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Article

Bark Beetle-Attacked and Wind-Damaged Norway Spruce (Picea abies (L.) Karst.) Trees as a Potential Raw Material for Extractives Recovery

by
Vanja Štolcer
1,2,
Ida Poljanšek
1,
Viljem Vek
1,* and
Primož Oven
1,*
1
Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
2
Kompetenzzentrum Holz GmbH, Altenberger Straße 69, 4040 Linz, Austria
*
Authors to whom correspondence should be addressed.
Forests 2026, 17(2), 183; https://doi.org/10.3390/f17020183
Submission received: 23 December 2025 / Revised: 23 January 2026 / Accepted: 27 January 2026 / Published: 29 January 2026
(This article belongs to the Special Issue Integrated Forest Products Biorefinery Perspectives)
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Abstract

Bark beetle infestations and other natural disturbances have increasingly affected Norway spruce (Picea abies (L.) Karst.) forests across Europe resulting in devaluation and decreased applicability of woody biomass of such trees. The aim of this research was to investigate the extractive content of bark beetle-attacked and dead wind-damaged Norway spruce trees relative to healthy trees, in order to assess their potential for extractives recovery. After harvesting, three discs were dissected along the stem height of each tree, and samples of sapwood, heartwood, knots, and bark were collected. Sequential extraction of the samples was performed using cyclohexane and acetone–water mixture in an accelerated solvent extractor. Lipophilic and hydrophilic extractives were determined gravimetrically, while total phenols and proanthocyanidins were measured by UV–Vis spectrophotometry. Results showed that knotwood contained the highest amounts of hydrophilic extractives and total phenols among investigated tissues. Knots of healthy trees contained the highest amount of hydrophilic extractives (52.4% w w−1), while knots of dead wind-damaged trees contained significantly higher content of total phenols (8.8% w w−1). The total phenols in bark beetle-attacked and healthy trees were 7.1% w w−1 and 7.2% w w−1, respectively. The sapwood and heartwood of dead wind-damaged trees had higher content of hydrophilic extractives (3.4% and 2.3% w w−1) than healthy and bark beetle-attacked trees. Bark from healthy trees contained more total phenols (2.7% w w−1) than bark of bark beetle-attacked trees, while proanthocyanidin contents in bark were comparable among three groups of trees. Our findings revealed that woody biomass from bark beetle-attacked and dead wind-damaged Norway spruce trees contains significant levels of phenolics, indicating high potential for extracting valuable compounds in biorefineries.

