Uncovering Mysteries of Fire Histories

Every place has a unique fire history. How often fires occur and how severe they are varies from region to region. For any one ecosystem, there are a wide range of "normal" forest conditions and fire activity.

Scientists evaluate if current fire activity is consistent with the area's natural fire history or if human activities like fire suppression have made an impact. Understanding a region's fire history helps guide development to better forest management.

Trees unveil mysteries of fire histories. There is no written record of every fire in every forest, so scientists reconstruct fire histories based on information contained in trees. Tree rings and fire scars show how frequently fires have occurred and with what level of severity.

Photo (right): Researchers uses an increment borer to take a core of annual rings. Credit: Meredith Gartner

 

Core sampling

Scientists use an increment borer, a drill-like instrument, to extract a thin section of a tree to count the growth rings to estimate age.

Photo: Example of a mounted and sanded tree core showing annual rings.Credit: Teresa Chapman

annual tree rings

Tree scars

 

fire scar on ponderosa pine cut fire scarWhen fires move through a forest, mature trees that survive the fire can be scarred. Fire scars form when heat from the fire kills a portion of the tree's cambium, the thin layer of cells responsible for growth of vascular cells, creating a scar that becomes visible as the rest of the tree continues to grow. Scientists remove a section of the scarred area and count the rings to determine when fire scarred the tree. Sometimes a tree will have multiple scars from multiple fires. Scars are usually found on the uphill side of the tree. As fire climbs up a slope, the flame tips reach around the trunk. The tips are the hottest part of the flames, so the uphill side of the tree is more likely to scar.

 

 

Picture (left): Fire scar. Credit:Meredith Gartner . Picture (right): Sample wedge removed from fire scar for dating year of fire in lab. Credit:Meredith Gartner

 

Reconstructing fire histories close up of fire scar year

 

Scientists uncover more complete fire histories when they study many trees from one area. When all the trees die in a high-severity fire, shade-intolerant (sun-loving) tree species can then occur. If seeds are available and climate conditions are right, new shade-intolerant seedlings will establish in a short time period (usually ~30 years). If most trees in a forest are close to the same age, a high-severity fire has probably previously moved through the area. Reconstructing fire histories Scientists uncover more complete fire histories when they study many trees from one area. When all the trees die in a high-severity fire, shade-intolerant (sun-loving) tree species can then occur. If seeds are available and climate conditions are right, new shade-intolerant seedlings will establish in a short time period (usually ~30 years). If most trees in a forest are close to the same age, a high-severity fire has probably previously moved through the area.

 

Picture (right): Close-up of annual resolution of fire year indicated by scar. Credit: Tom Veblen

 

In summary:
1) High-severity fire burns an area, killing all trees.

2) Nutrients, sunlight, water, and soil become available for new growth.

3) Lots of shade-intolerant tree species begin growing around the same time.

4) Dense stands of evenly-aged trees establish.