Trophic cascades and climate variability affect willow growth on Yellowstone's Northern Range

Background/Question/Methods   Understanding how perturbations in one trophic level affect other components in a food web forms a central challenge in contemporary ecology. In Yellowstone National Park, willows (Salix. spp) were heavily browsed by elk (Cervus elaphus) following the extirpation of wolves during the 1920s. Since wolves were reintroduced to Yellowstone in 1995, it has been widely suggested that elk browsing pressure has declined and willows have recovered on the Northern Range. Our findings have shown that recovery has been less widespread than originally believed. In this study, we investigate patterns in willow growth reconstructed from stem growth rings. We collected 130 stems from 18 established study sites across the Northern Range. Each stem was sectioned every 10 cm and the number of growth rings on each section counted. Multiple observers were used to estimate measurement error. Using relationships between stem age and height, we reconstructed the establishment and growth of these 130 stems. To evaluate which mechanisms might be responsible for differences between individuals, we developed hierarchical Bayesian models of stem height growth as a function of covariates including landscape metrics, climate variables, and estimates of elk and wolf abundance. Competing models were evaluated with DIC.

Results/Conclusions   Surveyed stems ranged from 0.5 to 4 meters in height and from 3 to over 20 years in age. Although age was a significant predictor of total stem height, age alone explained less than 10 percent of total variation in height. Models of stem growth most strongly supported by the data indicated that both climate variation and browsing pressure influenced stem heights through time. Further, we found landscape characteristics controlled large scale patterns of hydrologic conditions, explaining variability in stem growth between study sites. Despite declining elk numbers in recent years, browsing pressure remained high across much of the Northern Range, and stem increments reflected this general pattern. Overall, our results suggest that climate variability, landscape variation, and ecological interactions have interacted to shape willow growing conditions over the past two to three decades.