Mycorrhizal colonization of Pinus muricata from resistant propagules aftera stand-replacing wildfire

Colonization of mycorrhizal fungi was studied in a Pinus muricata forest on the coast in California, USA, burned by a stand-replacing wildfire in Octo ber 1995. Naturally established field seedlings of P. muricata were harvest ed 1 yr after the fire. The species composition of the mycorrhizal fungi on these field seedlings was dominated by Rhizopogon species, Wilcoxina mikol ae and Tomentella sublilacina. Bioassays, set up with soil collected immedi ately after the fire, were used to determine which mycorrhizal species had colonized the burned area from resistant propagules. The P. muricata seedli ngs in these bioassays were dominated by suilloid and ascomycetous fungi, t he same fungi which dominated the mycorrhizal flora of seedlings in pre-fir e bioassays derived from the same forest site, suggesting that resistant pr opagules were the primary inoculum source for naturally establishing seedli ngs. Drying of post-fire soil for 1 month raised the number of bioassay see dlings associated with Rhizopogon olivaceotinctus, while the number of bioa ssay seedlings associated with Rhizopogon ochraceorubens was reduced. Fire appeared to have either stimulated or provided a competitive advantage to R . olivaceotinctus, which increased in abundance on the post-fire bioassay a nd field seedlings. Soil collected from the burned area was diluted with st erile soil in three different concentrations, and the number and frequency of mycorrhizal taxa on bioassay seedlings decreased with increased dilution . Although precise quantification was not possible, propagules of the Rhizo pogon species were much more abundant than those of Tomentella or Wilcoxina species. Differences between the mycorrhizal associates of bioassay seedli ngs, naturally regenerated seedlings, and different inoculum sources are di scussed.