Little Willows at Great Heights

A couple species of willow grow as ground cover on the tundra.

NPS/C Turner

Spread across Rocky Mountain National Park’s alpine tundra is a carpet of life—at times quiet and discrete, at others loud and bursting with color. Amid this rugged landscape, with its craggy rock outcroppings and small but persistent snow fields, a few small, short, and determined species of willow (Salix spp.) make their home. These alpine willow plants persist against the intense summer sun, cold winter temperatures, and strong, harsh winds.

Often, the word ‘willow’ brings to mind a movie-like settings featuring a friendly, tall weeping willow tree—it’s long thin branches dangling to the ground, a tire swing hanging from its lower branches; or a young person, book in hand, enjoying its shade. But Rocky’s willows of the alpine, although related, are so very different. They are shrubby, growing short or staying low to the ground, with thick waxy leaves to keep in moisture. The smallest also have hairs along their short stems to break the raging winds. It’s a short season on the tundra; willows grow, flower, and seed within a few short months.

There are 17 confirmed species of willow found in the park. Four of these species are common within the park’s alpine tundra: Salix brachycarpa (barrenground willow), Salix reticulata (snow willow), Salix petrophila (alpine willow), and Salix planifolia (planeleaf willow). While the barrenground and planeleaf willows grow taller with a classic shrub look, the snow and alpine willows grow as a ground cover amid the grasses and wildflowers. Another species, Salix glauca (grayleaf willow), is likely present in the tundra and is known to hybirdize with S. brachycarpa.

White-tailed ptarmigan rely on alpine willow species for food and cover.

NPS/A Schonlau

Alpine-dwelling willows are important to several animal species that make the tundra home. One in particular, the white-tailed ptarmigan, relies on willow for food and cover. Short willows are especially important in the summer, while taller willows are used when other willows are under snow and the snow can provide a step up to the higher buds and shoots. Rocky areas with willow shrubs are often preferred nesting sites. The park’s white-tailed ptarmigan population declined severely in the mid-1970’s. Concerns about habitat degradation led to a few studies focused on vegetation, including willows, in the alpine tundra.

Elk forage on tundra plants, including willow, during the summer months.

NPS/A Schonlau

Elk also use willow. The majority of elk at Rocky Mountain National Park spend a good part of the summer in the alpine tundra, and their high population numbers prompted park management concerns of over-browsing. In 1971, park biologist, David Stevens, set up 12 vegetation transects, 8 of which were in the alpine tundra, to look at the impact that elk had on alpine plants. Stevens reported that almost all plant species declined, but whether elk were at the crux of this decrease could not be determined. Transect data was collected until 1996 and was analyzed again by researchers from USGS in 2006 (Zeigenfuss 2006). The 25-years of data collection did show an overall decrease in height and cover for both planeleaf and barrenground willow. Again, however, it couldn’t be determined if these changes had anything to do with over-browsing by elk.

Another interesting study conducted in the late 1970s, investigated diet selection in habitats frequented by elk during the summer months (Baker and Hobbs 1982). Using tame, trained elk, observers recorded the species of plants the animals grazed on, the number of bites they took of each species, and the parts of the plants on which the elk fed. Elk grazing in the alpine tundra were found to forage a variety of plants including sedges, rushes, tufted hairgrass, alpine avens, marshmarigold and willows. Barrenground and planeleaf willows accounted for approximately 11% of bites made by tundra grazing elk.

Questions about willows in the alpine continue to interest park managers. Willows have an important role to play in the alpine tundra. Understanding how changing conditions affect willow, and how changes in alpine willow cover and size influence the species that depend on them, will be valuable in addressing management concerns.

References:
Baker, D.L. and N.T. Hobbs. 1982. Composition and Quality of Elk Summer Diets in Colorado. Journal of Wildlife Management 46(3): 694-703

Zeigenfuss, L. 2006. Alpine Plant Community Trends on Elk Summer Range of Rocky Mountain National Park, Colorado: an analysis of existing data. U.S. Geological Survey, Open-File Report 2006-1122, 21 p.

Written by Carissa Turner, Continental Divide Research Learning Center
July 2020