Abstract: Based on 2×2 contingency table and with X²-and W-tests, this paper analyzed the inter-specific association of 20 main tree species in the Dipentodon sinicuscommunities in Yushe National Forest Park of Liupanshui, Guizhou Province. The results showed that a significant positive association was observed at the sampling levels of 50 m², 100 m², and 250 m² quadrates, and a sampling size of 100 m² was more suitable than that of 50 and 250 m² for the interspecific association analysis. The degree of interspecies association based on X²- test, together with the association coefficient (AC) and percentage co-occurrence (PC), showed that the indices applied could reflect the approximately same results, and multiple indices could remedy the insufficiency analyzed by a single index. The inter-specific associations betweenD. sinicus and other tree communities were comparatively significant, and mostly were of positive. The more intense the positive association, the more similar the environmental demand required, and the larger the value of niche overlap was.

Key words: Free-air CO₂ enrichment(FACE), Soil hydrolase, Soil oxidoreductase, Enzyme activity