**1. Introduction**

Wood is an anisotropic material, with respect to its anatomical, physical, and chemical properties, and is made up of different kinds of cells. Wood is degradable by fungi, microorganisms and heating [1]. Degradation of wood and its chemical structure are influenced by the temperature, oxygen available to the material, ambient pressure, wood type and shape, moisture content of the wood, and additives, such as inorganic substances, sorbed emissions, etc. [2–4]. Fedyukov et al. [5] progressively describe the decreasing of cellulose content of spruce wood with soil condition deterioration.

The structure of wood, chemical components (cellulose, hemicelulloses, lignin and extractives) and their relative mass proportion depending on the morphological region, kind of the tree, and age of the wood [1]. Variation can be found within a single tree from the center of the trunk to the bark, from the trunk to the top, between earlywood and latewood, and between sapwood and heartwood [6]. According to Fengel and Wegener [7], earlywood contains more lignin and inversely less cellulose than latewood.

Understanding the morphological and chemical heterogeneity of wood is important in its utilization, for example in the paper industry. Like wood, bark is an important source of raw material and chemical compounds [8]. The chemical composition of bark varies among the different tree species and depends on the morphological element involved. Many of the constituents present in wood also occur in bark, but their proportion is different. Bark can roughly be divided into the fraction: fibers, corok cells and fine substance (including parenchyma cells) [1]. Bark contains much more extractives than wood

**Citation:** Cabalová, I.; Bélik, M.; ˇ Kuˇcerová, V.; Jurczyková, T. Chemical and Morphological Composition of Norway Spruce Wood (*Picea abies*, L.) in the Dependence of Its Storage. *Polymers* **2021**, *13*, 1619. https:// doi.org/10.3390/polym13101619

Academic Editor: Salim Hiziroglu

Received: 4 May 2021 Accepted: 13 May 2021 Published: 17 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

from the trunk [9], considerable amounts of bioactive components such as antioxidants (polyphenols), and also structural polysaccharides such as pectins [10–12]. Degradation of biomass is influenced by photodegradation (UV light), high temperatures during storage in piles, microorganizms, etc., Routa et al. [13] describe the major reactions of lipophilic extractive compounds during wood storage, which can be divided into three types: (1) hydrolysis of triglycerides (rapid reaction) and steryl esters and waxes (proceeds slower); (2) oxidation/degradation/polymerization of resin acids, unsaturated fatty acids, and to some extent, other unsaturated compounds; and (3) evaporation of volatile terpenoids, mainly monoterpenes. of biomass is influenced by photodegradation (UV light), high temperatures during storage in piles, microorganizms, etc., Routa et al. [13] describe the major reactions of lipophilic extractive compounds during wood storage, which can be divided into three types: (1) hydrolysis of triglycerides (rapid reaction) and steryl esters and waxes (proceeds slower); (2) oxidation/degradation/polymerization of resin acids, unsaturated fatty acids, and to some extent, other unsaturated compounds; and (3) evaporation of volatile terpenoids, mainly monoterpenes. As any wood material, spruce wood is chemically complex and its physical, chemical and morphological characteristics are not uniform. In Slovakia, it is the most common

is different. Bark can roughly be divided into the fraction: fibers, corok cells and fine substance (including parenchyma cells) [1]. Bark contains much more extractives than wood from the trunk [9], considerable amounts of bioactive components such as antioxidants (polyphenols), and also structural polysaccharides such as pectins [10–12]. Degradation

*Polymers* **2021**, *13*, x FOR PEER REVIEW 2 of 12

As any wood material, spruce wood is chemically complex and its physical, chemical and morphological characteristics are not uniform. In Slovakia, it is the most common wood specie. In Norway spruce, approximately 95% of wood cell matrix is composed of tracheids, which can also be termed fibers. The average tracheid length is mainly influenced by tree age. As most wood properties, fibers length and width varies greatly both within and among trees, depending on its vertical and radial position (ring age) in trunk, and forest stand [14]. The tracheids length is shortest next to the pith, the increase with age is at first very rapid in the juvenile period, then slow down between the ages 10–30 and thereafter, as mature wood begins to form, increases very gradually with seasonal fluctuation [15]. wood specie. In Norway spruce, approximately 95% of wood cell matrix is composed of tracheids, which can also be termed fibers. The average tracheid length is mainly influenced by tree age. As most wood properties, fibers length and width varies greatly both within and among trees, depending on its vertical and radial position (ring age) in trunk, and forest stand [14]. The tracheids length is shortest next to the pith, the increase with age is at first very rapid in the juvenile period, then slow down between the ages 10–30 and thereafter, as mature wood begins to form, increases very gradually with seasonal fluctuation [15].

