"We've asked other researchers if they knew that bison browse on woody plants," says Patten. "Many people were surprised, but some researchers have witnessed bison eating willows and aspen when grass is not available."

Over the summer, the aspen grew a meter and a half.

(photo Jean Arthur)

Researchers have witnessed bison eating willows and aspen when grass is not available.

"It is significant when the tree is browsed down to a half meter or less, then exhibits incredible growth," says Meador.

Patten explains that it is like a pruned tree. The plant overcompensates and puts energy toward replacing the canopy growth. Meador points to a willow whose woody base diameter is 10 to 20 centimeters but whose height is only a meter and a half, indicating years of heavy browsing. Scars on the base indicate stripping—where ungulates peeled the bark for food.

"We are using willows and aspen as natural markers," says Patten.

He explains that the vegetationhydrology study is a small piece of a larger study. Decades ago, researchers noticed a decline in aspen and willow and recognized that changes in these vegetation communities in the northern range could reverberate through the wildlife community in complex and significant ways.

Duncan's study dovetails with a larger biocomplexity project in the Greater Yellowstone Ecosystem encompassing several universities and scientists. Patten explains that the biocomplexity study couples an understanding of the processes within natural systems of Yellowstone's northern range with changes adjacent to the park caused by human activities such as hunting, ranching and residential development.

"In the biocomplexity study, we look at the human component too," he says. "For example there has been an increase in the number of people in Paradise Valley, and an increase in the number of interactions with elk through expansion of ranching, fencing and new homes."

Karri Meador (left) and Levia Jones look at browse and growth patterns among aspen at Eagle Creek north of Yellowstone. Aspen and willow are considered barometers of ecosystem health on the park's northern winter range. It is significant when a tree is browsed down to a half meter or less, then exhibits incredible growth. (photo Jean Arthur)

Patten's family home of 50 years on the western edge of Yellowstone allowed him to watch the ecosystem's changes. He notes that because many humans live on the edge that wildlife stay in the park. With the introduction of wolves, the elk behavioral patterns have changed, and they may move out of the park. However, pressure from hunting tends to keep them within the park boundaries.

"The Greater Yellowstone area is highly valued and consequently heavily researched," says Meador. "There are multiple land—management goals from state and federal agencies as well as private land holders. One of the goals of this entire project is to really examine the linkages and interactions between the human and natural system."

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