Restoring aspen riparian stands with beaver on the northern Yellowstone winter range
McColley, Samuel David.
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Aspen (Populus tremuloides) on the Gardiner Ranger District, Gallatin National Forest, have declined over the last half-century. In an attempt to reverse this trend, beaver (Castor canadensis) were reintroduced in Eagle Creek in 1991. Beaver promote aspen suckering through their dam and lodge building activities. In 2005, I assessed the long-term effects of beaver on aspen stands and the associated riparian area in the Eagle Creek Drainage. Aerial photographs taken in 1990 and 2005 were used to compare changes in riparian area vegetation where beaver were reintroduced. Aspen canopy cover decreased (P<0.05) from 43% to 25% on Eagle Creek (29 ha) between 1990 and 2005. Willow (Salix spp.) cover increased (P<0.05) from 10% to 14% and alder (Alnus incana) cover and water surface area doubled during the same period. Aspen recovery was estimated by comparing vegetative changes among control sites with <10% beaver use (n = 5), active beaver sites (n = 6), sites abandoned for 1-3 years (n = 7), sites abandoned for 4-6 years (n = 4), and sites abandoned for 7-11 years (n = 5).Thirty, 1-m2 plots were used to determine aspen density and one 60-m2 belt transect was used to calculate size-class distributions at each site. Aspen stem densities in active sites and sites abandoned by beaver for 1-3 years were similar (2.6/m2) and increased (P=0.01) compared to all other sites (1/m2). In addition, sprout and sapling densities were greater (P=0.01) in these sampling areas. However, aspen suckers were not able to grow taller than 2m on sites absent of beaver for 4-11 years, which prevented aspen recovery. Ungulate herbivory on aspen was assessed by comparing differences in 14 fenced (3 x 3m) and unfenced (3 x 3m) areas over 2 growing seasons. Growth rate of aspen suckers was greater (P=0.001) in fenced areas (32cm/year) compared to unfenced areas (0.25cm/year) due to ungulate herbivory. Total ungulate density for Eagle Creek was equivalent to 17.6 elk/km2 in the winter of 2005-06. Beaver activity stimulated the growth of aspen sprouts and saplings, but ungulate herbivory prevented successful aspen recovery in Eagle Creek.