How Too Many Elk Drain Nitrogen From Rocky Mountain's Willows and Aspen

Willow browsed and damaged by elk. Image from the National Park Service's page.

The question

Elk graze plants and browse twigs and bark, and where they are unnaturally abundant, as they have been in Rocky Mountain National Park since the early 1990s for lack of predators, they can harm vegetation. Rangeland studies in the West have mostly measured changes in above-ground plant growth, which can be short-lived and can hide changes in roots or in the nitrogen stored in soil over the long term. Nitrogen is an essential nutrient that sets how productive an ecosystem is; losing it could reduce plant growth for years and change which species grow.

Kate Schoenecker of the U.S. Geological Survey and colleagues set out to measure both the direct effects of elk eating plants on their winter range and the less obvious ones: nitrogen in plants and soil, the amount and quality of leaf litter, soil conditions, and how fast microbes recycle nutrients.

An elk exclosure in the Kawuneechee Valley. Image from the National Park Service's page.

The study

The team used three exclosures the park fenced in 1963 and 12 they built in 1994, letting them compare elk effects after four and 35 years. They sampled inside the fences, where elk could not feed, and outside, where they could, measuring:

• nitrogen held above ground (in clipped plants) and below ground (in soil and roots);
• nitrogen deposited in leaf litter, and soil nitrogen available to plants;
• nitrogen eaten by elk;
• nitrogen returned by elk in dung and urine, by watching elk for 24-hour periods to see how they moved it between vegetation types.

The results

In willow and aspen, elk had a significant negative effect, causing a net transfer and loss of nitrogen from those communities. Browsing significantly reduced willow shrub size, plant biomass, above-ground nitrogen, litter and the nitrogen willows produce each year, and the nitrogen elk returned in their waste did not make up the loss. With less nitrogen available, the researchers saw substantial drops in the productivity and growth of willow, and probably aspen.

They recommend managing elk and their effects on the park's plants by reducing the elk population or preventing elk from concentrating in large numbers.

This summary rests on reports available when it was updated (January 2008) and is not a statement of park policy or a definitive account of the research. More on the park's research: nps.gov/romo.

Sources

• National Park Service, Rocky Mountain National Park, "Effects of Elk Herbivory" (updated January 2008). https://www.nps.gov/articles/effects-of-elk-herbivory.htm