Rune Halvorsen

Changes in the root-associated fungal communities along a primary succession gradient analysed by 454 pyrosequencing

We investigated changes in the root‐associated fungal communities associated with the ectomycorrhizal herb Bistorta vivipara along a primary succession gradient using 454 amplicon sequencing. Our main objective was to assess the degree of variation in fungal richness and community composition as vegetation cover increases along the chronosequence. Sixty root systems of B. vivipara were sampled in vegetation zones delimited by dated moraines in front of a retreating glacier in Norway. We extracted DNA from rinsed root systems, amplified the ITS1 region using fungal‐specific primers and analysed the amplicons using 454 sequencing. Between 437 and 5063 sequences were obtained from each root system. Clustering analyses using a 98.5% sequence similarity cut‐off yielded a total of 470 operational taxonomic units (OTUs), excluding singletons. Between eight and 41 fungal OTUs were detected within each root system. Already in the first stage of succession, a high fungal diversity was present in the B. vivipara root systems. Total number of OTUs increased significantly along the gradient towards climax vegetation, but the average number of OTUs per root system stayed unchanged. There was a high patchiness in distribution of fungal OTUs across root systems, indicating that stochastic processes to a large extent structure the fungal communities. However, time since deglaciation had impact on the fungal community structure, as a systematic shift in the community composition was observed along the chronosequence. Ectomycorrhizal basidiomycetes were the dominant fungi in the roots of B. vivipara, when it comes to both number of OTUs and number of sequences.

Climate change and spring-fruiting fungi

Most macrofungi produce ephemeral fruit bodies during autumn but some have adapted to spring fruiting. In this study, temporal changes in the time of spring fruiting in Norway and the UK during 1960–2007 have been investigated by statistical analyses of about 6000 herbarium and field records, covering 34 species. Nearly 30 per cent of the temporal variation in fruiting could be ascribed to spatial and species-specific effects. Correcting for these effects, linear trends towards progressively earlier fruiting were detected during the entire period in both Norway and the UK, with a change in average fruiting day of 18 days over the study period. Early fruiting was correlated with high winter temperatures in both countries, indicating that the observed phenological changes are likely due to earlier onset of spring. There were also significant correlations between climatic conditions in one year and timing of fruiting the following year, indicating that below-ground mycelia are influenced by climatic conditions over a longer time period before fruiting. Fruiting dates were, however, not strictly related to changes in vernal accumulated thermal time. Our results indicate that global warming has lead to progressively earlier fruiting of spring fungi in northwest Europe during the last half century.

Grazing responses in herbs in relation to herbivore selectivity and plant traits in an alpine ecosystem

Herbivores shape plant communities through selective foraging. However, both herbivore selectivity and the plant’s ability to tolerate or resist herbivory may depend on the density of herbivores. In an alpine ecosystem with a long history of grazing, plants are expected to respond to both enhanced and reduced grazing pressures, and the interaction between plant traits and changes in species abundance are expected to differ between the two types of alteration of grazing regime. To understand the mechanisms behind species response, we investigated the relationship between sheep selectivity (measured in situ), plant traits and experimentally derived measures of change in species abundance as a response to the enhancement (from low to high density) or cessation (from low to zero density) of sheep grazing pressure over a six-year time period for 22 abundant herb species in an alpine habitat in south Norway. Sheep selected large, late-flowering herbs with a low leaf C/N ratio. Species that increased in abundance in response to enhanced grazing pressure were generally small and had high root/shoot ratios, thus exhibiting traits that reflect both resistance (through avoidance) and tolerance (through regrowth capacity) strategies. The abundance of selected species remained stable during the study period, and also under the enhanced grazing pressure treatment. There was, however, a tendency for selected species to respond positively to cessation of grazing, although overall responses to cessation of grazing were much less pronounced than responses to enhanced grazing. Avoidance through short stature (probably associated with increased light availability through the removal of tall competitors) as well as a certain amount of regrowth capacity appear to be the main mechanisms behind a positive response to enhanced grazing pressure in this study. The plant trait perspective clearly improves our insight into the mechanisms behind observed changes in species abundance when the disturbance regime is altered.

