DOI: 10.6086/d18q3x
title: Voles mediate functional trait diversity along a resource gradient

description: Disentangling the effect of multiple ecological processes on plant trait
 composition is complicated by the fact that both top-down and bottom-up
 processes may affect similar traits. We examined the interacting role of
 resource variation and vole herbivory on functional trait patterns in an
 annual California grassland. We manipulated vole herbivory via exclosures
 at eight grassland sites along a steep resource gradient and measured
 plant composition and functional traits over three years. Plants with
 resource acquisitive functional traits were favored at sites with
 increasing resources. Vole herbivory influenced community-weighted mean
 (CWM) leaf nitrogen (N) and seed mass, suggesting these traits may mediate
 plant susceptibility to vole herbivory. After three years, CWM leaf N
 increased in the absence of the voles, as did CWM seed mass, although this
 increase in CWM seed mass only occurred at higher resource sites. Vole
 exclusion at high resources sites also increased the functional diversity
 of leaf N and seed mass by the end of the experiment. Overall,
 environmental filtering primarily structured the dominant plant trait
 strategies, but vole herbivory also influenced the functional diversity of
 traits that influence herbivore susceptibility, particularly at resource
 rich sites. Thus, habitat filtering and herbivory can operate on different
 dimensions of plant functional composition to influence the species and
 functional composition of communities.
description: <i>Study Site.</i> We conducted our experiment in the
 annual grasslands at the University of California Sierra Foothill Research
 Extension Center (SFREC), located in Browns Valley, California, USA (39º
 15' N, 121º 17' W). These communities are dominated by
 non-native annuals with low abundances of native species. The most
 abundant species included <i>Avena barbata, Elymus caput-medusae,
 Festuca perennis, Bromus hordeaceus</i>,<i> and Erodium botrys
 </i>(nomenclature follows Baldwin et al. 2012). The climate is
 Mediterranean with cool wet growing seasons (Sept-May) and hot dry summers
 (June-Aug).  Growing season rainfall increased over the course of our
 study more than doubled from 454mm to 625mm to 978mm for the 2014-2015,
 2015-2016, and 2016-2017 growing seasons, respectively. The dominant small
 mammal consumer is <i>Microtus californicus</i>, a small
 meadow vole that prefers habitat with a standing litter layer (Batzli
 & Pitelka, 1970; Ostfeld et al., 1985).  Other small mammal
 species at our study site predominantly reside in the more heavily cattle
 grazed pastures (Block & Morrison, 1990). Therefore, our design
 below reflects predominantly the effects of voles.
 <i>Sites. </i>We conducted our experiment at eight
 experimental sites that spanned a steep resource gradient.  We described
 this gradient (hereafter resource gradient) using a principal component
 analysis (PC1 which described 47.6% of the variation in environmental
 variables; Supporting Information Appendix S1, Figure S1). At one end of
 the gradient were low productivity sites (as characterized by aboveground
 primary productivity) with low soil nitrogen (N), but high micronutrients
 (i.e., magnesium, sodium, and calcium). At the other end of the gradient,
 sites had high productivity and soil N.  
