Studying Fire History Using Witness Trees

Photo Credit

Map created by Melissa Thomas-Van Gundy and Greg Nowacki

A map of the eastern U.S. shaded to show areas where pyrophilic percentage has been calculated.

Witness trees can be used to figure out what kinds of trees were in an area 100+ years ago. Historical notes describe witness trees and where they were located. Surveyors sometimes took other notes about the area like what kinds of plants were there, what the ground looked like, and if there was evidence of recent damage from things like wind or fire. 

For this project, the researchers used a new approach by classifying witness trees into two groups based on their known traits: those that do not tolerate fire (pyrophobic) and those that do (pyrophilic). By analyzing these trees and using statistical methods, the team has been able to map past fire patterns. This helps scientists understand historical fire environments better and provides valuable information for forest management.

The concept of ‘pyrophilic percentage’ was first explored for the Monongahela National Forest area in West Virginia. In this region, where direct measures of fire history are scarce, methods like analyzing witness trees become crucial. These trees reflect the long-term impact of recurring fire on the landscape.

Key Findings

• Witness trees can be used effectively to understand the makeup and fire history of past forests.
• In West Virginia, places that had more fires in the past tend to be at lower elevations.
• Understanding where fires happened a lot in the past can help us learn about how trees and other vegetation have changed over time.
• In the ‘prairie peninsula’ area of Indiana and Illinois, lakes and rivers were very important for stopping the spread of fires.
• In the Midwestern Tension Zone, scientists used witness trees to see how fires and temperature are linked. Fires north of the Midwestern Tension Zone were different from fires south of it.
• The maps created do not directly show where fires occurred in the past but do show the influence of fire on vegetation patterns.

Approach 

In 2024, the researchers published a scientific paper describing the technique they developed to map historic forests using witness trees. The paper also includes a full set of maps for all of the areas of the U.S. that have been analyzed so far.

This work has been done in stages over many years, starting with what is now the Monongehela National Forest in West Virginia. The scientists used information dating back to 1752 on about 22,000 witness trees to reconstruct what the forests of the past were like.

The team then looked at witness trees on national forests in the northeastern United States and mapped the historic forests in those areas. For this area, they were able to analyze how fire affected forests around Native American settlements.

Later analysis mapped past forests in Minnesota, Ohio, Indiana, Illinois and Wisconsin. For these midwestern U.S. states, the scientists calculated how important fire was in different areas. They also identified and mapped different versions of the Midwestern Tension Zone, an area where there was a transition between plants that depend on fire and plants that do not like fire.

In the pyrophilic percentage analysis, the maps show a clear relationship between pyrophilic percentage and elevation, with more fire-loving plants and trees at lower elevations. This pattern lines up with climate factors such as temperature, growing season length, and water availability, indicating a complex relationship between fire, vegetation, and climate.

Outcomes

• This research can help people understand how climate and fire work together to shape the trees and vegetation in different areas.
• Land managers can use this research in making decisions about using fire and allowing other disturbances in forests where fire was important in the past.