The climate is changing. Measurements show a trend towards rising air temperatures. Finally, the last ten years are among the warmest years in terms of weather observation in Slovenia and other central European countries. Moreover, we are experiencing increasingly intense extreme weather events (hail, storms and others; Oblak et al., 2021). We can likely expect even stronger and more pronounced climate change in the future (Bevacqua et al., 2021).

Climate change is already affecting the forest species composition and wood quality (Gričar et al., 2015). Increasingly higher temperatures and a changing precipitation regime are expected to affect tree species’ distribution in these forests significantly. Assuming limited migration, most European species would significantly decline suitable habitat areas. Most biome shifts focus on ecological issues, as changes in dominant tree species will also lead to changes in entire ecosystems and dependent organisms (Dyderski et al., 2018). Generally accepted models suggest that depending on the climate scenario, only the Mediterranean type of low-value oak forest will thrive on 21–60% (average 34%) of European forest area by 2,100. This will result in lower yields for forest owners and a shortage of raw material for the timber industry (Hanewinkel et al., 2013). In addition, it must be considered that CO₂ sequestration in such modified forests will also be reduced (Birdsey and Pan, 2015). Models showed that Abies alba, Fagus sylvatica, Fraxinus excelsior, Quercus robur, Quercus petraea and the non-native species Pseudotsuga menziesii, Quercus rubra and Robinia pseudoacacia could be considered climate winners (Dyderski et al., 2018).

Based on the data presented, one viable solution to improving the economic perspective of forest owners and related wood processing industries is the controlled introduction of non-native tree species such as Douglas fir (Eberhard et al., 2021) in Central Europe. The future use of these tree species must also be of long-term interest to the forest owner. The economic impact is primarily based on the relevant properties and the associated price of the forest assortments on the market. Most of the Slovenian wood processing industry is based on softwoods. Softwoods are of crucial importance for timber construction (roofs, skeletal structures, glue-laminated beams and cross-laminated timber) and the composite industry (particleboard and shuttering panels; Ansell, 2015). Today, the wood used for these purposes in Slovenia are mainly Norway spruce (Picea abies) and silver fir (Abies alba). However, their share in Slovenian forests decreases (Klopčič et al., 2017). Therefore, it is crucial to identify the relevant properties of a suitable wood species that could partially replace Norway spruce in the applications mentioned above. Douglas fir (Pseudotsuga menziesii) was defined as a potential candidate among different wood species. Douglas fir is already present in Europe and covers about 830,000 ha of forest area in Europe alone (Pötzelsberger et al., 2020; Eberhard et al., 2021). These tree species offer high productivity rates and thus ensure sustainable income for forest owners (Eilmann and Rigling, 2012).

The first documented introduction of a considerable number of non-native tree species on the territory of present-day Slovenia occurred in the late 18th and early 19th centuries. Initially, the species were planted in specialised plant collections, and the frequency of their planting was low. At the end of the 19th century, the planting frequency of some species increased, and several species were also introduced into forests (Brus and Gajšek, 2014). Most Douglas-fir plantations in Slovenia were established in the periods 1880–1940 and 1960–1990 in various parts of the country (Veselič et al., 2016). Douglas fir is currently the third most common non-native tree species, accounting for 0.05% (171,000 m³) of the total growing stock in Slovenian forests. It has clearly shown good adaptability under challenging conditions, e.g., resistance to ice storms, drought and bark beetle infestation in areas where Norway spruce was severely affected, as well as good growth performance. For this reason, Douglas fir has currently considered the most promising non-native tree species in Slovenian forests (Smolnikar et al., 2021).

