Quaking aspen (Populus tremuloides) ecology on forest service lands north of Yellowstone National Park
Kimble, David Stuart.
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The primary objective of this study was to determine if quaking aspen (Populus tremuloides) density and recruitment changed on the Gallatin National Forest north of Yellowstone National Park from 1991 to 2006. Three-hundred sixteen aspen stands were surveyed on the 560 km² study area. Secondary objectives were to determine if aspen density and recruitment were influenced by elk (Cervus elaphus) browsing, conifer establishment, and cattle (Bos spp.) grazing. A 202.3 m² circular plot was established within each stand. All aspen stems within each plot were categorized into size classes: sprouts (< 1 m), saplings (1-2 m), recruitment stems (> 2 m and < 5 cm diameter at breast height), and mature stems (> 2 m and > 5 cm diameter at breast height). Recruitment stems and mature stems have grown above the height at which elk generally browse. Recruitment stems have attained this height in the past 10-15 years.Therefore, important measures of aspen stand sustainability are changes in recruitment stem density and density of all stems > 2 m tall across the study area. Individual stands were further categorized as sustainable if the number of stems > 2 m remained stable or increased from 1991 to 2006. Sixty-three percent of stands are not recruiting sufficient numbers of stems to replace aging overstories. Mean recruitment stem density across the study area did not change from 1991 to 2006. Stems > 2 m density is the most meaningful measure of aspen sustainability at the landscape level. Mean stems > 2 m density declined 12% from 1991 to 2006. The average annual loss of stems > 2 m is four times the rate of canopy cover loss in surrounding areas in Montana and Wyoming. Areas with the greatest elk densities had the lowest recruitment stem densities, which indicates that elk herbivory was the primary reason for the 12% decline in stems > 2 m density. Conifer establishment and cattle grazing in aspen stands also contributed to this decline. Even though elk numbers on the Northern Yellowstone Winter Range have declined since wolf reintroduction, aspen recruitment has not increased at the landscape level.