METHODS

Species traits

Species selection, seed material and precultivation

The species used are flowering plants occurring in the grasslands plots of the German “Biodiversity Exploratories” . In each of three regions of Germany, the Schwäbische-Alb (southwestern Germany), Hainich-Dün (central Germany), and Schorfheide-Chorin (northeastern Germany), 50 plots (4×4 m) were selected in grassland habitats covering a wide range of land-use intensities. From 2008 to 2016, the vegetation composition of each of the 150 plots was assessed annually in late spring by estimating the cover of each species. We standardized the species names according to the accepted names in The Plant List 1.1 (http://www.theplantlist.org, accessed on 15th June 2019), using theTaxonstand package (Cayuela et al. 2017), to allow us to align the species names between different distribution and trait datasets (see below). In total, 364 vascular plant species have been identified in the plots. For 312 of those species, we were able to obtain seeds from commercial seed suppliers or botanical gardens for our experiments (see Appendix S1 in Supporting Information).
In two experiments, we measured functional traits on those species. Before the first experiment, we individually weighed 10 randomly chosen seeds of each of the 312 species. Then we did an indoor pot experiment to determine root morphology of the species, and an outdoor pot experiment to determine rooting depth. For both experiments, seeds were sown in plastic pots (7×7×6.5 cm) filled with peat soil. The pots were then placed in a phytochamber for two to three weeks (night/day 9/15 h; 18/21 ± 1.5°C; relative humidity 90 ± 5%) before transplanting the seedlings (for cultivation times, see Appendix S1). We additionally obtained data on aboveground traits (specific leaf area, height) and bud-bank size from several databases.

Experiment on root-system morphology

From May 1 to October 6, 2017, we performed a glasshouse experiment to measure root-system morphological traits of plants grown at either low or high nutrient levels. Because of the large number of species and the time-consuming measurements, we grew the plants in four temporally shifted (4-6 weeks) batches. We aimed to have each species represented in each batch, and to have a total of seven replicates per species and nutrient level across all batches (Appendix S1). The seedlings of the species that had germinated (N=238) were transplanted individually into plastic pots (1.3 L) filled with a mixture of sand and vermiculite (1:1 volume ratio). The pots were then randomly allocated to positions in two glasshouse compartments, and allowed to grow for four weeks (night/day 10/14 h; 22/28 ± 1.5°C; relative humidity 80 ± 15%). Plants were fertilized three times a week with either a low nutrient solution (40 ml with 1500 µM KNO3) or a high nutrient solution (40 ml with 12000 µM KNO3). The fertilizer was a modified version of the Hoagland recipe (see Appendix S2).
After four weeks of growth, plants were harvested. We grew the plants for four weeks to avoid roots becoming pot-bound, and to be able to analyse the entire root systems. After carefully washing off the substrate, the root system was cut below the collar and stored for <1 week in a plastic tube filled with distilled water at 4°C. Then, root systems were spread individually in a thin layer of water in transparent trays (11 cm × 11 cm) and scanned at 800 dpi with a flatbed scanner modified for root scanning (Epson Expression 10000 XL and 11000 XL). The images were analysed using the software WinRHIZOTM 2017a (Regent Instruments, Quebec, Canada) to obtain the total root length and root volume. Root systems were then oven-dried for >48 hours at 65°C and weighed. We calculated specific root length by dividing the total root length by the belowground dry biomass, and root tissue density by dividing the belowground dry biomass by the sum of the root volumes according to Rose (2017). The diameter of first-order roots (i.e. the most distal roots), thought to be most important for nutrient uptake , was determined by randomly sampling a distal root branch (or a portion of it) for each root system and calculating the mean of the external-internal links diameter obtained with the “Link analysis” function in WinRHIZO. We also dried and weighed the aboveground biomass of each plant, and calculated the root weight ratio (i.e. root biomass divided by total biomass).

