The critical threshold of Zygophyllum eurypterum shrublands changes caused by grazing pressure in summer rangelands of Kerman province, Iran

The threshold concept of vegetation changes has an important role in natural plant ecology, but there is little information about the nature of threshold response of vegetation to grazing in rangeland ecosystems. The objective of this paper is to introduce thresholds based on vegetation changes of grazing gradient. In order to define the threshold concept at Zygophyllum eurypterum shrublands of cold season rangeland in Kerman province, two linear models and three nonlinear models (exponential curve, a piecewise regression and a sigmoid logistic) have been monitored on vegetation data of this area. The best models are two nonlinear models (piecewise and exponential) for plant vegetation of this ecosystem. The Plant Functional Types have been changed by nonlinear model. Based on water point distance, the changes in the trend of plant functional types of scrub forests, shrubs and perennial grasses that are palatable, have increased but the annual and perennial forbs were in decline. The results show that the sigmoid logistic model is not suitable for this area. On the other hand, two nonlinear models (piecewise and exponential) are better suited for these kinds of grazing management.