The Habitats of European Oak (Quercus) in Poland and General Oak Wood Color Issues

Abstract

Oak wood color plays a critical role in veneer production, where visual consistency directly affects material value. However, production choices are still often based on experience rather than systematic scientific data. Although many studies have examined individual factors affecting wood color, such as species or drying conditions, few have brought together ecological and industrial perspectives. This review addresses that gap by examining how habitat, species characteristics, and processing parameters influence color variation in Quercus robur and Quercus petraea. A structured literature review was conducted using Web of Science, Scopus, and Google Scholar, complemented by industry observations. The results show that site-specific factors—such as soil type, forest type, and regional climate—can significantly affect oak wood color, in some cases more than genetic differences. Drying methods, wood age, and log storage also contribute to variations in color and homogeneity. These findings highlight the potential for better raw material selection and processing strategies, leading to improved quality, sustainability, and economic efficiency in veneer production. Remaining knowledge gaps—particularly in predictive modeling and veneer-specific studies—point to important areas for future research.

1. Introduction

Oak (Quercus spp.) is among the most economically and ecologically important hardwood species in Europe, widely used for its strength, durability, and attractive appearance. In Poland and much of Central Europe, Quercus robur (pedunculate oak) and Quercus petraea (sessile oak) are the most commonly harvested species, supplying key raw materials to the veneer and furniture industries. Due to their anatomical similarities and shared commercial classification, these species are often sold as undifferentiated stock [1,2,3,4]. However, manufacturers frequently encounter significant variation in wood color, which presents challenges in achieving the visual consistency demanded by end users. While processing techniques such as thermal modification and log grading are employed to manage color variation, the environmental and biological factors driving these differences remain poorly understood. As a result, production decisions often rely more on experience rather than on systematic scientific data as few studies have attempted to comprehensively connect harvesting conditions (such as habitat) with wood characteristics like color or practical applications [5,6].

From an industrial perspective, natural veneer products require high and repeatable aesthetic standards. Color is a particularly critical attribute in product classification and market valuation. While the origin of the raw material—especially forest site conditions—plays a substantial role in determining wood color, production processes such as boiling, drying, and cutting also contribute. These treatments are typically optimized for improving physical properties and dimensional stability, not for controlling color outcomes [7,8,9].

Even minor inconsistencies in color may lead to product downgrading or increased material loss during veneer production. A deeper understanding of pre-harvest factors—particularly habitat and site-specific variables—could enable producers to make more informed sourcing decisions and reduce waste. Natural compounds such as flavonoids and tannins, which influence wood coloration, vary with growing conditions and site chemistry. These differences are relevant not only to researchers but also to manufacturers, who aim to optimize visual outcomes, expected by customers, based on ecological input parameters [10,11,12]. Moreover, recent studies show that internal wood characteristics such as density and porosity—particularly around anatomical features like knots—also affect surface properties. For example, tensile wood, which forms in tension areas and is recognizable by its lighter color, denser cell walls, and glossy surface, has been associated with smoother veneer textures around knots. This correlation suggests that even subtle structural variations within a tree, shaped by mechanical stress or growth dynamics, may influence both surface quality and color uniformity of veneers. Recognizing and anticipating these features could support more targeted veneer selection and minimize sorting-related losses [13].

Natural variation in oak wood color is shaped by a complex interplay of physiological traits, environmental conditions, and chemical processes. While species identity may contribute to some extent, current evidence indicates that site-specific growing conditions—such as soil quality, moisture availability, and microclimate—along with external factors like light exposure, play a more decisive role. For example, Scalbert et al. [14] characterized the polyphenolic profile of Quercus robur, identifying key extractives involved in wood coloration. In contrast, Janin et al. [15] found that variation in wood color and ellagitannin content across oak populations was more strongly influenced by habitat and soil conditions than by genetic differences, highlighting the dominant role of environmental factors. Since heartwood coloration typically develops after 10 to 20 years, evaluating long-term genetic effects remains a challenge and would require extended observation in controlled trials [16].

Environmental influences, particularly light exposure, also play a critical role in wood color variation over time. Exposure to ultraviolet (UV) radiation has been shown to cause gradual surface discoloration in oak wood through chemical changes that affect its natural compounds. Zahri et al. [17] demonstrated that such exposure can alter the wood’s visual appearance, potentially reducing its aesthetic and commercial value. These effects, often linked to natural weathering processes, highlight the importance of understanding how external conditions interact with wood chemistry throughout its lifecycle. In products where appearance is essential—such as veneer and decorative wood—these factors must be considered in both raw material selection and long-term performance expectations.

This review was developed through close collaboration between forestry researchers and veneer industry professionals. It synthesizes current scientific knowledge and industrial observations to identify the key ecological, physiological, and technological factors influencing oak wood coloration. Findings are categorized into thematic areas: forest habitat types and site conditions, species-specific traits of Quercus robur and Quercus petraea, abiotic factors, biotic influences and industrial/technological processes (e.g., thermal modification, drying, storage methods).

2. Materials and Methods

This review was developed through a structured collaboration between forest scientists and veneer industry professionals, aiming to connect ecological research with real-world industrial applications. The joint objective was to investigate how forest habitat conditions, species distribution, and environmental variables influence the color characteristics of oak wood (Quercus robur and Quercus petraea), particularly in the context of veneer manufacturing where visual consistency and material grading are critical for product quality and market value.