Seasonal trends in the biomass and structure of bryophyte-associated fungal communities explored by 454 pyrosequencing

Bryophytes are a dominant vegetation component of the boreal forest, but little is known about their associated fungal communities, including seasonal variation within them. Seasonal variation in the fungal biomass and composition of fungal communities associated with three widespread boreal bryophytes was investigated using HPLC assays of ergosterol and amplicon pyrosequencing of the internal transcribed spacer 2 (ITS2) region of rDNA. The bryophyte phyllosphere community was dominated by Ascomycota. Fungal biomass did not decline appreciably in winter (P=0.272). Significant host-specific patterns in seasonal variation of biomass were detected (P=0.003). Although seasonal effects were not the primary factors structuring community composition, collection date significantly explained (P=0.001) variation not attributed to locality, host, and tissue. Community homogenization and a reduction in turnover occurred with the onset of frost events and subzero air and soil temperatures. Fluctuations in the relative abundance of particular fungal groups seem to reflect the nature of their association with mosses, although conclusions are drawn with caution because of potential methodological bias. The moss-associated fungal community is dynamic, exhibiting seasonal turnover in composition and relative abundance of different fungal groups, and significant fungal biomass is present year-round, suggesting a winter-active fungal community.

Arctic root-associated fungal community composition reflects environmental filtering.

There is growing evidence that root‐associated fungi have important roles in Arctic ecosystems. Here, we assess the diversity of fungal communities associated with roots of the ectomycorrhizal perennial herb Bistorta vivipara on the Arctic archipelago of Svalbard and investigate whether spatial separation and bioclimatic variation are important structuring factors of fungal community composition. We sampled 160 plants of B. vivipara from 32 localities across Svalbard. DNA was extracted from entire root systems, and 454 pyrosequencing of ITS1 amplicons was used to profile the fungal communities. The fungal communities were predominantly composed of Basidiomycota (55% of reads) and Ascomycota (35%), with the orders Thelephorales (24%), Agaricales (13.8%), Pezizales (12.6%) and Sebacinales (11.3%) accounting for most of the reads. Plants from the same site or region had more similar fungal communities to one another than plants from other sites or regions, and sites clustered together along a weak latitudinal gradient. Furthermore, a decrease in per‐plant OTU richness with increasing latitude was observed. However, no statistically significant spatial autocorrelation between sites was detected, suggesting that environmental filtering, not dispersal limitation, causes the observed patterns. Our analyses suggest that while latitudinal patterns in community composition and richness might reflect bioclimatic influences at global spatial scales, at the smaller spatial scale of the Svalbard archipelago, these changes more likely reflect varied bedrock composition and associated edaphic factors. The need for further studies focusing on identifying those specific bioclimatic and edaphic factors structuring root‐associated fungal community composition at both global and local scales is emphasized.

Mushroom's spore size and time of fruiting are strongly related: is moisture important?

Most basidiomycete fungi produce annual short-lived sexual fruit bodies from which billions of microscopic spores are spread into the air during a short time period. However, little is known about the selective forces that have resulted in some species fruiting early and others later in the fruiting season. This study of relationships between morphological and ecological characteristics, climate factors and time of fruiting are based upon thorough statistical analyses of 66 520 mapped records from Norway, representing 271 species of autumnal fruiting mushroom species. We found a strong relationship between spore size and time of fruiting; on average, a doubling of spore size (volume) corresponded to 3 days earlier fruiting. Small-spored species dominate in the oceanic parts of Norway, whereas large-spored species are typical of more continental parts. In separate analyses, significant relationships were observed between spore size and climate factors. We hypothesize that these relationships are owing to water balance optimization, driven by water storage in spores as a critical factor for successful germination of primary mycelia in the drier micro-environments found earlier in the fruiting season and/or in continental climates.

Amplicon-pyrosequencing-based detection of compositional shifts in bryophyte-associated fungal communities along an elevation gradient

Although bryophytes are a dominant vegetation component of boreal and alpine ecosystems, little is known about their associated fungal communities. HPLC assays of ergosterol (fungal biomass) and amplicon pyrosequencing of the ITS2 region of rDNA were used to investigate how the fungal communities associated with four bryophyte species changed across an elevational gradient transitioning from conifer forest to the low‐alpine. Fungal biomass and OTU richness associated with the four moss hosts did not vary significantly across the gradient (P > 0.05), and both were more strongly affected by host and tissue type. Despite largely constant levels of fungal biomass, distinct shifts in community composition of fungi associated with Hylocomium, Pleurozium and Polytrichum occurred between the elevation zones of the gradient. This likely is a result of influence on fungal communities by major environmental factors such as temperature, directly or indirectly mediated by, or interacting with, the response of other components of the vegetation (i.e. the dominant trees). Fungal communities associated with Dicranum were an exception, exhibiting spatial autocorrelation between plots, and no significant structuring by elevation. Nevertheless, the detection of distinct fungal assemblages associated with a single host growing in different elevation zones along an elevational gradient is of particular relevance in the light of the ongoing changes in vegetation patterns in boreal and alpine systems due to global climate warming.

