Exploring features of fire scars found on fossil trees will increase our knowledge of the ecological processes that occurred in ancient forests and the role of fire as an evolutionary force. We described the first fossil fire scar ever reported in a tree from the Late Triassic Chinle Formation of southeastern Utah in 2014. We also searched for other potential fire scars in the Petrified Forest National Park in northeastern Arizona, and found thirteen examples of fossil logs with external features resembling fire scars. Three well-preserved trees in the northern section of the park showed external features that closely matched those of modern fire scars, such as “cat face” shapes at their bases. We hypothesize that trees in ancient forests will show growth release after the tree recovers from the fire because of reduced competition and/or a pulse of nutrients released by the fire, and that water stress associated with a pre-fire drought will be reflected in the fossil wood anatomy, as in modern trees. To test these hypotheses we collected a fossil tree section showing the external characteristics of a fire scar from the Petrified Forest and prepared it for examination of the microscopic anatomy of the fossil wood.
The cellular structure of the petrified wood was well preserved in this specimen. A band of compressed and distorted tracheids (wood cells) was associated with the scarring event that is recorded in the external morphology of the tree. This band of small tracheids is similar to that found in the previously described fossil fire scar and comparable to features found in modern fire scars. In combination with other features, we therefore conclude that this is another ancient fire scar, the second example ever described. We are currently measuring the features of the fossil wood anatomy in this specimen (e.g., cell lumen diameter and wall thickness, density of tracheids and rays) to learn more about the growth of the tree before and after the fire event. We will determine if it exhibits a growth release following the fire, which was found in the previously described fossil specimen, and which is characteristic of modern fire scars. We will also look for signals of drought in the pre-fire fossil wood, which have been detected in the wood anatomy of modern trees in the years immediately preceding fires. Results from these measurements will be available soon.