## 3 Results
### Correlations between Leaf and Canopy Spectral Indices and Foliar Nutrient Contents  
Given that most of the observed variability in foliar nutrient contents took place over time between the early and late measurements (Figure 4A,B), we pooled together the results from both measurements in order to assess how the observed dynamics in foliar nutrient contents correlated with different spectral indices at the leaf level and how these relationships scaled up to the canopy level (Figure 7). In addition, correlations between different spectral indices and SLA, foliar pigment contents, and canopy FVC were used to investigate the factors underlying the correlations. A similar analysis was conducted separately for each of the measuring points (Figures 12 and 13). Correlations were clearly reflecting the grouping of nutrients introduced earlier, with Group 1 nutrients (macronutrients N, P, K, and Mg) being positively correlated to Cab and negatively correlated to Car/Cab, and Group 2 nutrients (including S and micronutrients Cu, Mn, and Zn) being negatively correlated to Cab and positively correlated to Car/Cab (see also Appendix E, Figure 15). When variability in foliar nutrient contents was reduced, such as in the case of Ca and Cd, or when examining the early and late measurements separately (Figures 12 and 13), correlations with spectral measurements weakened or disappeared. At the leaf level, the strongest correlations between VIs and nutrients were observed using the red-edge reflectance, which was negatively correlated with Group 1 micronutrients and Cab and positively correlated, albeit only slightly, with Group 2 nutrients. NDVI had only a weak negative correlation with Ca, Fe, and S, while MTCI was positively correlated with Group 1 nutrients and Cab and negatively but only slightly correlated with Group 2 nutrients. For fluorescence indices, F685 and the fluorescence peak ratio (F685/F760), Fratio, presented a similar pattern of correlation to those of red edge reflectance. In contrast, no correlations between nutrients and F760 were observed, other than minor positive correlations with Group 2 nutrients. For photoproduction-related indices, the CCI displayed again a similar pattern of correlation with Group 1, comparable to F685, Fratio, and red edge reflectance, whereas correlations with PRI were very weak and of inverted sign. In terms of carotenoids, the correlation between Car/Cab and PRI was slightly negative, as expected, but CCI was only negatively correlated to Car/Cab during the first measuring point (Appendix C, Figure 12). Overall, the strongest correlations  
Figure 7: Correlation matrix comparing leaf (L) and canopy (C) level spectral indices to leaf level nutrient and pigment contents. If the color of the correlation between the leaf and canopy stays the same, it indicates that the sign of the correlation scales up. If the color changes between the scales, the correlation is reversed when moving from one scale to another. The data used in the matrix is a combination of early and late measurement points (n = 52). Color denotes the Pearson correlation coefficient R-value, which is explained in the color chart on the right. All colored (non-white) squares are significant at the \(p\leq 0.05\) level. between foliar nutrients and spectral indices were found for Group 1 macronutrients N, P, K and Mg.  
At the canopy level, when pooling data over both early and late measurements, correlations between the red-edge and MTCI and foliar Group 1 macronutrient contents were drastically reduced compared to the leaf level, but a slight negative correlation between Group 1 nutrients and the NDVI appeared, which, at this scale, was positively correlated to FVC. In contrast, the canopy-level correlation between F685 or Fratio and foliar nutrient contents remained, but its sign inverted when moving from the leaf to the canopy scale.