Warming and rainfall reduction alter soil microbial diversity and co-occurrence networks and enhance pathogenic fungi in dryland soils.

In this 9-year manipulative field experiment, we examined the impacts of experimental warming (2 °C, W), rainfall reduction (30 % decrease in annual rainfall, RR), and their combination (W + RR) on soil microbial communities and native vegetation in a semi-arid shrubland in south-eastern Spain. Warming had strong negative effects on plant performance across five coexisting native shrub species, consistently reducing their aboveground biomass growth and long-term survival. The impacts of rainfall reduction on plant growth and survival were species-specific and more variable. Warming strongly altered the soil microbial community alpha-diversity and changed the co-occurrence network structure. The relative abundance of symbiotic arbuscular mycorrhizal fungi (AMF) increased under W and W + RR, which could help buffer the direct negative impacts of climate change on their host plants nutrition and enhance their resistance to heat and drought stress. Indicator microbial taxa analyses evidenced that the marked sequence abundance of many plant pathogenic fungi, such as Phaeoacremonium, Cyberlindnera, Acremonium, Occultifur, Neodevriesia and Stagonosporopsis, increased significantly in the W and W + RR treatments. Moreover, the relative abundance of fungal animal pathogens and mycoparasites in soil also increased significantly under climate warming. Our findings indicate that warmer and drier conditions sustained over several years can alter the soil microbial community structure, composition, and network topology. The projected warmer and drier climate favours pathogenic fungi, which could offset the benefits of increased AMF abundance under warming and further aggravate the severe detrimental impacts of increased abiotic stress on native vegetation performance and ecosystem services in drylands.

Influence of habitat and climate variables on arbuscular mycorrhizal fungus community distribution, as revealed by a case study of facultative plant epiphytism under semiarid conditions.

In semiarid Mediterranean ecosystems, epiphytic plant species are practically absent, and only some species of palm trees can support epiphytes growing in their lower crown area, such as Phoenix dactylifera L. (date palm). In this study, we focused on Sonchus tenerrimus L. plants growing as facultative epiphytes in P. dactylifera and its terrestrial forms growing in adjacent soils. Our aim was to determine the possible presence of arbuscular mycorrhizal fungi (AMF) in these peculiar habitats and to relate AMF communities with climatic variations. We investigated the AMF community composition of epiphytic and terrestrial S. tenerrimus plants along a temperature and precipitation gradient across 12 localities. Epiphytic roots were colonized by AMF, as determined by microscopic observation; all of the epiphytic and terrestrial samples analyzed showed AMF sequences from taxa belonging to the phylum Glomeromycota, which were grouped in 30 AMF operational taxonomic units. The AMF community composition was clearly different between epiphytic and terrestrial root samples, and this could be attributable to dispersal constraints and/or the contrasting environmental and ecophysiological conditions prevailing in each habitat. Across sites, the richness and diversity of terrestrial AMF communities was positively correlated with rainfall amount during the most recent growing season. In contrast, there was no significant correlation between climate variables and AMF richness and diversity for epiphytic AMF communities, which suggests that the composition of AMF communities in epiphytic habitats appears to be largely determined by the availability and dispersion of fungal propagules from adjacent terrestrial habitats.

Differential modulation of host plant delta13C and delta18O by native and nonnative arbuscular mycorrhizal fungi in a semiarid environment.

Native, drought-adapted arbuscular mycorrhizal fungi (AMF) often improve host-plant performance to a greater extent than nonnative AMF in dry environments. However, little is known about the physiological basis for this differential plant response. Seedlings of Olea europaea and Rhamnus lycioides were inoculated with either a mixture of eight native Glomus species or with the nonnative Glomus claroideum before field transplanting in a semiarid area. Inoculation with native AMF produced the greatest improvement in nutrient and water status as well as in long-term growth for both Olea and Rhamnus. Foliar delta18O measurements indicated that native AMF enhanced stomatal conductance to a greater extent than nonnative AMF in Olea and Rhamnus.delta13C data showed that intrinsic water-use efficiency in Olea was differentially stimulated by native AMF compared with nonnative AMF. Our results suggest that modulation of leaf gas exchange by native, drought-adapted AMF is critical to the long-term performance of host plants in semiarid environments. delta18O can provide a time-integrated measure of the effect of mycorrhizal infection on host-plant water relations.

Ecology of mycorrhizae: a conceptual framework for complex interactions among plants and fungi.

Mycorrhizae regulate elemental and energy flows in terrestrial ecosystems. We understand much of how mycorrhizae work, but integrating all possible mechanisms into a whole has proven elusive. Multiple evolutionary events and the long evolutionary history mean that different plants and fungi bring independent characteristics to the symbiosis. This variety results in extensive physiological variation. How do we integrate functional responses with diversity to understand the role of mycorrhizae in ecosystems? We review ecophysiological mechanisms of mycorrhizae and organize these into functional groups. Species-area relationships are not curvilinear, but resemble the "broken stick" model. We coupled functional groups with a metacommunity analysis to show how complex behavior can be generated using a simple matrix model of resource exchange. This approach provides insights into how we might integrate diversity and function across landscapes.