Wildfire appears to be a strong & natural driving force of ecosystems in our area. These historic adaptations from fire & the range of native plants & animals in our area due to fire are still present. To understand wildfire frequency in our area prior to European settlement before they plowed & cleared our area, we have to go way, way back.
Grasses first appeared in Africa & South America between 55 & 60 million years ago (GPWG, 2000; E. Kellogg, 2001). However, it would not be until around 30 million years ago that organized prairie ecosystems & communites began to really expand & dominate, first appearing in Chile (Udurawane, 2017). This is in the the same timeline that the Rockies were uplifted in the Laramide orogeny (English, Johnston, 2004). With a Rockies rain shadow (with less precipitation downwind of the Rockies), grassland migration & development continued to develop & expand, perhaps south to north into & in the United States. By around 20 million years ago, fossils show what forests left on the Plains were dying out or had died out or been eradicated by intensive grazing. The development of large, grazing animals at the time perpetuated the development of Plains grasslands. Mammalian rise & prairie dominance correspond with each other. The lower Plains rainfall & the animal grazing was more fitting for grasses, rather than trees. Shifting ocean currents with an increasingly cooler Pacific & the rise of mountains & thus ice sheets & montane ice led to a climate more suitable to grass in many areas of the world.
It is likely in a cyclical pattern that the grasslands of the Plains migrated east as the Rockies were thrust higher & higher. The grasslands probably contracted at times, too (with higher precipitation). Given the high amount of large grazing animals that evolved over millions of years, the Plains prairie ecosystem began to evolve & rely on browsing & distrubance to maintain itself. Lightning probably produced some fires, too, which burned away some woody vegetation.
When you look at the extent & diversity of large herbivores in the Plains & Midwest for such a long period of time, it is no wonder that there were clear windows for grassland vegetation to migrate eastward. This large part of the country east of the Rockies was also an immobile piece of the earth's crust called a craton. Craton & their lack of mobility tend to create flatter landscapes, which make ideal spaces for large mammal & grassland migration. More & more grass food & mammals that developed to digest prairie plants well led to larger & larger mammals. This led to more & more grazing, which only allowed prairie to expand. In fact, there were likely open savannas & prairies in large areas of the eastern U.S. at the time, except in steeper, swampy, more inaccesible areas for mammals.
Fast forward to the Pliestocene, which began roughly 3 million years ago & lasted until around 11,500 years ago when great ice sheets grew & moved southward in successive waves from the north & covered our area. This ice spread wide expanses of glacial till (mixed soils, rock & gravel) over massive areas of the northern Plains & over our entire viewing area. In each glacial recedence, as the climate grew warmer (& melting ice carved out deep ravines like those around Wildcat Creek, Wabash River & others), very dry climate followed (all the water was caught up in the ice). Also very strong gradient winds on the edge of ice sheets produced dust storms with very dry air that favored tundra vegetation & then grass. Between the glaciations, the climate warmed & returned to more of what you would expect today, along with very large grazing mammals. In fact the fauna was very much like the African savanna with large elephants, huge cats, bears, camels, sloths, etc. around here.Coring of old lake beds dating back to near 130,000 years ago in southern Illinois (following the third glaciation of our area) show oak & prairie plant pollen present, even before humans. The last glaciation, which receded over our area near 12,000 years ago, brought a similar situation as the three former coverages of ice with dry climate & strong winds over lots of flat glacial plains & receding glacial lakes. Wind-blown dunes of sand also occurred that were especially dry. Since the end of this last glaciation, there has been one main "prairie period" (period of rapid prairie & savanna development in our area) & two main sub periods. The main one was the "Hypsithermal" or "Xerothermic Period". Two sub periods have often been called the "Roman" & "Medieval Optimum" periods. These periods show a hotter, drier climate than today with rapid expansion of prairie & oak pollen in profiles of core in bogs & lakes in our region. This was also a period of rapid die off of Eastern Hemlock trees, which prefer cool, moist climates. Only small pockets exist on north-facing slops in deep hollows along a few streams in west-central & southern Indiana. Following this last glaciation, the last of the huge mammals died out rather mysteriously. This mass extinction of our local viewing area African savanna fauna left us with a lack of those grazers to chew & keep many prairies from being overtaken by trees. The cyclical expansion/contraction prairie climate was in place & we had flat glacial & lake plains over our area with some sandy & gravelly dunes & ridges (& an occasional wildfire from lightning) for grass. However, with no grazers, what maintained the prairies & savanna that lasted right up to the 1800s in our area? We will dig into that tomorrow! We will also dig into why Plains plants & plants native to the southeastern U.S. & western U.S. are native here in isolated, random populations!
Some typical North American grazers of a Pleistocene interglacial (ice free period before the next glaciation) (Groning, 2014):