How much difference did anthropogenic fire have on background natural fire regimes in eastern North America? Based on the limited data for the previous (Sangamon) interglacial, which occurred well before humans entered North America, vegetation in the prairie peninsula region developed a prairie-like vegetation, though perhaps less coverage relative to woodlands compared to the Holocene. Though I understand that in the current fragmented landscape, lightning contributes very little to the fire regime relative to human ignitions, I would expect that a simulation using larger unbroken blocks of fuel would have a greater potential for ignitions and for the rare large fire. Broken and moister landscapes would likely have had less fire and a more mesophytic vegetation without supplemental human ignitions. But lightning fire should have been sufficiently frequent in the most favorable landscapes to have evolved endemic fire-dependent species to begin with, unless they all somehow evolved within the last 15,000 years of human occupancy.
There are lightning-triggered fires in the boreal forest of Canada & AK today.
Lightning without rain could also have been a fire trigger in the past.
There are some issues with standard charcoal processing of lake sediments (using a 125 micron sieve) to reconstruct fire histories; I will have an upcoming paper on this.
Also, regarding the pre-Holocene, megafauna are an underappreciated massive shaper of landscapes.
There are several species of prairie vegetation that require fire to either germinate or release their seeds. Some of these species go for years without reseeding until a fire comes along. In addition, the longleaf pine's early stage of growth is fire resistant. While the pace of evolutionary changes is often rapid, I would argue that these species are indicative of sporadic wildfires across the landscape caused by lightning.
Yes, I would expect Megafauna would play a role that no longer operated in prairie during the Holocene, other than bison. And this would compensate for a reduced role of fire in maintaining open conditions. But specifically, I am still trying to understand whether any savannas and prairie would have developed at all in southern Michigan with or without the help of Native Americans and whether they would have been able to persist subsequent climate change when mesophytic species like beech expanded its importance. Certainly some savanna areas near former native villages as near Grand Rapids are more isolated from other savannas, suggesting a greater role of human ignitions. Less clear are larger landscape units from Kalamazoo south that are contiguous with prairie into Illinois. Insights from modeling adjacent areas do help.
Gregory,
Interesting. We know that flora communities retreated southward during the Pleistocene. Species like the cold temperature white spruce now found above the U.S./Canadian border migrated as far south as St. Louis according to archaeological evidence. The retreating southern margin of the ice sheet during the waning Pleistocene/early Holocene would have been more of a tundra-like region. This would slowly transition into prairie like that found today in portions of Alberta, Manitoba, and Saskatchewan, Canada, as the glaciers retreated northward. The slow northward recolonization of flora likely would have transitioned from prairie to savanna and finally to open woodland interspersed with small prairies. I would assume that there is sufficient pollen evidence in lake sediments and relict stream channel deposits of these transitions. I will have to see what I have on pollen studies in that region.
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