2023 Volume 11 Pages 76-92
The presence of heartwood is one of the most important parameters in economic value that can affect the final use of wood. For construction materials, the presence of a large heartwood is very necessary, considering this will have a positive effect on the basic properties of the wood. Meanwhile, its heartwood proportion has a negative impact on the quality of pulp and paper. Therefore, this study aims to review the methods of calculating the area of the heartwood by using suitable heartwood measurement methods for better stand management and selection of raw materials in the industry. The results of the review showed that non-destructive methods were developed to measure the area of heartwood-sapwood such as computer tomography (CT-image), electrical resistivity tomography (ERT), x-ray densitometry, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and near-infrared spectroscopy (NIRS), so far. This is different from the destructive methods that are commonly used for visual inspection of wood color and anatomical features. During the selection of methods, the factors to be considered include cost, time, tree damage, number of samples, and efficiency in applying the methods. The visual inspection is the best option for determining the area of heartwood and sapwood. But, when the heartwood and sapwood boundaries color is unclear, chemical indicators can be performed. In addition, the ERT method can be the second alternative for tropics species. Furthermore, the method used in a study of the presence of heartwood depends on the five limiting factors mentioned previously.
Wood formation is the process of a complex biological that involves the main developmental steps, i.e. (1) cell division of the secondary vascular cambium, (2) cell expansion, (3) deposition of the secondary cell wall, (4) programmed cell death (PCD), and (5) heartwood formation [1]. Heartwood formation is a form of senescence or PCD of parenchyma cells. The formation is often accompanied by physiological processes such as PCD [2], depletion of storage compounds [3, 4], xylem dehydration [5], and accumulation of heartwood substances [6, 7].
Physiologically, wood is divided into two areas: sapwood and heartwood. These areas are identified in many species, but their color, nature, and presence can vary [8]. Heartwood is the inner part of the trunk from the tree with dead cells and does not contribute to physiological activity such as respiration or water transport. In most tree species, its color is darker than the sapwood part and usually has lower water content [9]. Some species have been shown to have heartwood parts with higher natural durability compared to sapwood parts. Heartwood is the most valuable part of the wood because it often provides high natural durability or resistance to decay which is good for outdoor use. Furthermore, its color had a higher aesthetic value with the dark color compared to sapwood pale. It also has good wood stability (low shrinkage), high wood density, and mechanical properties (MOE and MOR) compared to sapwood with a high accumulation of wood extractives, which is embedded in the vessels of heartwood and parenchyma cells [10, 11, 12, 13, 14, 15, 16].
A previous study has shown that the heartwood often determines the wood’s aesthetic value and economics [17]. Some species such as mahogany and teak wood in the international timber market are expected to pay a premium for all heartwood of this species. This is because heartwood is the most marketable component that can be critical in the reputation of hardwoods [18, 19]. Therefore, the knowledge of heartwood formation is important due to its economic significance [17]. However, it also has a negative impact when used for pulp and paper materials such as pitch problems in a paper machine or pulp. This makes it necessary to understand the effect of the heartwood proportion on the properties of the final wood product for stand management and selection of raw materials in the industry [20].
In addition to the classical method, several approaches to sapwood and heartwood area determination have been developed. Quantification of heartwood areas is possible in some species through direct visual analysis of wood due to the marked differences in color between the sapwood and heartwood resulting from extractive accumulation. The heartwood is darker than the sapwood. Despite the presence of heartwood in certain species, there is little or no visually detectable difference between the color of the heartwood and the sapwood. In this condition, identification requires analysis of differences in chemical content such as pH variations between sapwood and heartwood [8, 21, 22, 23, 24]. Similarly, identifying anatomical properties such as tyloses can also differentiate heartwood and sapwood areas, but only in species closely associated with heartwood formation [25]. Some studies also used non-destructive methods such as computer tomography (CT-image), electrical resistivity tomography (ERT), x-ray densitometry, and time-of-flight secondary ion mass spectrometry (TOF-SIMS), and near-infrared spectroscopy (NIRS) to determine sapwood and heartwood boundaries based on certain wood properties, namely moisture content, wood density, and chemical properties. Therefore, this study aims to extensively review the various methods used in measuring the heartwood-sapwood boundary with the advantages and disadvantages of each method.
