Direct and indirect ground estimation of leaf area index to support interpretation of NDVI data from satellite images in hedgerow olive orchards

Abstract

In discontinuous crop soil coverage condition, as for many horticultural crops (e.g. fruit or olive orchard), the low spatial resolution of freely available reflectance data acquired from satellites, affects their agronomic use. Each reflectance pixel value is influenced not only by the reflectance response of the target crop but also by the percentage of pixel covered by the crop, as bare soil and/or spontaneous vegetation strongly affects whole pixel reflectance.In this context, when the analysis of crop conditions is the main agronomical target, the measurement of Leaf Area Index (LAI), supporting the estimation of the area covered by the discontinuous crop, could represent a crucial step helping the interpretation of remote sensed spectral data including related vegetation indices.From this perspective, a field experiment has been conducted on an 8.30 ha super intensive hedgerow olive orchard, monitoring (i) variation of NDVI from an 830 pixels’ grid of the Sentinel-2 imagery (10×10 m) during two years (2020–2021); (ii) LAI estimated by destructive measurement, and (iii) LAI estimated by light transmittance at soil level under the crop measured by a commercial low cost field instrument (LAI-Pen LP 100) with two light sensors (400 - 700 nm and 400 - 500 nm bands).Mean NDVI elaborated from Sentinel-2 imagery and referred to the whole orchard, varies from 0.28 to 0.81, along the two years, and was quite stable during the summer months for each year (0.28–0.36 in 2020 and 0.39–0.41 in 2021). LAI estimated by destructive measurements on single plants, ranged between 2.17 and 4.38. LAI estimated by LAI-Pen, was strongly related to LAI estimated by destructive measurements (R2=0.9473 n = 9)Finally, the average of LAI-Pen measurements referred to all the plants (n = 16) included in each pixel of the Sentinel 2 measurements’ grid, resulted directly correlated to pixel NDVI values collected in the same date.