Different bacterial communities in ectomycorrhizae and surrounding soil

Several eukaryotic symbioses have shown to host a rich diversity of prokaryotes that interact with their hosts. Here, we study bacterial communities associated with ectomycorrhizal root systems of Bistorta vivipara compared to bacterial communities in bulk soil using pyrosequencing of 16S rRNA amplicons. A high richness of Operational Taxonomic Units (OTUs) was found in plant roots (3,571 OTUs) and surrounding soil (3,476 OTUs). The community composition differed markedly between these two environments. Actinobacteria, Armatimonadetes, Chloroflexi and OTUs unclassified at phylum level were significantly more abundant in plant roots than in soil. A large proportion of the OTUs, especially those in plant roots, presented low similarity to Sanger 16S rRNA reference sequences, suggesting novel bacterial diversity in ectomycorrhizae. Furthermore, the bacterial communities of the plant roots were spatially structured up to a distance of 60 cm, which may be explained by bacteria using fungal hyphae as a transport vector. The analyzed ectomycorrhizae presents a distinct microbiome, which likely influence the functioning of the plant-fungus symbiosis.

Multiple Parallel Ordinations: the Importance of Choice of Ordination Method and Weighting of Species Abundance Data

Abstract Most species-abundance matrices subjected to ordination are analysed by applying one ordination method to one single weighting (transformation) of the raw abundance values. We argue that such an approach is sub-optimal for identification of species-environment relationships. In order to capture the full range of qualitative and quantitative components of variation unique to each species-abundance matrix, data sets covering the full range of abundance weights from presence/absence to raw abundance data should be subjected to ordination. Furthermore, two or more ordination methods should be used in parallel to enhance detection of artifacts in the results. We describe and exemplify a multiple parallel ordination (MPO) procedure, which ensures that both qualitative and quantitative properties of the data set are revealed. This procedure implies that different ordination methods are applied in parallel, each with different weightings of elements in the species-abundance matrix. Two species-abundance matrices are used to exemplify the MPO procedure, one of individual counts from a marine benthic study of mollusc and echinoderm species and one of percent cover of vascular plants, bryophytes and lichens from a study of boreal forest understory vegetation. Striking differences between point configurations obtained by different ordination methods and strong dependence of ordination results on the weights given to abundance are demonstrated. Properties of species and data sets that may explain the observed variation in ordination results are analyzed and discussed. We show that the widely held belief that changes in ordination structure with changes of weighting function are brought about only by highly abundant and frequent species is an over-simplification. We conclude that the MPO procedure - applying more than one ordination method and more than one weighting function is likely to enhance the user’s insight into the multivariate structure of species-abundance matrices resulting from ordination analyses.

Substantial compositional turnover of fungal communities in an alpine ridge‐to‐snowbed gradient

The main gradient in vascular plant, bryophyte and lichen species composition in alpine areas, structured by the topographic gradient from wind‐exposed ridges to snowbeds, has been extensively studied. Tolerance to environmental stress, resulting from wind abrasion and desiccation towards windswept ridges or reduced growing season due to prolonged snow cover towards snowbeds, is an important ecological mechanism in this gradient. The extent to which belowground fungal communities are structured by the same topographic gradient and the eventual mechanisms involved are less well known. In this study, we analysed variation in fungal diversity and community composition associated with roots of the ectomycorrhizal plant Bistorta vivipara along the ridge‐to‐snowbed gradient. We collected root samples from fifty B. vivipara plants in ten plots in an alpine area in central Norway. The fungal communities were analysed using 454 pyrosequencing analyses of tag‐encoded ITS1 amplicons. A distinct gradient in the fungal community composition was found that coincided with variation from ridge to snowbeds. This gradient was paralleled by change in soil content of carbon, nitrogen and phosphorus. A large proportion (66%) of the detected 801 nonsingleton operational taxonomic units (OTUs) were ascomycetes, while basidiomycetes dominated quantitatively (i.e. with respect to number of reads). Numerous fungal OTUs, many with taxonomic affinity to Sebacinales, Cortinarius and Meliniomyces, showed distinct affinities either to ridge or to snowbed plots, indicating habitat specialization. The compositional turnover of fungal communities along the gradient was not paralleled by a gradient in species richness.