Abstract: In the autumn_harvested dry crop fields of Middle Zhejiang province,170 random sampling fields of 17 sites on different ecological conditions were surveyed in detail with seven scales by visualization of weed dominance and the weeds important value to autumn_harvested crops was obtained. The principal components analysis(PCA)and the method of minimal spanning tree(MST)were used to reveal the ecological similarity of 22 main weeds in autumn_harvested dry crop fields.The results showed that the ecological similarity among Digitaria sanguinalis,Setaria viridis,Eleusine indica,Cyperus iria,Cleome viscosa and Echinochloa crusgalli var. hispidula,Cynodon dactylon,Acalypha australis,Cyperus rotundus was high, while Mazus japonicus,Centipeda minima,Luwdigia prostrata,Lindernia antipoda,Mollugo pentaphylla and Eclipta prostrata had the similar ecological demands.The fast shift of weed populations under succesive application of a certain herbicide in a long peroid of time resulted from two factors:the different sensitivity of various weeds to the herbicides applied and the different ecological demands of the weeds in crop fields. Therefore, it is important to study the ecological demand difference among various weeds.

Key words: soil erosion, soil organic carbon (SOC), black soil region, slope farmland.