*3.1. The Differences in the Spruce Wood and Bark in the View of Its Chemical Composition and a both Fibers Length and Width Distribution*

Wood is composed of cellulose, hemicelluloses, lignin, and extractives. From the results of the chemical analysis of spruce wood (Figures 2 and 3), we obtained the differences between the wood, as part of the trunk, and the top part of the tree. We compared the chemical composition of the trunk part in a height of 0.5 m and 4.5 m. We have taken samples (A, B, C) from the tree trunk, as shown Figure 1. The results indicate that there are differences in extractives content between A, B, and C in a height of 0.5 m. The biggest content of extractives, 1.68%, is in the part C (the last 25 years of growth), then A, 1.29%, and the last B, 1.04%. In a height of 4.5 m, there are no differences between the A and B part, and the amount of extractives in part C is comparable to the results in a height of 0.5 m. The amount of LIG, CEL and HEMI was very similar in a height of 0.5 and 4.5 m as well (Figure 2).

The analysis of chemical composition of wood and bark in different heights (Figure 3) exposed that the extractive content of both wood and bark is growing up with a height of the tree. The biggest extractives content of 32.35% was recorded in bark taken from the top part. Several authors mention the extractives amount of spruce bark from 23.5% to 28.3%, depending on the part of bark (inner bark from 17.3 to 38.7%, outer from 19.1 to 43.3%) [20–23]. The extractives content of the wood part is between 1% and 4.5% [24,25], while there is a difference between sapwood, values from 1.7% to 2.7% and heartwood, from 1.1% to 1.8% [26].

The amount of main chemical compounds: LIG (from 23.69 to 26.06%), CEL (from 39.01 to 42.51%), and HEMI (from 34.98 to 35.30%) were very similar in wood samples taken from the trunk (in a height from 0.5 to 4.5 m) compared to the top part. Sjostrom [27] mentions the lignin content of 27.5%, cellulose of 39.5 and hemicelluloses of 17.2% for Picea glauca; Wang et al. [28] showed a Klasson lignin of 28.2% for *Picea abies* (L.) H. Karst; Neiva et al. [29] showed a lignin content of 27.22%, polysaccharides of 71.18% for *Picea abies* (L.) H. Karst; Harris [30] showed a lignin from 28.0% to 30.8%, cellulose content from 38.1% to 40.3% for juvenile wood, and the lignin from 26.1% to 28.2%, cellulose from 40.2% to 42.7% for the mature wood of Norway spruce.

The biggest changes in chemical composition we noticed between bark trunk and bark top (Figure 3). As mentioned above, the extractive content was higher in the bark-top (32.35%) compared to the bark-trunk (22.16%), the lignin content was higher in bark-trunk (24.97%) compared to bark-top (15.46%), and the content of polysaccharides were similar (52.87% bark-trunk and 52.18% bark-top). Neiva et al. [29] determined the lignin between 26.86% and 29.92%, and polysaccharides content between 37.86% and 52.27%, depending on the bark fraction.

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**Figure 2.** Chemical composition (relative values) of spruce wood trunk at heights of 0.5 and 4.5 m. **Figure 2.** Chemical composition (relative values) of spruce wood trunk at heights of 0.5 and 4.5 m. **Figure 2.** Chemical composition (relative values) of spruce wood trunk at heights of 0.5 and 4.5 m.

**Figure 3.** Chemical composition (relative values) of spruce wood and bark at different heights. **Figure 3.** Chemical composition (relative values) of spruce wood and bark at different heights. **Figure 3.** Chemical composition (relative values) of spruce wood and bark at different heights.

