteins, are one of the specific contributions of IS which cannot be adequately replaced by multi- or super-spectral remote sensing systems, nor with radar systems. The question whether biochemical traits are more efficiently extracted based on carefully established empirical models, through the inversion of physically based reflectance models, or in a hybrid approach, still cannot be answered definitely.
Physically based approaches offer substantial conceptual advantages and most of the available models are capable of accurately reproducing the spectral characteristics of leaves or forest canopies as long as the required parameters are known (forward mode). However, the inversion of reflectance models is more critical. As long as the number of model parameters is limited and their impact on the reflectance signal not overlapping with other parameters, inversion provides reasonably accurate estimates (Omari et al. 2013). Models of optical leaf properties, such as the PROSPECT family, have thus been successfully used in many retrieval studies and are meanwhile accepted as replacement of traditional laboratory analysis (i.e., for measuring leaf water or pigment concentrations) (Dechant et al. 2017). Establishing empirical models for estimating leaf properties requires abundant high-quality calibration data covering the range of trait variation; once calibrated, they can build on a range of powerful regression algorithms able to deal with high dimensionality spectral data and equally reproduce laboratory quality (Martin et al. 2008; Serbin et al. 2014; Singh et al. 2015).
On canopy level, the use of both strategies becomes more difficult. Physically based models require a substantially higher number of parameters to adequately reproduce the scattering and absorption mechanisms within structurally complex forest canopy architectures. The behavior of canopy reflectance models in the forward simulation mode is convincing for a range of models, and increasingly powerful computational resources support the development of even more differentiated simulation concepts. However, difficulties arise when the physically based forest canopy reflectance models are inverted. Although the general principle has been demonstrated, the reported levels of accuracy in retrieving leaf biochemical traits from canopy observations are still limited (e.g., Koetz et al. 2004; Omari et al. 2013; Wang et al. 2018), owing to the ill-posedness of the problem. Strategically, this limitation has been treated by constraining the model inversion, either through including external measurements (e.g., by LAI estimation through LIDAR measurements), fixing some of the model parameters to “reasonable” values, or by imposing constraints (rules) on the inversion algorithm while retrieving the remaining (free) parameters. Wang et al. (2018), for example, filtered their lookup table and restricted the spectral range until acceptable results were found; Riaño et al. (2005) used species-specific dry matter content from measurements of dried leaves to constrain the inversion and get better EWT estimations, and Colombo et al. (2008) manually set different weights for different variables in the cost function of the inversion. The problem with these strategies is that physically based models then may become so much tuned to local conditions that they lose their inherent generalization capacities.
Although powerful retrieval strategies exist, employing empirical approaches for retrieving forest canopy properties becomes demanding, in particular with respect to collecting reference data from elevated tree crowns, and because of the conceptual difficulties in upscaling sampled leaf properties to the level of multi-storey forest canopies. The necessity to collect leaf samples across extended areas and diverse forest types adds further complications. However, a number of studies have demonstrated that this is feasible for a range of canopy traits and within considerably large areas (e.g., Asner et al. 2011; Martin et al. 2008; Ollinger et al. 2008; Singh et al. 2015). Hyperspectral systems or sample collectors mounted on unmanned aerial vehicles may become important tools for collecting reference data to span a bridge between ground-based measurements and space-borne spectroscopy.
Since canopy observations inherently combine leaf biochemical traits with the amount of leaves to estimate a bulk quantity, additional complexity emerges when the objective is to retrieve biochemical traits on leaf level from canopy reflectance data. Strategies for solving this problem that have been proposed (Colombo et al. 2008; Dotzler et al. 2015; Knyazikhin et al. 2013) could demonstrate that spatial differences in pigment and water concentrations identified on canopy scale are not necessarily a consequence of differences at leaf level. However, if the objective of the analysis of imaging spectroscopy data is to generate a map to be used for management or to characterize spatial patterns of important biological quantities, then it may make sense to exploit emergent relationships (e.g., differences between conifer and broadleaf species for estimating canopy N) to produce accurate maps. However, such maps will require good independent validation data and will only be valid within the range of measured conditions. Such work provides a basis for ongoing research and may yield results to enable better understanding of ecological patterns (Asner et al. 2017), but will require more study for associated models to have biophysical realism or to not be ill-posed (Townsend et al. 2013).
