Evaluation of the Basic Properties for the Korean Major Domestic Wood Species II. Tulip Tree (Liriodendron tulipifera) in Gangjin-gun, Jeollanam-do

1. INTRODUCTION

Although wood is an environmentally friendly material that is naturally produced through photosynthesis, it is also composed of a variety of cells that have unique properties, such as non-uniformity and anisotropy, which necessitate careful processing and utilization (Chong and Park, 2008). Given the variability in wood properties across species and even within the same species, depending on the growing region and age, it is essential to understand the characteristics of wood and its appropriate use to ensure efficient utilization of wood (Park et al., 2024). In a recent study, Park et al. (2024) evaluated the anatomical properties (length, width, and cell wall thickness for the tracheid), physical properties (specific gravity and shrinkage), mechanical properties (bending strength, compressive strength, tensile strength, shear strength, hardness), and chemical composition (ash, extract, lignin, total sugar content) of Korean red pine (Pinus densiflora) grown in Pyeongchang, Gangwon-do, Korea, with the objective of establishing a comprehensive database of wood properties for major tree species in Korea. This study aims to present the results of evaluating the fundamental material properties of the Tulip tree (Liriodendron tulipifera) cultivated in Gangjin, Jeollanam-do.

The Tulip tree is a deciduous broad-leaved tree that was introduced from North America in the 1920s and subsequently planted throughout Korea. It typically reaches an average height of 30 m with a diameter of 0.5 to 1.0 m. The Tulip tree is capable of withstanding frost and is not adversely affected by air pollution. It is relatively free from pests and has a long lifespan (Kim et al., 2007). In particular, it has been extensively cultivated throughout the Korea since the early 2000s due to its rapid growth, environmental adaptability, and high carbon absorption capacity (Lim et al., 2022). Representative successful forests include Chodang Forest in Gangjin-gun, Jeollanam-do, Soyang Forest in Wanju-gun, Jeollabuk-do, and Gumi National Forest in Gyeongsangbuk-do, and are known to thrive relatively well in western Gyeonggi-do, Chungcheongnam-do, Jeollanam-do, and Jeollabuk-do.

The Tulip tree has a history of utilization in a variety of applications, including furniture, packaging, interior decoration, veneer, and plywood. However, recent years have seen a shift in focus towards exploring its potential for a broader range of uses. In particular, given the rapid growth rate and plentiful resources of the Tulip tree, research has been actively pursued to identify optimal pretreatment conditions and assess its potential for biofuel production (Gwak et al., 2024; Kim et al., 2009, 2011, 2015, Shin et al., 2009). In recent studies, the specific gravity, strength, porosity, adhesion, and carbonization properties of Tulip tree have been evaluated to assess its potential as a building material (Jang et al., 2019; Kim et al., 2023; Lee and Eom, 2011; Lim et al., 2010; Song and Kim, 2022). Additionally, studies have been conducted to assess the characteristics of heat-treated Tulip trees as a means of addressing the inherent durability issues associated with Tulip trees (Chang et al., 2019; Kim et al., 2010; Yoon et al., 2009).

2. MATERIALS and METHODS

2.1. Target species

The study selected and used 40 Tulip tree logs with a diameter of 300 mm or more from Mountain 49-1 (N34.55°, E126.87°), Myeongju-ri, Chilyang-myeon, Gangjin-gun, Jeollanam-do, Korea (Fig. 1). The mean age of the logs used in this experiment was approximately 28 years.

Fig. 1. Tulip tree production site.

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2.2. Evaluation of base material properties

The anatomical properties (length, width, and cell wall thickness for the vessel element and wood fiber), physical properties (specific gravity and shrinkage), mechanical properties (bending strength, compressive strength, tensile strength, shear strength, hardness), and chemical composition (ash, extract, lignin, total sugar content) of the Tulip tree were analyzed. The evaluation of each wood property was performed in the same way as in previous studies (Park et al., 2024). In most cases, KS or ASTM standards were followed, as shown in Table 1, but for anatomical properties without standardized specifications, the experimental methods were determined by referring to previous studies (Kim et al., 2024; Lee and Bae, 2021; Lee et al., 2021a, 2021b, 2021c; Nam and Kim; 2021). The specimens used in this experiment were sawn from heartwood devoid of immature wood, as illustrated in Fig. 2, exhibiting annual rings parallel to the edges to account for the inherent heterogeneity and anisotropy of wood.

Table 1. Standard for the evaluation of wood properties

Property		Standard
Anatomical properties	Length of cell	-
	Width of cell	-
	Thickness of cell wall	-
Physical properties	Specific gravity	KS F 2198 (Korean Standards Association, 2016)
	Shrinkage	KS F 2203 (Korean Standards Association, 2020a)
Mechanical properties	Bending strength	KS F 2208 (Korean Standards Association, 2020d)
	Compression strength	KS F 2206 (Korean Standards Association, 2020b)
	Tensile strength	KS F 2207 (Korean Standards Association, 2020c)
	Shear strength	KS F 2209 (Korean Standards Association, 2020e)
	Hardness	KS F 2212 (Korean Standards Association, 2020f)
Chemical composition	Ash	KS M ISO 18122 (Korean Standards Association, 2015)
	Extractives	ASTM E 1690 (ASTM, 2021)
	Lignin	ASTM E 1758-01 (ASTM, 2020)
	Sugars

Adapted from Park et al. (2024) with CC-BY-NC.

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Fig. 2. Location of specimens collected from log. Adapted from Park et al. (2024) with CC-BY-NC.

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3. RESULTS and DISCUSSION

3.1. Anatomical properties

As for the anatomical properties of the Tulip tree, the length of the wood fibers was 1.33 mm in early wood and 1.74 mm in late wood; the width of the early wood fibers was 21.19 μm radially and 22.67 μm tangentially; and the width of the late wood fibers was 12.82 μm radially and 11.14 μm tangentially. The cell wall thickness of wood fiber was measured to be 3.24 μm in early wood and 5.40 μm in late wood.

The length of the vessel element was measured to be 0.75 mm in early wood and 0.71 mm in late wood, and the width of the early wood vessel element was measured to be 83.44 μm radially and 60.32 μm tangentially, while the width of the late wood vessel element was measured to be 59.73 μm radially and 47.29 μm tangentially. The cell wall thickness of the vessel element was measured to be 1.48 μm in early wood and 2.01 μm in late wood.

Fig. 3 shows an optical microscope image of three cross-sections of a Tulip tree to identify its cellular structure.

Fig. 3. Optical microscope images for each section (1% Safranine solution). (a) Cross section (×10), (b) radial section (×10), (c) tangential section (×10).

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4. CONCLUSIONS

This study evaluated the anatomical, physical, and mechanical properties, as well as chemical composition, of the Tulip tree (Gangjin-gun, Jeollanam-do), a representative forest tree species in Korea, with the aim of establishing a database of wood properties of major domestic trees (Table 2). Given the variability in the wood properties across different regions of cultivation, it is implausible that the wood properties of Tulip trees from a single region could accurately represent those of domestic Tulip trees. It is thus imperative to undertake a comparative and evaluative analysis of the wood properties of Tulip trees cultivated in disparate production zones, with a view to deriving representative wood properties of domestic Tulip trees. The results of this study can be employed as a foundation for such an endeavor. It is intended that, in the future, further data on the basic wood properties of a range of tree species and regions will be made available in order to create a database of wood properties by region for the major domestic tree species.