Abstract Pure culture inoculum (mycelium plus substrate) from each of six ectomycorrhizal fungi, all six fungi combined, and pine duff was mixed separately with 1:1 vermiculite-peat moss growing medium at 10 and 20 percent by volume, dispensed into 410 ml containers (Spencer-Lemaire Rootrainers ® ), and seeded with Pinus ponderosa or P. sylvestris . Seedlings were grown in a greenhouse at Bottineau, North Dakota, for 6 months, overwintered in the greenhouse, planted on a native grassland site at Denbigh, ND, and observed for 5 years. In the greenhouse, percent of seedling short roots ectomycorrhizal with Thelephora terrestris , Pisolithus tinctorius , mixed pure cultures, and pine duff ranged from 21 to 60. Cenococcum geophilum failed to synthesize mycorrhizae, and Rhizopogon roseolus and Suillus granulatus synthesized ectomycorrhizae on less than 5 percent of seedling short roots. Only one treatment, inoculation with 10 percent pine duff, increased stem diameters (24 percent) and weights (45 percent) of P. sylvestris seedlings. In the plantation, seedlings from P. tinctorius and pine duff treatments had over 48 and 65 percent of their short roots ectomycorrhizal, respectively, after 1 year, while those in other inoculation treatments averaged 20 to 41 percent of their rootlets ectomycorrhizal by all symbionts. Seedlings from pine duff treatment also had a greater percentage of short roots mycorrhizal than seedlings from P. tinctorius and control treatments after 2 and 3 years. Survival, stem diameters, current years' height increment, and biomass of P. ponderosa were increased after 2 and 3 years for seedlings inoculated with pine duff, Rhizopogon roseolus , or Suillus granulatus ; similar increases were obtained for P. sylvestris seedlings that were inoculated with pine duff. Forest Sci. 28:646-660.
Riffle et al. (Wed,) studied this question.