Historic and holocene forest disturbance in South-Central Utah

Paleoecological reconstructions provide important information regarding climate affects on vegetation and forest disturbance return intervals. In recent decades, bark beetles (Dendroctonus spp.) have rapidly and profoundly altered subalpine forest ecosystems across interior forests in western North America. Disturbance records for bark beetle epidemics extending beyond the most recent few centuries are absent from the paleoecological literature. The research presented here examines sedimentary pollen records from subalpine lake basins to assess both historic and Holocene disturbance by spruce beetle (D. rufipennis) and wildfire. It is evident that limited vegetative change has occurred over the last 9,000 years and climate is driving fire disturbance regimes rather than forest composition. As insolation-driven seasonal climate extremes ameliorated from the early to the middle Holocene, annual precipitation regimes transitioned from rain- to snow-dominated with perennial lakes developing in south-central Utah. The early Holocene was characterized by high fire peak magnitude and fire frequency, forced by strong seasonal temperature and moisture contrasts. The middle Holocene was relatively warm, with dry winters and wet summers, facilitating frequent, low-magnitude fire episodes. The late Holocene was relatively cool and wet driven by decreasing summer insolation and increasing amplitude and frequency of El Niño-Southern Oscillation (ENSO) variability. Landscape-scale stand-replacing fire disturbance becomes essentially absent over the last 3,000 years until the arrival of European settlers to the region. Pollen ratios from host and nonhost trees assessed during the historic period allow us to infer two high-severity spruce beetle epidemics during the Holocene at ca. 4,000 and 8,200 cal yrs BP.