Abstract:Tree competition states were examined in two 50 m × 50 m plots of natural middle-aged forests of Mongolian oak, which are located at Jincang Forest Farm, Wangqing Forestry Bureau, Jilin Province. Eight competition indices were selected to show the states of inter-and intra-species competition. The analysis results indicated that the trees competed strongly in both aspects of intra-species and inter-species, especially for Mongolian oak. White birch and black birch, the strongest competitive species for Mongolian oak, were also the most competitive species in the forest. But the competitiveness of Mongolian oak was at the middle level among the species. The results of the indices differed from each other due to they reflected the competition from different aspects. The competition intensity of Mongolian oak decreased as the diameter of the individual tree increased, and the relationship can be reasonably depicted by power function.