The European standard EN 350 (CEN, 2016) clearly distinguishes the durability of Douglas fir from North America from Douglas fir growing in Europe. The durability of Douglas fir from North America is 3 (moderately durable), while the durability of Douglas fir grown in Europe has a higher variation of 3–4 (slightly to moderately durable). Douglas-fir wood is generally characterised as the species with one of the best ratios between growth rates and quality (Perić et al., 2011). The wood properties are determined by numerous factors: site conditions, provenance, forest management and location in the tree (Pollet et al., 2017; Balanzategui et al., 2021; Golob et al., 2021). Therefore, the properties of Douglas fir from North America are not comparable to those of Douglas fir from Europe (Blohm et al., 2014). The excellent reputation of Douglas-fir wood is due to the exceptional quality of wood from old-growth forests in north-western North America, which are characterised by slow growth rates (Gartner et al., 1999). However, the characteristics of wood from more dynamically growing second-growth forests in Europe may be different. European forest owners frequently bring smaller logs to market with a higher proportion of juvenile wood (Vikram et al., 2011). The higher proportion of juvenile wood could be problematic, as this wood has less favourable mechanical properties than mature wood (Pollet et al., 2017). On the other hand, faster growth might also result in different durability, as the growth reflects in the anatomy-related water performance and presence of biologically active secondary metabolites.

Wood is the biochemical product of living trees, which consists of basic building elements, i.e., cellulose, hemicelluloses and lignin, representing approx. 95% of the wood (Holmbom et al., 2008). Wood also contains smaller amounts of non-structural compounds, known also as extractives (Vek et al., 2020). Extractives include a large variety of compounds that can be removed from the wood tissue by relatively simple extraction methods. Although the amount of extractives in most of the wood species is relatively low, these non-structural components are reported to possess antifungal, antimicrobial and antioxidant properties (Lu et al., 2016; Belt et al., 2017; Valette et al., 2017). Phenolic extractives that increase the wood durability are biosynthesised de novo at the boundary between sapwood and heartwood (Magel et al., 1994; Dellus et al., 1997; Kebbi-Benkeder et al., 2017). Taxifolin and dihydrokaempferol are reported as such compounds for wood of Douglas fir (Dellus et al., 1997). It is known that the amounts of extractives in the wood of all trees, as well of Douglas fir vary (Adamopoulos et al., 2005; Vek et al., 2020). Specific woody tissues contain more extractives than others, e.g., the wood of dead branches embedded in a stem (wood of knots) is one of the richest sources of polyphenols in nature (Willför et al., 2004). The literature reviews on the chemical composition of extractives of Douglas fir shows that sapwood, heartwood, knotwood and bark have already been investigated (Willför et al., 2003; Miranda et al., 2021). Flavonoids, lignans and oligomeric polyphenols were the main phenolic compounds in wood extracts of Douglas fir (Lindberg et al., 2004; Brennan et al., 2021). Oleson and Schwartz (2016) have categorised proanthocyanidins, phlobaphenes, flavonoids, waxes, terpenoids, phytosterols and lignans as the most abundant non-carbohydrate extractives in the wood and bark of Douglas fir. This review also shows that heartwood contains higher amounts of proanthocyanidins and flavonoids than sapwood. On the other hand, Douglas-fir sapwood and heartwood were reported to contain comparable amounts of waxes, terpenoids and phytosterols (Dellus et al., 1997; Oleson and Schwartz, 2016). In comparison with heartwood, extraction of sapwood with a mixture of acetone and water yield higher amounts of flavonoid glycosides and procyanidins (Dellus et al., 1997). The two goals of our chemical analysis were first to complement the existing literature data and to investigate the chemical composition of phenolic extractives in Douglas-fir heartwood samples of different ages, and then to study the possible correlation between the results of chemical analysis and the results of fungal testing.

The purpose of the respective document is to present information about material resistance and moisture dynamics of Douglas fir from Slovenian forests. This information is of great importance for assessing the suitability of the selected wood species in outdoor applications.

The anatomy of Douglas-fir wood is typical of conifers, with a sharp transition from early- to latewood. Resin canals with thick-walled epithelial cells are present (Wagenfuhr, 2007; Balanzategui et al., 2021; Figure 1). Annual ring widths varied from 1 cm in the juvenile wood to 1 mm in mature heartwood and in sapwood (Figure 2). Growth in the early years was fast compared to other softwoods growing in the region but comparable to growth rates of Douglas fir from other regions of Europe. In Belgium, for example the mean annual ring width varied from 3 mm to 8 mm (Pollet et al., 2017).

