Dynamics of species diversity and interspecific association in shrub layer of naturally restored woodland on northern slope of Qilian Mountains,Northwest China

By substituting spatial difference for time change,seven major natural communities representing the succession series of degraded woodland from herbage community to tree community at altitudes from 2468 to 2761 m on the northern slope of Qilian Mountains were selected to measure the species diversity in shrub layer by multiple indices,and the species overall interspecific association and dominant species interspecific association in the shrub layer of the communities at different succession stages were analyzed by variance analysis and χ2 test.Up to 19 shrub species belonging to 10 families and 16 genera were investigated.The results showed that in shrub layer,the species number and Gleason index were the greatest in Picea-Betula community,the Simpson index and Shannon-Wiener index were the greatest in Betula community,the evenness index was the greatest in Potentilla glabra community,and the dominance index was the greatest in Picea crassifolia community.Except for Betula-Populus community,all the other communities had a larger number of shrub species pairs with negative association than with positive association,reflecting the interspecific competition relationship of shrub species in the succession series.With the succession of the communities,the interspecific association of most shrub species pairs changed with succession stage,i.e.,insignificant at early stage,significant or very significant at middle stage,and null or insignificant at late stage.The species overall interspecific association was negative in the restored shrub community at early stage and in the broadleaved forest dominated by heliophytes,positive in the mixed coniferous forest dominated by heliophytes,negative in the mixed coniferous and broadleaved forest,and positive in the coniferous forest dominated by mesophytes.The changes in the species diversity and interspecific association among the shrub species pairs in the succession series could be related to the niche separation or niche overlapping caused by the stratification of community structure from the ground to the community's top crest.