Changing fire regimes in the Great Basin USA

Abstract

Wildfire is a natural disturbance in landscapes of the Western United States, but the effects and extents of fire are changing. Differences between historical and contemporary fire regimes can help identify reasons for observed changes in landscape composition. People living and working in the Great Basin, USA, are observing altered fire conditions, but spatial information about the degree and direction of change and departure from historical fire regimes is lacking. This study estimates how fire regimes have changed in the major Great Basin vegetation types over the past 60 years with comparisons to historical (pre-1900) fire regimes. We explore potential drivers of fire regime changes using existing spatial data and analysis. Across vegetation types, wildfires were larger and more frequent in the contemporary period (1991–2020) than in the recent past (1961–1990). Contemporary fires were more frequent than historical in two of three ecoregions for the most widespread vegetation type, basin and Wyoming big sagebrush. Increases in fire frequency also occurred in saltbush, greasewood, and blackbrush shrublands, although current fire return intervals remain on the order of centuries. Persistent juniper and pinyon pine woodlands burned more frequently in contemporary times than in historical times. Fire frequency was relatively unchanged in mixed dwarf sagebrush shrublands, suggesting they remain fuel-limited. Results suggest that quaking aspen woodlands may be burning less frequently now than historically, but more frequently in the contemporary period than in the recent past. We found that increased fire occurrence in the Great Basin is associated with increased abundance and extent of nonnative annual grasses and areas with high concentrations of anthropogenic ignitions. Findings support the need for continuing efforts to reduce fire occurrences in Great Basin plant communities experiencing excess fire and to implement treatments in communities experiencing fire deficits. Results underscore the importance of anthropogenic ignitions and discuss more targeted education and prevention efforts. Knowledge about signals of fire regime changes across the region can support effective deployment of resources to protect or restore plant communities and human values.