*2.3. Red Edge (680–780 nm)*

The red edge encompasses the region from the red reflectance minimum around 680 nm to the NIR shoulder at approximately 780 nm and indicates the sharp increase in reflectance from the VIS to NIR regions associated with strong chlorophyll absorptions and internal leaf structure (Figure 2). The inflection point of the slope in this region has been defined as the red edge position (REP) [70], and its strong correlation with chlorophyll concentration has seen it used as an indicator of stress and senescence in vegetation [71,72]. In the VIS-SWIR studies, the red edge region as represented by the 700–749 nm bin has the same rate of selection as the red minimum bin, whereas the VIS-NIR studies have a slightly higher red edge rate than red minimum. However, as previously stated, the delineation between the red minimum bin (650–699 nm) and the red edge bin (700–749 nm) means that bands selected from the lower point of the red edge would be included in the red minimum bin, potentially skewing red edge band selection rates.

The red edge region has been described as one of the most informative and frequently selected regions in a number of studies, where the authors have attributed its importance to its correlation with chlorophyll abundance, nitrogen concentration, water content, and structural features such as leaf area index (LAI) [3,10,11,73]. Additionally, significant variation of the red edge region between species has been documented after a first derivative transformation has been applied to the spectra [74]. The red edge has proven especially important in studies discriminating species with high levels of chlorophyll and high LAI values such as the giant reed (*Arundo donax*), in which a distinctive "red shift" is seen where the Red Edge Position (REP) is located at higher wavelengths [32,39]. This "red shift" mirrors the "blue shift" of the REP where its position is shifted towards the shorter blue wavelengths associated with a decrease in chlorophyll and used to monitor senescence or stress [75].

#### *2.4. Near Infrared (NIR) (700–1327 nm)*

The NIR is often defined to include wavelengths within the red edge region (680–780 nm) [42]: As this region has been previously discussed, this section focusses on the NIR plateau (780–1327 nm). The high reflectance of the plateau results from the scattering of photons within the leaf structure due to a change in the refractive index from liquid water to air within the inter-cellular spaces [76]. Two minor water absorption features at ~980 nm and ~1200 nm are the only major features of the

plateau. Along with water content, the depth and width of these absorptions can be influenced by the spectral recording method. Canopy scale spectra tend to produce deeper and wider absorption features compared to the leaf scale, at which absorption features can vary with leaf stack thickness [3]. High levels of intraspecific variability have been identified in the NIR and related to leaf age, water, and chlorophyll concentration, as well as herbivory, necrosis, and epiphyll cover [3,38]. Wavebands selected in studies reporting these high levels of intraspecific variation have generally been limited to the water absorption features [11,38], although it has been suggested to avoid band selection from within or near water absorption features due to this high level of within-class variability, specifically for Eucalypts [46,77,78]. Despite this, [3] reported greater interspecific variability in the NIR, particularly at the canopy scale, potentially related to species-specific photon scattering caused by differences in canopy architecture, a result also reported by other studies [68,69]. However, it has been suggested that the importance of the NIR and SWIR in [3] is linked to the time delay between leaf collection and spectral measurement, causing a decrease in water content and affecting waveband importance [58].

Even when the high selection rate of the red edge is included, the average selection rate of the NIR is close to half of that of the VIS, placing it third after the near SWIR. However, there are two small peaks in the rate of selection within the NIR, in bins 950–999 and 1150–1199, both of which are associated with water absorption features near 980 and 1200 nm. Despite having one of the lowest rates, some studies have reported that bands in the NIR plateau are the most strongly discriminating [45,52].

#### *2.5. Shortwave Infrared (SWIR) (1328–2500 nm)*

Based on the binned results (Figure 1) the SWIR can be divided into two distinct regions, the near SWIR (NSWIR) from 1350–1800 nm, including the strong water absorption feature at 1350–1450 nm, and the far SWIR (FSWIR) from 1800–2500 nm, including another strong water absorption feature from 1800–2000 nm. The wavebands associated with these water absorption features, that mark the start of the SWIR and separate the near and far SWIR, are often removed from spectra due to high levels of noise, as are the bands at the far end of the SWIR above 2400 nm. Selection rates within the NSWIR are on average the second highest, primarily caused by high rates of selection at 1350–1450 and 1700–1750 nm. This initial high selection rate, spanning two consecutive bins, is associated with the water absorption feature focused around 1400 nm. However, these bins are often removed in studies, primarily when hyperspectral imagery is used due to increased noise that is not as prevalent in lab or field spectra. Selection rates then drop in the mid-NSWIR bands before peaking again for the 1700–1750 nm bin, containing wavebands often associated with lignin, cellulose, tannins, and other biochemical constituents of foliar and non-foliar plant matter [19,79]. The FSWIR has the lowest average band selection rate, with its highest selection at bin 2250–2299 nm most likely associated with the weak absorption features of cellulose and lignin present at 2270 nm [19,79].

As the selection results suggest, wavebands selected from the SWIR are reported in the literature as being associated with water absorption [17,33,38,40,46–48,58] or the weak harmonic and overtone absorptions from biochemicals such as lignin, starch, and cellulose [9,40,42,46–48,52,58,80]. However, as described in regards to the NIR, the selection of bands in or near water absorption features may not be suitable for classification in field or lab spectra, due to high levels of intraspecific variance [46,77]. Additionally, bands selected from leaf scale spectra in the two major water absorption features would not be applicable to remotely sensed imagery as they coincide with low irradiance levels resulting from atmospheric water absorption. The observation of higher selection rates in the NSWIR compared to the FSWIR has previously been made with studies noting the importance of NSWIR bands and absence of selection from the FSWIR [9,42], even when visual differences between species were apparent [52]. Possible reasons for this reduced selection of the FSWIR could be high levels of LAI or leaf water content masking the biochemical features present in this region [81], or a high correlation between the FSWIR, NSWIR, and VIS bands [9].
