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).
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.
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.
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.
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.
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| 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
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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.
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| 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.
What would have been the natural/historic management of a prairie/savanna such as the south slope where steep topography would have created extremely intense grass fires? Is it fair to say that there wouldn't have been any trees on the slope? Or are you just being extra cautious about preserving the old bur oaks?