1. Introduction

Numerous harmful agents are causing damage to forest ecosystems globally, which can lead to the loss of entire forests or the destruction of individual trees [1]. In many regions of the world, the health and vitality of forests are being negatively impacted by the acceleration of climate change [2]. In recent years, climate change has increased the frequency and intensity of natural disasters such as wildfires, large-scale storms, heat waves, and droughts in European forest [3,4] resulting in an average of 43.8 million m3 of disturbed timber volume per year over the 70-year study period [5]. Disturbances have been responsible for 16% of average annual harvest in Europe over the past 20 years. Wind was responsible for 46% of all damage, followed by fire (24%), and bark beetles (17%) [5]. In Slovenia, 2.09 million m3 of trees were harvested in forests due to natural disasters in 2024, most of it due to wind (42%) and 35% due to bark beetles (735.369 m3) [6]. Warmer winters and drier summers benefit the spruce bark beetle, while decreasing spruce resistance. The bark beetle (Ips typographus (L.)), a member of the Scolytinae family, is the most devastating pest of Norway spruce. Although it is small, ranging from two to seven millimeters, it can cause multiple stages of infestation under spruce bark [7]. Depending on the health of the tree, bark beetle infestations can be divided into three stages: (1) the green stage occurs when the tree is attacked by bark beetles and the tree crowns remain green without any indications of infection, (2) the red stage is when a tree’s needles turn from green to yellow and red-brown, and they begin to fall to the forest floor, which indicates that tree is dying, and lastly, (3) the grey stage occurs years after the initial bark beetle attack, when the tree has lost all the needles, and the grey bark will remain at crown level [7,8,9]. Beetle attacks also bring blue-stain fungi, such as Endoconidiophora polonica, into the trees [7,10], which can contribute to the mortality of the attacked trees [11].
Due to natural disasters in recent years (ice storm in 2014, and windstorm in 2017) [12], Slovenian forests contained massive volumes of undervalued woody biomass and damaged fallen or standing trees that were underutilized and subject to degradation [13]. This woody biomass is an important functional and structural component of forest ecosystems, as well as a part of carbon storage and biodiversity protection [14,15]. Due to climate change, the need for sustainable development, resource preservation and the high costs of raw materials, the most complete and optimal use of woody biomass residues and side material flows of the forest-wood chain are becoming increasingly important [16]. The majority of economic implications of a bark beetle outbreak on wood markets are the result of massive salvage and sanitation harvesting activities, which initially increase both wood supply and temporary employment, processing activities, and wood exports. However, markets may eventually become flooded with wood as market participants attempt to liquidate both beetle-attacked and healthy wood in anticipation of future outbreak spread or lower wood prices [17]. Economic consequences include decreased commercial value of attacked trees and higher management costs [8]. The theoretical amount of low-quality wood in Slovenian forests that can be annually harvested and could enter the market is around 1,450,000 tons of dry matter per year, while the assessment of the actual market potentials amounts to only 32% and is hence considered as underutilized woody biomass [18]. Because bark beetle-attacked logging accounts for most of the tradable softwood in Europe, understanding the quality of the damaged wood is crucial for determining its potential uses [2]. Structural and physical–mechanical properties are relevant for engineering applications, whereas chemical properties of bark beetle-attacked Norway spruce woody biomass appear relevant for biorefinery processing.
The most obvious physical change of wood in Norway spruce bark beetle-attacked trees is blue or gray coloration of sapwood often referred to as blue-stained wood caused by the spread of ophiostomatoid fungi in the sapwood [19] whereas the color of heartwood is usually not altered. In the review on structural, physical and mechanical properties of wood in Norway spruce trees attacked by bark beetles, Hýsek et al. [2] stated that wood-decay fungi colonizing wood after bark beetle infestation can lead to negative changes in the structure and properties, reducing modulus elasticity, modulus of rupture, weight loss over time and changes in moisture content. In some examples, reducing energy consumption for producing wood particles from bark beetle-attacked trees or increasing surface free energy due to staining fungi can also improve paint or adhesive adhesion [2]. Kržišnik et al. [19] reported that the mechanical properties of bark beetle-attacked Norway spruce wood, severely colonized with blue-stain fungi, were almost unaffected, whereby the bending and compression strength, and sorption properties of attacked spruce wood were comparable to the properties of reference spruce wood. Performance of blue-stained wood against wood-decay fungi and water was reduced, which resulted in a reduced material resistance [19]. Löwe et al. [20] reported that tensile and compressive strength of Norway spruce attacked by the spruce bark beetle were affected both by the time the trees were left standing in the forest and the distance of the wood sample from the center of the trunk [20]. This research concluded that tested mechanical properties of heartwood from bark beetle-attacked trees without the presence of molds and fungi are not compromised and that this wood can also be used as a construction material [20]. Künniger et al. [21] found that spruce harvested 2–3 months after the bark beetle attack showed a decrease in surface elastic recovery after indentation during the Brinell hardness test, as well as a reduction in impact bending strength [21]. It was suggested that these results are a consequence of the incipient decay caused by wood-degrading fungi in addition to the already present sap-staining fungi in the sapwood of bark beetle-attacked spruce trees standing for a longer time in the forest [21].
Only few studies highlighted the chemistry of individual trunk tissues of bark beetle- attacked Norway spruce trees. Konôpková et al. [22] investigated the differences in biochemical composition of the wood, among healthy Siberian spruce trees (Picea obovata) and trees attacked by bark beetle across five experimental plots. Based on the use of the Soxhlet apparatus, using hot water to extract water-soluble holocellulose and lignin, and an alcohol-toluene solution to extract resinous compounds, it was demonstrated that bark beetle infestation had no significant influence on the content of holocellulose and lignin and, that non-attacked trees had a greater tannin concentration than attacked ones [22]. Salou-Quineche et al. [23] investigated the chemical composition of sapwood and bark of Norway spruce attacked by bark beetles and blue-stain fungi by using Soxhlet extraction with acetone as a solvent. The results showed that sapwood and bark from the spruce trees attacked by bark beetle and blue-stain fungi contained lower amounts of resin acids, triglycerides, steryl esters and fatty acids. Sterols were significantly higher in the sapwood affected by blue-stain fungi, and 10% lower in the affected bark. They also reported that sapwood from bark beetle-attacked spruce trees had greater carbohydrate:lignin (C:L) ratios compared to sapwood from non-attacked trees [23].
Literature review [22,23] revealed only fragmentary information on the extractive content in different trunk tissues of Norway spruce trees attacked by bark beetles. Data on content of extractives in heartwood and knots, which are proved to be tissues with the highest content of valuable bioactive polyphenolics in conifers [24,25,26,27,28,29,30,31,32], are practically missing for Norway spruce trees attacked with bark beetles. The aim of this research was therefore to study the content of lipophilic and hydrophilic extractives, total phenols, and proanthocyanidins in sapwood, heartwood, knots, and bark of Norway spruce attacked by the bark beetles. In addition to this group of trees, Norway spruce trees felled in windstorms and laid on the forest floor for nearly five years were included in the research due to absence on information of extractives content as well. For this purpose, three groups of Norway spruce trees were selected: healthy trees, bark beetle-attacked trees, and trees damaged in windstorm. Individual tissues were sampled and extracted, and extractive content was examined gravimetrically and spectrophotometrically in order to valorize the biorefining potential of underutilized Norway spruce woody biomass.

