Oak savannas: unburned patches in an otherwise complete burn

Given the right fuel and weather, oak savanna burns can be as complete as prairie burns. However, there are times when unburned patches remain after burning is otherwise complete. Although these unburned patches could be "touched off", the drip torch operator(s) have often moved on and don't have time to go back.

The oak savanna burn at Pleasant Valley Conservancy on March 21, 2017 was mostly successful, but distinct unburned patches were left in the white oak savannas on the lower slopes (Units 11D and 12A).

Yesterday I had time to go back and try to figure out why these areas did not burn.

Unburned patch on the lower slope of Unit 12A (White Oak Savanna)

There is plenty of fuel in this patch (oak leaves and prairie grass), and if "touched off" it should burn completely.

Since this unit faces south and has substantial steepness, it was backburned from the top of the hill to protect the large open-grown white oaks (over 150-years old). Because the wind was variable, it might have been blowing uphill at the time the fire line reached this point. Also, this patch is almost level, which could have influenced the fire line. Although the fire went out here, it burned around the edges and continued downhill.

This patch constitutes what is called a "refuge", where fire-sensitive organisms survive. My estimate is that about 85% of Unit 12A burned completely, leaving about 15% as refugia. In the adjacent Unit 11D the unburned areas were even more, amounting to about 50%.