Abstract: The competition among trees within a stand includes aboveground and underground parts.How to determine the zone of competition influence is important for predicting tree growth.We divide the zone into two parts: one part is defined as inside radius, the distance from nearest competitors to subject tree,in which, the competition influence is the sum of both aboveground and underground;the another part is defined as outside radius, the distance of competitors to subject tree,where the competition influence comes mostly from underground.Thus,the competition index,based upon the divided two radii,could be decomposed into different components.We used the data of manchurian ash(MA) (Fraxinus mandshurica) and dahurian larch(DL)(Larix gmelini) from pure and mixed stands to analyze the competition influence of aboveground and underground in inside and of underground in outside radius.The results show that for MA in pure stand,the competition effect was 59.06% from inside radius,and 40.94% from outside radius.For DL in pure stand,the competition effects in both radii were 71.31% and 28.69%,respectively.There was a stronger competition influence in inside than in outside radius.Underground competitions were the important issues for both species in pure stands.In a mixed stand of MA and DL,there were two species in both radii,and the effects of competition came from intraspecific and interspecific competitors.For subject tree of MA, the effect of interspecific competition was 49.55%,and that of intraspecific was 50.45%. For subject tree of DL,the effects were 81.11% and 18.89%,respectively.DL displayed a stronger interspecific competition than MA in the zone of competition influence, and was a main effect of competition on MA.For subject tree of MA, the effect of competiton from inside radius was 25.33%,which was the sum of competition influence from aboveground and underground,and about 74.67% was from outside radius.For subject tree of DL,both in inside and outside radii were 55.37% and 44.64%,respectively.Comparing with the information from both species and two radii,the root root competition from underground was an important component.DL grew well,and could reduce the impact of intraspecific competition for MA when both species mixed in a stand.