Andy F. S. Taylor

Two new species of milkcaps, Lactarius megaplinthogalus and L. rosascens , are described from montane forests in Xieng Khouang Province, northern Laos, Southeast Asia. Both species were collected in mixed forests dominated by Fagaceae and Pinaceae and are documented using detailed macro- and micromorphological observations in combination with ITS rDNA sequence data. Lactarius megaplinthogalus is distinguished by its large, stout, dark brown to blackish basidiocarps, very distant lamellae, sticky white latex that stains the lamellae pinkish to blackish, and thick-walled basidia. Lactarius rosascens is recognized by its yellow to honey-colored, scrobiculate pileus, abundant white latex rapidly turning bright pink to wine-red and intensely staining the lamellae, and a well-developed trichodermal pileipellis. Phylogenetic analyses support their recognition as distinct species and place the first species in L. subg. Plinthogalus (section Plinthogalus ) and the latter one in L. subg. Lactarius (unclassified at the sectional level). These findings highlight the exceptional and still underexplored diversity of milkcaps in Laos (and more generally in Southeast Asia) and underline the importance of continued mycological surveys and integrative taxonomic approaches in the region.

Hortiboletus (the former Xerocomus rubellus species complex) is one of the most taxonomically critical and difficult genera for species identification in the family Boletaceae. Here, we provide a detailed morphological and molecular re-assessment of European and Levantine species of Hortiboletus. A new species, H. hershenzoniae, is described from Israel. It is sister to H. engelii and associated with the evergreen oak Quercus calliprinos and potentially also with Q. ithaburensis. Based on the sequence retrieved from INSDC, this species is also found in Lebanon. Accurate morphological descriptions, comprehensive sampling, type studies, biogeography, macro- and microphotographs and a historical overview on the nomenclatural issues surrounding H. rubellus, H. bubalinus, H. engelii, and H. hershenzoniae are given. An epitype collection is designated for H. rubellus. A key is provided for identification of the European and Levantine taxa. In addition, we propose a novel taxonomic combination Hortiboletus flavorubellus, which is conspecific with Boletus rubellus var. flammeus, based on the DNA barcoding and phylogenetic analysis of type material. Boletus harrisonii is also shown to be conspecific with H. campestris. A multilocus phylogenetic analysis of four markers (ITS, LSU, tef1-α, and rpb2) reveals that Hortiboletus is a sister genus to Xerocomellus. Using the Genealogical Concordance Phylogenetic Species Recognition method, at least 19 phylogenetic species and eight putative phylogenetic species of the genus Hortiboletus can be delimited. Based on multilocus analysis, it contains from 24 to 25 species-level clades worldwide, 17 out of which represent known species, one newly described and potentially six to seven undescribed species. Tandem repeat insertions within the ITS region (both in ITS1 and ITS2) are reported for the first time, not only in the genus Hortiboletus, but in the entire subfamily Boletoideae. Their identification and characterisation were based on Tandem Repeat Finder analysis and visual assessment of the ITS alignment.