A systematic literature review was conducted using three major scientific databases: Web of Science [18], Scopus [19], and Google Scholar [20]. The search focused on peer-reviewed publications, forestry research papers, forest inventory reports, and industry-relevant technical documents addressing wood color variation, habitat influences, and processing impacts. The study was designed to reflect both ecological dynamics and practical challenges faced by veneer producers in sourcing and processing oak wood.

To ensure both scientific rigor and industrial relevance, the research process followed these key steps:

• Collaborative scope definition: The review framework was defined jointly by forestry researchers and veneer manufacturers. This ensured alignment between scientific investigation and industry needs, such as improving raw material classification, minimizing waste due to color mismatch, and enhancing surface appearance in high-end veneer products.
• Literature screening and selection: Sources were screened to exclude duplicates, non-peer-reviewed material, and studies not relevant to forest habitat, oak species characteristics, or wood color. Special attention was given to studies focused on Polish and Central European forest conditions, which reflect the primary sourcing regions for many veneer producers.
• Categorization into thematic areas: The selected literature was grouped into five primary themes, developed with direct input from industry experts:
  • Forest habitat types and site conditions;
  • Species-specific traits of Quercus robur and Quercus petraea;
  • Abiotic factors (e.g., soil, climate, light exposure);
  • Biotic influences (e.g., fungi, pathogens, insects);
  • Industrial and technological processes (e.g., thermal modification, drying, storage methods).
• Integration of forest management data: National datasets from the Polish Forest Data Bank, State Forests, and Forest Management Plans were incorporated to contextualize habitat variation, forest typology, and timber availability. These ecological data were cross-referenced with veneer production practices to identify key factors influencing raw material consistency.
• Industry validation and application: Findings were reviewed internally and double checked in veneer industry environment (Balti Spoon and Lubelski Fornir) to evaluate real-world implications for material sorting, color grading, and production efficiency. This input helped refine the interpretation of ecological data in relation to challenges encountered during procurement and processing, including seasonal variability, storage practices, and end-user expectations for surface appearance.

This integrated methodology—grounded in forest science and informed by industrial experience—provides a practical framework for understanding oak wood color variability. It also contributes to more sustainable and predictable veneer production strategies by aligning ecological parameters with manufacturing standards and market demands.

No statistical analysis was conducted by the authors, as this review compiles and synthesizes findings from existing literature and industry input rather than presenting original experimental data.

3. Results and Discussion

The following section integrates findings from the scientific literature with practical insights from the veneer industry. Where applicable, we indicate whether the information is literature-based or derived from professional experience.

3.1. Oak Wood Habitats in Poland

Pedunculate oak and sessile oak show no significant differences in growth rate or wood quality. Quercus robur and Quercus petraea are frequently treated together in ecological and forestry contexts due to their high morphological similarity, overlapping distributions, and shared silvicultural value. Natural hybridization between the two species is common, producing individuals with intermediate traits that complicate field identification. Both occupy similar ecological niches across temperate Europe, often co-occurring in mixed deciduous forests. They exhibit comparable growth habits, timber properties, and vulnerabilities to pests and diseases, including acute oak decline. These shared characteristics justify integrated management and conservation strategies [1]. Studies have shown that these species do have habitat-specific differences. The studies by Konatowska et al. [21] analyzes how site conditions—such as soil type, precipitation, temperature, and growing season length—affect the growth and distribution of Quercus robur (pedunculate oak) and Quercus petraea (sessile oak) in Poland, Q. robur prefers more fertile, moist sites, and Q. petraea favors drier, sandy soils. Importantly, under similar conditions, both species show comparable growth rates, and climate warming may benefit Q. petraea more than Q. robur [22,23].

Kowalkowski, referencing Kreutzer [24], identifies soil as one of the four main factors that define forest habitat quality, along with climate, biological influences, and topography. Pedunculate oak prefers fertile and well-drained soils, as it is megatrophic; in some sources, it is called a eutrophic species [25]. Sessile oak, on the other hand, accepts xeric (very dry)-to-moist conditions. It is classified by researchers as mesotrophic—that is, accepting moderately fertile habitat conditions. It is more tolerant to drought and poor soil than Pedunculate oak, but more sensitive to airless soil conditions. Optimal habitat conditions for both are fine-grained, sandy rusty brown soils and proper rusty soils. Both species can be found from northern Spain to southern Scandinavia and from Ireland to Eastern Europe. The pedunculate oak reaches the Urals. The natural range of the sessile oak overlaps with that of the pedunculate oak, but the eastern limit is Ukraine (Figure 1 and Figure 2) [25,26,27,28].

When outlining potential assumptions and research goals related to the connection between wood color, its origin, and potential applications, a specific regional classification of oak wood distribution in Central Europe was considered, particularly focusing on areas within Poland (through, for example, wood submissions) [29,30].

In Poland, pedunculate oak (Quercus robur) dominates over sessile oak (Quercus petraea). In practical applications, both species are combined, which is partially consistent with EN 13556 standard [2] and other nations [3,4]. European sessile and pedunculate oak are listed under entry 1.216, with the code QCXE and the botanical names Quercus petraea (Matt.) Liebl., Quercus robur L. Also, in the sale of raw material, the State Forests do not distinguish between pedunculate oak (Quercus robur) and sessile oak (Quercus petrea), combining both, in Quercus spp.

