It is a fractal frontier, patchy at every scale, with small patches within larger. And it is a frontier of fire, with each part checked or boosted by the ferocity and abundance of burning.

Cross Timbers

The contours of the Cross Timbers roughly track soils, a divide between grass-promoting limestone and the oak-favored sandstone. But they also trace a kind of biotic dry line, jumping west to east from 26” of rainfall annually to 42”. To the east, ahead of fronts, moisture surges up from the Gulf of Mexico and brings rainfall sufficient to sustain woodlands. To the west, weather systems draft air from the deserts of northern Mexico and west Texas; there is less moisture, and it promotes a regime suitable for shortgrass prairie that leaves its woods strung along streams as gallery forests. It is the middle ground, the belt of tallgrass prairies and implacable oaks, where the most vigorous fires meet the stiffest woods, a kind of tornado alley for flame. Only the most savage fire topkills the dominant trees; mostly the oaks freshly sprout, hydralike, into an enduring, oft-impenetrable thicket, a living seawall. Even the wildest fire surges break against it. Washington Irving famously described the outcome as a “cast iron” forest. He didn’t mean the phrase as a compliment.

Map courtesy of Tree-Ring Labratory University of Arkansas. Click here to access higher resolution interactive version of the map.

But he might have. The Cross Timbers endure, a patch of history as much as of geography. They remain the dominant ecotype of Oklahoma. And where they intercalate with prairie and city, they display the peculiar patch dynamics of a unique American fire regime. Each patch has its own internal regimes; but it is how they all link that defines combustion across the region today.

Prairie patch

For early explorers the association of fire and prairies was a given. Where you had one, you had the other, and precocious tourists like Irving expected to experience a sea of flame as much as swarms of bison. In fact, Irving’s French-Canadian guide exclaimed that, if a fire did not conveniently present itself, he would set one. 

The expansiveness of Barrens and of west-increasing grasslands in the form of prairie patches and peninsulas that spread into vast steppes both intrigued and baffled early Europeans, and it was no great leap from seeing how grass and fire associated to suggest that fire created the grasslands. Thomas Jefferson debated the issue with John Adams, and concluded that the practice of fire-hunting by the indigenes was a probable cause for the prairies. 

Still, the debate flourished between those who sought an explanation in soils and climate, and those who thought the answer lay in fauna and fire. Aldo Leopold commented on the “immemorial warfare” between the oaks and grasses in the Wisconsin savanna. And in his magisterial global survey Carl Sauer noted that temperate grasslands everywhere were sites of level and unbroken terrain, swept by windy westerlies and fire; he thought the burning anthropogenic. Yet it still seems implausible to some observers that aboriginal humanity, outfitted with spears and torches, could have prompted such immense effects.

One reply is to note the difference between creating such landscapes and maintaining them. Surely, fire – that most interactive of biotechnologies – worked in close coupling with other factors; but everywhere it has been removed from the grasslands east of the 100^th meridian, the scene has quickly overgrown with woody plants, and the further east, the more humid the climate, the more broken the land became with watercourses, the less effective lightning could be as a kindler and the more stubborn the resistances to fire spread. 

These are, however, circumstances of physical geography: they portray a dynamic of competing physical forces, of wind, weather, fine fuels, and flame. In this conception the thick combustibles power fires that hurl against heat-resisting boles, and their relative strengths determine whether grass or tree prevails. But Jefferson’s speculation had another side, that the fires were set for hunting, which bonds fire to a more biological etiology. There was little tall about prairie that was grazed, and those sites were most grazed which grew where they had most recently burned, since they were more accessible and far richer in protein. The fast combustion of flame had to compete with the slow combustion of metabolizing bison, elk, pronghorn, prairie dogs, and grasshoppers. 

In this scenario prairie patches had a dynamic of biological agents, within which the burning was embedded. In principle, this makes sense since fire, while not alive, is a creation of the living world. In practice, it meant that an intricate choreography of burning and grazing shaped the landscape. The outcome might well be characterized as fire ecology’s equivalent to physics’ three-body problem, an interaction void of any exact solution, with all the parts always mutually adjusting. 

Fire explanations favor triangles, however. And this third factor Jefferson also identified: people. They completed the cycle by setting the fires. It had to be so. Especially as the proneness of landscapes to propagate fires splintered to the eastward, as land roughened, watercourses multiplied, and humidity thickened, only people could have set enough fires. Remove any part of this prairie fire triangle and the fire would go out. 

The upshot is that those prairie patches were not only pyric landscapes: they were cultural landscapes. They remain so today.