Soil spore banks of ectomycorrhizal fungi in endangered Japanese Douglas‐fir forests

Abstract

Interactions between trees and ectomycorrhizal fungi are critical to the growth and survival of both partners. However, ectomycorrhizal symbiosis has barely been explored in endangered trees, and no information is available regarding soil spore banks of ectomycorrhizal fungi from forests of threatened trees. Here, we evaluated soil spore banks of ectomycorrhizal fungi from endangered Japanese Douglas‐fir (Pseudotsuga japonica) forests using bioassay approaches with congeneric P. menziesii and Pinus densiflora seedlings in combination with molecular identification techniques. Rhizopogon togasawariana was predominant in soil propagule banks and was found in all remaining P. japonica forests when assayed with P. menziesii, while no colonization of this fungus was observed on Pinus seedlings. Given the observed specificity of R. togasawariana for P. menziesii and its phylogenetic position within the Pseudotsuga‐specific Rhizopogon lineage, its geographical distribution is likely restricted to the remaining Japanese Douglas‐fir forests, indicating a high extinction risk for this fungus as well as its endangered host. Spore banks of R. togasawariana remained highly infective after preservation for 1 year or heat treatment at 70 °C, suggesting an ecological strategy of establishing ectomycorrhizal associations on regenerating Japanese Douglas‐fir seedlings after disturbance, as observed in other Rhizopogon–Pinaceae combinations. Therefore, the regeneration of Japanese Douglas‐fir seedlings may depend largely on the soil spore banks dominated by R. togasawariana, which has co‐evolved with the Japanese Douglas‐fir for over 30 million years. More attention must be paid to underground ectomycorrhizal fungi for the conservation of endangered tree species, especially in the era of human‐induced mass extinction.

Ectomycorrhizal fungal communities in endangered Pinus amamiana forests

“…amamiana populations. These findings generally support both our hypotheses, confirming the existence of an ECM fungus specific to the endangered pine species in its natural settings and the predominance of this fungus in soil spore banks [ 12 ]. Rhizopogon sp.1 is very likely to have a close ecological relationship with the endangered P .…”

“…Bioassay experiments were performed to assess the soil spore banks following an established method [ 11 , 12 ]. In these experiments, only soil samples collected in 2014 were used.…”

“…Molecular identification of ECM fungi was performed following Murata et al [ 12 ]. Briefly, each ECM tip sample was placed in a 2.0-mL tube containing a zirconia ball and then pulverized using a bead beater.…”

“…The species composition of ECM fungi often differs between spore banks and resident ECM roots, with soil spore banks usually composed of fewer ECM fungal species [ 8 , 10 – 12 ]. Especially in pine forests, soil spore banks are dominated by pine-specific Rhizopogon [ 8 , 10 , 13 – 15 ], because their spores have a longer life span and greater environmental resistance than do those of other ECM fungi [ 16 – 19 ].…”

“…ECM fungi of endangered trees have long been overlooked, and thus available information on these fungi is quite limited. However, our recent study in forests of Pseudotsuga japonica (Pinaceae), another endangered conifer species in Japan, found that soil spore banks are dominated by Rhizopogon togasawariana that coevolved with the endangered host for more than 30 million years [ 12 ]. Notably, this fungus was completely absent in resident tree roots [ 32 ].…”