Wetwood in black cottonwood (Populus Trichocarpa Torrey and Gray): the effects of microaerobic conditions on the development of decay
Creators: name: Gokhale, Atulchandra Anant
Publisher: name: University of British Columbia
Year: 2010

description: A study is described on the occurrence of wetwood in black cottonwood, found in the Lower Fraser Valley of British Columbia, Canada. Wetwood first appears when trees are 8 to 10 years old, and in 12 year and older trees, wetwood is always present. Further extension of wetwood is similar to that of a normal heartwood, A large number of microorganisms with different 0₂ requirements (i.e., aerobes as well as facultative and obligate anaerobes) were found in the wetwood. Wood-destroying fungi were absent in the wetwood. In the majority of the black cottonwoods examined, very low 0₂ concentrations (&lt;0.1% or microaerobic conditions, detected by a Field-lab 0₂ analyzer) were found in the wetwood during the summer. Generally the 0₂ concentration increased during the winter while the reverse was true for CO₂ (detected by gas chromatography). CH₄ was also present, but only in small quantities, in most trees. Mechanical wounding of wetwood resulted in an increase in the 0₂ concentration and decrease in the CO₂ and CH₄ concentrations. However, sealing of the wound re-established the original gas composition. Presumably the microbial flora of wetwood is primarily responsible for the microaerobic conditions. The ability of 2 wood-destroying fungi (Polyporus delectans Peck and Ganoderma applanation [Pers.] Pat.) to cause a weight loss in wood under microaerobic and aerobic conditions (10 weeks incubation each) was determined, using a soil-block experimental technique, Microaerobic conditions prevented weight loss in wood (average 0.1%) and therefore wood decay, whereas under aerobic conditions, the average weight loss was 41.7%. Special characteristics of wetwood such as high pH (average 7.8) or high moisture content (approximately 160%) did not contribute significantly to the decay resistance. Also, microorganisms associated with wetwood showed no antagonism to the growth of wood-destroying fungi. Exposure of wood-destroying fungi to microaerobic conditions subsequent to aerobic conditions arrested their growth and ability to cause weight loss. On the other hand, exposure of these fungi to aerobic conditions following the microaerobic conditions, induced a significant weight loss in wood (average 10.9%), but it was considerably less than when the fungi were grown under aerobic conditions alone (average 41.7%). The 2 wood-destroying fungi survived 10 weeks exposure to microaerobic conditions, A 13 weeks exposure to anaerobic conditions (&lt;0.002 % 0₂), however, resulted in the death of these, wood-destroying fungi. Eight wood-destroying fungi differed in their abilities to survive under anaerobic conditions; generally brown-rot fungi tolerated anaerobic conditions better than the white-rot fungi. Therefore, if anaerobic conditions exist in the tree trunks for long durations, some fungi (probably white-rot fungi) may die during that period while others (probably brown-rot fungi) may survive such an exposure and resume decay activity when 0₂ becomes available. These findings led to the concept that the microaerobic conditions found in the wetwood of black cottonwood may prevent the development of decay.