## 4 Discussion  
### Normalized Difference Red-Edge Index Best Predicts Cover Crop Biomass  
Our results indicate that the best VI for measuring the aboveground biomass of cereal grass cover crop species is the normalized difference vegetation index calculated using Sentinel-2 red-edge band 5 (NDVI_RE1), supporting our first hypothesis regarding the utility of the red-edge region for the improved biomass estimation of winter cover crops. Our findings are consistent with other studies that observed that the red-edge region was important for estimating crop biophysical characteristics, including LAI, the fraction of absorbed photosynthetically active radiation, fraction of vegetation cover, and biomass [17; 19; 25; 28; 29]. The red-edge region is a unique characteristic of healthy vegetation as it links the highly absorptive red region, which is sensitive to chlorophyll, and the highly reflective NIR region, which is linked to plant internal cellular structure [20; 76]. We found that band 5 (centered at 704 nm, 690-720 nm in the red-edge region) was most important for cover crop biomass estimation; the VIs containing band 5 consistently outperformed those from the middle red-edge (band 6, 733-747 nm) and the latter red-edge (band 7, 770-790 nm) region (Table 3). Despite this, we still observed NDVI_RE saturation at -1900 kg ha\({}^{-1}\) (-27% of our samples), which is similar to, although slightly higher than, previous thresholds observed for NDVI in cereal grass cover crops [14]. This finding may, in part, be because the index is normalized and is, therefore, bounded \(-1\) to +1, or because a reflectance below 720 nm is not strongly influenced by leaf layering when compared to a reflectance greater than 725 nm [76], indicating that shorter red-edge wavelengths are more likely to experience a saturation effect compared to longer red-edge wavelengths. Therefore, it appears that the shorter red-edge region is still sensitive to chlorophyll concentrations (up to ~705 nm) [77] but not as absorptive as the red region and is, therefore, slower to saturate as biomass increases. While our study focused on cover crops, our findings regarding the utility of NDVI_RE1 may be useful for future work dealing with the saturation of VIs in other contexts or for cereal grasses in other agricultural regions that are grown as commodity crops, although recalibration would be required for different species and regions. We also examined whether biomass-VI relationships varied seasonally (winter-spring) and relative to species. As expected, winter and spring relationships varied significantly (Appendix B), in accordance with other studies [14; 78]. Biologically, we anticipated this finding because cover crop growth is halted during the winter dormancy and plants experience chlorosis or yellowing of leaves due to frost damage. This results in biomass that is less detectable by vegetation indices, and the plants later recover their reflectance following spring green-up. We also observed significant species-level differences (Appendix B). Previous work on cash crops has shown that the relationship between VIs and biophysical characteristics, including chlorophyll content, LAI, and leaf inclination angle, are species-specific [79; 80]. Previous work also noted that the performance of VIs (NDVI and a red-edge index) to predict cover crop biomass estimations were species-dependent [17]. It should be noted that our cereal rye samples--which showed the most pronounced difference in slope and intercept from the other two species--may not be entirely representative of cereal rye broadly, because we observed patchy growth in several cereal rye fields included in this study. While we acknowledge the limitations of the present study, our results were consistent with other studies that have observed significant species-level differences in the NDVI biomass relationships among cereal grass species on a larger scale [78], possibly due to differences in growth form and leaf angle distribution.