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Abstract
In olive groves, vegetation ground cover (VGC) plays an important ecological role. The EU Common Agricultural Policy, through cross-compliance, acknowledges the importance of this factor, but, to determine the real impact of VGC, it must first be quantified. Accordingly, in the present study, eleven vegetation indices (VIs) were applied to quantify the density of VGC in olive groves (Olea europaea L.), according to high spatial resolution (10–12 cm) multispectral images obtained by an unmanned aerial vehicle (UAV). The fieldwork was conducted in early spring, in a Mediterranean mountain olive grove in southern Spain presenting various VGC densities. A five-step method was applied: (1) generate image mosaics using UAV technology; (2) apply the VIs; (3) quantify VGC density by means of sampling plots (ground-truth); (4) calculate the mean reflectance of the spectral bands and of the VIs in each sampling plot; and (5) quantify VGC density according to the VIs. The most sensitive index was IRVI, which accounted for 82% (p < 0.001) of the variability of VGC density. The capability of the VIs to differentiate VGC densities increased in line with the cover interval range. RVI most accurately distinguished VGC densities > 80% in a cover interval range of 10% (p < 0.001), while IRVI was most accurate for VGC densities < 30% in a cover interval range of 15% (p < 0.01). IRVI, NRVI, NDVI, GNDVI and SAVI differentiated the complete series of VGC densities when the cover interval range was 30% (p < 0.001 and p < 0.05).
Highlights
The Mediterranean basin contains 93.44% of the 10.24 million hectares of global olive cultivation (Olea europaea L.)
These changes, together with the continued existence of inappropriate practices linked to traditional land management, such as deep and continuous tillage in areas with high slopes or the elimination of vegetation ground cover (VGC) that protects the soil from torrential rains, aggravated the unsustainability environmental in the Andalusian olive groves [3,4]
We show that unmanned aerial vehicle (UAV) technology, together with image processing based on vegetation indices (VIs), makes it possible to remotely quantify the density of VGC produced spontaneously in olive groves
Summary
The Mediterranean basin contains 93.44% of the 10.24 million hectares of global olive cultivation (Olea europaea L.). The importance of vegetation cover is acknowledged in the cross-compliance system of the EU Common Agricultural Policy (CAP) (Regulation No 1306/2013), in the Good Agricultural and Environmental Conditions (GAEC) related to soil and the carbon stock, i.e., “minimum soil cover” (GAEC 4) and the “maintenance of soil organic matter level through appropriate practices including ban on burning arable stubble, except for plant health reasons” (GAEC 6) The application of these rules, which are mandatory for recipients of CAP assistance, requires the VGC density on the farms to be quantified. The quantification of land cover in woody crops, such as olive groves, is subject to an important limitation, because the trees partially cover the soil, which generates high error rates in remote sensing [23] This problem is exacerbated when the VGC presents heterogeneous densities, due to climatic seasonality and soil management (tillage vs no tillage).
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