Tom's Blog

Sunday, December 28, 2014

Bur oak survival of head fires in hill country

Pleasant Valley Conservancy is in the midst of the hill country of the Driftless Area of Wisconsin. We have a fine and extensive bur oak savanna on the upper part of our south-facing slope. Since bur oaks are known for their fire resistance, it seems reasonable that their presence here relates to historic fires that took place in this area.

We know from oral reports that the farmers in this area routinely burned their open slopes as a means of encouraging early green-up of grasses for their grazing animals. These were not carefully controlled burns, but essentially wildfires. The fire was almost always started at the
bottom of the hill and just allowed to burn. (Throw out a match and then go in for supper.) Depending on the conditions (fuel moisture, wind, R.H., topography) the fire could move quite rapidly. In this hill country, flame height depends more on topography than on any other factor. Even in the absence of wind, flame heights can be high (10 feet or more) and rate of spread can be rapid (50-100 feet per minute). You don't want to get near that sort of fire!

However, because of the linear shape of these "burn" units (see the air photo below), most of the fire would be a flanking fire getting ignited from either side of the original headfire.

It makes sense that the bur oaks on the upper part of our hill are the ones able to survive these vigorous (probably annual) fires.
The upper slope is dominated by numerous open-grown bur oaks. The fuel here consists primarily of warm-season (prairie) grass, which burns very hot.

When we first started burning our south slope in 1998, it burned very poorly. We could only get it to burn as a headfire. Since the fuel was light, this was a relatively gentle headfire and was very easy to carry out. We simply burned in fire breaks around the edges and then lit from the bottom. As the years went by the burns gradually got better. Then suddenly in 2005 the burn situation changed. We stood at the bottom of the hill and watched amazed as the fire roared up the hill with 10-15 foot flame heights. The whole burn was over in a few minutes. Snags and dead trees burned. Even some living bur oaks caught fire. But almost all of these living trees survived the fire without any significant damage and are still thriving.

This was probably the sort of fire the original landowners experienced every year.
Headfire on the south slope: typical of what fire must have looked like in historic times

We know from an examination of the 1937 air photo that the vegetation of the south-facing slope consisted of open prairie on the lower part and oak savannas on the upper part (see air photo below). Many of the trees shown in this air photo are still here but 77 years older. (The oldest tree we have found, based on coring, is 278  years old.)

USGS topo map overlaying the 1937 air photo for the south-facing slope. The burn unit is delineated as a narrow red line.  The individual bur oak trees can be seen scattered across the hillside. Many of these trees are still present today. The red and orange double lines show the position of the present service road. (ArcGIS map)
Where our terrain is less steep, many of the oaks are white instead of bur. These are areas that burn less hot. The photo below shows the distribution of the bur oak (yellow dots) and white oak (white dots). Note that all the large oaks on the south-facing slope are bur oaks.


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