The role of fire in the vegetation response to Little Ice Age climate change in the Big Woods of Minnesota

Background/Question/Methods

The Big Woods, an expanse of forest that included Acer, Tilia, Ostrya, and Ulmus species in southern Minnesota, developed between AD 1300 – 1500. The processes involved in the development of the Big Woods remain unclear, but an expansion of forest following a reduction in fire frequency as the result of climate change is a common feature of multiple hypotheses.  Recent landscape modeling suggests, however, that changes in fuels and the expansion of Big Woods populations were the cause, not the consequence, of reduced fire frequency. This modeling, therefore, brings into question the hypothesized role of fire. 

To study the importance of climate-vegetation-fire interactions within the area of the Big Woods, we examined the pollen and charcoal record from two lakes near the eastern margin of the forest (Bufflehead Pond and Minnetoga Lake). Both lakes are located on similar soils among multiple firebreaks and should have been equally protected from burning. 

Results/Conclusions

Charcoal counts record contrasting fire histories. Minnetoga experienced high charcoal influx (40 - 140 pieces cm^-2 year^-1), while Bufflehead experienced low charcoal influx (0.1 - 10 pieces cm^-2 year^-1). This indicates frequent or severe burning near Minnetoga and very little burning near Bufflehead.

In parallel, the Big Woods expanded quickly in the vicinity of Bufflehead (low charcoal influx) with squared chord distances (SCDs) between fossil pollen assemblages of 0.12-0.15 indicating a major vegetation shift. In contrast, the Big Woods did not develop around Minnetoga (high charcoal influx) with SCDs of <0.07. These differences underlie two conclusions. First, the data from Bufflehead show that a change in fire regimes did not contribute to the expansion of the Big Woods (at least locally), as this area had no substantial fires. Other ecological factors, such as the role of climate change in shifting the competitive balance among plant species, must then have been responsible for favoring the Big Woods taxa. Second, the absence of substantial vegetation change at Minnetoga confirms that frequent disturbance can minimize the vegetation response to changing climate.