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ŠUMARSKI LIST 7-8/2020 str. 39     <-- 39 -->        PDF

The altitude of study plots is approx. 163–165 metres above sea level. Although the terrain is typical lowland with small differences in altitude, such small, barely perceptible differences in altitude have a decisive impact on which species will thrive there (Nemesszeghy, 1986). The Polanski Log developed on gley soils on substrate of Holocene alluvial deposits of loam and clay (Lovrenčak, 1991). In gley soils groundwater is somewhere between 0–80 cm from surface all year, only in summer does the soil dry slightly (Nemesszeghy, 1986; Rauš, 1975). In depressions the groundwater comes to the surface, where it is stagnant or slow-flowing and where it remains longer than in slightly elevated areas (Wraber, 1951). These depressions are ideal black alder growing sites (Mlinšek, 1961; Vukelić and Rauš, 1998). Prekmurje has a sub-Pannonian climate with hot summers and cold winters. Mean annual temperature for 1963–2017 from the nearest meteorological station (Lendava, 190 m a.s.l.) was 10.5 °C, mean annual precipitation for the same period was 797.4 mm (ARSO METEO, 2018). Although precipitation is relatively low, 61% falls during vegetation period, which is favourable for forest vegetation (Nemesszeghy, 1986).
Sample plots were established at four locations. Each plot was divided into three 0.20 ha (40 x 50 m) fields representing different treatments: control, moderate thinning, high intensity thinning. The fields were separated by a 20 m buffer zone. In moderately thinned fields the strongest competitors of selected trees and some dominant trees of low quality were removed. In high intensity thinning fields all competitors and advanced regeneration of poor quality were removed (Kecman, 1999). As well-established, the thinning intensity decreased with the developmental stage of stands. For example, the thinning intensity of 40-year-old stands relative to the growing stock was approximately 13% and 20%, respectively.
Below is a brief description of each plot. Plot 4 is the oldest plot with trees aged 100–105 years. It was included in the experiment in 1967. In the subsequent years moderate and high intensity thinning fields were carried out three times (Table 2). After 1986 there were no more measures on this plot because the area on which the plot is located was designated a protected forest. After change of ownership it was changed to its present status of forest reserve in which logging is banned.
Plot 5 was formed in 1970 with the planting of two-year saplings with a density of 10,000 saplings/ha (Table 2). The trees are presently about 50 years old. Until age 12, when the first thinning was performed in the thinned fields, ground vegetation was cleared and removed. At stand age 18 the second and final thinning was conducted.
Plot 11 was established with planting of 10,000 saplings/ha in 1963 (Table 2). First thinning was conducted at stand age 16 in the moderately thinned field and stand age 12 in the high intensity thinning field. Subsequent thinning in both fields was conducted at stand ages 38, 51 and 57. The final thinning was also conducted in the control field, just before the final measurement in 2018.
Trees on plot 12 are currently 66–72 years old, at the start of the experiment they were approximately 17 years old. Since then three thinnings were conducted in moderate and high intensity thinning fields (Table 2). There were no measures in the last 30 years.
All trees in the fields were numbered and at each measurement their diameter at breast height (dbh) was measured using a diameter tape. Seven measurements were conducted on the plots (1967, 1973, 1979, 1983, 1993, 1998, 2018), except on plot 5, where five measurements were conducted (1975, 1980, 1993, 1998, 2018). Using IUFRO classification (Ouellet and Zarnovican, 1988), the trees were classified at measurement by social position, development class, evolutive trend, silviculural role, crown length and