Fungal colonisation on wood surfaces weathered at diverse climatic conditions.

Natural weathering test at two different European climatic zones were conducted to investigate simultaneously both, the fungal colonisation and weathering process of Scots pine wood (Pinus sylvestris L.). The hypothesis was that the wood performing differently in various climate conditions might affect fungal infestation. The colour changes, wettability, and glossiness were measured as indicators of weathering progress of wood together with an assessment of fungal diversity. Different intensities in weathering, occupancy, and colonisation of fungi on wooden surface were detected. A higher number of fungal species was found on wood exposed to the warm temperate climates compared to subarctic or boreal climates. The dominant fungal species in both locations were from the genera Cladosporium and Aureobasidium.

Decay Resistance of Surface Carbonized Wood.

Surface carbonization, or charring, of wood is a one-sided modification method primarily intended for protection of exterior cladding boards. The heavily degraded surface acts as a barrier layer shielding the interior from environmental stresses, and as such acts as an organic coating. To test the durability of surfaces created in this manner, unmodified, contact charred, and flame charred spruce and birch samples were exposed to the brown rot fungus Coniophora puteana and white rot fungus Trametes versicolor for a period of nine weeks. All sides of the samples except the modified surfaces were sealed to investigate the protective effect of the surface. Mass losses were greatest for unmodified references (up to 60% and 56% for birch and spruce, respectively) and smallest for contact charred samples (up to 23% and 32%). The wood below the modified surfaces showed chemical changes typical of brown rot and simultaneous white rot. The measured glucosamine content revealed fungal biomass in both the modified surface as well as the layers beneath. According to the recorded values, the fungal biomass increased below the surface and was higher for flame charred samples in comparison to contact charred ones. This is likely due to the more intact, plasticized surface and the thicker thermally modified transition zone that restricts fungal growth more effectively in contact charred samples in comparison to the porous, cracked flame charred samples. Scanning electron microscope images verified the results by revealing fungal hyphae in all inspected wood types and species.

Wood-Water Relations Affected by Anhydride and Formaldehyde Modification of Wood.

The moisture uptake of wood is influenced by accessible hydroxyl groups acting as sorption sites and the water-available cell wall space. To what extent do these mechanisms control the moisture uptake in wood needs to be addressed. For this purpose, we modified sorption site density and cell wall space by wood treatments with acetic anhydride or formaldehyde and investigated their effects on moisture uptake. Chemical changes at the cell wall level caused by the treatments were first determined by confocal Raman imaging. Following this, the deuterium exchange method was used to gravimetrically measure the hydroxyl accessibility, while the moisture uptake and the consequent swelling of the wood were determined by dynamic measurements of mass and dimensions within the hygroscopic range. The results showed that the effectiveness in reducing the moisture content of untreated wood across the hygroscopic range differed between the anhydride- and formaldehyde-modified wood. We also observed a poor correlation of accessible hydroxyl concentration in formaldehyde-modified wood with weight percentage gain and water uptake. Moreover, the dynamic mass and dimension analysis indicated that the reduction in swelling in formalized wood was affected by an unidentified mechanism in addition to reduced moisture content.

Chemical Characterization and Visualization of Progressive Brown Rot Decay of Wood by Near Infrared Imaging and Multivariate Analysis.

Brown rot fungi cause a type of wood decay characterized by carbohydrate degradation and lignin modification. The chemical and physical changes caused by brown rot are usually studied using bulk analytical methods, but these methods fail to consider local variations within the wood material. In this study we applied hyperspectral near infrared imaging to Scots pine sapwood samples exposed to the brown rot fungi Coniophora puteana and Rhodonia placenta to obtain position-resolved chemical information on the fungal degradative process. A stacked-sample decay test was used to create a succession of decay stages within the samples. The results showed that the key chemical changes associated with decay were the degradation of amorphous and crystalline carbohydrates and an increase in aromatic and carbonyl functionality in lignin. The position-resolved spectral data revealed that the fungi initiated degradation in earlywood, and that earlywood remained more extensively degraded than latewood even in advanced decay stages. Apart from differences in mass losses, the two fungi produced similar spectral changes in a similar spatial pattern. The results show that near infrared imaging is a useful tool for analyzing brown rot decayed wood and may be used to advance our understanding of fungal degradative processes.