This method is commonly used to distinguish between sapwood and heartwood area by the cross-section of the wood disk or increment core sample. The sapwood most often has a light color and low biological resistance, while the heartwood has a higher resistance to decay and is darker than sapwood [26, 27]. In some species where heartwood is apparent, it is because xylem parenchyma cells synthesize heartwood substances such as polyphenol groups, which respond to the color and wood’s natural durability before their death [8, 25, 28, 29].
Several wood species have contrasting colors between the sapwood and the heartwood due to natural changes in colors, which include Tectona grandis (T. grandis) (Figure 1a) [30], mahogany, acacia, eucalyptus, ebony, pine, larch, beech, black locust, and black walnut. The black walnut has a dark brown heartwood, while the color of black cherry varies from light to deep brown with high luster [31]. According to Bhat [32] the four-color groups for T. grandis in teak wood from the native area are (i) uniform golden-yellow-brown, (ii) yellow-dark brown, (iii) uniform gray-brown, and (iv) light uniform yellow.
There are several wood species with similar sapwood and heartwood colors such as Falcataria mollucana, Neolamarckia cadamba, Gmelina arborea (Figure 1b) [33], and red alder. It is visually difficult to determine sapwood and heartwood based on the wood color, therefore, a chemical reagent is needed to identify their area. Several studies have also used indicator coloring methods to investigate the heartwood-sapwood boundaries.
According to Kutscha and Sachs [34], some of the indicator chemicals used in several softwood species are listed in Tables 1 and 2. Each chemical indicator has a level of effectiveness ranging from effective (A) to useless (C/N) depending on the wood species of the softwood. Symbols C and N showed that they are not suitable or cannot distinguishable between sapwood-heartwood boundaries, therefore, no further testing is required. The colors given in Table 2 were only estimates as they refer to all tested species. In some tests, the color of the sapwood and heartwood can be pattern reversed from those given in Table 2, but in each of these cases, the sapwood-heartwood boundaries are still drawn. Furthermore, in some hardwood and softwood species such as eucalyptus, red pine, sugar maple, basswood, aspen, balsam fir, white cedar, and speckled alder chemical indicators used methyl orange (1%), bromocresol green, and basic fuchsin dye (0.1%) to distinguish sapwood from heartwood [34, 35, 36, 37, 38]. Similarly, stains such as safranin, indigo carmine or Schiff’s reagent, chlorine berberine, and fuchsin are used as direct recognition into the stream of transpiration and stain the cell walls of water-conducting tracheids and vessels [39, 40, 41, 42, 43]. There are several advantages and disadvantages of the method in the direct observation of the heartwood color by natural color changes and staining such as low cost. This is because it is observed with eyes without the use of special tools. However, its limitations include, namely, 1. Destructive when the measurement was carried out by felling trees [42], 2. The error is greater when the color between the sapwood and the heartwood is not contrasting or unclear [44], 3. Overestimating sapwood width when the disc is not round shape using wood core sampling [42, 45], 4. There is a need for a match between the indicator colors with certain species [34].
| No | Indicators | Color reaction | |
|---|---|---|---|
| Chemical | Sapwood | Heartwood | |
| 1 | Alizarine-Iodine | Green to yellow | Brown |
| 2 | Alizarine Red S (75 %) | Red | Yellow to orange |
| 3 | Ammonium Bichromate (6 %) | Orange to yellow | Brown |
| 4 | Benedict’s solution | Green | Brown |
| 5 | Benzidine | Yellow to brown | Red |
| 6 | Bromeresol Green (0.413 %) | Yellow to green | Green to brown |
| 7 | Bromphenol Blue (0.413%) | Brown | Blue |
| 8 | Fehling’s Solution | Green | Brown |
| 9 | Ferric Chloride (10 %) | Green | Brown to black |
