Data from: Interactions among herbivory, climate, topography, and plant age shape riparian willow dynamics in northern Yellowstone National Park, USA

Understanding how the environmental context modifies the strength of trophic interactions within food webs forms a central challenge in community ecology. Here, we demonstrate the necessity of considering the influence of climate, landscape heterogeneity and demographics for understanding trophic interactions in a well-studied food web in Yellowstone National Park, USA. We studied riparian willow (Salix spp.) establishment and stem growth reconstructed from tree rings on the northern range of Yellowstone over a 30-year period that included the reintroduction of a top-predator, the gray wolf (Canis lupus). We used climate variables (annual precipitation, stream flow, and growing season length), herbivore abundance, and landscape descriptors (elevation and topographic wetness index) to predict establishment and growth processes through time before and after the introduction of wolves. We fitted Bayesian hierarchical models to establishment data and time series of individual stem heights from 1980 to 2008. Explaining variability in establishment required models with stream flow, annual precipitation, and elk abundance. Climate, trophic, and landscape covariates interacted with stem age to determine stem height and growth rate through time. Growth rates of most stems ages (2+) declined after the reintroduction of wolves. However, stem growth rates naturally declined with age, and the decline we observed was coincident with faster growth rates for the youngest stems. Mean stem heights at age have remained relatively stable through time for most age classes. Estimated effects of landscape topography had approximately the same magnitude of effect on stem growth rate at age as elk abundance. Synthesis: We show that the effects of modification of a food web cannot be predicted by studying trophic dynamics in isolation. No single driver explained patterns of willow establishment and growth over the past three decades in Yellowstone. Instead, interactions among trophic forces, interannual climate variability and landscape topography together shaped how the ecosystem responded to perturbations. Top-down effects of ungulates on riparian woody vegetation must be considered in the context of plant age, and climate and landscape heterogeneity.Study site locations in Yellowstone National ParkThis file contains coordinates for 23 observational study sites on Yellowstone's northern range. Coordinates are given in UTMs, using the NAD 1983 UTM Zone 12N projection.SiteLocs.csvWillow establishment dataNumber of sampled willows that established by year, from 1980 to 2008. These data are pooled across all of the study sites.Establish_Data.csvWillow stem identifiersThis file contains identifying information for each willow stem collected and aged (stem number, site number, and species). Species (numbered 1-6) are: S. bebbiana, S. boothii, S. drummondiana, S. eriocephala, S. geyeriana, and S. pseudomonticola. Sites correspond to site numbers in Site Locations data file.StemData.csvWillow stem height through timeStem height data is a matrix of log(height) in cm HeightTime.csv. The first column lists years, and each subsequent column has height data for an individual numbered stem. Height is 0 in the year before the known sprouting year.HeightTime.cs