Abstract
Oriental spruce (Picea orientalis (L.) Peterm.), a refractory species of low natural durability, requires adequate impregnation to attain a longer service life in high-risk end uses (use classes 3 and 4). Improving the permeability of such species through laser-incising pretreatment is therefore important, provided that the physical and mechanical properties of the different wood portions (sapwood, transition zone, and mature wood) are not adversely affected. In this study, the weight loss, density, moisture content, shrinkage and swelling, and compressive strength parallel to the grain of these wood portions were evaluated after laser incising. The depth and diameter of the incised holes and the growth-ring characteristics were also examined. The sapwood and transition-zone samples had greater latewood layer width and higher latewood proportion than the mature wood. Laser incising reduced radial and tangential shrinkage and swelling in mature wood. Compressive strength parallel to the grain of the mature wood was lower in the laser-incised samples than in the controls. In the transition zone, the radial hole diameter was higher than the tangential diameter.
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Effects of Laser Incising on Compressive Strength and Physical Properties of Sapwood, Transition Zone, and Mature Wood of Oriental Spruce (Picea orientalis)
Davut Bakır 
,* and Abdullah Aydın
Oriental spruce (Picea orientalis (L.) Peterm.), a refractory species of low natural durability, requires adequate impregnation to attain a longer service life in high-risk end uses (use classes 3 and 4). Improving the permeability of such species through laser-incising pretreatment is therefore important, provided that the physical and mechanical properties of the different wood portions (sapwood, transition zone, and mature wood) are not adversely affected. In this study, the weight loss, density, moisture content, shrinkage and swelling, and compressive strength parallel to the grain of these wood portions were evaluated after laser incising. The depth and diameter of the incised holes and the growth-ring characteristics were also examined. The sapwood and transition-zone samples had greater latewood layer width and higher latewood proportion than the mature wood. Laser incising reduced radial and tangential shrinkage and swelling in mature wood. Compressive strength parallel to the grain of the mature wood was lower in the laser-incised samples than in the controls. In the transition zone, the radial hole diameter was higher than the tangential diameter.
DOI: 10.15376/biores.21.4.11167-11190
Keywords: Laser incising; Oriental spruce; Different wood portions; Physical properties; Compressive strength parallel to the grain
Contact information: Department of Forest Biology and Wood Protection Technology, Faculty of Forestry, Artvin Çoruh University, Artvin, Turkey; *Corresponding author: [email protected]
Graphical Abstract
INTRODUCTION
The high permeability of wood from different tree species and different wood regions within the same species (sapwood, transition zone, heartwood or mature wood, and juvenile wood) is of considerable importance for achieving effective penetration of preservative solutions into the wood structure. This guarantees an even distribution of treatment chemicals, achieving optimal retention levels. In this article, the term heartwood is used for the inner region of the stem where it is clearly differentiated from the sapwood, typically by a distinct colour change, whereas in species such as spruce and fir, in which no such colour boundary is formed, the corresponding inner region is referred to as mature wood; juvenile wood, by contrast, denotes the wood laid down during the first years of cambial activity around the pith and may therefore lie within either region. Although sapwood from many tree species can be successfully impregnated, the preservative treatment of heartwood using conventional impregnation methods is considerably more difficult due to several anatomical and chemical barriers. However, the mechanisms responsible for this low permeability differ between softwoods and hardwoods. In softwoods, the reduced treatability of heartwood is primarily attributed to pit aspiration and to the thickening and closure of pit membranes caused by incrustation with compounds such as lignans, whereas in hardwoods it is more commonly associated with the occlusion of vessels by tyloses and with the deposition of extractives and other occluding substances (Wang and DeGroot 1996; Matsumura et al. 1999; Messner et al. 2003; Panigrahi et al. 2018; Thybring et al. 2022).
Oriental spruce, one of the native tree species of Türkiye, is classified as a low-durability species and as a refractory-to-treatment wood species with respect to permeability. Not only its mature wood but also its sapwood, which would generally be expected to exhibit higher permeability, is difficult to impregnate (Bakır et al. 2022). Therefore, improving the permeability of refractory wood species is of substantial importance for the forestry sector. One of the approaches employed in the forest products industry to enhance the permeability of both refractory wood species and their heartwood portions is the application of various pretreatments. During impregnation processes, some pretreatments improve internal fluid flow in low-permeability wood (Nath et al. 2020a,b; Bakır 2022a). To achieve the minimum retention and penetration requirements in refractory wood species, various incising pretreatments are recommended prior to preservative treatment (Islam et al. 2007).
