Plant richness enhances plant-fungi network
Our analysis showed a striking pattern in the numbers of fungal OTUs statistically associated with each plant species (Fig 2a), and revealed that in relatively plant species-poor plant samples, fewer fungal OTUs were specifically associated with each plant species. This cannot be explained simply by more fungal OTUs being present in the samples with high plant species richness: from the total ‘pool’ of fungal OTUs detected in the soil is very large in all samples, there are more fungal OTUs present - relative to each plant species - in the low plant species richness samples (Fig 2b).
It is unclear exactly what forms these specific plant-fungi interactions take, except that most are positive (81.39%) rather than negative (18.61%). Positive interactions could be mutualisms (Seidl et al. 2009), benefitting both sides, or specialised commensalisms benefitting only one side. Specialised pathogen-host associations may also be expected to show up as positive, unless they have a strongly depleting effect on host plant abundance. Negative interactions may occur across trophic levels involving plant defense mechanisms, or antagonistic relations with other fungal species that are specialised on the same plant host(Chen et al. 2018) .
Compared to the whole soil fungal community, the plant-associated fungi had a distinct profile of taxonomic identities and functional guilds (Fig S7a). The FUNGUILD classification reveals that around half of the OTUs in the fungi-plant networks are of unknown ecology, with putative saprophytes (22.67%), ectomycorrhizal fungi (4.04%) and plant pathogens (2.45%) dominating amongst those that could be assigned by FUNGUILD. In terms of the functional composition of the whole soil fungal communities, saprotrophs (30.5%) and plant pathogens (13.6%) were the dominant functional guilds. In alpine habitats, ectomycorrhizal (ECM) fungi are a significant functional guild, forming mutualisms with some small lignified shrubs and herbaceous perennials, and playing role in carbon and nutrient cycling.
The striking decrease in numbers of fungal OTUs specifically associated with each plant species, along the declining plant species richness gradient, suggests that interactions (both negative and positive) between plants and the rhizosphere biota are less intense in the more physiologically stressful environments associated with low plant richness. Plant species richness was strongly associated with a gradient in mean annual precipitation and plant productivity according to our previous study on this area(Yanget al. 2017), so plant richness in fact could be seen as a general proxy for physiological stress on both plants and soil biota.
The trend in linked fungal OTUs per plant species thus suggests that under more stressful, low plant diversity conditions, narrowly specific interkingdom interactions are rarer, and if anything are replaced by more generalised interactions that do not tend to produce detectable OTU-plant species links. This may be because the maintenance energy requirements for survival – on both plants and fungi – are greater, with less excess carbon and other resources available for engaging in specific interactions. For instance, an unpredictable flow of photosynthate from frequent drought conditions may also preclude niche specialisation by fungi on an unstable resource. The concept that physiologically extreme conditions preclude large numbers of specialised interactions has long been discussed, for example in explaining gradients in insect and plant diversity (Richards et al. 2015).