Results of the Fiber Tester analysis (Figures 4 and 5) show that there are differences in both wood fibres length and width, mainly between samples A and B in a height of 0.5 and 4.5 m. The lower part of the trunk contains the highest amount of fines (>0.3 mm), from 45.94% (part B) to 52.38% (part A). The lowest content of long fibers (more than 1.01 mm) we determined in sample 0.5-A. There are also visible differences of fibers width in this sample (Figure 5). This part of the wood contains a higher amount of narrow fibers compared to sample 4.5-A. The average fiber length depends on the part of the trunk. The results show (Table 3) the smallest average fiber length and width in sample A, then B, and lastly C in a height of 0.5 m and 4.5 m as well. Both average fiber length and width are larger with the height of the tree. Results of the Fiber Tester analysis (Figures 4 and 5) show that there are differences in both wood fibres length and width, mainly between samples A and B in a height of 0.5 and 4.5 m. The lower part of the trunk contains the highest amount of fines (>0.3 mm), from 45.94% (part B) to 52.38% (part A). The lowest content of long fibers (more than 1.01 mm) we determined in sample 0.5-A. There are also visible differences of fibers width in this sample (Figure 5). This part of the wood contains a higher amount of narrow fibers compared to sample 4.5-A. The average fiber length depends on the part of the trunk. The results show (Table 3) the smallest average fiber length and width in sample A, then B, and lastly C in a height of 0.5 m and 4.5 m as well. Both average fiber length and width are larger with the height of the tree. Results of the Fiber Tester analysis (Figures 4 and 5) show that there are differences in both wood fibres length and width, mainly between samples A and B in a height of 0.5 and 4.5 m. The lower part of the trunk contains the highest amount of fines (>0.3 mm), from 45.94% (part B) to 52.38% (part A). The lowest content of long fibers (more than 1.01 mm) we determined in sample 0.5-A. There are also visible differences of fibers width in this sample (Figure 5). This part of the wood contains a higher amount of narrow fibers compared to sample 4.5-A. The average fiber length depends on the part of the trunk. The results show (Table 3) the smallest average fiber length and width in sample A, then B, and lastly C in a height of 0.5 m and 4.5 m as well. Both average fiber length and width are larger with the height of the tree.

**Figure 4.** Fibers length distribution of spruce wood trunk at heights of 0.5 and 4.5 m. Fibres length classes: >0.3; 0.31–0.5; 0.51–1.0; 1.01–2.0; 2.01–3.0; 3.01–6 mm*.*  **Figure 4.** Fibers length distribution of spruce wood trunk at heights of 0.5 and 4.5 m. Fibres length classes: >0.3; 0.31–0.5; 0.51–1.0; 1.01–2.0; 2.01–3.0; 3.01–6 mm. **Figure 4.** Fibers length distribution of spruce wood trunk at heights of 0.5 and 4.5 m. Fibres length classes: >0.3; 0.31–0.5; 0.51–1.0; 1.01–2.0; 2.01–3.0; 3.01–6 mm*.* 

**Figure 5.** Fibers width distribution of spruce wood trunk at heights of 0.5 and 4.5 m; (**a**) samples **Figure 5.** Fibers width distribution of spruce wood trunk at heights of 0.5 and 4.5 m; (**a**) samples **Figure 5.** Fibers width distribution of spruce wood trunk at heights of 0.5 and 4.5 m; (**a**) samples 0.5-A, 4.5-A; (**b**) samples 0.5-B, 4.5-B; (**c**) samples 0.5-C, 4.5-C.

0.5-A, 4.5-A; (**b**) samples 0.5-B, 4.5-B; (**c**) samples 0.5-C, 4.5-C. Wood from the trunk contains the highest amount of fines, approximately 50% (Figure 4) and its average length is 1.5 mm (Table 4). Tyrväinen [31] presents the average fiber length and its variation of Norway spruce trunk, namely inner heartwood 1.9 mm (1.28– 2.70 mm), middle zone 3.0 mm (1.69–3.88 mm), and outer sapwood 3.7 mm (2.80–4.29 mm). Harris [30] and Lönnberg et al. [32] mentioned the fiber width of Norway spruce as being between 15.0 and 28.5 µm in juvenile wood, and between 29.3 and 39.7 µm in ma-Wood from the trunk contains the highest amount of fines, approximately 50% (Figure 4) and its average length is 1.5 mm (Table 4). Tyrväinen [31] presents the average fiber length and its variation of Norway spruce trunk, namely inner heartwood 1.9 mm (1.28– 2.70 mm), middle zone 3.0 mm (1.69–3.88 mm), and outer sapwood 3.7 mm (2.80–4.29 mm). Harris [30] and Lönnberg et al. [32] mentioned the fiber width of Norway spruce as being between 15.0 and 28.5 µm in juvenile wood, and between 29.3 and 39.7 µm in ma-Wood from the trunk contains the highest amount of fines, approximately 50% (Figure 4) and its average length is 1.5 mm (Table 4). Tyrväinen [31] presents the average fiber length and its variation of Norway spruce trunk, namely inner heartwood 1.9 mm (1.28–2.70 mm), middle zone 3.0 mm (1.69–3.88 mm), and outer sapwood 3.7 mm (2.80–4.29 mm). Harris [30] and Lönnberg et al. [32] mentioned the fiber width of Norway spruce as being between 15.0 and 28.5 µm in juvenile wood, and between 29.3 and 39.7 µm in mature wood. According to our results (Figure 6), the biggest content of fraction (from

