Alicia Montesinos-Navarro

The network structure of plant–arbuscular mycorrhizal fungi

Ecological network theory predicts that in mutualistic systems specialists tend to interact with a subset of species with which generalists interact (i.e. nestedness). Approaching plant-arbuscular mycorrhizal fungi (AMF) association using network analyses will allow the generality of this pattern to be expanded to the ubiquitous plant-AMF mutualism. Based on certain plant-AMF specificity recently suggested, networks are expected to be nested as a result of their mutualistic nature, and modular, with certain species interacting more tightly than others. Network analyses were used to test for nestedness and modularity and to compare the different contribution of plant and AMF to the overall nestedness. Plant-AMF networks share general network properties with other mutualisms. Plant species with few AMFs in their roots tend to associate with those AMFs recorded in most plant species. AMFs present in a few plant species occur in plant species sheltering most AMF (i.e. nestedness). This plant-AMF network presents weakly interlinked subsets of species, strongly connected internally (i.e. modularity). Both plants and AMF show a nested structure, although AMFs have lower nestedness than plants. The plant-AMF interaction pattern is interpreted in the context of how plant-AMF associations can be underlying mechanisms shaping plant community assemblages.

Plant facilitation occurs between species differing in their associated arbuscular mycorrhizal fungi

Complementary beneficial effects of different arbuscular mycorrhizal fungi (AMF) can result in a more efficient exploitation of the soil nutrients available, thus influencing plant communities. Here, we hypothesize that plant-AMF specificity is mediated by phylogenetic constraints defining possible interactions, and that plant-AMF interaction patterns can influence plant-plant facilitation specificity. We reanalyzed previous data describing plant-plant and plant-AMF interaction at the community level to specifically test for a phylogenetic signal on plant and AMF interactions and for a relationship between plant-plant facilitation specificity and plant species differences in their AMF associates. Closely related AMF operational taxonomical units (OTUs) tend to interact with the same plant species, but there is not a significant signal in the interaction through the plant phylogeny. This indicates that the similarity in the AMF associates of two plant species is independent of their phylogenetic relatedness. Interestingly, plant-AMF interactions match plant facilitation specificity, with pairs of plant species recruiting more frequently under each other tending to have different AMF associates. An increment of AMF diversity in the rhizosphere, as a result of plant-AMF and plant-plant selectivity, is suggested as a potential driver of plant-plant facilitation. This study highlights the role of plant-AMF interactions in shaping plant community assemblages.

Temporal variation in genetic diversity and effective population size of Mediterranean and subalpine Arabidopsis thaliana populations

Currently, there exists a limited knowledge on the extent of temporal variation in population genetic parameters of natural populations. Here, we study the extent of temporal variation in population genetics by genotyping 151 genome‐wide SNP markers polymorphic in 466 individuals collected from nine populations of the annual plant Arabidopsis thaliana during 4 years. Populations are located along an altitudinal climatic gradient from Mediterranean to subalpine environments in NE Spain, which has been shown to influence key demographic attributes and life cycle adaptations. Genetically, A. thaliana populations were more variable across space than over time. Common multilocus genotypes were detected several years in the same population, whereas low‐frequency multilocus genotypes appeared only 1 year. High‐elevation populations were genetically poorer and more variable over time than low‐elevation populations, which might be caused by a higher overall demographic instability at higher altitudes. Estimated effective population sizes were low but also showed a significant decreasing trend with increasing altitude, suggesting a deeper impact of genetic drift at high‐elevation populations. In comparison with single‐year samplings, repeated genotyping over time captured substantially higher amount of genetic variation contained in A. thaliana populations. Furthermore, repeated genotyping of populations provided novel information on the genetic properties of A. thaliana populations and allowed hypothesizing on their underlying mechanisms. Therefore, including temporal genotyping programmes into traditional population genetic studies can significantly increase our understanding of the dynamics of natural populations.

