Inoculum potential of Rhizopogon spores increases with time over the first 4 yr of a 99-yr spore burial experiment.

In disturbed or pioneer settings, spores and sclerotia of ectomycorrhizal fungi serve as the necessary inoculum for establishment of ectomycorrhizal-dependent trees. Yet, little is known about the persistence of these propagules through time. Here, live field soil was inoculated with known quantities of basidiospores from four pine-associated species of Rhizopogon; these samples were then buried in retrievable containers, and pine seedling bioassays of serially diluted spore samples were used to measure spore viability. In the first 4 yr, no evidence of loss of spore viability was found in the four Rhizopogon species tested, but all four species exhibited dormancy in which a maximum of 1-8% of their spores were initially receptive to pine roots. There were some differences between species in overall inoculum potential of their spores, but all species broke dormancy at a statistically similar rate. This result provides evidence for spore dormancy in a common ectomycorrhizal genus, but it also precludes our ability to estimate the longevity of the spores accurately. Nevertheless these results, coupled with the observed patterns of Rhizopogon spore banks, suggest that at least decade-long durations are likely. As this experiment progresses, the true longevity of the spores will eventually be revealed.