In order to eliminate the effects of temporal variation and avoid the potential influence of exceptional factors such as extreme climate or measurement error, the average of the ustainability values of each group was used. These averages were denoted as the grading standards. To ensure the restoration of ecological sustainability in the research site and avoid unsolvable difficulty from exorbitant expectation, two restoration levels are set, i.e. low level and high level which are corresponding to the grading standards of Group II and Group I, respectively (Table 6).