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.

 Could paleoecology mark the age of humanity?

Nell Badgley

It is not uncommon to hear today’s times described as ecologically unprecedented.

Human-driven climate changes, species die-offs, and land use changes have starkly transformed our environment, so much that some scholars suggest this warrants the definition of a new geologic epoch– the Anthropocene, or “the age of humanity.” Yet are today’s changes truly unprecedented in the context of Earth’s history? 

Defining the Anthropocene on a continental scale 

Our current geologic epoch called the Holocene began approximately 11,700 years ago, as Earth transitioned out of its previous epoch known as the Pleistocene. This transition occurred over thousands of years and was marked not only by rapid warming, but also by a number of rapid ecological changes. A 2023 study by M. Allison Stegner and Trisha L. Spanbauer sought to illuminate whether the rapid vegetation changes that took place during the Pleistocene-Holocene transition are comparable to the rapid changes we are seeing today as a result of human influence. With this, we might better understand the question of whether today’s “unprecedented” changes might mimic the beginning of a new epoch like one Earth has experienced before.

In their paper “North American pollen records provide evidence for macroscale ecological changes in the Anthropocene”, Stegner and Spanbauer utilized sediment pollen records from the Neotoma Paleoecology Database to track changes in 7 elements over the past ~12,000 years: how taxonomic richness changed over time (SQS), dates of first and last appearances of species in the pollen record (FAD/LAD), short-term (1,000y) loss and gain of plants, and abrupt community changes (AC) to plant diversity.

 

 

Authors of the paper specifically elaborated on changes to FAD/LAD, as these changes are commonly used as markers for epoch transitions. As seen in Figure 1, FADs are elevated during the Pleistocene-Holocene transition around 11.7k YBP (years before present) and begin to increase in the past ~200 years, alongside LAD increases beginning ~1000 YBP. Furthermore, authors point to similar patterns in ACs and taxonomic richness (peaks around

Holocene-Pleistocene transition and in the last ~200 years) as further evidence of similarly abrupt changes occurring both during the last epoch transition and in recent centuries. 

What does this mean for Iowa?

Stegner and Spanbauer compared the past epoch transition to the rapid human-driven vegetation changes that have defined an era of human influence. But how does the

Anthropocene, a large-scale geologic epoch, translate locally? This paper began to break down regional changes based on 4 ecoregions but left out a significant swath of North America in these divisions. Notably, the Great Plains and Midwest are largely omitted from the paper’s dataset, meaning forested ecosystems are prioritized over the grassland and prairieland. As the pace of change defines Stegner and Spanbauer’s work, one may wonder whether the paleoecological history of the prairie-dominated ecosystems differed from the forest ecosystems highlighted in this research. 

We don’t have to be left completely in the dark as to Iowa’s paleoecological history, however, as a study by Baker et al. in 1996 took a different approach to understanding how the Holocene took shape in Northeast Iowa specifically. While Baker et al. relied on (fossil) pollen data in part, they also utilized plant macrofossils to reconstruct the specifics of past Holocene vegetation, mapping shifting forest, prairie, and human-disturbed ecosystems from around 12,000 years ago to today. Baker et al. attribute these changes to shifting climate, precipitation, temperature and fire regimes. But recent changes over the past few centuries certainly have shown evidence that humans have made their mark– disturbed ground and emerging cropland are key indicators that Iowa’s ecology had begun to shift as a result of humanity.

Despite this, what makes Stegner and Spanbauer’s work unique is their approach in highlighting species turnover (LAD/FAD). Whereas the work of Baker et al. has elucidated comprehensive species-composition level changes, Stegner and Spanbauer have highlighted markers crucial in understanding the epoch transitions–particularly the emergence of the Anthropocene. 

Unprecedented?

Ultimately, the abrupt vegetation changes that mark the potential Anthropocene are unprecedented because they are driven so markedly by human influence but are not so different in the context of what North America has seen over the last 12,000 years. The evidence is clear that humanity has made its mark on the continent’s vegetation, possibly also staking our place in geologic history.

Works cited 

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.

Stegner, M. A., & Spanbauer, T. L. (2023). North American pollen records provide evidence for macroscale ecological changes in the Anthropocene. Proceedings of the National Academy of Sciences, 120(43), e2306815120.

