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ŠUMARSKI LIST 7-8/2020 str. 38     <-- 38 -->        PDF

traditional black alder thinning models in Slovenia (Mlinšek, 1961; Nemesszeghy, 1986; Kecman; 1999) advocated shorter rotations, 50–60 years (Table 1). Subsequent studies found that thinning promotes too many crop trees, which requires a significant investment of effort and reduces collective stand stability (Spiecker and Spiecker, 1988; Schütz, J.-Ph., 1996; Arnič et al., 2018). Therefore, they started to develop crop tree situational thinning, which involves one-off selection of a smaller number of trees – which must be the most vigorous and best-quality trees – and final or semi-final spacing of crop trees (Ammann, 2013). Crop tree situational thinning focuses on favouring part of tree population with the highest quality potential; in other segments it is restricted to ensuring stand stability (Roženbergar et al., 2008; Arnič et al., 2018). Development of this thinning method started in the 1990s (Claessens et al., 2010) but it was not yet been tested for black alder stands in Slovenia.
Authors of thinning models with fewer selected trees (Table 1) recommend similar rotations as authors of traditional models, but with less frequent measures (Lockow, 2003; Immler, 2004; Claessens, 2004; Fennessy, 2004; Claessens et al., 2010), and lower densities (70-120 trees / ha) whereas Nemesszeghy (1986) and Rauš (1975) recommend a final density of 250-350 trees / ha, of which 160 are selected trees / ha. Most authors of traditional selection thinning models and models with a smaller number of selected trees advocate early thinning of alder stands, to be conducted no later than year 10 or at 6 metres of branch free stem (Table 1).
In Slovenia, the stands of pure black alder are mostly located in the Pannonian region, where they form larger forest complexes in Murska Šuma, Črni Log and Polanski Log (Brus, 2015). Polanski Log and Črni Log were studied in the past by Mlinšek (1961) and Nemesszeghy (1986), who contributed to a transition from coppicing to high forest with regular thinning and the production of better-quality wood with larger diameter. Stand regeneration with coppicing was efficient, but the management method promoted irregular stem growth and accelerated root rot infection. After the Second World War it also turned out that black alder forest can represent an important input for the wood industry, and in recent years demand for high-quality black alder wood has been growing (Utschig, 2003). It is therefore important to develop optimal thinning models for managing such stands.
In 1967 sample plots were established in Polanski Log to monitor two different intensities (moderate and high intensity) of selection thinning compared to a control (no thinning). At present, the stands on these plots are mature and suitable for analysis. This study therefore analysed the development of densities, growing stock, basal area and diameter increments to determine the outcomes of different thinning intensities. We wanted to determine which method is best for pure black alder stands and the scope of differences between tended and untended plots. The results were compared with recommendations and findings of authors of traditional thinning models, and thinning models based on a smaller number of crop trees and less frequent intervention.
STUDY AREA AND METHODS
PODRUČJE ISTRAŽIVANJA I METODE
Study area – Područje istraživanja
Floodplain forests of black alder (Alnus glutinosa (L.) Gaertn.) account for just 0.4% (approx. 4708 ha) of total forest area in Slovenia (Čater et al., 2001). The largest black alder areas are found in Russia, Belarus and Ukraine (Pernar et al., 2012, quoted from Zalesov, 2008). The study was conducted in Prekmurje, Slovenia’s north-eastern most region (Figure 1). Adjacent to Mala Polana is the Polanski Log forest, which forms a 1,100-hectare complex of lowland floodplain pure black alder forest.