Multidimensional Exploration of Wood Extractives: A Review of Compositional Analysis, Decay Resistance, Light Stability, and Staining Applications

Abstract

Extractives, which naturally evolve as fundamental defense mechanisms in wood against environmental stresses, hold an essential place in the field of wood conservation science. Despite their low content in woody substrates, extractives are chemically complex and can be extracted accurately by solvents with different polarities, covering key components such as aliphatic, terpenoid, and phenolic compounds. The application of solvent extraction allows for the effective recovery of these extracts from forestry waste, thereby creating new opportunities for their reuse in wood modification and enhancing the economic value and potential applications of forestry waste. In the wood industry, extractives not only act as efficient preservatives and photo-stabilizers, significantly improving the decay resistance and photodegradation resistance of wood, but also serve as ideal dyes for fast-growing wood due to their abundant natural colors, which lend the product a distinct aesthetic appeal. The aim of this paper is to provide a comprehensive review of the origin and distribution characteristics of wood extractives and to examine the impact of solvent selection on extraction efficiency. At the same time, the mechanism of extractives in enhancing wood decay resistance and slowing down photodegradation is deeply analyzed. In addition, specific examples are presented to illustrate their wide utilization in the wood industry. This is intended to provide references for research and practice in related fields.

1. Introduction

Wood, a renewable material of natural origin, has played an indispensable role in human history. For centuries, wood has been widely used in various fields, particularly in furniture making and building construction, due to its attractive appearance, excellent strength-to-weight ratio, and outstanding insulation properties.

As with alternative base materials, however, wood has its natural imperfections. Wood is susceptible to environmental effects that can cause performance degradation and shorten its service life. In the natural environment, the external erosion of wood is a complex process with various factors acting together [1]. The humid environment encourages wood to absorb water, creating favorable conditions for the growth of fungi and other microorganisms [2]. In addition, environmental factors such as oxygen molecules and ultraviolet light can accelerate chemical reactions on the surface of wood, changing its color and properties. Moreover, atmospheric pollutants can also cause wood to darken and reduce its aesthetic value [3]. Furthermore, wood is subject to internal stresses due to swelling from water absorption and shrinkage from water loss, resulting in cracks, deformation, and weakened physical structure [4]. In the natural environment, wood biodegradation, light radiation and oxidative reactions, and physical swelling and shrinking combine to exacerbate the rate of wood erosion and degradation [5].

Fungal infestations are the number one killer of wood, breaking down wood cell walls into nutrients for their own uptake and survival. These fungi include (a) wood-destroying fungi (“brown rot”, “white rot”, and “soft rot” fungi), which degrade structural polymers in the wood cell wall, leading to a loss of wood strength [6]. Brown rot fungi typically target hardwoods, focusing mainly on cellulose and hemicellulose [7,8]; white rot fungi tend to attack hardwoods and can depolymerize all three cell wall polymers [9,10]. Soft rot fungi decompose holocellulose and pectin present in the wood [11]. (b) Wood-staining fungi and molds, which, through the secretion of a variety of enzymes and pigments, break down the inclusions in the thin-walled cells of the xylem, mainly discoloring the wood and affecting its aesthetic properties [2]. (c) Bacteria degrade parenchyma cells, leading to the increased permeability of the wood [12]. The mechanical properties of decayed wood typically decline significantly during the initial stages of degradation [13]. Wood undergoes a series of adverse changes after being attacked by fungi. These include changes in chemical composition, loss of mass, decreased mechanical properties, changes in natural color, increased permeability, and increased vulnerability to insect attack. These changes severely compromise the integrity of the original design purpose for which the wood was intended to be used [14].

Among the various factors that degrade wood, light is particularly damaging to the properties of wood, especially radiation in the UVB band (280–315 nm) of ultraviolet light [15]. The photodegradation of wood occurs when photons of visible and ultraviolet light possess sufficient energy to break chemical bonds, resulting in the depolymerization of lignin and cellulose polymers in the cell walls [16]. As UV light cannot penetrate deep into the wood, photodegradation affects the surface layer of the wood more. Photodiscoloration is a primary issue affecting the aesthetic properties of wood, as a small number of chemical changes can produce an intense color change. The intensity of color change in wood is initially elevated during photoaging and gradually decreases over time [17]. Under light, the wood surface color changes to yellow and brown, with a general decrease in brightness. If some degraded substances in the wood are washed away by the combined action of water, the surface eventually turns gray [18]. Previous studies have indicated that UV light can penetrate the wood surface up to a depth of 70 µm [19]. However, additional recent research has suggested an increased penetration limit of approximately 140 µm for UV light in wood [20,21]. This not only leads to discoloration and the cracking of the wood surface but may also influence the structural integrity of the wood [22,23].

In order to adapt to the challenging external environments, wood has developed countermeasures over millions of years of growth and evolution. Wood extracts, as a secondary metabolite of wood, have an outstanding contribution to the resistance to external biotic and abiotic erosion, although at a low content [24]. Wood extracts include primary and secondary metabolites. Primary metabolites are physiologically active compounds that are responsible for growth or cell development [25]. Secondary metabolites are derived from the metabolism of primary metabolites during heartwood formation and make up the major part of the extractives [26]. Wood extracts contain a variety of complex organic compounds, such as polyphenols, flavonoids, terpenoids, etc., which exhibit a wide range of biological activities. These extracts can help trees resist various biological attacks, including termite gnawing and fungal infections [27,28]. In addition, extractives provide protection against abiotic factors that attack the wood, such as UV radiation and oxidative reactions [29]. Some extracts also impregnate and fill the cell wall, acting as a barrier to prevent water from spreading through certain parts of the cell wall and slowing down the swelling and shrinking of the cell wall [30,31]. The study by Salamon et al. [32] also agreed that the shrinkage of the heartwood of Pseudotsuga menziesii (Mirb.) Franco was less than that of the sapwood and may depend more on differences in extractive content than on differences in density.

Secondary metabolites play a key role in plant growth, development, and defense against infection or damage [33]. The high natural decay resistance of some wood species is more related to their high levels of specialized extractives [34]. Teak is one of the species known to be extremely resistant to white and brown rot fungi [35]. Chávez-Salgado and Rodríguez Anda et al. [36,37] analyzed the chemical composition of teak extracts and found that the decay resistance was mainly attributed to quinones and anthraquinones in the extracts. The analysis of extractives contained in Burkholderia africana heartwood was investigated by Neya et al. [38] and showed that the fats, waxes, and squalene present in the extracts have excellent antifungal properties. It is true that a close correlation has been established between the ability of wood to resist fungi and its extract content and diversity. Kirker et al. [39], after removing extractives from natural resistant wood in their experiments, found that the durability of extracted wood samples decreased to be comparable to that of non-durable wood. By applying extractives from durable species to non-durable species, a transfer of durability can be achieved, increasing the durability of non-durable wood [40,41,42]. In recent years, the use of traditional chemical preservatives has been restricted or banned, prompting researchers to actively seek out and develop natural preservatives. Learning about the natural compounds in durable wood may provide fresh inspiration for the development of more effective wood preservatives and additional wood protection chemicals.

Extracts are the most light-sensitive chemical components of wood, and the photodegradation process is initially accompanied by the consumption of the most antioxidant components of the extracts [43]. For high extractive content wood, even one hour’s exposure to sunlight can result in measurable color changes [15,44]. In recent years, in order to investigate the role played by wood extracts in the photodegradation of wood, many scholars have carried out a large number of studies. Pandey [45] exposed extracted and unextracted wood specimens to artificial light sources for radiation. The results showed that the unextracted wood absorbs UV light during the initial stages of photodegradation, resulting in rapid color changes on the wood surface. However, the unextracted wood reduces lignin photodegradation over an extended period of time during weathering. Nzokou et al. [46] exposed extracted and unextracted wood specimens of black cherry, red oak, and red pine to artificial weathering. In the study, it was found that some of the extractives contained in the wood could act as antioxidants and provide some protection to the wood surface against weathering degradation. The role of extractives in the photodegradation of wood has been extensively studied by Chang et al. [47,48,49]. The analysis of substances in the extracts has demonstrated the photostabilizing effects of certain compounds contained within them. Interestingly, the extract can also be used as a natural light stabilizer in wood that is less light-stable to slow down the photodegradation of the wood. [50]. When some researchers mixed extractives as an additive with acrylic polyurethane coatings, they found that the coatings with extractives had better light stabilization than those without extractives [51,52,53]. This indicates that extractives have great potential for application as wood light stabilizers [54,55,56].

Since extracts are abundant in chromophores such as phenolic and carbonyl groups, they constitute the main components responsible for the color of wood [57]. Extracts can alter the color of dyed materials by reflecting and absorbing visible light [58,59]. Natural dyes generally offer a more environmentally friendly option than synthetic dyes and provide distinct colors and textures. Generally, fast-growing timber is lighter in color, which is one of its drawbacks as a material for furniture and interior decoration. Dyeing dark wood using its extracts can give fast-growing timber a color appearance similar to that of high-value timber [60]. The use of the extract as a natural dye is also one of its potential applications.

As a natural product, wood extract can be derived from wood and used as an alternative. Industrial wood processing inevitably produces significant amounts of waste, such as bark, wood chips, and trimmings. The utilization of wood processing residues is extremely limited, with some used as fuel for fires and others disposed of as waste in landfills. Obtaining bioactive compounds from these residues can reduce the environmental impact and yield economic benefits [61,62,63]. To obtain bioactive compounds from wood, it is necessary to select the appropriate solvent and extraction method. There are various methods to obtain wood extracts (e.g., Soxhlet extraction, microwave-assisted extraction, ultrasound-assisted extraction, pressurized liquid extraction, accelerated solvent extraction, etc.), and Santos et al. [64] reviewed these extraction methods for obtaining wood extracts.

This paper focuses on the properties and applications of wood extractives in wood. Section 2 summarizes the sources and distribution of wood extract, discusses the different solvents used to extract the wood, and classifies the different components in the extract. Section 3 focuses on providing an overview of the aspects related to the decay resistance of the extracted. Section 4 reviews the light stability aspects of the extractions. Section 5 temporarily describes the potential of the extract as a natural colorant. Section 6 provides a conclusion and prospect on the application of wood extractives.

2. Wood Extractives

2.1. Source and Distribution

Extracts mainly originate from the intercellular and intracellular lumen of wood, rather than cell wall material [65,66]. It is to be noted that the extractives are not homogeneously distributed throughout the lumen of the wood tissue, but occur in special resin and latex canals [25]. Their contents are mainly determined by tree species, and the types and contents of secondary metabolites produced by different tree species vary due to differences in genetic characteristics. In addition to variations in tree species, the extracted content also varies depending on factors such as wood origin, tree age, trunk position, cutting season, storage time, transportation mode, and extraction conditions [67,68,69,70,71]. In general, the amount of wood extractives in tree species from temperate regions is low, whereas the amount of wood extractives from tree species from tropical regions accounts for a higher percentage [72]. According to the 653 species in the CIRAD database, the average value of total extractives of tropical hardwood is 7.6% (4.6% for ethanol-benzene extractives) [73].