Fungal Degradation of Extractives Plays an Important Role in the Brown Rot Decay of Scots Pine Heartwood.

Scots pine heartwood is known to have resistance to wood decay due to the presence of extractives, namely stilbenes and resin acids. However, previous studies have indicated that these extractives are degradable by wood decaying fungi. This study aimed to investigate the relationship between extractive degradation and heartwood decay in detail and to gain insight into the mechanisms of extractive degradation. Mass losses recorded after a stacked-sample decay test with brown rot fungi showed that the heartwood had substantial decay resistance against Coniophora puteana but little resistance against Rhodonia placenta. Extracts obtained from the decayed heartwood samples revealed extensive degradation of stilbenes by R. placenta in the early stages of decay and a noticeable but statistically insignificant loss of resin acids. The extracts from R. placenta-degraded samples contained new compounds derived from the degraded extractives: hydroxylated stilbene derivatives appeared in the early decay stages and then disappeared, while compounds tentatively identified as hydroxylated derivatives of dehydroabietic acid accumulated in the later stages. The degradation of extractives was further analysed using simple degradation assays where an extract obtained from intact heartwood was incubated with fungal mycelium or extracellular culture fluid from liquid fungal cultures or with neat Fenton reagent. The assays showed that extractives can be eliminated by several fungal degradative systems and revealed differences between the degradative abilities of the two fungi. The results of the study indicate that extractive degradation plays an important role in heartwood decay and highlight the complexity of the fungal degradative systems.

Extractive concentrations and cellular-level distributions change radially from outer to inner heartwood in Scots pine.

The heartwood of many wood species is rich in extractives, which improve the wood material's resistance to biological attack. Their concentration is generally higher in outer than inner heartwood, but the exact radial changes in aging heartwood remain poorly characterized. This investigation studied these radial changes in detail in Scots pine (Pinus sylvestris L.), using radial sample sequences prepared from three different trees. Stilbene and resin acid contents were first measured from bulk samples, after which the extractive contents of individual heartwood annual rings were investigated using Raman spectroscopy and fluorescence microscopy. Raman imaging and fluorescence microscopy were also used to study the cellular-level distributions of extractives in different annual rings. Although there were substantial differences between the trees, the content and distribution of stilbenes seemed to follow a general radial trend. The results suggest that stilbenes are absorbed into heartwood tracheid cell walls from small stilbene-rich extractive deposits over several years and then eventually transform into non-extractable compounds in aging heartwood. Resin acids followed no consistent radial trends, but their content was strongly connected to the frequency of large extractive deposits in latewood tracheid lumens. The results highlight the variability of heartwood extractives: their content and distribution vary not only between trees but also between and even within the annual rings of a single tree. This high variability is likely to have important effects on the properties of heartwood and the utilization of heartwood timber.

Cellular level chemical changes in Scots pine heartwood during incipient brown rot decay.

The heartwoods of many wood species have natural resistance to wood decay due to the accumulation of antifungal heartwood extractives. The natural durability of heartwoods has been extensively investigated, yet very little information is available on the initiation of heartwood decay. This experiment examined the onset of Rhodonia placenta brown rot decay in Scots pine heartwood in order to identify the key changes leading to heartwood decay. An imaging approach based on Raman imaging and multivariate image analysis revealed that the degradation of heartwood began in the innermost cell wall layers and then spread into the remaining cell walls and the middle lamella. Pinosylvins were extensively degraded in the cell walls, middle lamella and extractive deposits, while unidentified material most likely consisting of hemicelluloses and/or lipophilic extractives was removed from the inner cell wall layers. Changes similar to inner cell wall degradation were seen in the remaining cell walls in more advanced decay. The results indicate that the key change in incipient heartwood decay is the degradation of antifungal heartwood extractives. The inner cell wall degradation seen in this experiment may serve a nutritive purpose or facilitate the penetration of degradative agents into the cell walls and middle lamella.