Heat-Treated Wood from Grand Fir Provides the Same Quality Compared to Silver Fir

Abstract

Grand fir (Abies grandis/Dougl. ex D. Don/Lindl.) represents the North American species that has the potential to replace and complement to some extent several commercial European species, in particular, Norway spruce and silver fir. This is not only due to its high production potential but also because of its favorable effect on the soil compared to spruce or pine. We tested sample trees from these tree species growing in the same location for physical and mechanical wood properties and evaluated the effect of the thermal treatment (180 °C and 200 °C) on their wood. Wood density, swelling, surface properties, strength, and stiffness were the properties used to find the differences among species. Grand fir obtained higher values for density and compressive strength compared to silver fir. For the remaining properties, these tree species are comparable, except for toughness, which was significantly lower for grand fir. Grand fir wood was even comparable with Norway spruce in the case of density and compressive strength. The thermal treatment resulted in a decrease in density, swelling, wettability, modulus of rupture, and toughness while increasing compressive strength. The effect of the thermal treatment was similar for all tested species. From this perspective, grand fir represents a potential substitute for the timber of endangered European commercial conifers.

1. Introduction

Norway spruce is facing serious problems due to climate changes, and a suitable substitution poses a challenge to European forestry. One of the possibilities is silver fir, which is the focus of forestry research [1,2,3], although introduced tree species are also under consideration [4]. The giant fir appears to be a promising species. Grand fir (Abies grandis/Dougl. ex D. Don/Lindl.) occupies a large natural range on the northwest coast of the North American continent. It ranges across the USA (in the states of Washington, Oregon, California, Idaho, and Montana) and Canada (in British Columbia, especially on Vancouver Island) [5]. It is ostensibly one of the most important introduced tree species with high production potential [6,7,8] and was investigated in Czech forestry quite early [9]. This interest was related to the decline of the native silver fir and the efforts to include other fir species or their hybrids in the stand composition [10,11]. Grand fir is characterized by intensive growth, and on suitable sites, it can outcompete even the most productive tree species of the temperate zone—Douglas fir [12]. Moreover, it provides raw timber material with considerable potential use [13,14,15,16].

Grand fir’s wood is whitish in color and has no heartwood. It is similar in appearance to the wood of silver fir, with one difference, the width of the annual rings [17]. Due to the nature of growth, the annual rings are usually quite wide. Spruce wood is similar in structure and color but is also characterized by the presence of resin canals, which affect some of its properties and, consequently, the use of the wood. The wood of all three species is described as light, with a low resistance to biotic agents. In its natural range, the wood of grand fir is used for inferior purposes, mainly for the production of pulp and paper. It is also characterized as less strong [18]. Wagenführ [17] reports the use of the wood for constructional purposes and joinery, such as windows and doors. The properties of wood from grand fir’s native areas are detailed and well known [19]. However, studies evaluating the properties of this species from other continents where it has been successfully introduced are scarce. In Central Europe, Moliński and Raczkowski [20] addressed some of the physical and mechanical properties of grand fir wood. Lukášek et al. [15] evaluated wood shrinkage. Hapla et al. [13] conducted research on the oven-dry density of grand fir, followed by research on identical test material that focused on variability in wood density [14]. The variability in the density of grand fir, depending on location, was assessed by [21]. Wood strength was investigated by [22]. To some degree, these studies also attempt to assess the suitability of grand fir as a wood replacement for European conifers. However, it should be pointed out that the comparison with native species is based solely on the literature, not on experimental data.

The purpose of modifying wood is to improve some of its properties, particularly in relation to the processing and subsequent use of the wood [23]. Thermal modification of wood is one of the most widespread and environmentally friendly methods that uses only basic physical and chemical principles without the use of additional extraneous chemicals [24,25]. Heat treatment is mainly used for wood species with low natural durability [26], increases dimensional stability, and reduces wood wettability [27], however, it can also follow aesthetic aspects [28]. The disadvantages of such modified wood include a reduction in strength, and the wood is not recommended for use in building structures that are subject to stress [23,24]. Of the several technologies currently used in the industry, the most widely applied is the Finnish Thermo Wood technology [24,29].