Differences in tree species have a significant impact on the extracted content. In some species, the extractive content can be up to one-third of the wood mass, such as the Larch extractives up to 30% [74], and the total extractives of the Euphorbiaceae family Excoecaria parvifolia up to 35% [75]. However, in different species, extractives are rarely found, for example, spruce, with only 0.9%–1.5% [76], and Brazilian palm (with bark containing 1.55% and heartwood containing 1.33%) [77]. Huang et al. [78] evaluated the extracts of 22 African hardwood species and the differences in yields between species under the same extraction conditions were highly significant, ranging from 1.69% to 18.21%. Kebbi-Benkeder et al. [79] studied knotwood extracts from 12 European softwood and hardwood species. They concluded that extracts from softwood species, with the exception of oak, were more abundant than those from hardwoods in 12 species.

There were significant differences in the distribution of extracted contents from different parts of the trees. Usually, the largest proportions of extractives are concentrated in the bark, needles, and leaves. Then come the branch parts, followed by the heartwood. The sapwood contains the least amounts of extractives. The extract content of two common cypress species (Cupressus sempervirens L. and Cupressus arizonica Greene) was determined experimentally by Terzopoulou et al. [80]. As expected, in both species, the extractive content of the bark material was significantly higher than the extractive content of the respective wood. In the case of Cupressus arizonica Greene, the extract content of the bark was an astonishing 27.59%. The extracts of heartwood, sapwood, bark, twigs, and needles of the seven most common tree species from alpine regions were analyzed by Piccand et al. [81]. The most abundant tissues in the extracts were bark and needles accounting for 18.6%–39.0% of the total, while sapwood extracts had a very low percentage of 5.2%–7.5%. In their study, Miranda et al. [82] observed that the distinguishing chemical feature of the bark of Eucalyptus sideroxylon is its high content of extractives, particularly polar extractives abundant in phenolic substances. The bark usually contains higher concentrations of extractives than the stem of the tree. This phenomenon can be attributed to the fact that the bark, which acts as a barrier between the plant body and direct contact with the external environment, is more vulnerable to threats from biotic and abiotic factors. This continuous stress condition prompts the plant to accumulate large amounts of metabolites in the bark to enhance its resistance [83]. Vek et al. [84] analyzed extracts from silver fir sapwood, heartwood, knotwood, and branchwood and found that higher extractive content was observed in the knotwood. This is mainly related to the special physiological function of the junction between the branches and the main stem, in an area that is typically considered highly susceptible to mechanical damage and pest infestation. It is noted that the xylem tissue at the base of the branch provides constitutive protection through the accumulation of high concentrations of the extract. Miranda et al. [85] evaluated oak heartwood and sapwood extractives and found that the solvent extractables in the heartwood were approximately twice as high as in the sapwood. Gierlinger and Wimmer [86] examined the radial distribution of European larch extractives by microsampling wood blocks, which were then subjected to FTIR analysis. Their results showed that in mature larch, the amount of wood extractives increased linearly from the pith to the heartwood/sapwood boundary. In general, the heartwood portion of the xylem contains a higher amount of extractives than the sapwood. The formation of heartwood and the accumulation of extractives are important strategies for trees to adapt to their environment and enhance their own protective mechanisms.

The content of extracts from trees of the same species within the family, growing in different geographical areas, may also vary considerably due to factors such as climate and altitude [74]. Additionally, the extract content in the same tissue of the same species may also vary seasonally [12,37,87,88,89]. Mbakidi-Ngouaby et al. [90] analyzed the metabolites of Pseudotsuga menziesii wood from the same location during different seasons and found that the content of extractives was the highest in sapwood during autumn and the lowest in summer. The yield of heartwood extract was the highest in spring and the lowest in summer, while the transition zone between heartwood and sapwood was the highest in summer and the lowest in winter. The age of the tree also has an effect on the extractive content, as the ducts of the formative layer of older trees are more likely to split and multiply into parenchymal cells, which leads to an increase in the formation of parenchymal cells [91]. Consequently, older trees generally contain higher amounts of extractives.

2.2. Solvent Extraction

With a deeper understanding of extractives from trees, the efficient and non-destructive extraction of these components from wood has become a critical issue. Solvent extraction is the most widely used method to obtain bioactive molecules from wood, and extracts can be separated from wood by both polar and non-polar solvents, with their attraction to solvents varying according to polarity [64,92]. Some commonly used solvents for different wood extraction and extraction yields are summarized in Table 1. The yield of an extract depends on its solubility in a specific solvent; however, no single solvent can effectively extract all compounds from wood due to their differences (hydrophilic and lipophilic) [27,93]. If the target component is a polar compound, a polar solvent would be a better choice (e.g., water, methanol, ethanol, acetone, etc.); if the target compound is non-polar, a non-polar solvent would be more appropriate (e.g., dichloromethane, benzene, toluene, cyclohexane, petroleum ether, etc.) [94].

The corresponding solvent can be chosen for the targeted extraction of different substances in the extract. For example, toluene–ethanol removes waxes, fats, and some resins; hot water removes tannins, gums, sugars, starch, and pigments; and ethanol extracts consist of phenolics, terpenoids, fats, and carbohydrates [95]. Syahidah et al. [96] investigated the polarity of Cempedak wood extracts and found that the total extractive content was 12.16%, with 11.07% extracted by polar solvents and only 1.09% by non-polar solvents. Currently, methanol and ethanol are effective and widely used solvents for extracting phenols and flavonoids from wood. Methanol was chosen as the solvent for wood extraction because of its strong polarity, which gives it an advantage in extracting various polar molecules from wood samples [97,98,99]. Ethanol is more widely used as an extraction solvent than methanol. Ethanol has been studied as a greener solvent that is more environmentally friendly and it is proclaimed as safe in accordance with the European Food Safety Authority (EFSA) [100,101,102]. In Eucalyptus sideroxylon bark, ethanol and water extracted approximately 97% of the total extracted polar compounds [82]. Significantly higher extraction rates were observed when using aqueous ethanol and aqueous methanol solutions compared to organic solvents alone [103]. Shabir et al. [104] used various solvents to extract the bark of Gold Mohar [Delonix regia (Bojer ex Hook.) Raf.]. The order of yield of the extracts was as follows:80% methanol > deionized water > 80% ethanol > absolute methanol > absolute ethanol > absolute ethanol > 80% acetone > absolute acetone. Because methanol–water mixtures are highly polar, they show greater efficacy in extracting polar phytochemicals such as phenols and flavonoids.

Although polar solvents tend to extract more substances, the use of non-polar solvents as extraction solvents is indispensable. For example, hexane and petroleum ether are more suitable for extracting non-polar compounds. This choice, based on the principle of “similarity and solubility”, ensures that the desired chemical components can be efficiently separated from complex biological matrices. Hexane is suitable for the extraction of non-polar, extremely lipophilic compounds. For instance, Kacik et al. [105] utilized hexane to extract a wide range of terpenes from fir wood. Oktavianawati et al. [94] used a range of solvents with different polarities for the comprehensive extraction of metabolites from Gonystylus bancanus wood, and more terpenoids were obtained in n-hexane, dichloromethane, and ethyl acetate. Most of these compounds are volatile and non-polar, which makes them more soluble in non-polar solvents than in polar solvents.

Water is not a good solvent for extracts because water extracts have lower yields, but organic solvents are harmful to the environment. However, water can be as effective an extractant as an alcohol-containing solution at high temperatures and under pressure [106,107]. This could be an environmentally friendly method that can aid in the development of wood extraction.

The selection of the appropriate solvent is critical to obtaining a higher extraction rate, which directly impacts the yield and quality of the extract. However, the proper treatment of the wood before extraction can also significantly increase the rate at which the wood can be extracted. In general, smaller sample sizes have higher extraction yields, which can be attributed to the intrinsic diffusion capacity of the solvent. Yamamoto and Hong [108] used the same extraction method and solvent to extract 24 broadleaf timber species from Malaysia. The extracted samples were processed as powder and blocks. Unsurprisingly, all the samples processed as powder had a higher extraction rate than those processed as blocks. Sládková et al. [109] treated spruce (Picea abies) bark with different sizes of 0.3, 1.0, and 2.5 mm for extraction, and the results showed that higher yields were obtained for the smaller-sized samples. In practice, various extraction factors, such as the physical state of the sample, solvent selection, extraction process, choice of equipment, and environmental factors, significantly influence the content and composition of the extract [110,111].

Table 1. Extraction and yield of tree species by different solvents (ASE: accelerated solvent extraction, M: minutes; H: hours; D: days; -: unspecified. Multiple solvent distribution extraction 1. step: first step, 2. step: second step, and so on.).

