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.