The effect of heat treatment on spruce wood, which is the most significant commercial European tree species, has been well studied and described [25,27]. There are some studies that deal with heat treatment in European silver fir wood or other fir species. In their study, Allegretti et al. [30] evaluated the impact of heat treatment on mass loss, dimensional stability, durability, and strength of Norway spruce and silver fir wood. They achieved a beneficial effect of increased dimensional stability with minimal impact on wood toughness. The effect of this modification on the wood density of Abies bornmülleriana ttf. was investigated by [31]. They found a decrease in density in relation to temperature. In another study focusing on thermal modification of Abies bornmülleriana, Gündüz et al. [32] concluded that this modification negatively affects compressive strength and hardness. Selected physical properties, modulus of rupture (MOR) and elasticity, and compressive strength of the modified wood in this fir species have been investigated by [33] or [34]. They found a positive effect on dimensional stability and compressive strength but a negative effect on MOR. Korkut [35] also confirmed a decrease in strength characteristics in Abies bornmülleriana. Kučerová et al. [36] evaluated the color changes in fir (Abies alba Mill.) wood due to heat. Yildiz et al. [37] found a decrease in compressive strength in spruce wood due to heat, and Shi et al. [38] confirmed a decrease in MOR in fir wood due to high temperatures. Heat modification has the greatest negative impact on toughness [34]. In the case of grand fir wood, the impact of heat modification on its properties has not yet been comprehensively investigated.

As the studies above show, grand fir has a high production potential with a minimal negative impact on forest ecosystems. In the Czech Republic, this tree species can fulfill a variety of functions, primarily timber production in quantities superior to native tree species, as well as soil improvement, landscaping, ornamental, and many other functions [39,40]. Along with some other introduced tree species, it could prove beneficial under changing ecological conditions [12,41].

There is insufficient research assessing the quality of grand fir wood in Europe, especially in relation to native species. This study aims to evaluate the potential of grand fir wood in terms of possible substitution of commercial European conifers’ timber in the processing industry, namely silver fir and Norway spruce. We also investigated the effect of thermal modification on the wood properties of these species as one of the environmentally friendly wood treatments to enhance selected performance properties. We focused on wood properties that are important from the point of view of processing and use.

2. Materials and Methods

2.1. Material

For the purpose of this study, we selected specimen trees of North American grand fir and native European silver fir to assess the effect of heat treatment on selected wood properties. In addition, we included Norway spruce in our experiment, not only as a representative of the most important European commercial tree species but also as a conifer with a similar wood structure, with one significant difference compared to fir, the presence of resin canals. All of the specimen trees used in this study came from the same site. The design of the experiment consequently minimized the influence of altitude, exposure, or nutrient availability and allowed only the effect of wood anatomy (species) or heat treatment to be analyzed.

The stand studied was located in the School Forest Enterprise in Kostelec nad Černými lesy. This enterprise is a special-purpose facility of the Czech University of Life Sciences in Prague. It was founded in 1935, approximately 25–50 km southeast of Prague. The number of the studied stand section was 703D5a, covering an area of 3.05 ha, with an average altitude of 400 m above sea level. The group of forest habitat types was 4H1 (Fagetum illimerosum mesotrophicum), and the slope was 3%. The species composition was 40% Norway spruce, 20% European beech, 20% grand fir, 15% European larch, and 5% silver fir (

Table 1. Basic dendrometric indicators.

Species	Species Share (%)	Mean Diameter (cm)	Mean Height (m)	Standing Volume (m3·ha−1)
Norway spruce	40	19	21	144
European beech	20	18	18	43
Grand fir	20	25	24	97
European larch	15	27	21	50
Silver fir	5	17	17	15

). The age of the stand was 55 years.

