Its difficult to schedule a class field trip a month in advance which turns out to be the day of a fire, but this happened April 12 when I took my Forest fire and disturbance ecology class to the University of Minnesota's Cedar Creek Ecosystem Science Reserve, 35 miles north of the Twin Cities. The area is on the Anoka Sand Plain, with soils that are 95% or more sand, and the pre-European settlement vegetation known in the Midwest as sand savanna, historically dominated by bur oak with some northern pin oak. When we arrived, they were doing a backburn in one of the 'burn units', units of 5-20 acres in size which have been burned at different frequencies in an experiment going on for 46 years. The area was savanna in the early 1900s, but due to fire exclusion since that time, had grown up into oak woodland. The experimental burn frequencies to see how fire frequency affects vegetation have been 8/10 years, 5/10 years, every 3 years, once every 5 years, once a decade and never burned (the experimental controls). Photos by Kalev Jogiste, visiting Scholar from Estonia.
Fig. 1. Backburn in progress, unburned on left and backburn on the right, in an area burned 5/10 years:
Fig. 2. Burn crew member lighting/observing fire--beige areas are made by the University of Minnesota mascot, namely gopher mounds with exposed sand
Fig.3, heat from the fire creates an impressionist painting
Fig. 4. Burn crew watches fire approach the northern pin oak and bur oak woodland
Fig. 5. Fire enters the woodland.
Fig. 6. Lee Frelich talks to the class about bur oak ecology in another burn unit (10 days post burn) burned 8/10 years for the last 46 years. Bur oak tree is about 150 years old.
Fig. 7. The class examines a 250 year old bur oak. Many of these bur oaks had been overtopped by the fast-growing, relatively-large northern pin oak during the fire exclusion period, but now, at least on the frequently burned units, are once again on the open savanna (this area burned 8/10 years). Bur oaks growing on units with low burn frequency have a higher mortality rate than those on the high-frequency units, due to being shaded by the pin oaks.
Fig. 8. A fire scar has formed (white charred area at base of northern pin oak in middle of picture), because another tree had blown down with its crown near the base of the tree, allowing the fire to burn there fore the 15 or so minutes necessary to cause a scar. The bark and cambium have been killed in the charred area and the remainder of the bark will fall off, creating a scar shaped like a Gothic church door. Grass fires do not last long enough (<1 minute) to scar oaks. The pin oaks are being scarred, rotting at the base, then falling during thunderstorm winds, and causing other pin oaks to be scarred, thus accelerating the process of the woodland opening up at 40+ years after the burning regime began. This is increasingly leaving only bur oaks behind as the dominant tree in areas burned with high frequency.
Fig. 9. Example of a northern pin oak that blew down due to rotted fire scar at base, and causing an area of high fire intensity where the crown landed on the ground, in an area burned 8/10 years. Note how open the savanna is after the high fire frequency for 46 years.
Fig. 10. Gopher mounds and a gopher snake going after its prey.
Fig. 11. Areas burned 8/10 years (left) and never burned in the last 46 years (right). Note thin barked tree species such as serviceberry and red maple invading the unburned woodland, as well as the density of brush.
Figure 12. A unit burned every three years for 46 years. Note that it is restored to bur oak dominance with occasional pin oaks, but that the understory is dominated by 3 foot tall hazel (brown in color at this time of year), rather than grasses as in left side of Fig. 11 which is burned 8/10 years (green understory is dominated by grasses), which gradually removes hazel and allows grasses to dominate the understory.
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