 <i>Experimental Design.</i> To evaluate the effects
 of voles on plant trait composition, in the summer of 2014, we initiated a
 field experiment where we manipulated the presence of voles. At each site,
 we established a set of paired plots (9x9m), one fenced to exclude small
 mammals and one unfenced control to allow small mammal access. The
 exclosures were constructed of 0.64cm welded wire dug 60cm into the ground
 around the perimeter of the plot. The fence extended about 90cm
 aboveground and was topped with galvanized sheet metal with a 20cm face to
 prevent voles from climbing over the fence. Within each of the paired
 plots, we randomly established a set of six 0.5m x 0.5m subplots to assess
 the effects of voles on plant trait composition. We additionally set up an
 electric fence around the 14m x25m experimental area at each site to
 exclude cattle. <i>Plant traits. </i>To
 examine the distribution of functional traits in communities we sampled
 5-10 individuals of the dominant and subdominant resident species  across
 the  sites for plant functional traits (following Pérez-Harguindeguy et
 al. 2013). The samples were collected from within the non-cattle grazed
 experimental area but not from within any experimental plots. Samples were
 collected at peak biomass from April-May in 2015 and 2016. We measured
 maximum plant vegetative height, specific leaf area (SLA; leaf area/dry
 leaf mass), leaf water content (LWC; 1- leaf dry weight/leaf fresh weight)
 and seed mass.  The sampled leaves were additionally processed for tissue
 carbon and nitrogen content to estimate leaf N content and carbon to
 nitrogen ratios. These traits are strong indicators of resource use and
 plant growth. SLA is positively correlated with a species relative growth
 rate and tissue N (P B Reich, Walters, & Ellsworth, 1997; Westoby,
 Falster, Moles, Vesk, & Wright, 2002); leaf water content is
 negatively correlated to water stress (Farooq, Wahid, Kobayashi, Fujita,
 & Basra, 2009); plant height is often indicative of competitive
 interactions for light (Westoby, 1998); and greater leaf N and lower carbon to
 nitrogen ratios can be linked to higher food quality (Westoby,
 1999). To account for potential trait differences in species that occurred
 across the environmental gradient, we sampled individuals for as many
 species as possible at both the low and high end of the gradient. To estimate a species’ seed mass, we
 first took 10 samples with the same number of seeds (i.e. the number of
 seeds was either 50 or 75 seeds for a species, depending on seed
 availability). For each sample, we calculated the average seed weight by
 dividing the total weight of the sample by the number of seeds. The
 species level average was then the average of those 10 estimates.
 (All species trait data are available in Table S1.) In total, we sampled traits on 54
 different grassland species (24 of these in both habitat types), which
 made up on average 98% of the species composition in a given plot (mean,
 range of species cover: 2015: 99.25 (89-100), 2016: 98.9 (93.5-100), 2017:
 98.75 (84-100). <i>Community
 sampling</i>. From 2015-2017, at peak biomass (Apr-May) we sampled
 the plant species composition within each subplot. To estimate vole
 activity, in 2016 and 2017, we recorded the frequency of vole activity
 (i.e. runways, burrows, and droppings), along 8 10m long transects within
 each larger control plot at each site. For each subplot, we then
 calculated the community weighted mean (CWM) for each individual plant
 trait and the functional dispersion of each trait. CWM is measured as the
 mean of species trait values present in the community, weighted by the
 relative abundance of each species (Lavorel et al., 2008). Functional
 richness (FRic) estimates the dispersion of species in trait spaces
 without accounting for species abundance and is estimated as the convex
 hull volume (Villeger, Mason, & Mouillot, 2008). Functional
 dispersion (FDis) is the average distance to the centroid in multivariate
 trait space that is weighted by species relative abundances (Laliberte
 & Legendre, 2010). For those species that occurred across the
 environmental gradient we used the species level trait data for a plot
 that best matched its position along the environmental gradient for these
 calculations.
description: There are four files for this data set: 1) Site environmental
 data, 2) Plot community species cover data 3) Plant species trait data and
 4) Vole runway activity data. Site Environmental Data:
 Three soils samples up to 10cm in depth were collected from each large
 9mx9m plot (n=16) and processed for soil resources. These data are
 reported here. Additionally, the mean live aboveground biomass and mean
 litter from six 10cmx50cm subplots are reported. These data were included
 in a principal components analysis that generated two PCA axis. The first,
 PCA1 was used as the resource gradient described in the
 manuscript. Plot Community Species Cover Data: With
 each block, 12 0.5m 0.5m plots were sample in each of three years (2015,
 2016, 2017). For this sampling the visual cover of each observed species
 was recorded, with cover adding over 100 to allow for multiple canopy
 layers. These data are the cover estimates for each plant species in each
 plot over time. The Site.Prod column indicates the how these data were
 stratified to match up with species trait that were collected at the low
 or high end of the resource gradient. Plant species
 trait data: These data are species-level trait data for species observed
 at the field site. Specific Leaf Area (SLA), leaf water content (LWC),
 leaf area (Area.cm2), leaf dry matter content (LDMC), maximum vegetative
 height (Height.max.cm) were sampled on 5-10 different individuals in the
 field. These plant leaves were used for leaf nitrogen (Leaf.N), leaf
 carbon (Leaf.C), and carbon to nitrogen ratios (C.N.Ratio). Seed mass was
 extracted from the Kew database, Baker seed collection at the Jepson
 Herbarium, site collected samples, or purchased seed.
 Vole Runway data: These data are the average runways observed in
 8 transects sampled in the presence of voles (i.e. open plots).