Trophic cascades from wolves to alders in Yellowstone

• In northern Yellowstone, alder was suppressed by elk browsing.
• Wolf reintroduction in 1995–1996 could cause a trophic cascade benefiting alder.
• Alder recruitment occurred along six streams soon after wolf reintroduction.
• A trophic cascade may have reversed the long-term trend of alder suppression.

We explored possible interactions among gray wolves (Canis lupus), Rocky Mountain elk (Cervus elaphus), and thinleaf alder (Alnus incana spp. tenuifoli) in northern Yellowstone National Park. We developed an alder age structure based on annual growth rings for plants growing along six streams in areas accessible to ungulates on the northern range. Alder stems (n = 412) along the six streams originated only after wolf reintroduction. By 2013, 80% of the sampled alders along these streams were taller than 2 m, in contrast with a historical pattern of height suppression by ungulate herbivory. This pattern of alder recruitment is consistent with a trophic cascade whereby new alder growth occurred across all study streams within several years after wolf reintroduction. Although declines in elk density since wolf reintroduction likely contributed to the release of alder from herbivory, the immediate onset of new alder recruitment following wolf reintroduction indicates that behavioral responses to predation may also have been an important component in the resulting trophic cascade. These results suggest that predator conservation could play a role in the management and ecological restoration of riparian areas.