Species	Location	Method	Time	Solvent	Yield	Reference
Tectona grandis L. f.	Heartwood	Soak	12 H	Ethanol	9.17%	Brocco [112]
		Boil	2.5 H	Water	6.56%
Pterocarpus macrocarpus Kurz.	Heartwood	Soxhlet	24 H	70% acetone	13.68%	Zhang [113]
				Water	9.96%
				Dioxane	13.31%
Acacia confusa Merr.	Heartwood	Soak	21 D	70% acetone	9.2%	Chang [114]
				Toluene/ethanol (2/1, v/v)	2.6%
Cunninghamia lanceolata (Lamb.) Hook.	Bark	Soak	2 H	1% NaOH	21.26%	Peng [52]
				Water	2.46%
		Soxhlet	7 H	95% ethanol	5.04%
Burkea africana	Heartwood	Soxhlet	16 H	Diethyl ether	3.1%	Neya [38]
				Acetone	14.8%
				Toluene/ethanol (2/1, v/v)	18.1%
Pterocarpus santalinus	Bark	Soxhlet	-	Methanol	1.05%	Kumar [60]
				Acetone	2.59%
				Methanol/water (70:30)	2.85%
				Methanol/acetone/water (40:40:20)	4.39%
		Boil	2 H	Water	26.22%
Neobalanocarpus heimii P.S. Ashton	Bark	Soxhlet	5 H	Toluene/industrial methylated spirit (2:1)	23.50%	Kadir [77]
	Heartwood				9.16%
Prunus africana (Hook.f.) Kalkman	Heartwood	Soxhlet	15 H	Hexane	0.3%	Mburu [115]
				Dichloromethane	1.1%
				Toluene/ethanol (2:1 v/v)	4.4%
				Acetone	3.4%
				Water	4.8%
Palaquium gutta (Hook.f.)/ Pometia pinnata J.R. Forster & J.G. Forster	Heartwood	Soxhlet	6 H	Petroleum ether	6.15% (P.g) 5.55% (P.p)	Kadir [116]
				Absolute ethanol	8.87% (P.g) 6.39% (P.p)
				Absolute methanol	9.12% (P.g) 7.44% (P.p)
Quercus vulcanica Boiss.	Bark Sapwood Heartwood	Soxhlet	12 H	Cyclohexane	2.0% (Bark) 0.22% (Sap) 0.42% (Heart)	Balaban [67]
			12 H	Ethanol/benzene (1/2, v/v) (1. step), ethanol (2. step)	8.35% (Bark) 4.56% (Sap) 6.13% (Heart)
Olea europaea L.	Bark	ASE	-	Ethanol/water (70:30 v/v)	21%	Faraone [83]
	Wood				9%
Acacia mangium Willd.	Bark	Soak	-	Acetone/H2O (7/3)	37.9%	Makino [117]
Acacia auriculiformis A. Cunn. ex Benth.					28.6%
Acacia mearnsii De Wild.					52.7%
Salix rorida Lacksch.					34.9%
Thujopsis dolabrata (L. f.) Siebold & Zucc.					17.8%
Triplochiton scleroxylon K.Schum.	Heartwood	Soxhlet	12 H	Dichloromethane (1. step), Acetone (2. step), Toluene/ethanol (2:1, v/v) (3. step), Water (4. step)	4.4%	Saha [93]
Baillonella toxisperma Pierre					16.6%
Distemonanthus benthamianus Baill.					16.1%
Pterocarpus soyauxii Taub.					16.7%
Erythrophleum ivorense A. Chev.					17.7%
Cinnamomum sp.	Heartwood	Orbital shaker	8 H	Absolute methanol	10.9%	Kadir [27]
Canarium littorale Blume					2.83%
Eugenia griffithii Duthie					5.57%
Scorodocarpus borneensis (Baill.)					2.98%
Cupressus sempervirens L./ Cupressus arizonica Greene	Xylem	Soxhlet	-	Ethanol–toluene	6.74% (C.s) 10.44% (C.a)	Terzopoulou [80]
	Bark				14.09% (C.s) 27.59% (C.a)
Picea abies L.	Needles	Supercritical CO2	-	CO2	3.3%	Bukhanko [118]
	Branches				2.4%
	Bark				5.3%
Juniperus virginiana L./ Juniperus occidentalis Hook./ Juniperus ashei J. Buchholz	Heartwood	ASE	-	Hexane	4.78% (J.v) 4.26% (J.o) 6.60% (J.a)	Tumen [119]
				Methanol	9.56% (J.v) 7.32% (J.o) 11.27% (J.a)
				Ethanol	7.94% (J.v) 6.24% (J.o) 10.34% (J.a)
Castanea sativa Mill	Xylem	ASE	-	Ethanol/water (70:30, v/v)	12.5%	D’Auria [120]
		Soxhlet	7 H	Ethanol/toluene (1:2, v/v)	7.4%
		Autoclave	20 M	H2O	4.2%
Pterocarpus angolensis/ Pterocarpus macarocarpus Kurz/Pterocarpus soyauxii	Heartwood	Hot reflux	8 H	Water	10.06% (P.a) 14.31% (P.m) 7.44% (P.s)	Cai [121]
				70% ethanol	20.15% (P.a) 28.39%/ (P.m) 28.59% (P.s)

Table 1. Extraction and yield of tree species by different solvents (ASE: accelerated solvent extraction, M: minutes; H: hours; D: days; -: unspecified. Multiple solvent distribution extraction 1. step: first step, 2. step: second step, and so on.).

Species	Location	Method	Time	Solvent	Yield	Reference
Tectona grandis L. f.	Heartwood	Soak	12 H	Ethanol	9.17%	Brocco [112]
		Boil	2.5 H	Water	6.56%
Pterocarpus macrocarpus Kurz.	Heartwood	Soxhlet	24 H	70% acetone	13.68%	Zhang [113]
				Water	9.96%
				Dioxane	13.31%
Acacia confusa Merr.	Heartwood	Soak	21 D	70% acetone	9.2%	Chang [114]
				Toluene/ethanol (2/1, v/v)	2.6%
Cunninghamia lanceolata (Lamb.) Hook.	Bark	Soak	2 H	1% NaOH	21.26%	Peng [52]
				Water	2.46%
		Soxhlet	7 H	95% ethanol	5.04%
Burkea africana	Heartwood	Soxhlet	16 H	Diethyl ether	3.1%	Neya [38]
				Acetone	14.8%
				Toluene/ethanol (2/1, v/v)	18.1%
Pterocarpus santalinus	Bark	Soxhlet	-	Methanol	1.05%	Kumar [60]
				Acetone	2.59%
				Methanol/water (70:30)	2.85%
				Methanol/acetone/water (40:40:20)	4.39%
		Boil	2 H	Water	26.22%
Neobalanocarpus heimii P.S. Ashton	Bark	Soxhlet	5 H	Toluene/industrial methylated spirit (2:1)	23.50%	Kadir [77]
	Heartwood				9.16%
Prunus africana (Hook.f.) Kalkman	Heartwood	Soxhlet	15 H	Hexane	0.3%	Mburu [115]
				Dichloromethane	1.1%
				Toluene/ethanol (2:1 v/v)	4.4%
				Acetone	3.4%
				Water	4.8%
Palaquium gutta (Hook.f.)/ Pometia pinnata J.R. Forster & J.G. Forster	Heartwood	Soxhlet	6 H	Petroleum ether	6.15% (P.g) 5.55% (P.p)	Kadir [116]
				Absolute ethanol	8.87% (P.g) 6.39% (P.p)
				Absolute methanol	9.12% (P.g) 7.44% (P.p)
Quercus vulcanica Boiss.	Bark Sapwood Heartwood	Soxhlet	12 H	Cyclohexane	2.0% (Bark) 0.22% (Sap) 0.42% (Heart)	Balaban [67]
			12 H	Ethanol/benzene (1/2, v/v) (1. step), ethanol (2. step)	8.35% (Bark) 4.56% (Sap) 6.13% (Heart)
Olea europaea L.	Bark	ASE	-	Ethanol/water (70:30 v/v)	21%	Faraone [83]
	Wood				9%
Acacia mangium Willd.	Bark	Soak	-	Acetone/H2O (7/3)	37.9%	Makino [117]
Acacia auriculiformis A. Cunn. ex Benth.					28.6%
Acacia mearnsii De Wild.					52.7%
Salix rorida Lacksch.					34.9%
Thujopsis dolabrata (L. f.) Siebold & Zucc.					17.8%
Triplochiton scleroxylon K.Schum.	Heartwood	Soxhlet	12 H	Dichloromethane (1. step), Acetone (2. step), Toluene/ethanol (2:1, v/v) (3. step), Water (4. step)	4.4%	Saha [93]
Baillonella toxisperma Pierre					16.6%
Distemonanthus benthamianus Baill.					16.1%
Pterocarpus soyauxii Taub.					16.7%
Erythrophleum ivorense A. Chev.					17.7%
Cinnamomum sp.	Heartwood	Orbital shaker	8 H	Absolute methanol	10.9%	Kadir [27]
Canarium littorale Blume					2.83%
Eugenia griffithii Duthie					5.57%
Scorodocarpus borneensis (Baill.)					2.98%
Cupressus sempervirens L./ Cupressus arizonica Greene	Xylem	Soxhlet	-	Ethanol–toluene	6.74% (C.s) 10.44% (C.a)	Terzopoulou [80]
	Bark				14.09% (C.s) 27.59% (C.a)
Picea abies L.	Needles	Supercritical CO2	-	CO2	3.3%	Bukhanko [118]
	Branches				2.4%
	Bark				5.3%
Juniperus virginiana L./ Juniperus occidentalis Hook./ Juniperus ashei J. Buchholz	Heartwood	ASE	-	Hexane	4.78% (J.v) 4.26% (J.o) 6.60% (J.a)	Tumen [119]
				Methanol	9.56% (J.v) 7.32% (J.o) 11.27% (J.a)
				Ethanol	7.94% (J.v) 6.24% (J.o) 10.34% (J.a)
Castanea sativa Mill	Xylem	ASE	-	Ethanol/water (70:30, v/v)	12.5%	D’Auria [120]
		Soxhlet	7 H	Ethanol/toluene (1:2, v/v)	7.4%
		Autoclave	20 M	H2O	4.2%
Pterocarpus angolensis/ Pterocarpus macarocarpus Kurz/Pterocarpus soyauxii	Heartwood	Hot reflux	8 H	Water	10.06% (P.a) 14.31% (P.m) 7.44% (P.s)	Cai [121]
				70% ethanol	20.15% (P.a) 28.39%/ (P.m) 28.59% (P.s)

In the extraction of bioactive molecules from wood, the extraction method is as important as the choice of solvent. Traditional methods such as impregnation and Soxhlet extraction are usually very time-consuming and require large amounts of solvent [93]. In addition, these methods are often used for grinding the product, which increases the means of treatment and complicates subsequent processing [122]. However, there are alternative techniques such as accelerated solvent extraction, microwave assisted extraction, and ultrasound assisted extraction [120,123,124]. There techniques show significant reductions in extraction time and solvent consumption and result in improved extract recovery [125]. Supercritical extraction using CO₂ is one of the very friendly extraction methods. Because the extract obtained is solvent-free, there is no need to remove any solvent. The process of extracting bioactive molecules from plant species, whether by classical or alternative extraction techniques, involves a series of steps such as technology selection, screening and identification, extraction, separation, characterization, and large-scale production [64,126]. In the technique selection stage, the solubility of the desired substance in the chosen solvent, the process conditions, and the co-extraction of unwanted compounds must be considered [127].

2.3. Component Analysis

The composition of wood extracts is crucial for understanding the nature of wood; certain specific extracts can also be used as a reference for the chemical classification of trees [128,129,130]. For example, the cypress family is the only source of terpene compounds (trophenones) of an aromatic nature, including thuyaplicins found in the heartwood of cedar [128]. It is well known that the composition of extracts from different tree species varies according to their biological characteristics and growing environment. However, they are generally composed of similar chemical components such as fatty acids, terpenoids, lignans, flavonoids, tannins, and other compounds. These compounds are primarily composed of low molecular substances and can be classified into the following three main groups [131]: (1) aliphatic compounds, such as fatty alcohol, fatty acid glycerides, sugars, proteins, alkaloids, and gliadins, are mainly found in parenchymal cells; (2) terpenoids, including volatile oils and resin acids, are mainly found in the resin canal; (3) and aromatic compounds. Most polyphenolic compounds with one or more phenolic hydroxyl substitutions are mainly found in the bark and heartwood [63].

