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ŠUMARSKI LIST 9-10/2019 str. 44     <-- 44 -->        PDF

Likewise, giving a comprehensive review on potential impact of climatic changes on forest vegetation shift in Slovenia, Kutnar and Kobler (2011) stated that the share of thermophilous forests, which are less economically interesting, will increase significantly (from the present 14% to a range between 50% and 87%), replacing the currently predominant mesic forests. This situation is particularly worrying concerning European beech, which although cover an extensive natural range, and which is also known to be vulnerable to drought (Rose et al. 2009). Indeed, Lakatos and Molnar (2009) documented mass mortality of beech trees in Hungary, as a consequence of a damage chain, caused by drought period from 2000 to 2004, while Czúcz et al. (2011) found that 56-99% of existing European beech forests in Hungary might fall outside their bioclimatic niche by 2050. Finally, using Ellenberg’s climate quotient (EQ) Stojanović et al. (2013) showed that 90% of beech forests in Serbia (southeast Europe) may be outside their present bioclimatic niche until the end of 21st century. Therefore, it seems that alternative forests tree species, such as hornbeam, will need to be used intensively in the future in order to meet raising demands of wood industry.
Using of hornbeam, which is known for its high density, hardness, toughness and wear-resistance (Fodor et al. 2017), would be justified from both ecological point of view, as well as taking into account projected climate change effects on forest ecosystems in Central and Southern Europe. Namely, certain studies on a long-term natural forest dynamics in a mixed stands evidenced a quantitative increase of late-successional species, such as Carpinus betulus, whereas parallel decline was observed for economically more important tree species (e.g. Picea abies, Fraxinus excelsior, Quercus robur, etc.) (Bernadzki et al. 1998). Finally, according to aforementioned EQ, European beech is dominant tree species at the areas with EQ less than 30 (Ellenberg, 1988), whereas higher EQs (i.e. regions with dryer and warmer climate) are more suitable for hornbeam growing. Although very rare, certain studies evidenced naturalization and formation of large pure stands by this species in the regions with EQ over 30, thus suggesting that climatic change might positively affect the distribution of hornbeam in the future (Jensen et al. 2004).
Thermal modification of wood at high temperatures causes permanent changes in chemical and physical properties (Aytin et al. 2015; Welzbacher et al. 2009). Thermal modification of wood increases its moisture resistance, improves dimensional stability, enhances resistance against biological deterioration, and contributes to uniform colour change from original to dark brownish tones (Kollmann et al. 1975; Stamm, 1956; Tjeerdsma et al., 1998; Kotilainen et al., 2000; Yildiz, 2002; Avadi et al., 2003; Rousset et al., 2004; Hill, 2006; Živković et al. 2008; Sinković et al., 2011; Sahin, H. T. et al. 2011; Mitani and Barboutis, 2014). Heating changes wood colour acquiring a darker tonality which is often caused by the formation of color degradation produced from hemicelluloses (Sehlstedt-Persson 2003, Sundqvist 2004). The colour of wood is important from the aesthetic point of view.
However, thermal modification decreases mechanical properties of wood. Mass loss, lower density of thermally modified wood are caused by heating regime, procedure, duration, relative humidity and wood moisture content (Vernois, 2001; Alén et al., 2002; Esteves et al., 2007; Candan et al., 2013; Laine et al., 2016; Lykidis et al., 2016; Li et al., 2017; Boruvka et al., 2018). These undesirable changes may limit application of thermally modified wood in wood construction.
Although there are scientific papers with measured reduced values of hardness of beech and hornbeam wood after heat treatment (Yildiz, 2002; Gunduz et al., 2009), their results are difficult to compare with each other due to the use of different treatment regimes. This paper aims to check and compare the decrease in the density and hardness of Brinell wood due to the equal treatment of beech and hornbeam wood with high temperature. Beech and hornbeam were selected precisely because of the aforementioned impacts of climate change as well as predictions on the distribution of beech and hornbeam in South East Europe.
The aim of this article was to investigate and compare Brinell hardness on three main sections of thermally modified and unmodified beech wood an hornbeam wood and to determine wood mass and wood density reduction after thermal modification.
MATERIAL AND METHODS
MATERIJAL I METODE
Research was carried out on beech wood and hornbeam wood. Trees selected for research came from the Papuk region and originate from the same economic unit which means they had the same conditions for growth. Trees were selected according to HRN ISO 3129:1999. One bark to bark core, length of one meter, was cut out from each tree. Then the cores were sawn in half its length. A half of core was used to make samples of unmodified beech and hornbeam wood and the other half was thermally modified at 200 ºC for 48 hours. The entire process from the beginning of thermal modification to the cooling of the heath chamber lasted for 72 hours.
After the thermal modification cores were sawn into specimens 30 mm × 30 mm × 20 mm (R, T, L) for the purpose of testing wood hardness. Only the samples without any natural wood defects (bark, cracks, reaction wood) where taken in consideration. Three series of samples were prepared, depending on the investigated cross section. The hardness of wood was investigated according to Brinell (HRN ISO 3350:1999) on cross, radial and tangential section. Wood density was investigated according to HRN ISO