DOI: 10.5061/dryad.mj7t7
title: Data from: Interactions among herbivory, climate, topography, and plant age shape riparian willow dynamics in northern Yellowstone National Park, USA

description: 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.
description: 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.csv