| 10 | Ferric Nitrate (1 N) | Blue to green | Brown to gray |
| 11 | Ferrous Ammonium Sulfate (1 N) | Blue to green | Gray |
| 12 | Ferrous Sulfate (1 N) | Blue to green | Blue to gray |
| 13 | Harleco indicator | Red to orange | Pink to brown |
| 14 | Hydrochloric acid-Methanol | Yellow | Purple to brown |
| 15 | Iodine 2 (0.5 %) | Yellow to green | Orange to brown |
| 16 | Methyl orange (1 %) | Orange to yellow | Red to orange |
| 17 | Perchloric acid (40 %) | Green | Dark green to brown |
| 18 | Per-Osmic Acid (1 %) | Black | Brown |
| 19 | Phenol-HCl-Ethanol + U.V. | Red | Brown |
| 20 | Potassium Iodide-Iodine | Brown to green | No change |
| 21 | Triplex Soil Indicator | Green to yellow | Green to yellow |
| Species | Solution numbers | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | |
| Cedar | |||||||||||||||||||||
| Alaska-cedar | C | C | C | B | B | C | C | C | C | C | B | B | C | ||||||||
| Incense-cedar | A | A | A | B | A | B | B | A | C | A | C | A | |||||||||
| Port-Orford-cedar | A | A | C | C | B | A | B | A | B | B | A | A | |||||||||
| Western-redcedar | A | B | A | A | B | B | B | B | A | ||||||||||||
| Firs | |||||||||||||||||||||
| Subalpine fir | N | A | B | C | C | C | B | B | N | A | A | ||||||||||
| Grand fir | A | C | N | A | B | C | C | C | C | C | C | B | A | A | |||||||
| Noble fir | B | N | A | B | C | B | B | B | C | N | A | C | A | ||||||||
| Pacific silver fir | B | A | N | A | B | C | C | C | C | C | C | C | A | C | A | ||||||
| California red fir | N | B | B | N | A | B | B | B | C | C | B | C | N | A | C | A | |||||
| Shasta red fir | C | C | C | N | B | B | C | B | B | C | B | C | C | N | C | A | B | C | A | ||
| White fir | B | B | C | A | A | B | C | B | C | C | B | C | A | C | B | B | A | ||||
| Hemlock | |||||||||||||||||||||
| Mountain hemlock | A | B | B | B | C | A | |||||||||||||||
| Western hemlock | C | B | B | C | N | N | B | B | B | C | N | N | C | A | A | B | N | C | |||
| Pines | |||||||||||||||||||||
| Jeffrey pine | B | B | A | C | C | C | C | C | B | C | C | ||||||||||
| Lodgepole pine | B | B | A | A | A | A | A | C | A | A | C | A | |||||||||
| Ponderosa pine | A | A | A | B | A | A | A | C | C | B | B | A | B | A | |||||||
| Sugar pine | B | A | B | A | A | A | B | B | A | B | A | ||||||||||
| Western white pine | C | B | A | A | A | A | C | B | A | A | |||||||||||
| Spruce | |||||||||||||||||||||
| Engelmenn spruce | C | C | C | C | B | C | C | C | C | B | C | C | B | C | C | ||||||
| Sitka spruce | C | A | B | B | B | A | N | A | A | B | A | ||||||||||
| Douglas-fir | A | B | A | B | B | B | B | A | A | B | B | B | A | B | A | A | A | C | B | B | B |
| Western larch | B | A | B | B | B | A | B | A | B | C | C | A | A | A | B | A | |||||
| Redwood | A | A | A | A | A | B | A | A | B | B | A | A | A | A | B | C | A | B | A | ||
A: means effective; B: fairly effective; C: Not useful; N: not useful according to literature; 1: Alizarine-Iodine; 2: Alizarine Red S (75 %); 3: Ammonium Bichromate (6 %); 4: Benedict’s solution; 5: Benzidine; 6: Bromeresol Green (0.413 %); 7: Bromphenol Blue (0.413%); 8: Fehling’s Solution; 9: Ferric Chloride (10 %); 10: Ferric Nitrate (1 N); 11: Ferrous Ammonium Sulfate (1 N); 12: Ferrous Sulfate (1 N); 13: Harleco indicator; 14: Hydrochloric acid-Methanol; 15: Iodine 2 (0.5 %); 16: Methyl orange (1 %); 17: Perchloric acid (40 %); 18: Per-Osmic Acid (1 %); 19: Phenol-HCl-Ethanol + U.V.; 20: Potassium Iodide-Iodine; 21: Triplex Soil Indicator.
The determination of the area of the sapwood-heartwood is not only based on the difference in the color of the wood but also on anatomical features. However, there are also anatomical cell features such as the definition of heartwood as defined by the IAWA committee that needs to be considered. Heartwood is the center layer of wood when the tree is still standing which no longer contains living cells with reserve materials (non-structural carbohydrates and lipids) that have been converted to heartwood (IAWA) [46]. Moreover, in the heartwood, features of cellular are this region lacks living cells.
