2.3 Calculations and statistics
The contents of N and P in vegetation were calculated as the weighted means of the contents of N and P, respectively, in branches and leaves of all plants in each investigated quadrat, and the biomass of branches and leaves of each species was used as the weighting coefficient. After the N and P contents of vegetation were calculated, the N:P ratios of vegetation were determined.
To calculate the species diversity of the A. ordosicacommunities, the number of A. ordosica plants in each 4×4 m quadrat was transformed into the number per of 1×1 m subquadrat. Then, the number of A. ordosica plants per square meter and the number of herb plants per square meter were used to calculate the species diversity of A. ordosica communities.
The Shannon-Wiener index (H ’) was used to characterize the species diversity of the community. The formula used to calculate this index was as follows:
where N is the total number of individuals of a species in the quadrat, S is the number of plant species in the quadrat, andni is the number of individuals of the i th species.
For the regressions of variables of the A. ordosica and S. alopecuroides communities, SPSS 20.0 software was used, and Excel 2016 was used to plot the results. For the regression of variables of theS. bungeana communities, the component + residual plot of the regression of vegetation N:P ratio and total plant density was performed in R (version 3.5.3; R Core Team, 2019).