Previous studies on laser incising have primarily focused on increasing wood permeability (Ruddick 1991; Voulgaridis et al. 2014; Bakır 2022a, 2023; Winandy et al. 2022), the effects of laser irradiation on wood anatomical characteristics (Wang et al. 2013; Nath et al. 2020a, 2022; Bakır 2023), the influence of laser incising on the strength properties of wood (Suzuki et al. 1996; Morrell et al. 1998; Kortsalioudakis et al. 2015; Sharapov et al. 2019; Arulvel et al. 2023), enhancement of wood drying rates through laser-incising pretreatment (Simpson 1987), the effects of laser processing parameters on wood properties (Nath et al. 2022), the impact of laser incising on durability and leaching resistance (Bakır 2026), and the chemical characterization of wood tissues affected by laser-induced thermal modification (Barcikowski et al. 2006; Wang et al. 2013; Kúdela et al. 2023).
In a previous study investigating the effects of different pretreatment methods (mechanical incising, laser incising, and biological incising) on the physical and mechanical properties of refractory Oriental spruce and European larch (Larix decidua Mill.) sapwood specimens (Bakır 2022b), the effects of these pretreatments on density, mass loss, dimensional stability, and compressive strength parallel to the grain were found to vary depending on the wood species. For example, while laser and biological incising significantly reduced the compressive strength parallel to the grain of Oriental spruce and European larch sapwood specimens, mechanical incising did not cause any significant reduction in strength. These findings suggest that mechanical pretreatment should be prioritized for refractory wood species intended for applications in which strength properties are of critical importance. Furthermore, laser incising, which produced favorable improvements in dimensional stability, appears to be a more suitable pretreatment for wood products intended for applications where dimensional stability is a primary concern. The study also demonstrated that laser incising caused greater mass loss in European larch sapwood than in Oriental spruce sapwood and was more effective in reducing wood density in European larch than in Oriental spruce (Bakır 2022b). The findings of such previous studies, which were conducted exclusively on sapwood specimens, indicate that pretreatments applied to different tree species or different wood regions within the same species may exert varying effects on the physical and mechanical properties of the resulting wood materials. This observation constitutes the primary motivation and focus of the present study.
Although numerous recent studies have investigated micro-scale technological approaches (e.g., mechanical incising, laser incising, microwave treatment) and biotechnological approaches (e.g., bacteria, enzymes, wood-decaying fungi, and sap-stain fungi) for enhancing wood permeability, and despite the substantial body of literature devoted to improving the treatability of refractory wood species, no study has been identified that specifically examines the effects of laser incising on the physical and mechanical properties of different wood regions of spruce. Therefore, the present study aimed to evaluate the effectiveness of laser incising in improving the dimensional stability and compressive strength parallel to the grain of sapwood, transition zone, and mature wood of Oriental spruce. In addition, growth-ring characteristics of these wood regions and the relationships between wood type and the depth and diameter of laser-drilled cavities formed on radial and tangential surfaces were comparatively investigated. All experimental findings were analyzed and discussed in a comparative framework to provide a comprehensive assessment of the response of different wood regions to laser incising.
EXPERIMENTAL
Test Materials
Defect-free wood specimens representing different wood portions of Oriental spruce, namely sapwood, transition zone, and mature wood, were used in this study. The specimens were obtained from a single Oriental spruce tree (Picea orientalis (L.) Peterm.) growing in Artvin Province, located in the Eastern Black Sea Region of Türkiye. The tree was approximately 122 ± 5 years old and had a diameter at breast height (DBH) of 40 cm.
