The combined positive effects of two dominant species in an arid shrub-herbaceous community: implications from the performance of two associate species

To test the hypothesis that the species could be more beneficial in association with two-nurse species than with single-nurse species, we surveyed an arid shrub-herbaceous community primarily composed of two dominant species, Achnatherum splendens and Nitraria tangutorum and two associate species, Phragmites australis and Reaumuria soongorica in Northwest China. The performance (frequency, abundance, and size) of the two associate species in neighbored patches (patches formed by a dominant species which were closely adjacent to the patches formed by the other dominant species), isolated patches (patches formed by one dominant species with no neighbor), and open areas was compared to analyze the individual and combined effects of the two dominant species. A. splendens and N. tangutorum appeared to have reciprocal facilitation effects when growing adjacent to one another, as evidenced by the increased size of neighbored patches over isolated ones. The individual effects of A. splendens on the associate species should be generally neutral, since the frequency, abundance, size, and co-occurrence frequency of the two associate species in isolated A. splendens patches and open areas were the same (except for the significantly larger size of R. soongorica in isolated A. splendens patches). The individual effects of N. tangutorum were positive to the associate species R. soongorica, as evidenced by the fact that the frequency and abundance of R. soongorica was significantly higher (P < 0.05), and the size of two associate species was significantly larger (P < 0.05), in isolated N. tangutorum patches than in open areas. However, the frequency and abundance of P. australis, and the co-occurrence frequency of the two associate species in isolated N. tangutorum patches and in open areas were the same. In comparison to the isolated patches, there were significantly higher frequency and abundance (P < 0.05), larger sizes (P < 0.05), and higher co-occurrence frequency (P < 0.05) of the two associate species in neighbored N. tangutorum patches. Since the neighbored patches could be influenced by both dominant species, the combined effects of A. splendens and N. tangutorum were identified as positive and over-additive. We suggested that the indirect facilitation or synergistic effects could account for the combined effects of the two dominant species.