Host specificity of fungal pathogens covaries with climate: Implications for intraspecific plant interactions

Plants of the same species harbor shared enemies that can limit population growth and promote plant species coexistence, a phenomenon known as conspecific negative density dependence (CNDD). Host-specific microbial pathogens are known drivers of CNDD, yet how these plant-microbe interactions vary with environmental context is less explored. Further, microbial pathogens and mutualists may jointly contribute to the relative strength of CNDD, yet most studies focus only on pathogens. We used ITS metabarcoding to characterize soil and foliar fungal functional groups for individual trees of three dominant conifers in forests where the strength of CNDD has been shown to be greater at lower elevations relative to higher elevations. We hypothesized that trees within mesic low elevation forests accumulate more host-specific fungal pathogens compared to trees found in the more xeric high elevation forests. Conversely, we hypothesized that trees found in the high elevation forests accumulate more host-specific mutualists compared to those found at low elevations. We tested these hypotheses with the fungal community sequencing data by evaluating three independent metrics of plant-associated fungal functional groups-alpha diversity, community weighted mean (CWM), and host specificity-that together address the degree to which each functional group is associated with each focal tree. We found soil pathogen alpha diversity, CWM, and host specificity to all decline in climates associated with increasing elevation. We also found foliar fungal pathogen host specificity, but not diversity or CWM, to decline in climates associated with increasing elevation. In contrast, we found no significant relationships between any of our three metrics for ectomycorrhizal fungi and climate. Our results suggest that across a gradient known to negatively covary with CNDD strength, pathogen accumulation is most prevalent in the mesic, lower elevation climates. These results are consistent with pathogenic fungi contributing to observed landscape level variation in CNDD.