Above is a typical Bigwoods Remnant Forest in SE Minnesota

Bigwoods of Minnesota

A Brief History

Oak Savanna/Woodland

The Bigwoods were a large area of deciduous forest surrounded by prairie that occupied much of south-central Minnesota. The existence of this tongue of forest surrounded by prairie -- and its associated fires -- was the cause of much discussion in the 19th century and has been the subject of several research projects since.

J. McAndrews in the 1960s and Eric Grimm again in the 1980s reconstructed the long-term history of the Bigwoods based on pollen analysis and changes in sedimentation. Both suggested that a reduction in fire frequency coincident with the onset of the 'Little Ice Age' ( cool/moist period from ~1250 AD to 1850) was the trigger for the expansion of the Big Woods; a competing hypothesis is that a reduction in Native American populations due to disease was responsible for a drop in fires but there is very little data to address this hypothesis directly. Grimm showed that the borders of the Bigwoods corresponded with the location of rivers and lakes and other features of the landscape that would have acted as fire breaks.

My students and I are trying to better understand the relationship of fire to the rise of the Big Woods, examining a range of proxies from the sediments of 17+ lakes in and around the Bigwoods area. Our work (see Umbanhowar 2004 in references below) suggests that afforestation of the region -- primarily signaled by an increase in the amount of oak pollen -- began as early as 1150 AD during the Medieval Climate Anomaly (900-1250 AD, warm and dry) and then accelerated with the on-set of the Little Ice Age. Forest may have begun expanding during a dry year because the intensity and frequency of grassland fires were reduced by the extended period of aridity/drought (grass productivity is strongly positively correlated with moisture). Once established, oaks suppressed the grasses creating the necessary conditions for forest expansion that was encouraged by the cooler/moister conditions of the Little Ice Age. It should be stressed that this is a working hypothesis and has not been directly tested.

I have also been working with Phil Camill (Carleton), Christoph Geiss (Trinity College), and Becky Teed (LRC, University of MInnesota) on cores from two lakes in the Bigwoods (Kimble Pond and Sharkey Lake) both of which have sediment records that extend 10-12000 years. We have published several papers based on these two sites. The results from this work indicate that the Big Woods has been a mosaic of woodland and grassland since at least 9000 years ago and that changes in fire (frequency and/or intensity) may be the product of climate-driven changes in fuels.

Selected References

Close-up of Prairie in Fall

Prairie Fire

(St. Olaf College Homepage)