Abstract: By using two competition index models with diameter at breast height (DBH) and crown width as parameters respectively, the competitive units based on Voronoi diagram were applied quantitatively to analyze the intra- and inter-specificcompetition of Pinus tabulaeformis f. shekannesis and Quercus liaotungensis (Liaotung oak) in the P.tabulaeformis  f. shekannesis community on Loess Plateau. The results showed that P. tabulaeformis  f. shekannesis population suffered intense intra-specific competition for DBH and intense inter-specific competition for crown width. The maximum competitive stress appeared in the fast-growing period with DBH<10 cm, and the stress affected tree growth significantly. Liaotung oak population suffered greater inter-specific competition for crown width at DBH<5 cm, and greater intra-specific competition for crown width at 5 cm<DBH<10 cm. The inter-specific competition for crown width had no significant effects on the tree growth of Liaotung oak, indicating that P. tabulaeformis  f. shekannesis couldn’t exert the intense and continuous competition for crown width on Liaotung oak. The competition intensity of the two tree species declined with increasing diameter class of the trees. The relationship between the competition intensity (CI) and DBH (D) of target trees followed the equation CI=AD^B.

Key words: Songnen Plain, Ecotone, Ecologic landscape geochemistry, Ecological coupling