The tree was felled using a chainsaw, leaving approximately 30 cm of the stem base embedded in the soil. The north-south orientation of the tree was marked prior to felling. Breast height was not assumed as a nominal value; prior to felling, a height of 1.30 m was measured on the standing tree from ground level on the uphill side using a steel measuring tape, and the sampling position was marked on the stem. A single 5-cm-thick disc was then removed from this marked breast-height position and was used only to determine the age of the tree and its growth-ring characteristics, and to identify the boundaries of the sapwood, transition zone, and mature wood regions. The test specimens themselves were not cut from this disc; they were prepared from radial strips obtained from the sapwood, transition zone, and mature wood regions of a single log taken from the same Oriental spruce tree. Radial strips containing the pith were cut from this disc along the north-south axis. The surfaces of the strips were sanded to obtain smooth measurement planes, and growth-ring widths were measured in both the northern and southern directions with a precision of 0.01 mm using an Eklund-Brinell microscope. The widths of both earlywood and latewood layers were determined from the bark toward the pith. Subsequently, the procedures described below were carried out to identify the sapwood, transition zone, and mature wood regions within the stem of Oriental spruce.
Identification of Juvenile Wood, Mature Wood, Transition Zone, and Sapwood
The transition from juvenile wood formation to mature wood generally occurs between ages 5- and 20-years (Wolcott et al. 1987; Wang and Stewart 2012). Based on this information, growth rings were counted from the pith outward using an Eklund-Brinell microscope. Specimens corresponding to the first 20 growth rings, representing juvenile wood, were excluded from the study because juvenile wood does not fully exhibit the characteristic properties of the species and generally possesses inferior quality characteristics (Bakır 2019).
The distinction between sapwood, transition zone, and mature wood was performed using a moisture meter (GANN Hydromette HT 85 T), which enables moisture measurements at predetermined intervals on the transverse surface of the wood. Measurements were taken on the freshly cut transverse surface of the log, starting from the outermost region adjacent to the bark and proceeding inward to the pith at intervals of approximately 40 to 50 mm along the radius. No fixed absolute moisture threshold was adopted; the boundaries were instead located from the shape of the resulting radial moisture profile. The outer region adjacent to the bark gave the highest readings and was assigned to sapwood; the readings then decreased moderately over an intermediate band, which was assigned to the transition zone, and they fell markedly in the inner region, which was assigned to mature wood. Because these resistance-type readings were used only as a relative indicator for locating the boundaries between the wood portions, and not as absolute moisture values, the reduced accuracy of resistance-type measurements on transverse surfaces did not affect the data reported in this study; all moisture contents given in the Results were determined gravimetrically by the oven-dry method. Measurement on the transverse surface was necessary because the objective was to map the moisture gradient along the radius, which cannot be obtained from a radial or tangential face. The electrodes were driven to a constant depth at every measuring position, and identical species and temperature corrections were applied throughout. In addition, thin sections cut from this disc were examined against transmitted light, and the differences in transparency were used to distinguish sapwood, transition zone, and mature wood regions (Fig. 3), following the method described by Sandberg (2009) and Bakır (2019).
Wood specimens utilized in the present study were subsequently prepared from the identified sapwood, transition zone, and mature wood regions (Fig. 3). All specimens were cut to dimensions of 100 mm × 25 mm × 25 mm (length × width × thickness). A total of 54 specimens were used, comprising 18 sapwood specimens, 18 transition-zone specimens, and 18 mature-wood specimens. In the radial direction, the transition zone was defined as the band lying immediately inward of the sapwood and immediately outward of the mature wood; it therefore corresponds to the intermediate band across which both the moisture readings and the translucency of the disc changed progressively from sapwood values to mature wood values, rather than to a sharp boundary line (Fig. 3). Counted from the pith, growth rings 1 to 20 were classified as juvenile wood and excluded from the study, rings 21 to 63 were assigned to mature wood, rings 64 to 77 to the transition zone, and rings 78 to 122 to sapwood. The transition zone was thus appreciably narrower than the two adjacent regions, comprising 14 growth rings compared with 43 rings for mature wood and 45 rings for sapwood. This is also reflected in the sampling: The sapwood and mature wood regions were each wide enough to yield two radial strips of 25 mm × 25 mm, whereas only a single strip could be obtained from the transition zone (Fig. 3b). All transition-zone specimens were consequently taken from this single band and from corresponding growth-ring positions along the stem, so that they represent the full radial width of the zone rather than a selected part of it. All specimens were sawn with consideration of the radial and tangential anatomical directions.
