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.