*2.1. Spectral Range*

Of the studies that met the rules for inclusion in this review, 38 used hyperspectral data spanning most of the range 350–2500 nm. However, a number of studies utilised devices that recorded a more restricted wavelength range between 350 and 1100 nm, generally from 400 to 800 nm or 1000 nm (Table 2). These studies are presented separately as the absence of Shortwave Infra-red(SWIR), and much of the Near Infra-red(NIR) has shown to have an influence on waveband selection for the Visible (VIS) and partial NIR [54]. Although selection rates in the VIS/NIR studies appear similar to those from broader wavelength hyperspectral studies there are some notable differences. The initial peak in selection rates present in both sets is shifted towards shorter blue wavelengths, and a greater importance of the red edge over the red minimum is evident for the VIS/NIR studies. However, the overall pattern is the same with two peaks in the rate of selection at both the blue/green and red reflectance minima, with yellow wavelength bands having the lowest selection rate, save for the sub-400 nm bands that appear in a very limited number of studies. Although the VIS/NIR studies do not cover the full NIR region, selection rates for the red edge and shorter wavelength NIR are closely matched between both groups (Figure 1). The overall higher rates present in the VIS/NIR table results from the smaller number of studies in that group, with selection rates tending to decrease as more studies are added. Additionally, the relatively small number of studies included in the VIS/NIR group prevents the analysis of specific subsets, such as canopy and leaf. The following discussion of selection rates refers to VIS/SWIR studies (Table 1) and is generally applicable to the VIS/NIR studies, although particular discussion of the VIS/NIR studies is included when required.

**Figure 1.** Waveband selection binned at 50 nm intervals for the VIS/SWIR studies (3502500 nm) green, VIS/NIR studies (350–1100 nm) blue. Orange filled cells represent waveband regions removed from a study due to noise. Selection rate is the percentage of studies that selected a given 50 nm region for species classification. Each row of the table is an individual study, with each column being a 50 nm range bin. Green/blue shaded bins represent at least one waveband being selected from within that range, while orange shaded bins represent removed wavelength regions (e.g. major water absorption regions). Wavelength bins were only removed if the entire 50 nm region was removed due to noise/atmospheric effects in that particular study.

#### *2.2. Visible (VIS; 400–700 nm)*

Primarily a region of low reflectance in living foliage, typically as low as 5%–10% with the exception of the green peak at ~550 nm where reflectance can be more than twice that of surrounding wavelengths (Figure 2). Reflection in the visible wavelengths is dominated by absorptions from foliar pigments. Differences in leaf pigments between species have been identified by many studies as important factors for discrimination [39], despite variability in the VIS being generally low compared to longer wavelengths [53,55]. Of the pigments, chlorophyll a and b have the strongest influence over absorption in this region, followed by those of carotenoids and anthocyanins whose effects are predominantly masked by that of chlorophyll. The visible region is one of the most influential regions for classification, with the vast majority of studies in this review selecting bands from within it. The visible wavelengths can be divided into three regions of high discriminatory value, spanning almost the entire visible range: the blue/blue-green edge (400–499 nm), the green peak centred around 550 nm, and the red reflectance minimum (650–700 nm) (Figure 2). Of these, the red reflectance minimum, specifically bands near 680 nm has previously been identified as the most commonly selected and critical band centre for crop type discrimination [56]. The continued selection of 680 nm, along with neighbouring bands in later studies has validated the importance of this region amongst agricultural crop studies [5,25,47,48,57], as well as for other vegetation types [10,17,18,26,27,29,30,46,58,59]. In addition to the obvious relationship with chlorophyll, absorption in the red region has been related to anthocyanin content, a foliar pigment responsible for the red colouration in leaves [60], particularly evident in juvenile leaves of certain species [30].

**Figure 2.** Example hyperspectral reflectance of 3 species of tree and key broad regions of the electromagnetic spectrum (400–2400 nm).

The green region has the second highest selection rate amongst both the VIS and entire measured spectrum (Figure 1). Wavebands selected in this region tend to be focused around the green reflectance peak at approximately 550 nm, which is strongly correlated with chlorophyll content [61]. The green peak, either manually chosen as a spectral variable as a representation of chlorophyll content or selected via feature selection, has demonstrated importance in classifying species [9,30,42,62,63]. Additionally, absorption in wavebands within the green region adjacent to the reflectance peak is associated with xanthophylls and anthocyanins. Xanthophyll pigments protect against photo-oxidation of the photosynthesis reaction centres during high light conditions [64], resulting in short term changes in reflectance at 531 nm. This band, along with 570 nm, makes up the photochemical reflectance index [65]. Anthocyanins can be estimated by an index using anthocyanin's absorption maximum near 550 nm, and a band from the red edge, usually 700 nm [66]. Although not necessarily associated with these additional pigments, studies have selected bands along the leading edge of the green reflectance peak between 500,550 nm [10].

Selection from the blue region (400–449 nm) has the third highest rate in the VIS region, though the blue-green edge (450–499 nm) has an almost equal rate of selection to the green region (55.8% and 58.8%, respectively). The importance of blue bands has been established for discriminating within groups of conifers, and between conifers and broadleaf species [67,68], though its inclusion

in approximately half of the studies, many of which include non-coniferous species, indicates its importance in general for a wider range of vegetation types. Some of these non-coniferous studies focused on the savanna ecosystem, where blue bands along with the red reflectance minimum and red edge were informative [10,29]. Blue wavelengths are strongly influenced by chlorophyll absorption, along with carotenoid absorption features present in the 450–499 nm region. Carotenoids have proven important for the discrimination of senescent leaves, when the decay of chlorophyll and the diminishing of the strong chlorophyll-absorption feature reveal the carotenoid absorption feature [18].

However, studies have noted that strong similarities between the visible reflectance of different species can decrease the significance of VIS wavelengths for classification purposes. In one such study, the NIR region was more informative for distinguishing species than the VIS, with spectral differences in the VIS region being non-significant between species [69]. Additionally, in a study of tropical trees, Rivard et al. [54] performed feature selection and classification on various datasets derived from the same original spectra. One dataset included the wavelengths 350–2500 nm, another excluded the VIS, while another excluded the SWIR. Although it was found that the full spectrum produced greater overall classification accuracy, and both reduced datasets produced lower overall accuracies, individual accuracies for certain species remained high. The classification model excluding the VIS region maintained high accuracies for six out of 20 species, whereas the model excluding the SWIR maintained high accuracies for five out 20 species. Although the importance of the VIS region has been described by many authors and is clearly seen in the binned data, studies such as [54] demonstrate that wavelength importance is dependent on the species included in the study.
