Abstract: The age status of four Stipa L. species, S. baicalensis Roshev., S. grandis P. Smim., S. krylovii Roshev., and S. klemenzii Roshev. in Inner Mongolia plateau was analyzed using a transect sampling method. The ontogeny of Stipa species was divided into five stages: the seedling, juvenile, reproductive, presenile, and senile. Dynamics of the population age speetra were determined mainly by the appearance of new individuals and their death rate. The survival rate of the seedlings was very low while that of file adults was high. The considerable longevity of bunch life allowed the old individuals to accumulate in the population. Both S. grandis and S. krylovii had higher proportion of seedlings than others. The proportion of the juveniles and the reproductives in the population decreased from S. baicalensis community to S. klemenzii community, but the old individuals including the preseniles and the seniles showed a reverse pattern. The aging process occurred in bunch level was characterized by the fragmentation of bunch and the within-bunch- self-thinning of the tillers. The process of within-bunch-self-thinning can be described by power equation Y: aXb. Meanwhile as a response to enviroument changes especially to the fluctuation of annual precipitation and its seasonal distribution both the numbers of reproductive shoots and vegetative shoot mass within a bunch were variable.

Key words: Inner Mongolia plateau, Stipa species, Population, Age status, Within-bunch-self-thinning of the tillers