Assessment of Physical and Mechanical Properties Considering the Stem Height and Cross-Section of Paulownia tomentosa (Thunb.) Steud. x elongata (S.Y.Hu) Wood

The aim of this study is to analyze the properties of Paulownia tomentosa x elongata plantation wood from Serbia, considering the influence of the stem height (0 to 1 m and 4.5 to 6 m above soil level—height spot) and radial position from the pith to bark (in the core, near the bark, and in between these zones—cross-section spot). The results show that most properties are improved when the samples were taken from upper parts of the tree (height spot) and from the near bark spot (cross-section spot). The mean density measured 275 kg/m3 at the stem height between 4.5–6 m and 245 kg/m3 for the samples collected from 0–1 m trunk height. The density had the highest value on the spot near bark (290 kg/m3), for the mature wood at a height of 4.5–6 m, and near pith had a mean density of 230 kg/m3. The Brinell hardness exhibited highest values in the axial direction (23 N/mm2) and near bark (28 N/mm2). The bending strength was 41 N/mm2 for the trunk’s height range of 4.5–6 m and 45 N/mm2 in the cross-section, close to cambium. The three-point modulus of elasticity (MOR) of the samples taken at a stem height of 4.5 to 6 m was up to 5000 N/mm2, and on the spot near bark, the MOR measured 5250 N/mm2. Regarding compressive strength, in the cross-section, near the pith, the mean value was the highest with 23 N/mm2 (4.5–6 m), whilst it was 19 N/mm2 near bark. The tensile strength was, on average, 40 N/mm2 for both 0–1 m and 4.5–6 m trunk height levels and 49 N/mm2 between bark and pith. The screw withdrawal resistance measured 58 N/mm for the samples extracted at a stem height of 4.5 to 6 m and 92 N/mm for the specimens collected near pith. This study stresses the influence, in short-rotation Paulownia timber, of indicators, such as juvenile and mature wood (difference emphasized after the fifth year of growth) and height variation, on the physical and mechanical properties of sawn wood. This study will help utilize more efficient sustainable resources, such as Paulownia plantation wood. This fast-growing hardwood species from Europe is adequate as a core material in sandwich applications for furniture, transport, sport articles, and lightweight composites, being considered the European Balsa.