Biological diversity in mountain ecosystems has been increasingly studied over the last decade. This is also the case for mountain soils, but no study to date has provided an overall synthesis of the current state of knowledge. Here we fill this gap with a first global analysis of published research on cryptogams, microorganisms, and fauna in mountain soils above the treeline, and a structured synthesis of current knowledge. Based on a corpus of almost 1400 publications and the expertise of 37 mountain soil scientists worldwide, we summarise what is known about the diversity and distribution patterns of each of these organismal groups, specifically along elevation, and provide an overview of available knowledge on the drivers explaining these patterns and their changes. In particular, we document an elevation‐dependent decrease in faunal diversity above the treeline, while for cryptogams there is an initial increase above the treeline, followed by a decrease towards the nival belt. Thus, our data confirm the key role that elevation plays in shaping the biodiversity and distribution of these organisms in mountain soils. The response of prokaryote diversity to elevation, in turn, was more diverse, whereas fungal diversity appeared to be substantially influenced by plants. As far as available, we describe key characteristics, adaptations, and functions of mountain soil species, and despite a lack of ecological information about the uncultivated majority of prokaryotes, fungi, and protists, we illustrate the remarkable and unique diversity of life forms and life histories encountered in alpine mountain soils. By applying rule‐ as well as pattern‐based literature‐mining approaches and semi‐quantitative analyses, we identified hotspots of mountain soil research in the European Alps and Central Asia and revealed significant gaps in taxonomic coverage, particularly among biocrusts, soil protists, and soil fauna. We further report thematic priorities for research on mountain soil biodiversity above the treeline and identify unanswered research questions. Building upon the outcomes of this synthesis, we conclude with a set of research opportunities for mountain soil biodiversity research worldwide. Soils in mountain ecosystems above the treeline fulfil critical functions and make essential contributions to life on land. Accordingly, seizing these opportunities and closing knowledge gaps appears crucial to enable science‐based decision making in mountain regions and formulating laws and guidelines in support of mountain soil biodiversity conservation targets.

There is an urgent need for woodland expansion to help enhance and restore vital forest ecosystem services. However, the process of natural regeneration of woodlands is often slow, requiring active reforestation approaches. Factors limiting regeneration are complex, but there is increasing evidence that natural woodland expansion onto sites that have been treeless for a long period of time may be limited by the loss of microbial woodland‐soil properties. This study explored a potential role for mycorrhizal associations in facilitating obligate ectomycorrhizal (ECM) tree establishment, with potential implications for active reforestation practices. We determined the spatial distribution patterns of ECM tree seedlings – Pinus sylvestris and Betula spp. – outwith a woodland, in relation to existing dual‐mycorrhizal Salix plants. There was very strong evidence that ECM tree seedlings were more likely to be found growing within groups around Salix than as isolated individuals, supporting the view that Salix drives the current spatial distribution patterns of ECM tree establishment. We suggest that the ability of Salix to form dual AM and ECM associations allows it to become established across the open AM dominated vegetation. Over time these plants become colonised by ECM fungi via aerial spores and then can act as a source of inoculum for obligate ECM tree seedlings, which arrive as seeds in the vicinity of Salix. This study highlights the potential role of dual‐mycorrhizal Salix as a nurse plant in supporting secondary forest succession of obligate ECM boreal/temperate woodland species onto adjacent open areas through nucleation. Overall, we concluded that there is potential for existing ECM networks to aid tree establishment and propose a nature‐based approach to maximize reforestation efforts by targeting existing symbiotic fungal networks as nucleation points for tree planting. Our findings contribute to understanding the ecological dynamics involved in woodland expansion and restoration.

Despite the importance of healthy soils for human livelihood, wellbeing, and safety, current gaps in our knowledge and understanding of biodiversity in soil are numerous, undermining conservation efforts. These gaps are particularly wide in mountain regions where healthy soils are especially important for human safety and yet evidence is accumulating of ongoing degradation, posing significant threats to ecosystem functioning and human settlements. To analyse these gaps in detail, we synthesise current research on the global diversity of microorganisms, cryptogams, and invertebrates in mountain soils above the treeline. This synthesis is based on a semi-quantitative survey of the literature and an expert-based analysis. Our work reveals not only deficiencies in geographic cover but also significant gaps in taxonomic coverage, particularly among soil protists and invertebrates, and a lack of (functional and ecological) description of the uncultivated majority of prokaryotes, fungi, and protists. We subsequently build on this overview to highlight opportunities for research on mountain soils as systems of co-occurring species that interact in complex environmental matrices to fulfil critical functions and make essential contributions to life on land. Closing gaps in biodiversity research in mountain soil is crucial to enhance our understanding and to promote laws and guidelines advancing international soil biodiversity conservation targets in mountains. Addressing sparse and biased data, recognizing the impact of environmental changes on mountain ecosystems, and advocating dedicated policies are essential strategies to safeguard mountain soils and their biodiversity. GLOSSARY:

