Statistical analysis
To assess if the soil microbiome from native (Europe) and introduced (South America) sites exerts a differential effect on fitness-related traits (survival, biomass, and number of flowers) of T. officinale seedlings originated in either native and introduced ranges (i.e. , France and Chile), we compared their survival, biomass and produced flowers after grown on treated soils (M+, M- or Mr). For this, we run an aligned rank transformation ANOVA, as implemented in the “art” function from the ARTool R-package (Kay et al. 2021), on the fitted linear mixed-model (LMM) relating plant performance (survival, biomass, or number of flowers) with the three experimental factors (soil origin, seed origin and treatment). This experiment consisted of a block design, each including individuals from three different populations by country, hence we included the block and the population in the model as a nested random factor. For multiple a-posteriori comparisons the same package also implements the function “art.con”, which uses the Holm method for p -value corrections. In addition, to test whether the soil microbiome maintain (or not) a consistent effect on the performance of T. officinale among different populations from the different positions in the latitudinal gradient, we also followed a LMM framework to determine the linear and quadratic relations between each population’s latitude and the mean survival, biomass, and number of flowers denoted experimentally by their individuals, as well as for their integrated “performance index”. These models were implemented using the “lme” function in the nlme R-package (Pinheiro et al. 2023) including the population as a random factor in the error model structure. Normality assumptions were verified for each fitted model residuals in LMMs using the Shapiro-Wilk test (R Core Team 2023).