Thinning experiments in narrow-leaved ash (Fraxinus angustifolia Vahl.) plantations: 10-year results

Narrow-leaved ash (NLA) has a wide natural distribution with ecologically different areas; however, in Sakarya, Turkey, the bottomland forests are now mostly dominated by pure NLA plantations. In this study, two thinning experiments were carried out in 22- and 36-year-old NLA plantations in Sakarya in order to determine the effects of different thinning intensity (unthinned or the control, removal of 22.3 and 39.4% of the basal area) on mean diameter, basal area and volume growth of the whole stand, those of the 100 largest trees in the stand and trees of initial diameter size class after 10 years. A randomized block design with three replications was used in both experiments. Ten-year results of selective low thinning treatments indicated that thinning significantly increased diameter increments, which were positively correlated with the thinning intensity in both plantations when compared to the control. Heavy thinning increased the increment of mean stand diameter but did not significantly affect the basal area or volume of the stand in either of the experiments. However, in the first experiment, basal area and volume increments differed significantly among treatments, increasing with thinning intensity for the 100 largest trees. In the second experiment, although not statistically significant, the basal area and volume increments also increased with thinning intensity for the 100 largest trees. Moreover, all measured increments were higher in the younger plantation than in the older. These results showed that different thinning intensities applied in NLA plantations can produce similar basal area and volume increments. Consequently, to maximize diameter growth and increase crop tree value, heavy thinning treatments can be recommended, particularly for young NLA plantations at bottomland sites.