DOI: 10.5061/dryad.rjdfn2zcv
title: Species provenance and traits mediate establishment and performance in an invaded grassland

description: In many invaded grasslands, dominant exotic species can produce large
 amounts of litter that modify local abiotic conditions and species’
 interactions. These novel conditions can reduce native species abundance
 and promote the persistence of exotic species, yet the strength of this
 disparity may be influenced by how consumer pressure interacts with litter
 accumulation. Consumers may exacerbate this disparity by preferentially
 targeting native species or by promoting heterogeneity in microhabitats
 due to their movement and small-scale ground disturbances that favours
 fast growing exotic species. How species respond to litter accumulation
 and consumer pressure may depend on either evolutionary differences,
 whereby exotics species may benefit from a lack natural predators, or by
 functional differences, in which species’ physiological traits may confer
 fitness advantages to low-light conditions or herbivory or granivory
 pressure. We examined the impact of litter presence and small mammal
 herbivory on the establishment and reproduction of functionally diverse
 exotic versus native species seeded across sites that naturally vary in
 resource availability in an annual invaded California grassland. We
 assessed whether seed mass and leaf nitrogen content (LNC) were predictive
 of successful establishment and reproduction. Litter accumulation affected
 exotic and native species differently, with litter significantly
 decreasing native recruitment and reproduction, while exotics were largely
 unaffected. Small mammals had a slight positive effect on the
 establishment of native species when litter was present but did not
 influence exotic species. Regardless of species provenance, larger seeded
 species established at a higher density while species with lower leaf
 nitrogen content had a higher density of reproductive individuals. Native
 species that successfully established and reproduced were functionally
 different in LNC than the resident community, while successful exotic
 species were functionally more similar to the resident community in LNC.
 Our study demonstrates that exotic species outperformed native species
 regardless of the presence of litter or herbivory pressure. Without the
 removal or thinning of litter, it is likely that exotic species will
 continue to dominate, resulting in a positive feedback that further
 favours the persistence of exotic species within this invaded grassland
 system.
description: <em>Study site</em> This study was
 conducted in annual grasslands at the University of California Sierra
 Foothill Research and Extension Center (SFREC), located in Browns Valley,
 California, USA (39º 15' N, 121º 17' W). The site has a
 Mediterranean climate, with a cool wet growing season (September – May)
 and host dry summers (June – August). Temperature and precipitation over
 the course of the experiment varied over the three growing seasons:
 2014-2015, 2015-2016 and 2016-2017, with 454mm (mean 15.5 ο C), 625mm
 (mean 14.9 ο C) and 978mm (mean 14 ο C) recorded, respectively (PRISM
 Climate Group 2004). Vegetation at the site is dominated by exotic annual
 species, with low abundances of native annual species. The most abundant
 exotic species, <em>Elymus caput-medusae</em>, produces large
 amounts of litter, which remains on the landscape due to slow
 decomposition and low palatability (Nafus and Davies 2014). Other dominant
 exotics species include: <em>Avena barbata</em>,
 <em>Bromus hordeaceus</em> and <em>Erodium
 botrys</em> (nomenclature follows Baldwin et al. 2012). Plant
 consumers in this system include mule deer (<em>Odocoileus
 hemionus</em>) and small mammals such as field voles
 (<em>Microtus californicus</em>) and gophers
 (<em>Thomomys bottae</em>) (Block and Morrison
 1990). <em>Experimental
 design</em> To assess the influence of litter,
 small mammals and resources on native and exotic species recruitment and
 reproduction, in the summer of 2014 we established experiments at eight
 sites at either end of a productivity/soil resource gradient. Within each
 site, we either excluded rodents or not within paired 9m x 9m plots, one
 of which was fenced to exclude small mammals and the other which remained
 unfenced to allow small mammal access. Within each pair of plots, we
 established 16 subplots (0.5m x 0.5m) that were randomly assigned to one
 of eight unique treatments. The treatments represent a factorial
 combination of: 1) litter treatment (removed or left intact), 2) seed
 addition (yes or no) and 3) Origin of species in seed mix (native or
 exotic). The first set of of eight subplots were seeded in each site in
 summer 2014 for a total of 64 subplots. The second set of eight subplots
 were seeded in the summer of 2015 and provide a replicate over time (total
 of 128 subplots sampled over 2 years). <em>Plant
 traits</em> To evaluate species responses based
 on functional traits, we measured functional traits that are known to
 impact germination and growth, which included seed mass (Moles and Westoby
 2004) specific leaf area (SLA), leaf water content (LWC), maximum plant
 height, leaf nitrogen concentration (LNC) and carbon to nitrogen ratio
 (C:N) (Navas and Violle 2009). We measured traits on 5 - 10 individuals of
 most species outside the treatment plots, avoiding trait estimations in
 experimental plots where treatments could influence trait expression.
