Discussion
On average, roadsides harbored an increased share of AM plant species in
the total vegetation cover and a decreased share of EcM and ErM
vegetation (Fig. 2). This mostly followed our initial expectations as
road conditions and management are known to be detrimental to the more
perennial and often tall and/or woody vegetations types that commonly
associate with EcM and ErM fungi, while more ruderal and opportunist
plant species are most often AM and tend to be more successful in these
environments (Müllerová et al. 2011; Smith and Read 2010). These
differences were especially true in colder environments, i.e. at high
elevations and high latitudes, but were less pronounced or even reversed
at the upper end of the temperature gradient where roadside vegetation
showed lower AM vegetation cover than in the adjacent vegetation.
However, despite these intra-regional fluctuations, we did observe that
for almost all regions the effect of disturbance on the regional scale
followed the global trend of increased AM vegetation along roadsides
coupled with decreased ErM and EcM vegetation (when present; Table 2).
Only Argentina was an exception as we found no correlation between road
disturbance and changes in mycorrhizal type, which was not the case in
other regions where the vegetation entirely or almost entirely AM
associated or NM. These findings suggests that the impact of mountain
road disturbance on mycorrhizal type distribution follows a systematic
pattern that can be influenced, but is mostly not overruled by, local
factors. As we observed in a recent study in the northern Scandes
(Clavel et al. 2020), this change in AM vegetation cover is likely also
reflected in the prevalence of the AM fungi themselves, as disturbance
in these cold-climate mountains strongly promoted both the colonization
of roots by arbuscular mycorrhizas as well as overall AM fungal
diversity.
While largely constant in its direction, the strength of the roadside
disturbance effect on mycorrhizal types varied between regions (Table 2,
Fig. 2, Fig. 3) as it was lowest, but still present, in regions already
dominated by AM and NM plants, and in warmer regions. This was most
obvious in the three regions where EcM and ErM vegetation was entirely
absent (Argentina, Chile and India) as could be expected since there is
less margin for increases in AM vegetation cover when they already
dominate the regional vegetation. Conversely, the regions with large
proportions of EcM- and ErM vegetation cover, such as Norway or
Tenerife, exhibited the largest differences in mycorrhizal associations
between the vegetation of the roadside and of the adjacent plots (Fig.
3). In a similar fashion the effect of road disturbance on mycorrhizal
type distribution was on average strongest in plots with lower average
soil temperature, which is to be expected as EcM and ErM plant species
are more common in colder conditions (Brundrett and Tedersoo 2018).
However, this pattern was less clear at a regional level with examples
of warmer regions such as Tenerife and colder regions such as China not
following the global pattern (Fig. 2). Despite these region-specific
patterns, disturbance was overall the strongest predictor of mycorrhizal
proportion, above elevation and temperature, and this for all
mycorrhizal types. This confirms, at the global level, findings from
previous regional studies showing that local anthropogenic disturbance
predicts plant species composition better than macro-environmental
drivers (Fuentes-lillo et al. 2021; Lembrechts et al. 2016). The
distribution patterns of the different mycorrhizal types that we
observed across our sites was overall consistent with those previously
reported in the scientific literature across climate and elevation
gradients (Barcelo et al. 2019; Soudzilovskaia et al. 2017).
