Does fungal competitive ability explain host specificity or rarity in ectomycorrhizal symbioses?

Abstract

AbstractTwo common ecological assumptions are that host generalist and rare species are poorer competitors relative to host specialist and more abundant counterparts. While these assumptions have received considerable study in both plant and animals, how they apply to ectomycorrhizal fungi remains largely unknown. To investigate how interspecific competition may influence the anomalous host associations of the rare ectomycorrhizal generalist fungus,Suillus subaureus, we conducted a seedling bioassay.Pinus strobusseedlings were inoculated in single- or two-species treatments of threeSuillusspecies:S. subaureus,S. americanus, andS. spraguei. After 4 and 8 months of growth, seedlings were harvested and scored for mycorrhizal colonization as well as dry biomass. At both time points, we found a clear competitive hierarchy among these species:S. americanus>S. subaureus>S. spraguei, with the competitive inferior,S. spraguei, having significantly delayed colonization relative toS. americanusandS. subaureus. In the single-species treatments, we found no significant differences in the dry biomasses of seedlings colonized by eachSuillusspecies, suggesting none of these species was a more effective plant symbiont relative to each other. Taken together, these results indicate that the rarity and anomalous host associations exhibited byS. subaureusin natural settings are not driven by inherently poor competitive ability or host growth promotion, but that the timing of colonization is a key factor determining the outcome of ectomycorrhizal fungal competitive interactions.