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).