Fire-scarred fossil tree from the Late Triassic shows a pre-fire drought signal.

Exploring features of wood anatomy associated with fire scars found on fossil tree trunks is likely to increase our knowledge of the environmental and ecological processes that occurred in ancient forests and of the role of fire as an evolutionary force. In Petrified Forest National Park, Arizona, where Late Triassic fossil trees are exposed, we found 13 examples of fossil logs with external features resembling modern fire scars. One specimen with the unambiguous external features of a fire scar was collected for analysis of its fossilized wood. A light-colored band composed of compressed and distorted tracheids was associated with the scarring event. Cell lumen diameter and cell wall thickness in the pre-scarring fossilized wood show a response similar to that described in modern trees experiencing drought conditions. Tracheids in the post-scarring wood are initially smaller, and then become larger than average following a recovery period, as is often observed in modern conifers following fire. The responses in external morphology and wood anatomy to drought and fire were similar to those of some modern trees and support the view that some forests may have experienced conditions favoring the evolution of fire-adapted traits for more than 200 million years.