Influence of the climate gradient
The aforementioned effect of disturbance on mycorrhizal type
distribution was strongest in plots with low mean annual soil
temperature as shown in Figure 2 and as indicated by a positive
interaction between disturbance and temperature (Table 1). While the
percentage of AM vegetation cover was on average higher in roadsides,
this pattern tended to be reversed at the upper ranges of the regional
temperature gradients where the percentage of AM vegetation cover in
high temperature plots was lower in the roadside when compared to the
adjacent vegetation, with NM vegetation instead being higher in these
roadside plots. This effect was less clear at the inter-regional level:
while the effect on disturbance was mostly higher in cold regions
compared to warmer regions, both Australia and especially Tenerife where
outliers with comparatively high average temperatures as well as a
strong effect of road disturbance on the proportion of mycorrhizal
association types observed in the vegetation (Fig. 2, Table 3).
Overall, disturbance had larger and more consistent effects on the
proportion of mycorrhizal association types than elevation or
temperature. Using variation partitioning, we found that disturbance
explained 9.8% of the total variation in AM vegetation cover and mean
annual soil temperature 4.0%. For EcM, these numbers were 2.8% for
disturbance and 2.2% for temperature, for ErM these were 21.6% against
7.9% respectively, and 1.0% against 0.6% for NM. Again, we found
similar results when replacing mean annual soil temperature with
elevation as the environmental explanatory variable: 8.9% for
disturbance and 1.2% for elevation in AM vegetation and respectively
2.7% and 1.9% for EcM, 15.3% and 1.3% for ErM, 3.0% and 1.9% for
NM. The direction of the temperature effect on the proportion of
mycorrhizal association types also greatly varied across regions for all
mycorrhizal types except ErM, while the disturbance effect was
consistent in its direction across all regions for all four mycorrhizal
types (Table 2).