The area of the Stipa grandis steppe in Inner Mongolia is 2798081 hm２. Based on the genetic variation, the adaptability of Stipa grandis under grazing pressure was significant. Changes to genetic diversity of the Stipa grandis population under different grazing pressures were studied by Inter-Simple Sequence Repeat (ISSR). The plant materials were collected from a series of grazing gradients of the Stipa grandis steppe in Dalinuoer national nature reserve in Inner Mongolia. The location of the reserve is 116°38′～116°41′E and 43°25′～43°27′N , and it has abundant vegetation types; Leymus chinensis is the constructive species. Dominant species include Stipa grandis, Cleistogenes squarrosa，Artemisia frigida, with the companion species being Potentilla acaulis. Four grazing gradients were identified, from herdsmen residence to enclosure site according to the grazing pressure. They were no grazing (CK enclosure site), light grazing (LG), moderate grazing (MG) and heavy grazing (HG). Young leaves of each Stipa grandis were collected during the growing season.
The results showed that Stipa grandis has abundant genetic diversity although some polymorphic loci were missing. At the same time new polymorphic loci emerged when grazing pressure increased. 10 primers were used, with 74 bands produced in total and 65 of the bands being polymorphic. The total percentage of polymorphism was 89%. With the increase of grazing pressures, the percentage of polymorphic loci of the Stipa grandis population decreased. The percentage of polymorphic loci was 62.2% in the no grazing (CK) population, 64.9% in the light grazing (LG) population, 58.1% in the moderate grazing (MG) population and 56.8% in the heavy grazing (HG) population. The genetic diversity of the population from highest to lowest by the Shannon’s information index is as follows: light grazing (0.3486), no grazing (0.3339), moderate grazing (0.3249) and heavy grazing (0.2735), with the same distributional pattern as the Nei’s genetic diversity index. The test showed that as grazing pressures increased, the change of genetic diversity decreased. The genetic differentiation coefficient among the population (Gst) was 0.1984, which showed small partial genetic diversity (19.8%) present among populations. Gene flow (Nm*) between primers varied from 0.9806 to 3.4463 and the mean Gene flow (Nm*) was 2.0202. The UPGMA cluster figure that was constructed based on the genetic distance matrix showed four populations that became genetically closer at each step. The first group was the moderate grazing (MG) population and the heavy grazing (HG) population, group two consisted of the no grazing (CK) populating and the light grazing (LG) population, then the two groups closed together.