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ŠUMARSKI LIST 1-2/2019 str. 50     <-- 50 -->        PDF

The effects of thinning intensity and cross-section height on the mean stem diameter increment of the co-dominant and dominant trees were significant (p <0.05). However, the effect of the interaction of thinning intensity × cross-section height on the mean stem diameter increment was not significant (p >0.05). The mean stem diameter increment of the co-dominant trees with heavy treatments (5.8 cm) was greater than that of the control plots (4.4 cm). The mean stem diameter increment of the dominant trees was similar in the control and moderate treatment plots (5.8 cm), whereas it was highest in the heavy treatment plots (6.5 cm) (Table 2).
The highest diameter increments in both stem classes were determined for the cross-section at heights of 0.30 m and 17.30 m without distinction of thinning intensity. The diameter increments and diameter increment percentages between stem heights of 1.30-13.30 m were very close. However, the diameter increment percentages in the sections at 15.30 m and higher had increased (p <0.05) (Table 3).
In the NLA plantation, the greater increments for d1,30 and mean stem diameter were in the heavily thinned plots. There have been a number of studies showing that the intensity of thinning increases the breast height diameter in NLA (Çiçek et al., 2013) and other broadleaved species (Andrašev et al., 2012; Bobinac and Andrašev, 2006; Bréda et al., 1995; Clatterbuck, 2002; Hibbs et al., 1995; Juodvalkis et al., 2005; Kerr, 1996; Makineci, 2005; Mayor and Rodà, 1993; Meadows and Goelz, 2002; Medhurst et al., 2001; Özbayram, 2015; Tüfekçioğlu et al., 2005). On the other hand, Eler (1988) reported that the diameter increments at 0.30 m and d1.30 and in the middle part of the stem were greater for the heavy thinning than in the control plot. This positive effect of thinning intensity on the diameter increment can be explained by the fact that that the remaining trees in the stand benefited from the increase in light, water and nutrients.
For all classes and treatments, the highest diameter increments were observed in the sections at heights of 0.30 and 17.30 m (Table 3). Trees give priority to thickening at the bottom of the stem in order to survive (Smith et al., 1997). Therefore, it is believed that the diameter increment will be greater at the 0.30 m section than at other section heights. In some studies, it has been noted that the diameter of the stem increased rapidly at the bottom (Pukkala et al., 1998; Tasissa and Burkhart, 1998), followed by an increase in the upper parts of the stem (Hilt and Dale, 1979; Mäkinen and Isomäki, 2004a; Mäkinen and Isomäki, 2004b; Pukkala et al., 1998; Tasissa and Burkhart, 1998). On the other hand, the increase in diameter at 17.30 m could have been caused at this height because it represents the area where the branches are most concentrated. It is necessary for the crown to thicken and grow to form a solid connection with the tree structure in order to shield it against mechanical effects. Kalıpsız (1998)