2. Materials and Methods

2.1. Trees Selection and Material Sampling

Three different groups of Norway spruce trees were included in our study. The first group was represented by six healthy spruce trees and the second group was six trees of Norway spruce attacked by the bark beetle. These Norway spruce trees were harvested in the forest area of Plešivška Kopa in Slovenia (altitude 1362 m) in September 2021 (Figure 1a). The average height of selected standing trees was 29 m, with an average diameter at breast height of 43 cm. All healthy trees appeared vital, had intact bark without visible insect entrance holes and their crowns showed no signs of drying or needle discoloration. Trees attacked with spruce bark beetles had green and partially yellow-red colored needles, with one tree having no needles. The bark was partially present, with visible entrance holes and galleries beneath it. From each tree, three discs were dissected along the height of the tree; first at the base of the tree (0 m), second at 8 m and third at 16 m. Two additional discs containing knotwood were taken from the crowns of the harvested trees (Figure 1b). The third group of the woody biomass was represented by dead Norway spruce trees, sampled at the forest area of Plešivec, Slovenia (altitude 1245 m), which was damaged in the windbreak on 11th and 12th December 2017. We selected three dead wind-damaged trees lying on the forest floor from which three discs containing knotwood were dissected in July 2022. Cross-sections of disks revealed that sapwood was soft and moist, while the heartwood was still partially firm and soft at some places, and the bark was locally present.
All dissected discs have been processed in the same way. First, they were air-dried for four weeks. Discs taken in 2021 were air-dried at an average temperature (T) of 15 °C and the relative humidity (RH) of 83%, while discs taken in 2022 were air-dried at an average temperature of 24 °C and the relative humidity of 79%. From each air-dried disc, one sample of sapwood and heartwood was taken with a battery drill using an 8 mm diameter drill bit and stored in a cold and dark space in labeled sealed plastic bottles. The discs containing knots were cut longitudinally in order to obtain knotwood samples with a drilling technique. Bark was collected already in the field from trees attacked by bark beetles and from dead wind-damaged trees. Bark of healthy trees was removed from dissected discs. The bark from all three groups was then separately grounded to the same size as the wood samples. The samples were dried for 24 h in a vacuum dryer at a temperature of 60 °C.

2.2. Accelerated Solvent Extraction

The extraction of the samples of softwood (SW), heartwood (HW), knotwood (K) and bark (B) from all discs was performed in an accelerated solvent extraction (ASE 350) apparatus (Thermo Scientific Dionex, Waltham, MA, USA) at temperature 100 °C and pressure 103.42 bar as previously described [24,33,34]. An amount of 1 g of each dried sample was placed into a 10 mL stainless steel extraction cell, which was sealed with filter paper, a metal frit and a plug screw. The samples were sequentially extracted, first with cyclohexane (Carlo Erba Reagents S.A.S, Val de Reuil, France) to remove lipophilic compounds, and then with an acetone/water mixture (95/5, v/v) (Avantor Performance Materials B.V., Deventer, the Netherlands). All samples were extracted with two 5 min static cycles [34]. The content of lipophilic and hydrophilic extractives was determined gravimetrically by drying 10 mL of each extract to a constant mass (mg/g dw) [35].

2.3. Spectrofotometric Analysis

The content of total phenols and proanthocyanidins was measured calorimetrically with Lambda UV–Vis spectrophotometer (Perkin-Elmer, Walthan, MA, USA). The content of total phenols was determined according to a protocol already described [36,37]. Amounts of 2 N Folin–Ciocalteu phenol reagent (aq) and an aqueous solution of sodium carbonate (75 g/L) were added to each wood extract. The incubation was performed in 4.5 mL disposable macro cuvettes, which were closed with a 10 × 10 mm polyethylene lid [35]. After 2 h of incubation, the absorbances were measured at a wavelength of 765 nm, where the content of total phenols was expressed as gallic acid equivalents per mass of dry wood. The proanthocyanidin content in the bark samples was measured using the vanillin assay as described by Hrovatič et al. [35], with absorbance measured at 500 nm [37,38]. The content of proanthocyanidins was expressed in (+)-catechin equivalents per mass of dry wood [35].