2.3.1. Aliphatic Compounds

Aliphatic compounds are a class of organic compounds characterized by a chain-like carbon skeleton, unlike terpenes and aromatic compounds that possess cyclic structures. Aliphatic compounds in wood extracts mainly include fatty acids, resinous acids, waxes, alcohols, sterols, and some other components such as sugars and proteins [132]. The structural differences between saturated and unsaturated fatty acids depend on the presence or absence of double bonds [128]. Saturated fatty acids have all their carbon atoms saturated with hydrogen atoms, forming a long carbon chain structure without any double bonds. On the other hand, unsaturated fatty acids contain one or more unsaturated double bonds [133]. The study by Ramos Azevedo Oliveira et al. [134] focused on analyzing the chemical composition of lipophilic extracts obtained from Acacia mearnsii De Wild. wood at different ages. The GC-MS results indicated that fatty acids were the predominant chemical class in these lipophilic extracts, with contents ranging from 27.1 to 95.0 mg/kg. The composition of the lipophilic extracts from the heartwood and bark of Eucalyptus pellita is highly interesting, with four groups of compounds (i.e., free fatty acids, sterol/steroids, triterpenoids, and other compounds) being detected in all parts of the tree by Arisandi et al. Among them, long-chain fatty acids were the major lipophilic constituents in the heartwood, while sterol/steroids were identified as the primary constituents in the bark extracts [135]. The fatty acid content of mahogany sapwood (Swietenia mahagoni (L.) Jacq.) was 16.70%, while the heartwood fraction had a content of 13.70%. From sapwood to heartwood, the fatty acid concentration tended to decrease, and the composition included oleic and linoleic acid [91]. Spruce bark contains significant amounts of resinous acids with a wide range of antimicrobial properties. Ristinmaa et al. [136] suggested that these resinous acids may function as “gatekeeper” molecules in the degradation of wood fungi. This is because the α diversity of the bacterial community initially decreases dramatically during bark degradation and then returns to normal after the metabolism of resin acids begins. These lipophilic compounds in the extracts often cause a “pitching” problem during the production process in the pulp and paper industry [137]. In addition, fats and waxes impact the pulping process and the wettability of wood [138], as they occupy potential space for water and form a hydrophobic film on the surface of the wood or inside it, which hinders the absorption and diffusion of water [139].

2.3.2. Terpenoid Compounds

Terpene compounds play a vital role in trees, not only protecting them from diseases and microorganisms but also communicating and interacting with other organisms in their surroundings by producing specific odors [128]. Terpenoids are modified terpenes with different functional groups and oxidized methyl groups moved or removed at different positions, which are widely distributed in plants and microorganisms and represent a large and diverse class of secondary metabolites [140]. Terpenoids can be classified according to the number of isoprene units; they include monoterpenes (C₁₀), sesquiterpenes (C₁₅), diterpenes (C₂₀), triterpenes (C₃₀), and other additional terpenes (Figure 1) [141]. Some terpenes are classified as volatile (VTs) due to their lower molecular weight and higher volatility, such as monoterpenes and sesquiterpenes. Other terpenes, such as diterpenes, are classified as semivolatile or nonvolatile [142]. These differences mainly stem from the number of isoprene units in their molecular structures.

Monoterpenes and sesquiterpenes are found primarily in the essential oils of plants and are highly aromatic and biologically active. The most common monoterpenes are α- and β-pinene found in firs and pines [143]. Falleh et al. [144] stated that monoterpenes (C₁₀) typically constitute the major molecules in essential oils, with their proportion potentially reaching as high as 90% of the entire essential oil. Several studies have shown that the antimicrobial activity of many essential oils extracted from plants is related to the presence of monoterpenes in their composition [145,146,147]. Monoterpene hydrocarbons in Pinaceae are the main constituents of volatile oils, which are not only aromatic but also have antimicrobial and anti-inflammatory properties [147]. Sesquiterpenoids are the most diverse terpenoids [148]. They are often present in volatile oils in plants as alcohols, ketones, lactones, etc., and constitute the main components of the high-boiling fraction of volatile oils [149]. Conifers, especially Pinaceae, produce large amounts of oleoresin, which mainly consists of a variety of compounds, among which terpenoids occupy an important position [150,151]. Diterpenoids are the main constituents of resins, and a few can be found in volatile oils with high boiling points. The diterpene resin acids in oleoresins can remain in the tree to form hardened substances that seal tree wounds, prevent water and nutrient loss, and resist invasion by pathogens and pests [152]. Triterpenes are derivatives of the C₃₀ precursor squalene and are used in organisms as important intermediates in sterol biosynthesis. Lupinol is a triterpene that exhibits chemopreventive, cytotoxic, and antioxidant effects [128].

Figure 1. Classification of terpenes [145].

Figure 1. Classification of terpenes [145].

2.3.3. Aromatic Compounds

Aromatic compounds are mostly polyphenols substituted with one or more phenolic hydroxyl groups. There is a wide variety of phenolic compounds in wood, and based on the basic skeleton of the molecular structure, the phenolic compounds in wood extracts can be classified into several categories, as shown in

Table 2. Major classes of phenolic compounds in wood extracts [154].

Class	Structure
Simple phenolics, benzoquinones	C6
Hydroxybenzoic acids	C6-C1
Hydroxycinnamic acids, phenylpropanoids	C6-C3
Napthoquinones	C6-C4
Stilbenes, anthraquinones	C6-C2-C6
Flavonoids, isoflavonoids	C6-C3-C6
Lignans, neolignans	(C6-C3)2
Condensed tannins (proanthocyanidins or flavolans)	(C6-C3-C6)n

. In the structural formula, C₆ represents an aromatic ring, C₁ represents a carbon atom, and C₂ and C₃ represent the number of carbon chain atoms [153].

Hydroxybenzoic acid is one of the two subclasses of phenolic acids and typically has a C₆-C₁ structure. Commonly found in plants are p-hydroxybenzoic acid (PHBA), protocatechuic acid (PRA), vanillic acid (VA), gallic acid (GA), and syringic acid (SYA) [155]. Phenylpropanoid derivatives (C₆-C₃) constitute an important class of low-molecular-weight phenolic compounds. The most important phenylpropanoid is hydroxycinnamic acid, another subclass of phenolic acids, commonly known as p-coumaric, ferulic, and caffeic acids [156]. Phenolic acids can exist in the free state or form conjugated structures with sugar residues (such as glucose, galactose, rhamnose, arabinose, and xylose), amines, organic acids, or other phenols through ester or ether bonds. Also, they can exist in an insoluble form, bind to components of the plant cell wall (such as arabinoxylan, arabinogalactan and cellulose) by esterification and etherification, and constitute cross-linked structures with lignin [157,158]. Lignans are a class of natural compounds that polymerize from two molecules of phenylpropanoid derivatives (i.e., C₆-C₃ monomers) and are widely distributed throughout the plant kingdom. Their monomers are linked by a central carbon (C₈) atom; most of them are free, and a few are present in the xylem and resin of plants by combining with sugars to form glycosides [159]. Phenylpropane dimers are classified as lignans or neolignans based on the presence or absence of an 8,8′ bond between the phenylpropanoid monomers (Figure 2) [160]. Lignans and demethyl lignans are commonly biosynthesized and deposited in large quantities in the heartwood region of trees as a metabolic event in heartwood formation and can be used to prevent heartwood decay caused by decay fungi [161]. Similar to other secondary metabolites, lignans are defense mechanisms that have evolved in plants as a response to external environmental stresses, and they play an important role in defending against external animals and microbes.

Quinones are a class of compounds with two ketone groups (C=O) that can be categorized according to the number of benzene rings in their structure, such as benzoquinone, naphthoquinone, and anthraquinone [163]. Benzoquinone is a simple phenol with a molecular backbone of C₆. Quinones in wood are found primarily as benzoquinone, and they can exist in the plant body either as free compounds or bound as glycosides. Quinones exhibit high resistance to termites and fungi, and can be found in the wood of certain tropical species, particularly teak. The presence of anthraquinone (C₆-C₂-C₆) and other quinone derivatives, such as naphthoquinone (C₆-C₄), contributes to the natural durability of teak [164,165]. Stilbene typically consists of two benzene rings connected by an ethylene bridge and has a C₆-C₂-C₆ backbone. Depending on the relative position of the ethylene bridge between the two benzene rings, it can exist in either the Z-configuration or the E-configuration [166]. Stilbenes, including the well-known stilbene resveratrol, are defense compounds produced by some plants in response to pathogen attacks and other stresses [167]. These compounds are mainly found in conifers, have antifungal properties, and are anti-free radical agents and metal chelators.

Flavonoid compounds are one of the most studied secondary metabolites in wood, and they consist of a tricyclic structure in the form of C₆-C₃-C₆ with different substituents such as hydroxyl and methoxy. They can be classified into six subgroups: flavanols, flavanones, flavones, isoflavones, flavonols, and anthocyanidins forming oligomers and polymers [168]. Flavonoids play multiple roles in the life cycle of wood, including protection against UV damage, defense against external predators such as insects, and the regulation of growth hormone transport and metabolism [169]. Tannins are a class of polymeric phenolic compounds, usually with a relatively high molecular weight and a backbone structure of (C₆-C₃-C₆)_n. Depending on the chemical structure, tannins can be broadly classified into two categories: hydrolyzable tannins and condensed tannins (proanthocyanidins). Tannins are prominently characterized by their ability to strongly adsorb proteins, and their presence in vesicles and waxes inhibits the digestive enzyme libraries of pathogens and herbivores [170]. Most plants (such as Acacia spp., Maples, and Crustacea) contain hydrolyzable and condensed tannins [171].

3. Decay Resistance of Extracts

The abundant chemical content of wood, although not directly involved in core biological processes such as growth or photosynthesis, profoundly influences the color characteristics of wood and exhibits excellent defense mechanisms against pests and pathogenic microorganisms [172]. The concept that extractives deposited in wood are a major source of resistance to fungal attack was first demonstrated by Hawley, Fleck, and Richards [34]. Subsequent researchers have demonstrated that (a) extracts from durable heartwood are more toxic than extracts from the sapwood of the same tree; (b) depending on the species of wood and the type of toxic compounds involved, extractions with hot water or organic solvents, or both, considerably reduce the decay resistance of the heartwood; (c) and the toxicity of extracts obtained from the heartwood of different species is generally in keeping with the decay resistance of these species [12].

