Abstract: [Objective] This study aims to explore the dynamic changes of the diversity and interspecific relationships of vegetation communities in Caijiachuan small watershed, located in Loess Plateau in western Shanxi Province which has been closed for 35 years. The findings of this study will provide a theoretical basis and technical supports for the closed-to-nature restoration and reconstruction vegetation in the Loess Plateau.[Method] In July and August, 2016, both line transect and quadrate sampling methods were employed to survey the status vegetation in six different habitats (including sunny slope, semi-sunny slope, shady slope, semi-shady slope, hilltop and ditch sole) in the small watershed. The survey data was compared with those obtained in 1996 (closed for 15 years), 2007 (closed for 26 years),and 2016 (closed for 35 years). The dynamic changes of the diversity and interspecific relationships of vegetation communities were analyzed by comparing indices in the periods from 1996 to 2007 (considered as the first period) and from 2007 to 2016 (considered as the second period).[Result] On a time scale, the Jaccard index, which indicates the degree of similarity, increased significantly in each of the six habitats in the second period. The increase in the habitats of semi-sunny slope, sunny slope, semi-shady slope, shady slope, hilltop and valley bottom were 69.6%, 27.3%, 18.1%, 48.5%, 5.2%,and 1.0% respectively. The Cody index, which indicates the degree of species replacement, decreased in almost all of the six habitats except the hilltop, where it was kept the same in both the first and second periods. The decrease in habitats of semi-sunny slope, sunny slope, semi-shady slope, shady slope, and valley bottom were respectively 41.2%, 29.4%, 28.6%, 50.0%, and 22.7%. In 2016, the Jaccard index of habitat gradient was in an order of shady slope-semi shady slope > sunny slope-semi sunny slope > valley bottom-shady slope, while the Cody index of habitat gradient of shady slope-semi-shady slope=sunny slope-semi-sunny slope < hilltop-sunny slope. The interspecific relationships of vegetation community in the semi-sunny slope in 1996 were not significantly related with that in 2007 (P> 0.05). Similar phenomenon also happened in the sunny slope and the valley habitat. However, significant positive correlations (0.01<P<0.05) of the interspecific relationships of vegetation community were found between 1996 and 2007 for the other three habitats. The interspecific relationships of vegetation community in the valley bottom in 2007 was significantly positively related with that in 2016 (0.01 < P < 0.05). While, extremely significant positive correlations (P < 0.01) were found between 2007 and 2016 for the other habitats. The association coefficient (AC) representing the junction strength between communities of the hilltop habitat was decreased in the second period compared with the first one. While, the AC of the other five habitats increased significantly.[Conclusion] The improved rate of vegetation abundance in the closed small watershed in the second period from 2007 to 2016 was slower than that of the first period from 1996 to 2007. The community similarity between the shade slope and semi-shady slope were increasing, and the rate of species replacement was slowing down. The same situation existed on the sunny slope and semi-sunny slope. For long run, there was a certain correlation between vegetation communities in the same habitat, and the structure of the communities gradually turned to be stable, even though they might still be the stage of progressive succession.

Key words: the Loess Plateau, closed watershed, β-diversity, community association, dynamic changes