Fire regime shift reveals 17th century socio-ecological change in the Rio Grande del Norte National Monument, New Mexico

Abstract

Humans have influenced ecosystems for millennia, and social changes can have notable ecological effects. Recent human influence on ecosystems is exemplified by the collapse of fire regimes circa 1900 in the western United States due to increased grazing, fire suppression, and declining Indigenous fire use. Climate can also drive fire regime changes, which is reflected by increasing area burned across the West in recent decades associated with warming and increased aridity. In the southwestern United States during the mid-1600s, large social changes accompanied Spanish colonization. These changes included increases in domestic livestock (e.g., sheep and horses), declines of Indigenous populations and cultural burning, and exploitation of Puebloans that led to the 1680 Pueblo Revolt, an Indigenous uprising that expelled Spanish colonizers for 12 years. The ecological effects of these large social changes are not well studied. The goal of this study was to use tree-ring fire histories to investigate fire dynamics of a sagebrush-dominated landscape in the Rio Grande del Norte National Monument (RGDN) in northern New Mexico, including the effects of historical social and climatic changes. We reconstructed fire history on four forested mountains that rise above the sagebrush/grass plateau. We asked: (1) What was the pre-1900 fire frequency, season, and fire–climate relationship in the RGDN? (2) Was there evidence for fire spread in the sagebrush/grass flats between the mountains? and (3) Did regional social or climate changes influence fire regimes? From 1400 to 1679, we found frequent fire within forests (8–12-year median fire intervals [mfi]) and potential evidence of spreading fire through the sagebrush/grass ecosystem based on greater-than-expected intermountain fire synchrony. However, beginning in the mid-1600s and continuing after the Pueblo Revolt of 1680, there was (1) a significant decline in fire frequency, including 38–166-year mfi and no fire for >350 years at two sites; and (2) only one synchronous fire between mountains. During this 17th century period of socio-ecological change there was no persistent shift in climate. We propose that reduced Indigenous ignitions and increased domestic grazing, which reduced herbaceous fuels and fire spread, are likely causes of the extraordinary early and persistent reduction in fire.