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ŠUMARSKI LIST 7-8/2020 str. 46     <-- 46 -->        PDF

crown length (χ2 = 0.209; p = 0.901) and quality (χ2 = 7.328; p = 0.120). Regardless of thinning model, the majority of these trees were in the predominant layer (73%), highly vigorous (85%), sociologically progressive trees (73%) with average crown length (46%) or slightly lower quality (51%).
The regression model predicted that diameter increment increased with thinning intensity (p < 0.001), diameter at the start of the experiment (p < 0.001) and with vitality classes at the start of the experiment (p < 0.001). The effects of the thinning intensity and vitality were weaker than that of the initial diameter (Figure 8). The effect of thinning was also reflected by the higher proportion of good and medium quality trees between the beginning of the experiment and year 2018 (49% vs. 62%). The differences in the diameter increment between the three trunk quality classes also increased (Figure 8d).
The densities, growing stock and basal area determined in this study are in line with the findings and recommendations of Nemesszeghy (1986) and Mlinšek (1961), who developed a traditional selection model for black alder thinning. As expected, the results correspond to a lesser extend to recommendations of more contemporary models with a lower number of selected trees and less frequent intervention. Some authors of these models recommend basal area values after second or third thinning of 15 m2/ha (Claessens, 2004) and decrease of densities until stand age 20–30 years to 200–300 trees/ha (Claessens et al., 2010) or selection of 300 trees/ha by stand age 18 (Immler, 2004). On our plots, such densities were not even achieved at the end of rotation, which Nemesszeghy (1986) recommends be at stand age of 50–60 years (Table 1).
Statistical model used in the study (LMM) proved thinning intensity, diameter at the start of the experiment and vitality class at the start of the experiment as the most important factors affecting the diameter increment. In all cases the relation was positive. Trees in moderately thinned fields and control fields had average annual diameter growth increment of 0.33 cm/year, while trees in high intensity thinning fields averaged 0.37 cm/year. The selected 100 largest-dbh trees grew faster, as expected. In control fields their annual diameter increment was 0.46 cm/year, in moderately thinned fields it was 0.02 cm/year higher, and in high intensity thinning fields it averaged 0.50 cm/year. Malus (2012) recorded similar annual diameter increment (0.34 cm) with dendrochronological analysis of cut trees in Polanski Log. Claessens et al. (2010) report than in the best growing sites, such as Slovenia, north Germany and south France, black alder can reach a dbh of 40–50 cm in 40–65 years. This means that annual diameter increment should be at least 0.6 cm or even in excess of 1 cm. The trees in our plots did not even achieve the annual diameter increment values that Claessens et al. (2002) measured for dominant trees at the end of rotation on less productive sites in Belgium. There, dominant trees in non-thinned stands grew 0.4 cm/year and trees without competition 0.7 cm/year.
This shows that annual diameter increment of dominant trees in our plots was at least 0.5 cm lower than expected for black alder on such sites. At the same time, the differences among thinning models in this study were small, which may indicate that predominant black alders are characterised by a similar diameter increment for extended periods regardless of thinning measures. Favourable social differentiation and small differences in growth patterns of predominant crop trees was also indicated for spruce, beech, ash and maple by Ammann (2004). We also determined that there were no significant differences among thinning models in terms of stratification, vigour, tendency, crown length and quality of dominant trees, whereby it is necessary to account for inadequate consistency of measures in thinning fields: from two to a maximum of four thinnings were conducted, whereas Nemesszeghy (1986) and Kecman (1999) recommend from five to seven throughout the rotation period. Still, such intervention frequency may not be justifiable in today’s economic terms.
Excluding plot 4, where intervention started at a significantly higher age, the first thinning on our plots was conducted later, at stand ages 12-18. Subsequent thinning was conducted at very different ages (Table 2) and even less in line with the recommendations of authors of traditional thinning models (Mlinšek, 1961; Nemesszeghy, 1986; Kecman, 1999).
It is also notable than in recent years thinning has not been conducted systematically, as labour costs rose and wood prices fell (Roženbergar et al., 2008; Arnič et al., 2018). The thinning delay was partially influenced also by the change of ownership. In spite of all, the thinning performed exemplifies a representative situation from the past practice of thinning of lowland forests. In the future it seems worthwhile to check the appropriateness of crop tree situational thinning models for black alder in Slovenia.
We also attribute the low annual diameter increment to excessive densities, short crowns and developed epicormic sprouts. As many as 43% of trees in the plots had short crowns and there were no trees with long crowns at all. There was also a high share (38%) of trees of the lowest quality with developed epicormic sprouts. There may also be other reasons not dealt with in this study. Malus (2012) for example found that above-average irradiation and above-average water levels reduced diameter increment of black alder. Hydrological improvements may theoretically affect growth as well, but Levanič (1993) ruled that out in Polanski Log.