2.4. Statistics

For the statistical analysis of the extractive content we used Statgraphics 19 software (Statgraphics Technologies Inc., USA). The values were first evaluated for normal distribution, and the analysis of variance (ANOVA) and Multiple Range Test using Fisher’s least significant difference (LSD) procedure at a 95% confidence level were performed.

3. Results

3.1. Extractives in Sapwood

The average content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in sapwood among woody biomass groups differed statistically significantly (ANOVA, p < 0.0001) as seen in Figure 2. The content of hydrophilic extractives in the sapwood of healthy and dead wind-damaged trees was significantly higher than that of lipophilic extractives. There were no statistically significant differences between the average contents of lipophilic and hydrophilic extractives in the sapwood of bark beetle-attacked trees. Dead wind-damaged woody biomass (D) contained the highest average amount of hydrophilic extractives (33.60 mg/g), and the content was significantly different from the content of hydrophilic extractives in the sapwood of healthy trees (H) and bark beetle-attacked trees (A), between which there were no significant differences. The average content of lipophilic extractives in sapwood was not significantly different among three groups of Norway spruce trees.

3.2. Extractives in Heartwood

Figure 3 shows statistically significant differences (ANOVA, p < 0.0001) in the average content of lipophilic and hydrophilic extractives in the heartwood among the analyzed woody biomass groups. The heartwood of dead wind-damaged woody biomass contained an average of 23.43 mg/g of hydrophilic extractives, which was significantly higher than that in the heartwood of healthy Norway spruce trees and bark beetle-attacked trees, where the contents were comparable. The average content of hydrophilic extractives in the heartwood was significantly higher than that of lipophilic extractives in healthy and dead wind-damaged trees, respectively. There were no significant differences between the content of lipophilic and hydrophilic extractives in the heartwood of bark beetle-attacked trees.

3.3. Extractives in Knotwood

Our results indicated a statistically significant difference (ANOVA, p < 0.0001) in the average content of lipophilic and hydrophilic extractives in knotwood among the studied woody biomass groups (Figure 4). The average content of hydrophilic extractives was significantly higher than the content of lipophilic extractives within each woody biomass group. The average content of hydrophilic extractives in knots of healthy trees was 524.30 mg/g, which was statistically significantly higher than the content of hydrophilic extractives in knots of bark beetle-attacked trees. The content of hydrophilic extractives in knots of dead wind-damaged woody biomass was comparable to that in knots of healthy trees and bark beetle-attacked trees. In comparison to hydrophilic extractives, the content of lipophilic extractives was essentially lower (20.02–39.70 mg/g) in knots of three groups of Norway spruce trees.

3.4. Extractives in Bark

The results of extractive content in the bark of healthy trees, bark beetle-attacked trees, and dead wind-damaged trees (Figure 5) showed no statistically significant differences between these biomass groups (ANOVA, p > 0.05). Nevertheless, analysis showed a significant difference in the content of lipophilic (51.28 mg/g) and hydrophilic (105.82 mg/g) extractives in the bark of healthy trees.

3.5. Content of Total Phenols and Proanthocyanidins

Table 1 shows the average quantities of total phenols and proanthocyanidins in different tissues of analyzed woody biomass groups. Results showed that there were statistically significant differences in the total phenol contents among and within the analyzed groups (ANOVA, p < 0.0001). The highest average of total phenolic content in analyzed Norway spruce biomass groups was in knots, which was significantly higher than the total phenol contents in sapwood, heartwood, and bark, respectively (Table 1). Knots of dead wind-damaged woody biomass contained 87.53 mg/g of total phenols, which was significantly the highest content, while the contents of total phenols in knots of healthy trees and bark beetle-attacked trees were comparable. The bark of healthy trees had 27.21 mg/g of total phenols, and the content was significantly higher than that in the bark of bark beetle-attacked trees and the bark of dead wind-damaged woody biomass, where the total phenolic contents did not differ significantly from each other. The bark of healthy trees also had a significantly higher content of total phenols than the sapwood and heartwood. The bark of bark beetle-attacked trees and trees of dead wind-damaged woody biomass contained comparable quantities of total phenols to those in sapwood and heartwood. Sapwood and heartwood contained the lowest amount of total phenols; nevertheless, there were no significant differences in the content of total phenols among the analyzed groups and between the groups. The average content of proanthocyanidins in the bark of three Norway spruce groups was comparable.

