Rarity in sequestrate fungi (truffles): A function of niche width or productivity

Background/Question/Methods

Sequestrate fungi (truffles) develop in an enclosed form underground where the mature fruiting bodies are eaten by small mammals that disperse the spores. Although they are not easily observed, truffles are seasonally abundant in association with their mycorrhizal hosts. Among all truffle species, however, some are common while others are rare, as defined by frequency of occurrence. We tested two hypotheses to explain rareness. H1: Rare truffle species may have a narrow niche that is highly specific to particular host trees or to certain abiotic conditions for the host (e.g., elevation and precipitation). A truffle species with a narrow niche may be locally abundant. H2: Rare truffles may occur under environmental conditions of high truffle productivity as indicated by high truffle biomass, numerous fruiting bodies, or high species richness. We surveyed for sequestrate fungi at 40 sites in the eastern Siskiyou Mountains, the western Cascades, and the interior valleys between them in southern Oregon, with vegetation ranging from predominantly conifer through mixed conifer-hardwood to hardwood. We identified species by morphology and DNA sequences.

Results/Conclusions

Species of rare truffles differed in niche preference and in response to productivity. Rhizopogon subpurpurascens (Basidiomycota) occurred at three sites in the Siskiyou Mountains at 900-1300 m in predominantly conifer vegetation (Pinus ponderosa, P. lambertiana, Pseudotsuga menziesii), with 1-9 fruiting bodies per site. Nine species of sequestrate fungi were present at these sites. These data indicate support for H1—a limited niche consisting of a narrow host range and environmental conditions, limited truffle diversity, and local abundance. High elevation conditions may separate potential sites and limit dispersal. In contrast, Cazia flexiascus (Ascomycota) occurred at three sites in the interior valley at 500-600 m in mixed conifer-hardwood vegetation, chiefly Quercus garryana but also Pinus Ponderosa and Pseudotsuga menziesii, with 1-2 fruiting bodies per site. These sites were among the most species-rich with 15 species of sequestrate fungi. These data support H2—occurrence of the rare species in a widespread niche when truffle biodiversity was high. These results indicate that no single explanation can account for rarity among truffles. Although, their similarity of habitat (belowground), morphology (sequestrate), and functional guild (mycorrhizal) suggests that they might respond to common environmental factors, that is not the case. Truffle rarity must be investigated on a species-by-species basis.