Experiment on rooting depth

From the 15th of May to the 10th of October 2018, we performed an outdoor pot experiment to measure the maximum rooting depth of the species. Up to five seedlings of the species that had germinated (n=197; Appendix S1) were transplanted individually into 120 cm high plastic grow tubes (Tubex ® Standard Plus, http://www.tubex.com/products/tree-shelters/tubex-standard-treeshelters/specification.php) the bottom of which was closed with a thick piece of cotton tissue. The tubes were filled with a mixture of sand and vermiculite (1:1 volume ratio) up to a height of 115 cm. The tubes came in five diameter classes (8.0, 8.4, 10.0, 10.8 and 12.0 cm) stacked in each other. To avoid that tube diameter would be confounded with species identity, each of the five seedlings per species was planted in a different tube-diameter category. We placed the tubes upright in a randomized design in the Botanical Garden of the University of Konstanz (47°41’24.0”N 9°10’48.0”E; see Appendix S10 for pictures).
We planted 734 plants, but due to early mortality we had to replace 126 of them within the next three weeks. The growth period therefore ranged from 16 to 19 weeks. The experiment took place during the summer of 2018 (mean temperature: 19.5 °C, min/max 2.5/37.4°C; relative humidity: mean 74%, min/max 22.7/100%). All the plants were fertilized once a week with 60 ml of a standard nutrient solution (1‰ Universol® Blue, Nordhorn, Germany), and watered regularly from above. We harvested the plants in October. Each tube was carefully sliced open, and we measured the distance from the top of the substrate to the deepest root.

Traits from databases and data imputation

Data on the aboveground traits specific leaf area (236 species) and height (232 species) were obtained from the LEDA database (Kleyeret al. 2008; https://uol.de/en/landeco/research/leda, accessed on 26th August 2019). In addition, data on bud-bank size (219 species) was obtained from Klimešová et al.(2016).
Although for each of the traits we had data for 197 (rooting depth) to 312 (seed weight) species, the number of species with complete trait data was 163. Therefore, we did phylogenetically informed imputation of missing data for the 242 species that were grown in at least one of our two experiments. Imputation was realised for 5.6% of trait values and details about the procedure can be found in Appendix S4. The phylogenetic tree of the species used, their standardized trait values and phylogenetic signal can be found in Appendix S5, S6 and S7.

Species abundance and occurrence frequency

To quantify each species’ success from local scale abundance to global distributions, we used four different data sources.

The Biodiversity Exploratories

To obtain information on local abundance and occurrence frequency of our study species in German grasslands, we used data from the Biodiversity Exploratories grassland-composition surveys. In each of the three regions, c. 500 so-called grid plots (GPs) and a subset of those, the 50 so-called experimental plots (EPs), have been monitored for biodiversity measures. The plots are 50 m × 50 m, and in each of those there is a subplot of 4 m × 4 m, in which the relative abundance of each plant species has been determined. In the 1494 GPs, vegetation was sampled once from 25 May to 15 August 2007. In May 2009, 138 plots were re-assessed and earlier relevés were discarded the because vegetation had been recorded too late in the season of 2007 which led to unreliable data . Of our 242 study species, 213 were present in that census of the GPs (Appendix S1), and, when present in a plot, they covered on average 2.8% of the plot (min: 0.27%; median: 1.44%; max. 17.16%). For the 150 EPs, the vegetation data were collected annually between mid-May and mid-June from 2008 to 2016, and we averaged the data across years. Of our 242 study species, 240 were present in the EPs vegetation survey, and, when present in a plot, they covered on average 1.05% of the plot (min.: 0.09%; median: 0.33%; max.: 13.05%). Two study species,Spergula arvensis and Taraxacum campylodes , had been selected because their names were included in an earlier version of the vegetation survey due to misidentification. While there are ten times more GPs than EPs, the latter includes data over a longer time period. For both the GPs and EPs, we used two distribution metrics for each species: the occurrence frequency defined as the number of plots in which a species is present divided by the total number of plots, and the local abundance defined as the mean cover of a species across all the plots were it is present.