Summary: Many fungi that form ectomycorrhizas exhibit a degree of host specialisation, and individual trees are frequently colonised by communities of mycorrhizal fungi comprising species that fall on a gradient of specialisation along genetic, functional and taxonomic axes of variation. By contrast, arbuscular mycorrhizal fungi exhibit little specialisation. Here, we propose that host tree root morphology is a key factor that gives host plants fine‐scale control over colonisation and therefore opportunities for driving specialisation and speciation of ectomycorrhizal fungi. A gradient in host specialisation is likely driven by four proximate mechanistic ‘filters’ comprising partner availability, signalling recognition, competition for colonisation, and symbiotic function (trade, rewards and sanctions), and the spatially restricted colonisation seen in heterorhizic roots enables these mechanisms, especially symbiotic function, to be more effective in driving the evolution of specialisation. We encourage manipulation experiments that integrate molecular genetics and isotope tracers to test these mechanisms, alongside mathematical simulations of eco‐evolutionary dynamics in mycorrhizal symbioses.

Traditional strict separation of fungi into ecological niches as mutualist, parasite or saprotroph is increasingly called into question. Sequences of assumed saprotrophs have been amplified from plant root interiors, and several saprotrophic genera can invade and interact with host plants in laboratory growth experiments. However, it is uncertain if root invasion by saprotrophic fungi is a widespread phenomenon and if laboratory interactions mirror field conditions. Here, we focused on the widespread and speciose saprotrophic genus Mycena and performed (1) a systematic survey of their occurrences (in ITS1/ITS2 datasets) in mycorrhizal roots of 10 plant species, and (2) an analysis of natural abundances of ¹³ C/ ¹⁵ N stable isotope signatures of Mycena basidiocarps from five field locations to examine their trophic status. We found that Mycena was the only saprotrophic genus consistently found in 9 out of 10 plant host roots, with no indication that the host roots were senescent or otherwise vulnerable. Furthermore, Mycena basidiocarps displayed isotopic signatures consistent with published ¹³ C/ ¹⁵ N profiles of both saprotrophic and mutualistic lifestyles, supporting earlier laboratory‐based studies. We argue that Mycena are widespread latent invaders of healthy plant roots and that Mycena species may form a spectrum of interactions besides saprotrophy also in the field.

Non‐native plant pests and pathogens are increasing exponentially, causing extirpation of foundation species. The impact of large‐scale declines in a single host on associated biodiversity is widely documented. However, the impact of multiple host loss on biodiversity and whether these impacts are multiplicative has not been assessed. Ecological theory suggests that systems with greater functional redundancy (alternative hosts) will be more resilient to the loss of sympatric hosts. We test this theory and show its importance in relation to pest/pathogen impact assessments. We assessed the potential impact on biodiversity of the loss of two widely occurring sympatric European tree species, Fraxinus excelsior and Quercus petraea/robur, both of which are currently threatened by a range of pests and pathogens. At the UK scale, the total number of associated species at risk of extirpation from plant diseases affecting these two sympatric hosts is greater than the sum of the associated species at risk from declines in either host alone. F. excelsior hosts 45 obligate species (species only found on that host) and Q. petraea/robur hosts 326. However, a decline in both these trees would impact 512 associated species, across multiple taxon groups, a 38% increase. Assessments at a local scale, 24 mixed F. excelsior–Q. petraea/robur woodlands revealed that these impacts may be even greater due to a lack of functional redundancy. Only 21% of sites were able to provide functional redundancy for F. excelsior and Q. petraea/robur associated species which can use other tree species. In most woodlands, the tree species required to provide functional redundancy were not present, although the site conditions were often suitable for them to grow. Synthesis. Understanding of functional redundancy should be applied to assessments of pests/pathogens impact on biodiversity. In risk assessments, higher impact scores should be given to pests/pathogens affecting hosts occurring with other host plant species already impacted by pests/pathogens. Current pest/pathogen risk assessment approaches that ignore the cumulative, cascading effects shown in this study may allow an insidious, mostly overlooked, driver of biodiversity loss to continue.

