Summary The root‐associated habit has evolved on numerous occasions in different fungal lineages, suggesting a strong evolutionary pressure for saprotrophic fungi to switch to symbiotic associations with plants. Species within the ubiquitous, saprotrophic genus Mycena are frequently major components in molecular studies of root‐associated fungal communities, suggesting that an evaluation of their trophic status is warranted. Here, we report on interactions between a range of Mycena species and the plant Betula pendula. In all, 17 Mycena species were inoculated onto B. pendula seedlings. Physical interactions between hyphae and fine roots were examined using differential staining and fluorescence microscopy. Physiological interactions were investigated using 14C and 32P to show potential transfer between symbionts. All Mycena species associated closely with fine roots, showing hyphal penetration into the roots, which in some cases were intracellular. Seven species formed mantle‐like structures around root tips, but none formed a Hartig net. Mycena pura and Mycena galopus both enhanced seedling growth, with M. pura showing significant transfer of 32P to the seedlings. Our results support the view that several Mycena species can associate closely with plant roots and some may potentially occupy a transitional state between saprotrophy and biotrophy.

Aim Polar and alpine ecosystems appear to be particularly sensitive to increasing temperatures and the altered precipitation patterns linked to climate change. However, little is currently known about how these environmental drivers may affect edaphic organisms within these ecosystems. In this study, we examined communities of plant root‐associated fungi (RAF) over large biogeographical scales and along climatic gradients in the North Atlantic region in order to gain insights into the potential effects of climate variability on these communities. We also investigated whether selected fungal traits were associated with particular climates. Locations Austria, Scotland, Mainland Norway, Iceland, Jan Mayen and Svalbard. Taxa Root fungi associated with the ectomycorrhizal and herbaceous plant Bistorta vivipara. Methods DNA metabarcoding of the ITS1 region was used to characterize the RAF of 302 whole plant root systems, which were analysed by means of ordination methods and linear modelling. Fungal spore length, width, volume and shape, as well as mycelial exploration type (ET) of ectomycorrhizal (ECM) basidiomycetes were summarized at a community level. Results The RAF communities exhibited strong biogeographical structuring, and both compositional variation as well as fungal species richness correlated with annual temperature and precipitation. In accordance with general island biogeography theory, the least species‐rich RAF communities were found on Jan Mayen, a remote and small island in the North Atlantic Ocean. Fungal spores tended to be more elongated with increasing latitude. We also observed a climate effect on which mycelial ET was dominating among the ectomycorrhizal fungi. Main conclusions Both geographical and environmental variables were important for shaping root‐associated fungal communities at a North Atlantic scale, including the High Arctic. Fungal OTU richness followed general biogeographical patterns and decreased with decreasing size and/or increasing isolation of the host plant population. The probability of possessing more elongated spores increases with latitude, which may be explained by a selection for greater dispersal capacity among more isolated host plant populations in the Arctic.

Although only a relatively small proportion of plant species form ectomycorrhizae with fungi, it is crucial for growth and survival for a number of widespread woody plant species. Few studies have attempted to investigate the fine scale spatial structure of entire root systems of adult ectomycorrhizal (EcM) plants. Here, we use the herbaceous perennial Bistorta vivipara to map the entire root system of an adult EcM plant and investigate the spatial structure of its root associated fungi. All EcM root tips were sampled, mapped and identified using a direct PCR approach and Sanger sequencing of the ITS region. A total of 32.1 % of all sampled root tips (739 of 2302) were successfully sequenced and clustered into 41 OTUs. We observed a clear spatial structuring of the root associated fungi within the root system. Clusters of individual OTUs were observed in the younger parts of the root system, consistent with observations of priority effect in previous studies, but were absent from the older parts of the root system. This may suggest a succession and fragmentation of the root-associated fungi even at a very fine scale, where competition likely comes into play at different successional stages within the root system.

Ecosystems all over the world are being affected by climate change (Parmesan, 2006;Pecl et al., 2017). High-latitude and high-altitude ecosystems are particularly exposed because of arctic amplification and elevation-dependent warming (Pepin et al., 2015;Serreze & Barry, 2011). In temperate and boreal regions, one of the most striking ecological transitions in high-elevation areas is the change from boreal lowland forests to alpine highlands without forests (Figure 1a). This marked ecotone, termed the forest line (here) or treeline, varies in altitude and species composition across the globe, and has in many regions moved towards higher elevations during the last few decades due to climate warming (Beckage et al., 2008;Chen et al., 2011;Harsch et al., 2009). Although many mountain regions are influenced by land use, the uppermost alpine forest lines are mainly temperature-driven (Körner, 2012). Recent studies have also documented positive climate feedbacks from expanding alpine forest lines (de Wit et al., 2014); an increase in the elevation of alpine forest lines will probably increase the local temperature and therefore accelerate the ongoing expansion further (Rydsaa et al., 2017).

The number of plant species regarded as non-mycorrhizal increases at higher latitudes, and several plant species in the High-Arctic Archipelago Svalbard have been reported as non-mycorrhizal. We used the rRNA ITS2 and 18S gene markers to survey which fungi, as well as other micro-eukaryotes, were associated with roots of 31 arctic plant species not usually regarded as mycorrhizal in Svalbard. We assessed to what degree the root-associated fungi showed any host preference and whether the phylogeny of the plant hosts may mirror the composition of root-associated fungi. Fungal communities were largely structured according to host plant identity and to a less extent by environmental factors. We observed a positive relationship between the phylogenetic distance of host plants and the distance of fungal community composition between samples, indicating that the evolutionary history of the host plants plays a major role for which fungi colonize the plant roots. In contrast to the ITS2 marker, the 18S rRNA gene marker showed that chytrid fungi were prevalently associated with plant roots, together with a wide spectrum of amoeba-like protists and nematodes. Our study confirms that arbuscular mycorrhizal (AM) fungi are present also in arctic environments in low abundance.