**2.2. Reflectance indices**

Another spectral method for chlorophyll content estimation is based on reflectance measurements to create pigment indices. Such indices take into account between two and four spectral bands and have shown high accuracy. Despite the literature offers several pigment indices, the majority of them have been tested in just specific plant species or vegetation type. As a result, they have become plant or vegetation specific. Estimations of chlorophyll content based on reflectance indices have been widely used [23–25, 33, 39–42].

Chlorophyll indices are increasingly being used in crops and forest assessments but also in ecology and Earth science. Several calibration models have been described in the literature, most of which, however, have been calibrated and validated in few or closely related plant species with a limited number of samples. Under these conditions, most of the models can only be applied to specific species and environmental conditions [23, 32, 43]. There is no scientific consensus as to whether a universal model can be found that can be applied for species-rich forest stands in different latitudes, phenological stages, and leaf structures [17]. Feret et al. [25] noted this limitation of the spectral indices and proposed new indices for chlorophyll and carotenoid estimation. They were based on a vegetation dataset collected in various ecosystems around the world including a wide variety of plant physiology and leaf structure.

#### **2.3. Radiative transfer models: PROSPECT model**

Based on the relationship between reflectance and the biochemical and biophysical properties of the leaves and canopies, models have been created in order to simulate the interaction of the light with the plant leaves through the radiative transfer theory. The leaf optical properties spectra (PROSPECT) model describes radiative transfer within a broadleaf with a plate model [44]. Plate models treat internal leaf structure as sheets or plates and calculate multiple reflections of diffuse radiation between these interfaces [13]. PROSPECT is based on the representation of the leaf as one or several absorbing plates with a rough surface giving rise to isotropic scattering. The model estimates the directional-hemispherical reflectance and transmittance of leaves across the solar spectrum from 400 to 2500 nm [45].

A leaf structure parameter of the model is represented by *N*, which is the number of compact layers specifying the average number of air/cell wall interfaces within the mesophyll. The leaf biophysical parameters of the model are represented by chlorophyll *a* + *b* content (*Cab*) and equivalent water thickness (*Cw*). The latest versions of the model include the parameters dry matter content (*Cm*) and brown pigments content (*Cbp*). Inversion of PROSPECT revealed good agreement between measured and predicted leaf chlorophyll concentrations [13, 45].
