Origin of a park-forest vegetation mosaic in the Wind River Range, Wyoming

Vegetation of the montane and subalpine zones of the Rocky Mountains is a mosaic of conifer forests and large (1 ha to several km{dollar}sp2{dollar}) treeless "parks" dominated by sagebrush (Artemisia spp.), grasses, and other forbs. This study presents three hypotheses for park origin and tests them using paleoecological methods.; The "permanent site hypothesis" states that the park-forest vegetation mosaic results from 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 "fire hypothesis" states that parks can originate following forest fire. These hypotheses are tested using fossil pollen in the sediments of small ponds to reconstruct local vegetation change at sites in the existing park-forest mosaic.; Surface samples of sediments from small ponds in parks and forests were collected to establish criteria to separate pollen assemblages from these two vegetation types. The surface sample study demonstrates that pollen samples can be separated using the conifer:herb pollen ratio and multivariate analyses. Park-forest vegetation patterns on the scale of 100s of meters can be detected in the pollen assemblages from small ponds.; Fossil pollen extracted from the sediments of five small ponds in and around Fish Creek Park (elev. 2750 m) in the Wind River Range, Wyoming is used to reconstruct regional vegetation changes and the more recent changes in the local park-forest mosaic. Following deglaciation, the area supported alpine tundra, followed by treeline vegetation by 9500 yr BP. From ca. 8500-6000 yr BP, an open lodgepole pine (Pinus contorta) parkland occupied the area, perhaps in response to climate conditions warmer than today. Park vegetation originated only in the last 3000 years. Two of the three hypotheses for park origin are rejected. The "fire hypothesis" is not rejected, and cooling climate over the last several thousand years, combined with removal of forest cover may explain the origin of Fish Creek Park.