## 4 Discussion

Table 22: \(\Delta E^{*}_{ab}\) (Equation (8)) values between damaged wood and Norway spruce and Scots pine reference wood. in biological degradation because of harvest 5-7 months after the fire [21]. The effect of the biological and chemical degradation of wood translates into changes in viscoelastic behavior [43]. Clear positive correlations between density and mechanical properties were noted in our study, as in numerous previous studies [44,45]. In sapwood (mature wood), the mechanical properties were also higher than in heartwood (juvenile wood), particularly in pine. Structural differences between the mature wood and juvenile wood of softwoods have been reported in several compilation works [45, 46, 47]. Forest fire caused a decrease in the moisture content in spruce sapwood due to its thin bark and partly dead wood (high mortality of trees). The moisture gradient between sapwood and heartwood after the fire was larger in pine wood than in spruce. The radial position and basic density had significant effects on the moisture content. Density and the moisture content were partially negatively correlated for pine but not for spruce. In both species, the moisture content decreased from the butt to the top. When modeling the moisture content, the RMSE values were relatively high in both species, and the model's explanatory power was higher in Scots pine. The slight difference in harvesting time may have influenced the moisture content difference between the burned trees and reference trees, with the former being harvested in November and the latter in February (except for small-log spruce trees). In Northern Europe, the moisture content of spruce and pine is the highest in the middle of winter and usually 2-4% lower in November than in February [48,49]. In Norway spruce and Scots pine sapwood, a perceptible color difference due to the fire was observed. In heartwood, the \(\Delta E\) values indicated slight changes in color but were mostly below the perceptibility level of \(\Delta E^{\star}_{ab}=2\) for wooden surfaces [50]. The wood color was the darkest in the burned Norway spruce wood of all studied wood materials. Changes in wood color might indicate some chemical changes in the wood at high temperatures during the forest fire or some staining due to soot and char. The increase in the relative proportion of darker latewood with an increased cambial age explains the difference between heartwood and sapwood. In Scots pine, the color difference due to heartwood formation is naturally visible between sapwood and heartwood. A sharp boundary between heartwood and sapwood occurs naturally in Scots pine, while in Norway spruce, yellowish heartwood is not clearly distinguished from sapwood [51,52]. The differences in color parameters between heartwood and sapwood can mainly be explained by their different chemical compositions. The color of Scots pine heartwood is usually brown with a hue of red. The color of sapwood, especially earlywood, is typically very light, with a hue of yellow in both species. In addition, the proportion of darker latewood is higher in heartwood than in earlywood. It is well known that the temperature level and burning time of the exposed area during a fire are critical factors for changes in the physico-mechanical and chemical properties of wood [12,23]. We had no information available on the thermal circumstances during the forest fire, but we assumed that the temperature elevation stayed lower than in the prescribed burning. The following temperatures were measured in experiments in the forest: 400-500 \({}^{\circ}\)C at ground level, 220 \({}^{\circ}\)C at the 3 m level, 150 \({}^{\circ}\)C at the 6 m level, and 45 \({}^{\circ}\)C at the 9 m level [15]. In a forest fire, bark provides protective support for wood, slowing or inhibiting the rise in temperature and the chemical and physical alteration of wood.