1 Introduction
Various forecasts have predicted a steady increase in the global demand for fibrous materials in the wood-based industry until at least 2050 (Sikkema et al.
2021; Kircher
2022). On the other hand, the natural forests have declined as wood supply over the years, while plantation forests have increased steadily and they will predominate in large parts of the world as future sustainable source of wood (FAO
2022). For this, there is a rapidly growing interest, worldwide, in plantations of fast-growing wood species that can be used for production of industrial wood (McEwan et al.
2020), for conservation of natural ecosystems (Pirard et al.
2016) as well as for carbon fixation (Bredemeier et al.
2015). As one of the fastest-growing tree species in the world, the genus
Paulownia Siebold & Zuccarini, has attracted enormous interest in recent years (Jakubowski
2022).
The genus
Paulownia, belonging to the family
Paulowniaceae and indigenous to China and East Asia, consists of nine species (Hassler
2022); the most important used and studied species are
Paulownia elongata S. Y. Hu,
Paulownia fortunei (Seem.) Hemsl. and
Paulownia tomentosa (Thunb.) Steud. (Zhao-Hua et al.
1986; Jakubowski
2022). Different hybrids have also been developed, such as
Paulownia hybrid Cotevisa 2 produced in Spain, which is the result of the cross between
Paulownia elongata x Paulownia fortunei (Esteves et al.
2022; Jakubowski
2022).
Paulownia species or hybrids have been introduced in North America, Australia, Japan, Iran, Kyrgyzstan and Kenya (Jakubowski
2022). In Europe, its presence is documented in Spain, Portugal, Hungary, Turkey, Croatia, Italy, Serbia, Romania, Northern Ireland, Czech Republic, and Ukraine (Jakubowski
2022).
Paulownia is a deciduous tree capable of achieving very high growth rates under favorable conditions (Zhao-Hua et al.
1986; Flynn and Holder
2001).
The wood of
Paulownia is defined as straight grained, with gloss after being planed, light, soft, inodorous, and with few knots (Zhao-Hua et al.
1986).
Paulownia wood is traditionally used for a variety of applications, such as furniture, construction, musical instrument, ship building, aircraft, packing boxes, cabinet making, molding, kitchen items and paper (Zhao-Hua et al.
1986; Akyildiz and Kol
2010; Jakubowski
2022).
Paulownia wood also performs well in the production of plywood, LVL or blockboards, which act as a core layer between veneers and as an ingredient for the production of lightweight particleboards (Bee et al.
2005; Jakubowski
2022). Nowadays,
Paulownia plantations oriented towards chip production for biomass or wood-based panels are gaining in popularity (Fernández-Puratich et al.
2017; Esteves et al.
2022). There is currently a lot of research being conducted that is aimed at furthering the use of
Paulownia wood in the production of wood plastics and composites and in the production of biopolymers (Jakubowski
2022).
The radial cross-section of a stem can be divided into three zones: juvenile wood (JW), transition wood and mature wood; but most of the time only juvenile wood and mature wood are considered (Alteyrac et al.
2006). The juvenile wood is the secondary xylem formed during the early life of a tree and all trees have juvenile wood, but it has little significance when the timber supply is primarily old-growth trees grown in old natural forest ecosystems (Kretschmann et al.
1993). In these old trees, the juvenile wood core is small because early growth is suppressed by competition from surrounding trees and thus, the juvenile wood proportion in the total volume is small (Tsoumis
2009; Ruano and Hermoso
2021). Otherwise, fast-growing trees that reach sawn timber size are harvested at an early age on short rotations in plantations and they contain a bigger core of juvenile wood in comparison with harvested timber from natural forests (Dowse and Wessels
2013; Nawrot et al.
2014; Wessels et al.
2014; Hermoso et al.
2016; Boruszewski et al.
2017).
It is well known that juvenile wood characteristics contribute to undesirable solid wood properties (sawn timber, veneer and wood-based panels) (Zobel
1984), due to its low bending strength and dimensional instability upon drying (Moore and Cown
2017). Juvenile wood often has different properties compared to mature wood (Barbour
2004). The age of transition between juvenile and mature wood has been difficult to determine because transition is gradual and not abrupt and the estimate of the transition to mature wood may vary depending on the property used (Alteyrac et al.
2006; Clark et al.
2006; Marbun et al.
2020). The duration of the period when juvenile wood is produced, the rate of transition from juvenile wood to mature wood, and the differences in properties between juvenile wood and mature wood differ among species (Bao et al.
2001) and are all important in determining the suitability of wood for specific end uses (Zobel and Sprague
1998).
Differences in chemical, physical, anatomical, and visual characteristics have all been reported for various species, and the importance of each depends on the wood product to be manufactured. So, pith-to-cambium variation in anatomical wood traits, such as ring width and latewood proportion, differences in cell wall thickness, microfibril angle and fiber length are frequently described in terms of juvenile and mature wood zones and are used to estimate transition age (Burdon et al.
2004; Lachenbruch et al.
2011; Darmawan et al.