Sensu stricto, empirical approaches can only be applied to data sets that are comparable to the training data. First attempts in transferring empirically defined models across large distances to similarly structured forests were contradictory. However, it was demonstrated in this paper that applying PLSR coefficients determined by Serbin et al. (2014) to leaf and needle samples from Western Europe was satisfactory, although outperformed by training a PLSR model on the samples themselves. Transferring PLSR coefficients for canopy estimates (Singh et al. 2015) was clearly less successful, likely due to spectral calibration differences between the AVIRIS and HySpex images and different strategies in scaling leaf traits to canopy level. Free access to collections of spectra and biochemical properties from forests around the world, similar to the Spectranomics database (Carnegie Institution for Science), will be a key element for enhancing satellite-based studies of forest ecosystems in general, and for the development of transferable relationships that go beyond locally restricted studies.
In case the existing technical and methodological incompatibilities can be reduced, e.g., through a more consistent data quality provided by space-borne instruments, it is expected that empirical models can be established which are sufficiently generalizable and transferable between similar forest ecosystems. Since they are computationally fast and therefore suited to be applied in large area monitoring schemes, empirical approaches may establish themselves a valuable alternative to computationally more complex inversions of physically based models.
Hybrid approaches complement pure empirical or physically based approaches. When physically based reflectance models can reproduce canopy reflectance well in the forward mode, these simulated spectra can feed a parametric or nonparametric empirical model to be applied to image data. If too little reference data could be collected in the field to establish a robust statistical relationship, the field data can be used to validate the hybrid model instead. However, a major prerequisite is that globally distributed regular observations with space-borne imaging spectrometers become available to foster the required cross-comparison exercises.
Successful applications of Hyperion data to map canopy chlorophyll content (Omari et al. 2013), canopy water content (White et al. 2007; Zhang et al. 2012), and foliar N (Coops et al. 2003; Martin et al. 2008; Smith et al. 2003; Townsend et al. 2003) prove the great potential of space-borne imaging spectroscopy for forest ecosystem studies. Also the derivation of canopy liquid water content during atmospheric correction, a truly spectroscopic approach, shows the unique value of imaging spectroscopy (Asner et al. 2016; Green et al. 2006; Thompson et al. 2019).
In this perspective, the Italian PRISMA (to be launched in 2019) and the German EnMAP missions (to be launched in 2020) constitute cornerstones in the efforts to regularly map diverse forest ecosystems from space with hyperspectral capacities. Their availability will provide an essential contribution toward optimizing the approaches discussed in this paper. The US-American SBG mission (to be launched in 2025) and the European CHIME mission (under design) will extend the regional coverage of previous satellite systems toward a truly global scope. While their scientific value is not in doubt, the question whether hyperspectral satellite observations will provide added value for many operational forestry applications in comparison with the enormous observation capacities of multispectral sensor systems like Sentinel-2 or Landsat remains open.
Notes
Acknowledgements
The study was supported within the framework of the EnMAP project (Contract No. 50 EE 1530) by the German Aerospace Center (DLR) and the Federal Ministry of Economic Affairs and Energy, and the CalTech Jet Propulsion Laboratory (Contracts 1579654 and 1590148). The authors thank Willy Werner, Dorothee Krieger, Bernhard Backes, Martin Schlerf, Johannes Stoffels, Sandra Dotzler, Barbara Paschmionka, Marion Lusseau, Max Gerhards, and many others who helped gather the data presented here. We also thank the two anonymous reviewers for highly constructive comments that helped improve the manuscript.
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