Tom's Blog

Saturday, March 23, 2019

Protecting trees in a bur oak savanna burn

 The south-facing slope at Pleasant Valley Conservancy is a good example of a bluffprairie. Such bluff prairies are common in the Driftless Area. However, although most of these sites are called “prairie”, most of them are actually bur oak savannas.

In a well-restored site, fire is the principal management tool, but there are significant difficulties when trees are present. Because of the steep topography (slopes up to 75 degrees), even in the absence of wind, head-fire flame heights can be 15-20 feet (occasionally even higher).

The bur oak trunk is very fire resistant, but its branches are less so and with high flame heights can be easily damaged. Thus, despite the recognized fire tolerance of the bur oak, even large trees can become seriously harmed.

When we first started burning the South Slope at PVC, the fuel was very spotty and fire did not carry well, so we mostly burned as a head-fire. Annual burns, accompanied by brush control and overseeding, gradually turned the South Slope into a tallgrass savanna. It was about burn year 5 that fire with high flame heights carried up into the bur oaks. Lots of branches caught on fire, and mop-up was a major problem.

Since then we have tried to back burn the South Slope, especially its bur oak component. A few years later we added tree-centered spotfiring as an additional protection, especially for the smaller trees.

The photos below show how we did the South Slope burn this year.

Starting the burn at the top of the South Slope. Depending on fire behavior, a second drip torch may be used to speed up the process.

Far end of the ridge. Backburning around ancient bur oaks

Above: Tree-centered spot firing small bur oaks. The blackened area protects the saplings from a  possible head fire.

Above. Later stage. The blackened areas have grown larger.

Above. Later stage. The black areas have coalesced and the continuous fire line is backing through the tallgrass prairie.

Once the bur oaks are completely in the black, the burn can be speeded  up by lighting a head-fire from the bottom.

The bur oaks are now "safe" in the black. The lower area is darker because it was burned as a head-fire.
The burn was done on March 21, 2019. Amanda was the burn boss.