A one-meter section was extracted from each tree and then cut into 30-mm thick planks on a band saw. This material was left to dry naturally. Once the moisture content had sufficiently decreased, test samples were produced. Two basic types were made from the planks. The first type, measuring 20 × 20 × 300 mm (tangential × radial × longitudinal), was used to determine the toughness, modulus of elasticity, and modulus of rupture (MOR). After performing the tests on the mechanical properties of the timber, 30-mm long sections were cut from the solids. These test samples were used to assess the density, dimensional stability (volumetric swelling), compressive strength, and moisture content of the wood. This type of test sample was prepared in three series, specifically, reference test samples (without heat treatment), samples treated at 180 °C, and samples treated at 200 °C. All three series of test samples were cut consecutively in a given plank to eliminate any difference between trees or the width of the annual rings within a tree. The second type of test sample, 100 × 20 × 300 mm (width × height × length), was used to assess wettability and brightness (Figure 1A). In this case, only two series of test samples were prepared, as these were non-destructive experiments, and after measuring the properties of the untreated (reference) bodies, the series were heat-treated at the appropriate temperature, at 180 °C and 200 °C. The parallelism of the test samples in the sawn timber was also strictly observed in the production of this type of body.

2.2. Thermal Modification

Thermal modification of the test samples was accomplished using the A Type KHT thermo chamber (Katres Ltd., Jihlava, Czech Republic). Two degrees of thermal modification were employed, 180 °C and 200 °C. Due to the different sizes of the two types of test samples used, it was necessary to treat each type with the appropriate temperature separately. The initial moisture content of the test bodies was approximately 8%–10%. The course of the individual phases, i.e., heating, modification, and cooling, along with all other related procedures, was carried out in accordance with the Finnish ThermoWood^® production technology [42]. The duration of the primary treatment phase for both temperature levels used was 3 h.

2.3. Tests

We evaluated the mechanical and physical properties of the wood, including surface properties. All property tests were achieved using standardized procedures. Wood density at 12% moisture content was determined according to [43], volumetric swelling [44], and brightness [45]. Changes in color were detected by a CM-600d spectrophotometer (Konica Minolta, Osaka, Japan). Surface wettability was determined as the contact angle of a water droplet with the surface measured 5 s after dropping, using the Krüss DSA30E (KRÜSS GmbH, Hamburg, Germany). For the mechanical properties, we tested modulus of elasticity [46], MOR perpendicular to the fibers [47], compressive strength along the fibers [48], and toughness [49]. All samples tested for density and mechanical properties were acclimatized in an air temperature of 20 °C ± 2 °C and a relative air humidity of 65% ± 5% using the ClimeEvent C/2000/40/3 climatic chamber (Weiss Umwelttechnik GmbH, Reiskirchen, Germany). The humidity content of the samples was determined according to Ref. [50]. The universal testing machine TIRAtest 2850 (Tira GmbH, Schalkau, Germany) was used to determine the MOE, MOR, and compressive strength (Figure 1B). Toughness was measured using the Charpy impact test (CULS, Prague, Czech Republic).

2.4. Statistical Analysis

Basic descriptive statistics and two-factor analysis of variance (ANOVA) were used in STATISTICA Version 13.4.0.14 (TIBCO Software Inc., Palo Alto, CA, USA) to demonstrate trends of the properties and characteristics under study. A uniform significance level of α = 0.05 was applied for all statistical analyses.

3. Results

The density of heat-untreated grand fir wood reached 475 kg·m⁻³. A detailed summary of the results for the tested species, including the effect of heat treatment and variability of properties, is given in Appendix A. The highest density value was achieved by spruce, but there was no statistically significant difference from grand fir. In contrast, silver fir reached a significantly lower value when compared to these species (Figure 2A). Thermal treatment caused a decrease in density (mass loss) for all evaluated tree species. The higher the temperature used, the higher the loss. Spruce showed the greatest loss (1.9% and 4.9% for 180 °C and 200 °C, respectively), while the lowest effect of temperature on wood density was for the silver fir (

Table 2. Changes in properties (in %) for each species in relation to the degree of heat treatment.