ture wood. According to our results (Figure 6), the biggest content of fraction (from 45%

0.5-A, 4.5-A; (**b**) samples 0.5-B, 4.5-B; (**c**) samples 0.5-C, 4.5-C.

45% wood-trunk to 60.85% wood-top) is in the width class from 15.1 to 30.0 µm and the average width of the wood trunk is 24.33 µm and bark 21.02 µm (Table 4). wood-trunk to 60.85% wood-top) is in the width class from 15.1 to 30.0 µm and the average width of the wood trunk is 24.33 µm and bark 21.02 µm (Table 4). wood-trunk to 60.85% wood-top) is in the width class from 15.1 to 30.0 µm and the aver-

The results of the bark analysis (Figures 6 and 7) show the differences mainly in fibre length distribution. The results of fibers width distribution were very similar and the largest proportion of fibers is in the class from 15.1 to 30 µm (67.53–69.17%). Bark from the trunk contains a high amount of fines (65.9%) and a lower amount of longer fibers compared to the bark from the top part of the tree. Bark contains a higher amount of shorter fibers to 0.5 mm (65.88% bark-top, 80.81% bark-trunk), than the wood part. The results of the bark analysis (Figures 6 and 7) show the differences mainly in fibre length distribution. The results of fibers width distribution were very similar and the largest proportion of fibers is in the class from 15.1 to 30 µm (67.53–69.17%). Bark from the trunk contains a high amount of fines (65.9%) and a lower amount of longer fibers compared to the bark from the top part of the tree. Bark contains a higher amount of shorter fibers to 0.5 mm (65.88% bark-top, 80.81% bark-trunk), than the wood part. age width of the wood trunk is 24.33 µm and bark 21.02 µm (Table 4). The results of the bark analysis (Figures 6 and 7) show the differences mainly in fibre length distribution. The results of fibers width distribution were very similar and the largest proportion of fibers is in the class from 15.1 to 30 µm (67.53–69.17%). Bark from the trunk contains a high amount of fines (65.9%) and a lower amount of longer fibers compared to the bark from the top part of the tree. Bark contains a higher amount of shorter fibers to 0.5 mm (65.88% bark-top, 80.81% bark-trunk), than the wood part.


**Table 3.** The average fibre length and width of wood samples.

**Table 3.** The average fibre length and width of wood samples.

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**Figure 6***.* Fibres width distribution of spruce wood and bark at different heights. Fibres width classes: 8.4–15.0 µm; 15.1–20.0 µm; 20.1–30.0 µm; 30.1–45.0 µm*.*  **Figure 6.** Fibres width distribution of spruce wood and bark at different heights. Fibres width classes: 8.4–15.0 µm; 15.1–20.0 µm; 20.1–30.0 µm; 30.1–45.0 µm. **Figure 6***.* Fibres width distribution of spruce wood and bark at different heights. Fibres width clas-

**Figure 7***.* Fibres length distribution of spruce wood and bark at different heights. **Figure 7***.* Fibres length distribution of spruce wood and bark at different heights. **Figure 7.** Fibres length distribution of spruce wood and bark at different heights.

*3.2. The Differences in the Spruce Wood and Bark after Storage* 

*3.2. The Differences in the Spruce Wood and Bark after Storage* 

ses: 8.4–15.0 µm; 15.1–20.0 µm; 20.1–30.0 µm; 30.1–45.0 µm*.* 

#### *3.2. The Differences in the Spruce Wood and Bark after Storage*

successively with increasing time [33–35].

