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ŠUMARSKI LIST 9-10/2021 str. 56     <-- 56 -->        PDF

apparent after a longer period. Due to the gradual and slow development of protective vegetation cover, it is impossible to notice the full effects of the performed works on the NDVI from 1972. The map of the distribution of excessive and intensive erosion from 1970 (Figure 3) confirms the spatially extensive representation of bare land and areas with very low vegetation cover (Figure 2).
Low values of the NDVI index indicate bare soil areas and/or areas where sparse vegetation is present (Table 1) (Aquino et al., 2017). The high intensity of the erosion process in the studied area in the past correlates with the vegetation cover condition (Figures 2 and 3). In 1972, over 55 % of the area was in the classes of the bare soil and very low vegetation cover (Figure 4). Simultaneously, about 50 % of the area was in the categories of excessive and intensive erosion (Table 2). According to Yansui et al., 2015 vegetation cover plays a significant role in protecting soil from erosion. Areas with low values of NDVI index completely coincide with the areas in excessive erosion category, according to the erosion map (Figure 2 and 3) in 1970.
According to Wang et al. (2008), afforestation is the most efficient measure of vegetation restoration and erosion control. Over time, as a result of afforestation in Grdelica Gorge and Vranjska Valley, the situation changes by reducing the presence of areas in the classes very low and low vegetation coverage at the expense of classes moderately low to moderately high vegetation coverage (Figure 2 and 4). In 1986 changes in the percentage distribution of areas with higher NDVI index values were observed, in relation to 1972 (Figure 2 and 4). Namely, during the 1980s, the positive effects of biological works became more noticeable. In 1986, there was a slight increase in areas with very low and low vegetation coverage, while bare lands were less represented. When compared vegetation coverage in 1996 with the previous period (1986) there was a decrease in the area in the low class, and significantly increased the area in the class moderately low (about 30 %). After the performed biological works, there is a change in the extent of vegetation cover. More precisely, with the formation of vegetation cover, the classes of lower coverage pass over time into higher coverage classes. In 2017,