	180 °C/REF *			200 °C/REF *
	Silver Fir	Spruce	Grand Fir	Silver Fir	Spruce	Grand Fir
Density	−1.6	−1.9	−1.8	−3.6	−4.9	−4.0
Volumetric Swelling	−10.8	−14.3	−10.5	−27.0	−34.1	−26.3
Brightness	−35.8	−32.7	−34.1	−57.6	−55.2	−54.2
Wettability	132.8	73.6	128.8	140.7	71.6	123.7
Modulus of Elasticity	2.9	5.8	7.7	−1.1	2.6	0.7
Modulus of Rupture	−1.2	−6.1	−2.0	−29.3	−24.1	−24.3
Toughness	−21.6	−24.4	−30.3	−56.6	−44.6	−47.9
Compressive strength	−2.5	6.2	11.3	3.2	10.9	16.3

* REF = referential test samples, i.e., without heat treatment; “−” denotes a decrease in a property.

).

Spruce wood shows the most considerable dimensional changes (16.5%) related to volumetric swelling in the reference samples. In contrast, the lowest values were shown by the grand fir wood (14.1%), with a statistically evident difference from both silver fir and spruce (Figure 2B and Appendix A). Thermal modification reduces volumetric swelling the most for spruce wood by more than one-third (34.1%) for the highest temperature grade. For both fir species, the effect of thermal modification on dimensional changes is similar, with 27.0% and 26.3% for the highest temperature grade for silver fir and grand fir, respectively (Table 2).

Differences in brightness values between tree species were minimal, although statistically significant, due to low variability. In any case, grand fir reached the lowest value for untreated wood (Figure 2C and Appendix A), with a statistically significant difference from the other species. Silver fir wood undergoes the greatest color changes when thermally modified. Again, it should be emphasized that in this feature, the differences between tree species are minimal (Table 2).

Spruce wood has the highest contact angle of wetting (62.5°), which significantly exceeds both firs. This is probably due to the presence of resin. There is a minimal, statistically insignificant difference between the fir species (Figure 2D and Appendix A). Thermal treatment caused an increase in contact angle (decrease in wettability) for all three species but erased the differences between the wood of spruce and both fir species by the first temperature grade. Moreover, the applied temperature does not play a significant role in this case since a further increase in temperature did not increase wettability. The effect of both temperatures is similar (Figure 2D and Table 2), i.e., caused by the variability of the wood in general.

Spruce wood shows the highest values of the modulus of elasticity (9252 MPa), much higher (statistically proven) than the wood of both fir species. Although grand fir reached the lowest value (8246 MPa), the difference between the firs is not statistically demonstrable (Figure 2E and Appendix A). The treatment with the first temperature grade causes an increase in MOE, with the highest positive effect obtained for grand fir (7.7%). However, the value is statistically insignificant compared to silver fir. A further increase in temperature causes a decrease in MOE for all tree species. In the case of spruce and grand fir, the values still did not fall below the reference sample value (Table 2). Nevertheless, the differences in values caused by heat treatment are not statistically significant for individual tree species. Even after heat treatment, the MOE value of spruce significantly exceeds that of both fir species (Figure 2E).

Similar to the MOE, spruce shows the highest MOR values (84.9 MPa), statistically significant when compared to the strength of both fir species. Grand fir had the lowest strength value (73.6 MPa), but this difference is not statistically significant compared to the strength of silver fir (Figure 2F and Appendix A). In contrast to MOE, heat treatment causes a decrease in strength already by the first degree of treatment. At the first heat treatment degree, the difference is not yet statistically distinguishable from untreated wood. The second temperature degree already causes a significant decrease in MOR. The greatest decrease was found in silver fir (29.3%) and the lowest in spruce (24.1%), comparable to grand fir (24.3%) (Table 2). Spruce showed the highest strength values, even after heat treatment. The values of both fir species are still very similar after heat modification (Figure 2F).