Laser Incising Pretreatment
Laser incisions were produced on all four longitudinal surfaces of each Oriental spruce specimen derived from different wood portions, that is, on both radial surfaces and both tangential surfaces (four incised faces per specimen); the two transverse (cross-sectional) end surfaces were not incised. The incisions were produced using a CO₂ laser system (VLS6.60, Universal Laser Systems, Scottsdale, AZ, USA). Throughout this article, the terms “drilling” and “laser incising” refer exclusively to the non-contact removal of wood material by the focused laser beam: the beam heats the wood locally to the point of pyrolysis and vaporisation, and the resulting ablation of the wood substance leaves an open hole of defined diameter and depth in the surface. No mechanical drill bit, needle, or other cutting tool was used at any stage of the treatment, and the hole geometry is therefore governed by the beam and process parameters rather than by the dimensions of a tool. The laser incising was conducted at a power output of 60 W, a speed setting of 4.0, and a focal distance of 30 mm. The same incision pattern was applied to all specimens, as illustrated in Fig. 1b.
Fig. 1. Laser-incised wood specimens (a), and schematic representation of the spacing between incisions produced in the lateral and longitudinal directions according to the laser-incising pattern (b). The same pattern was applied to all four longitudinal surfaces of each specimen, that is, to both radial and both tangential surfaces; the transverse end surfaces were not incised.
The incision density was established at 10.000 incisions/m² for the laser-incising process (Fig. 1b). All incisions were produced with a depth of 1.0 mm and a diameter of 2.5 mm (Fig. 1a), following the methodology reported by Islam et al. (2008). This depth was selected deliberately rather than being limited by the capability of the equipment. Laser incising is a surface pretreatment intended to create a network of discrete openings that increases the accessibility of the outer wood tissue to fluids, and not a perforation of the cross-section, so that the intended effect is generated in the region immediately beneath the treated surface. Because the incisions were applied to all four longitudinal surfaces of each 25 mm cube, the treated zone extends inward from every lateral face, and the proportion of the specimen affected by the treatment is therefore considerably greater than the nominal depth on a single face would suggest. The choice was further guided by earlier experimental evidence obtained on the same species. In a previous study in which laser incisions were produced to a depth of 10 mm in Caucasian spruce and European larch specimens, the compressive strength parallel to the grain was reduced significantly, and the reduction was attributed to the removal of load-bearing material and to the thermal damage extending deep into the cross-section (Bakır 2022b). Because the present study was designed to isolate the response of the three wood portions to an identical laser treatment, a depth that would itself dominate the strength response was avoided; at 1.0 mm the ablated volume remains small, as confirmed by the measured weight-loss values, and the mechanical integrity of the specimens is preserved. Restricting the depth to 1.0 mm additionally ensured that an identical setting could be reproduced in all three wood portions; the penetration depths measured stereomicroscopically did not differ significantly among sapwood, transition zone, and mature wood (Table 3), confirming that the comparison among portions was made under equivalent geometrical conditions. The laser incising was carried out on specimens in the air-dry condition. Prior to treatment, all specimens, irrespective of wood portion, were conditioned at 20 ± 2 °C and 65 ± 5% relative humidity until constant mass was attained, so that each specimen had reached moisture equilibrium with the same surrounding environment before being incised; the corresponding moisture contents are reported in Table 2. Any remaining difference in moisture content between the wood portions in this equilibrium state reflects a genuine difference in the hygroscopic behavior of sapwood, transition zone, and mature wood rather than a difference in specimen preparation, and it is discussed as such in the Results and Discussion section.
Weight loss was determined gravimetrically. The specimens were dried in an oven at 60 °C for three days until constant mass was reached and were then weighed to the nearest 0.01 g to establish the reference mass prior to laser incising. After this reference weighing, the specimens were reconditioned at 20 ± 2 °C and 65 ± 5% relative humidity until they had regained the air-dry state and constant mass was again attained, and the laser incising was performed only after this equilibrium had been reached. The oven-drying step therefore served solely to provide a moisture-free reference mass for the gravimetric calculation and did not represent the condition of the specimens at the time of treatment. Following laser incising, the identical drying and weighing procedure was repeated to obtain the corresponding reference mass after treatment. A drying temperature of 60 °C was selected in preference to the conventional 103 ± 2 °C in order to avoid any thermal degradation or loss of volatile constituents during the drying stage itself. Because both weighings were performed on specimens in the same dry condition, the calculated weight loss is independent of any difference in moisture content; it reflects the mass removed by ablation together with any mass lost through thermal degradation and volatilization of wood constituents within the heat-affected zone generated by the laser. The percentage weight loss resulting from the laser incising was calculated using Eq. 1:
(1)
where WL is weight loss of the specimen (%); W₀ denotes oven-dry weight of the specimen before treatment (g); and W₁ is oven-dry weight of the specimen after treatment (g).