FloraWeb

For information on the occurrence frequency in all of Germany, irrespective of habitat type, we obtained data from the German plant distribution atlas of NetPhyD and BfN (2013) through the FloraWeb database . For each species, we extracted the number of grid cells in which the species has been reported. Each grid cell is about 130 km², and there are 2995 grid cells in total. Of our 242 study species, 236 had grid-cell data available (Appendix S1).

Euro+Med PlantBase

To obtain information on the extent of the native distribution in all of Europe and its adjacent regions, we used Euro+Med PlantBase (http://www.emplantbase.org/home.html, accessed 1st June 2019). This on-line database provides information on the presence of vascular plant taxa in 117 regions (mostly countries) covering all of Europe and the Mediterranean regions of North Africa and the Near East. Of our 242 study species, 238 species were found in Euro+Med PlantBase, and for those we extracted the total number of regions with native occurrences (Appendix S1). The four remaining species, Cerastium nutans , Erigeron canadensis ,Matricaria discoidea and Medicago × varia , are not native to the region.

GloNAF

As 238 of our 242 study species are native to Europe, we also assessed the extent of their global occurrence as naturalized alien species, using the Global Naturalized Alien Flora (GloNAF) database, version 1.2 . GloNAF is a compendium of lists of naturalized alien plant species for 1029 regions covering >80% of the terrestrial ice-free surface. Of our 242 study species, 222 species had at least one record in GloNAF. For those species, we extracted the number of regions in which they are naturalized, and for the 20 species without GloNAF records, we set the number of GloNAF regions equal to zero.

Statistical analyses

All statistical analyses were performed in R version 3.6.1 . To test whether more abundant and more widespread species have particular traits values, we used generalized linear models in which the response variables were the different measures of species success and the predictors were a selection of trait mean values. For number of occurrences in GloNAF regions, and in grassland GPs and EPs, we used negative binomial error distributions (with a log-link function). As the number of occurrences in FloraWeb grid cells and Euro+Med regions did not follow negative binomial or Poisson error distributions, we instead analysed the proportion of FloraWeb grid cells and Euro+Med regions in which a species has been recorded, with a binomial error distribution. To account for overdispersion, we used the ‘quasibinomial’ setting, correcting the standard errors. For analyses of the mean local abundance (i.e. the cover proportion) of the species in the GPs and EPs, we used a gamma conditional distribution (with log-link function).
For each success measure, we used a model with nine traits as predictors. We a priori chose traits that represent different aspects of plant functioning and that had relatively low correlations between them (all r ≤ |0.51|, Appendix S3) to minimize multicollinearity (the maximum generalized variance-inflation factor of a model was 3.55). We used the following traits: individual seed weight (measured on seeds ordered for the experiments), specific root length, root tissue density and first order root diameter (measured in the root-system morphology experiment), maximum rooting depth (measured in the rooting-depth experiment), and bud-bank size, height and specific leaf area (from trait databases). Seed weight was log transformed. To facilitate interpretation of and comparison between model coefficients, each trait was scaled to a mean of zero and a standard deviation of one . To test for potential non-linear effects of traits, orthogonal polynomial terms of second degree (i.e. quadratic terms) were also included for each trait, using the poly function. To estimate the proportion of variance explained by the models, we calculated delta R² values, applicable to all distributions and link functions, according to using the package MuMIn . To assess whether belowground traits can explain more than aboveground traits, we also extracted delta R² values for models using only the three aboveground predictors and the three belowground predictors with the highest standardized coefficients overall separately. To account for phylogenetic non-independence of the study species, the models were also run using phylogenetic relatedness of species as a variance-covariance matrix (for details, see Appendix S4). Although the significances of the trait effects differed in some instances between the non-phylogenetic models and the phylogenetic ones, the directions of the effects were largely the same in both types of models (compare Fig. 1 and Appendix S8). Therefore, we only present the results of the non-phylogenetic analyses in the main text.