What can paleoecology learn from anthropology?

Johanna Swanson

You can always find amazing things in the back of a museum. It could be bones, founding documents, or in this case, an old test tube of burnt wood which can help color the picture of what Iowa flora was 1500 years ago. In their paper, William Green and Kathryn Parker, Pre-Contact Use of Balsam Fir, show that some scraps of forgotten charcoal can provide novel information about the historic range of the balsam fir.

            During the 1920’s, archeological excavations occurred at Seeberger cave in Jackson County, Iowa. Archeologists and paleoecologists have plenty of experience identifying animal remains, however, it is less common to look at the remains of burned wood from human campsites. In fact, this is the first paper I have read that tries to identify the species of charcoal used in a hearth fire. This technique has implications for future paleoecology studies, as it means that trees that are less likely to be near lakes or don’t have wind born pollen have a chance to be identified as part of a fossil record. Using electron microscopy Green and Parker determined the wood to be from 300 AD and of the genus Albies, more commonly known as firs.

            What is bizarre is that there are no fir trees native to Jackson County in the present. Most fir trees are either in the north, the pacific coast, or mountain ranges, with one exception. Abies balsama has some small remnant populations that reside in northeast Iowa over a 100 km away in a habitat unique to the driftless area called algific talus slopes.

An image from Green and Parker’s paper showing the distance from Seeberger Cave to the nearest balsam firs.

A diagram from the Iowa Natural Heritage showing how algific talus slopes work.

            Algific talus slopes stay cool throughout the year thanks to air flowing through underground ice caves that escapes onto the hillside. As these hills stay colder throughout the year, lifeforms like the Iowa Pleistocene snail, northern monkshood, and balsam fir can survive much further south than they could into areas like Iowa and Illinois. While all remaining balsam firs live on talus slopes farther to the north, some talus slopes are within 20 kilometers of Seeberger Cave.

A diagram from Green and Parker’s paper showing talus slopes in/around Jackson County, with Seeberger Cave marked as a star.

            It would have been significantly easier to transfer and use balsam fir from less than 20 km away then hauling it over 100 km from a site in Northeast Iowa. The presence of balsam fir in this cave suggests that algific talus slopes serving as a paleorefugia much further south than previously imagined. Instead of already being transitioned into a prairie and oak savannah matrix, parts of Iowa held on to biotic systems that reflect a far older environment from the ice age.

Additionally, this article shows the importance of cross disciplinary work. This research suggesting that balsam fir populations existed further south than previously thought didn’t come from traditional paleontological techniques, but instead from anthropologists using archeological field techniques nearly a century ago. Archeologists have long known that human debris can hold valuable clues to a location’s history, and this has applications well beyond reconstructing humanity’s past. I hope that Green and Parker’s work encourages others to look at archeological samples as potential datapoints for measuring past floral and faunal populations.

            While Green and Parker’s conclusions about balsam fir populations are fascinating, there are some major flaws in their theory that I would be remiss to ignore. There are three hypotheses for how this wood got into Seeberger cave.

1) The wood was transported in from much further north like Minnesota or further east or west.

2) The wood was transported in from talus slopes in Northeastern Iowa.

3) The wood was transported in from a nearby slope.

            I agree that the wood likely isn’t from far away, but I do have some doubts over whether it could be from Northeastern Iowa. Native Americans had complicated and sophisticated trading networks all over North America, and for items of ritual importance like balsam fir (which the article suggests is being used for religious purposes at Seeberger Cave) there could be economic reasons to justify transporting it long distances. Additionally, as Chief Iowa Archeologist John Doershuk once put it; “It isn’t that big of a deal for native people to just walk to a site for a couple of weeks to get something they want”. Perhaps this article shows that balsam firs and algific talus slopes extended further south in the past, or that Native Americans had complex trade networks earlier than we thought.

 

Works Cited

Iowa Natural Heritage Foundation. A global treasure. https://www.inhf.org/about-us/blog/2025/09/11/a-global-treasure. 

Green, W., & Parker, K. E. (2025). Precontact use of balsam fir (Abies balsamea) in Iowa, USA. Ethnobiology Letters, 16(1), 56–69. https://doi.org/10.14237/ebl.16.1.2025.1935.