(iii) Plant community-, environment- and soil-mediated relationships between elevation and above- and belowground plant pathogens
To explore how soil properties, plant community and environment conditions mediated the relationship between elevation and above- and belowground plant pathogens, we used the “piecewiseSEM” package (v. 2.1.2; Lefcheck, 2016) to build a structural equation model (SEM) based on a series of linear mixed- effects models to test the relationship between elevation and PL and sfpOTUs including paths that were mediated by the plant community and environment (i.e. elevation may first influence plant community characters, soil properties and environmental conditions, which then indirectly change plant fungal pathogen communities) (Halliday et al., 2021; Fig. 2a). We did not include MDT in the SEM, given that there was a strong collinearity between MDT and elevation (Pearson’ r= 0.985). We calculated the standardized path coefficients (scaled by their mean and standard deviation) and corresponding significance (P values) for each path of the final models. Statistically significant path(s) (P < 0.1) highlight potentially important mechanisms generating relationships between elevation and above- and belowground plant pathogens. We used the Fisher’s C test to test the goodness-of-fit of the SEM. We then used partial residual plots to interpret the relationships highlighted in the model (Grace, 2006). In brief, we extracted the residuals of each variable in SEM and then set the residuals of plant pathogen related variables (i.e. PLand sfpOTUs ) as functions of residuals of explanatory variable (i.e. Elevation , Soil PCA1 , Evenness andProneness ) in a series of linear models (Grace, 2006). Significant results of residual analysis correspond to significant paths in the SEM.