Several studies have been conducted on the process of heartwood formation with an anatomical approach. This includes Nakaba et al. [47] who reported a study of ray tracheids and ray parenchyma cells of Pinus rigida and Pinus densiflora. They also stated that the survival rate of ray parenchyma cells decreased from 100% to 0% in the latewood (two annual rings) of Pinus rigida and Pinus densiflora. In other species, Nakaba et al. [2] observed the death of a parenchymal cell in the pith area of the branch of Robinia pseudoacacia during the process of heartwood formation. The results showed that the large parenchyma cells within one year died in the part of inner pith. Other parenchymal cells died within 4 years, where the time of death depends on the cell type. Xylem axial parenchyma cells close to the pith die first. Furthermore, Nobuchi et al. [48] discovered that there were no living cells in the heartwood region while living cells were present in the sapwood in the Robinia pseudoacacia. They concluded that the completion of heartwood takes many months, however, the colored heartwood region expands in a rather short period.
According to Spicer [49], the death of axial and ray parenchyma cells in the transition zone (between sapwood and heartwood) is characteristic of heartwood formation. Cell metabolism is active before death, indicating cell death events that are responsible for the best characteristics of heartwood. Spicer [49] mentioned that when the axial conduits are blocked through gel secretion and the formation of tyloses in hardwood and tylosoids in softwood, the permeability will decrease. In sapwood, this blockage may also form in response to injury.
Wilson and White [50] reported that in hardwoods during heartwood formation, water permeability decreases and the sapwood vessels become increasingly blocked. Open and occluded vessels in the transition zone can be identified using an incident light microscope with a magnification of 50–100× [35, 51, 52, 53]. Therefore, the distance from the sapwood–cambium boundary was measured as the sapwood depth where all the sapwood vessels were blocked and the water vertical transport stopped was measured precisely with a digital caliper [53].
Cherelli et al. [54] also conducted the identification of heartwood and sapwood areas in eucalyptus species E. grandis (18-year-old), Corymbia citriodora (28-year-old), and Eucalyptus tereticornis (35-year-old). The cross-section of the disks was polished and analyzed with 10× increase of a stereo-microscope. Tyloses is characteristic of the heartwood part as shown in Figure 2. Fine-tipped pens were used to mark the boundaries of the sapwood and heartwood and high-quality disc photographs were taken with a digital camera at 1 cm2 scale (graph paper). Digital image processing is used to analyze the photos with computational algorithms for measuring the area of heartwood and sapwood.
Figure 2: Vessels observed on the polished cross section of Eucalyptus grandis wood (scale: 200 μm); (a) vessels filled with tyloses, disc area characterized as heartwood, (b) vessels without tyloses, disk area characterized as sapwood [54]
Several chemicals have been used to detect the anatomical features of the wood. These include acetocarmine, which was used to detect nuclei [2, 48, 56, 57], iodine–potassium for starch grains [2, 55, 58, 59], potassium permanganate for polyphenols [47], and safranin for cell wall thickness observations [2, 45, 54, 55]. Meanwhile, Nobuchi et al. [48] used safranin for nuclei observation, IKI for starch grains, and Sudan IV for observation of lipid droplets.
Observations have shown that there are several advantages and disadvantages of the anatomical features. The advantages were more detailed because it was observed that the anatomical composition of the wood the cell structure such as parenchyma cell, nuclei, starch grains, and vessel or tracheid. Meanwhile, the disadvantages include (1) Destructive when observed through the cross-section of the disc, (2) Time consuming to prepare and analyze sample slices as well as the probability of damage incurred during sample preparation [60], and (3) Requires a lot of equipment (inflexible) such as a microscope, knife, fixation solution, and chemical color indicator [60].
2.3 Computer tomography (CT-image)The method of measuring heartwood and sapwood boundaries with Computer Tomography (CT-image) was carried out on digital photos using image analysis techniques. CT image in the wood industry was used for the first time to monitor wood density and also describe the moisture content distribution of wood [61]. CT image can be used to detect and distinguish wood features such as heartwood, sapwood, defects, and decay, as well as wood that has a unique density and moisture content.
