**2.5. The red-edge position (REP)**

The red-edge position is a unique feature of green plants related to leaf chlorophyll content and to LAI. REP is defined as the inflection point (or sharp change) of the low red reflectance caused by chlorophyll absorption near 680 nm and high infrared reflectance governed by the internal structure of leaves near 750 nm [51]. REP has been used as an indicator of chlorophyll content in vegetation, as increasing chlorophyll content implies an enlargement of the chlorophyll absorption peak: this moves the red-edge to longer wavelengths while a decrease in chlorophyll shifts the red-edge toward shorter wavelengths [12]. However, the REP has been reported not to be an accurate indicator of chlorophyll content in vegetated areas showing high chlorophyll content values because of the asymptotic relationship between REP and chlorophyll content [52, 53].

Several methods have been proposed to estimate REP from spectral data coming from field and satellite sensors. Dawson and Curran [54] developed a three-point Lagrangian interpolation technique, but this method has shown some problems when the reflectance spectrum exhibits more than one maximum in its first derivative [51]. Another method was developed by Guyot and Baret [55], which applies a linear model to the red-NIR slope. This method has been reported to be robust when it was applied to various datasets [11]. A third method identifies the red-edge inflection point as the maximum of a curve fitted to the first derivative of the reflectance spectrum. This method has been closely related to chlorophyll content per unit area at leaf and canopy level [56] and has shown sensitivity to detect vegetation stress by quantifying changes in chlorophyll content [57].
