The expansion process of a Stellera chamaejasme population in a degraded alpine meadow of Northwest China

The expansion of poisonous plants can change vegetation community structures and affect grassland ecosystem service values. Stellera chamaejasme is one of the most important poisonous plants and has rapidly expanded in the arid areas of Northwest China in recent decades. The objective of this study was to elucidate the expansion process and model of an S. chamaejasme population. Therefore, we classified the S. chamaejasme population into five classes based on coverage: 31–40%, 41–50%, 51–60%, 61–70% and 71–80%. We investigated the spatial distribution patterns and the size compositions of S. chamaejasme under different coverages. The results show that the spatial distribution pattern of S. chamaejasme under low coverage (31–40%) at all study scales (0–100 cm) was random; the spatial distribution pattern translated to a clumped distribution from a random distribution at some scales, and the clumped distributions gradually became obvious, with coverage increasing from 41–50% to 61–70%; the spatial distribution tended to be random at all study scales when coverage was increased further (71–80%). However, the spatial distribution patterns were closely related to the size composition of the S. chamaejasme population. In particular, the quantity of older individuals had a significant impact on the variation of the spatial distribution patterns of S. chamaejasme. The spatial distribution pattern varied from a random distribution to a clumped distribution and then returned to a random distribution with increasing coverage (from 31–40% to 71–80%), and this may indicate that the S. chamaejasme patches experienced patch formation and extension and merged with each other.