 However, of the total 33 seeded species, six species could not be found
 outside of the treatment plots, and for these species we had to sample
 individuals from plots representing each of the treatment
 combinations.  <em>Species recruitment and
 reproduction sampling</em> We assessed the
 colonization of each species in every subplot by counting the number of
 individuals of each added species that established at peak biomass in
 April-May 2015 and 2016. We assessed reproduction by counting the number
 of reproductive stems of added species. To capture differences in
 phenology among species, we sampled all plots once a week between
 April-May. We estimated the total number of seeds produced by individuals
 of each added species by first randomly selecting 2-4 individuals per
 species within four different subplots (two subplots in the rodent
 exclosure and two subplots in the open plot), to obtain a total of 8-16
 individuals per species per site. For each individual, we then estimated
 per capita seed production by counting all viable seeds. If seeds were not
 yet present, we counted the total number of spikelets or inflorescences
 for an individual and multiplied this number by an estimated number of
 seeds per spikelet/inflorescence. This multiplier was either obtained by
 field estimates of fecundity of individuals outside of plots that produced
 seed or extracted from a published source. If species did not have at
 least 8 individuals within seed addition subplots, we sampled all
 individuals that were present. <em>Community
 sampling</em> We estimated the percent cover of
 resident species (species that were not included in the seed mix but
 recruited naturally) within all seed addition subplots in 2015 and 2016.
 Species cover was visually estimated for each species in every subplot,
 and total cover was allowed to go over 100 to allow for multiple canopy
 layers. We sampled traits for resident species, following the methodology
 to that of the seeded species (see above). We calculated the resident
 community weighted mean for seed mass, SLA, LWC, LNC and C:N within all
 seed addition subplots spanning each treatment combination and year. We
 calculated trait differences of seed size and LNC between each species
 present in the seed addition groups from the community weighted mean (CWM)
 of resident invaded communities (i.e. trait difference = species mean
 trait value – CWM value, thus a negative trait difference indicates higher
 trait values of the added species compared to the resident community and
 vice versa for positive trait differences).
description: There are three files for this data set: 1) Species stem count
 data, 2) Plant species trait data, 3) Plot community weighted mean
 data. <strong>Species stem count
 data</strong>: Recuitment data for native and exotic species that
 were seeded into seed addition subplots. This data includes: the number of
 stems per species that established (stem count), the probabilty of
 establishment (yes or no) the number of reproductive stems and seed output
 across two litter treatments (removed or left intact) and two rodent
 treatmetns (open or excluded).
 FunEco_Dryad_SFREC_species_stem_seed_count_data.csv
 <strong>Plant species trait data</strong>: These data
 are species-level trait data for species observed in the experimental
 sites at the Sierra Foothills Research Extension Center. Specific leaf
 area (SLA), leaf water content (LWC), maximum plant height, leaf nitrogen
 concentration (LNC) and carbon to nitrogen ratio (C:N) were sampled on 5 -
 10 individuals of most species outside the treatment
 plots. FunEco_Dryad_SFREC_species_plant_trait_data.csv
 <strong>Plot community weighted mean data</strong>:
 Community weighted mean values for seed mass, SLA, LWC, LNC and C:N within
 all seed addition subplots spanning each treatment combination and year
 for both resident and added communities. Species mean trait values for
 seed mass, SLA, LWC, LNC and C:N are also included.
 FunEco_Dryad_SFREC_community_weighted_mean_data.csv