WOUNDS AND CANKERS Not only do openings in the bark allow decay fungi and insects to enter trees, but also they can weaken a tree and make it more hazardous. Wounds are mechanical injuries while cankers are localized areas of dead cambium and bark caused by microorganisms, usually fungi. Large, old, deep wounds are associated with more decay than small, young, shallow ones. Wounds at groundline are more likely to have associated decay than those higher up. Carving wounds made on thin-barked species such as aspen, birch, and beech can seriously injure trees (Figure 3). Figure 3 - Initials carved in bark allow entry of diseases and insects that can further injure trees. Figure 4 - Common stem cankers: a. Target or Nectria canker (Nectria galligena) occurs on many hardwood species. Because cankers are caused by a living organisms, they often continue to enlarge year after year (Figure 4), while a wound may begin to callus over soon after it is made. Some main stem cankers continue to enlarge until the tree is girdled. Wounds and cankers can be weak points on a trunk and their position relative to the direction of prevailing winds influences the risk they represent. A tree is more likely to break at a wound or canker if it is either facing or opposite the direction of the prevailing wind. Most trees fail during winds of greater than 40 mph, but there always are exceptions. Trees have been known to fall on perfectly still days. Figure 4 - Common stem cankers: b. Eutypella canker (E. parasitica) occurs mainly on sugar maple. Figure 4 - Common stem cankers: c. Hypoxylon canker (H. mammathum) occurs on aspen.

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