Results
The common garden experiment showed that the soil microbiome modulated
fitness-related variables, particularly aboveground biomass of T.
officinale plants (Suppl. Table 2). Nevertheless, this effect was more
pronounced in T. officinale plants growing in non-native South
American soils, where individuals from both origins exhibited a
significant reduction in biomass when the soil microbiome was removed
(M+ vs. M−). Specifically, plants
from the native range (Europe) that grew in South American
M− soil accumulated less biomass and produced fewer
flower heads than their counterparts growing in the untreated soil
(M+). Plants from the introduced range (South America)
that grew in M− South American soil not only
accumulated less biomass and produced fewer flowers but also exhibited
reduced survival (Fig. 1). Conversely, when T. officinale plants
were grown on native soils, only the biomass of the South American
plants appeared to be affected by the soil microbial removal; in all
other cases, the loss of the soil microbiome did not result in
significant performance responses, either for European or South American
plants (Fig. 1). In all cases in which soil microbiome removal affected
the performance of T. officinale , the reinoculation treatment
successfully restored the previously lost performance in the plants
(Mr vs. M+; Fig. 1). There was also
a subtle but significant genotype effect (i.e. , associated with
origin) on the aboveground biomass of T. officinale . Irrespective
of the soil treatment, the South American plants presented on average,
greater biomass than their European counterparts (Fig. 1).
The soil microbiome influenced the performance of T. officinaleplants along the latitudinal gradient in their invasive range (Fig. 2).
While plants grown in control (M+) and reinoculated
(Mr) soils showed similar responses in terms of
performance variables along the latitudinal gradient, the performance of
plants in sterilized soil (M−) deviated from the
referred pattern (Fig. 2a). Across the entire latitudinal gradient,
survival of plants was high in M+ and
Mr treatments but low in the M−treatment. Additionally, only under this latter treatment, survival
rates were lower at the extremes of the gradient compared to the center
(Suppl. Table 3). Overall, there was a positive association between
plant aboveground biomass and latitude, increasing from south to north
(Fig. 2); nonetheless, the biomass of plants in the northern extreme was
lower when grown M− soil (Fig. 2b). The number of
flowers per plant was higher in the middle of the latitudinal gradient
and lower at both extreme south and north (Fig. 2c). However, despite
this overall pattern, the removal of the soil microbiome was associated
with a significant reduction in the number of flowers per plant across
the entire south-north distributional range (Fig. 2c). The same pattern
of responses to variation in latitude and soil microbiome treatment was
observed in integrated index of plant performance (Fig. 2d).