## 3 Results and Discussion
### Relationships between Radar Signals and Vegetation Parameters  
In this section, the backscattered signal was analyzed as a function of vegetation parameters (LAI, HVE, BIO, and VWC). Grassland plots contain approximately 20 different species of vegetation. The main vegetation species are grasses (_Dactylis glomerata L., Lolium perenne L., Poa pratensis L., Holcus lanatus L., Arrhenatherum elatius L., Festuca pratensis L., Setaria glaucus L.,_ and _Paspalum dilatatum Poir_), legumes (_Medicago lupulina L., Trifolium repens L., Trifolium pretense L., Lotus corniculatus L.,_ and _Viccia cracka L._), and diverse dicotyledons (_Plantago lanceolata L., Taraxacum officinalaWeber., Tragopogon pratensis L., Galium mollugo L., Galium verum L., Daucus carota L., Achellea millefolium L., Pastanica silvestris L.,_ and _Rumex acetosa L._) [84]. At plot scale, the vegetation structure geometry is homogeneous. The biomass levels of these species vary during the growth season. In the first growth period, grass species are dominant (60%-65%). Grass biomass levels decrease in the second and third growth periods. However, legume and diverse dicotyledon biomass levels increase from 35%-40% in the first period to 55% in the third period [69]. An important change in morphology is observed, especially in grass species, when vegetation exceeds approximately 50 cm (LAI about 2 m\({}^{2}\)/m\({}^{2}\), BIO about 1.5 kg/m\({}^{2}\) and VWC about 1.2 kg/m\({}^{2}\)); inclined elements (panicle, small leave, _etc._) randomly oriented at the top of the plant begin to appear. The VWC and plant morphology are the main vegetation variables that affect the radar response [85]. It was therefore essential to study the relationship between the radar signal and vegetation parameters (LAI, HVE, BIO and VWC) separately according to two vegetation classes (LAI, HVE, BIO and VWC lower and higher than 2 m\({}^{2}\)/m\({}^{2}\), 50 cm, 1.5 kg/m\({}^{2}\), and 1.2 kg/m\({}^{2}\), respectively). To reduce the effect of soil moisture on the analysis of the backscattered radar signal, relationships between radar signals and vegetation parameters were traced according to three classes of soil moisture (Mv \(<\) 0.2, 0.2 \(<\) Mv \(<\) 0.3 and Mv \(>\) 0.3 cm\({}^{3}\)/cm\({}^{3}\)). Figure 9 shows the behavior of the SAR X-band signal according to LAI. The results showed that for soil moisture between 0.10 and 0.20 cm\({}^{3}\)/cm\({}^{3}\), the radar signal in HH and HV decreases with LAI for LAI lower than 2 m\({}^{2}\)/m\({}^{2}\), and increases as the LAI increases from 2 m\({}^{2}\)/m\({}^{2}\) to 5 m\({}^{2}\)/m\({}^{2}\). These variations in the radar signal (decrease and increase) are higher in HV polarization than in HH polarization (Figure 9 first row). The decrease in radar signal for LAI lower than 2 m\({}^{2}\)/m\({}^{2}\) is related to an increase in the attenuation of the soil contribution; this is more important than the enhanced contribution from the vegetation canopy [80,81,86]. In addition, the increase in the vegetation contribution as a function of the LAI, combined with the decrease in the soil moisture contribution (Mv between 0.10 and 0.20 cm\({}^{3}\)/cm\({}^{3}\)), results in a slight increase in the radar backscatter with LAI for values greater than 2 m\({}^{2}\)/m\({}^{2}\). For Mv between 0.20 and 0.50 cm\({}^{3}\)/cm\({}^{3}\), the radar signal slightly decreases in HH and HV polarization for LAI between about 0.1 and 5.0 m\({}^{2}\)/m\({}^{2}\) (Figure 9 second row and third row). Indeed, for Mv between 0.20 and 0.50 cm\({}^{3}\)/cm\({}^{3}\), the decrease of the soil contribution when LAI increases between 0.1 and 5.0 m\({}^{2}\)/m\({}^{2}\) is of the same order than the increase of the vegetation contribution. Many studies have analyzed the behavior of radar signals (Ku, X, C and L bands) as a function of LAI [87, 88, 89, 90, 91, 92, 93].