Changes of abiotic and biotic factors along elevational gradients
In our field survey, abiotic and biotic factors (soil properties, plant community characters and environment conditions) greatly varied along elevational gradients, consistent with past studies (e.g. Lomolino, 2001), and those abiotic and biotic factors, in turn, explained relationships between elevation and above- belowground plant pathogens. By integrating this field survey with a systematic meta-analysis, we showed a general, negative association between elevation and soil pathogen richness, but did not find support for a general association with foliar fungal pathogen richness, foliar fungal diseases or soil pathogen relative abundance. Our meta-analysis further suggests that the elevation range of sampling may potentially shape which relationships are observed between elevation and soil fungal pathogens. Larger elevation gradients mean that studies are comparing among different habitat types with different sets of species, whereas smaller elevation gradients are usually making comparisons across environmental conditions within a single habitat type. These results provided important evidence that elevational patterns of plant pathogens are consistent despite great variation in plant and pathogen species pools.
In our field survey, we did not find any evidence supporting a direct association between elevation and foliar fungal diseases at either the host population or community levels. These results are inconsistent with a previous study in the Swiss Alps along a 1101 m elevational gradient, which found that elevation not only associated with disease directly, but also indirectly regulated plant diseases by shifting the relationship between host plant composition and disease (Halliday et al., 2021). The differences between these two studies might be due to variation in climate (higher temperature and precipitation in the Swiss Alps), vegetation type (our site was a typical alpine meadow, while the Swiss Alps had more montane habitat), and/or elevation range of sampling (1101 m for Swiss Alps vs. 800 m here). Elevation is also associated with a series of changes in plant communities, soil properties and other environmental factors (Lomolino, 2001). These factors may have both positive (e.g. warming; Siebold and Tiedemann, 2013; Liu et al., 2019) and negative (e.g., dilution effect; Mitchell et al., 2002; Rottstock et al., 2014; Halliday et al., 2020) effects on foliar fungal diseases. These effects could also offset each other, and environmental gradients can further modify how host community structure affects disease (Halliday et al., 2021), resulting in imperceptible associations between elevation with foliar fungal diseases. Hence, the relationship between elevation and foliar fungal diseases was difficult to explain using temperature, precipitation, latitude and elevation range of sampling in our meta-analysis.