Significant differences in growth rates are also reflected in density. Densities of Douglas fir from Celje were lower than densities from Planina. The lowest density was measured in juvenile wood, which is consistent with the distribution of annual rings. The lowest density was determined by Celje’s Douglas-fir tree (342 kg/m³) with the broadest annual rings. The average density of Douglas-fir juvenile wood from Celje was 384 kg/m³, while the density of juvenile wood from Planina was slightly higher (527 kg/m³). The densities of juvenile wood are consistent with the literature data (Pollet et al., 2017). On the other hand, the density of mature wood was slightly higher, varying between 516 kg/m³ (Celje) and 568 kg/m³ (Planina). These values are also consistent with reference data for Douglas-fir wood (Wagenfuhr, 2007; Poletti et al., 2019). The density of Douglas fir is slightly higher than the average density of Norway spruce from Slovenia (457 kg/m³) and comparable to the density of Scots pine (578 kg/m³; Figure 3). The density and annual ring width are reflected in the MoE. Juvenile wood from Celje had the lowest MoE (6,766 MPa). In contrast, much higher values were obtained for mature wood and sapwood. The highest values were obtained for sapwood from Planina (14,724 MPa). The MoEs of mature heartwood from Planina (12,577 MPa) and Celje (11,882 MPa) are comparable to that measured on Norway spruce (12,024 MPa). The modulus of elasticity of mature Douglas-fir wood from Slovenia is comparable to the mechanical properties of Douglas-fir wood from Netherlands (Polman and Militz, 1996).

The amounts of extractives in the wood samples were measured to be 2.05% (SD = 0.42) for sapwood (SW), 3.49% (SD = 0.56) for mature heartwood (MW), and 2.48% (SD = 1.63) for juvenile heartwood (JW). Statistical analysis revealed that MW samples’ extraction gave significantly larger extractives than SW samples (LSD test). The content of measured lipophilic extractives was low, and the differences among the samples were not statistically different (ANOVA, p > 0.05; Figure 4). It was found that MW samples contain significantly larger amounts of hydrophilic extractives and total phenolic extractives than SW samples (LSD test; Figure 4). Even though the numbers were higher for the MW samples, significant differences in the content of hydrophilic extractives between MW and JW samples were not statistically confirmed (LSD test; Figure 4). The content of phenolic extractives in SW, MW and JW samples was measured to be 0.53% (SD = 0.09), 1.62% (SD = 0.43) and 1.03% (SD = 0.54). The differences in the contents of total phenols among SW, MW and JW samples were statistically significant (ANOVA, p < 0.01). These results correspond to the estimates for amounts of flavonoids present in sapwood and heartwood od Douglas fir, i.e., 0.5 and 2% of the oven-dried mass, respectively (Oleson and Schwartz, 2016). The HPLC analysis confirmed presence of catechin (tr = 6.5 min), epicatechin (tr = 8.0 min), homovanillic acid (tr = 8.2 min), coumaric acid (tr = 9.5 min), taxifolin (tr = 9.6 min), ferulic acid (tr = 10.0 min), secoisolariciresinol (tr = 11.7 min), pinoresinol (tr = 12.5 min) and matairesinol (tr = 13.0 min; Figure 5). These compounds have already been reported to be present in stem wood and knotwood of Douglas fir (Willför et al., 2003; Lindberg et al., 2004; Oleson and Schwartz, 2016). Isolariciresinol, lariciresinol and nortrachelogenin have also been reported to be present in smaller amounts in wood extracts of Douglas fir (Willför et al., 2003; Brennan et al., 2021).

