Large-scale studies that examine the responses of ectomycorrhizal fungi across biogeographic gradients are necessary to assess their role in mediating current and predicted future alterations in forest ecosystem processes. We assessed the extent of environmental filtering on interior Douglas-fir ( Pseudotsuga menziesii var. glauca (Beissn.) Franco) ectomycorrhizal fungal communities across regional gradients in precipitation, temperature, and soil fertility in interior Douglas-fir dominated forests of western Canada. We also examined relationships between fine-root traits and mycorrhizal fungal exploration types by combining root and fungal trait measurements with next-generation sequencing. Temperature, precipitation, and soil C:N ratio affected fungal community dissimilarity and exploration type abundance but had no effect on α-diversity. Fungi with rhizomorphs (e.g., Piloderma sp.) or proteolytic abilities (e.g., Cortinarius sp.) dominated communities in warmer and less fertile environments. Ascomycetes (e.g., Cenococcum geophilum ) or shorter distance explorers, which potentially cost the plant less C, were favored in colder/drier climates where soils were richer in total nitrogen. Environmental filtering of ectomycorrhizal fungal communities is potentially related to co-evolutionary history between Douglas-fir populations and fungal symbionts, suggesting success of interior Douglas-fir as climate changes may be dependent on maintaining strong associations with local communities of mycorrhizal fungi. No evidence for a link between root and fungal resource foraging strategies was found at the regional scale. This lack of evidence further supports the need for a mycorrhizal symbiosis framework that is independent of root trait frameworks, to aid in understanding belowground plant uptake strategies across environments.

Plants alter soil biological communities, generating ecosystem legacies that affect the performance of successive plants, influencing plant community assembly and successional trajectories. Yet, our understanding of how microbe‐mediated soil legacies influence plant establishment is limited for primary successional systems and forest ecosystems, particularly for ectomycorrhizal plants. In a two‐phase greenhouse experiment using primary successional mine reclamation materials with or without forest soil additions, we conditioned soil with an early successional shrub with low mycorrhizal dependence (willow, Salix scouleriana) and a later‐successional ectomycorrhizal conifer (spruce, Picea engelmannii × glauca). The same plant species and later‐successional plants (spruce and/or redcedar, Thuja plicata, a mid‐ to late‐successional arbuscular mycorrhizal conifer) were grown as legacy‐phase seedlings in conditioned soils and unconditioned control soils. Legacy effects were evaluated based on seedling survival and biomass, and the abundance and diversity of root fungal symbionts and pathogens. We found negative intraspecific (same‐species) soil legacies for willow associated with pathogen accumulation, but neutral to positive intraspecific legacies in spruce associated with increased mycorrhizal fungal colonization and diversity. Our findings support research showing that soil legacy effects vary with plant nutrient acquisition strategy, with plants with low mycorrhizal dependence experiencing negative feedbacks and ectomycorrhizal plants experiencing positive feedbacks. Soil legacy effects of willow on next‐stage successional species (spruce and redcedar) were negative, potentially due to allelopathy, while ectomycorrhizal spruce had neutral to negative legacy effects on arbuscular mycorrhizal redcedar, likely due to the trees not associating with compatible mycorrhizae. Thus, positive biological legacies may be limited to scenarios where mycorrhizal‐dependent plants grow in soil containing legacies of compatible mycorrhizae. We found that soil legacies influenced plant performance in mine reclamation materials with and without forest soil additions, indicating that initial restoration actions may potentially exert long‐term effects on plant community composition, even in primary successional soils with low microbial activity.

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Whitebark pine (Pinus albicaulis Engelm.; WBP) is an endangered subalpine tree species and requires associations with ectomycorrhizal fungi (ECMF) for survival and growth. Despite this obligate dependence, there are gaps in the identification of ECMF that associate with WBP. In addition, ECMF rarely feature in assessments of recovery actions and little is known about the relationship between ECMF and the insects and pathogens affecting WBP. We used next-generation sequencing to characterize ECMF occurring in soil and mycorrhizal root tip samples from naturally occurring mature WBP trees and seedlings as well as planted WBP seedlings in the Columbia Mountains of Interior British Columbia, Canada. ECMF data was paired with data on tree age, tree health and soil conditions. Thirty-three species and twenty-one genera of ECMF were identified with medium or high confidence from mycorrhizal root tip samples. Major groups were: generalist ascomycetes [Cenococcum, Meliniomyces (=Hyaloscypha)], Atheliales (Piloderma, Amphinema, Tylospora), non-ascomycetous generalists (e.g., Amphinema), associates of high-elevation conifers (species of Cortinarius, Russula) and Suilloids (Suillus, Rhizopogon). Differences in WBP ECMF with other, drier and southerly regions that have been studied previously, were consistent with a distinct forest type and an endemism hypothesis. Soil at the planting site and planted seedlings hosted a reduced ECMF community or were non-ectomycorrhizal, which can be explained by site factors and is expected to affect seedling survival. ECMF composition on mature trees was correlated with tree health, which may have implications for WBPs resistance to pathogens and signals that ECMF are affected by the decline of their host. Understanding the ecology of WBP ECMF and their relationship with tree performance is essential for WBP recovery efforts.