4. Discussion

The main focus of our research was the analysis of extractive content in particular tissues of Norway spruce trees attacked by the spruce bark beetle, as well as in dead spruce trees left on the forest floor for nearly five years after a windstorm, whereas healthy trees served as a reference. Our results showed a higher amount of hydrophilic extractives compared to lipophilic ones in all investigated tissues of healthy Norway spruce, which is consistent with previous findings [28,30,34,39]. In our study, the sapwood of healthy trees contained an average of 2.3% (w w−1) of hydrophilic extractives, the heartwood 1.2% (w w−1), while knots contained the highest amount of hydrophilic extractives (52.4% w w−1). Willför et al. [30] investigated extractive content in different spruce species and reported that the sapwood of healthy Norway spruce contained 0.25% to 0.52% (w w−1) of hydrophilic extractives, the heartwood 0.40% to 2.6% (w w−1), while knots of living branches contained 17% to 21% (w w−1), and dead knots 14% to 17% (w w−1) of hydrophilic extractives [30]. Lignans and oligolignans were identified as the main components of hydrophilic extractives in spruce knots [34]. The concentration of lignans in spruce sapwood was negligible, while the spruce heartwood contained up to 0.5% [39].
The sapwood of healthy trees in our study contained an average of 1.4% (w w−1) lipophilic extractives, the heartwood 0.5% (w w−1), while knots contained 4% (w w−1) of lipophilic extractives. Willför et al. [34] reported that Norway spruce sapwood contained relatively small amounts of native and modified resin acids (0.6–3.9 mg/g and 0.05–0.18 mg/g), diterpenyl alcohols (0.1–0.3 mg/g), and sterols (0.8–1.5 mg/g), while the amount of fatty acids was higher (4.6 mg/g to 10.9 mg/g) [34], corresponding to 0.6%–1.7% of lipophilic extractives. In contrast, heartwood was shown to contain 0.3%–0.7% of lipophilic extractives, represented by 0.8–2.4 mg/g of native and 0.09–0.13 mg/g of modified resin acids, 0.1–0.2 mg/g of diterpenyl alcohols, 0.7–1.2 mg/g of sterols, and 1.2 mg/g to 2.8 mg/g of fatty acids [34]. The amount of lipophilic extractives in most spruce knots was similar to that in stem heartwood, whereas stem sapwood contained larger amounts of esterified fatty acids than knots [34]. Nisula [39] reported that spruce sapwood contained 0.06% to 0.2% resin acids, heartwood 0.05% to 0.2%, living knots 0.02% to 0.05% and dead knots 0.02% to 0.4% [39]. The content and composition of examined groups of extractives in sapwood, heartwood and knots of healthy trees reflects their structural and physiological function in the living tree. Sapwood is “the portion of the wood that in the living tree contains living cells and reserve materials (e.g., starch)”, heartwood denotes “the inner layers of wood which, in the growing tree, have ceased to contain living cells and in which the reserve materials (e.g., starch) have been removed or converted into heartwood substances [40], and a knot “is a branch base that is embedded in the wood of a tree trunk, whereas organization of the branch is similar to that in the main stem [41]. The summative content of extractives in Norway spruce sapwood is 1.7%–2.7%, in heartwood 1.1–1.8, and in knots 6.8%–13.7% as reviewed by Routa et al. [42]. A relative high content of extractives in knots could be interpreted as protection against the ingress of pathogens in case of branch breakage [43,44].
The results of the extractives content indicated specific differences among the tissues of bark beetle-attacked Norway spruce trees. The sapwood of bark beetle-attacked trees contained an average of 1.5% (w w−1) of hydrophilic extractives, the heartwood 1.3% (w w−1), while knots contained 42.2% (w w−1) of hydrophilic extractives. The content of hydrophilic extractives in the heartwood of bark beetle-attacked trees was comparable to that of healthy trees. The heartwood is usually not attacked by the bark beetle and blue-stain fungi, as it contains extractives that protect against microbial attack. Knots of bark beetle-attacked trees contained a lower average amount of hydrophilic extractives (42.2% w w−1) than the knots of healthy trees (52.4% w w−1), whereas the content of lipophilic extractives was essentially lower (2.0% w w−1).
The extractive content in tissues of dead wind-damaged woody biomass showed that the sapwood contained an average of 3.4% (w w−1) of hydrophilic extractives, the heartwood 2.3% (w w−1), while the knots contained an average of 44.4% (w w−1) of hydrophilic extractives. Both the sapwood and heartwood of dead wind-damaged woody biomass contained higher amounts of hydrophilic extractives compared to corresponding tissues of healthy trees, which could be ascribed to the different stages of decay, during which degradation of wood polysaccharides and lignin may have occurred [45,46,47]. The average contents of hydrophilic and lipophilic extractives in knots of dead wind-damaged trees were comparable to those in healthy trees. Willför et al. [34] reported that even dead knots of living trees contained large amounts of resin acids, close to 2% (w w−1), as well as high levels of free diterpenyl alcohols [34]. These findings may explain the comparable contents of lipophilic extractives observed in knots of dead wind-damaged and healthy Norway spruce trees in our investigation.
