Response patterns of arbuscular mycorrhizal fungal communities to shrub encroachment in subalpine grasslands

Shrub encroachment strongly alters grassland structure and function, yet its effects on arbuscular mycorrhizal fungi (AMF), remain poorly understood. We examined how different shrub species influence herbaceous root AMF communities and mycorrhizal colonization in two subalpine grasslands dominated by two herbaceous species, Patzkea paniculata and Bromus erectus . Across a shrub cover gradient, we characterized AMF via high-throughput sequencing of the partial large subunit rRNA gene and quantified root mycorrhizal colonization. In P. paniculata or B. erectus grasslands, AMF community responses were shrub-specific, with communities forming two distinct groups associated with each encroaching shrub species. Across the shrub cover gradient, AMF community turnover consistently shifted with increasing shrub cover, although the magnitude and direction of responses depended on shrub and grassland identity. These community shifts were associated with variation in soil carbon and in one context, soil pH. AMF species richness also responded to shrub cover, but patterns varied among grassland-shrub combinations. At the operational taxonomic unit (OTU) level, shrub encroachment induced contrasting changes in the abundance and distribution of AMF taxa, while shifts in higher taxonomic levels were more limited. In contrast, mycorrhizal colonization remained stable across shrub-cover gradients, with no detectable relationships with soil properties. Our results show that shrub encroachment alters the composition of herbaceous root-associated AMF communities rather than the extent of root colonization, highlighting a decoupling between AMF community structure and mycorrhizal colonization and the key role of AMF community in mediating belowground responses and potential impacts on grassland functioning.