Shifting Ground: How Humans and Climate Co-Wrote the Story of the Prairie

Nehir Ergun

What do pollen grains, ancient campfires, and prairie grasses have in common? They all hold clues to one of North America’s longest-running experiments — how humans and nature shaped each other across the prairie-forest ecotone. In a recent study, Briere and Gajewski (2023), zoomed in on this dynamic zone to uncover the Holocene’s hidden history of feedback loops between people and place.

Their goal was to answer a fundamental question: What is the temporal relationship between major shifts in the prairie-forest ecotone and significant changes in human population density? They proposed two competing hypotheses:

• Hypothesis 1 – Climate as the Primary Driver: Large-scale climate changes controlled the ecotone. Humans simply adapted.
• Hypothesis 2 – Humans as a Significant Contributor: Human activity, especially the use of fire, played a major role in maintaining and expanding the prairies.

To test these, the team analyzed pollen and charcoal from lake sediment cores across the north-central U.S. and southern Canada, tracking 11,700 years of vegetation and fire history. They then compared these patterns with archaeological data on human populations.

The Big Picture: Climate Sets the Stage

When Briere and Gajewski retrieved pollen data from the Neotoma database—an international, community-curated repository for paleoecological records—to reconstruct past vegetation, an interesting story began to emerge.

Across the Holocene, the prairie–forest ecotone shifted repeatedly in response to changing temperature and moisture. During warmer, drier periods, prairie expanded; during cooler, wetter intervals, forests reclaimed the landscape.

These patterns strongly pointed toward climate as the primary long-term driver—supporting Hypothesis 1. Not only did the timing of major vegetation transitions align closely with well-known climate fluctuations, but the authors also noted a second key detail: neither vegetation changes nor fire frequency corresponded with their estimates of human population size. If humans had been the dominant driver (as in Hypothesis 2), increases in population should have produced clear, parallel increases in fire activity and vegetation change. Instead, no such alignment appeared. This mismatch further reinforced the conclusion that broad-scale ecological shifts were governed mainly by climate, not by human influence.

Figure 1: Midwest population estimate in relation to paleoenvironmental reconstructions. (a) Average mean temperature of the warmest month anomaly calculated from individual pollen-based reconstructions. (b) Composite oxygen isotope record generated from series N, O, and Q. (c,d) The second and first components, respectively, of a principal components analysis (PCA; Fig. 2) of pollen records from across the region. (e) Charcoal composite record gener ated using the pfComposite function of the paleofire package in R. (f,g) Summed probably distribution (SPD) plots of archaeological radiocarbon dates for the Midwest (arbitrary units). (f) SPD zoomed-in to facilitate the view of fluctuations in the earlier part of the study period.

 

Figure 1 synthesizes multiple paleoenvironmental and archaeological datasets to explore how climate, fire, vegetation, and human population interacted across the Holocene in the Midwest (Briere and Gajewski). The top panels show climate proxies: (a) temperature anomalies indicating the warmest mid-Holocene interval, and (b) oxygen isotope (δ¹⁸O) records reflecting moisture availability, with drier conditions in the mid-Holocene and wetter conditions later. The middle panels (c, d) represent pollen-based reconstructions of vegetation composition, showing a shift from forest to prairie dominance during warmer, drier periods and a gradual return of forests as conditions cooled and moistened. The charcoal composite (e) tracks fire activity, which peaked alongside prairie expansion, linking climate and vegetation to increased burning. Finally, the summed probability distribution (f) of radiocarbon-dated archaeological sites reflects human population density, which rose during the Late Holocene, coinciding with higher fire activity and landscape management.

 

The Human Spark: A Case Study from Iowa

But that’s not the whole story. The charcoal records revealed fires that couldn't be explained by climate alone. This is where a pivotal piece of evidence comes into play, perfectly illustrating the mechanism Briere and Gajewski detected.

A landmark 1996 study by Baker et al. focused specifically on the paleoecology of northwestern Iowa. By examining lake sediments, they reconstructed a detailed history of the region's vegetation and fire regimes (Figure 2). The North American Macrofossil Database was used to identify the plant macrofossil taxa in this region. Their findings were striking. Around 3,000 years ago, despite a climate that was becoming wetter and more favorable to forests, the oak savannas and prairies in Iowa expanded. How? The charcoal data showed a significant increase in fire frequency that was decoupled from climate. The researchers concluded this was clear evidence of anthropogenic fire—fire deliberately set by Native American communities to manage the landscape for game hunting and to maintain open, productive lands.

This Iowa case study is a microcosm of the broader pattern Briere and Gajewski observed. It shows that humans weren't just passive inhabitants; they were active land managers.

Figure 2: Map of upper Midwest showing location of sites
mentioned in text, glacial boundaries, and pre-settlement vegetation.
 

The Takeaway: A Co-Adapted Landscape

So, what's the verdict? Briere and Gajewski’s continental-scale analysis confirms that climate set the boundaries, but humans played an increasingly active role in managing what happened within them.

The relationship evolved into a powerful feedback loop: humans influenced vegetation through intentional fire, and those managed, open landscapes, in turn, provided the resources people depended on. The prairie-forest ecotone wasn’t just a passive responder to environmental change. It was a living, shifting frontier of co-adaptation—shaped first by the forces of climate and later fine-tuned by millennia of human ingenuity and ecological knowledge. The legacy of those ancient fires is still written in the land, if we know how to look for it.

Figure 3: This diagram shows the interactions among Climate, Humans, Fire, and Vegetation, illustrating how each factor influences vegetation dynamics — particularly relevant to Holocene vegetation changes in Iowa.

 

To conclude the findings of these studies, during the Holocene, changes in Iowa were driven by interacting influences of climate, fire, and humans (Figure 3). Early in the period, cooler and wetter conditions supported forests, but as the climate became warmer and drier during the mid-Holocene, frequent natural fires promoted prairie expansion. In the late Holocene, increasing human activity—especially intentional burning—reinforced fire regimes that maintained grasslands even as the climate grew cooler and moister again. Overall, climate set the broad environmental conditions, while fire and human actions amplified and sustained the dominance of prairie vegetation across Iowa.

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References

Briere, M. D., & Gajewski, K. (2020). Human population dynamics in relation to Holocene climate variability in the North American Arctic and Subarctic. Quaternary Science Reviews, 240, 106370.

Baker, R. G., Bettis III, E. A., Schwert, D. P., Horton, D. G., Chumbley, C. A., Gonzalez, L. A., & Reagan, M. K. (1996). Holocene paleoenvironments of northeast Iowa. Ecological Monographs, 66(2), 203-234.