Bark denotes all tissues outside the vascular cambium of trees, shrubs, or lianas, which fulfills among others the transport and (re)distribution of photosynthates and signaling molecules, mechanical support, and protection against abiotic and biotic factors [48]. Bark is very rich in all groups of extractives, with the highest content of condensed tannins and polysaccharides, especially in red pine and spruce [49]. Our study showed that the bark of bark beetle-attacked trees contained an average of 10.4% (w w−1) hydrophilic extractives, while the bark of dead wind-damaged woody biomass contained 5.7% (w w−1) of hydrophilic extractives. These values did not differ significantly from those in the bark of healthy trees (10.06% w w−1), which is consistent with previously reported content of hydrophilic extractives [26]. Krogell et al. [50] indicated that Norway spruce inner bark contained 166 mg/g of hydrophilic extractives, while the outer bark contained 45.7 mg/g of hydrophilic extractives [50]. On the other hand, Halmemies et al. [51] reported that fresh bark of Norway spruce contained 29.9% of hydrophilic extractives [51]. The average content of lipophilic extractives in the bark of bark beetle-attacked trees was 8.3% (w w−1), comparable to that in the bark of healthy trees. According to the literature [50], lipophilic extractives in Norway spruce were more abundant in the outer bark (23.5 mg/g of dry bark) than in the inner bark (12.0 mg/g of dry bark) [50]. Burčová et al. [52] reported that Norway spruce bark obtained as waste during debarking contained mainly resin acids (2.8%–31.7%), fatty acids (4.8%–5.3%), terpenes (1.6%–2.7%), and stilbenes (0.8%–1.0%) [52]. Previous studies on Norway spruce bark storage have shown that extractive content decreased most rapidly during the initial weeks of storage [51,53,54,55]. Halmemies et al. [51,54] observed a dramatic decline in hydrophilic extractives and a more gradual decrease in lipophilic extractives [51,54]. Our results are not consistent with these findings, as the bark of dead wind-damaged trees lying on the forest floor for five years contained comparable amounts of extractives as healthy bark, probably due to longer exposure of woody biomass to weather conditions and different biological activity in the forest environment.
Recent studies have shown that knots of conifers are an exceptionally rich source of bioactive polyphenolic compounds [25,26,27,28,30,32,34,56,57,58], whereby most investigations have focused on healthy trees. Research on Scots pine woody biomass—including broken tree parts, wood residues, and knots—has demonstrated that this underutilized woody biomass can also serve as an important source of phenolic extractives [56,59]. Our study showed that knots of bark beetle-attacked trees and knots of dead wind-damaged trees left on the forest floor for five years contained high amounts of total phenols. The content of total phenols in knots of healthy and bark beetle-attacked trees was comparable (7.2% and 7.1% w w−1), while knots of dead wind-damaged trees contained a significantly higher amount (8.8% w w−1) of total phenols. Norway spruce contains up to 30% of polyphenols in knots, which have antioxidative and antibacterial properties [26,57]. Our results confirm these findings. Willför et al. [34] similarly reported that lignans occur in especially high concentrations in Norway spruce knots compared to the stemwood [34]. On the other hand, our results showed that the bark of bark beetle-attacked and dead wind-damaged trees contained significantly lower amounts of total phenols (1.1% and 1.0% w w−1) than the bark of healthy trees (2.7% w w−1). Strizincova et al. [60] reported that total phenolic content in spruce bark varied between 19.14 and 29.48 mg GAE per 1 mL of bark extract, while Spinelli et al. [61] found values between 0.77 ± 0.02 and 54.97 ± 2.00 mg GAEs/g dw, depending on the extraction method used [61]. Norway spruce bark extracts contain stilbenes, high concentrations of several stilbene glucosides and their aglycons, lignans, flavonoids, and tannins [26,62]. The content of proanthocyanidins in bark was low in all investigated groups, with no significant difference: 0.81% (w w−1) in bark beetle-attacked trees, 0.95% (w w−1) in dead wind-damaged trees, and 0.91% (w w−1) in healthy trees. Piccand et al. [49] reported that spruce bark contained 3.6% of proanthocyanidins [49]. Our results on extractive content in the bark of bark beetle-attacked and in dead wind-damaged trees could be partially compared with studies on the effects of Norway spruce biomass storage on extractive content [51,53,54,55]. These studies show that a certain content of extractive compounds—especially phenolics—is still retained after prolonged storage and exposure to the weather conditions, which could coincide with our results on the content of extractives in the bark of bark beetle-attacked trees and dead wind-damaged woody biomass of Norway spruce that was left in the forest for five years.

