The effect of different methods of rangeland utilization on vegetation characteristics of arid rangelands (Case study: Zirkouh Rangelands, South Khorasan Province)

Introduction: Each of the different exploitation and management methods in rangelands have different effects on the rangeland ecosystem functions. Incorrect and inappropriate ways of rangeland use, especially in the last few decades, have caused severe damage to natural resources. There is a few research on the effects of grazing on sand habitats of desert areas. The aim of the current research is to investigate the effect of different methods of rangeland utilization on ecological characteristics, including the percentage of vegetation cover, plant density, species richness, diversity, evenness and dominance, and species similarity in the studied rangelands.
Material and methods: This research was conducted in the outskirt, seminomadic, settled nomadic and enclosure in Zirkouh, South Khorasan province. In each rangeland, the list of plant species, density and vegetation cover of each species was determined separately. Also, the range condition was evaluated by the modified four-factors method. In addition to numerical indices, the ranking of species diversity was investigated using the Renyi diversity ordering curve. In order to check the species similarity between the four studied rangelands, Jaccard's similarity index was used, and hierarchical cluster analysis (Euclidean distance and Ward's method) was used to better display the species similarity. Finally, the relationship between vegetation cover and the number of species and biodiversity indices was tested by Pearson's correlation coefficient.
Results and discussion: Based on the modified four factors method, the condition of outskirt and seminomadic rangelands were moderate and settled nomadic rangeland was poor and enclosure rangeland, has been evaluated as good. The results of ANOVA showed that the effect of grazing methods on the vegetation cover (at the 0.05 level) and the total and average species richness (at the 0.01 level) were significant. Enclosure rangeland had the highest plant cover percentage and plant density (54.73% and 80, respectively) and the settled nomadic rangeland had the lowest plant cover percentage and plant density (15% and 5, respectively). The highest species richness belonged to enclosure rangeland and there was no difference between other rangelands. Compared to other rangelands, the seminomadic rangeland had the highest proportion of native plants and the lowest proportion of increaser plants in the plant composition of total richness. The highest proportion of increaser plants was observed in settled nomadic rangelands. In terms of species diversity, enclosure rangeland had the highest Shannon-Wiener diversity index (2.03) and Simpson diversity index (0.80) compared to other rangelands. The results of the correlation coefficients of vegetation cover and the number of species and species diversity indices showed that, in general, there is a significant positive correlation between the diversity indices and vegetation cover and the number of species. In addition, there is a significant negative correlation between the species evenness and dominance indices, as well as vegetation cover and the number of species. The results of the Jaccuard similarity index show that the highest degree of species similarity is observed between the outskirt and the settled nomadic rangelands (62%).
Conclusion: The results showed that the overgrazing around the livestock pen in the settled nomadic rangeland is clearly visible and it has led to a significant decrease in vegetation cover, plant density and species diversity. Resting in rangelands (in the enclosure) and delaying grazing (in seminomadic rangeland) can also enhance vegetation and biodiversity. In terms of the percentage of vegetation cover, density and plant composition, the seminomadic rangeland is located next to the enclosure. The most proportion of forage species was observed in the plant composition of the seminomadic rangeland. If we accept the hypothesis that range enclosure causes the plant succession towards the climax stage and in range management, the sub-climax stage is always better than the climax stage, in this case, in terms of livestock and range management, the temporary settlement of nomads will be more beneficial than range enclosure.