The aim of this paper is to characterize chemical composition (extractives, lignin, polysaccharides, cellulose and hemicelluloses) and morphological properties (fibers length and width) of different parts of Norway spruce wood (*Picea abies* (L.) Karst.) and to evaluate the rate of changes in these properties after wood storage. The aim of this paper is to characterize chemical composition (extractives, lignin, polysaccharides, cellulose and hemicelluloses) and morphological properties (fibers length and width) of different parts of Norway spruce wood (*Picea abies* (L.) Karst.) and to evaluate the rate of changes in these properties after wood storage.

#### **2. Experimental 2. Experimental**

#### *2.1. Materials 2.1. Materials*

Norway spruce wood (*Picea abies* L. Karst.) was harvested in the middle part of Slovakia (Zvolen region) in June. The 65-year-old tree was cut 0.5 m from the ground and its height was 25 m. For the experiment, we used a 5 m long trunk part (the diameter of 32 cm—thicker part and the diameter of 29.5 cm—thinner part) and the top part of tree, represented the waste biomass (the diameter of 12 cm—thicker part and the diameter of 7.5 cm—thinner part). We took samples (cut with an all diameter) from the trunk in a height of 0.5, 1.5, 2.5, 3.5 and 4.5 m from the ground and both wood and bark from the top. Equally, we took samples A, B, C (Figure 1) in a height of 0.5 and 4.5 m, whereas sample A represent juvenile wood, which is within the first 20 years of growth; B sample, the next 20 years; and C sample, the rest. Norway spruce wood (*Picea abies* L. Karst.) was harvested in the middle part of Slovakia (Zvolen region) in June. The 65-year-old tree was cut 0.5 m from the ground and its height was 25 m. For the experiment, we used a 5 m long trunk part (the diameter of 32 cm—thicker part and the diameter of 29.5 cm—thinner part) and the top part of tree, represented the waste biomass (the diameter of 12 cm—thicker part and the diameter of 7.5 cm—thinner part). We took samples (cut with an all diameter) from the trunk in a height of 0.5, 1.5, 2.5, 3.5 and 4.5 m from the ground and both wood and bark from the top. Equally, we took samples A, B, C (Figure 1) in a height of 0.5 and 4.5 m, whereas sample A represent juvenile wood, which is within the first 20 years of growth; B sample, the next 20 years; and C sample, the rest.

**Figure 1.** Samples A, B, and C used for analysis. **Figure 1.** Samples A, B, and C used for analysis.

After trunk sampling, we obtained five wood pieces with a length of approximately 1 m. We debarked the first one. A part of the bark was used for the experiment and the rest for storage. The rest (four) of the wood pieces (in the bark) were also used for storage. After 2, 4, 6 and 8 months of storage, we always analysed one piece of wood (the first

sample was wood and the next sample bark from this piece) and bark, which was stored separately. Specifications of the samples are described in Table 1.


**Table 1.** Samples signification.

During storage, the conditions were as follows (Table 2):


**Table 2.** Conditions during wood storage.

#### *2.2. Methods*

2.2.1. Chemical Composition of Wood

The samples were disintegrated into sawdust, and fractions 0.5 mm to 1.0 mm in size were used for the chemical analyses. The extractives content (EL) was determined in a Soxhlet apparatus with a mixture of ethanol and toluene (2:1) according to ASTM D1107-96 [16]. The lignin content (LIG) was determined according to Sluiter et al. [17], and the cellulose content (CEL) was determined according to the method by Seifert [18], and the holocellulose content according to the method by Wise et al. [19]. Hemicelluloses (HEMI) were calculated as the difference between the holocellulose and cellulose contents. Measurements were performed on four replicates per sample. The results were presented as oven-dry wood percentages.

#### 2.2.2. Fibers Length and Width

Two hundred-millilitre mixtures of concentrated CH3COOH and 30% H2O<sup>2</sup> (1:1, *v*/*v*) were poured onto the wood samples (weight = 10 g and dimensions = 20 mm × 2 mm × 2 mm). Then, the samples were refluxed for 3 h, suction filtered through a sintered glass filter (S1), and washed with distilled water. An L & W Fiber Tester (Lorenzen and Wettre, Kista,

Sweden) was used to determine the fibre dimensional characteristics. This measurement is based on the principle of two-dimensional imaging technology. The measurement technology is automated, allowing for frequent and rapid analysis of the fibre quality. The instrument measures various fibre properties, such as the length and width of the fibres, fine portion (from 0.1 mm to 0.2 mm). Measurements were performed on a single replicate per sample, and the number of fibres within each population of the replicate ranged from 19,182 cells to 21,128 cells.

#### **3. Results and Discussion**
