Summary The aboveground impacts of climate change receive extensive research attention, but climate change could also alter belowground processes such as the delicate balance between free‐living fungal decomposers and nutrient‐scavenging mycorrhizal fungi that can inhibit decomposition through a mechanism called the Gadgil effect. We investigated how climate change‐induced reductions in plant survival, photosynthesis and productivity alter soil fungal community composition in a mixed arbuscular/ectomycorrhizal (AM/EM) semiarid shrubland exposed to experimental warming (W) and/or rainfall reduction (RR). We hypothesised that increased EM host plant mortality under a warmer and drier climate might decrease ectomycorrhizal fungal (EMF) abundance, thereby favouring the proliferation and activity of fungal saprotrophs. The relative abundance of EMF sequences decreased by 57.5% under W+RR, which was accompanied by reductions in the activity of hydrolytic enzymes involved in the acquisition of organic‐bound nutrients by EMF and their host plants. W+RR thereby created an enhanced potential for soil organic matter (SOM) breakdown and nitrogen mineralisation by decomposers, as revealed by 127–190% increases in dissolved organic carbon and nitrogen, respectively, and decreasing SOM content in soil. Climate aridification impacts on vegetation can cascade belowground through shifts in fungal guild structure that alter ecosystem biogeochemistry and accelerate SOM decomposition by reducing the Gadgil effect.

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26The epiphytic vascular flora is scarce and facultative in semiarid Mediterranean 27 ecosystems, thus covering diverse taxonomic groups. However, differently to terrestrial 28 conditions, little is known about the factors driving mycorrhizal communities in 29 epiphytic environments. Here, we investigated the arbuscular mycorrhizal fungi (AMF) 30 harboured by 31 plant species occurring in the trunks of Phoenix dactylifera. We 31 wanted to ascertain if host identity and plant functional traits shape mycorrhizal 32 communities. Specifically, we tested the plant life-cycle (perennial versus annual), the 33 plant life-form (herbaceous versus woody), the plant origin (exotic versus native) and 34 the plant species. 35 The roots were examined by molecular and phylogenetic analysis of AMF community. 36 The plant affiliation to species strongly influenced the AMF assemblages. Plant life-37 form and plant life-cycle also shaped AMF interactions. The AMF community differed 38 between annual and perennial species and higher AMF richness was detected in 39 perennial plants. The indicator species analysis revealed three Operational Taxonomic 40 Units belonging to the Glomeraceae, associated with annual species. However, the 41 epiphytic plants associated with AMF irrespective of whether they were native or not, 42 probably because here no functional differences derive from plant origin. 43 44 IMPORTANCE 45 Arbuscular mycorrhizal (AM) symbiosis has a decisive role in plant nutrient and water 46 uptake by plants, with particular importance in stressful environments. Under semiarid 47 conditions, the facultative epiphytic flora should cope with harsh conditions. While 48 numerous studies have been conducted on factors driving terrestrial AM assemblages, 49 3 the epiphytic environment remains unexplored. We offer new insights into composition 50 of AM communities as shaped by epiphytic plant host identity and functional traits. 51 52 53 54 Keywords: Facultative epiphytes; arbuscular mycorrhizal fungi; diversity; SSU rDNA; 55 semiarid ecosystems; 56 57 58 59Epiphytic habitats are considered as extreme plant environments due to the large fluxes 60 of temperature and low water and nutrient availability they are subjected to. In these 61 conditions, symbiotic associations such as mycorrhizal symbiosis could be crucial 62 because of their widely demonstrated role in plant nutrient and water uptake in 63 terrestrial habits, with particular importance in stressful environments (38). 64Furthermore, the availability of compatible and suitable mycorrhizal fungi might be a 65 key factor constraining the development and distribution of epiphytic plants and vice 66 versa. 67In semiarid Mediterranean ecosystems the epiphytic vascular flora is scarce and 68 facultative or accidental, mainly occurring on the trunks of certain palm species. Here, 69 the microhabitat conditions that originate in the cut leaves formed by the pruning of 70 dead or old leaves allow water and debris accumulation, which enables occasional plant 71 establ...

We carried out a 4-year manipulative field experiment in a semiarid shrubland in Southeastern Spain to assess the impacts of experimental warming (W), rainfall reduction (RR) and their combination (W+RR) on the composition and diversity of arbuscular mycorrhizal fungal (AMF) communities in rhizosphere soil using singlemolecule real-time (SMRT) DNA sequencing. Across climate treatments, we encountered 109 AMF OTUs that were assigned to four families: Glomeraceae (93.94%), Gigasporaceae (2.19%), Claroideoglomeraceae (1.95%) and Diversisporaceae (1.92%). The AMF community composition and diversity indices at OTU level were unaffected by the climate manipulation treatments, except for a significant decrease in AMF richness in the W treatment relative to the control. In contrast, AMF family richness decreased significantly in all the climate manipulation treatments relative to the control treatment. Members of the Gigasporaceae and Diversisporaceae families appeared to be highly vulnerable to intensification of heat and drought stress, as their abundances decreased by 67% and 77% respectively, in the W+RR treatment relative to current ambient conditions. In contrast, the relative abundance and dominance of the Glomeraceae family within the AMF community increased significantly under the W+RR treatment, with Glomeraceae being indicator family for the W+RR treatment. The interaction between warming and rainfall reduction had a significant effect on AMF community structure at family level. These findings provide new insights into AM fungal community responses to climate warming and drying in dryland ecosystems.