Abstract:By using the improved competition index model,the intraspecific and interspecific competitions of Pinus tabulaeformis from Ziwuling Mountains in Qingyang City of Gansu Province were quantitatively analyzed.The results show that with the increase of the size of objective tree and the distance between plants,the intraspecific competition intensity decrease,because of the population density regulation during self thinning.In the community,there are many other species,but their diameter at breast height is less than that of P.tabulaeformis,as a result,the competition intensity of intraspecific was more excessive than that of interspecific.The order of competition intensity is intraspecific＞Quercus liaotungensis＞Toxicodendron vernifluum＞Ulmus davidiana var.japonica.The relation between competition intensity and individual growth of objective tree nearly conformed to power function,and the changing of competition intensity is very small when the diameter of the objective tree grows upto 25 cm.The model can simulate and predict the intraspecific and interspecific competition efficiently.The study provides a new approach to measure the competition intensity.