2.1 Field investigation
We chose the Ningxia Habahu National Nature Reserve, China, as the research area. The Reserve belongs to a semi-arid region with geographical coordinates of 106°53′10″E–107°38′10″E, 37°38′36″N–38°02′00″N. The Reserve has an elevation of 1,300–1,622 m, an average annual temperature of 7.7°C, an average annual precipitation of 282 mm and an average annual evaporation of 2,250 mm.
In the research area, a successional series exists because of grazing disturbance (He et al., 2016). The plant communities in early successional, mid-successional and late successional stages of the succession series were Artemisia ordosica , Sophora alopecuroides and Stipa bungeana communities, respectively. The soil below the A. ordosica community included semi-fixed and fixed sandy soil, and the main plant species in the community includedCorispermum declinatum , Heteropappus altaicus ,Cynanchum thesioides , Inula salsoloides , Astragalus melilotoides , Thermopsis lanceolate , Allium mongolicum ,Aneurolepidium dasystachys , Calamagrosis epigeijos ,Ixeris chinensis , Bassia dasyphylla , Salsola collina and Hedysarum mongolicum. The soil below the S. alopecuroides community was fixed aeolian sandy soil, and the main plant species in the community included A. scoparia , Poa sinoglauca , Pennisetum centrasiaticum , A. dasystachys ,Echinops gmelini , Chenopodium glaucum , C. acuminatum , Oxytropis psammocharis , Tribulus terrestris ,I. chinensis , Setaria viridis , Peganum nigellastrum , Lactuca tatarica , A. tenuissimum ,Euphorbia humifusa , E. kozlovi var. angustifoliaand E. esula . The soil in the area of the S. bungeanacommunity was light sierozem and sierozem, and the main plant species included Cleistogenes squarrosa , P. sinoglauca ,Glycyrrhiza uralensis , Agropyron mongolicum , O. aciphylla , Gypsophila davurica , Lespedeza davurica ,Hedysarum brachypterum , Haplophyllum davuricum ,Linum perenne , Polygagla tenuifolia , Iris tenuifolia , Laguchilus ilicifolius , Potentilla bifurcaand Cynanchum hancockianum.
In July 2016, we designed a 2×10 km belt transect to investigate features of the three communities.
For the A. ordosica community, we established 25 sampling plots along the belt transect. In each plot, a 50 m sampling line was randomly established, and a 4×4 m quadrat was set at distances of 0 m, 25 m, and 50 m along the sampling line to investigate the number and biomass of the A. ordosica plants in each quadrat. The plants were cut along the root stock, and a balance was used to measure the biomass ofA. ordosica . Then, a 1×1 m subquadrat was randomly established within each 4×4 m quadrat, and the number and biomass of each herb was recorded. Each herb plant in each subquadrat was cut along the root stock, and a balance was used to measure the biomass. The branches and leaves of each plant species in each subquadrat were collected; in each 4×4 m quadrat, a soil sample to 20 cm depth was collected.
For each of the S. alopecuroides and S. bungeanacommunities, 25 sampling plots were established in the belt transect, respectively. In each plot, a 50 m sampling line was randomly established, and a 1×1 m quadrat was set at was set at distances of 0 m, 25 m, and 50 m along the line. The number and biomass of each herb plant in each quadrat were measured. Each herb was cut along the root stock, and a balance was used to measure biomass. Samples of branches and leaves of each plant in each quadrat were collected and brought back to the laboratory. In every 1×1 m quadrat, a soil sample to 20 cm depth was collected.