On the other hand, in the aforementioned standard (EN 13556) [2], this term refers to other species of oak [31]-white, red and Japanese (items 1.217, 1.218, 1.219, respectively):

-

Quercus spp. including Q. alba L and other species;

-

Quercus spp. including Q. rubra L. and other species;

-

Quercus spp., especially Q. mongolica Fisch. ex Turcz. var. Grosseserrata Rehd. and Wils.

The resources of European oak in standing timber were estimated to average around 6.9% in 2023 (Figure 3 and Figure 4).

The optimal age for oak wood, ideal for producing the highest quality material, is at least 80 years old [31]. In Poland, the average age of oak stands in forests managed by the State Forests is 62 years, while those under private management are 55 years old [33]. This suggests that sustainable forest management practices are in place, and there is a positive outlook for future raw material availability, assuming there are no major legislative restrictions on access. According to the Forest Management Plans, the recommended cutting age for oak trees varies between 120 to 160 years, depending on the forest district [34].

The development of oak wood’s structural properties (including color) is strongly influenced by the age of the tree and the ecological conditions in which it grows. Trees that reach maturity in well-balanced soil and moisture regimes tend to yield timber of superior technical quality, including higher density, durability, and workability—characteristics essential for high-grade wood products. Given that the current average age of oak stands in Poland remains significantly below the recommended felling age, much of the harvested material may not yet exhibit the full range of these desirable attributes. The practice of extending the rotation period to between 120 and 160 years, as prescribed in forest management plans, is therefore critical not only for ensuring long-term timber supply but also for optimizing wood quality at the time of harvest.

The importance of wood color is especially relevant for the veneer industry. Since veneers are thin slices of wood used for visible surfaces in furniture and interior finishes, uniformity in color and appearance is essential. Regional and site-based factors—such as soil type, moisture levels, and forest habitat class—can influence the natural color of oak wood, as observed in the differences between sessile and pedunculate oak across various forest types. Understanding how these environmental variables affect wood color can help veneer producers select and sort raw material more effectively, improving product consistency and reducing waste during manufacturing.

According to the Forest Data Bank [34,35], the largest oak-dominated forest areas in Poland are found in fresh forests (Ff) and fresh mixed forests (Fmf). Smaller oak populations also occur in upland (Uf), moist (Mf), mixed moist (Mmf), fresh mixed coniferous (Fmcf), and riparian (Rf) forests. Sessile oak tends to prefer fresh mixed (Fmf) and eutrophic (Fe) habitats, while pedunculate oak is more commonly found in eutrophic forests with varying moisture, such as Ff, Mf, and Rf. Both species grow best in fine-grained soils, though sandy rusty brown soils and typical rusty soils are also considered suitable. Habitat and soil variation in these mixed forests significantly influences wood quality and should be considered in material sourcing for veneer production. Habitat and soil variability in mixed forests—where oak often grows alongside species like pine, spruce, and fir—have also been explored in detail by Lasota et al. [36,37,38,39].

3.2. Forest Management and Changes in Oak Wood

Habitat factors like climate, biotic influences, soil conditions, and topography can greatly affect tree growth. When these factors change drastically, they can even lead to the death of entire forest areas. Extreme weather events can directly harm species and also create conditions that make trees more susceptible to biotic factors like pests and diseases. On the other hand, biotic factors can weaken trees, making them more prone to secondary pest attacks. This weakened state, where a tree’s natural defenses are no longer strong enough to fend off pathogens, is called physiological stress [40]. As Oszako [41] explains, oak trees usually die for three reasons: defoliation (caused by insect feeding, pathogens, or frost), frost damage, and drought. Kuźminski et al. [42] provide a more in-depth analysis of the causes and symptoms of oak dieback.

These ecological stressors underscore the importance of proactive, site-specific forest planning. While habitat factors like soil, topography, and climate directly influence tree growth and wood quality, their sudden disruption—through drought, pests, or frost—can severely damage forest stands, as seen in the oak dieback events reported in Krotoszyn [43]. This has clear consequences for industries dependent on high-grade timber. By incorporating environmental variability into Forest Management Plans (FMPs), forest districts can better anticipate risks, maintain forest health, and secure the long-term availability of veneer-quality oak. In this way, ecological planning and economic sustainability are closely intertwined.

In Poland, every state-managed forest district operates under a Forest Management Plan (FMP), which accounts for topography, soil type, water availability, and regional forest classifications [24,44,45,46,47]. These environmental factors not only guide sustainable forest use—they also influence the quality of the wood.

For oak, conditions like slope position, moisture balance, and altitude affect growth rate, wood density, and color. These traits are especially important to the veneer industry, which relies on slow-grown, straight oaks with uniform grain and consistent tone.

By integrating ecological assessments with long-term planning, FMP help ensure that oak forests remain both resilient ecosystems and reliable sources of high-grade timber. For veneer producers, that means a sustainable supply of wood that meets the demands of precision manufacturing and design.

As forest management practices continue to evolve, the European Union has introduced important conservation initiatives, including the Natura 2000 network and the EU Biodiversity Strategy for 2030, aimed at protecting critical habitats and enhancing biodiversity across member states. While these strategies are designed to preserve forest ecosystems, they also prompt concerns about their potential effects on active forest management and timber quality. Striking a balance between these conservation efforts and the need for sustainable forest management is essential to ensure that both ecological health and timber-dependent industries can prosper in the long term.

