Pure black alder stands in Slovenia make up a small part of total forest land but are nevertheless ecologically and economically significant. Steering the development of such stands is possible with appropriate silvicultural models. In 1976, first research plots were established in Polanski log for this purpose. Each of the plots was divided into three fields with different treatment: no intervention, moderate thinning and strong thinning. In 2018, the diameter at the breast height was measured and the social status, vitality, tendency, silvicultural role, crown length and overall quality of trees were estimated. The height of ten trees in each of the fields was also measured. The findings showed lower diameter increment of trees than expected. There were no statistically significant differences in diameter increment of the trees that were in the fields with moderate thinning and those with no intervention; these trees had an average diameter growth of 0.33 cm/year. There were, however, significant differences in the diameter increment between the trees that have undergone strong thinning (0.37 cm/year) and those with the other two treatment regimes. The diameter growth of dominant trees compared to all trees was higher; still there were no statistically significant differences among treatments. This may be due to inconsistent thinning or insufficient intensity. Compared to silvicultural models with fewer crop trees, the density and basal area of the analyzed stands were significantly higher, while the diameter increment was lower.
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