The chemical composition of extractives presents in bark and knotwood of Douglas fir have already been comprehensively investigated (Miranda et al., 2021). However, there is hard to find exact information on individual phenolic compounds present separately in sapwood and heartwood (Dellus et al., 1997). Literature data on differences in the composition of extractives between mature and juvenile heartwood are completely missing. The measured quantities of the identified compounds are presented in Table 2. Taxifolin was found and confirmed as the most abundant compound present in the wood extracts of Douglas fir (Figure 5). The differences in the content of taxifolin among the sapwood and heartwood samples were statistically significant (ANOVA, p < 0.05; Table 3). The highest amounts of taxifolin gave the extraction of MW. The lowest amounts were measured in SW samples (LSD test) (Figure 5). The coumaric acid and especially ferulic acid were found to be characteristic for sapwood samples. These two simple phenolic acids were reported to be characteristic also for the resin of Douglas fir (Holmbom et al., 2008; Figure 5). Extraction of sapwood and heartwood samples gained somehow comparable amounts of lignans secoisolariciresinol and pinoresinol with no significant differences in the contents among the samples (ANOVA, p > 0.05, LSD test). Matairesinol was measured in the wood extracts in traces (Table 2). As well, catechin, epicatechin and homovanillic acid were present in the wood extracts of Douglas fir in traces (Table 3). Wood and bark extracts of Douglas fir are known to have antimicrobial properties (Lindberg et al., 2004; Välimaa et al., 2007). Acetone extractives of Douglas-fir knotwood were also demonstrated to possess a high antioxidative potency and radical scavenging capacity (Willför et al., 2003). In the frames of the study, taxifolin was found as a phenolic compound with high antioxidative potency and radical scavenging capacity (Willför et al., 2003). Taxifolin is also reported as a de novo synthesised compound of Douglas-fir heartwood that increases wood durability (Dellus et al., 1997). Higher resistance of the mature heartwood of Douglas fir to the fungal decay (Table 4) could be therefore explained by higher amounts of phenolic extractives and taxifolin in these tissues.

The relative humidity is an important factor affecting the properties and performance of wood. When RH is high, wood absorbs water vapour from the air. The importance of air humidity increases when the air temperature is below or near the dew point. The relationship between wood MC and RH is shown in Figure 6. Wood MC increases with increasing RH. Since there was no significant difference between the Douglas-fir from Celje and Planina, only the results from Planina are shown. The surfaces of the tested wood samples are more polar than water molecules and therefore show increased water absorption at low relative humidities (0–10%). Once a single (mono-)layer of water is formed, the additional adsorption increasingly resembles the condensation of water. At high humidities, i.e., above 70%, adsorption is enhanced by tiny surface pores (mesopores with pore diameter 2 nm–50 nm). These attract water molecules on more than one side, i.e., capillary condensation. This leads to hysteresis in this humidity region caused by the reluctant release of adsorbed water (Mangel, 2000). There is no significant difference between juvenile and mature heartwood’s sorption and desorption curves. On the other hand, the moisture content of sapwood is higher. For example, the MC of heartwood at 95% RH ranged from 19.36 (juvenile heartwood) to 19.83 (mature heartwood). The moisture content of the sapwood was significantly higher (21.93%; Figure 6). This is consistent with the data in the literature (Jankowska et al., 2016). Chemical analysis shows that sapwood contains significantly fewer extractives than heartwood (Figure 4). Generally, wood species with higher extractives content have lower equilibrium moisture content (Hernández, 2007).

We determined the MC of wood exposed above the saturated water atmosphere for modelling purposes after 24 h of conditioning. The RH above saturated water ranges between 98 and 100%. This value reflects the ability of the wood to absorb water from the air. The reference Norway spruce wood absorbs 16.8% of the oven-dry mass in 24 h. On the other hand, a difference was found between the wood from Celje and Planina. On average, the MC of wood exposed above the water for 24 h from Planina is significantly lower than the MC of wood from Celje. For example, the MC of mature heartwood from Planina was 12.5%. In contrast, the MC of mature heartwood from Celje was comparable to the MC of spruce (17.2%; Table 4). These differences can be partially attributed to the different densities. Due to the higher porosity of wood from Celje, water vapour diffusion occurs more rapidly than in denser wood with lower porosity (Sjökvist et al., 2019).