2015; Moore and Cown
2017), but also different physical properties related to dimensional stability of sawn timber, such as density, longitudinal shrinkage, or anisotropic shrinkage coefficient, have been studied for different species (Harris
1977; Oliver-Villanueva and Becker
1993; Pliura et al.
2005; Zhang et al.
2021). One of the most serious problems in drying and utilization of young plantation timber is warp in the form of twist, crook and bow (Oliver-Villanueva and Becker
1993; Kliger
2001; Johansson and Kliger
2002; Moore and Cown
2017).
In summary, in fast rotation timbers, anatomical parameters like fiber length, microfibrillar angle, vessel diameter/percentage and ring width appear to be the best indicators of age demarcation between juvenile and mature wood, although maturation age often varies among the properties (Bhat et al.
2001). However, for solid wood uses like sawn timber for construction or furniture or rotary veneer for plywood or LVL, or other ultra-light wood-based panels, other physical properties are of key relevance for drying and mechanical transformation processes, like dimensional stability, especially longitudinal shrinkage and anisotropic coefficient, i.e. relation between radial and tangential shrinkage (Oliver-Villanueva and Becker
1993; Geimer et al.
1997; Ab Latib et al.
2020). Therefore, this research combines the analysis of morphological and anatomical characteristics and physical properties of
Paulownia wood produced in very short rotations in intensive plantations with optimal growth conditions to obtain results that allow us to establish the limit between juvenile and mature wood in order to optimize their industrial transformation processes, their use as value-added solid wood products in furniture, construction and packaging, and the production of biomass and cellulose.
Bearing in mind that the new Paulownia plantations established in different world regions have as their main objective the production of high quality wood in very short rotations for use in added-value products, the objective of the research is to reveal radial variations in wood anatomical structure and physical properties, and to determine the beginning of the transition between juvenile and mature wood of Paulownia hybrid Cotevisa 2, as a clone widely used in the Mediterranean area in fast-growing plantations.
4 Conclusion
The present work aimed to analyse radial variations in wood anatomical structure and physical properties of
Paulownia hybrid Cotevisa 2 from fast-growing plantations. Based on the results of this study, the following conclusions can be drawn:
a)
Paulownia hybrid Cotevisa 2 trees can grow very quickly in intensive plantations with localized irrigation on land previously used for agriculture in the Mediterranean area, reaching DBH dimensions of approx. 30 cm in less than 7 years, thus allowing the production of minimum dimensions of logs for use in sawmills and for veneer and plywood.
b)
While the stems grow straight, with a very small percentage of bark and free of knots thanks to quality pruning, the main quality problem lies in the presence of a large hollow pith, which runs eccentrically in the trunk length, especially in the first years of growth of the young trees. Undoubtedly, lengthening the rotation time by a few years would significantly reduce this defect, increasing the quality wood of the logs for the best performance in the sawing and rotary veneer processes.
c)
Annual growth of the xylem is very accelerated during the first years, even reaching more than 40 mm of radial growth per year. However, after the 5th year, the ring width stabilizes significantly around 10 mm of annual radial growth.
d)
Although the paulownia wood is extremely light in weight, significant differences are observed between the first rings and the stabilization of the values from the 5th growth ring onwards, both for oven-dry density (approx. 0.285 g/cm3) and for basic density (approx. 0.265 g/cm3).
e)
Fiber saturation, maximum moisture content and cell wall thickness and proportion also show a similar behavior, with significant differences between the first four growth rings and after the 5th ring.
f)
Regarding the dimensional stability, the differences are very clear and consistent in all directions and at the volumetric level. However, the most important result is the significant variation in the anisotropic coefficient of shrinkage, which is of key importance for the industrial process of drying sawn timber. Due to the sharp drop in tangential shrinkage and the practically constant behavior of radial shrinkage from pith-to-bark, the wood goes from being very nervous wood during the first four years of growth to being calm wood from the 5th year onwards.
As the final conclusion, although we do not have data after the 6th ring, all results indicate that from the 5th year of growth, the wood of Paulownia hybrid Cotevisa 2 stabilizes in some key parameters such as the radial growth of the rings, the structure and proportion of the vessels, the structure of the cell wall, the density of the material and the behavior in terms of dimensional stability in the drying process. That is why, for practical purposes, we can indicate that the transition from juvenile to mature wood can begin from the 5th year. Taking into account the growing conditions in the experimental plantation and the production rotation established originally for 7 years, (DBH of 30 cm in the lower part of the trunks), the total proportion of juvenile wood reaches more than 60% of the total volume of the trunk in the first sections for sawn or veneer wood. Following the observed growth regime, if the rotation was extended to 10 years, the DBH would increase to 35–40 cm and the proportion of juvenile wood would decrease to approximately 40%. Finally, if it was extended to 12 years, the DBH would reach 40–45 cm and the proportion of juvenile wood would barely be 25–30%, which would considerably increase the yield, in terms of quality of the roundwood produced, in this type of plantation, with eminently productive purposes for supplying sawmills, veneer and plywood mills, as well as other wood-based panels for furniture, construction and packaging, and even if it is destined to the production of biomass and cellulose.
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