Plant–soil feedbacks regulate plant productivity and diversity, but potential mechanisms underpinning such feedbacks, such as the allocation of recent plant assimilate, remain largely untested especially for plants forming tripartite symbioses. We tested how soils from under alder Alnus glutinosa and beneath other species of the same and different families affected alder growth and nutrition, and colonization of roots by nitrogen‐fixing Frankia bacteria and ectomycorrhizal fungi. We also measured how the soil environment affected carbon capture and allocation by pulse labelling seedlings with ¹³CO₂. We then tested for linkages between foliar nutrient stoichiometry and carbon capture and allocation and soil origin using statistical modelling approaches. Performance of alder and nitrogen nutrition were best on home and birch Betula pendula soils (both Betulaceae), whereas performance on Douglas fir Pseudotsuga menziesii (Pinaceae) soil was poor. Plants growing in P. menziesii soil were virtually devoid of Frankia and ectomycorrhizas, and the natural abundance ¹⁵N signatures of leaves were more enriched indicating distinct nitrogen acquisition pathways. Seedlings in these soils also had smaller ¹³C fixation and root allocation rates, leading to smaller ¹³C respiration rates by microbes. Statistical models showed that the best predictors of foliar N concentration were ¹³C allocation rates to fine roots and net CO₂ exchange from the mesocosms. The best predictors for foliar phosphorus concentration were net CO₂ exchange from the mesocosms and soil origin; seedlings in home soils tended to have greater foliar phosphorus compared to birch soils while seedlings from Douglas fir soils were no different from the other treatments. Foliar phosphorus concentration was not correlated with plant available or total soil phosphorus for any of the soils. Home soils also resulted in distinct ectomycorrhizal communities on seedlings roots, which could be responsible for greater foliar phosphorus concentration. Our findings show how the association of alder with nitrogen‐fixing Frankia relieved nitrogen limitation in the seedling triggering a performance feedback loop. We propose that relief of nitrogen limitation likely increases plant phosphorus demand, which may promote the formation of ectomycorrhizas in nutrient‐deficient soils. The formation of tripartite symbioses therefore generates positive plant–soil feedbacks, which enables plants to acquire mineral nutrients otherwise inaccessible in trade for carbon. A free Plain Language Summary can be found within the Supporting Information of this article.

For successful colonization of host roots, ectomycorrhizal (EM) fungi must overcome host defense systems, and defensive phenotypes have previously been shown to affect the community composition of EM fungi associated with hosts. Secondary metabolites, such as terpenes, form a core part of these defense systems, but it is not yet understood whether variation in these constitutive defenses can result in variation in the colonization of hosts by specific fungal species. We planted seedlings from twelve maternal families of Scots pine (Pinus sylvestris) of known terpene genotype reciprocally in the field in each of six sites. After 3 months, we characterized the mycorrhizal fungal community of each seedling using a combination of morphological categorization and molecular barcoding, and assessed the terpene chemodiversity for a subset of the seedlings. We examined whether parental genotype or terpene chemodiversity affected the diversity or composition of a seedling's mycorrhizal community. While we found that terpene chemodiversity was highly heritable, we found no evidence that parental defensive genotype or a seedling's terpene chemodiversity affected associations with EM fungi. Instead, we found that the location of seedlings, both within and among sites, was the only determinant of the diversity and makeup of EM communities. These results show that while EM community composition varies within Scotland at both large and small scales, variation in constitutive defensive compounds does not determine the EM communities of closely cohabiting pine seedlings. Patchy distributions of EM fungi at small scales may render any genetic variation in associations with different species unrealizable in field conditions. The case for selection on traits mediating associations with specific fungal species may thus be overstated, at least in seedlings.