Tall willow thickets return to northern Yellowstone
Level 3 - non-randomized controlled study
Level 3 by design analogy (longitudinal repeat-measures ecological field study).
OpenAlex W3023657411 · doi:10.1002/ecs2.3115
What was done
The authors evaluated changes in willow (*Salix* spp.) height, canopy cover, and browsing pressure in northern Yellowstone National Park across three time periods: 1988–1993 (high elk density), 2001–2004 (early recovery following carnivore restoration), and 2016–2018. They compared willow growth between stream sites and wet meadow sites, assessed the impact of elk and bison browsing, and surveyed stream reaches for the establishment of willow thickets (defined as groups of 5 or more willows >200 cm spring height and <2 m apart).
What was found
Willow response varied substantially by habitat type: - Meadow vs. stream sites: Willows in wet meadows did not increase in height, whereas stream-side willows increased significantly, exceeding a mean summer height of 200 cm by 2001–2004 and a mean spring height of 200 cm by 2016. - Height distribution: The overall willow height distribution shifted from predominantly short to bimodal, with a new peak at 300–400 cm. - Bison browsing: In areas where bison congregated, willows remained suppressed below 120 cm due to summer browsing. - Thicket formation: Willow thickets were present in all surveyed stream reaches, occupying between 22% and >80% of willow patches across sites.
Why it matters
This study provides long-term field evidence that reduced elk browsing following carnivore restoration facilitates passive riparian vegetation recovery even under warming conditions, while highlighting that recovery is spatially patchy and mediated by differing herbivore dynamics.
Limits
The abstract does not report the exact number of plots, transects, stream reaches, or individual willows measured. The study design is observational and non-randomized, making it difficult to fully isolate carnivore-mediated trophic cascades from hydrological, microclimatic, or localized site differences. Furthermore, recovery was limited to stream channels, with wet meadow willows failing to rebound.
Cited by
- supports Reintroducing wolves to Yellowstone National Park after 70 years altered large herbivore abundance, willow and tree growth, and beaver behavior.