RESULTS
All of our species success metrics were significantly related to at
least four of the nine traits considered, including both above- and
belowground traits (Fig. 1). The abundance measures in the grassland GPs
and the occurrence in the EPs were associated with the largest number of
traits (Fig. 1). The delta R2 values ranged from 0.02
for the model on occurrence frequency in the native range (i.e. the
proportion of occurrences in Euro+Med) to 0.41 for the model on
occurrence frequency outside their native range (i.e. the number of
occurrences in GloNAF; Fig. 1; Table 1). When we reduced the models to
either include only the three aboveground traits or the three best
belowground traits, the variation explained by the belowground-trait
models was equal or higher than the variation explained by the
aboveground-trait models for all species success metrics, except
frequency of occurrence in the grassland EPs (Table 1).
Root-tissue density was a significant predictor in all models (Fig. 1).
Species with low root-tissue densities were consistently more successful
than species with high root-tissue densities across all spatial scales
considered (i.e. all linear coefficients were negative and significant;
Fig. 1). For occurrence frequency outside the native range, the
coefficient of the quadratic term was also significantly negative (Fig.
1), indicating that this success metric was highest for species with
intermediate root-tissue densities (Fig. 1, Appendix S9).
Specific leaf area (SLA) was a significant predictor in most models, but
the direction of the effect on species success changed with the spatial
scale considered (Fig. 1). Occurrence frequencies outside their native
range, in their native range, in Germany, and in the grassland GPs and
EPs increased with SLA, and sometimes slightly decreased again at higher
SLA values. However, the abundance in the GPs decreased with SLA and the
abundance in the EPs was highest at intermediate SLA values, although
these effects were only marginally significant (Fig. 1, Appendix S9).
Bud-bank size was also a significant predictor in most models, but its
effects on species success varied with spatial scale (Fig. 1). Bud-bank
size was negatively associated with occurrence frequency of species
outside their native range, whereas occurrence frequencies in their
native range and in Germany were lowest at intermediate bud-bank sizes
(Fig. 1). However, occurrence frequencies as well as abundances in the
grassland GPs and EPs increased linearly or asymptotically with bud-bank
size (Fig. 1, Appendix S9).
The other traits were only significant predictors in some of the models
of species success (Fig. 1). Seed weight had no significant associations
with success at the larger spatial scales, but occurrence in Germany was
higher for species with lower seed weights. Occurrence frequency and
abundances in the GPs and EPs, on the other hand, were highest at higher
seed weights (Fig. 1, Appendix S9). Maximum rooting depth was
non-linearly positively associated with occurrence frequency outside the
native range, and was positively associated with occurrence frequencies
in the native range and Germany, but not in the EPs and GPs (Fig. 1,
Appendix S9). Root-weight ratio was marginally negatively associated
with occurrence frequency outside the native range, with an optimum in
the mid-lower range of the trait, which was also the case for occurrence
frequency in GPs and abundance in EPs (Fig. 1, Appendix S9). First-order
root diameter had no significant effects on any of the occurrence
frequencies, but had significant negative effects on abundance in the
GPs and EPs (Fig. 1, Appendix S9). Specific root length only had a weak
marginally significant effect for occurrence frequency in Germany (Fig.
1, Appendix S9).