Did Yellowstone willows grow more after wolves returned?

Details

Wolves (Canis lupus) were reintroduced to Yellowstone National Park in 1995–1996. Researchers had proposed that wolves could trigger a trophic cascade by changing elk (Cervus elaphus) density, habitat selection and behavior, potentially changing plant communities used by elk.

The researchers sampled two willow species, Salix boothii and S. geyeriana, on Yellowstone’s northern winter range. They asked whether willow growth increased after wolf reintroduction, whether elk browsing affected growth, and whether any increase exceeded what climate and hydrological factors alone could explain—potentially indicating a trophic cascade caused by wolves.

Using stem sectioning techniques to quantify historical growth patterns, they found an approximately twofold increase in stem growth-ring area after wolf reintroduction in both willow species.

Climate and hydrological factors alone could not explain the increase; wolf presence on the landscape was a significant predictor of stem growth beyond those abiotic factors.

Growth-ring area was positively correlated with the previous year’s ring area and negatively correlated with the percentage of twigs browsed from the stem during the winter before growth. This indicated that elk browsing impeded stem growth.

The results were consistent with the hypothesis of a behaviorally mediated trophic cascade on Yellowstone’s northern winter range following wolf reintroduction.

The researchers suggested that the community-altering effects of wolf restoration support ecological-process management in Yellowstone National Park.