Abstract:

Aims Inter-specific interactions in plant communities can be characterized by their spatial relationships. Because these interactions are usually asymmetrical, i.e., one species exerts more influences on the other, asymmetric spatial patterns should be expected. Our objective was to study how species spatially adapt to arid conditions in four typical arid communities that differed in growth-form composition (tree-grass, shrub-grass, shrub-semishrub and forb-grass).

Methods We selected four representative arid plant communities near Sand Lake in Xidatan, Pingluo County, Ningxia Hui Autonomous Region and measured the position, height and crown diameter of each individual in a 30 m × 30 m plot in each community. We applied the bivariate Ripley’s K function in spatial association analysis and developed a new method named pattern control analysis (PCA) for investigating asymmetric spatial relationships. PCA was derived from the assumption that the controlled species should be distributed around the controlling species regularly at a specific scale. Two simulated community patterns were used to test the new method.

Important findings In communities ofElaeagnus angustifolia-Achnatherum splendens (EA), Nitraria tangutorum-A. splendens (NA) and A. splendens-Sophora alopecuroides (AS), significant positive spatial associations were detected between the dominant species (at scales of 0-15 m, 0-7.5 m and 0-7 m, respectively), and species pairs with positive spatial association significantly differed with each other in growth form. In the community of Reaumuria soongorica-Kalidium foliatum (RK), negative spatial association was detected (at the scale of <15 m) between the two dominant species, which are similar in growth form. Pattern control analysis showed a relationship between the dominant species in the three communities with positive spatial associations at scales of 0-6.5 m, 0-0.65 m and 0-1.5 m in EA, NA and AS, respectively. The pattern-controlling species in each community are larger in size and produced better developed microhabitats. However, no such relationship was detected between the dominant species of similar growth form and comparable microhabitats in RK. Results indicate that certain spatial relationships, especially pattern-controlling relationships, might be mechanisms for adaptation within plant communities. This might help in understanding inter-specific interactions within plant communities.

Key words: point pattern analysis, Ripley’s K function, pattern control analysis, inter-specific relationship