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The amount of extractives is influenced by wood storage. Tree bark is a biological, heterogeneous material whose composition is changing. Immediately after tree harvesting, the amount of volatiles declined, and degradation continued during wood storage. The amount of extractives is influenced by wood storage. Tree bark is a biological, heterogeneous material whose composition is changing. Immediately after tree harvesting, the amount of volatiles declined, and degradation continued during wood storage. The amount of extractives is influenced by wood storage. Tree bark is a biological, heterogeneous material whose composition is changing. Immediately after tree harvest-

According to the results (Figures 8 and 9), it is visible that the biggest changes in the chemical composition can be obtained in bark (stored separately and as a part of the trunk). Figure 10 explains the results of chemical composition of wood trunk, while values are very comparable. Spruce wood retained very good durability in terms of its chemical composition after eight months of storage. Several authors studied wood with different natural aging time and the results found that the proportion of saccharides gradually decreases (mainly due to the hemicelluloses degradation), and the content of lignin increases successively with increasing time [33–35]. According to the results (Figures 8 and 9), it is visible that the biggest changes in the chemical composition can be obtained in bark (stored separately and as a part of the trunk). Figure 10 explains the results of chemical composition of wood trunk, while values are very comparable. Spruce wood retained very good durability in terms of its chemical composition after eight months of storage. Several authors studied wood with different natural aging time and the results found that the proportion of saccharides gradually decreases (mainly due to the hemicelluloses degradation), and the content of lignin increases successively with increasing time [33–35]. ing, the amount of volatiles declined, and degradation continued during wood storage. According to the results (Figures 8 and 9), it is visible that the biggest changes in the chemical composition can be obtained in bark (stored separately and as a part of the trunk). Figure 10 explains the results of chemical composition of wood trunk, while values are very comparable. Spruce wood retained very good durability in terms of its chemical composition after eight months of storage. Several authors studied wood with different natural aging time and the results found that the proportion of saccharides gradually decreases (mainly due to the hemicelluloses degradation), and the content of lignin increases

**Figure 8.** Changes in the composition (relative values) of spruce bark from trunk after its storage of 2, 4, 6, and 8 months. **Figure 8.** Changes in the composition (relative values) of spruce bark from trunk after its storage of 2, 4, 6, and 8 months. **Figure 8.** Changes in the composition (relative values) of spruce bark from trunk after its storage of 2, 4, 6, and 8 months.

**Figure 9.** Changes in the composition (relative values) of spruce bark after its storage of 2, 4, 6, and 8 months. **Figure 9.** Changes in the composition (relative values) of spruce bark after its storage of 2, 4, 6, and 8 months. **Figure 9.** Changes in the composition (relative values) of spruce bark after its storage of 2, 4, 6, and 8 months.

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**Figure 10.** Changes in the spruce wood composition (relative values) after its storage of 2, 4, 6, and 8 months. **Figure 10.** Changes in the spruce wood composition (relative values) after its storage of 2, 4, 6, and 8 months. **Figure 10.** Changes in the spruce wood composition (relative values) after its storage of 2, 4, 6, and 8 months.

The degradation of main chemical components of both bark stored separately and bark as a part of the trunk, was obvious. During eight months of storage, a decreasing of extractives occurred (bark of 80.96%, bark trunk of 73.69%) and HEMI (bark of 67.52%, bark trunk of 49%), and increasing of LIG (bark of 45.65%, bark trunk of 60.9%) and CEL (bark of 65.73% and bark trunk of 69.11%). Bergström and Matison [36] in their study describe a decrease in the extractive content during storage, roughly halving during the first four weeks. The biggest losses noticed during this period were in the amounts of hydrophilic, and lipophilic as well. From chemical components, the stilbenes are very sensitive to degradation [37]. According to Routa et al. [13], after eight weeks of pine bark storage, we obseved types of extractive substances, which are predominate in the bark: triglycerides, steryl esters, sterols, resin acids, and fatty acids. The degradation of main chemical components of both bark stored separately and bark as a part of the trunk, was obvious. During eight months of storage, a decreasing of extractives occurred (bark of 80.96%, bark trunk of 73.69%) and HEMI (bark of 67.52%, bark trunk of 49%), and increasing of LIG (bark of 45.65%, bark trunk of 60.9%) and CEL (bark of 65.73% and bark trunk of 69.11%). Bergström and Matison [36] in their study describe a decrease in the extractive content during storage, roughly halving during the first four weeks. The biggest losses noticed during this period were in the amounts of hydrophilic, and lipophilic as well. From chemical components, the stilbenes are very sensitive to degradation [37]. According to Routa et al. [13], after eight weeks of pine bark storage, we obseved types of extractive substances, which are predominate in the bark: triglycerides, steryl esters, sterols, resin acids, and fatty acids. The degradation of main chemical components of both bark stored separately and bark as a part of the trunk, was obvious. During eight months of storage, a decreasing of extractives occurred (bark of 80.96%, bark trunk of 73.69%) and HEMI (bark of 67.52%, bark trunk of 49%), and increasing of LIG (bark of 45.65%, bark trunk of 60.9%) and CEL (bark of 65.73% and bark trunk of 69.11%). Bergström and Matison [36] in their study describe a decrease in the extractive content during storage, roughly halving during the first four weeks. The biggest losses noticed during this period were in the amounts of hydrophilic, and lipophilic as well. From chemical components, the stilbenes are very sensitive to degradation [37]. According to Routa et al. [13], after eight weeks of pine bark storage, we obseved types of extractive substances, which are predominate in the bark: triglycer-

