Phylogenetic relatedness of saprotrophic fungal communities affects decomposition rates

Background/Question/Methods

Global change is significantly reducing biodiversity worldwide, with potential consequences for ecosystem functioning. Ecosystem-level consequences of biodiversity loss of macroorganisms are well understood, while those of microbial systems are not. Biodiversity and ecosystem functioning relationships may be fundamentally different for microbial communities as these taxa are many orders of magnitude more diverse than macroorganisms. Thus, phylogenetic diversity, which encompasses diversification over evolutionary time periods, may be a better estimate than species richness to predict the effects of diversity loss in microbial systems. Most previous studies have manipulated microbial species richness and have been conducted in the laboratory setting, making it difficult to understand these processes in natural systems. We manipulated species richness and phylogenetic relatedness of saprotrophic fungi in situ in a boreal forest to understand how biodiversity contributes to mass loss over time.

Results/Conclusions

Litter decomposition rates (as total mass loss) after two months were significantly higher in the least phylogenetically-related fungal assemblages (F = 3.540, P = 0.031). Specifically, the 50–75% relatedness treatment lost more mass than the 75–90% and 90–100% relatedness treatments (Duncan post hoc P < 0.05) Likewise, cellulose loss was also highest in these treatments after one year (F = 4.401, P = 0.014). There were no effects of species richness on mass loss (F = 2.323, P = 0.076). Two processes may explain these results: competition between closely-related fungal taxa, and phylogenetic conservation in cellulose decomposition. Closely-related assemblages significantly underyielded in mass loss after two months compared to more distantly-related assemblages (F = 5.125, P = 0.016). Cellulose loss was also significantly phylogenetic conserved at this time point (Variance Contrast = 201.397, P = 0.046). Because closely-related assemblages lost 25% less mass than the most distantly related assemblages, 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.