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Above: Annual rings of a Douglas-fir tree injured by two fires. The rings growing before the injury in 2003 were filled with resin to create a boundary from infection at the injury site. Wood grew over the dead cambium, enclosing the injury. Then the tree was injured in 2007 and woundwood again enclosed the injury. Note that the bark apparently remained intact both times. Photo: U.S. Forest Service.

From the U.S. Forest Service

When trees are injured they develop physical and chemical boundaries around the injury wound to resist infection. Trees also grow new wood to close over the injured place. Injuries caused by fires result in fire scars and we use the patterns of scarring among many trees to understand when and how often fires burn.  This research helps to understand the biological process of fire scar formation and use it to improve fire history analysis.

Fire history information is used to interpret the ecological role of fire and other disturbance events in ecosystems and is required in environmental assessments. Fire scars in different tree species don’t all look the same, which can lead to confusion about whether irregular growth in wood is a scar. This research examined the process fire scar formation and the anatomy of fire scars in conifer and hardwood trees.

In three species of trees that survived wildfires near Missoula, Montana (western larch, Douglas-fir, and ponderosa pine) we found that injuries happened even when bark was not charred. This was also true of oak trees we examined that burned in a prescribed fire in Ohio hardwood forests. It would be impossible to know that the tree had a scar by just looking at it. Where a tree’s bark was rough, both hardwoods and conifers sometimes had several small injuries next to crevices in bark where heat could get to living cells more easily. Hardwood trees produce chemicals to resist infection, mostly phenols. Conifer trees produce resin (terpenes) as a physical and chemical barrier to block infection.

Key Findings

• The stems of western larch, Douglas-fir, and ponderosa pine trees can be injured from fire even when the bark isn’t visibly charred. Heat alone causes the injuries, especially where bark is thinner.
• Western larch, ponderosa pine, and Douglas-fir ordinarily make resin in cells called resin ducts. After an injury western larch and ponderosa pine produce many extra (traumatic) resin duct cells, and in turn these ducts produce and transport a lot of resin. Traumatic resin seems to be a better disinfectant than the usual resin. In larch traumatic resin ducts are especially large and effective at transportation. Douglas-fir trees don’t produce traumatic resin ducts, so after an injury they produce relatively less resin, and less effectively transport it. They tend to die more easily than larch or ponderosa after a fire.
• In years following a fire, wood grows around the edges of the injury. The density of wood cells is higher at the edge of the injury and when they grow, the tree ring at that point is unusually wide. If the injury is small, within a few years it can close over and be invisible from the outside. Sometimes injuries are never visible from the outside.
• Knowledge about why these differences occur in a particular species can help us understand how scars are formed, why some trees survive or die after injury, and determine whether irregularities in wood anatomy are likely to be a fire scar.