117-6 Late Holocene Intensification of Regional Burning Linked to Puebloan Agriculture in the Southern Rocky Mountains

Abstract:

Dry coniferous forests in the Southern Rocky Mountains are experiencing widespread increases in tree mortality and high-severity wildfires. Understanding the societal implications of these changes requires a long-term context for interactions among climate, vegetation, wildfire, and humans. However, high-resolution sedimentary archives are rare from elevations that are most sensitive to change. Whereas most sedimentary records from the Southern Rocky Mountains are from high-elevation lakes, dramatic changes, including high-intensity wildfire and drought-related tree mortality are centered in warmer, lower-elevation forests and woodlands. We addressed this limitation with a multiproxy record from Santa Fe Lake, NM, a small subalpine lake. We combined pollen analysis to document local to regional vegetation, with macroscopic charcoal to reconstruct local wildfire history, and microscopic charcoal and molecular markers to infer regional biomass burning. Santa Fe Lake formed as alpine glaciers receded in the Sangre de Cristo Mountains after 14,000 cal yr BP. Pollen data indicate that ponderosa pine forests, the dominant lower-elevation, forest type, expanded after 12,200 cal yr BP. Subalpine forests reached Santa Fe Lake by 10,300 cal yr BP. The influx of both macroscopic charcoal and levoglucosan increase after 10,000 cal yr BP and remain high until about 5500 cal yr BP. However, microscopic charcoal influx, an indicator of regional burning, is generally low during this period. This result may mean that during the warmest period of the Holocene, most biomass burning occurred in subalpine forests. Macroscopic charcoal and levoglucosan influx decline from about 5500-2000 cal yr BP as microscopic charcoal influx remains low. Influx of microscopic charcoal and levoglucosan, but not macroscopic charcoal, more than double after 2000 cal yr BP, indicating an increase in regional, but not local, burning. This late Holocene increase in regional burning coincides with the expansion of maize agriculture by ancient Puebloans and probably resulted from the use of fire to open lower-elevation forests for cultivation. Maize pollen, an unambiguous but rare cultural indicator, is consistently present in sediments from 800-350 cal yr BP (1150-1600 cal yr CE), which demonstrates the intensity of agriculture at lower elevations near Santa Fe Lake. Our data record millennial-scale interactions among climate, vegetation, and wildfire but also demonstrate that the largest changes in regional burning during the Holocene resulted from human impacts.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-18152

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