plant communities in Xiaolongmen Forest Farm,Dongling Mountain,Beijing,China,were studied with 2 abundance measures (i.e. coverage and importance value) to examine the effects of sample size on diversity measures,and with 3 methods designed to determine critical number of quadrats ( CNQ ) (i.e.the number of quadrats when the diversity measures become relatively stable).The results show that (1) it is suitable to study plant community diversities with coverage and importance value as 2 abundance measures,a...