Spatial heterogeneity is an important feature in grassland ecosystems. Understanding spatial heterogeneity and species richness is important for determining the degree of grassland degradation and for promoting the recovery of grassland ecosystems. We conducted field surveys in a Leymus chinensis grassland, an Achnatherum splendens grassland, and an Artemisia ordosica shrub grassland in Yijinhuoluo-Qi, Ordos, China, in August 2010. First, we established study plots composed of 90 quadrats along a 45 m transect in each of the three grasslands; each quadrat was 50 cm × 50 cm and was divided into four 25 cm × 25 cm sub-quadrats. We recorded all plant species within each sub-quadrat. Then, we harvested the aboveground biomass along each transect every 5 m, classified the content by species, and weighed the oven-dried material.We analyzed vegetation data using the power law to examine characteristics such as species composition, species diversity, biomass, and spatial distribution in the three types of grassland. Our results indicated that all three types of grassland followed the power law and exhibited a strongly heterogeneous distribution of vegetation (versus a random distribution). The index values of spatial heterogeneity and species diversity for the entire community in the A. ordosica shrub grassland were higher than those for the L. chinensis grassland, and those for the L. chinensis grassland were higher than those for the A. splendens grassland. The species richness and species diversity per 50 cm × 50 cm quadrat in the L. chinensis grassland were higher than for the A. ordosica shrub grassland; the A. splendens grassland had the lowest values. The average per-quadrat biomass in the A. ordosica shrub grassland was significantly higher than that in the A. splendens grassland, whereas the values in the L. chinensis grassland did not differ significantly from the other two grassland types. The average biomass and species diversity per quadrat increased with the spatial heterogeneity for the entire community in all three types of grassland. We analyzed the species compositions in these three grassland types from the viewpoint of biological succession and found that the grasslands changed greatly after enclosure with a fence to exclude grazing for 10 years. Inside the enclosure, annuals such as Salsola collina and poisonous, harmful plants such as Cynanchum komarovii decreased greatly, while highly nutritious grasses and legumes with good palatability for livestock became the main species. This indicates that the grassland vegetation had recovered to some degree by 2010 after establishing the enclosure in Yijinhuoluo-Qi.
Recently, the power law has been used widely in Japan for calculating the quantitative species composition and spatial heterogeneity of constituent plant species. However, this method has not been used much in China, which has a vast area of grassland. The power law requires only a survey to count the binary occurrence of each species in quadrats, and the survey is simple and convenient for observing many quadrats (i.e., >50-100 quadrats) in a short time. This method also avoids the destruction of vegetation during observations because it is based only on visual observation.