This study presents the analysis of mean tree crown defoliations of different tree biosocial groups belonging to the main stand prepared on the basis of observations from years 1991, 1993 and 1995 in the Zielonka Experimental Forest Division. The objective of investigations was to determine statistically significant or non-significant differences between them. In relation to 1991, an improvement of tree crown condition was observed in successive years of observation in consecutive individual Kraft classes and groups of Kraft classes expressed by a decreasing value of defoliation. A smaller number of evaluated tree crowns in 1993 had a distinct effect on defoliation variability, which was significantly higher than in the remaining two years of observation – 1991 and 1995. The better the condition of crowns, the more statistically significant differences of mean defoliations. This process was more conspicuous in younger stands. The change of tree crown defoliation of the same Kraft classes but different age classes observed in successive years of observation was statistically significant which proves that the improvement in stand condition was apparent and unequivocal. When the thinning was considerable, differences in tree crown defoliations of the same Kraft classes but different age classes in a given year of observation were not statistically significant. The improvement in crown condition initially revealed significance of differences only in the case of groups of Kraft classes (1993) and later also in individual Kraft classes (1995). The obtained results did not give a clear answer as to the usefulness of individual Kraft classes for the evaluation of stand defoliation. In order to obtain a clear answer to the above question, it is necessary to carry out investigations on a wider sample collected from different objects.

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