Postglacial climate, vegetation, and fire dynamics of western Montana

Abstract

In recent decades, fires in the western U.S. have grown in size and severity, a trend expected to continue, raising concerns about future forest ecosystems. Fire is a primary disturbance in northern Rocky Mountain forests, but human activities--like logging, fire suppression, igniting fires, and ecosystem fragmentation--combined with anthropogenic contributions to climate change, have altered the natural fire regime. To contextualize these changes, it is essential to understand the co-evolution of forests and fire and to examine past shifts in fire regimes to better grasp the consequences of increased fire activity for forest ecosystems today. This study provides pollen and charcoal data from twelve locations across western Montana and presents vegetation and fire histories from forest types and regions that are underrepresented in the existing paleorecord. Chapter 2 investigates high-elevation records from an ice patch and pond on the Beartooth Plateau, offering insights into ecosystem changes at upper treeline. Chapter 3 examines a middle-elevation site in a mesic mixed conifer forest, exploring the stability of these ecosystems. Lastly, Chapter 4 discusses a network of sites from low to high elevation, revealing regional shifts in vegetation and fire activity in response to climate. Climate has been the primary driver of ecosystem change over the past 14,000 years, and fire activity has responded to both climate and site-specific conditions. As summer insolation levels rose at the end of the late-glacial period, forests were established, fuel biomass increased, and wildfires became more frequent. Open Douglas-fir parkland prevailed at low elevations during the warm, dry early Holocene, lower treeline moved upslope, and upper treeline shifted downslope in some areas. The openness of the vegetation was controlled by summer drought, and an attendant decline in fire activity reflected a lack of fuel connectivity. Forest density increased after 6000 cal yr BP and closed forests persisted throughout the cold, wet late Holocene. This period experienced the highest fire activity as a result of ample biofuel and increased short-term climate variability.