5. Conclusions

The investigation of extractive content in the bark beetle-attacked and dead wind-damaged Norway spruce showed that this undervalued biomass contains notable amounts of extractives, particularly phenolics in knots. The contents of total phenols in the knots of healthy and bark beetle-attacked trees were comparable, whereas knots from dead wind-damaged trees contained significantly higher amounts of total phenols as a result of the wood degradation process. Although the study was limited by a relatively small number of sampled trees, particularly of dead wind-damaged woody biomass, the findings indicate that low-quality woody biomass from Norway spruce trees damaged by natural disturbances and bark beetle attacks has significant potential for extracting valuable compounds. To fully evaluate the potential of this specific Norway spruce feedstock, future research should include trees from different sites and altitudes, with more repeated measurements per sample. Ensuring similar tree heights across all categories of woody biomass and analyzing a large number of knots would further strengthen the study. In the case of bark beetle-attacked trees, more sampled trees in different stages of attack would improve the robustness of the study. To further characterize the chemical composition of extractives, we plan to perform TLC, HPLC, and GC-MS analyses, and future studies with more detailed tissues-specific analyses are warranted to confirm these results. Despite these constraints, our findings show that woody biomass from bark beetle-attacked and dead wind-damaged Norway spruce trees contains significant quantities of phenolics, indicating a high potential for extracting valuable compounds. To demonstrate how laboratory-scale results translate to a more realistic scenario, we have assessed the quantities of total phenols that could be obtained from extracting knots of bark beetle-attacked trees under the following assumption. If 733,162 m3 of wood was felled in Slovenia due to bark beetle attack in 2024, and assuming that the volume of knots in conifer stems is approximately 0.5%–2% [63], with only 5% of these knots available for extraction, the amount of total phenols produced annually would be 10.5–42 t. The usability of extracts obtained from tree tissues, especially polyphenolics, is versatile; they can be used as ingredients in functional foods and cosmetics and have great potential as antioxidants in technical products or as natural biocides and fungicides, replacing synthetic alternatives [64]. Valorization of woody biomass streams originating from damage caused by natural disturbances can be integrated into the wood processing industry or more advanced biorefinery industrial systems. Production of extractives should rely on existing facilities, such as water-based tannin extraction, where available, or on new biorefineries [65], whereas recovery of extractives should precede fractionation of the main wood components.

Author Contributions

Conceptualization, P.O., V.V., I.P. and V.Š.; methodology, V.V., V.Š. and I.P.; software, V.V.; validation, V.Š., V.V. and P.O.; formal analysis, V.Š.; investigation, V.Š., V.V. and P.O.; resources, V.Š., V.V. and P.O.; data curation, V.Š.; writing—original draft preparation, V.Š.; writing—review and editing, V.Š., I.P., V.V. and P.O.; visualization, V.V. and V.Š.; supervision, P.O. and V.V. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the Slovenian Research and Innovation Agency (ARIS) and Ars Pharmae d.o.o. for funding project L4-2623 (ArsAlbi), as well as ARIS for supporting research program P4-0015 (Wood and lignocellulosic composites). The authors also acknowledge the University of Ljubljana for supporting M-Era.Net project BaPur and project CRP V4-2512 Simulation and optimization of potential valorization routes for lower quality wood in Slovenia, financially supported by ARIS and the Ministry of Economy, Tourism, and Sport of the Republic of Slovenia.

Data Availability Statement

The data presented in this study are available from the corresponding author upon request. The data are not available publicly as they are from a project for obtaining specific research results.

Acknowledgments

V.Š. is acknowledged to the Interdisciplinary Doctoral Study Program Biosciences, scientific field Wood and Biocomposites, at the University of Ljubljana, Biotechnical Faculty. We would like to thank the Slovenia Forest Service, especially Avgust Kunc and Franci Breg from the Department of Slovenia Forest Service in Slovenj Gradec, for providing the trees for this research, and for organizing the harvesting. Many thanks also to their employees for carrying out the harvesting and for their assistance with the sampling in the field. Peter Hrovatič and Anže Lopatič are acknowledged for help in the laboratory.