When we talk about protecting oak forests, we often focus on their ecological role—home to countless species, buffers against climate change, reservoirs of biodiversity. But there’s another side to these landscapes: the wood itself. How an oak grows—its soil, age, health, and growth rate—directly affects the color, grain, and quality of the timber it produces. Conservation, in this sense, is not just about nature. It is also about preserving the character and value of one of Europe’s most sought-after materials.

This matters deeply to the veneer industry. To produce high-end veneer—used in fine furniture, interiors, and architecture—manufacturers need tall, straight, slow-grown oaks. These trees thrive in healthy, diverse forests that are carefully managed over long cycles. Practices like selective thinning support both ecological goals and timber quality. But if policies lean too far toward strict non-intervention, the result can be faster, uneven growth and lower-grade wood—costly for both producers and end users.

The European Union has made major commitments to forest conservation, particularly through the Natura 2000 network and the EU Biodiversity Strategy for 2030 [48,49,50,51,52,53,54]. Natura 2000 spans all EU countries and protects oak-rich habitats of high conservation value. In Poland, detailed monitoring campaigns in 2006–2008 and 2015–2018 have provided essential data on forest condition, structure, and species composition [51,52,54].

The 2030 Strategy, part of the broader European Green Deal, aims to protect at least 30% of EU land and sea areas, with one-third under strict protection. It prioritizes restoration, ecological corridors, and safeguarding Europe’s remaining old-growth forests [53,54]. While generally welcomed, especially in Poland, the strategy also raises important questions. Reducing active forest management could lead to more pest outbreaks or altered forest dynamics—particularly under a changing climate [54].

For the veneer industry, the message is clear: sustainable, responsible forest management supports both nature and craft. Oak forests, when properly cared for, can continue to deliver ecological value and the fine raw material that has helped shape Europe’s cultural and design heritage for centuries.

3.3. Changes in Wood Coloration

Abiotic factors, unlike biotic ones, allow for greater control over the final appearance of wood. Treatments such as artificial weathering (using UV light, heat, and humidity) [55], thermal modification [56,57], and various chemical methods—including dyeing, bleaching, and greying techniques [58]—are commonly used to produce desired tones or surface textures. These controlled processes are vital in the veneer industry, where uniform coloration and predictable results are often required. The impact of both natural and artificial processes on wood color has been extensively studied. Exposure to external factors such as water, UV radiation, oxygen, air pollution, and temperature fluctuations extended of exposure time, can significantly affect the condition of the wood, leading to changes in its color and structure. Numerous studies conducted under controlled laboratory conditions, simulating natural aging processes, help to predict how wood materials will behave after many years of use. Research by Kropat et al. [55] explored how various methods of accelerating aging processes closely replicate natural conditions. The team also emphasized that the results of wood exposure to external factors should not always be viewed as degradation. Accelerating biochemical changes can have valuable applications in the industry, as it can enhance the aesthetic qualities of wood, making it more desirable [59]. Moreover, accelerating these changes can also contribute to color stabilization and uniformity, as noted by Béatrice George et al. [60], who highlighted the benefits of thermal or photochemical “pre-weathering”.

As Różanska et al. [58] highlight, accelerated “aging” methods are employed to achieve specific aesthetic or mechanical changes in wood. These methods are divided into mechanical techniques, which alter the surface structure—such as brushing, bleaching, chisel structuring, or rubbing—and chemical techniques, which involve the use of color-changing agents like graying, leaching, or smoking with ammonia. In their study, Różanska et al. [58] applied various aging treatments to Pinus sylvestris and Quercus spp. wood to observe changes in color, gloss, surface roughness, abrasiveness, and scratch resistance [60,61].

In the context of wood exposure, it is important to mention the process of photodegradation, which refers to the breakdown of chemical compounds when exposed to light, typically UV radiation. This process can lead to sun stains, color changes, and alterations in the mechanical properties of wood. Zahri et al. [17] studied the high levels of phenolic extractives in pedunculate and sessile oak, which are particularly susceptible to photodegradation and discoloration under UV exposure [17,59]. Photodegradation in oak was also examined by Tomak et al. [62], who found that, unlike exotic species, native oak species showed more significant changes in color and roughness of annual growth rings as a result of UV exposure. The researchers emphasized that the test results are influenced by many variables, including the species, radiation intensity, radiation source, sample size, heartwood-to-sapwood ratio, test duration, relative humidity, oxygen levels, and ambient temperature. These factors must be considered when analyzing wood’s response to UV exposure (

Table 1. Selected abiotic degradation factors for wood.