Both the quantity and quality of extractives play a role in the durability of wood, but their relative contributions vary from substrate to substrate. Numerous studies have demonstrated that the extractive content plays a major role in resistance; however, the percentage of extractive content is not directly related to durability, and an increase in concentration does not necessarily increase the decay resistance of extractive-treated wood species [173,174]. Park et al. [175] found, in decay experiments on heartwood and sapwood of Paulownia tomentosa, that sapwood samples showed a lower rate of weight loss than heartwood samples in the case of Fomitopsis palustris fungal attack. Generally, the extractable components of wood are found in the heartwood, while the sapwood is more susceptible to fungal attack [176,177]. However, the results of this study indicate that the heartwood also decays. As already concluded in some studies, the individual components of the extract may confer resistance, rather than the presence of the extract in large quantities. While higher levels of extractives can indicate natural resistance, the composition of these extracts may hold greater significance [40,112,178].

3.1. Evaluation of the Decay Resistance of Extracts

Secondary metabolites derived from wood have been extensively studied as antimicrobial agents. Most experiments assessing the fungicidal effect of such extracts have typically employed three methods: (1) comparative experiments between extracted and unextracted wood in the same fungal environment; (2) the extraction of secondary metabolites from wood to assess their effect on the growth of pure fungi in agar; (3) and the impregnation of specific woods with extracts from durable tree species to assess their decay resistance.

3.1.1. Evaluation of Decay Resistance of Extracted and Unextracted Wood

The effectiveness of the extractives in protecting the wood can be visualized by comparing the performance of wood with extractives removed and retained in the presence of fungus.

Table 3. Mass loss of extracted and non-extracted tree samples exposed to fungi or termites (multiple solvent distribution extraction 1. step: first step, 2. step: second step, and so on).

Species	Solvent	Fungi or Termites	Mass Loss (Unextracted)	Mass Loss (Extracted)	Reference
Tectona grandis	Ethanol:toluene (2:1 v/v)	Reticulitermes flavipes, Trametes versicolor, Rhodonia placenta	0.30% (R.f) 2.90% (T.v)	1.93% (R.f) 9.53% (T.v)	Hassan [180]
Cedrus deodara			1.93% (R.f) 8.62% (R.p)	33.4% (R.f) 48.94 (R.p)
Baillonella toxisperma	Dichloromethane (1. step), Acetone (2. step), Toluene:ethanol (2:1,v/v) (3. step), Water (4. step)	Trametes versicolor, Trametes coccinea, Coniophora puteana, Rhodonia placenta	3.3% (T.v) 2.1% (T.c) −0.3% (C.p) 2.1% (R.p)	5.6% (T.v) 3.0% (T.c) 0.4% (C.p) 3.5% (R.p)	Saha [93]
Distemonanthus benthamianus			0.9% (T.v) 0.1% (T.c) −0.4% (C.p) 0.1% (R.p)	26% (T.v) 16% (T.c) 20% (C.p) 8.0% (R.p)
Pterocarpus soyauxii			1.9% (T.v) 0.1% (T.c) 0.4% (C.p) 0.2% (R.p)	40% (T.v) 18% (T.c) 0.5% (C.p) 16% (R.p)
Erythrophleum suaveolens			1.5% (T.v) 0.4% (T.c) 0.2% (C.p) 0.8% (R.p)	15.9% (T.v) 17% (T.c) 0.8% (C.p) 0.9% (R.p)
Prunus africana	Dichloromethane,	Macrotermes natalensis	0.3%	23.6%	Mburu [115]
	Acetone,			13.6%
	Toluene/ethanol (2:1 v/v),			10.3%
	Water			22.2%
Burkea africana	Toluene/ethanol, Mixture (2/1, v/v),	Coriolus versicolor	2.5%	1.2%	Neya [38]
	Diethylether			2.2%
	Acetone			2.9%
Thuja plicata	Hexane(1. step), Methanol(2. step)	Coptotermes formosanus, Postia placenta	4.1% (C.f) 1.5% (P.p)	20.6% (C.f) 35.4% (P.p)	Taylor [181]
Chamaecyparis nootkatensis			1.4% (C.f) 0% (P.p)	6.4% (C.f) 29.0% (P.p)

summarizes the mass loss of selected woods with extractives removed compared to those with extractives retained in the fungal environment. Many studies have shown that durable wood with extractives removed becomes susceptible to decay [12,179]. Saha Tchinda et al. [93] removed extractives from five species of wood and exposed them to fungi. The results show that the samples with extractive removal show a larger mass loss compared to the control samples. Before extraction, all wood samples to be studied were classified as extremely resistant to fungal attack relative to the standard (category 1). After extraction, Distemonanthus benthamianus, Pterocarpus soyauxii, and Erythrophleum suaveolens were weakened to moderately durable resistance to fungal attack (category 3). Kirker et al. [39] used eight natural durable wood species and one non-durable control wood, removed the extractives, and exposed them to six common wood-rotting fungi and termites and unextracted wood blocks of the same species. The results showed that almost all wood species exhibited higher mass loss due to fungi or termites when the extractives were removed. The mass loss of the durable species samples with extractives removed was comparable to that of the non-durable control species. These experimental results suggest that extractives have a critical impact on wood durability.

The presence of extractives does play a pivotal role in the natural durability of wood; however, their influence is not yet sufficient to fully account for the outstanding durability of some specific wood species that remain after undergoing the extraction process [38]. The microscopic distribution of the extract within wood cannot be disregarded. In-depth in situ studies and the precise localization of the distribution of these extracts in wood, particularly their impact on fungal colonization, are essential for a comprehensive understanding of the specific functions performed by extracts in wood [39]. Furthermore, in addition to the antimicrobial effects of extractives, other components or material properties of the wood contribute to its overall natural resistance [182]. For example, physical barriers can provide additional protection, and woods such as Tectona grandis, Cedrus deodara, and Adenanthera microsperma retain their resistance to decay even after extraction. The resistance may be attributed to other factors such as high density and hardness, or rigorous extraction may fail to remove chemicals that are not easily extracted from some hardwood species [12,180].

3.1.2. Inhibition Assessment of Extracts in Agar Plate

The inhibitory effect of wood extracts on fungi can be effectively assessed by directly adding these extracts to a fungal agar medium. The inhibition zone method is a simple and inexpensive test that provides accurate and reliable results, allowing the determination of the inhibitory effect of an extract by visualizing the size of the inhibitory circle.

Table 4. Inhibition of fungi by extracts in agar plates.

Species	Solvent	Concentration	Fungus	Inhibition Rate	Reference
Cedrela fissilis	Methanol	0.5 µg/µL	Trametes trogii, Pycnoporus sanguineus, Chaetomium globosum	25.12% (T.t) 27.84% (P.s) 29.57% (C.g)	Vovchuk [183]
	Acetone			24.34% (T.t) 30.11% (P.s) 16.20% (C.g)
	Ethyl acetate			31.62% (T.t) 32.28% (P.s) 29.66% (C.g)
	Dichloromethane			33.77% (T.t) 42.92% (P.s) 52.31% (C.g)
	Hexane (1. step) Ethyl, Acetate (2. step)			32.61% (T.t) 31.62% (P.s) 35.22% (C.g)
Cunninghamia lanceolata	Hexane	2.5 g/L	Trametes versicolor, Irpex lacteus, Gloeophyllum trabeum, Postia placenta	82% (T.v) 47% (I.l) 54% (G.t) 44% (P.p)	Xu [185]
	Ethyl acetate			100% (T.v) 42% (I.l) 43% (G.t) 83% (P.p)
	Methanol			100% (T.v) −47% (I.l) 51% (G.t) 65% (P.p)
Tectona grandis	Hot water	0.125%	Postia placenta	31.92%	Brocco [112]
		0.25%		37.74%
		0.50%		49.17%
		1.0%		58.73%
		2.0%		86.13%
		4.0%		100%
Neobalanocarpus heimii King P. S. Ashton	Hot water	4% (w/v)	Trametes versicolor, Lentinus sajor-caju, Coniophora puteana	61.43% (T.v) 66.53% (L.s) 59.63% (C.p)	Kadir [186]
Cotylelobium lanceolatum Craib				55.27% (T.v) 60.28% (L.s) 48.88% (C.p)
Madhuca utilis (Ridley) H.J.Lam ex K. Heyne				49.67% (T.v) 57.62% (L.s) 43.22% (C.p)
Shorea curtisii Dyer ex King				74.44% (T.v) 80.33% (L.s) 80.33% (C.p)
Prunus africana	Dichloromethane	100/500 ppm	Coriolus versicolor Poria placenta, Aureobasidium pullulans	87%/100% (C.v) 100%/100% (P.p) 100%/100% (A.p)	Mburu [115]
	Acetone			76%/87% (C.v) 78%/100% (P.p) 100%/100% (A.p)
	Toluene/ethanol (2:1 v/v)			73%/86% (C.v) 45%/71% (P.p) 88%/91% (A.p)
	Water			45%/100% (C.v) 47%/100% (P.p) 75%/100% (A.p)
Morus alba L.	Methanol, Acetone, Ethyl acetate, Chloroform	4 g/L	Coriolus versicolor, Gloeophyllum trabeum	75%/98%/100%/12% (C.v) 91%/54%/100%/29% (G.t)	Li [187]
Fraxinus mandshurica Rupr.				19%/47%/48%/17% (C.v) 69%/35%/-11%/31% (G.t)
Sabina chinensis (L.) Antoine				25%/60%/70%/57% (C.v) 74%/55%/85%/54% (G.t)
Larix principis-rupprechtii Mayr				67%/60%/36%/70% (C.v) 33%/48%/26%/73% (G.t)
Burkea africana	Toluene/ethanol (2/1, v/v)	100/1000 ppm	Coriolus versicolor, Poria placenta, Gloeophyllum trabeum, Coniophora puteana	14%/47% (C.v) 46%/46% (P.p) 17%/31% (G.t) 30%/58% (C.p)	Neya [38]
	Diethylether			4%/29% (C.v) 17%/51% (P.p) 12%/31% (G.t) 9%/58% (C.p)
	Acetone			1%/32% (C.v) 1%/17% (P.p) 47%/76% (G.t) 18%/46% (C.p)

summarizes the inhibitory effect of wood extract against wood rot fungus on agar plates. Numerous scholars have used this method to investigate the bacteriostatic properties of wood-derived extracts. Brocco et al. [112] selected hot water extracts of 20-year-old teak heartwood and conducted a series of Petri dish fungal inhibition tests against the brown rot fungus Postia placenta. The different concentrations of hot water extract used in the tests significantly reduced the growth area of Postia placenta in the Petri dishes compared to the control plates (no extract). It is noteworthy that even at a lower concentration of 1%, these extracts exhibited more than 50% fungal growth inhibition, and their fungal inhibition was even more effective when the concentration was raised to 2%, 4%, and 8%. Vovchuk et al. [183] extracts of Cedrela fissilis heartwood were extracted using several different solvents and tested for Petri dish fungal inhibition. The results showed that all the tested cedar heartwood extracts exhibited significant inhibition against xylophagous fungi, including Chaetomium globosum (soft rot), Trametes trogii (white rot), and Pycnoporus sanguineus (white rot). Kumar et al. [60] evaluated the bioactivity using the bark extract of Pterocarpus santalinus. Petri dish bioassay studies showed that Pterocarpus indicus bark extract showed good inhibition of fungal growth. Lv et al. [184] found that the extract of Dalbergia retusa exhibited a strong dose-dependent inhibition against decay fungi. All these studies showed that the fungal inhibitory effect usually increased with increasing concentrations of the extracts, demonstrating the potential application of wood extracts in the prevention of wood decay.