Heartwood and sapwood have different physical and technological properties for industrial applications, including color, durability, suitability for chemical treatment, and moisture content [62, 63, 64, 65, 66, 67]. Since the area and width of heartwood and sapwood are considered parameters of wood quality, several reports on using CT images to identify the border of heartwood and sapwood directly correlated their area and widths with the tree, stand, or site inventory variables [68]. Longuetaud et al. [69] reported that the width of the sapwood extracted from CT images of Norwegian spruce trunks was associated with tree height and diameter at breast height as well as relative crown height.
According to Rojas et al. [65], the moisture content and concentration of certain inorganic and extractive materials in sugar maple heartwood are higher than in sapwood. Areas with high moisture content such as heartwood appear brighter on CT images. Because water has a higher density than wood. However, species such as Picea mariana with the heartwood of lower density and moisture content compared to the sapwood [64], appear dark in CT images. In both cases, CT images are appropriate to differentiate between heartwood and sapwood due to the differences between their densities and moisture contents. However, this difference is less strong in dry wood, which makes fresh samples to be required for investigation [62].
Grundberg and Grönlund [70] described the boundary between sapwood and heartwood by applying a gray level-based (GL) threshold to the image with a low pass filter and calculating the gradient image using a Roberts filter to highlight the boundary. This method allows the delineation of heartwood and sapwood boundaries with a mean radius of 12° every 10 mm along the log expressed in polar coordinates from the pith. Moreover, the method was also used in further researches by Björklund [62], Hagman and Grundberg [71], and Chiorescu and Grönlund [72]. Similarly, CT-images have been extensively used to identify the density of wood and moisture content of wood that is directly related to sapwood and heartwood, as well as their boundaries [62, 65, 69, 73, 74, 75].
Although computer tomography provides a non-destructive method, it is an expensive technique for the determination of sapwood area [76, 77, 78], and has limited application to only a few stems in a stand [79]. Furthermore, it is not sensitive when there is a slight change in density along the cambium-pith direction [69, 80]. Other types of low-cost sensors were investigated, due to the high cost of CT scanning equipment. This includes the electrical resistivity tomography (ERT), which was a geophysical method based on the electrical resistivity of the material or its converse and electrical conductivity related to the structure of the object. This method, ERT was used to identify decay in trees [81] and discolored wood [82].
The attenuation of a collimated radiation beams from several directions was measured by CT image. The radiation attenuation increases due to increased density and moisture content in the wood. According to Ridder [83], the coefficients of absorption measured from a range of angles can be used to calculate a two-dimensional map of wood density in the standing tree. Rust [78] measured wood density on standing trees in the field with two identical portable devices. Horizontal plane 1.3 m above the ground as the object being measured. Meanwhile, heartwood and bark had lower coefficients, whereas absorption coefficient higher than 7.2×104 cm–1 were assumed to be sapwood. This technique was used on 105 trees in 7 stands of Scots pine.
2.4 Electrical Resistivity Tomography (ERT)The methods used to identify heartwood and sapwood are based on their different physical properties. Similarly, variations in electrical resistivity (Rs) associated with differences in wood moisture content in the heartwood and sapwood are used as the basis for distinguishing the two parts of the wood. However, in some species such as English oak electrolyte concentration can be the main contributor [84]. ERT was considered moderately non-destructive of the wood because only a small nail is required to be inserted through the bark until it entered the sapwood of about 0.2 cm into the sapwood and the nail is removed after measurement. The ERT has been used in several studies to determine the width of sapwood [79, 84, 85, 86, 87, 88]. Distribution of Rs and wood properties, which includes moisture content, density, concentration of electrolytes (Na+, Mg2+ K+, etc.), and pH (Figure 3), provides insight into the causal relationship of this study. ERT found the sharpest change from Rs in sapwood to heartwood at the sapwood-heartwood boundary.
Figure 3: Schematic illustration of a cross-section of an undamaged tree trunk (a), distribution of electrical resistivity (b) along the diameter of the cross section of a tree trunk and the slope of the electrical resistivity distribution curve (c). Perpendicular lines on plots b and c are the center of the stem; two dotted lines and a rectangle are the boundaries between sapwood and heartwood [79]
Wang et al. [79] reported the advantages and limitations of using ERT, where it has several advantages compared to conventional methods. These include (1) Many conventional methods only give results at a specific measurement point, whereas this method provides the direct spatial distribution of sapwood in a trunk cross-section. (2) ERT is a relatively non-destructive method because it requires inserting the nail into the tree trunk at a shallow depth of approximately 0.2 cm and removing the nail after measurement. (3) Since it only takes 15 to 30 min to complete one measurement per tree [85, 87, 88], this tool is very portable and efficient to use. Thus, in a large number of trees, it is suitable for measurement.
