Fig. 6.
Selected spectral indices derived from hyperspectral images vs. tree crown discolouration and defoliation as assessed during field campaigns. Data from field campaigns in 2014/11 and 2015/12 are compared with images collected on 2015/01/28 and 2015/12/02, respectively, for the NDVI (a), CI (b), TCARI/OSAVI (c) and PRI (d). Sample sizes (nlevel) for discoloration levels are: n0 = 410, n1 = 5, n2 = 8, n3 = 16, n4 = 62, n5 = 162; those for defoliation levels are: n0 = 27, n1 = 24, n2 = 106, n3 = 215, n4 = 124, n5 = 167. Level 0 corresponds to trees without any sign of declining health, while other scores reflect the percentage of the crown displaying poor health: level 1 = [1–25%], level 2 = [26–50%], level 3 = [51–75%], level 4 = [76–99%], level 5 = 100%.