Questions: The formerly overgrazed Inner Mongolia steppe was subject to retrogressive succession. Today, Artemisia frigida and Potentilla acaulis are two dominant species in different phases of successive degradation. To investigate the impact of grazing intensity on spatial community structure, we investigated the small-scale spatial association between A. frigida and P. acaulis at zero, light, medium and heavy sheep grazing, and proposed factors involved in the spatial associations between these two species along a grazing intensity gradient.
Location: The Inner Mongolia Grassland Ecosystem Research Station of the Chinese Academy of Sciences.
Methods: Four grazing intensities were selected: zero, light (1.33 sheep/ha), medium (4.0 sheep/ha) and heavy (6.7 sheep/ha). After 13 years of grazing three 2 m × 2 m quadrats with 100 × 100 cells of size 2 cm × 2 cm were randomly selected in each treatment in July and August 2002. The presence of A. frigida and P. acaulis in each cell was recorded and the positions of the individuals were mapped using Cartesian coordinates in each quadrat. The small-scale spatial associations between A. frigida or P. acaulis were quantified with the L12(d), J12(d) functions (both derived from K12(d), the former indicating the type of the spatial association, the latter indicating the strength of the spatial association), using Monte Carlo simulations.
Results: A. frigida was negatively associated with P. acaulis at short distances (0 - 100 cm) under zero and light grazing, and negatively or independently under medium and heavy grazing. Increasing grazing intensities suppressed the peak negative associations. More intense grazing enhanced the tendency towards independent distribution of these two species.
Conclusions: The small-scale spatial associations between A. frigida and P. acaulis were significantly different at four different intensities of sheep grazing. Grazing disturbance, clonal growth habit of species, and interspecific competition are the main factors leading to a difference of spatial associations between these two species at different grazing intensities.