Abiotic	Physical	- Accelerated “weathering”—a laboratory method to predict the effects of natural weathering, but over a shorter period of time, involving monitoring and exposure to defined levels of UV light, humidity and temperature [55]. - Thermal modification (ThermoWood)— heating the wood to a specific temperature for a predetermined period of time. The wood color changes depend on the temperature, heating time and species [56,57].
	Chemical	- Dyeing—the most common paints are based on metal salts, such as copper, iron, chromium, nickel, manganese, cobalt and zinc (potassium dichromate, ferrous sulfide and ferric chloride); - Whitewashing with 15% hydrogen peroxide solution; optionally, with the addition of ammonia or 6%–10% oxalic acid solution; - Greying (a) with a solution obtained after soaking steel wool in apple cider vinegar for several days, or (b) with tannin, gallic acid, pyrocatechol or oak tannins; - Leaching with a soap emulsion; - Smoking with ammonia [58].
	Mechanical	- Brushing—structuring wood by rubbing a brush over the surface along the grain; - Sandblasting, glazing (glass beads blasting) or soda blasting; - Creating scratches, damage marks, imitating insect tunnels (with an axe hammer, nails, chisels or other tools); - Rubbing—applying two layers of paint and rubbing the outer layer until the bottom layer becomes visible with a dull knife or sandpaper; - Obtaining the effect of cracked acrylic paint by applying a layer of polyvinyl acetate glue on the first layer of paint; then, after the glue has partially dried, applying a second layer of paint—this layer cracks, along with the drying of the paint and glue [58].
Natural	Physical	- Natural weathering—when wood or wood products are exposed to external factors such as natural radiation, fluctuations in temperature and humidity, as well as interactions with fungi, bacteria, and insects, various changes can occur. These changes may include visible alterations like color shifts, roughness, cracks, nicks, and cavities, but they can also involve less apparent effects such as changes in strength or chemical composition [55]. Internal weathering can be considered a variation of these changes. - Pre-weathering—exposure to natural conditions (over a certain period of time), preceding its finishing; it can cause chemical and physical changes to the surface of wood-what can affect adhesion properties [63].

).

Biotic factors—such as fungi, bacteria, and insects—have been shown to play a significant role not only in the decomposition of wood but also in the changes observed in its color. As these organisms interact with wood through various biological and chemical processes, they often cause visible alterations, including staining and discoloration. Research presented in the following sections demonstrates a strong correlation between shifts in microbial and fungal communities and the resulting color transformations in wood, under both natural weathering and controlled laboratory conditions. These findings emphasize how biotic activity influences not only the structural integrity but also the visual appearance of wood.

Among the key contributors to these changes are naturally occurring bacteria and fungi (Table 2) [64]. This connection was a central focus of research conducted by Davor Kržišnik et al. [65], who were among the first to carry out comparative studies on the color changes in wood and wood products under laboratory-simulated outdoor conditions (artificial weathering). Their experiments involved exposure to blue-stain fungi such as Aureobasidium pullulans and Dothichiza pithyophila, as well as assessments under natural weathering conditions. While their results showed a high correlation between microbial presence and color changes, they also revealed that the patterns of discoloration observed under laboratory conditions could not be entirely predicted or generalized.

Several recent studies have focused on emerging biological threats to trees, particularly those caused by pathogenic bacteria linked to Acute Oak Decline (AOD) and root-damaging oomycetes. These organisms, often associated with stress and decline in hardwoods such as oak, have become increasingly prominent across Europe. In parallel, long-standing pressures from common insects and fungi continue to play a critical role in forest health. Within the framework of Manion’s disease spiral, these agents are classified as predisposing, inciting, or contributing factors in the decline process, further highlighting the need for interdisciplinary research and advanced diagnostic tools to monitor biotic-induced changes [66,67].

Among these changes, one of the most visible and commercially significant is the discoloration of wood. Mieszkin et al. [68] observed that bacterial communities in heartwood and phloem differ considerably from those in surrounding soil, both in diversity and composition. Functional testing revealed that these wood-dwelling bacteria often show low metabolic activity but a higher prevalence of cellulose-degrading capabilities, indicating a degree of specialization to the wood environment. Shigo [69] also noted this trend. Dominant bacterial phyla—such as Proteobacteria, Actinobacteria, Bacteroidetes, and Acidobacteria—have been consistently identified in decaying wood [63,70,71]. Notably, the composition of these microbial communities may vary by tree species [72,73] or decomposition stage [65], which in turn may influence the type, intensity, and pattern of wood discoloration.

Despite growing interest, the specific role of bacteria in visual degradation remains underexplored. Most existing knowledge is extrapolated from soil microbiology, where organisms such as Silvibacterium bohemicum (Acidobacteria), Streptomyces (Actinobacteria), and Burkholderia (Proteobacteria) are recognized for their ability to break down lignocellulosic compounds [74,75,76,77,78,79]. However, relatively few functional studies have been conducted on bacterial strains isolated directly from decaying wood [80,81,82]. This raises important questions about how microbial composition—and specifically bacterial metabolic activity—affects not only the breakdown of structural components but also the development of stains, blotches, and uneven coloration that impact the commercial value of wood products.

Biotic factors, particularly fungi and insects, are directly responsible for a wide range of surface color changes. Fungal species such as Aureobasidium pullulans and Dothichiza pithyophila are known for their blue-stain effects, while Aspergillus niger, Penicillium spp., and Trichoderma spp. produce black or green lesions under favorable moisture conditions [65]. These stains are not merely cosmetic defects; in industries such as veneer production, furniture manufacturing, and wood finishing, any deviation in color uniformity is often deemed unacceptable. Even minor microbial or insect-induced discoloration can render high-grade wood unsuitable for decorative applications [68,83]. Insects like Anobium punctatum (common furniture beetle) are also associated with surface damage and potential color change due to tunneling and microbial spread.

Table 2. Selected biotic degradation factors for wood.