The Fiber Tester analysis (Figures 11 and 12) shows changes in fibers length and width distribution of both wood and bark. Sample of wood stored for eight month contains lower amount of fibers longer than 1.01 mm (decrease of 23,9%). The both average fiber length and width of samples decreased during storage (Table 4) because of wood and bark degradation. The Fiber Tester analysis (Figures 11 and 12) shows changes in fibers length and width distribution of both wood and bark. Sample of wood stored for eight month contains lower amount of fibers longer than 1.01 mm (decrease of 23,9%). The both average fiber length and width of samples decreased during storage (Table 4) because of wood and bark degradation. ides, steryl esters, sterols, resin acids, and fatty acids. The Fiber Tester analysis (Figures 11 and 12) shows changes in fibers length and width distribution of both wood and bark. Sample of wood stored for eight month contains lower amount of fibers longer than 1.01 mm (decrease of 23,9%). The both average fiber length and width of samples decreased during storage (Table 4) because of wood and bark degradation.


**Table 4.** The average fibre length and width before and after storage for 8 months. **Table 4.** The average fibre length and width before and after storage for 8 months.

**Figure 11.** Fibres length distribution of spruce wood trunk and bark before and after its storage of 8 months. **Figure 11.** Fibres length distribution of spruce wood trunk and bark before and after its storage of 8 months. **Figure 11.** Fibres length distribution of spruce wood trunk and bark before and after its storage of 8 months.

**Figure 12.** Fibres width distribution of spruce wood trunk and bark before and after storage for 8 **Figure 12.** Fibres width distribution of spruce wood trunk and bark before and after storage for 8 months.

#### months. **4. Conclusions**

	- of the tree. 5. The both average fiber length and width is higher with a height of the tree.
	- lignin and cellulose. Bark stored separately degraded faster than bark stored on the trunk. 7. In view of the chemical composition of the wood from the trunk retained very good durability during the storage for eight months.

compared to raw wood. The both average fibers length and width decreased during storage. **Author Contributions:** I.Č. conceived and designed the experiments; M.B., V.K. and T.J. carried out **Author Contributions:** I.C. conceived and designed the experiments; M.B., V.K. and T.J. carried out ˇ the laboratory experiments; I.C. analyzed the data, interpreted the results, prepared figures, and ˇ wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

the laboratory experiments; I.Č. analyzed the data, interpreted the results, prepared figures, and **Funding:** This research received no external funding.

wrote the manuscript. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Acknowledgments:** This work was supported by funding from Slovak Scientific Grant Agency VEGA (Contract 1/0397/20 (100%)).

**Acknowledgments:** This work was supported by funding from Slovak Scientific Grant Agency **Conflicts of Interest:** The authors declare no conflict of interest.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

	- Applied Sciences: South Karelia, Finland, 2011; 47p. 3. Hill, C.A.S. *Wood Modification-Chemical, Thermal and Other Processes*; John Wiley & Sons Ltd.: Chicester, UK, 2006.
	- 4. Reinprecht, L. *Wood Deterioration, Protection and Maintenance*; John Wiley & Sons: Chichester, UK, 2016.

VEGA (Contract 1/0397/20 (100%)).