Grand fir wood exhibits the lowest value of toughness (5.2 J·cm⁻²), while the highest is achieved by silver fir (7.1 J·cm⁻²). However, it is statistically indistinguishable from spruce (Figure 2G and Appendix A). There is a steep decline in toughness with increasing temperature. The second temperature degree virtually unified the values of all three tree species, and the average values obtained are thus very similar (Figure 2G). Silver fir shows the greatest impact of temperature on the decrease in toughness (56.6%), while spruce shows the lowest decrease (44.6%) (Table 2).

Grand fir has the highest compressive strength of the evaluated wood species (42.0 MPa). However, the difference in the spruce strength is minimal and statistically insignificant. Silver fir had the lowest compressive strength (39.1 MPa), which is a statistically significant difference from the remaining tree species (Figure 2H and Appendix A). Thermal treatment had a significant positive effect on compressive strength, except for silver fir wood, where the effect was not conclusive. For both grand fir and spruce, the compressive strength increases with temperature. The second temperature degree increased the compressive strength of grand fir to 48.8 MPa, the highest value of all the species examined. However, it was statistically indistinguishable from spruce (Figure 2H). The highest increase in compressive strength due to heat is for grand fir, with a 16.3% increase for the second temperature degree (Table 2).

If we look at grand fir as a prospective replacement for silver fir or spruce wood, its potential is evident in Figure 2. In all cases, grand fir showed comparable values to silver fir, and in some cases, even to spruce. This is particularly noticeable in density or compressive strength. The only characteristic where grand fir falls short of the other species is toughness. This feature generally limits the use of wood for construction purposes, especially when subjected to impact (dynamic) stress. Given its quality and based on the properties assessed, spruce wood is the best performing wood of all in terms of density and strength characteristics.

If we consider the applications of the three types of wood, the positive impact of thermal modification on most properties is also evident (Table 2). The slight decrease in density is compensated by a significant reduction in dimensional changes due to the reduction in water content of the wood, an increase in the contact angle of wetting (and thus achieving lower wettability), and an increase in compressive strength. This positive effect is offset by a reduction in MOR and toughness. The modulus of elasticity is minimally affected by the modification. In any case, grand fir cannot be regarded as inferior based on this comparison. For most properties, it is at least comparable or even superior to silver fir, and in some cases, to spruce as well.

4. Discussion

A high wood density value of heat-untreated grand fir is a surprising finding of this study (475 kg·m⁻³). Up to this point, its wood has been considered less valuable in this respect, especially compared to silver fir or spruce (450 kg·m⁻³ and 470 kg·m⁻³, according to Ref. [17]. The values achieved are higher than those reported for Abies grandis by Ref. [17]. A significantly lower value for grand fir was also confirmed by Ref. [51]. Research from areas where grand fir occurs naturally also reports lower density values [19]. The high density also results in higher strength characteristics than those reported for this species in the previous literature, often reaching the values reported for silver fir [17]. Concerns about lower wood quality for this non-native species in Europe are thus unfounded.

In the case of our study, thermal modification resulted in a decrease in wood density for all species studied. This finding was temperature-dependent, with the percentage decrease in density for the second temperature degree being more than double that of the first degree [24]. The decrease in density is primarily attributed to wood decomposition due to high temperatures, resulting in mass loss. Mass loss, therefore, affects wood density and, consequently, some of the mechanical properties. Gündüz et al. [31] demonstrated decreasing density in relation to temperature for Abies nordmanniana (Stev.) Spach. The mass loss that increases with higher temperatures was reported for Abies alba by Ref. [36] or Ref. [52]. Kol et al. [33] obtained similar results to our study for Abies bornmülleriana. The decrease in wood density for the lowest temperature was 1.7%, and for the highest temperature, 5.4%. The maximum mass loss of 6.5% due to thermal treatment for Norway spruce and silver fir is also mentioned by Ref. [30]. They did not reveal any significant difference between these species. The fact that differences in mass loss due to temperature between conifers are minimal is confirmed by [53] for Abies alba, Picea abies Karst., and Larix decidua Mill.