Group of Factors	Name	Impact
Fungi	Aureobasidium pullulans	- They can colonize coniferous and hardwood trees, roundwood, lumber, finished wood and wood products, provided the moisture content is high enough; - Can manifest in blue lesions as with exposure to Aureobasidium pullulans and Dothichiza pithyophila (blue stain fungi), in black lesions as with Aspergillus Niger, or in green lesions as with Penicillium spp. and Trichoderma spp. [65].
	Dothichiza pithyophila
	Aspergillus Niger
	Penicillium spp.
	Trichoderma spp.
Bacteria	Brenneria goodwinii Gibbsiella quercinecans,	- The main cause of AOD, a phenomenon that is increasingly being documented in Europe–decline of oak trees [68].
Insects	Coleoptera (beetle)	- The most common consumer of xylem and phloem; - Considered to be a major pest of wooden heritage and libraries; - A total of 105 wood-feeding species identified; - Anobium punctatum is the most common beetle species in museums, libraries and cultural heritage sites [83].
	Blattodea (cockroaches and termites)	- In hot climate, termites, along with beetles, are considered to be a major pest that attacks wood; - Threats posed by termites are usually limited to tropical and subtropical regions, but are also found in temperate zones, including Europe, New Zealand and Republic of Korea; - The harmfulness of cockroaches is also noted, but these insects, according to studies, mainly attack starch paste, which is used to repair wood, rather than the wood itself [83].
	Hymenoptera (ants, bees, wasps)	- In Norway, in addition to beetles, Anobium punctatum and Hylotrupes bajulus, as well as two species of carpenter ants, Camponotus herculeanus (with widespread distribution) and C. ligniperdus (in the south of the country), are considered the main agents of damage the country’s cultural heritage; - Ants do not feed on wood—they make nests in wood decayed by fungi (secondary colonizers) and can spread to healthy wood; - Wasps and bees also attack wood—there are reports on wood wasps in Korea damaging the country’s cultural heritage [83].
	Diptera, Ephemeroptera, Lepidoptera	- Wood-destroying species can also be found in these families, although not much attention is given to them [83].

Table 2. Selected biotic degradation factors for wood.

Group of Factors	Name	Impact
Fungi	Aureobasidium pullulans	- They can colonize coniferous and hardwood trees, roundwood, lumber, finished wood and wood products, provided the moisture content is high enough; - Can manifest in blue lesions as with exposure to Aureobasidium pullulans and Dothichiza pithyophila (blue stain fungi), in black lesions as with Aspergillus Niger, or in green lesions as with Penicillium spp. and Trichoderma spp. [65].
	Dothichiza pithyophila
	Aspergillus Niger
	Penicillium spp.
	Trichoderma spp.
Bacteria	Brenneria goodwinii Gibbsiella quercinecans,	- The main cause of AOD, a phenomenon that is increasingly being documented in Europe–decline of oak trees [68].
Insects	Coleoptera (beetle)	- The most common consumer of xylem and phloem; - Considered to be a major pest of wooden heritage and libraries; - A total of 105 wood-feeding species identified; - Anobium punctatum is the most common beetle species in museums, libraries and cultural heritage sites [83].
	Blattodea (cockroaches and termites)	- In hot climate, termites, along with beetles, are considered to be a major pest that attacks wood; - Threats posed by termites are usually limited to tropical and subtropical regions, but are also found in temperate zones, including Europe, New Zealand and Republic of Korea; - The harmfulness of cockroaches is also noted, but these insects, according to studies, mainly attack starch paste, which is used to repair wood, rather than the wood itself [83].
	Hymenoptera (ants, bees, wasps)	- In Norway, in addition to beetles, Anobium punctatum and Hylotrupes bajulus, as well as two species of carpenter ants, Camponotus herculeanus (with widespread distribution) and C. ligniperdus (in the south of the country), are considered the main agents of damage the country’s cultural heritage; - Ants do not feed on wood—they make nests in wood decayed by fungi (secondary colonizers) and can spread to healthy wood; - Wasps and bees also attack wood—there are reports on wood wasps in Korea damaging the country’s cultural heritage [83].
	Diptera, Ephemeroptera, Lepidoptera	- Wood-destroying species can also be found in these families, although not much attention is given to them [83].

3.4. Production-Driven Factors Determining the Change of Wood Color

While biotic factors—such as fungi, bacteria, and insects—play a key role in wood discoloration through natural degradation, production-driven abiotic factors, though not previously addressed, also contribute significantly to unwanted color changes, particularly during the early stages of storage and processing. One important aspect to consider is the impact of secondary processes on wood color, especially those occurring in the logyard environment [84]. Tarotinsky [85] drew attention to the formation of sun stains—discolorations that develop when veneer oak logs are left unprotected or insufficiently shielded from environmental exposure. These stains, which became more pronounced from June to September, penetrated up to 50–100 mm into the wood from the log’s end surface. To reduce such effects, he emphasized the importance of proper log storage and efficient use of raw materials. Malaszewski [86] further recommended processing oak wood during the winter–spring period (up to the end of May) and, afterward, switching to species less sensitive to weathering. In response, modern production practices have evolved to minimize sun-related staining. These include winter felling, tight log stacking to reduce light and heat exposure, orienting logs away from direct sun, and sprinkling logs with water to cool them and stabilize temperature in the raw material yard (Figure 5). The image shows logs stacked tightly in a timber yard—a simple but effective practice widely used to protect wood from early discoloration, especially sun stains. When logs are arranged in dense, compact piles like this, much of their surface is shielded from direct sunlight, air movement, and temperature extremes. This helps reduce the risk of uneven drying, cracking, and surface staining caused by prolonged exposure to heat and light.