It is noteworthy that the inhibitory effect exhibited by wood extracts in Petri dishes is influenced by a combination of factors, including the choice of extraction solvent, the adjustment of extract concentration, and the specificity of the fungal species [184,188]. This remarkable inhibitory activity likely stems from the synergistic effect of multiple compounds rather than the isolated effect of a single substance [6].

3.1.3. Antimicrobial Assessment of Extract-Impregnated Non-Durable Wood

The extracts of durable wood species have been shown to possess deterrent effects against fungi, bacteria, and termites and can be used to protect susceptible wood species [189,190]. Researchers are working on extracting large amounts of potent antimicrobial substances from naturally durable wood species and attempting to transfer them to non-durable wood species for the cross-species transfer of resistance [39,191,192]. This environmentally friendly and efficient wood conservation strategy has attracted much attention and intensive research. Li et al. [193] extracted the heartwood, sapwood, and bark of Larix olgensis var. changpaiensis using five different solvents. The extract was subsequently used as a preservative in the treatment Populus ussuriensis Kom, which was subsequently exposed to the white rot fungus and brown rot fungus. The results revealed that the methanol and acetone extracts from the heartwood had a better inhibitory effect on the white rot fungus, and aqueous extracts from the bark had a better inhibitory effect on both fungi. Juniper is known for its pleasant fragrance and resistance to mold and mildew. Tumen et al. [119] used extracts of these species to impregnate wood species that were also affected, making them effective against subsequent attacks by decay organisms. Kirker et al. [40] impregnated two types of non-durable wood without extractives using ethanol–toluene extractives from eight durable wood species and compared the mass loss due to fungal degradation between the extractive-treated blocks and the untreated controls. The results showed that in some cases, treatment with extracts from durable wood species could reduce the percentage of mass loss due to the exposure to decay fungi. Rodrigues et al. [194] investigated the natural decay resistance of seven wood extracts from the Amazonian forest. When tested for brown rot, Pinus sylvestris treated with 5.1 g/L of Handroanthus serratifolius showed improved resistance to rot, resulting in only a 1.6% loss of wood mass. Onuorah [195] used extracts of very durable tropical hardwood species (Milicia excelsa and Erythrophleum suaveolens) impregnated into the non-resistant species Ceiba pentandra Gaertn, and then exposed the treated samples to soil containing decay fungi (Lenzites trabea and Polyporous versicolor). The results showed an effective inhibition of L. trabea or P. versicolor infestation at higher extract concentrations, but there was no significant difference with the results obtained with the control at lower concentrations.

However, studies performed in laboratory settings have shown that the fungal degradation of wood does not occur unconditionally. Moisture content in wood is one of the key factors in fungus degradation, and the fungus is only effective in degrading wood if certain conditions are met. In temperate regions, the moisture content of wood must be greater than 20 percent, whereas in the tropics, this threshold is raised to over 30 percent for the fungus to completely initiate the degradation process [196]. If the moisture content of the wood is below these thresholds, it becomes “difficult” to assess the degree of fungal infestation by mass loss data. In addition, additional conditions necessary for fungal infestation include elevated concentrations of oxygen and low concentrations of carbon dioxide, which collectively influence fungal growth and degradation activities on wood [93].

3.2. Analysis of Decay-Resistant Components of Extracts

Phenolic compounds and terpenoids have good inhibitory activity against wood-rotting fungi, e.g., stilbene helps the wood resist decay by brown-rotting fungi [197], and rosin forms complexes with enzymes to inhibit the enzymatic hydrolysis of wood by white-rotting fungi [198]. Therefore, the presence of phenols and terpenoids in the extracts of the studied species could explain their natural resistance to fungal attack [196].

Natural flavonoids have been shown to have strong inhibitory effects on fungal growth [199]. According to Tascioglu’s analysis of the chemical composition of natural extracts, the elevated antifungal activity of mimosa and argan extracts was attributed to their high tannin content [173]. In addition, up to 14 constituents were successfully identified in both extracts, including catechols, epicatechins, and gallic acid, compounds that are well known for their fungal inhibitory activity [200,201,202]. The preservative effect of tannins was further investigated by Silveira et al. [203], who treated Acacia mearnsii wood with tannins and the traditional preservative CCB, and then exposed it to the fungus Pycnoporus sanguineus. The experimental results showed that the preservative effect of tannin was comparable to that of CCB preservative in wood treated with tannin at concentrations of 5% and 10%, demonstrating the remarkable potential of tannin in the field of preservation.

The quinones in the extract effectively suppress termites and wood rot fungi. The high decay resistance of teak is closely related to the presence of various chemical compounds, particularly quinones. Romanis was the first to publish a paper on the chemical properties of teak in 1887. In this paper, he isolated a unique compound known as methyl anthraquinone [204]. Through further research, it has been found that methyl anthraquinone has a significant “repellent effect” on termites [205]. According to Simatupang et al. [206], tectoquinone (2-methyl anthraquinone) exhibits insecticidal properties, especially against termites. Thulasidas et al. [164], in their study, showed that naphthoquinone was the most important single compound, conferring resistance to two brown rot fungi (i.e., Polyporous palustris and Gloeophyllum trabeum) in teak. Anthraquinone and squalene, which are abundant in teak extracts; terpenes, which are present in cedar heartwood extracts, cedrol; and naphthalene, which are present in cedar heartwood extracts, are compounds that exhibit extremely potent biological activity against insects and other organisms [207,208]. Similarly, sesquiterpenes have been found to have significant food refusal and repellent effects on subterranean termites and trigger illegal behavioral responses [180].

It has been stated in some previous studies that essential oils are also effective in protecting wood from fungi and insects [209]. Fidah et al. [210] tested the antifungal activity of Cedrus atlantica essential oil against four wood-rotting fungi and demonstrated its significant antifungal activity using the ring of inhibition method, which was further analyzed chemically using GC-MS. The results showed that its main components include E-γ-Atlantone (19.73%), E-α-Atlantone (16.86%), 5-Isocedranol (11.68%), 9-Isothujopsanone (4.45%), Cedranone (4.13%), and Z-α-Atlantone (4.02%). Chittenden et al. [211] confirmed in wood resistance tests that eugenol and cinnamaldehyde in essential oil extracts are potentially benign wood preservatives for the treatment of wood that has not been exposed to wet conditions. Kartal et al. [212] exposed wood samples treated with specific essential oil compounds to brown rot fungi (Tyromyces palustris), white rot fungi (Trametes versicolor), and subterranean termites (Coptotermes formosanus). The results of their experiments revealed an essential finding: preparations containing cinnamic acid, ferulic acid, and cinnamaldehyde exhibited extremely potent antifungal activity. In addition, during their testing, they found that formulations containing squalene and wood tar oil were equally effective against wood-degrading fungi.

3.3. Principle of Corrosion Resistance of Extracts

3.3.1. Inhibition of Fungal Degradation Enzymes

Fungi secrete a variety of degradative enzymes during growth and metabolism, which are capable of finely cleaving the complex and diverse organic compounds within the wood, thereby converting them into an indispensable source of nutrients for fungal growth [10]. Specific components in the extracts can precisely target and tightly bind to the active site of the fungal degrading enzyme, forming a barrier that effectively prevents normal contact between the enzyme and the substrate intended for degradation [213,214]. This direct and specific binding mechanism significantly weakens the catalytic activity of the enzyme and reduces the efficacy of the fungus in degrading wood. Gallic acid and tannins have been shown to be potent inhibitors of laccase and peroxidase, and gallic acid has no direct effect on the detoxification process of laccase but only indirectly reduces its detoxification process by inhibiting the enzyme activity [215]. Tannins irreversibly bind to active proteins, leading to the formation of precipitates. Alternatively, tannins can interact with enzymes and potentially act as inhibitors [216]. Voda et al. [217], in their study, indicated that thymol and carvacrol are the essential oil phenols with the most antifungal activity and that the mechanism of the toxicity of phenolic compounds to fungi is mainly based on the inhibition of fungal enzymes containing -SH groups in the active site [197].

In addition to binding directly to the active site of the enzyme, wood extracts exhibit more complex regulatory mechanisms that affect the catalytic activity of the enzyme by subtly adjusting the microenvironment in which it resides. These components are capable of altering key factors such as the pH, oxygen concentration, and ionic strength of the surrounding environment, setting an adverse stage for the catalytic reaction of the enzyme. In addition, they may bind non-specifically to the inactive region of the enzyme, and this binding not only disturbs the internal structural stability of the enzyme but also interferes with its normal functional operation, thus further weakening the catalytic efficiency of the enzyme. It has been reported that tropolone, hinokitiol, and tannins can form stable chelates with metal ions [218], depriving enzymes of essential metal ions for their activity and inhibiting microbial growth [219]. However, the redox cycle of iron is considered to be an important step in catalysis [220], and one of the possible mechanisms by which hinokitiol inhibits enzyme activity is that the compound binds to iron, which is necessary for the catalysis of the enzyme [221]. Li et al. [222] showed that vanillin can inhibit cellulase activity at appropriate concentrations, and a comparative study with three compounds structurally similar to vanillin confirmed that phenolic hydroxyl and aldehyde groups play an important role in inhibiting cellulase activity. The mechanism of the antifungal activity of flavonoids may be related to the cross-linking of fungal enzymes; inhibition of enzymes (such as cellulases, xylanases, and pectinases); chelation with metals required for enzyme activity; and formation of rough hard, near-crystalline structures that act as physical barriers against fungal attack [202,223].

