Despite the critical roles fungi play in the functioning of ecosystems, especially as symbionts of plants and recyclers of organic matter, their biodiversity is poorly known in high-latitude regions. In this paper, we discuss the molecular diversity of one of the most diverse and abundant groups of ectomycorrhizal fungi: the genus Lactarius Pers. We analysed internal transcribed spacer rDNA sequences from both curated sporocarp collections and soil polymerase chain reaction clone libraries sampled in the arctic tundra and boreal forests of Alaska. Our genetic diversity assessment, based on various phylogenetic methods and operational taxonomic unit (OTU) delimitations, suggests that the genus Lactarius is diverse in Alaska, with at least 43 putative phylogroups, and 24 and 38 distinct OTUs based on 95% and 97% internal transcribed spacer sequence similarity, respectively. Some OTUs were identified to known species, while others were novel, previously unsequenced groups. Non-asymptotic species accumulation curves, the disparity between observed and estimated richness, and the high number of singleton OTUs indicated that many Lactarius species remain to be found and identified in Alaska. Many Lactarius taxa show strong habitat preference to one of the three major vegetation types in the sampled regions (arctic tundra, black spruce forests, and mixed birch-aspen-white spruce forests), as supported by statistical tests of UniFrac distances and principal coordinates analyses (PCoA). Together, our data robustly demonstrate great diversity and nonrandom ecological partitioning in an important boreal ectomycorrhizal genus within a relatively small geographical region. The observed diversity of Lactarius was much higher in either type of boreal forest than in the arctic tundra, supporting the widely recognized pattern of decreasing species richness with increasing latitude.

Atmospheric nitrogen (N) fixation by Alnus tenuifolia can account for up to 70% of the N accumulated during vegetation development along river floodplains in interior Alaska. We assessed disease incidence and related mortality of a recent outbreak of fungal stem cankers on A. tenuifolia across three regions in Alaska during the 2005 growing season, and determined the impacts on N-fixation rates, nodule biomass, and stand-level N-fixation inputs. The highest percentage of ramets colonized or dead with canker was found on Tanana River plots, suggesting the epidemic is most severe in the Fairbanks region. A positive relationship between % basal area loss to canker and % canopy loss provides a simple means for assessing stand-level mortality associated with disease in the field. Although specific N-fixation (SNF) rates were not influenced by canker disease incidence of individual genets, live nodule biomass beneath alder canopies was inversely correlated with the percentage of ramets dead or with main ramet canker. Variations in SNF and live nodule biomass translated to differences in N-fixation inputs, which ranged from 22 to 107 kg N ha-1 y-1 across study regions. Nodule biomass was reduced by incidence of canker disease and related mortality an average of 24% across all sites, which translates to N input reductions of 8, 16, and 33 kg N ha-1 y-1 for the three regions, respectively. During the 2008 growing season, we resurveyed the Tanana River plots and found that of the ramets larger than 4-cm diameter having main ramet canker in 2005, 74% are now dead; and for those without main ramet canker in 2005, 25% have developed main ramet canker, and 8% are dead. Thus, it is likely that N-fixation inputs have declined further below what we estimated for 2005.

In long-lived N-fixing plants, environmental conditions affecting plant growth and N demand vary at multiple temporal and spatial scales, and symbiont assemblages on a given host and patterns of allocation to nodule activities have been shown to vary according to environmental factors, suggesting that hosts may alter partner choice and manipulate symbiont assemblages based on shifting plant needs. This study assessed economic trade-offs among N-fixing symbionts of thin-leaf alder (Alnus tenuifolia) by examining whether alderFrankia associations change in response to the up-regulation (P fertilization) and downregulation (N-fertilization) of N-fixation activities, and whether these changes are associated with differences among Frankia partners in their relative C cost and/or N benefit to A. tenuifolia.Relative to control plots, alder in þP plots had significantly higher nodule biomass and Nfixation rates; these parameters were significantly lower in þN plots, translating to stand-level N inputs that were more than an order of magnitude greater in þP than þN plots. Nodule respiration and N-fixation rates were positively correlated, and analyses revealed that alder employs mechanisms to increase the efficiency of C use when N fixation is up-regulated. Of the eight Frankia OTUs (operational taxonomic units) identified, two were dominant, with significant differences in Frankia OTU composition across samples being explained by fertilization treatment. Dominant OTUs had similar up-and down-regulatory responses to treatments but differed in C costs of N fixation, while the most abundant sub-dominant failed to up-regulate N-fixation rates in þP plots. Differences among Frankia OTUs in traits relevant to plant performance may form the basis for host choice and explain successional shifts in alder-Frankia assemblages. We suggest that manipulation of Frankia assemblages is an adaptation for a species such as A. tenuifolia that maintains a high dependence on atmospheric N across radically different growth environments. Because of the strong effects of thin-leaf alder on soil N contents, it appears that host changes in C partitioning to nodule activities, including the physiological consequences of host specificity for Frankia assemblages, results in alder both driving and responding to environmental heterogeneity at small to large spatial scales.

Summary• Although critical for the functioning of ecosystems, fungi are poorly known in high-latitude regions. Here, we provide the first genetic diversity assessment of one of the most diverse and abundant ectomycorrhizal genera in Alaska: Russula.• We analyzed internal transcribed spacer rDNA sequences from sporocarps and soil samples using phylogenetic methods, operational taxonomic unit (OTU) delimitations and ordinations to compare species composition in various types of boreal forest.• The genus Russula is highly diverse in Alaska, with at least 42 nonsingleton OTUs (soil) and 50 phylogroups (soil + sporocarp). Russula taxa showed strong habitat preference to one of the two major forest types in the sampled regions (black spruce and birch-aspen-white spruce), and some preference for soil horizon.• Our results show that the vast majority of Russula species are present in the soil samples, although some additional taxa are expected to be found with extended sampling. OTU diversity in black spruce forests was only one-third of the diversity observed in mixed upland forests. Our findings suggest that some of the diversity is niche based, especially along host and successional axes, because most OTUs predictably occurred in specific habitats, regardless of geographical location.