## 5 Phenotyping and Monitoring of Trees' Biophysical Parameters
In particular, a method for the recovery of NPK (Nitrogen, Phosphorus, and Potassium) canopy content in olive trees by using MS UAV imagery under different fertirrigation treatment was developed. Different retrieval techniques partial least squares regression (PLSR), artificial neural network (ANN), support vector regression (SVR), and gaussian process regression (GPR) were tested for NPK leaf content estimation by exploiting data extracted from UAV imagery as input variables and the results of chemical analyses as reference. The results showed the best performance of the ANN approach. To our best knowledge, there are only three studies involving the use of satellite imagery to calculate biophysical parameters of the canopy [176; 177; 178]. [PERSON] et al. [176] evaluated the potentialities of using airborne hyperspectral sensors and Quickbird satellite imagery for determining and mapping tree canopy size, canopy transmittance, and LAI of single olive trees. Tests were carried out in both traditional and intensive olive groves. The dimensional parameters of the canopy, area, and volume were measured. The projected canopy volume of each tree was calculated using the coordinates of the tree silhouette: the tree canopy was ideally divided into eight sectors obtained by eight equidistant trapezoids which were placed around the central-vertical axis of the olive tree. The volume was obtained by summarizing each trapezoid using the second Pappus-Guldinus theorem [179]. The projected canopy area was obtained, firstly marking the perimeter of the orthogonal projection of the tree canopy onto the soil surface using a pole in eight points, generating eight equidistant transects from the center of the tree. Finally, the canopy area was calculated by summing the areas of circular sectors for each transect. [PERSON] et al. [178] used Sentinel-2 images for estimating olive biophysical parameters as LAI, chlorophyll and water contents and mesophyll structure by using the image inversion method based on an approach coupling radiative transfer modeling and a probabilistic estimation technique MCMC (Markov Chain Monte Carlo) [180]. Active microwave remote sensing imagery has also been employed, taking advantage of its ability to penetrate vegetation canopy and reach the ground surface, providing vegetation information. [PERSON] et al. [177] assessed a methodology for characterizing vegetation canopies using optical imagery and microwave scattering models. In particular, the authors integrated biophysical variables of olive tree canopies, derived from optical images, into microwave (RADARSAT 2 imagery) scattering models. The researches focused on phenotyping and monitoring of olive trees' biophysical parameters are summarized in the Table 4 below. In this table, a preponderance in the use of UAVs is observed. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Platform** & **Sensor Type Used \({}^{*}\)** & **Aim of the Study** \\ \hline
[178] & \begin{tabular}{c} Satellites Sentinel-2 and \\ PlanetScope \\ \end{tabular}  & MS & Estimation of olive biophysical parameters \\ \hline
[169] & UAV DJI SPARK & RGB & Estimation of the tree row volume in super-intensive \\ \hline
[149] & UAV Mikrokopter & Tetracam ADC Snap (MS) & \begin{tabular}{c} Discriminating olive cultivars with different \\ scion/rootstock combinations \\ \end{tabular}  \\ \hline
[14] & UAV - \({}^{*}\) & Tetracam MCA-6 (MS) & Estimation of parameters such LAI, chlorophyll \\ \cline{3-3}  & & Thermovision A40M (TH) & \begin{tabular}{c} Estimation of biophysical and geometrical \\ parameters of olive trees under different \\ irrigation regimes \\ \end{tabular}  \\ \hline
[164] & UAV DJI S1000 & \ \begin{tabular}{c} Coolpix P7700 (RGB) \\ Tetracam ADC-lite (MS) \\ \end{tabular}  & \begin{tabular}{c} Estimation of biophysical and geometrical \\ parameters of olive trees under different \\ irrigation regimes \\ \end{tabular}  \\ \hline
[115] & UAV DJI S1000 & \ \begin{tabular}{c} Coolpix P7700 (RGB) \\ Tetracam ADC-lite (MS) \\ \end{tabular}  &