3.3.2. Disrupts the Cell Wall and Cell Membrane of Fungi

As the important barriers of fungal cells, the cell wall and cell membrane not only guard the survival of the cell but also play an irreplaceable role in the physiological metabolism of the cell, material transport, signal communication, DNA replication, and other life activities [224]. Wood extracts are rich in flavonoids, which can bind non-specifically to phospholipids in cell membranes, significantly changing the permeability of cell membranes [225]. This subsequently triggers the disruption of the osmotic pressure balance inside and outside the cell, leading to the leakage of key intracellular components (proteins, nucleic acids, enzymes, etc.), weakening the cell structure, interfering with metabolism, and ultimately contributing to cell apoptosis or necrosis [226]. Phenols contained in the essential oils of plants can enter the cell walls or cell membranes of fungi, affecting the function of the membranes and releasing cell contents. o-Vanillin induces cell wall and cell membrane damage by reducing the content of β-1,3-glucan and cell permeability. In addition, o-vanillin altered the hydroxyl functional groups in the mycelial cell wall and decreased the protein content in the mycelial cell wall, leading to an increase in cell membrane permeability [227]. The presence of hydroxyl groups in the extract can form hydrogen bonds that affect the enzyme. These changes in the enzyme structure lead to the rupture of the cytoplasmic membrane, which inhibits fungal growth [93].

3.3.3. Inhibit Protein and Nucleic Acid Biosynthesis

Proteins, as the central enablers of the various metabolic and regulatory pathways on which living cells depend, are intricately linked to the activities of life. An intensive and comprehensive understanding of the complex activities of living organisms requires research at the protein level and the exploration of the platforms of proteomics and genomics. Phenol, which is abundant in tree extracts, is effective at denaturing proteins at lower concentrations; however, these phenolics significantly contribute to the precipitation of proteins when the concentration is elevated to higher levels [228,229]. Haslam and Spencer et al. [230,231] conducted an insightful study on the reaction mechanism of polyphenols with proteins and found that the combination of plant polyphenols with proteins can cause protoplasm coagulation in microorganisms, thereby exhibiting biological activities such as antiviral effects and enzyme inhibition. Cinnamomum camphora extract can inhibit functions such as mRNA shearing and ribosome synthesis in brown rot fungi [228]. Quercetin exhibits significant inhibitory activity against fungi and bacteria, and its antimicrobial mechanism mainly involves damaging the cell wall, affecting protein synthesis and expression and inhibiting nucleic acid synthesis [232]. By affecting the gene expression of the fungus, the extract inhibits the synthesis of key proteins and enzymes, which in turn prevents the normal division of the mycobacterial cells. This process not only inhibits the gene expression of growth-related proteins and enzymes but also ultimately leads to the disintegration and death of the fungus [233].

3.4. Applications and Challenges

In the wood protection industry, traditional wood preservatives are being withdrawn from the market due to their harmful effects on the environment [234]. The need for environmentally friendly preservatives to protect wood from biodegradation is increasing, and research is necessary to develop a new generation of improved wood preservatives to extend the life of wood materials. Inhibiting fungal growth is one of the most interesting properties of wood extracts for wood preservation [207]. Research conducted in recent decades has shown that wood extracts are highly effective in wood preservation. The extracts of natural durable wood species have been used to inhibit a wide range of organisms against bacterial, fungal, and insect attacks [235]. Commercially available products such as Cedarshield^® and Termilone^® are currently used as wood preservative products, but their effectiveness has not been proven in the academic literature [39].

As an alternative to chemical preservatives, natural extracts have demonstrated significant advantages in terms of environmental protection and safety. However, the practical application of natural extracts as wood preservatives has revealed certain drawbacks and challenges that cannot be ignored. Some of the compounds contained in the extracts, such as polysaccharides, resins, and low-molecular carbohydrates, may accidentally become “food” for certain species of fungi or insects. High levels of primary metabolites (especially sugars) may increase the colonization of wood by fungi, especially staining fungi (blue staining, mold) [25]. Instead of preventing them from attacking the wood, they may inadvertently contribute to accelerated decay. Furthermore, the preservation mechanisms of natural extracts are more complex than those of chemical preservatives, making it difficult to achieve precise “targeted defense” against specific fungal or insect species [181]. Future research could focus on the deep separation and purification techniques of natural extracts, aiming to accurately extract key components with efficient antiseptic activity and explore their unique antiseptic mechanisms.

4. Photostability of Extracts

4.1. Photochemical Reactions in Wood

From a chemical point of view, wood, a natural material, is mainly made up of three core components: cellulose, hemicellulose, and lignin, which are all subject to degradation processes when exposed to sunlight, especially UV. They all undergo degradation when exposed to sunlight, particularly ultraviolet light [19]. In this complex chemical reaction, lignin, with its abundant chromophore structure, absorbs the majority of UV radiation energy (up to 80%–95%), making it the most susceptible component to photodegradation [236,237]. With the gradual photodegradation of lignin as an important cell wall filler, cellulose and hemicellulose, which were originally protected by it, are exposed to more direct radiation and thus become the main targets of attack in the subsequent degradation process [238].

In addition to the major components (cellulose, hemicellulose, and lignin), the minor components of wood (extractives) are also susceptible to UV radiation [239]. This may be due to some special links between extractives and lignin. During biosynthesis, both lignin and polyphenolic compounds in extractives undergo some common intermediates [240]. For example, phenylalanine is converted to trans-cinnamic acid by the enzyme phenylalanine deaminase (PAL) [241,242]. This is followed by hydroxylation, methylation, and other reactions to form a series of hydroxycinnamic acid-like compounds, such as caffeic acid and ferulic acid [243,244]. These compounds are both precursors for lignin biosynthesis and important components of polyphenolic compounds [245]. This implies that they have certain similarities in their chemical structures and properties, and therefore may exhibit similar reaction patterns during photodegradation. For example, both lignin and polyphenols contain multiple phenolic hydroxyl groups, and these groups are susceptible to oxidation under light conditions to produce quinones or other oxidation products [60,246].

After additional studies, it is now clear that one of the key mechanisms of lignin photo-yellowness lies in the mutual coupling of photocatalysis and the direct photoreaction of chromophores in lignin, which ultimately promotes the formation of quinone compounds and alternative chromophores [247]. When wood is exposed to UV light, it initiates the conversion of lignin on the wood surface into carbonyl and phenoxy radicals. Phenoxy radicals, as the key intermediates of the photo-yellowing phenomenon, are exceptionally active and highly susceptible to oxidation reactions with oxygen in the air to form colored compounds [199,248]. Due to the extremely elevated chemical reactivity inherent in free radicals, phenoxy radicals continuously attack the surrounding unreacted lignin molecules, thereby accelerating the photodegradation of lignin [249,250]. It has also been shown that these free radicals may subsequently lead to the photo-oxidation of cellulose and hemicellulose [251]. Specifically, when lignin absorbs UV energy, it undergoes a change in state to the highly reactive excited state of lignin [252]. A chain reaction occurs when excited state lignin meets oxygen. On the one hand, it directly interacts with oxygen to produce carbonyl derivatives as products [45,47]; on the other hand, excited state lignin also efficiently transfers the absorbed energy to the oxygen molecule, which prompts its conversion into the highly oxidizing single-linear oxygen [253]. Subsequently, this singlet oxygen acts as a powerful oxidizing agent and additionally reacts with the unreacted lignin, inducing its cleavage into the original lignin radical and oxidized derivatives. This process forms a circular photodegradation chain reaction [254,255]. A series of light-induced lignin degradation processes is shown in Figure 3.

4.2. The Role of Extracts in Photo-Radiation

Due to the strong absorption of UV–visible light by wood extracts, this makes it one of the factors influencing the photodiscoloration of wood. The mechanism of the protective effect of wood extracts under UV irradiation is complex and has not been rigorously established. It is certain, however, that the extract does act as an absorber of light, thereby delaying the rate of lignin degradation. Wood species with high levels of extractives exhibit more pronounced initial discoloration after UV irradiation because the extractives themselves contain chromogenic groups that absorb UV light and undergo degradation [45]. However, the presence of extractives can reduce the extent of wood discoloration during long-term exposure [29,47,256]. Furthermore, the extract has a protective effect on the surface properties of the wood. When comparing wood with extractives removed and wood with extractives retained under the same weathering conditions, it was observed that wood with retained extractives tended to exhibit better surface properties [239].

Nzoko et al. [46] suggested that the discoloration of wood during weathering is partly associated with the type and content of extractives, while Diouf et al. [43] proposed that the stability of wood color during weathering is not directly related to its overall extractive content. Some other researchers have suggested that certain components in the extract may have a stronger photoprotective effect compared to the extracted content. Chang et al. [253] isolated and studied flavonoids extracted from the heartwood of Acacia confusa, and they found that okanin and melanoxetin exhibited excellent UV-absorbing abilities. In addition, the action effectively disrupts the excited lignin, further preventing the photodegradation of the intact lignin. Peng et al. [257] found that tannins can act as UV absorbers and free radical scavengers during photodegradation, in addition to their hydrophobic nature and ability to reduce the water absorption of wood.

4.3. Photoprotective Mechanism of Extracts

Under high levels of UV irradiation, plants produce photoprotective secondary metabolites such as phenolic acids, condensed tannins, and flavonoids to mitigate the induced damage [258,259,260]. Phenolic extractives in wood can react with singlet oxygen and free radicals generated during the photo-oxidation of lignin, so these phenolic extractives are photo-oxidized and lignin remains protected [261]. In addition, Chang et al. suggested that the extract is oxidized before the lignin during wood photo-oxidation to form oxidized derivatives. For example, flavonols are oxidized to B-ring o-quinone intermediates, peroxides, and other oxidation derivatives via catechol metal interactions after UV irradiation, which indirectly leads to a reduction in the oxidation of lignin [48,262]. The extracts have been shown to possess efficacies, including UVA absorptivity, a capacity for single-linear oxygen burst, and phenoxy radical scavenging efficacy [263].

During photochemical reactions in wood, lignin absorbs most of the UV light to produce phenoxy radicals and lignin radicals. In addition to lignin, cellulose absorbs 5%–20% of UV light [248], resulting in the formation of hydrogen peroxide radicals and formyl radicals [249,264]. The chromogenic groups present in wood extracts have the ability to absorb specific wavelengths of UV radiation, thereby protecting the wood and mitigating the harmful effects of UV light on organisms [265,266]. Chang et al. [262] found that the treatment of Cunninghamia lanceolata sapwood powder with extractives of Acacia confusa heartwood extracts was effective in inhibiting the production of free radicals in wood under light. This is mainly because certain chemical components in the extractives, such as flavonoids, can reduce the formation of free radicals in wood, thereby inhibiting the photodegradation of the wood. Diouf et al. [43] used three methods to measure the antioxidant capacity of wood extracts and found that exposure to light preferentially depleted the most antioxidant extract compounds. It is known that the photodegradation of wood produces single-linear oxygen, which accelerates the process of wood photodegradation. However, flavonoids containing catechol or pyrogallol in the extract can either physically quench them or be chemically quenched by flavonoids that contain a C=C bond at the C₂-C₃ position of the C-ring [267].

