ORIGIN OF A PARK–FOREST VEGETATION MOSAIC IN THE WIND RIVER RANGE, WYOMING

Abstract

The vegetation of the montane and subalpine zones of the Rocky Mountains is a mosaic of conifer forests and large (1 ha to several square kilometers) treeless “parks” dominated by sagebrush (Artemisia spp.), grasses, and forbs. Three hypotheses for the origin of parks are proposed. The “permanent site hypothesis” states that the park–forest vegetation mosaic is a result of differences in physical characteristics of sites. In the “remnant hypothesis” parks are thought to be remnants of vegetation that was widespread under previous climate conditions. The “replacement hypothesis” states that parks replace forest vegetation in response to disturbance, climate change, or a combination of these two factors. Patterns in the past distribution of park and forest vegetation in the vicinity of Fish Creek Park (elevation 2750 m) were used to test these hypotheses. Fossil pollen extracted from the sediments of five small ponds in and around Fish Creek Park was used to reconstruct Holocene vegetation changes. Changes in vegetation were reconstructed through the use of multivariate analyses and pollen ratios derived from modern surface samples and by comparison with pollen data from other studies. The pollen record indicates that shortly after deglaciation (∼11000 yr BP) the area supported alpine tundra, followed by whitebark pine–spruce–fir parkland at ∼9500 yr BP. From ∼8500 to 6000 yr BP, a pine parkland occupied the area, perhaps in response to climate conditions warmer than today. By 5000 yr BP a mixed pine–spruce–fir forest resembling the modern subalpine forest near Fish Creek Park probably replaced the pine parkland at all five sites. The modern park vegetation originated only within the last ∼2500 yr. The conversion to park vegetation may not have been synchronous at all three sites, and the replacement of forest by park did not always result in a long-term conversion to park vegetation. The timing and pattern of changes in the vegetation mosaic eliminate the permanent site and remnant hypotheses and suggest instead that climatic cooling over the last several thousand years, possibly combined with removal of forest cover by fire or some other disturbance, could explain the origin of Fish Creek Park.