Simulated impact of paleoclimate change on Fremont Native American maize farming in Utah, 850–1449 CE, using crop and climate models

Our paper in Quaternary International just went online today. The pithy takeaway: Ancient dryland farmers in Utah adapted to warming and drying during the MCA but were unable to adapt to increased variability at the MCA-LIA transition, and abandoned the area for maize farming. Because increased variability is one of the near certainties for temperate semi-arid and Mediterranean zones under global warming, the AP experience offers a cautionary example of the limits of low-technology adaptation to environmental change.


The Fremont were members of an expansive maize-based Ancestral Puebloan (AP) cultural complex who disappeared from Utah between the 12th and 13th centuries CE. This period brackets that of a climatic transition in the Southwest from the warm, dry Medieval Climate Anomaly (MCA, ca. 850–1350 CE) to the cool, hydro-climatically variable Little Ice Age (LIA, ca. 1350–1850 CE). We simulated maize (Zea mays) crop productivity for Fremont AP archaeological sites in Utah between 850 and 1449 CE using a process-based crop model driven by climatologies from a statistically downscaled a climate model. We compared the model-simulated crop yields to time-series of archaeological site occupations given by spatially discrete, chronologically summed probability distributions (SPDs) of radiocarbon-dated Fremont artifacts. We found that the anomalous abandonment of different sites throughout Utah may be explained by site-specific combinations of reduced mean yield due to volatile year-to-year yields caused by increasing temperature variability, increasing hydro-climatic variability, and loss of soil quality, which depended on crop management strategy. In other words, we model the elimination of the Fremont AP ecological niche by exogenous influences of temperature and precipitation variability at the MCA-LIA transition and endogenous degradation of soil from organic carbon and nitrogen loss. Our method has broad applicability to contexts of low-technology, dryland farming human-environmental interactions.


Figure. EPIC-simulated yield for Fremont corn based on 7 different management strategies, rainfed (non-irrigated) case, for a site in the upper reach of Range Creek, Utah. The top row shows mean annual maize yields; the bottom row shows yield standard deviation over a 21-year moving window. The purple shading shows the summed probability distribution (SPD) of ~20 calibrated radiocarbon dates on maize-associated artifacts recovered from Range Creek sites (provided by D. Metcalfe). All maize simulation data is based on averages of multiple simulation runs, with different parameterizations for crop fertilization, plant spacing, and optimum growing degree days, driven by three members of the CESM Last Millennium Ensemble. The simulated maize and SPD were produced from independent datasets: the simulated maize curves were generated from simulated climatologies, and the SPD was produced from radiocarbon ages only.

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