Nitrogen fertilization has a small but detectable effect on the spectral properties of needles in Norway spruce

Optical remote sensing has been proposed as a tool to guide precise application of fertilizers; however, the effects of nitrogen (N) fertilization on the spectral properties of coniferous needles from mature forests are not known. In this extensive pilot study, we present how N fertilization affects the spectral properties, nutrient content, and morphological properties of Norway spruce needles collected from a controlled fertilization experiment in Finland. We found small mean differences (<3 % across the 400–2500 nm range) in reflectance, transmittance, and albedo spectra between the needles collected from fertilized and unfertilized plots. In addition, the results suggest a saturation in the spectral changes beyond moderate (<200 kg N ha−1) levels of N fertilization. The needle spectra enabled predicting morphological properties using partial least squares regression. Specific leaf area and water content showed moderate-to-good generalization (R² values of 0.7 and 0.8, respectively) with low-to-very-low relative RMSEP-% values (6.2 % and 2.2 %, respectively) and narrow confidence intervals (4.8–7.4 % and 1.5–2.8 %, respectively). Nitrogen fertilization had a statistically significant effect on spectra only in current-year needles. We found that an absorption feature centered between 1112–1272 nm showed the best accuracy (83.3 %) in differentiating current-year needles collected from fertilized and unfertilized plots. Our results highlight that N fertilization has a small yet detectable effect on the spectral properties of needles in Norway spruce trees which could be used to develop remote sensing applications for assessing N fertilization in forests.