Response Of Black Alder (Alnus Glutinosa (L.) Gaertn.) To Selective Thinning Of Various Intensities: A Half-Century Study In Northeastern Slovenia

Pure black alder stands are specific and require adapted silvicultural models. To determine the best intensity of selective thinning in such stands, research plots were established in Polanski Log in 1967. Three thinning intensities were selected: control, moderate and high. These stands are presently in a mature phase. In 1967, 1973, 1979, 1983, 1993, 1998 and 2018 diameter at breast height was measured and social status, vitality, tendency, silvicultural role, crown length and overall quality were estimated. The differences in black alder responses to thinning intensities were analysed and the results were compared with recommendations for selective high thinning and newer crop tree situational thinning models. Diameter increments were lower than expected regardless of thinning intensity. In moderately thinned plots and control plots diameter increment was the same (0.33 cm/year); high intensity thinning plots showed higher increment (0.37 cm/year). Dominant trees had slightly higher increment regardless of thinning model. Compared to thinning models with a lower number of crop trees, density and basal area of studied stands were significantly higher and diameter increments lower. We attribute the small diameter increments and small differences among thinning models to insufficient intensity and partially to inconsistent thinning. The results indicate that thinning must be of higher intensity and the largest-diameter trees which display the best vigour, quality, tendency, and which have well formed, long crowns, must be promoted from the beginning.