Bok Sowell

Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range

Abstract Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km2 of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height > 2 m and diameter at breast height < 5 cm) represent recent growth of aspen sprouts above elk (Cervus elaphus) browsing height, whereas overstory stems (all stems > 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P = 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P = 0.0001), conifer establishment (P = 0.0001), and cattle (Bos spp.) grazing (P = 0.016) contributed to the decline in overstory stem densities when analyzed using a mixed effects model of log transformed medians. Eighty percent of the clones were classified as having medium to high browsing levels in 1991, whereas 65% of the clones received a similar rating in 2006, possibly due to the reduced NYWR elk population. Aspen recruitment has increased in some 2–10 km2 areas, but not consistently. Our study found that a trophic cascade of wolves, elk, and aspen, resulting in a landscape-level recovery of aspen, is not occurring at this time.

Characteristics of shrub morphology on nest site selection of Greater Sage-Grouse (Centrocercus urophasianus) in high-elevation sagebrush habitat

ABSTRACT Selection of nest sites directly influences reproductive success for Greater Sage-Grouse (Centrocercus urophasianus); thus, regional evaluation of how this species selects nest sites is necessary for effective habitat management. We evaluated fine-scale nest site selection of Greater Sage-Grouse in the Centennial Valley of southwest Montana. We conducted vegetation surveys at nest sites (n = 90) of radio-tagged Greater Sage-Grouse and paired random locations across 2 breeding seasons (2014–2015). The majority of nests were located under mountain big sagebrush (Artemisia tridentata ssp. vaseyana), three-tip sagebrush (A. tripartita), and basin big sagebrush (A. tridentata ssp. tridentata) shrubs. We used generalized linear models and information theory to evaluate competing hypotheses about nest site selection. Our top model indicated that nest site selection was primarily associated with nest shrub morphological characteristics and cover provided by the nest shrub. Mountain big sagebrush and three-tip sagebrush provided twice the amount of lateral cover that basin big sagebrush shrubs provided. Our results suggest that herbaceous cover was unimportant at fine scale nest site selection of Greater Sage-Grouse in our study area. Managers should focus on conserving large intact stands of mountain big sagebrush and three-tip sagebrush habitats because they provided the most lateral cover and supported the majority of nest sites.

Beaver Habitat Selection for 24 Yr Since Reintroduction North of Yellowstone National Park

ABSTRACT Beavers (Castor canadensis) disappeared from drainages north of Yellowstone National Park in the mid-1900s because of trapping, a potential tularemia outbreak, and willow (Salix spp.) stand degradation by ungulates. Beavers were reintroduced in 1986 after a 40-yr absence with inventories of active-beaver structures completed each fall after reintroduction for 24 consecutive yr. We used this inventory to evaluate the expansion of beaver populations in a riparian environment recovering from past overuse by ungulates. Specifically, we investigated the density of active-beaver colonies and dams, the change in willow cover, and habitats associated with beaver expansion since reintroduction. Successful establishment and expansion of beavers indicate that sufficient resources were available to the population despite the suboptimal condition of riparian vegetation. Carrying capacity on third-order streams was reached approximately 14 yr after reintroduction (2000) with an average annual density of 1.33 (95th percentile = 1.23 —1.44 active colonies/stream km) between 2000 and 2010. The average annual density of beaver dams during this time was 2.37 (2.04 — 2.71 active dams/stream km). Despite the beaver population being at carrying capacity in meadows since 2000, willow cover increased by 16% between 1981 and 2011. We speculate that beaver activities, together with reduced ungulate browsing from predation and habitat loss, combined to increase willow cover. Willow cover and height were positively associated with colony longevity, but numerous other influencing variables included secondary channels, sinuosity, stream depth, and sandbar width. Our results provide evidence that beaver reintroduction can be successful in riparian areas where willow stand condition is less than optimal and that beavers might ultimately improve willow condition. We suggest that reducing ungulate use of overgrazed riparian environments will facilitate the reestablishment of beaver populations. We also provide managers with habitats that should be identified in an environment targeted for reintroduction.