The aforementioned relationship between disturbance and the different
mycorrhizal types seems to suggest that ErM and EcM plant species are
more sensitive to the effects of road disturbance when compared to AM
plant species, thus resulting in increased success for AM plant species
along roadsides in environments where they would usually be competing
with EcM and ErM plant species (Segre et al. 2016). This would also help
explain the discrepancies between our results of increased AM vegetation
cover in disturbed roadside conditions and the results of previous
studies on disturbance in agricultural settings, where physical
disturbances in the form of tillage led to reduced colonization by AM
fungi (van der Heyde et al. 2017; Schnoor et al. 2011). This disparity
would then be explained by the effect of altered biotic interactions
(through a proportionally higher disruption of EcM and ErM plants)
outweighing the negative effects of physical disturbance in a setting
that is not already entirely AM-dominated. Supporting this hypothesis is
the fact that the proportion of AM vegetation cover in the roadside
plots tended to be comparatively lower when the proportions of AM
vegetation cover in the adjacent vegetation was high. This pattern also
aligned with the relationship between soil temperature and road
disturbance, i.e., the fact that the difference between mycorrhizal
proportions in the roadside and in the adjacent plots grew smaller
following the regional temperature gradients, and higher following the
regional elevational gradients. In both cases, when reaching the upper
limits of the temperature gradients and at the highest saturation of AM
natural vegetation, the effect of disturbance tended towards reversing,
with lower proportions of AM vegetation cover along roadsides than in
the adjacent vegetation (Fig.2, roadside plot line in red crossing over
the adjacent vegetation line in green; Fig. 3, points below the
1/1-line, blue line falling below 1/1-line). These congruent patterns
are to be expected as EcM and ErM vegetation tends to be most common in
cooler conditions and at higher elevations (Barcelo et al. 2019; Bueno
et al. 2021; Smith and Read 2010). Understanding these interactions and
the general patterns that result from them is a crucial first step in
disentangling the nexus between disturbance and changes in mycorrhizal
type distribution.
While we cannot come to a definitive conclusion as to the mechanisms
driving the aforementioned increase in AM plants to the detriment of EcM
and ErM plants due to the strictly observational nature of our study, a
possible hypothesis would be that roadside disturbance leads to
increased soil nutrient mineralization which AM-fungi are more apt at
taking advantage of (Read et al. 2004; Sulman et al. 2017). Indeed, it
is no coincidence that most ruderal plant species are associated with
AM-fungi. The fact that these ruderal AM plant species (as well as NM
species) are generally faster growing and more disturbance tolerant due
to their ability to take advantage of vegetation gaps (García de León et
al. 2016), could begin to explain the patterns we observed: increased
nutrient availability, less extreme pH and reduced competition in
roadsides all act in favor of AM plant species.
Importantly, we also observed a clear difference in mycorrhizal
association type between native and non-native plants species: almost
all non-native plant species were NM or AM, with 0.2% of EcM non-native
plant species being the exception. This matches the results of previous
studies in the U.S. that found non-native plant species to be mainly AM
or NM (Pringle et al. 2009). Although as we saw earlier that EcM/ErM
dominated vegetation saw a greater increase in AM plant abundance upon
disturbance, increases in non-native vegetation were more pronounced in
AM-dominated communities. Additionally, non-native species had greater
potential for establishing in the native community when that native
community was already dominated by AM-associated native plant species
(Fig. 4, Fig. S2). This suggests that plant communities that
predominantly associate with the same mycorrhizal fungi as the
non-native plant species are also more susceptible to potential
invasions by these non-native plant species (Yang et al. 2013), either
directly through interactions between mycorrhizal fungi, or indirectly
through the selection for other associated plant traits, such as
ruderality. The importance of anthropogenic disturbances in facilitating
the success of non-native plant species outside of their natural range
is well documented (Jauni, Gripenberg, and Ramula 2015; Lembrechts et
al. 2016). However, the mechanisms linking these disturbances to
increased non-native success remain poorly understood. We suggest that
anthropogenic disturbances being favorable to the most common types of
mycorrhizal association amongst non-native species could partly help
explain the pattern of increased non-native species success in
roadside-adjacent environments, especially in communities already
dominated by AM or NM-associated species.
However, it remains important to keep the limitations of our study in
mind when considering potential mechanisms that drive changes in plant
species composition and non-native success. Indeed, our observations
remain based on a proxy of mycorrhizal abundance shown previously to
have limitations (Brundrett and Tedersoo 2019): aggregated datasets such
as FungalRoot are bound to accumulate small errors and imprecisions and
should be used with caution when looking at distribution patterns. This
is partly why we limited our analyses to broad categories of mycorrhizal
associations. Nevertheless, using large collections of datasets remains
valuable as long as direct mycorrhizal measurements are not sufficiently
available. Our results should thus be seen as a first investigation of
global relationships between anthropogenic disturbances, mycorrhizal
distribution and their potential impacts on non-native plant invasions,
and the resulting observations as potential openings for further studies
investigating how human activities can impact mycorrhizal communities
and plant-fungal interactions.