Microaerobic and Anaerobic Bacterial Activities Involved in Formation of Wetwood and Discoloured Wood

High numbers (10 6 -10 7 cellsjg) of strictly and facultatively anaerobic bacteria were found in wetwood sam pies taken from Eastern cottonwood (Populus deltoides Bartr.) and American elm (Vlmus americana L.). Typical products of anaerobic fermentations (methane, alcohols, fatty acids) and indications of pectinolytic activities (pectinolytic bacteria, free pectinolytic enzymes, destroyed pit membranes) were detected and characterise this type of wetwood as an anoxic habitat in wh ich pectin, as weIl as starch and possibly monomeric wood sugars are subjected to nearly complete an aerobic degradation. In other types of wetwoods, as in fir, spruce, oak and hemlock, anaerobic conditions are less expressed. The different appearances of wetwood in different tree species are discussed as possible consequences of differences in wood structure and substrate availability. Bacterial activities influence the chemical conditions (alkaline or acidic, aerobic or anaerobic) under which polymerisations of phenolic compounds to form visible stains and discolourations will take pI ace to a hirger or minor extent.