Plant phylodiversity enhances soil microbial productivity in facilitation-driven communities

The classical relationship between biodiversity and ecosystem functioning can be better understood when the phylogenetic component of biodiversity is considered. We linked plant phylodiversity and ecosystem functioning in a water-limited gypsum ecosystem driven by plant facilitation. We tested whether (1) plant facilitation relaxes the abiotic filter imposed by gypsum, allowing the establishment of non-gypsophyte plant species, and consequently increasing plant phylodiversity, and (2) plant phylodiversity influences soil microbial productivity. Our data revealed that the gypsophyte Ononis tridentata spatially determines a macrophytic mosaic, ameliorates the microenvironment, and maximizes plant richness and phylodiversity through facilitating non-gypsophyte species. Beyond the direct effect of the nurse plant on soil microbial biomass, activity, and respiration, the analyses suggest a direct effect of plant phylodiversity (MPD) on these general indicators of soil microbial productivity. Plant diversity (Shannon index) neither correlated with the mentioned parameters nor with specific indicators of C, N and P cycling. This is the first report of a relationship between producer phylodiversity and decomposer productivity, which supports phylogenetic diversity as a relevant player of the ecosystem functioning.

Modularity Reveals the Tendency of Arbuscular Mycorrhizal Fungi To Interact Differently with Generalist and Specialist Plant Species in Gypsum Soils

ABSTRACT Patterns in plant–soil biota interactions could be influenced by the spatial distribution of species due to soil conditions or by the functional traits of species. Gypsum environments usually constitute a mosaic of heterogeneous soils where gypsum and nongypsum soils are imbricated at a local scale. A case study of the interactions of plants with arbuscular mycorrhizal fungi (AMF) in gypsum environments can be illustrative of patterns in biotic interactions. We hypothesized that (i) soil characteristics might affect the AMF community and (ii) there are differences between the AMF communities (modules) associated with plants exclusive to gypsum soils (gypsophytes) and those associated with plants that show facultative behavior on gypsum and/or marly-limestone soils (gypsovags). We used indicator species and network analyses to test for differences between the AMF communities harbored in gypsophyte and gypsovag plants. We recorded 46 operational taxonomic units (OTUs) belonging to nine genera of Glomeromycota. The indicator species analysis showed two OTUs preferentially associating with gypsum soils and three OTUs preferentially associating with marly-limestone soils. Modularity analysis revealed that soil type can be a major factor shaping AMF communities, and some AMF groups showed a tendency to interact differently with plants that had distinct ecological strategies (gypsophytes and gypsovags). Characterization of ecological networks can be a valuable tool for ascertaining the potential influence of above- and below-ground biotic interactions (plant-AMF) on plant community composition.

Soil Characteristics Driving Arbuscular Mycorrhizal Fungal Communities in Semiarid Mediterranean Soils

ABSTRACT We investigated communities of arbuscular mycorrhizal fungi (AMF) in the roots and the rhizosphere soil of Brachypodium retusum in six different natural soils under field conditions. We explored phylogenetic patterns of AMF composition using indicator species analyses to find AMF associated with a given habitat (root versus rhizosphere) or soil type. We tested whether the AMF characteristics of different habitats or contrasting soils were more closely related than expected by chance. Then we used principal-component analysis and multivariate analysis of variance to test for the relative contribution of each factor in explaining the variation in fungal community composition. Finally, we used redundancy analysis to identify the soil properties that significantly explained the differences in AMF communities across soil types. The results pointed out a tendency of AMF communities in roots to be closely related and different from those in the rhizosphere soil. The indicator species analyses revealed AMF associated with rhizosphere soil and the root habitat. Soil type also determined the distribution of AMF communities in soils, and this effect could not be attributed to a single soil characteristic, as at least three soil properties related to microbial activity, i.e., pH and levels of two micronutrients (Mn and Zn), played significant roles in triggering AMF populations. IMPORTANCE Communities of arbuscular mycorrhizal fungi (AMF) are main components of soil biota that can determine the productivity of ecosystems. These fungal assemblages vary across host plants and ecosystems, but the main ecological processes that shape the structures of these communities are still largely unknown. A field study in six different soil types from semiarid areas revealed that AMF communities are significantly influenced by habitat (soil versus roots) and soil type. In addition, three soil properties related to microbiological activity (i.e., pH and manganese and zinc levels) were the main factors triggering the distribution of AMF. These results contribute to a better understanding of the ecological factors that can shape AMF communities, an important soil microbial group that affects multiple ecosystem functions.

