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 (ϳ11 000 yr BP) the area supported alpine tundra, followed by whitebark pine-spruce-fir parkland at ϳ9500 yr BP. From ϳ8500 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 ϳ2500 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.

Disturbance interactions can impact resilience mechanisms of forests

“…Several studies have demonstrated alternate stable states exist in the Rocky Mountain subalpine. Conifer seedlings may aid in the establishment of future seedlings (through shading of grasses or eventual overtopping of P. tremuloides; Stahelin 1943, Nyland 1998), but both P. tremuloides stands (Crawford et al 1998) and grasslands (Schauer et al 1998, Lynch 1998 can effectively exclude seedlings. P. tremuloides is potentially self-replacing indicating long-term dominance (Crawford et al 1998); P. tremuloides seedlings are prevalent within the burned area, and seedling densities are insensitive to the compounding effects of the blowdown/burn (B. Buma, unpublished data).…”

Ecological types of the eastern slope of the Wind River Range, Shoshone National Forest, Wyoming

“…Lastly, the replacement hypothesis suggests that parks are the result of permanent changes in vegetation composition from forest to park following forest fire and/or climate change. Lynch (1998) concluded that the replacement hypothesis best explained the persistence of park vegetation in the northwestern WRR and provided the following explanation. A disturbance event (forest fire or windthrow) that resulted in the removal of forest cover occurred ~3500 to 2500 years BP and corresponded with a cooling trend that favored park vegetation over the regeneration of pioneer tree species.…”

“…Park-forest vegetation refers to a mixture of non-forested and subalpine forest vegetation that occurs in a mosaic across mountainous landscapes. Park-forest vegetation is a common vegetation type across the mountains of Wyoming and southwestern Montana (Patten 1963;Despain 1973;Doering and Reider 1992;Whitlock 1993;Lynch 1998). Parks are sometimes related to environmental conditions not conducive to conifer regeneration, including poorly drained soils in valley bottoms; exposed ridges; and hot, dry, south-facing slopes (Lynch 1998).…”

“…Park-forest vegetation is a common vegetation type across the mountains of Wyoming and southwestern Montana (Patten 1963;Despain 1973;Doering and Reider 1992;Whitlock 1993;Lynch 1998). Parks are sometimes related to environmental conditions not conducive to conifer regeneration, including poorly drained soils in valley bottoms; exposed ridges; and hot, dry, south-facing slopes (Lynch 1998). However, parks also occur on sites with similar environmental conditions to those of surrounding conifer forests, and a variety of ideas and hypotheses have been developed to explain the persistence of parks on these sites.…”

“…This ET occurs in an area of park-forest vegetation (Lynch 1998) On shoulders and summits, parent materials are residuum.…”