Fire, Climate, and Social-Ecological Systems in the Ancient Southwest: Alluvial Geoarchaeology and Applied Historical Ecology

Abstract

Although human land use in the industrial and post-industrial world has had demonstrable impacts on global climate, human land use may also improve or reduce the resilience of ecosystems to anthropogenic and natural climate change. This dissertation tests the hypothesis that low severity anthropogenic burning by prehistoric and protohistoric indigenous societies in the ponderosa pine forests of east-central Arizona improved the resilience of these forests to low frequency climate change. I use sedimentary charcoal, phosphorus, stable carbon isotopes, and palynology to reconstruct changes in fire regimes over the last 1000 years from seven radiocarbon dated alluvial sequences in five watersheds across a gradient of indigenous land use and occupation histories. Paleoecological evidence from occupied watersheds is consistent with small-scale, agricultural burning by Ancestral Pueblo villagers (between AD 1150-1325/1400) and anthropogenic burning by Western Apaches to promote wild pant foods (ca. AD 1550-1900) in addition to naturally frequent, low severity landscape fires. Statistical reconstructions of climate driven fire activity from tree-ring records of annual precipitation indicate that Southwestern forests were vulnerable to increased fire severity and shifts to alternative stable states between AD 1300-1650. In watersheds that were unoccupied or depopulated by AD 1325, paleoecological and sedimentological evidence is consistent with an increase in fire severity, whereas areas occupied and burned by indigenous people until AD 1400 did not yield evidence of increased fire severity. These results suggest that anthropogenic burning by small-scale societies may have improved the resilience of Southwestern forests to climate driven environmental changes.