Nurse plants transfer more nitrogen to distantly related species

Plant facilitative interactions enhance co-occurrence between distant relatives, partly due to limited overlap in resource requirements. We propose a different mechanism for the coexistence of distant relatives based on positive interactions of nutrient sharing. Nutrients move between plants following source-sink gradients driven by plant traits that allow these gradients to establish. Specifically, nitrogen (N) concentration gradients can arise from variation in leaf N content across plants species. As many ecologically relevant traits, we hypothesize that leaf N content is phylogenetically conserved and can result in N gradients promoting N transfer among distant relatives. In a Mexican desert community governed by facilitation, we labelled nurse plants (Mimosa luisana) with 15 N and measured its transfer to 14 other species in the community, spanning the range of phylogenetic distances to the nurse plant. Nurses established steeper N source-sink gradients with distant relatives, increasing 15 N transfer toward these species. Nutrient sharing may provide long-term benefits to facilitated plants and may be an overlooked mechanism maintaining coexistence and increasing the phylogenetic diversity of plant communities.

Fungal phylogenetic diversity drives plant facilitation

Plant–plant facilitation is a crucial ecological process, as many plant species (facilitated) require the presence of an established individual (nurse) to recruit. Some plant facilitative interactions disappear during the ontogenetic development of the facilitated plant but others persist, even when the two plants are adults. We test whether the persistence of plant facilitative interactions is explained by the phylogenetic diversity of mutualistic and non-mutualistic fungi that the nurse and the facilitated species add to the shared rhizosphere. We classify plant facilitative interactions as persistent and non-persistent interactions and quantify the phylogenetic diversity of mutualistic and non-mutualistic fungi added by the plant species to the shared rhizosphere. Our results show that the facilitated species add less phylogenetic diversity of non-mutualistic fungi when plant facilitative interactions persist than when they do not persist. However, persistent and non-persistent facilitative interactions did not differ in the phylogenetic diversity of mutualistic fungi added by the facilitated species to the shared rhizosphere. Finally, the fungal phylogenetic diversity added by the nurse to the shared rhizosphere did not differ between persistent and non-persistent interactions. This study suggests that considering the fungal associates of the plant species involved in facilitative interactions can shed light on the mechanisms of persistence for plant–plant interactions.

Community structure informs species geographic distributions

Understanding what determines species’ geographic distributions is crucial for assessing global change threats to biodiversity. Measuring limits on distributions is usually, and necessarily, done with data at large geographic extents and coarse spatial resolution. However, survival of individuals is determined by processes that happen at small spatial scales. The relative abundance of coexisting species (i.e. ‘community structure’) reflects assembly processes occurring at small scales, and are often available for relatively extensive areas, so could be useful for explaining species distributions. We demonstrate that Bayesian Network Inference (BNI) can overcome several challenges to including community structure into studies of species distributions, despite having been little used to date. We hypothesized that the relative abundance of coexisting species can improve predictions of species distributions. In 1570 assemblages of 68 Mediterranean woody plant species we used BNI to incorporate community structure into Species Distribution Models (SDMs), alongside environmental information. Information on species associations improved SDM predictions of community structure and species distributions moderately, though for some habitat specialists the deviance explained increased by up to 15%. We demonstrate that most species associations (95%) were positive and occurred between species with ecologically similar traits. This suggests that SDM improvement could be because species co-occurrences are a proxy for local ecological processes. Our study shows that Bayesian Networks, when interpreted carefully, can be used to include local conditions into measurements of species’ large-scale distributions, and this information can improve the predictions of species distributions.

Correction: Community structure informs species geographic distributions

[This corrects the article DOI: 10.1371/journal.pone.0197877.].