Passage VI

Two studies examined how forest fires would have burned during the late Paleozoic era, when Earth's atmosphere contained an elevated O₂ content of more than 21% by volume.

Araucaria tree leaves and Pinus tree dowels (wood pieces) and needles, all from modem trees, were used in the studies to most closely model trees in Paleozoic forests.

Study 1

A 10 mg sample of Araucaria leaves was heated at a rate of 40°C/min in an atmosphere containing 21% O₂ by volume. The mass of the sample was measured every 2 sec during heating. These procedures were repeated in a second trial in an atmosphere containing 35% O₂ by volume. Then each trial was repeated using a 10 mg sample of paper instead of Araucaria leaves (see Figure 1). The rate of mass loss was directly proportional to the rate of combustion.

Study 2

In each of several trials, a 75 cm long chamber was filled either with dowels or needles, both from a Pinus tree. Each sample of dowels or needles had been dried and then soaked in water to bring to the desired water content by weight. A mixture of O₂ gas and N₂ gas was continuously supplied to the chamber. The sample was ignited at 1 end of the chamber.

For any trial in which the sample burned completely, the flame spread rate was recorded. Those trials represented conditions under which a forest fire would have burned out of control. A failed burn (F) was recorded far any trial in which the fire in the chamber extinguished itself before the sample burned completely (see Table 1).

Figure and table adapted from Richard Wildrnan et al... “Burning of Forest Materials under Late Paleozoic High Atmospheric Oxygen Levels." ©2004 by the Geological Society of America.