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).