DOI: 10.5061/dryad.6wwpzgn2n
title: Functional assembly of grassland plant species in native communities in Spain and recipient communities in California

description: A major aim in invasion ecology is to understand the role of exotic
 species in plant communities. Whereas most studies have explored the
 traits of exotic species in the context of the introduced community,
 functional comparisons of entire assemblages of species in their native
 and introduced communities have rarely been analysed. Taking advantage of
 the unidirectional invasion of plant species of European origin (i.e.
 colonizers) into California, this study aims to investigate the relative
 importance of plant traits, environmental factors, and invasion status in
 biological invasions. We compared the functional structure (i.e. trait
 composition and diversity) along with resource availability gradients in
 recipient and native Mediterranean grassland communities in California and
 Spain, respectively. Traits were related to resource use in above- and
 belowground organs and reproductive strategy. We also investigated how
 niche differences vary along environmental gradients between coexisting
 colonizer and native species assemblages within communities. There were
 clear differences in the functional structure of Mediterranean grassland
 communities between regions, which were associated with the resource
 availability gradient. Paradoxically, the most acquisitive communities
 occurred in resource-poor sites, highlighting that rapid acquisition and
 use of resources permit species to cope with environmental stress through
 stress avoidance. In Spain, colonizer species had greater SLA than
 non-colonizers. Yet, differences between colonizer and non-colonizer
 species in Spain for other traits were mostly absent and did not change
 along the gradient. This might be a result of the greater native species
 richness as a consequence of the agricultural practices that have taken
 place in Europe for millennia and reflect that the entire species pool of
 grasslands is adapted to agricultural landscapes. In comparison, in
 California, colonizer species were more acquisitive in their use of
 resources than natives under favourable conditions, but functionally
 converged in resource-limited sites. This result in the recipient region
 underscores that the importance of niche differences between native and
 colonizer species as a community assembly mechanism is strongly subject to
 the influence of habitat filtering. Trait comparisons are
 context-dependent, and a correct interpretation of filtering processes in
 community assembly requires a regional perspective.
description: From March 2018 to May 2018,
 we sampled 60 vegetation plots across 4 sites in Spain; and from March
 2019 to May 2019, we sampled 60 vegetation plots across 4 sites in
 California. Sites were weekly monitored and sampled during the two weeks
 following the flowering blooming peak; therefore on mature plants. We
 selected sites that, over the last 30 years, had been moderately grazed by
 wild and domestic ungulates and had not been burned, ploughed, or planted.
 In each site, we collected plant species composition and relative cover
 abundance in fifteen plots (0.50 × 0.50 m) located along a 1 km
 transect. For each
 combination of species × site, we collected 10 flowering individuals of
 the species that contributed to 90% of the cumulative cover
 and measured 8 traits representative of plant resource-use and acquisition
 strategies: Specific Leaf Area (SLA),
 Leaf Dry Matter Content (LDMC), Leaf Nitrogen Content (LNC), Isotopic
 Carbon Fraction (δ13C),
 Specific Root Length (SRL), Root Dry Matter Content (RDMC) and
 Reproductive Height (distance from the latest inflorescence to the
 ground). Measurements were performed following the protocols
 included in the handbook for standardised measurement of plant functional
 traits (Pérez-Harguindeguy et al., 2013). Additionally, seed mass was retrieved from the Seed Information
 Database. We classified species as “colonizers” if they were
 native to Spain and known to be successfully established as exotic species
 elsewhere. We also recorded species growth form and life form
 traits. We estimated
 average soil nutrient concentration per site by taking 5 cores: percentage
 total nitrogen (N) by Kjeldahl digestion, organic matter (OM) by the
 Walkley and Black method, and available phosphorus (P) by the Olsen
 method.
description: Trait and soil data are available in csv format, and abundance
 data in txt format.