Understory vegetation,one of the important parts of forest ecosystem structure,has some effects on material cycling,biodiversity,succession,and development of forest ecosystem.To study the structure and biomass dynamics of understory vegetation,investigations were conducted on the typical plantations of the mixed alder(Aluns cremastogyne) and cypress(Cupressus funebris) at 10,15,20,25,and 30 year cypress plantation succeeded from the mixed plantations in the hilly areas of central Sichuan basin.Results showed that species abundance in shrub and herb layers increased from 10th to 20th years.However,from 20th to 30th years,species abundance in shrub layer declined significantly,and in herb layer,it did not change significantly.Heights of shrub and herb layers increased significantly from 10th to 15th years and decreased slowly from 15th to 30th years.Spatial heterogeneity of shrub height in the samples at the same age was higher.Coverage degrees of shrub and herb layers increased significantly before 15th year and then declined sharply,but did not change significantly from 20th to 30th years.Coverage degree of herb layer was higher and its spatial heterogeneity was lower compared with shrub layer.Biomass of shrub layer increased significantly before 15th years,then decreased from 15th to 20th year,and eventually stabilized from 20th to 30th years.Biomass of herb layer fluctuated from 10th to 30th years and tended to decline with plantation age.The results indicate that structures and functions of understory vegetation tend to degenerate with growth and development of the mixed alder and cypress plantations.