Competition between trees exists due to limited resources required for tree growth. The growth dependent competitive situation of each tree is described in growth models by calculating distance dependent and distance independent competition indices, CIs. The first aim of the research was to compare distance dependent and distance independent CIs and find the best variant with highest predictive capacity to simulate basal area and height growth of pine trees under Lithuanian conditions. The second aim was to describe the impact of competition on the tree diameter and height growth of pine trees by using those CIs that showed the highest predictive capacity. The research was based on data collected from eighteen permanent sample plots. The study evaluated 2 distance independent and 18 distance dependent CIs using partial correlation analysis.

The results showed that the distance dependent CIs had a higher predictive capacity for basal area growth of pine trees. The CI proposed by Biging and Dobbertin (1992) combined with the selection method of an inverse search cone at height to crown base with opening angle of 80 degrees is recommended for developing basal area increment models used in single tree level growth simulators in Lithuania.

While modelling tree height growth, distance dependent CIs did not show better results than the distance independent CIs. Hegyi's (1974) distance independent CI scored the highest partial correlation coefficients in predicting mean annual height increment. Generally, poor ability of competition indices to predict periodic mean annual height increment was observed under growth conditions prevalent in Lithuania.

The results also showed that an increase in competition always had a negative impact on tree diameter growth, but a low level of competition increased tree height growth compared to no competition and additional competition had a negative impact to tree height growth.

Key words: basal area, competition-dependent, competition indices, diameter, distance dependent, distance independent, growth, height, increment, tree