Broken by bison, aspen saplings have a tough time in northern Yellowstone

In northern Yellowstone National Park, saplings of quaking aspen, an ecologically important tree in the American West, are being broken by a historically large bison herd, affecting the comeback of aspen from decades of over-browsing by elk.

Findings of the U.S. National Science Foundation-supported research led by Luke Painter of Oregon State University were published in Ecology and Evolution.

“I’ve studied the response of aspen in northern Yellowstone to the reduction in elk after the wolves were brought back and found that during this time, bison increased and have begun to affect aspen,” Painter said. “Now we’re showing strong evidence of a previously unreported behavior of bison bulls breaking aspen saplings.”

The saplings were tall enough to escape most browsing by elk and thus likely to grow into trees, but bison broke them off at a low height, he said. Other saplings were killed when bison scraped off the bark with their horns.

Quaking aspen largely reproduces by root sprouts, a process known as suckering, and stands of aspen are often a single organism connected by the trees’ common root system. Fire stimulates aspen reproduction from both roots and seeds.

For much of the 20th century, Painter said, aspen sprouts were unable to grow into trees because they were eaten by elk during winter. But at the end of the century, when wolves were reintroduced and the numbers of other large predators such as grizzly bears and cougars increased, elk numbers in the northern part of Yellowstone went down, bringing relief to the aspen.

“Some young aspen began growing into saplings, which was an indication they were no longer being consumed by elk and were likely to grow into mature trees,” Painter said. “It was a trophic cascade that changed the Yellowstone ecosystem, creating conditions that could bring it closer to what it was historically, with more aspen, willow and beaver, which depend on these plants. But the tremendous increase in bison over the last two decades has added a new turn to the story.”

Bison have long been known to have strong effects on their environment, Painter said. Among those is removing and suppressing shrubs and trees by eating, trampling and breaking them — and as bison numbers have greatly risen in northern Yellowstone in the last two decades, their effects on plants have also increased. In places where bison are present in large numbers, they are hindering some aspen stands from replacing their dying trees.

“These results clearly show that conservation and management decisions must consider interactions among these ecologically important species, and how changes in the population size of one species will impact the rest of the community,” said Kari Segraves, a program director in NSF’s Division of Environmental Biology. “If we truly want to restore an ecosystem, we must strike that balance.”

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