Stereomicroscopic Evaluation of the Laser-Incised Holes
The purpose of these evaluations was to characterize the geometry of the individual laser-incised holes and to determine whether this geometry differed among the sapwood, transition zone, and mature wood portions. The radial and tangential surfaces of the specimens (100 mm × 25 mm × 25 mm) were used for these examinations and measurements. The evaluations were performed at low magnification on the intact specimen surfaces, on features that are visible to the unaided eye; the stereomicroscope was used solely to magnify these features and to allow calibrated dimensional measurement of them. No sectioning or cell-level (light or electron microscopy) examination of the wood tissue was carried out, and the term “macroscopic” is therefore used here in this sense throughout the text. These evaluations were conducted using images captured with a BAB Bscamera digital camera mounted on a BAB Image Analysing Systems BAB-STR80 stereomicroscope. The acquired images were analyzed using the BsCap image-processing software (Fig. 2).
To ensure an accurate assessment of the effects of laser incising on different wood portions of Oriental spruce, the macroscopic analyses and the determination of physical and mechanical properties were performed on the same specimens. For this purpose, three parameters were measured on the laser-incised specimens selected for this evaluation: (i) the penetration depth of the incised holes, (ii) their tangential diameter, and (iii) their radial diameter, determined on both the radial and the tangential surfaces. These measurements were used to compare the response of the sapwood, transition zone, and mature wood portions to the identical laser-incising treatment, and the resulting mean values were evaluated statistically together with the weight loss, density, dimensional stability, and compressive strength data obtained from the same specimens.
Fig. 2. Measurement of the tangential/radial diameters (b) and penetration depths (c) of laser-incised holes on the radial and tangential surfaces of the specimens using a stereomicroscope (a)
Physical and Mechanical Properties
The effects of laser incising were evaluated on the density, moisture content, compressive strength parallel to the grain, and dimensional stability, including swelling and shrinkage behavior, of the Oriental spruce specimens obtained from the different wood portions.
Specimens with dimensions of 30 mm × 20 mm × 20 mm (longitudinal × radial × tangential) were prepared from the original samples (100 mm × 25 mm × 25 mm) and used for the determination of physical and mechanical properties. The reduction of the cross-section from 25 mm × 25 mm to 20 mm × 20 mm was not carried out symmetrically. Material was removed from two adjacent faces only, 5 mm being cut from one radial and one tangential surface, so that the opposite radial and tangential surfaces were left in the condition in which they had been laser incised. Each test specimen therefore retained two intact incised surfaces, together with the incisions themselves and the thermally affected material surrounding them, and the physical and mechanical properties reported below were determined on specimens in which the treated zone was preserved rather than machined away.
Swelling and shrinkage measurements were performed using a digital caliper with an accuracy of 0.01 mm at three predetermined and marked locations on each specimen before and after water immersion. Swelling and shrinkage values were determined in accordance with TS ISO 13061-16 (2021) and TS ISO 13061-14 (2021), respectively.
Density was determined stereometrically in accordance with TS ISO 13061-2 (2021), both at a moisture content of 12% (D₁₂) and in the oven-dry condition (D₀). For each specimen the mass was determined to the nearest 0.01 g and the volume was calculated from the external dimensions, each of which was measured to the nearest 0.01 mm with a digital caliper at three marked positions along the specimen. The same procedure was applied before and after laser incising, and the marked measuring positions ensured that the dimensions were recorded at identical locations in both measurements, so that any dimensional change caused by the treatment would be captured. Because the volume was obtained stereometrically from the external dimensions, the voids created by the incisions are included in the calculated volume; the reported values therefore represent apparent density. Compressive strength parallel to the grain was determined in accordance with TS ISO 13061-17 (2019).
