Abstract: To investigate the effects of flooding disturbance on the interspecific relationships among grassland plant species, a comparatively thorough study was conducted on Sanjiadian National Rangeland in the territory of Da'an city, Jilin Province, which was partly flooded in1998. The study site was located in the south of Songnen plain, Northeastern China, dominated by Leymus chinensis grassland. Along the flooding gradients (from un flooded to heavily flooded) formed on an extensive mild slope taken as the test site, four flooded transects coded T₁, T₂, T₃ and T₄ respectively,but subjected to the flood of different durations were designed, and also an un flooded one coded T₀ was set as a control at relatively higher elevation. Before flooding occurred in1998, the slope had almost uniform soil and L.chinensis dominated vegetation. Contingency table was used to calculate the interspecific association indice,while Spearman rank correlation coefficient was adopted to analyze the interspecific covariance. The results showed that flooding disturbance had little effect on the association type for most species pairs, but had some effect on the rank coefficients of inter specific co variation. Different species pairs varied greatly in their responses to the flooding disturbance, suggesting that the inter specific co varying relations were not only dependent on environmental perturbation or disturbance factors, but also on (probably to a larger extent) the biological & ecological properties of the species that comprised the species pairs. The interspecific association types that could be determined between Potentilla paradoxa and all the other plant species were always positive,and little affected by flooding disturbance. As a main companion species, P.paradoxa had a broad ecological breadth, and weak selectivity for habitat. Contrarily, the interspecific association types that could be determined between Phragmites australis and others plant species (in addition to Scirpus planiculmis) were always negative, probably due to the biological properties of the plant. P.australis was a kind of clonal plant with long rhizomes, and enlarge its population through vegetative propagation by rhizomes. When in good water conditions, P. australis had a powerful competitive ability, thereby, imposed competitive or exclusive effects on the other plant species, thus formed the negative correlations between the plant and the other, which could be little affected by flooding disturbance. Interspecific covariance was not only influenced by environmental or disturbance factors, but also dependent on the different responses to the changing environments during the disturbances between the component species that comprised the species pairs. The two plant species that had similar and dissimilar water demands would tended to be positive and negative covariance respectively,if water conditions changed towards an extreme. Interspecific covariance based on the numerical properties of the communities were much more sensitive to the external disturbance than the interspecific associations calculated only based on the binary data of “presence or absence of species”.

Key words: Flooding, Disturbance, Interspecific relationships, Association, Co variation, Songnen Steppe