Despite the advantages of ERT, there were some limitations of this method, which include (1) The configuration of the stem cross-section can lead to errors in estimating the width of the sapwood because it is difficult to determine trees with a good round shape (symmetrical). (2) ERT devices are more expensive than additional increment borers and staining solutions, although cheaper than CT-image [68]. Furthermore, ERT did not perform to trees under very wet conditions [79].
2.5 X-ray DensitometryIn previous studies, X-ray was used to examine pieces of wood with the specific purpose of quantitatively evaluating the properties of different species [80]. Other laboratories developed applications of X-ray densitometry for related areas such as wood science and technology as well as the environment. Meanwhile, visual separation of heartwood and sapwood for some species is very difficult because of the slight contrast in color. X-ray densitometry is useful for identifying heartwood-sapwood boundaries based on different wood properties and X-ray attenuation levels such as in 29-year-old Corymbia citriodora [80].
Wood samples (10×10×10 mm) were obtained from heartwood and sapwood parts for identification of sapwood-heartwood boundaries. The sections were polished using a shear microtome for tyloses microscopic examination. Based on ANOVA analysis, the average wood density of the heartwood was significantly greater than the sapwood. The higher wood density in heartwood is due to the development of tyloses in the vessel’s lumens and the heartwood substances deposition such as tannins and oils in the radial parenchyma cells. The heartwood color becomes darker, with lower permeability and greater wood durability.
Heartwood from xylem cells which contain tyloses and chemical components, there was greater attenuation of X-rays. Meanwhile, functional xylem with open vessels was present at a lower sapwood density. The potential of X-ray densitometry to separate the heartwood and sapwood boundaries of eucalypts was due to the differences in chemical composition and anatomical features. This is used for identifying heartwood-sapwood boundaries based on different properties of wood and levels of X-ray attenuation [80]. These differences in sapwood and heartwood are reflected in the attenuation of X-rays because of the wood density variations. Longuetaud et al. [69] and Tomazello et al. [80] stated that it was non-sensitive when there was a slight change in density along the cambium-pith direction. Furthermore, it is a destructive method, which requires the felling of trees to make a wood sample in the stick form (10 × 10 × 10 mm) and includes expensive methods.
2.6 Time of flight secondary ion mass spectrometry (TOF-SIMS)The identification of the chemical differences between heartwood and sapwood is necessary to understand the heartwood formation mechanism. Polysaccharides, cellulose, hemicellulose, and lignin are the main wood component structural [89]. A previous study has shown that less cellulose and more lignin content are generally found in heartwood [90]. To understand wood chemistry and environmental science, the distribution of mineral elements from sapwood to heartwood is also important to investigate. One of the tools to identify wood mineral components is the secondary ion mass spectrometry (SIMS) method. Meanwhile, Saito et al. [89] investigated the mineral distributions (Ca, Mg, Na, Al, and K) and lignin in Hinoki cypress (Chamaecyparis obtusa) wood.
The results showed that the levels of most of the elements, namely Ca, Mg, Na, Al, and K were significantly larger in sapwood and transition zone compared to heartwood. Meanwhile, the content of K was lower in sapwood compared to heartwood (Figure 4). Furthermore, there was a drastic increase or decrease of Ca, Al, and NA levels in the transition zone (T1, T2, and T3). There was a significant increase in the K concentration in the H1 heartwood ring immediately after the T3 ring. This observation showed that changes in elemental concentration are related to the heartwood formation that occurred in the transition zone, while the lignin content is constantly distributed in all parts of the wood. Therefore, TOF-SIMS is a convenient tool to detect the differences in chemical components between sapwood and heartwood in wood without chemical pretreatment. In another study, Saito et al. [91] detected sapwood that was visually indistinguishable from heartwood in discolored ancient wood from the Horyuji Temple, Japan by the same tool (TOF-SIMS.) The study was carried out in Hinoki cypress wood by analyzing extractive compounds, namely hinokinin, hinokiresinol, hinokione, and honokiol. The results showed that the hinokinin compound is a good indicator to distinguish between sapwood and heartwood in Hinoki wood without chemical pretreatment. This indicated that TOF-SIMS analysis is used to determine the presence or absence of sapwood in discolored wood and visualize the distribution of various chemicals in the wood [91].