In addition to water vapour, outdoor wood is frequently exposed to capillary water (Rice and Phillips, 2001; Žlahtič-Zupanc et al., 2018). Therefore, special attention was paid to assessing this part of water performance. Namely, two types of tests were performed, short-term water uptake, determined in axial planes and long-term water uptake, determined by long-term immersion of the samples, with water penetrating the samples in all directions. Special attention is paid to penetration in axial planes, as these surfaces are usually the weakest point for both water penetration and fungal attack (Žlahtič Zupanc et al., 2019). Short-term water uptake into sapwood (Celje 0.0981 g/cm²; Planina 0.1185 g/cm²) was comparable to that determined on Norway spruce wood samples (0.1234 g/cm²). On the other hand, the water uptake to Douglas-fir heartwood was much lower. Namely, water uptake of 0.0210 g/cm² was determined for mature heartwood from Celje (Table 4). The main reason for the low uptake can be attributed to aspirated pits and the presence of extractives (Bolton and Petty, 1977). The same phenomenon can be observed in other conifers as well (Reinprecht, 2016). A similar effect was observed in samples immersed in water for 1 h. Water uptake in the Douglas-fir sapwood samples is in the same range as in spruce wood, while lower values were obtained in heartwood samples. The results of short- and long-term capillary water take also showed that water uptake was lower on samples from Planina than on samples from Celje (Table 4). The lower water uptake can be ascribed to the Planina samples’ higher density and lower porosity (Figure 3).

One of the main objectives of the manuscript was to determine the durability of the Douglas fir from Slovenia to wood-decaying fungi. The wood decay fungi used caused considerable mass loss of 35% (Gloeophylum trabeum) and 20% (Trametes versicolor) in the reference Norway spruce samples (Table 4). This indicates that the fungi were vital and that the wood was susceptible to fungal decay. Less decay of the Norway spruce samples exposed to the white-rot fungus T. versicolor is expected, as conifers are less susceptible to white-rot than brown-rot decay (Schmidt, 2006). The mass loss of the Douglas-fir sapwood after 16 weeks of exposure was slightly lower than the mass loss of the reference Norway spruce. Spruce wood exposed to G. trabeum lost 35%, while Douglas-fir sapwood samples lost between 27% (Celje) and 26% (Planina). A lower mass loss was observed in heartwood samples. Mature wood samples exposed to G. trabeum lost between 14% (Celje) and 16% (Planina). The T. versicolor showed a lower ability to degrade mature Douglas-fir heartwood, while the decay activity of Fibroporia vaillantii on heartwood was comparable to that of G. trabeum (Table 4). Better durability of the Douglas-fir wood can be ascribed to the higher concentration of biologically active extractives (Table 2).

Mass loss is a basis for the classification of durability according to the standard CEN/TS 15083–1 (CEN, 2005a). Based on the criteria of the respective standard, both sapwood and heartwood can be classified in durability class (DC) 4. Durability is on the borderline between DC 3 and DC 4, which is consistent with the standard EN 350 (CEN, 2016). Besides basidiomycetes, wood is frequently exposed to decay caused by terrestrial microorganisms, soft-rot fungi, and bacteria (Blanchette, 2000). Therefore, the parallel specimens were placed in the compost soil. The sapwood samples lost approximately 18% of the mass and 28% MoE. On the other hand, heartwood samples showed mass loss ranging from 9% (mature heartwood, Celje) to 14% (juvenile heartwood, Celje). However, the exposure did not result in significant losses of MoE. One of the possible explanations for this could be that the microorganisms eroded and abraded the surface, resulting in mass loss, but not loss of MoE.

Besides the decay fungi, the wood is also exposed to blue stain and sapstain fungi. All samples were completely covered with blue stains after laboratory exposure and rated with grade 3 according to EN 152 standard. On the other hand, exposure in the condensation chamber showed more significant differences (Table 5). The sapwood of Scots pine was covered entirely with sap stain after 5 weeks of exposure. On the other hand, fungal degradation progressed most slowly in Norway spruce wood. Spruce wood samples were rated 1 after 9 weeks of exposure. The disfigurement of Douglas-fir wood proceeded more rapidly. The sapwood reached an average value of 3.2 and the heartwood 2.2. The difference between the sites was not noticeable.

