The impact of meteorological drought and grazing on the vegetation structure in aridlands (Case study: Nakhab Forest, Khosf County)

Drought and grazing rangelands play an important role in changing the structure of vegetation and losing the biodiversity of natural ecosystems. In this research, in order to determine the effects of drought and livestock grazing on the vegetation structure of rangelands located in the Nakhab Forest of South Khorasan Province, species abundance, biodiversity, and range condition and trend were evaluated. For this purpose, sampling was done in 80 plots (2×2 m) located on four transects of 150 m using conventional methods in a two-year period (between the water years of 2019-2020 and 2020-2021). According to the standard precipitation index (SPI) and the normal percentage index (PN), 2019-2020 was very wet and 2020-2021was a mild drought. According to the physiognomy method, the plant type of the study area is Hammada salicornica and the important species include Aeluropus littoralis, Haloxylon persicum, and Zygophyllum atriplicoides. In terms of the percentage of vegetation, litter, bare soil, and stones and gravel, there is no significant difference between the grazed rangeland and enclosure in two years. The results show that in the two sampling years, the grazed rangeland has a higher species density than the enclosure. Enclosure rangeland has benefited class I and III species and livestock grazing has caused a significant decrease in class I and III plants and an increase in class II plants. Livestock grazing caused a significant decrease in the density of grass, forb, and shrub plants and did not affect the density of shrubs. The results showed that in the very wet year, there is no difference in species richness between the grazed range and the enclosure. Nevertheless, the enclosure has more species diversity than the grazed range. However, in the mild drought year, all biodiversity indices were not different between the grazed range and the enclosure. This indicates that in drought conditions, the enclosure does not help to improve species diversity.