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ŠUMARSKI LIST 5-6/2019 str. 43     <-- 43 -->        PDF

with the increase in thinning intensity especially in younger leaved stands (Mayor and Rodà, 1993; Bréda et al. 1995; Hibbs et al. 1995; Rytter, 1995; Kerr, 1996; Medhurst et al. 2001; Clatterbuck, 2002; Meadows and Goelz, 2002; Juodvalkis et al. 2005; Makineci, 2005; Tufekcioglu et al. 2005; Rytter and Werner, 2007, Çiçek et al. 2013). The positive effects of thinning intensity on diameter increment can be attributed to the availability of more light, water, and nutrients for the thinned trees. The response given by the ecosystem to the thinning treatment was found to be associated with the increased diameter increment, water, and nitrogen use efficiency among the thinned trees, and the increased net photosynthesis ratio (Wang et al. 1995). It is possible to attribute the higher diameter increments in the Maçka-Yeºiltepe experiment in comparison with Vakfıkebir to lower stand age, or in other words, to the high growth potential. In the study supporting this study, Carus and Çiçek (2007) reported that the diameter increment in Oriental Beech decreased with the competition index among trees and the increasing plantation age. Thinning at the young stage enables individual trees to grow faster and develop more resistance to biotic and abiotic damages.
In our study, significant differences were determined between basal area and stem volume increments and thinnings in each plantation (Table 2). RBAI and RVIs were found higher in Yeºiltepe experiment (27.02 - 41.08%) compared to Vakfıkebir experiment (24.93 - 28.88%). The highest increments were found in strong thinning in both plantations. In the experiments, it was determined that RBAI and RVIs grew along with the increase in thinning intensity. However, in the old plantation Vakfıkebir experiment, RBAI and RVIs of moderate and strong thinnings gave close results and were statistically similar (Table 2). In the study carried out by Umut et al. (2000) on young Oriental Beech stands, moderate (20%) and strong thinning (40%) treatments were applied, and it was concluded that the treatments applied were effective in increasing the basal area but there was no significant relationship between the increase in treatment intensity and the increase in basal area. This result complies with the old plantation Vakfıkebir experiment. However, younger plantation Maçka-Yeºiltepe experiment does not comply. In Maçka-Yeºiltepe experiment, RBAI and RVIs increased with the increase in thinning intensity, and the other thinnings except for strong thinning were found statistically similar (Table 2). Since young plantations have higher growth potential compared to older plantations, they have a higher diameter, height, and basal area increments. Similar to the results of this study, it was seen that thinning increased the basal area growth in some leaved tree species (Cañellas et al. 2004; Pretzsch, 2005; Boncina et al. 2007), and in other studies, it decreased the basal area and volume increments (Simard et al. 2004; Çiçek et al. 2013). The importance of early silvicultural treatments on the future growth of young leaved stands (natural or plantation) has been emphasized in many studies (Schönau and Coetzee, 1989; Juodvalkis et al. 2005; Rytter and Werner, 2007; Matiæ et al. 2003). Schönau and Coetzee (1989) suggested that thinning should start early, recommended thinning at frequent intervals, and noted that the first thinning should be heavier than later ones. Early thinning can result in greater growth response, provided that the residual trees are vigorous (Oliver and Larson, 1996). The change in the basal area, volume and biomass increments in the reaction revealed by the stand against thinning can be explained by thinning intensity, thinning type, stand maintenance, stand age, growing environment and the differences between tree species (Çiçek et al. 2013). Our results support the known information about the effects of thinning on stand production.
We can say that the annual height increment in both plantations except for the control plot in Vakfıkebir experiment was not affected by the thinning intensity (Table 2). Similar results were obtained in various leaved tree species (Graham, 1998; Medhurst et al. 2001; Rytter and Werner, 2007). Except for very high and very low stand densities, stand density has significant effects on diameter growth, but it has no effect on height growth. In this study, the other thinnings except for the control plot in the older Vakfıkebir plantation were found statistically similar. The thinnings in the younger Maçka-Yeºiltepe plantation were found statistically similar. Along with the increase in the interval-distance of trees in fast-growing broad-leaved trees, the tree height may increase, decrease or remain unchanged (Alcorn et al. 2007; DeBell et al. 1996; Fang et al. 1999; Kerr, 2003; Pinkard and Neilsen 2003).
Variations in height growth with changes in available growing space could be attributed to ontogeny, to the range of tested spacing treatments, or to species. Height growth plays an important role in morphological acclimation to light competition (Lanner, 1985), with plants tending to allocate more photosynthate to height than diameter growth, which results in increasing stem slenderness (Benomar et al. 2012). On the other hand, height growth occurs early in the season when resources are not limited, and diameter growth occurs in summer when resources that restrict photosynthesis are limited (Wang et al. 1995).
Thus, stand density reduction by thinning increases soil water availability in summer, which primarily affects diameter increment. Bréda et al. (1995) found that thinning enhanced radial growth as a result of less severe water deficits in the thinned plot in late summer than in the control plot. Soil water measurements in our experimental plots showed that volumetric soil water contents were higher in thinned plots than in unthinned plots from July through September (Çiçek et al. 2010). Stone et al. (1999) also reported that thinning increased soil volumetric water content between May and August in the first year after thinning.