Macrofossil and brGDGT evidence of forest state changes after fires and cooling in the Snowy Range, Wyoming since 6 ka

Large-scale forest changes are currently taking place across the western United States as the result of insect outbreaks, fires, and drought, but the long-term consequences of these disturbances remain unknown. Permanent state-shifts may be possible if new climate conditions prohibit recovery to past forest states. Paleoecological records can help anticipate such outcomes by examining past effects of climate change and disturbance.

This case study examines macrofossil and fossil pollen evidence that past events caused sub-alpine forests in the Medicine Bow Mountains in southeast Wyoming to shift permanently to open alpine meadows sometime in past centuries or millennia. Specifically, a high-elevation plateau known as Libby Flats today contains a large area of meadows covered with the logs from large trees killed in the past but never replaced by new forest growth. Dense forests cover surrounding areas, even at higher elevations, indicating that the area can sustain multiple forest or meadow states depending upon past disturbances and climate changes.

Small ponds located within Libby Flats yielded sediment cores extending to ~6200 and ~3400 calibrated years before present (cal. yr BP). Changes in the methylation of brGDGTs (branched glycerol dialkyl glycerol tetraethers) from the longer core indicate cooling over the past 6000 years, which coincides with a reduction in the concentration of spruce ( Picea ) and fir ( Abies ) needles in the sediment from both ponds. Basal ages of sediments from 5 additional small, snow-dependent ponds in the area also indicate an increase in available moisture over the past 5000-1000 years. Increased snow drifting on Libby Flats may have contributed to a reduction in tree cover and needle deposition by burying seedlings and shortening the snow-free growing season. Needle concentrations drop sharply to a minimum in both cores around 875 cal. yr BP immediately following a major increase in charcoal accumulation rates, indicating that fire played a critical role in the final loss of forest cover near the ponds and creating a permanent state change on the landscape. This event supports the hypothesis that disturbance can trigger lasting changes in ecosystem states if they accelerate changes favored by long-term climate trends.