Truco Kernel

The temperature was the least significant climatic factor in this study. Temperature had a significant effect on the mould rating in the statistical analysis where data of all species were included (Table 7), but the separate analyses of all wood materials showed that this only applied to spruce heartwood and spruce sapwood (Table 11 - 13). Interaction effects between temperature and the other factors were also only significant on spruce. The effects of temperature were most prominent in climate 1, 2, 7 and 8, but the RH probably deviated from the set value in two of these climates (1 and 8), which made it difficult to determine whether it was the temperature or the other factors that affected the mould growth. When the statistical analyses were performed without climate 1, 3 and 8, temperature was no longer significant in any of the tested models. This made it challenging to draw any conclusions about the effect of temperature based on the statistical analyses.

Although the temperature overall seemed to have little effect on the mould growth, it probably had an effect on the wood moisture content. Lowering the temperature decreases the wood drying rate, and this led to higher wood moisture content during the exposure period (Figure 10). The temperature may also have affected the actual relative humidity in the climatic chambers because of the water spraying. This was especially evident in climate 8, set at 10 °C, 65 % RH and 4 hours wetting period. The air-regulating system in the chambers struggled to remove the moisture supplied during the water spraying. Since colder air have less ability to hold moisture than warm air, the relative humidity increased more during the water spraying in the climates with 10 °C than in the climates with 25 °C.

Earlier research has shown that humidity in general may affect fungal development more than what temperature does (Viitanen and Ritschkoff 1991). According to this theory, the specimens in the colder climates should have had more mould growth than the specimens in the warmer climates, since the colder climates generally had higher wood moisture content. This did not happen in all cases in this study, which was especially evident in climate 7 and 8. For pine heartwood (Figure 13), spruce heartwood and spruce sapwood (Figure 14),

climate 7 reached higher mould rating than climate 8, although the wood moisture content

(Figure 11) and measured relative humidity (Table 4) was generally higher in climate 8 than in climate 7. Hence, it seems that a higher temperature actually had a positive effect on the

(more durable wood materials and less humid climates). For each wood material in this study, the growth curves for the two climates with 85 % RH and 4 hours wetting period were nearly identical, and this supports the findings by Viitanen and Ritschkoff (1991), suggesting that the effect of temperature on mould growth is suppressed at high humidity levels.

As mentioned in the introduction, different moulds have different optimum temperatures, and the results therefore depend on which moulds are present. The spore suspension used in this study was made of three fungi and what actually grew on the specimens was not examined. Both Cladosporium cladosporioides and Aureobasidium pullulans are known to have optimum temperature between 25 and 30 °C (Käärik 1980; Sedlbauer 2001), but moulds from

Ulocladium may favour a colder environment (Mattsson 2004). The presence of Ulocladium

may thus have been a contributing factor to the low significance of temperature in this study, and identifying which fungi actually were present on the different specimens could have revealed this.

4.1.5 Wood properties

Wood material

In this study, aspen and pine sapwood were most susceptible to mould growth. Pine heartwood and spruce heartwood were least susceptible to mould growth. The water permeability and the content of nutrients and extractives may explain some of the differences in durability against mould growth. A more permeable material has a more open structure and absorbs water more rapid. During the water spraying, the more permeable species may have absorbed enough moisture to increase the time of wetness of the wood surface and therefore increase the mould growth. Aspen and pine sapwood had highest MC in several climates, but lowest MC in the driest and warmest climates. The calculated MC is based on weight measured several hours after the water spraying. The exact amount of absorbed and desorbed humidity for each specimen is therefore not known, but the more permeable wood species have presumably reached a higher MC during this period.

The differences between species may be smaller under more favourable conditions and the differences between species may be evened out over longer time. Climate 5 seemed to have very favourable conditions for the moulds used in this study. The differences between species were smaller in climate 5 than the other climates. Rüther (2011) found no difference in mould rating between Norway spruce, Scots pine and aspen after three and a half year of outdoor

exposure facing west; all species had achieved the highest possible mould rating (growth on over 70 % of the surface). West was the cardinal point with the most mould growth in the study (Rüther 2011).

Density

In this study, density had significant effect on mould growth on aspen, but not the other species. This study was only designed to test the effect of density on the spruce specimens, and density was therefore not expected to have a major effect on aspen and pine. Increased density significantly increased the mould rating on aspen, but the results may have been influenced by other wood properties. The heartwood proportion in the specimens of aspen was not known so the density may have been confounded with this. Although heartwood and sapwood of aspen are visually similar, there may be differences in the physical properties; heartwood of aspen may be less permeable than sapwood of aspen (Wengert 1976). However, the result in this study showing that density had no significant effect on the mould growth on spruce is interesting, and supports previous research stating that high density does not necessarily mean high resistance against fungi (Brischke et al. 2006; Gobakken et al. 2011).

Heartwood and sapwood

Sapwood was significantly more susceptible to mould growth than heartwood for both pine and spruce in this study (Table 14). Since no effect of density was found for pine and spruce, the assumptions that heartwood proportion is more important than density for the resistance against moulds are strengthened. The results showing that pine sapwood was more susceptible to mould growth than pine heartwood corresponds with the findings of Blom et al. (2013). Pine sapwood is known to be more permeable than pine heartwood (Øvrum and Flæte 2008), and the measured mean MC corresponds well with this; the MC during the exposure period was higher in pine sapwood than pine heartwood in several of the studied

climates, but the MC was lower for pine sapwood than pine heartwood in climates where the

specimens got the chance to dry (Figure 11). The low permeability combined with the increased extractive content may explain the increased durability for pine heartwood.

Results showing that spruce sapwood may be more susceptible to mould growth than spruce heartwood have also been found in earlier studies (Sandberg 2008; Blom et al. 2013). Spruce sapwood had slightly higher mean MC than spruce heartwood in several climates (Figure 11).

sapwood (Sivertsen and Vestøl 2010). The difference in mean MC between spruce heartwood and spruce sapwood was largest in climate 5, 6 and 7. These climates were also the ones with largest difference in mean mould rating between spruce heartwood and spruce sapwood after 91 days (Figure 9).