## 3 Results
### Normalized Difference Vegetation Index (NDVI)  
In general, we find moderate NDVI values (between 0.2 and 0.5) throughout the Ouled Hanneche forest for all three dates (Figure 5). Excluding places covered with snow during winter, the map of the seasonal variation of NDVI indicates that some territories in the forest have a high value of NDVI of above 0.6 for the three periods considered. These are the dense forest formations. This underlines the capacity of these territories to maintain ES related to active vegetation and the amount of aboveground biomass. These services are, for example, the climate-regulation service resulting from carbon storage and sequestration and the erosion-regulation service, since the NDVI is a useful indicator for predicting the risk of erosion and quantifying the state of soil fertility [30].  
The NDVI values are distributed according to the type of plant formation and its floristic composition. They were extracted from the different existing plant formations based on our field surveys. The results are shown in graphical form in Figure 6.  
The formations with a high NDVI are the _Cedrus atlantica_ forests, with a mean value for the surveys corresponding to this species close to 0.6 for the periods in May and June. _Acer monspessulanum_ matorrals have a relatively constant average NDVI value in each period, close to 0.55. The NDVI of _Pinus halepensis_ forests and _Quercus rotundifolia_ matrorals has an average value of 0.5. Open and more or less clear formations growing on rocky ground, such as steppe formations and _Juniperus phohenica_ matorral, have among the lowest NDVI  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Plant Species** & **R** & **Plant Species** & **R** & **Plant Species** & **R** \\ \hline _Quercus lx_ & 42/68 & _Rosa canina_ & 15/68 & _Pinus halepensis_ & 9/68 \\ _Juniperus oxycdrus_ & 29/68 & _Globularia alyupum_ & 14/68 & _Rhamnus lycoides_ & 9/68 \\ _Juniperus phohenica_ & 27/68 & _Cedrus atlantica_ & 13/68 & _Pistacia atlantica_ & 8/68 \\ _Cistus albidus_ & 26/68 & _Genista microcephala_ & 12/68 & _Thapsia garganica_ & 7/68 \\ _Pistacia lentiiscus_ & 20/68 & _Astragalus_ sp. & 12/68 & _Lonicera impelaxa_ & 7/68 \\ _Ampelodesmos mauritancius_ & 19/68 & _Jasminum fraticans_ & 11/68 & _Teucrium polium_ & 7/68 \\ _Phillyrea angustifolia_ & 18/68 & _Crataegus monogyna_ & 11/68 & _Bunium_ sp. & 6/68 \\ _Santolina rosmaritifolia_ & 17/68 & _Rosmarinus officinalis_ & 10/68 & _Bupleurum_ sp. & 6/68 \\ _Stipa tenacissima_ & 16/68 & _Gallium_ sp. & 9/68 & _Eryngium_ sp. & 6/68 \\ _Acer monspessulanum_ & 16/68 & _Crataegus azarolus_ & 9/68 & _Lomelosia stellata_ & 6/68 \\ _Calicotome spinosa_ & 15/68 & _Asparagus acutifolius_ & 9/68 & _Aegilops geniculata_ & 6/68 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Main species identified in the forest of Ouled Hannèche.  
values (between 0.2 and 0.3 on average). The Erme is made up of geophytes, i.e., plants whose vegetative buds spend the unfavorable period (winter here) in dormancy buried in the ground and whose aerial parts only appear at the beginning of the favorable period in spring [81]. This explains their negative NDVI values for the date of 17 February 2017.  
Figure 4: Map of geomorphon classes in the Ouled Hannèche forest.  
Figure 5: Seasonal changes in NDVI in the Ouled Hannèche Mountains in 2017.