Statistical Analyses
Statistical analyses were conducted using analysis of variance (ANOVA) to evaluate differences among the experimental groups. When significant differences were detected, mean comparisons were performed using Tukey’s honestly significant difference (HSD) test at a 95% confidence level. All statistical analyses, including ANOVA and Tukey’s HSD tests, were carried out using JMP statistical software version 5.0.1a (JMP 2002; SAS Institute Inc., Cary, NC, USA).
RESULTS AND DISCUSSION
Determination of Wood Regions for Specimen Sampling
Sapwood, transition zone, and mature wood were identified based on differences in transparency and moisture content.
Radial strips with cross-sectional dimensions of 25 mm × 25 mm were subsequently prepared so that they corresponded to the same growth rings along the stem. Taking into account the saw kerf loss of approximately 0.3 mm during cutting, only two strips could be obtained from each of the sapwood and mature wood regions. In contrast, because of its limited radial extent, only one strip could be obtained from the transition zone (Fig. 3).
Fig. 3. Identification of sapwood, transition zone, and mature wood in Oriental spruce. (a) Thin sections cut from the breast-height disc and examined against a light source to reveal differences in transparency; (b) Determination of sapwood, transition zone, and mature wood regions along the stem for specimen sampling based on differences in transparency and moisture content
Determination of Growth-Ring Characteristics
The growth-ring width, latewood width, and latewood proportion of the sapwood, transition zone, and mature wood specimens were determined and are presented in Table 1. Although no significant difference was observed between the growth-ring widths of the transition-zone and mature wood specimens, a significant difference was found between these wood portions and the sapwood specimens (Table 1). Similarly, no significant difference was detected between the sapwood and transition-zone specimens in terms of latewood width and latewood proportion; however, both wood portions differed significantly from the mature wood specimens.
As shown in Table 1, the growth-ring widths of the transition-zone and mature wood specimens were greater than those of the sapwood specimens. In contrast, the sapwood and transition-zone specimens exhibited higher latewood widths and greater latewood proportions than the mature wood specimens. These findings may help explain why laser incising caused a more pronounced reduction in density in specimens obtained from mature wood than in those obtained from sapwood. Nath et al. (2020a) reported that the presence and distribution of earlywood and latewood layers significantly influence the drilling characteristics, that is, the geometry and quality of the holes ablated by the beam, during CO₂ laser processing. Furthermore, Nath et al. (2020b) demonstrated that material removal resulting from surface melting and ablation during CO₂-TEA (Transversely Excited Atmospheric) laser machining of pine wood was greater in the low-density earlywood layer than in the denser latewood layer. The same authors also reported that lasers operating at shorter wavelengths tend to provide more efficient machining performance in high-density wood materials.
Based on the growth-ring measurements obtained in the present study, mature wood specimens contained a higher proportion of low-density earlywood than sapwood specimens. Consequently, the greater amount of earlywood within mature wood may have increased the susceptibility of these specimens to laser-induced ablation and material removal. This phenomenon is therefore proposed as a possible contributing factor to the greater reduction in density observed in mature wood following laser incising; because the mechanism was not evaluated directly in this study, it should be regarded as a plausible explanation rather than a demonstrated cause.
Because the three wood portions differ systematically in both their anatomical organization and their chemical composition, a brief account of these differences is warranted here, since they underlie the differing responses to laser incising reported in the following sections. Anatomically, the most relevant distinction in a softwood such as Oriental spruce concerns the proportion and the dimensions of the earlywood and latewood tracheids. Earlywood tracheids possess wide lumina and thin cell walls, whereas latewood tracheids are narrow-lumened and thick-walled, so that the proportion of latewood largely governs the density of a growth ring (Gindl 2001). The measurements presented in Table 1 show that this proportion decreased from the sapwood (29.8%) through the transition zone (23.0%) to the mature wood (16.4%), while the growth-ring width increased in the same direction, from 1.24 to 1.71 mm. The mature wood examined here therefore consisted predominantly of wide, thin-walled earlywood tracheids, whereas the transition zone occupied an intermediate position between the two extremes. Because the coupling of laser energy into the material is governed largely by the density and the cell-wall thickness of the tissue encountered, this radial gradient provides a direct anatomical basis for expecting the ablation response to differ among the three portions.