Stem Decays and Stains
Figure 196. Figure 196. Evidence of decay is marked by fungal conks or cavity nesting holes.

Decay of wood is caused by fungi that utilize the cell wall components of xylem as a source of nutrition. These fungi have both beneficial and damaging influences on forest stands. In living trees, decay fungi break down tissues in branches, roots, and stems of trees. They play an irreplaceable role in the earth's carbon cycle by returning to the atmosphere billions of tons of carbon each year and provide valuable habitat for many species of wildlife. Although vital to ecosystem functioning, decay can render trees useless for wood products and create hazard trees in recreation sites.

Many decay fungi discussed here decay the heartwood of living trees, others decay the wood of dead trees, and some grow on both live and dead trees. Most decay fungi do not interfere with the normal growth of live trees since they stay in the heart or xylem wood of the host. However, they can affect tree structure and contribute to wind breakage and windthrow as decay progresses. There are some decay fungi that cause mortality. These fungi begin decaying the host as a heart rot and later move into the sapwood and cambium, which cuts off the flow of water and nutrients, girdling the tree. Other fungi decay the wood of dead trees and can also decay dead sections of live trees, such as a dead top or old wound on the bole.

Decay levels in live trees are highly correlated with age. For example, high amounts of decay are typically found in sites where conifer trees are 150+ years old. Susceptibility to stem decay fungi varies with tree species. Some hardwoods—like cottonwood and aspen—are the most susceptible to stem decay in the Southwest, and Douglas-fir, white fir and spruce are in general more susceptible than ponderosa or southwestern white pine. However, susceptibility is also dependent on site quality and host genetics; silviculturists have observed some 70-year-old ponderosa pine sites with high levels of decay.