In conclusion, when wood is exposed to UVA light, some of the substances in the extract can act as UVA absorbers to absorb the energy of the UVA light and reduce lignin-derived free radicals. However, even if lignin-derived free radicals are still formed, substances such as condensed tannins and flavonoids can act as scavengers of these free radicals to inhibit their formation. In addition, single-linear oxygen generated by photosensitization of oxygen during lignin photoreaction can also be quenched by specific compounds in the extract as single-linear oxygen quenching agents, thus ultimately reducing the photodegradation of lignin [49].

4.4. Applications and Challenges

Photochromic discoloration has become an important economic issue for the wood industry as the specification of wooden products is now more demanding. In addition, stricter environmental legislation requires the development of environmentally friendly clear coatings that minimize the use of chemicals, balancing aesthetics with environmental protection.

Previous studies have indicated that wood extracts have significant potential for application as natural light stabilizers [53,114,262,268]. Chang et al. [50] used Acacia heartwood extract as a light stabilizer in comparison with EV80 and EV93. The results showed that the heartwood extract treatment inhibited the increase in ΔE* value of wood after longer irradiation time, and the inhibitory effect was comparable to that of EV80. Peng et al. [269] added bark extract as a light stabilizer to wood flour/polypropylene composites. It was shown in a 1200 h accelerated UV-weathering experiment. The composites containing the bark extract show photostabilizing properties and successfully mitigate the photodegradation of the composites. By comparison with a commercial antioxidant, butylated hydroxytoluene, the extract exhibited excellent UV absorption and a similar free radical inhibition. In their study, Cetera et al. [270] found that chestnut (Castanea sativa Mill.) extracts from native and thermally modified wood could be used as a means of protecting poplar (Populus spp.) and spruce (Picea abies Karst.) from UV radiation. The authors also point out that if the surface modification carried out by the extracts is protected by a waterproof membrane. The photostabilizing effect of the extracts on light-colored wood species would be more promising. However, natural extracts have the disadvantage of being water-soluble and prone to leaching from the wood. Some researchers have added wood extracts to coatings to both prevent leaching of the extracts and enhance the color stability of the wood during artificial weathering [51,52,271,272,273,274]. Hsiao et al. [261] used the chelation of extracts with metals, which was able to alleviate the leaching of extracts without affecting their photostability. Chang et al. [114] used a renewable polymeric material, refined Oriental Lacquer (ROL), and Acacia heartwood extract as a protective coating for wood. The photodegradation phenomenon of the ROL film was reduced by the addition of the heartwood extract. Compared to untreated ROL films, the addition of a certain amount of heartwood extractive addition already showed noticeable lightfastness improvement and excellent film properties. Özgenç et al. [53,275,276] used bark extracts from several different trees and commercial UV absorbers for comparison. The results showed that some acrylic paint systems containing bark extract provided similar protection to those containing commercial UV absorbers under artificial weathering test conditions. Peng et al. [52] explored the photostabilization mechanism of adding wood extracts to organic coatings, as shown in Figure 4. The composite coating forms a barrier between UV light and the wood, which effectively protects the wood from photodiscoloration and photodegradation.

The application of this extract to wood as a light stabilizer demonstrates the potential to enhance the stability of wood under light exposure. However, this application is not without its challenges. Firstly, extractives can absorb light, which leads to severe initial discoloration problems (as explained earlier). In addition, different types of extractives absorb and respond to light to varying degrees, which complicates the prediction and control of wood discoloration. Future research could optimize the composition of extractives by removing those susceptible to discoloration while retaining those with light-stabilizing properties.

5. Staining of Extracts

As far as wood quality is concerned, color is a factor that has been studied and may influence its commercialization [277]. Wood products from virgin forests are favored in part because of their attractive grain and rich natural colors [278]. Overall, lighter-colored wood is associated with lower natural resistance and reduced market acceptance. Technologies are needed to enhance the value of these timbers by increasing their natural resistance and enriching their original color [279]. Wood extracts have the potential to be used as environmentally friendly dyes, and utilizing darker-colored wood extracts can dye neutral-colored fast-growing timber, giving it a similar chromaticity to that of slow-growing timber and enhancing its aesthetic appeal [280]. The dyeing of wood helps to make it uniform in color, highlights the grain, and enhances the aesthetic properties of wood [281]. However, since the introduction of synthetic dyes, many environmental challenges have arisen. To make matters worse, common wastewater treatment processes do not completely remove dyes from wastewater, thus magnifying the risk of environmental contamination [282,283]. In contrast, natural dyes are more biodegradable and less harmful to the environment [284]. Therefore, exploring and applying natural dye resources has become a worthy research direction in the future.

The color molecules in wood extracts have a variety of properties that give them the potential to be used as alternatives to synthetic dyes. The dyes made from sustainable, biodegradable and UV-absorbing wood extracts are more attractive for wood dyeing applications [285,286]. The barks of Tectona grandis, Pterocarpus santalinus, Pinus pinaster, and Dalbergia cohinchinensis have been used for wood staining due to their good UV resistance and staining properties [55,268]. Kumar et al. [60] used Pterocarpus santalinus bark extracts to stain Hevea brasiliensis (rubberwood) and showed that when applied to fast-growing timber rubberwood, Pterocarpus santalinus bark extracts could give the fast-growing timber a tone comparable to that of Pterocarpus santalinus timber, increasing the value of fast-growing timber from these plantations. Brocco et al. [287] stained teak sapwood and pine sapwood with extracts of teak heartwood and found that the teak sapwood stained very closely to the color of the teak heartwood, and pine sapwood stained to a brownish-yellow color that accentuated the brownish hue of pine. This staining effect resulted in a smaller difference between the color of the tested timber and the 20-year-old teak heartwood, which could reduce the undesirable characteristics associated with light-colored timber in the market. Bi et al. [278] used extracts of heat-treated larch (Larix gmelinii (Rupr.) Kuzen.) to stain poplar (Populus tomentosa Carr.). The laccase-catalyzed grafting was used to improve the color stability. The extracts of grafted poplar showed satisfactory color stability under various environmental conditions. However, bio-based dyes are limited by poor color fastness and a constrained palette of shades. These limitations can be addressed through the use of mordants, which enhance the binding of natural dyes to the object being dyed, giving it excellent wash fastness [288]. Chang et al. [289] combined Acacia confusa heartwood extracts with metal ions as a natural stain for treating cedar sapwood. The results showed that the combination of heartwood extract with CuCl₂ and FeCl₂ resulted in chelated structures, leading to red-brown and dark brown appearance of the wood, respectively. Notably, samples treated with CuCl₂ and FeCl₂ mordant heartwood extracts exhibited excellent heat and water color fastness.

The coloring properties of natural extracts are also commonly used in textiles. The active molecules contained within them have the potential to impart a wide range of functional properties to textiles, such as antimicrobial, UV protection, and aromatic properties. Tsouka et al. [290] evaluated the dyeing properties of an aqueous extract of Alnus glutinosa leaves on cotton and wool fiber. The results of the study showed that the samples dyed with this natural dye exhibited high lightfastness, with a performance comparable to that of synthetic dyes, such as reducing dyes or metal complex dyes. In addition, the dyed samples showed excellent protection against ultraviolet radiation. Dhanania et al. [291] used gallnut (Quercus infectoria) (as a bio-mordant) and an aqueous extract of babul bark (Acacia nilotica) as natural dyes for dyeing cotton fabrics. The color components present in the gallnut and bark extracts gave a medium to dark brown color to the cotton fabrics. In addition, the preferential absorption of UV light by gallic acid in gallnut and bark indicated good color fastness and UV protection of the dyed cotton fabrics. Inprasit et al. [292] successfully extracted two functional compounds from neem (Azadiracta indica), a dye from the bark and an antimicrobial agent from the leaves. These extracts were applied to the dye treatment of hemp fabrics. The results showed that the treated fabrics exhibited significant antimicrobial activity against Staphylococcus aureus and that this antimicrobial property remained effective after up to 15 washing cycles. Islam et al. [293] utilized Mahogany sawdust extract as a natural dye for dyeing cotton fabrics, and the dyeing effect and fabric durability were significantly improved by metal mordant.

One question we need to consider is whether natural dyes are better. Although natural dyes have been in use since centuries, there are very few ecotoxicological studies available. Although natural dyes are often considered to be more environmentally friendly, it does not necessarily mean that they are less toxic than synthetic dyes [294]. However, more ecotoxicological studies are needed to support the safety and sustainability of natural dyes.

At present, the use of wood extracts for coloring is still at an exploratory stage. Firstly, there is the issue of color stability, as extracts absorb UV light and leach out through rainwater, causing color changes that can affect aesthetics and durability. Secondly, there is the limitation of the dye shade. Compared to chemical dyes, wood extracts are generally lighter in color depth and more difficult to blend to achieve the desired shade, meaning that even if a large amount of extract is used, the desired dyeing effect may not be achieved. However, these challenges are also being gradually overcome, and the molecular structure of extractives can be altered by modifying them to improve their stability in water. Combining different types of wood extract may be able to produce more vibrant shades. Despite the drawbacks of using extractives as natural colorants in practice, they still offer an environmentally friendly and natural alternative to coloring wood.

6. Conclusions and Prospect

Wood extractives, as an important and integral part of the wood, exhibit a wide range of chemical diversity and functionality through various solvent extraction methods. These extractives not only contain a wide range of biologically active components but also have the potential to significantly impact wood properties. In terms of decay resistance, it contains specific compounds that effectively resist microbial attacks and slow down the decay process. When used as a preservative, the extractives provide an environmentally friendly solution that reduces the use of chemical preservatives and is more beneficial to both the environment and human health. The extracts are also notable for their performance in light stabilization. Wood extractives can be used as light stabilizers to absorb UV light and reduce the degradation of key wood components, thus slowing down the effects of photodegradation on wood. Natural coloring also shows great potential. Dyeing wood with extractives gives light wood a rich color and preserves its natural texture, meeting both aesthetic and environmental needs.

The uses of wood extracts do not stop there. These extracts contain natural bioactive compounds that give them a range of desirable properties. They are also used in various fields, as shown in Figure 5, such as pharmaceuticals [295], cosmetics [296], the food industry [297], the light industry [298,299], and environmental treatment [300,301]. Our goal is the development and utilization of wood resources. Through the synergistic development of forestry and industry, basic chemical molecules will be produced from renewable, plant-based chemicals [302]. The mobilization of extractable resources will be achieved more quickly if the forest chemical industry is actively developed by the many players in the forestry and wood-related industries.

In the future, as research on wood extractives continues to mature and technology advances, we have reason to believe that they will play an even broader and more important role in the areas of wood protection, processing, and landscaping. At the same time, further development and exploitation of the potential of wood extractives will also contribute new strength to the sustainable use of wood resources and environmental protection.