Here the relative NDVI was calculated by subtracting the
NDVI on the last day of the growing season from the
original NDVI values.
The main inference drawn is that relating the NDVI to
the phenological development pattern of the plants gives a
better and more general relationship. The leaf production
period is a greening period, but unless the LAI exceeds
some threshold, the effect of background reflection can
cause considerable deviations. On the contrary, the leaf
senescence period is a yellowing period and even though the
below canopy effect may be negligible, the yellow leaves
will affect the overall reflectance of the vegetation, causing
distortions similar to the background NDVI. The same
numerical LAI value during the leaf production and leaf
senescence period may not result in the same NDVI
because, in the early season, leaves are all green while in
the latter stage, the colour of the leaves will be a mixture of
green, yellow and even brown (dead leaves). Since the onset
of greening or yellowing will differ from year to year
(depending of weather factors), the extrapolation of a LAI–
NDVI determined for particular year to other years may be
erroneous.