Physical Properties of Juvenile Wood of Two Paulownia Hybrids Fizi č ka svojstva

• There is a growing trend in the world of planting fast growing species (rotations 5 to 10 years). Their primary purpose is the production of wood ﬁ bers and biomass, but they certainly represent the potential in making solid wood products as well. One of the fast-growing species is Paulownia sp., a species of extremely fast growing wood. Plantation breeding of Paulownia sp. in Croatia is increasing, although there is a little knowledge about the technical properties of Paulownia wood and its end use is questionable. This paper presents preliminary results of some physical properties of juvenile wood of two Paulownia hybrids planted in the area near the town of Glina in the Republic of Croatia. One hybrid is 9501 ((Paulownia fortunei × Paulownia elongata) × (Paulownia fortunei × Paulownia tomentosa)) and the other hybrid is Shan Tong (Paulownia fortunei × Paulownia tomentosa). The aim of this study was to investigate physical properties of juvenile wood of two Paulownia hybrids from one site in Croatia, to determine differences in physical properties of wood between two hybrids and to evaluate the correlation between density and shrinkages of each hybrid. Signi ﬁ cant differences in oven dry density, basic density and density at maximum MC, between the two hybrids were determined. There is no statistically signi ﬁ cant difference in longitudinal, radial, tangential and volumetric shrinkages between the two hybrids.


UVOD
The trend of increasing demand for wood raw material is becoming of great global concern. The answer to this would be to plant more fast growing trees of different species, in order to conserve native forests and to ensure adequate supplies of wood. Paulownia sp. is an example of very adaptable genus requiring minimal management after the fi rst few years (El-Showk, 2003). It has been cultivated throughout Asia for centuries, with China having the longest history (Ates et al., 2008).
Paulownia wood is considered to be extremely fast growing, especially in the juvenile phase of growth. Under optimal conditions, Paulownia sp. trees can produce useful timber within fi ve to six years, measuring 30-40 cm in diameter after ten years (Olson and Carpenter, 1982;Zhao-Hua et al., 1986). It is widely used for various purposes, with excellent prospects for pulp and biomass (Vilotić et al., 2015;Icka et al., 2016;Vusić et al., 2018). However, as a solid wood, it is suggested to be used in products that are not subject to great loads during exploitation (Šoškić et al., 2003) and not for structural purposes (Koman et al., 2017).
Due to its high adaptability, new markets are developing rapidly for plantation grown Paulownia sp. in many countries. In Croatia, the most commonly propagated are Paulownia hybrids Shan Tong and 9501. Currently, there are no Paulownia hybrid trees with known origin older than four years in Croatia. Drvodelić (2018) investigated their propagation by root cuttings, where the difference in rooting percentage between two hybrids depended on the cutting thickness and drying procedure. However, technical properties of hybrids 9501 and Shan Tong have not been investigated so far in Croatia. In addition, there are limited data on technical properties of wood of different Paulownia species (Ayhildiz and Kol, 2010; Kiaei, 2013; San et al., 2016;Komán et al., 2017).
Therefore, data on properties of Paulownia hybrid wood grown on the territory of the Republic of Croatia are needed. This information could determine whether Paulownia sp. is profi table for cultivation and use as a raw material for industrial purposes.
The aim of this study was to investigate physical properties of Paulownia hybrids 9501 and Shang Tong juvenile wood from one site in the Republic of Croatia, to determine differences in physical properties of wood between two hybrids and to evaluate the correlation between density and shrinkages of each hybrid.

MATERIJALI I METODE
For the purpose of this research, two 4-year old Paulownia hybrids were taken from the area near the town of Glina in Croatia. One hybrid is 9501 ((Paulownia fortunei × Paulownia elongata) × (Paulownia fortunei × Paulownia tomentosa)) and the other hybrid is Shan Tong (Paulownia fortunei × Paulownia tomento-sa). Five test trees of each hybrid were taken from the experimental stand. All trees were chosen as representative of the stand according to HRN ISO 3129:2015.
Four disks were cut at breast height (1.3 m), upwards to the crown, from each tree. Disks were approximately 5 cm thick and 10 cm in diameter. Maximum number of test samples were cut from each disk, according to HRN ISO 3129:2015.
Statistical analysis of data and their comparison were carried out in Statistica 8. Data were analyzed and presented as the minimum, mean and maximum values, as well as standard deviation. The analysis of variance (ANOVA) was used to determine whether there are any statistically signifi cant differences between the means of investigated wood properties of two Paulownia hybrids. Duncan's multiple range test (DMRT) was applied to test statistical signifi cance at α = 0.05 level. The simple linear regression model was used to analyze the relationship between density and shrinkage.

REZULTATI I RASPRAVA
Statistical values of Paulownia hybrids 9501 and Shan Tong juvenile wood, as well as the summary of analysis of variance (ANOVA) for oven dry density, basic density, density at maximum moisture content (MC), longitudinal, radial, tangential, and volumetric shrinkage are shown in Table 1, Table 3, Figure 1 and Figure 2.
Physical properties of wood, especially wood density and dimensional stability, are important factors affecting wood quality (Ištok et al., 2016). Mean oven dry density of hybrid 9501 is 249 kg/m 3 and of hybrid Shan Tong is 237 kg/m 3 ( Table 1). The analysis of variance (ANOVA) indicated that there is signifi cant difference in oven dry density between the two hybrids (    (Welch, 1932 and1934;Kelsey, 1963;Hann, 1969;Skaar, 1988). Based on the work of Harris and Meylan (1965), the major cause of the variation in longitudinal shrinkage is varying microfi bril angle. Table 2 shows the comparison of investigated values of shrinkage with other authors.
In this study, regression equations between oven dry density and shrinkage, as well as, statistical significance between two properties are shown in Table 4. The relationship between oven dry density and shrinkage is statistically signifi cant only for radial and tangential shrinkage of hybrid 9501 and radial shrinkage of hybrid Shan Tong. Negative correlation between oven dry density and longitudinal shrinkage in both hybrids was determined. Radial, tangential and volumetric shrinkage showed positive correlation w ith oven dry density. Measured shrinkages showed weak correlation coeffi cient with oven dry density, except radial and volumetric shrinkage of hybrid 9501. Similar results were reported by Šoškić et al. (2003) for Paulownia elongata and Paulownia fortunei wood.

ZAKLJUČAK
Preliminary result of juvenile wood of two Paulownia hybrids showed statistically signifi cant differences between mean values of some investigated physical properties.
Signifi cant differences in oven dry density, basic density and density at maximum MC, between hybrid 9501 and hybrid Shan Tong were determined. However, these differences in mean values of densities amounted only to about 6 %.
There is no statistically signifi cant difference in longitudinal, radial, tangential and volumetric shrinkages between the two hybrids.
Radial, tangential and volumetric shrinkage values of both investigated hybrids are similar to references for some Paulownia wood species.
Both hybrids should be investigated after fi ve to ten years when they reach larger diameters.

Acknowledgements -Zahvala
The examined issues constitute a part of the project: Kratkoročna fi nancijska potpora istraživanju 2018; Istraživanje strukturnih, fi zičkih svojstava i biološke otpornosti drva klonova paulovnije (Shortterm fi nancial support for research 2018; Investigation of structural and physical properties and biological resistance of Paulownia hybrids) (Paulownia Siebold et Zucc.). This work was fi nanced by the University of Zagreb, Croatia.