Initial phylogenetic relatedness of saprotrophic fungal communities affects subsequent litter decomposition rates

Ecosystem-level consequences of biodiversity loss of macroorganisms are well understood, while the repercussions of species extirpation in microbial systems are not. We manipulated species richness and phylogenetic relatedness of saprotrophic fungi in situ in a boreal forest to address this issue. Litter decomposition rates (as total mass loss) after 2 months were significantly higher in the least phylogenetically related fungal assemblages. Likewise, cellulose loss was also highest in the most distantly related treatments after 1 year. There were marginal effects of species richness on mass loss that only affected decomposition after 2 months. At the end of 1 year of decomposition, most fungal communities had collapsed from their original diversity to two species, mainly in the Penicillium or Hypocrea clades. Two concurrent processes may explain these results: competition between closely related fungal taxa and phylogenetic conservation in cellulose decomposition. Our results suggest that phylogenetic relatedness of fungal communities may be a more appropriate metric than species richness or community composition to predict functional responses of fungal communities to global change.