However, the limitations of this method include expensive tools [53] and it is destructive because of the felling of the trees to make test samples in TOF-SIMS. It also consumes more time due to the preparation of wood samples and analysis of the results of chemical components after testing [89, 91].
Figure 4: The distribution of the relative intensity of ions originating from elements (a–e), Na, Mg, Al, K, and Ca, and (f) lignin, in adjacent growth rings from sapwood to heartwood (S7–H5) at Hinoki samples. Data were observed from two positions (solid line and dotted line) in each growth ring. Shades of gray indicate a transition zone between sapwood and heartwood [89]
NIRS has been used in several fields such as pharmaceutical, food, wood, and polymer spectroscopy. Meanwhile, its technique is based on electromagnetic radiation, which covers wavelengths from 750 to 3,000 nm [92]. According to Flæte and Haartveit [93], the heartwood and sapwood from pine wood can be correctly classified using the partial least square regression model (PLS). In addition, NIRS is also an affordable dan rapid technique for estimating the density and moisture content of solid wood samples compared to CT and ERT [92, 94, 95, 96, 97]. The industries of plantation, pulp, and paper have used NIRS to separate heartwood from sapwood in softwood [92, 93, 98].
Pfautsch et al. [53] compared macroscopic and microscopic methods with the NIRS technique to investigate the accuracy of the NIR method in angiosperm species such as Eucalyptus and Corymbia species. Microscopically, 80 samples of wet wood core were installed to a wooden board using epoxy resin so that the core wood fibers were perpendicular to the surface of the board. A binocular light microscope at 50× magnification was used to investigate the vessels and wood structures after the wood core was hand-planed. The visibility of occluded and open vessel conduits was used as the basis for determining heartwood/sapwood. To remove contamination, a clean transverse surface is made and the core is thawed for 20 min. Subsequently, the wood cores were trimmed to a length of approximately 6 cm from the bark end by removing excess heartwood. Individual cores were transported past a fiber-optic array (1×3 mm, R × T) at the bark end and scanned at 1 mm spatial resolution. For each 1 mm spatial increment, a 64-scan NIR spectrum was obtained with a spectral range of 4,000–10,000 cm-1, which is equivalent to 1,000–2,500 nm at a spectral resolution of 8 cm-1.
A series of NIR spectra per 1 mm can be obtained along the radial length from bark to pith each 20 min of measurement at a rate of approximately 30 min per 100 mm of core length. Furthermore, immediately after the NIR analysis in the macroscopic method, methyl orange solution (1%), was used to color the wood cores. This reacts to the pH changes in wood and caused the heartwood to turn red, while the sapwood remained orange. The partial least squares regression-discriminant analysis (PLS-DA) model was carried out to identify sapwood-heartwood. According to the three methods, macroscopic and microscopic are generally similar in value and accuracy when compared to NIRS. Therefore, further improvements in sapwood depth prediction using NIR required more intensive model calibration and validation.
Sandberg and Sterley [92] reported that sapwood and heartwood of Norway spruce that was not different in color under dry conditions were properly separated using NIR spectroscopy and multivariate data analysis of 44 specimens. It was stated that the visible wavelength spectra have a significant influence on the prediction to separate between heartwood and sapwood of Norway spruce. However, several factors need to be considered and included in the calibration set to build a viable predictive model. This showed that further study is required to verify the effect of density, moisture content, color, and chemical content on the models.
There is a need to understand that NIRS is cheaper and faster than CT and ERT in measuring wood properties, however, there it has some limitations. These include (1) NIRS is less accurate when used to measure the area of sapwood of hardwood due to the variety of structure and composition of hardwoods. This has caused variation in wood properties such as wood density and moisture content that increases [99] or decreases [100] with distance from the bark. (2) Inaccuracy of the NIRS method to estimate sapwood depth, causes fatal errors in calculating the area of sapwood in hardwoods, where the sapwood is relatively narrower than softwoods [53].