These kinds of storage methods are especially important for wood that is intended for high-value uses, like veneer or furniture, where natural color and appearance matter most. By keeping the logs cooler and limiting light exposure, producers can better preserve the wood’s original tone and prevent blotchy or streaked discoloration from forming too early in the process. In short, good stacking is not just practical—it is a key step in maintaining the quality and visual appeal of wood from the moment it is felled to the moment it is processed.

Next, the softening stage—often referred to (depending on context) as conditioning, steaming or boiling—is a key step in veneer production, helping to make logs more pliable and easier to process. However, this stage can also lead to unwanted black discoloration, often caused by iron ions reacting with natural components in the wood, such as tannins and phenolic acids [65]. These ions can come from the water used during the process, but also from the materials used in soaking pits, pipelines, or other equipment that come into contact with the logs. In some cases, similar dark stains can appear when the veneer comes into contact with a damaged peeling knife, especially if the blade chips while passing through dense wood or knots—leaving behind small metal fragments that create black specks on the surface [65,83]. Additionally, condensation from iron-rich steam can settle on the veneer and cause further discoloration if not properly managed [68]. Thankfully, these issues are preventable. One of the most effective ways to reduce staining is to thoroughly clean the logs before conditioning, especially when they are covered in mud or other iron-containing debris. The quality of process water should also be monitored, and if iron levels are high, treatment or filtration may be necessary. Using stainless steel or concrete for soaking pits and water pipes can help avoid contamination from corroding materials. Proper care and maintenance of peeling knives are also important, including regular checks for damage and ensuring blades are evenly heated. In practice, many producers have found that stainless steel knives, while more costly upfront, significantly lower the risk of staining and help maintain the visual quality of veneer products [65,83].

Numerous studies have established a connection between wood species, drying parameters, and changes in wood color. For instance, Bukara et al. [87] conducted research on color changes during the conventional drying of sessile and pedunculate oak (in essence, pedunculate oak showed faster drying but more discoloration than sessile oak). while Sadoth Sandoval-Torres [88] demonstrated the chemical processes that cause yellowing in pedunculate oak during vacuum drying and the effect of temperature on the brightness of the dried material. The study quantitatively showed that during plain vacuum drying of oak wood, lightness (L*) increased with temperature, especially between 46 °C and 80 °C, while antioxidant capacity significantly decreased. During plain vacuum drying of oak wood (Quercus pedunculata), increasing the temperature from 46 °C to 80 °C led to a noticeable increase in lightness (L* value rose by several units) and a hue shift from red toward yellow (h* angle reached around 80 °C).Other research has focused on the browning processes in oak wood like here by Charriet at al. [89].The study was done on pedunculate oak (Quercus robur L.), focusing on its heartwood–which contains a lot of natural compounds called ellagitannins. During drying at 30–60 °C with high wood and air moisture, the wood often turns brown. Researchers found that in the brown areas, there were lower amounts of ellagitannins and more large molecules formed from their breakdown. As ellagitannins are extractives responsible for color and UV resistance in oak, their breakdown directly contributes to browning and visual unevenness. This shows that heat and moisture cause ellagitannins to break down, which leads to the color change [90]. From practical side, in veneer production, this uneven browning can impact the classification of high-grade material, requiring additional sorting or surface correction, which increases production costs.

Using low-pressure drying can help prevent the browning or the relationship between color changes, the drying process, grain structure and the degree of deformation of oak lamellas The study showed that drying caused visible changes in the surface color of both beech (Fagus sylvatica L.) and oak (Quercus robur L.) wood, with beech being more affected. The color difference, measured using the ΔE* parameter, reached values as high as 11.5 in beech tangential samples with defects—indicating a highly perceptible change—while oak samples generally ranged between 4.4 and 7.3. These changes were more noticeable in samples with defects and in tangential sections, especially in beech wood. Oak wood showed less variation overall, with ΔE* values typically staying below 7.0, suggesting moderate perceptibility. However, the discoloration was confined to the surface layer and could be removed during machining, meaning the changes do not affect the final appearance of the finished wood products [90]. However, there is limited research on the color changes of veneer specifically during the drying process, although similar processes have been observed in thicker wood types like boards and lamellas.

Controlled application of heat (high temperatures) is widely used in the wood industry [56,57]. Temperatures above 150 °C influence color alteration but also change wood properties. Thermal processes below 100 °C, on the other hand, affect color change, but the process is slower. The range of 90–130 °C is only of theoretical importance. In industry, the temperature applied is between 160–260 °C because of the desired speed of the processes. Dry thermal treatment induces a color change in the wood throughout, without the use of external chemical agents. This process can induce color modification in cases of unattractive or inhomogeneous initial wood coloration. A well-known thermal modification method is Thermo Wood^® [91].

From professional experience, these studies make it clear that even relatively small changes in drying conditions—like temperature or moisture—can have a visible impact on oak wood color. The increase in brightness (L*) during vacuum drying, as shown by Sandoval-Torres, might be seen as an advantage in veneer production, where lighter tones and uniform surfaces are often preferred. But the color does not just get lighter—it also shifts in tone, moving from reddish to yellowish hues. Depending on the intended use, that could be a benefit or a problem.

