## 3 Results
### Illumination Condition and Vegetation Indices
The analysis of the Pearson correlation between IC and VI on a pixel-basis showed both negative and positive trends in the area clearly associated with IC. The correlation EVI-IC showed a higher range of values (\(-\)0.55-0.97) than the correlation NDVI-IC (\(-\)0.25-0.73), confirming that EVI is more sensitive than NDVI to changes in IC. Positive correlation values were more abundant than negative ones, describing overall positive trends for both VI, meaning that for an observed increase in IC, VI also increases. However, not all correlations were significant. The EVI-IC correlation was significant (\(p<0.05\)) with positive correlations for most of the pixels and negative correlations for a few pixels (Figures 9 and 10). Areas with positive EVI-IC correlations correspond to pixels with low mean IC in Figure 7a, suggesting that EVI is very sensitive to increase when IC increases in poorly illuminated areas, showing the strongest correlation in pixels with the lowest IC (Figure 8a,b). A small number of pixels showed significant values for negative correlations between EVI-IC in well-illuminated areas. This can be related to the appearance of tree shadows between canopy levels when IC conditions change, which has been found in other studies [40]. The response of EVI to trees shadowed by other trees would be lower. However, higher resolution imagery would be needed to describe the canopy structure and the distribution of canopy shadows with different IC to account for this effect. The correlation NDVI-IC was significant only for some areas that showed positive correlations and not in areas with negative or no correlations (Figures 9 and 10). The strength of the positive correlations NDVI-IC was also lower compared to those in EVI-IC. The increase of NDVI with increased IC was also found in pixels with low mean IC and low IC variation, previously described as having a parallel orientation to that of the sunlight and limited to a well-defined aspect range. It is known that NDVI saturates rapidly with high biomass levels and dense canopies, and is less sensitive to changes in IC, which would explain the low correlations NDVI-IC found [7,11]. However, the fact that NDVI seems to be sensitive to changes in areas with low mean IC and low IC variation is a new finding that needs to be further investigated. Because the correlation is calculated with all IC and VI values available for each pixel and these pixels correspond to different dates in different seasons and years, the correlations for both VI are reflecting intrinsic temporal patterns associated with IC. Figure 8: Scatter plot showing mean IC and aspect (\({}^{\circ}\)) in (**a**) and IC and slope (\({}^{\circ}\)) in (**b**) at the pixel level. Figure 9: Spatial distribution of the Pearson correlation values between EVI-IC (**a**); Pearson correlation values between NDVI-IC (**b**); significance values (\(p<0.05\)) for Pearson correlation between EVI-IC (**c**); and significance values (\(p<0.05\)) for Pearson correlation between NDVI-IC (**d**). Each pixel shows the result of the 39 selected images. When the relationship between EVI-IC and NDVI-IC is described comparing the mean values for each selected image (\(n=39\)), EVI and IC showed a significant and positive dependence (R\({}^{2}=0.5715\), \(p<0.01\)) (Figure 11), whereas NDVI and IC showed a very small to almost no dependence (R\({}^{2}=0.075\), \(p<0.09\)) (Figure 12). While these results corroborate the higher sensitivity of EVI and the lower sensitivity of NDVI to the effect of IC, they are not able to explain alone the VI-IC variations found when we examine the image at the pixel level. Figure 11: Scatter plot of the mean illumination condition (IC) versus the mean EVI value for each of the 39 selected images. Figure 10: Scatter plot showing p values and Pearson correlation coefficient values for the correlation EVI-IC (**a**) and NDVI-IC (**b**). The black vertical line represents \(p=0.05\), being all points to the left significant at \(p<0.05\).