Interest in the ecology of ectomycorrhizal (ECM) fungi has increased considerably, but little is known about interspecific interactions among ECM species. We examined competitive interactions between Rhizopogon occidentalis and R. salebrosus at Point Reyes National Seashore, California, USA. At three field sites, species abundances were compared in single- and two-species treatments on Pinus muricata seedlings inoculated with spores. Competition for root tips was assessed using real-time polymerase chain reaction (PCR) of internal transcribed spacer rDNA. In general, we found strong competitive exclusion of R. salebrosus by R. occidentalis, with ≥ 75 % of the seedlings in the two-species treatment colonized exclusively by R. occidentalis after 5 and 10 months. However, on the seedlings that were co-colonized, we observed no significant difference in the abundances of R. salebrosus and R. occidentalis, suggesting that once R. salebrosus was established, it was no longer competitively inferior. There were no significant differences in survival, growth, or percentage leaf nitrogen of seedlings colonized with either Rhizopogon species, but both growth and percentage leaf nitrogen were significantly higher for ECM than non-EC
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