At the same time, the browning observed in the study by Charrier et al. shows how sensitive oak is to the chemical changes that happen during drying. The breakdown of ellagitannins—a natural group of compounds in oak—can lead to permanent color changes. And while some of that surface darkening might be removed during finishing, deeper or uneven discoloration often means material has to be downgraded or rejected.

For veneer producers, this means drying is not just about removing moisture—it is a critical step that directly shapes the appearance and value of the final product. If the temperature is too low, the process is slow; too high, and the wood might turn too dark or lose its natural tone. That is why controlled thermal treatments, like ThermoWood^®, are sometimes used to intentionally change color. But these need to be balanced carefully, as they also affect strength and workability [90,91].

From an industry perspective, all of this points to one conclusion: to make the most of oak as a high-value material, we need to treat drying and thermal processing not just as technical steps, but as key tools for shaping color and consistency. Paying attention to color metrics like L*, a*, b*, and ΔE* can help producers better understand and manage these changes—before they show up as quality issues at the end of the line.

3.5. Correlation of Habitat with Wood Color

Few researchers have focused on the correlation between habitat and wood color, so this remains an area for further exploration. For example, Halalisan Aureliu-Florin et al. [92] studied the color variations of different oak species in Romania, but they did not consider specific regions of origin. They provided a clear link between oak origin and wood color in Romania: native species tend to have darker, richer heartwood with more red and yellow tones, while exotic oaks like Quercus robur and Quercus petraea are noticeably lighter and less vibrant, likely due to differences in wood chemistry or adaptation.

Similarly, Mosedale et al. [93] explored whether color variation in European oak is more influenced by genetics or the location where the wood was harvested. The research concluded that while genetics do influence wood properties in European oak—especially ellagitannin concentration and wood density—wood color is more strongly affected by the environment, particularly the site where the tree is grown. Site differences had a greater impact on color variation than species or genetic differences, suggesting that location plays a dominant role in determining wood color, even though there is some genetic influence.

The impact of wood origin on property changes has also been studied in other species. Katri Luostarinen [94] examined both environmental and internal factors affecting the color of birch during the drying process, analyzing variables such as origin (from two harvesting sites) and the influence of different drying methods on color changes (e.g., darkening). Birch wood tends to darken during drying, especially below 30% moisture content. Lightness (L) decreases, while redness (a) and yellowness (b) increase, with the most pronounced changes observed under high temperatures and vacuum drying. In contrast, room-temperature drying helps preserve a lighter color. Factors related to wood origin—such as winter felling, fertile sites, and extended log storage—also contribute to lighter final coloration. This darkening effect is linked to the oxidation and polymerization of proanthocyanidins under moderate moisture and elevated temperature.

Wood color is shaped more by environmental conditions than by genetics, though both play a role. Studies on Romanian and European oak [92,93] show that site-specific factors such as soil and climate have a stronger influence on color than species-level genetic differences. In birch, too, both origin and drying conditions affect final color, with lighter tones associated with winter felling, fertile sites, and room-temperature drying [94]. These findings underscore the need for further research into how habitat affects wood appearance. This relatively overlooked area holds significant potential—especially for industries like natural veneer production, where regional color variation could support more informed material selection and processing strategies [95,96,97,98]. Unlocking this knowledge could lead to meaningful improvements in both product quality and operational efficiency.

From a wood industry point of view, this group of studies underscores a critical yet underexplored point: where a tree grows can matter more than what species it is when it comes to wood color [99]. The observation that native oaks in Romania tend to produce darker, more saturated heartwood, while non-native species are lighter and less vibrant, suggests a strong interaction between environmental factors and wood chemistry.

The findings by Mosedale et al. [93] strengthen this view, showing that even though genetics—like ellagitannin levels—play a role, site conditions such as soil, microclimate, and moisture availability are stronger drivers of color variation. This has major implications for industries like veneer production, where visual homogeneity is key to material value. If environmental conditions can be linked to predictable color traits, sourcing decisions could become more data-driven. The birch study adds another layer: origin affects not just the raw color, but how wood responds during drying. Variables like winter felling or log storage time influence the final tone, especially under high-temperature or vacuum drying. This highlights how habitat and processing interact, and that understanding both is essential for managing color outcomes.

The assumption that wood species and genetics matter can be verified based on the research methods used [100]. Habitat-specific factors often provide manufacturers with more useful information, especially when trying to achieve uniform color. More targeted research allows for the proper selection of raw materials and processing strategies, which is especially valuable in high-end veneer markets where small differences in color can lead to significant economic differences.

4. Conclusions

Oak wood quality, particularly color, is strongly influenced by habitat and processing conditions—factors that are critical in veneer production where consistency and appearance are key. While natural variation adds value, uncontrolled differences can limit usability in high-end applications.

European species like Quercus robur and Quercus petraea, though often used interchangeably, may respond differently to site conditions and processing, affecting color traits. Yet, these differences remain underexplored in practical applications.

To improve sorting, grading, and processing outcomes, further research should focus on the following:

• Clarify habitat–color relationships using colorimetric and environmental data;
• Examine species-specific responses to drying and modification methods;
• Explore how genetics and site conditions jointly shape wood color;
• Validate findings in real-world veneer and furniture production.

Collaboration across forestry, wood science, and industry will be essential to optimize oak use and ensure high-quality, sustainable products.