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ŠUMARSKI LIST 7-8/2020 str. 41     <-- 41 -->        PDF

trees, classified as the 100 largest-dbh crop trees per hectare. To compare the diameter structure, we classified trees into diameter classes (5 cm): 1 (0–4.9 cm), 2 (5–9,9 cm), 3 (10–14,9 cm), 4 (15–19,9 cm), 5 (20–24,9 cm), 6 (25–29,9 cm), 7 (30–34,9 cm), 8 (35–39,9 cm), 9 (40–44,9 cm), 10 (45–49,9 cm), 11 (50–54,9 cm), 12 (55–59,9 cm), 13 (60–64,9 cm), 14 (65–69,9) and 15 (70–74,5 cm). The growing stock was calculated using modified French tables for even-aged stands (Schaeffer tariffs) as usual in Slovenia (Kotar, 2007). We adopted the tariff class from the Slovenian Forest Service. Diameter increment was calculated based on the difference between the first and last diameter measurement. Diameter increment was modelled with linear mixed-effects model (LMM) where three repetitions (i.e. plots) were considered as random factors. Final model was selected following a top-down approach (Zuur et al., 2009). For model diagnostics of all model types, we examined confidence intervals of parameters and analysed sets of graphical summaries proposed by Robinson and Hamann (2011).
By design the plots were of different ages at the start of the experiment, which made it possible to analyse the development of pure black alder stands from stand initiation to over 100 years of age. In Polanski Log black alder stands are regenerated by planting; study plots 5 and 11 had 10,000 saplings/ha planted (Figure 3). The first data on diameter structure (Figure 2) is available several years after planting. At stand age of approximately 7 years, most trees on plot 5 were in diameter class 1 irrespective of the thinning model.
For plot 11 the first data on diameter structure is available at stand age approximately 13 years. At that point the majority of trees were in diameter class 2 and 3, with negligent differences among thinning models. The next diameter structure data is available for plot 12 at stand age 17. Irrespective of the thinning model, trees in diameter classes 2, 3 and 4 dominated. Between stand age 10 and 20, densities on plots were between 1,500–2,985 trees/ha in high intensity thinning fields and 1,920–3,630 in control fields (Figure 3). In the same period growing stock was 54–177 m3/ha in high intensity thinning fields and higher in control fields, 109–213 m3/ha (Figure 4). Basal area at this stand age was 12–22 m2/ha in high intensity thinning fields and 18–26 m2/ha in control fields (Figure 5).
Until approximately stand age 30, densities dropped to 625–1,235 trees/ha in high intensity thinning fields and 1,305–1,890 trees/ha in control fields (Figure 3). Growing stock increased to 194–356 m3/ha in high intensity thinning fields and 294–438 m3/ha in control fields (Figure 4). Basal area