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The establishment of common mycelial networks by mycorrhizal fungi shared between host plants depends on the ability of neighboring plants to enter into mycorrhizal associations with compatible fungal species. Such compatibility is governed by the potential mycorrhiza specificities of the symbionts. Mycorrhiza specificities exist along a continuum from low specificity (association with multiple partners) to high specificity (association with one or few partners). Although the ability of symbionts to form mycorrhizas may be largely governed by host-fungus gene interactions as influenced by co-evolutionary events, mycorrhizal associations in natural ecosystems can also be influenced by environmental factors (e.g. soil) and biological factors (e.g. different neighboring host species), phenomena referred to as “ecological specificity.” For example, in natural settings, mycorrhizal fungi often express “host preference” wherein fungi may be more common on a particular host in mixed-host settings than would be expected by random species assemblage within the fungal and plant communities. Mycorrhiza specificity phenomena significantly influence plant community dynamics, particularly plant succession. Early seral plants can positively affect the establishment of later-seral plants by maintaining commonly shared mycorrhizal fungi, and thus affecting the function of common mycelial networks over time. Such knowledge provides guidance for ecosystem managers to maintain “legacy” early -seral plants that benefit later-seral plants via shared mycorrhizal fungus species. Understanding specificity phenomena is also crucial for predicting the successful migration of plants and compatible mycorrhizal fungi during climate change. We review mycorrhiza specificity terminology and types of specificity phenomena, and suggest use of common terms to provide consistency in addressing this research topic. We also provide extensive examples from diverse ecosystems on the ability (or inability) of neighboring plants to develop common mycelial networks.

In most ectomycorrhizal (EM) community studies involving molecular identification methods there is a poor correspondence between fungi that appear dominant as sporocarps and those that appear dominant on EM roots and the species richness belowground is higher than that above ground. As a consequence, many fungi from root tip samples remain unidentified. In most studies, genetic data from multiple samples of an EM morphotype collected from various sampling locations are compared to genetic data from one to a few sporocarps of each species for identification purposes. The mismatch between above- and belowground species richness may be influenced by these different sampling efforts. To address this, intra-specific variation in the ITS region first investigated in Karen et al. (1997) is revisited here, but at a spatial scale in which variation is expected to be low. Sporocarps were collected across a 7 km region of the Oregon Dunes National Recreation Area in western North America. ITS—RFLP data are presented for 3–18 sporocarps from each of 44 EM species in 18 genera. A total of 311 sporocarps were analyzed. Fifty-three ITS—RFLP types were observed. Of the 44 species, 38 (86% of total) yielded a single, species specific, RFLP type. No 2 species had the same RFLP type. Polymorphic ITS—RFLP types were observed in six species (14%). The following three species had two ITS-RFLP types with one type dominating: Inocybe lacera, Laccaria proximo, and Rhizopogon subcaerulescens. The following three species had three RFLP types with no type dominating: Laccaria laccata, Lactarius deliciosus, and Tricholoma flavovirens. A phylogenetic analysis of ITS sequences in Tricholoma revealed that two of the RFLP types in T. flavovirens were apparently the result of intra-specific variation, while the third RFLP type was likely a cryptic species. All the other RFLP types observed in Tricholoma represented unique phylogenetic species. These results suggest that ITS—RFLP data from single samples (sporocarp or EM) are robust for characterizing most of the species at this scale. However, restriction endonucleases detect a limited amount of existing nucleotide variation and thus have limited value to detect cryptic species. Therefore, additional analyses of sequence data should be added to the RFLP matching technique to identify unknown RFLP types. These data also suggest that polymorphic RFLP types within species do not adequately explain the mismatch between above- and belowground views of EM species richness.