Factors describing wettability and inherent durability were calculated based on experimental analyses. The wettability of wood materials is a factor that indicates how the wood will behave during rain events. Wood that wets less and dries faster is less susceptible to fungal decay than wood absorbs more water during rain events and dries slowly. Sorption properties, permeability, possible tyloses formation, pit aspiration and other anatomically determined properties contribute to water penetration (Humar et al., 2020). As shown in Table 3, the moisture performance of Douglas-fir sapwood is comparable to that of Norway spruce. On the other hand, the k_wa factor of Douglas-fir heartwood is higher and is about 2 for both juvenile and mature heartwood, comparable to the moisture performance of European larch (Humar et al., 2019). It is known that the moisture resistance of heartwood is better than that of sapwood. In conifers, this is primarily due to pits aspiration and the process associated with heartwood formation (Bolton and Petty, 1977).

In addition to moisture performance, the k_inh factor, which characterises resistance to microbiological degradation, also plays a significant role in the performance of wood in outdoor environments. This durability primarily reflects the presence of biologically active extractives and biocides. Like the k_wa factor, the k_inh factor is also limited to 5. The durability of Douglas-fir mature heartwood varies between 2.9 (Celje) and 2.6 (Planina), which is slightly better than the value determined for European larch wood (1.8; Humar et al., 2019).

Based on the procedure proposed by Meyer-Veltrup et al. (2017), the material resistance dose (D_Rd) was calculated assuming a critical dose independent of wood species (D_crit = 325 d). This number corresponds to the number of days with optimal conditions for fungal decay before the first signs of deterioration appear. The resistance dose of Douglas-fir sapwood (436 days—Celje; 481 days—Planina) was higher than that of the reference Norway spruce. A much higher resistance dose was determined in the heartwood. The corresponding value ranged from 2007 days (mature heartwood from Celje) to 1,500 days (mature heartwood from Planina; Table 3). The difference between juvenile and mature heartwood was not significant. These values indicate that Douglas-fir heartwood will perform much better outdoors than the reference Norway spruce wood. The analysis indicated that the main reasons for the observed difference are the better water performance and inherent durability of Douglas fir. In contrast to the traditional durability classification, the dosimeter-based resistance model applied here showed the excellent performance of Douglas-fir heartwood, indicating that this material can be used for outdoor applications.

The average density of Douglas fir from two Slovenian sites was 489 kg/m³ (sapwood), 507 kg/m³ (mature heartwood) and 380 kg/m³ (juvenile heartwood). The low density of the juvenile heartwood can be attributed to the fast growth in the first decade.

The chemical analysis revealed the highest amounts of hydrophilic extractives and phenolic compounds in younger heartwood parts, i.e., mature heartwood. These tissues also contain the highest amounts of taxifolin. Higher resistance of mature heartwood against wood-decaying fungi can be therefore explained with the higher content of phenolic extractives and taxifolin.

The durability of Douglas-fir wood heartwood was like the durability of European larch and can be classified in the group of less durable wood species (DC 4). On the other hand, Douglas-fir heartwood exhibits good water exclusion efficacy, which contributes to a high resistance dose that can be attributed to pit aspiration. Based on durability and moisture performance tests, factors affecting inherent durability and wetting ability were calculated for Douglas fir. Douglas-fir sapwood exhibited a higher resistance dose (D_Rd = 459) than Norway spruce (D_Rd = 325). An even higher resistance dose was noted in Douglas-fir heartwood. There was a minor difference between juvenile (D_Rd = 1,621) and mature (D_Rd = 1754) heartwood. This indicates that Douglas fir has potential for outdoor use, where it can at least partially replace European larch and Norway spruce. As the population of Douglas fir in Slovenia is one of the southernmost populations in Europe, it can be expected that the performance of the Douglas fir from locations north of Slovenia will become more and more comparable to the values presented in the respective article because of the climate changes. Data presented herein can be used as an input factor for the prognosis of the future properties of Douglas fir from northern and central Europe.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

MHu and VV designed the study and wrote the manuscript. BL, DK, VV, EK, and MHo carried out the experiments. RB found suitable trees and organised the harvesting of the trees. MHu and RB were responsible for project management. All authors contributed critically to the drafts and gave final approval for publication.

The study was supported by Slovenian Research Agency (ARRS) within research program P4-0015 (Wood and lignocellulosic composites) and the infrastructural centre (IC LES PST 0481–09). Part of the published research was also supported by the Ministry of Agriculture, Forestry and Food in the frame of the project V4-1818.

The 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.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.