In several studies, color is used as an indicator to determine the area of the heartwood and sapwood. Generally, the heartwood color is darker than the sapwood which is yellow or pale. The dark color of the heartwood is caused by extractives, especially phenolic extractives, which contribute to its natural durability. There is a need to cut the tree to inspect the color of the wood in cross-section. However, this method is destructive because it requires felling the trees and consumes time, limiting the samples tested. An increase in the core sampling is an alternative to the heartwood-sapwood color, which is more non-destructive and reduces workload. In addition, a wood cores sample can be taken in a shorter time compared to a disk sample.
Although increment cores can perform a complete sample of wood when it extends from the pith to the bark, it is limited only by the length of the drill bit and the ability to extract the sufficient core. They have dominated several studies for decades throughout a century because of their non-destructive nature, which has been used as a standard method for determining ecosystem biomass, investigating climate history through dendrochronological analysis, and assessing wood quality. Gao et al. [101] stated the use of the increment borer to collect chest-high core samples has been the standard method for obtaining the intrinsic properties of wood in standing trees since the 1960s. However, the operating factors of the drill can minimize the sampling of drill bits, both the selection of the tool and the fatigue factor of the operator. Drill diameters (4–12 mm), with larger diameters providing the best samples when larger quantities of wood are required [102, 103]. Meanwhile, core removal also affected tree health because it leads to the entry of decay and disease.
In the wood cores inspection for wood color, not all wood has a clear corer color between heartwood and sapwood, therefore, a color indicator is required. This color indicator helps to clarify the color of the heartwood and sapwood based on the pH response and chemical composition of a particular wood. However, their suitability with certain types of wood species needs to be considered because each species has a different response to color indicators. According to IAWA, the heartwood is distinguished by color and the activeness of the wood cells (parenchyma cells). Therefore, a microscopic method is used to clarify the anatomical features of the wood under a microscope such as a nucleus and vessels. The chemical components such as non-structural carbohydrates (NSC), wood elements, lignin, and certain extractives also need to be investigated to classify sapwood from heartwood. Recently, the TOF-SIMS method was carried out to identify the presence of sapwood and heartwood based on chemical components.
Sampling through wood disks or corer still causes damage to trees. Recently, the development of an advanced non-destructive method was carried out without extracting the wood cores. This was conducted by observing the difference in moisture content and wood density using the CT image and ERT methods to estimate the sapwood in a standing tree. The moisture content of heartwood is known to be lower than that of sapwood due to blockage in vessels or tracheid by tylosis or certain extractives for water permeability to decrease. The blockage and accumulation of extractives also cause higher heartwood density than sapwood. This underlying the operations of the CT image and ERT methods, making them more accurate in estimating sapwood depth. However, the method is significantly expensive, which makes it necessary to search for other inexpensive tools to estimate the sapwood.
The NIRS is much cheaper and faster than CT-image and ERT in measuring moisture content and wood density. However, it is more accurate when used on softwood species, where the wood structure is more uniform than hardwood. Furthermore, NIRS requires a validation process for the predictions in measuring wood properties to be more accurate. It also requires wood samples extracted from standing trees (incremented cores) to calculate wood density. Similarly, x-ray densitometry also requires a certain sample size in form of sticks.
Apart from visual inspection and anatomical features, the use of CT-image, ERT, and TOF-SIMS methods is still intensively being carried out in temperate species, while x-ray densitometry and NIRS have been used in tropical countries such as Brazil and Indonesia [80, 104]. Furthermore, there are several considerations such as cost, accuracy, tree damage, time, and easy application of the tools in determining heartwood and sapwood area to optimize the end use of the wood. For each of these considerations, the number of tree samples and different sampling procedures need to be carefully designed to suit the expected conditions since each method has its advantages and disadvantages.
Based on the development of instruments for determining the area of heartwood-sapwood, foresters or users are required to evaluate the wood qualities from forest plantation for genetic improvement, forest management, and optimal use of timber. This takes a long time and is significantly complicated due to the classical method that needs to be carried out in the laboratory. Meanwhile, time, cost, accuracy, efficiency, and tree damage need to be considered in choosing a method because each method has its advantages and disadvantages. According to the previous description, the visual inspection method is the best option for determining the area of heartwood and sapwood, because it is easy, fast, and inexpensive. When the color of heartwood and sapwood boundaries is unclear, chemical indicators can be applied. The ERT can be the second alternative in tropics species, because it is cheaper than CT-image, no need to extract samples from the trees, and can easily measure a large number of trees. Therefore, the number of tree samples and different sampling procedures need to be carefully designed to suit the expected conditions in further study.
