Spatial pattern and association of shrub species in gravel hilly and rocky low mountain desert dominated by relict Helianthemum songaricum in China

Helianthemum songaricum is a tertiary relict plant and an endemic species found in the central of Asia. It is the only species of the Cistaceae family found in China and categorized as a rare and endangered one. However, little is known about how this strong xerophytic cushion plant maintains community structure and what are underlying mechanisms. Therefore, the present study investigated the structure and dynamics of H. songaricum desert in the west Ordos of Inner Mongolia Plateau in China by analyzing the intra- and interspecific spatial associations and distribution patterns of the major woody species in two H. songaricum communities with significantly different soil conditions. By using point pattern analysis based on the univariate and bivariate transformations of Ripley’s L function, we revealed that both facilitation and competition acted in the H. songaricum desert, but facilitation occurred more frequently in a more water-stressed rocky habitat than less water-stressed gravel habitat, while the opposite was observed for competition. The whole population of H. songaricum and the individuals of different size classes were distributed randomly and regularly at most of the spatial scales in the gravel habitat, while tended to distribute clearly clumped in the rocky habitat. These differences in spatial distribution patterns of H. songaricum in the two habitats could be related to the differences in the neighborhood effects of intraspecific individuals and other woody species on H. songaricum. The lower number of H. songaricum juveniles and intra- and interspecific competition detected to the juveniles in both H. songaricum communities indicated limited regeneration of H. songaricum population. These findings have important implications for H. songaricum population protection.