Oxygen Deficit Strategy for Boats

Several months ago, a boatbuilder friend asked me why sapwood is prone to rot in boats but in the living tree we generally observe sound sapwood surrounding sometimes decayed heartwood. At the time, I offered the quick answer that sapwood in the tree is a “living” substance, while heartwood, consisting of non-living cells, is, like all dead organisms, prone to decay—except, of course, in tree species that deposit decay-resistant chemicals when sapwood is converted to heartwood. I felt that this was an unsatisfactory answer and said I would ponder the question further and perhaps develop a column from my thoughts. I forgot about the conversation until it recently shifted from my long-term brain repository to active-currency mode. After a bit more thinking and research, here is a deeper dive into the question. Multiple Defenses The primary defense that trees have to thwart invasive insect and pathogenic organisms is the production of an outer covering of bark. Tree barks can be quite variable in structure and thickness. Often, the longest-lived trees develop the thickest barks. Coastal redwoods, for example, can have bark up to a foot thick in the oldest trees. Another strategy used by some trees is the production of a thin but exfoliating bark in which the outer layers are continually shed, ridding the tree of potential pathogens. Mechanical injury to the bark from external sources is rare in undisturbed forests but can be a serious issue during poorly managed logging operations. The principal way that pathogens can enter a tree is through naturally occurring broken branch stubs, as shown in the accompanying photograph. But, as the photo demonstrates, the bark cambium develops a collar to seal the sapwood so that potential decay fungi are shunted directly into the heartwood. Of course, bark closure at branch stubs or other wounds takes some time. Forest pathologists have documented ways in which the living sapwood mounts its own defense during that lag. When sensing fungal invaders, scattered living parenchyma cells develop chemical responses and begin a process of compartmentalization to contain the infection and thwart further spread. These elaborate mechanisms are most well-developed in broad-leaved hardwood trees. Oxygen Deprivation Millions of years ago, before hardwood tree species appeared on Earth, many conifers had adapted a much simpler and more effective means of preventing sapwood decay in the living tree. As many readers of this column know, wood-decay fungi require three prerequisites: a food source, moisture, and oxygen. The food source, of course, is wood—either cellulose (brown-rot fungi) or lignin (white-rot fungi). Since the sapwood is the conduit that transports water from the roots to the branches and leaves, moisture is a given. This means that the most effective way of thwarting fungal attack is by reducing oxygen to levels below which decay fungi cannot continue to operate. The critical moisture content (MC) needed to reduce oxygen levels below what is required for decay fungi to survive and grow is a bit of a mystery. Any level above about 30 percent will mean the cell walls are saturated. As MC increases above that level, free-oxygen levels wil