Heat treatment resulted in a decrease in volumetric swelling, almost identical for both fir species. The values of swelling decrease with increasing temperature. The difference (decrease) between the temperature grades is more than twice as large. Thus, heat treatment contributes to an increase in the dimensional stability of the wood, reducing swelling [27,54]. High temperature changes the chemical structure of the wood, reducing the amount of free hydroxyl groups, which are responsible for swelling. Depolymerization primarily affects hemicelluloses, which contain the highest amount of free hydroxyl groups [34]. Similarly to our results, Kol [34] achieved a decrease of 40.6% in volumetric swelling due to thermal modification for Abies bornmülleriana compared to unmodified wood.

The decrease in wettability, or increase of the contact angle, is markedly more pronounced in the case of fir wood. In contrast to the previous properties, the difference between the treatment temperatures used is no longer as significant. Heat treatment has a positive effect on reducing wettability [24], i.e., there is an increase in the wetting contact angle, as has been confirmed, e.g., for spruce [55]. An increasing contact angle as a function of temperature is reported by Ref. [56] for Eastern redcedar.

Brightness decreased for both fir and spruce wood almost identically, dependent on temperature degree. Wood gets darker due to the thermal treatment [24], an ancillary effect of the thermal modification, regardless of the wood species. This has been confirmed by Ref. [32] for fir, spruce [52,57,58], and Douglas fir wood [59]. The effect of species on this characteristic is negligible, as confirmed in the case of the comparison of fir, spruce, and larch by Ref. [53]. The higher the temperature, the darker the color [36]. Except for aesthetic importance, the color could be regarded as an indicator of the thermal modification intensity because of its close relationship with mass loss [30].

The modulus of elasticity increased with the first temperature degree, which was most pronounced in the case of grand fir. At higher temperatures (second degree of treatment), there was a reduction in MOE, but only when compared to the first degree. When compared to the reference sample (untreated wood), the MOE values are still higher, except for silver fir, for which the positive effect of heat treatment is the smallest. A similar pattern of MOE as a function of treatment temperature is reported for Abies alba by Ref. [52]. Improvement in MOE due to the thermal treatment (200 °C) is cited by [38] for fir wood and for spruce [58]. An insignificant effect of the treatment on toughness for Norway spruce and silver fir is reported by Ref. [30]. Kačíková et al. [57] confirmed a decrease in MOE for spruce wood with increasing temperature. A 9.5% decrease in MOE for Abies bornmülleriana due to heat was found by [34], lower than the decrease in MOR. A far lesser impact of heat treatment on MOE compared to MOR is confirmed by Ref. [58] or Ref. [24].

In contrast to MOE, heat treatment has a negative effect on the modulus of rupture, starting from the first degree of treatment, regardless of the wood species. The thermal treatment resulted in a decrease in MOR, as stated in other studies. The decrease in MOR for spruce wood was confirmed by Ref. [57]. Fir wood MOR was reduced by 4.4% at 180 °C and 10.8% at 200 °C [33]. In our research, the decrease was not as steep for the first temperature, but at 200 °C, the decrease was substantial. In contrast to our results, Shi et al. [38] reported a more pronounced MOR decrease of 37% for fir wood, and Korkut et al. [60] a decrease of 32.68% for pine wood. However, no significant change in bending strength (modulus of rupture) and stiffness for Norway spruce and silver fir was concluded by [30]. Kučerová et al. [52] even reported a higher MOR value for 200 °C compared to the reference sample for Abies alba wood. A positive effect of temperature on MOR for spruce and pine was also obtained by Ref. [58].