Conflicts of Interest

Author Vanja Štolcer is employed by the company Kompetenzzentrum Holz GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors declare that this study received funding from Ars Pharmae d.o.o. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Forests 17 00183 g001
Figure 1. (a) The location of the study side, (b) trees of Norway spruce attacked by the bark beetle after harvesting, (c) discs taken from the attacked trees, (d) samples before the analysis, and (e) samples before the UV–Vis spectrophotometry.
Figure 1. (a) The location of the study side, (b) trees of Norway spruce attacked by the bark beetle after harvesting, (c) discs taken from the attacked trees, (d) samples before the analysis, and (e) samples before the UV–Vis spectrophotometry.
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Figure 2. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the sapwood of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a–c) indicate statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
Figure 2. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the sapwood of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a–c) indicate statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
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Figure 3. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the heartwood of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a–c) indicate the statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
Figure 3. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the heartwood of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a–c) indicate the statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
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Figure 4. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the knotwood of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a–c) indicate the statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
Figure 4. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the knotwood of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a–c) indicate the statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
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Figure 5. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the bark of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a,b) indicate the statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
Figure 5. The content of lipophilic (Elip) and hydrophilic (Ehyd) extractives in the bark of Norway spruce: healthy trees (H), bark beetle-attacked trees (A) and dead wind-damaged trees (D). Different letters at the top of the error bars (a,b) indicate the statistically significant differences at a 95% confidence level (Fisher’s least significant difference test).
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Table 1. Average content of total phenols in individual tissues of three groups of Norway spruce woody biomass, and proanthocyanidins in the bark samples. Results are expressed in milligrams of extracted sample per gram of dried wood (mg/g, dw). Healthy trees (H), trees attacked by the spruce bark beetle (A) and dead trees (D).
Table 1. Average content of total phenols in individual tissues of three groups of Norway spruce woody biomass, and proanthocyanidins in the bark samples. Results are expressed in milligrams of extracted sample per gram of dried wood (mg/g, dw). Healthy trees (H), trees attacked by the spruce bark beetle (A) and dead trees (D).
SampleTotal Phenols [mg/g]
HAD
Sapwood2.54 a
(2.62)		6.46 a
(6.71)		4.72 a
(6.35)
Heartwood8.94 a
(4.97)		4.07 a
(3.65)		0.96 a
(1.05)
Knotwood71.67 c,d
(19.03)		71.43 b,d
(16.73)		87.53 b,e
(12.65)
Bark27.21 b
(9.58)		10.65 a
(3.73)		10.27 a
(6.78)
SampleProanthocyanidins [mg/g]
HAD
Bark9.11 a
(7.55)		8.07 a
(7.88)		9.53 a
(7.96)
Note. There are corresponding standard deviations of average values in parentheses. Different letters within the same column (a–e) indicate statistically significant difference at a 95% confidence level (Fisher’s least significant difference test).
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MDPI and ACS Style

Štolcer, V.; Poljanšek, I.; Vek, V.; Oven, P. Bark Beetle-Attacked and Wind-Damaged Norway Spruce (Picea abies (L.) Karst.) Trees as a Potential Raw Material for Extractives Recovery. Forests 2026, 17, 183. https://doi.org/10.3390/f17020183

AMA Style

Štolcer V, Poljanšek I, Vek V, Oven P. Bark Beetle-Attacked and Wind-Damaged Norway Spruce (Picea abies (L.) Karst.) Trees as a Potential Raw Material for Extractives Recovery. Forests. 2026; 17(2):183. https://doi.org/10.3390/f17020183

Chicago/Turabian Style

Štolcer, Vanja, Ida Poljanšek, Viljem Vek, and Primož Oven. 2026. "Bark Beetle-Attacked and Wind-Damaged Norway Spruce (Picea abies (L.) Karst.) Trees as a Potential Raw Material for Extractives Recovery" Forests 17, no. 2: 183. https://doi.org/10.3390/f17020183

APA Style

Štolcer, V., Poljanšek, I., Vek, V., & Oven, P. (2026). Bark Beetle-Attacked and Wind-Damaged Norway Spruce (Picea abies (L.) Karst.) Trees as a Potential Raw Material for Extractives Recovery. Forests, 17(2), 183. https://doi.org/10.3390/f17020183

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