The most striking negative effect of heat treatment on mechanical properties is in the case of toughness, where we observe a steep decrease starting at the first degree. When exposed to high temperatures, the wood becomes more brittle, and its toughness decreases. Kol [34] reports a decrease of 10.5% for Abies bornmülleriana wood. A decrease in toughness in fir wood was reported by Ref. [35]. A similar decrease in toughness (46.22%) due to heat treatment, as in our study, was obtained in Pinus sylvestris L. [60].

The most pronounced positive effect of the treatment on mechanical properties was observed for compressive strength, especially in grand fir. Even at the second temperature degree, all of the three wood species showed an increase in strength. In contrast to the modulus of rupture, thermal treatment has a positive effect on compressive strength. It was reported by Ref. [27], as well as [30]. For Abies bornmülleriana wood, Kol et al. [33] received an increase of 6.6% and 12.6% for 180 °C and 200 °C, respectively. These results resemble spruce from our study. Grand fir obtained even higher figures. The conclusion reached by Ref. [32] for fir or for spruce [37] that compressive strength decreases with increasing temperature was not confirmed.

The effect of heat treatment on the aforementioned wood properties is as follows. First, it is the decomposition of the basic chemical components of wood, primarily hemicelluloses, or their transformation into other chemical substances. Increasing temperature causes a degradation of the hemicelluloses by deacetylation followed by depolymerization of these compounds. Dehydration of carbohydrates also reduces the total content of hydroxyl groups. Although cellulose is more resistant than hemicelluloses, amorphous cellulose becomes degraded and the crystallinity of cellulose increases. Extractives (e.g., resin) volatilize away from the wood or, alternately, are degraded [23,27,36]. A steep decrease in the hemicellulose content of spruce wood with increasing temperature is reported by Ref. [37]. The decomposition of chemical components leads to a reduction in weight, and the loss of structural components and changes in cell wall structure are subsequently reflected in a decrease in strength characteristics [57]. The higher the temperature, the greater the impact of modification. A frequently used characteristic to assess the impact of thermal modification is the color of the wood. Color changes in heat-treated wood are associated with degradation of hemicelluloses and depolymerization of cellulose, resulting in loss of strength [36]. Thus, a negative side effect of heat modification is the deterioration of mechanical properties [24]. In particular, the fracture properties of wood deteriorate, and its brittleness increases due to the loss of amorphous polysaccharides. Thus, the decomposition of hemicelluloses can be considered the primary cause of the loss of mechanical strength [60]. Another effect of heat treatment, reflected in strength characteristics or dimensional changes, is the reduction of equilibrium moisture content (EMC) due to the decrease of available hydroxyl groups in wood [27]. As the EMC value decreases, mechanical properties improve, and the loss of cell wall structural substances is compensated to some extent [52]. This is the reason why some of the mechanical properties improve with heat, primarily at lower modification temperatures.

5. Conclusions

Unlike silver fir, the grand fir is considered a fast-growing tree species with wide annual rings, which is the principal reason why its timber is referred to as inferior. This was in no way confirmed in our study. On the basis of the characteristics evaluated, we can state that under the same habitat conditions, the quality of the wood of grand fir is comparable to silver fir and even to spruce. For density and compressive strength, grand fir reached higher values than the native species of fir. It is comparable to silver fir in the other variables evaluated, the only exception being toughness, where grand fir significantly underperformed both silver fir and spruce. Thermal modification of the wood has a positive effect on reducing dimensional changes, reducing wettability, and increasing compressive strength. It has a negative effect on reducing density, brightness, MOR, and toughness. The effect on the modulus of elasticity is statistically inconclusive. Concerning wood species and heat treatment, all tree species behave similarly. In some cases, particularly at the higher temperature grades, the properties have even been homogenized from the initial statistically different values. Therefore, we see no fundamental reason why the wood of grand fir should not be used for the same purposes as silver fir or spruce.