The remarkable diversity of fungi in soils is thought to be maintained by differentiation of fungal species across substrates, space and time. However, patterns of fungal diversity and distribution within and across ecosystems are poorly understood at regional and global scales. Ectomycorrhizal (EM) and saprotrophic fungi are two diverse functional groups of fungi that regulate key nutrient cycling processes in many forest ecosystems. We tested the hypothesis that EM and saprotrophic fungi spatially segregate across the upper layers of organic soil horizons in an Alaskan boreal forest and a tropical forest in Guyana, South America. We separately collected underlying soil containing highly-decomposed organic material (0-5cm) and recently fallen leaf litter that had been decomposing in litter bags for one year from each ecosystem. We extracted DNA from each soil fraction, created cDNA libraries using fungal-specific primers and sequenced the resulting clones.

Results/Conclusions

In both forests, we found significant separation of EM and saprotrophic fungi across soil horizons. Ectomycorrhizal fungi were predominantly found in the underlying soil layers, while saprobes were mostly detected in the recently decomposed litter. Overall, the tropical forest had twice as many fungal species as the boreal forest, but both ecosystems had approximately the same number of EM taxa. Since tropical forests regionally contain higher plant diversity by at least an order of magnitude compared to boreal forests, it is possible that the gradient in saprotrophic fungal species richness tracks plant diversity. Fungal communities were distinct between ecosystems, with few overlapping species. Out of the approximately 50 total fungal families detected, only six were found in both forests, suggesting that the distribution of taxa may not be ubiquitous. Our results are consistent with other findings from boreal forests, but this is the first test of this hypothesis in a tropical forest, and the first direct comparison of fungal diversity and distribution across boreal and tropical forests. Since these ecosystems are both significant drivers of climate, evaluating patterns of fungal diversity and distribution is critical for understanding fluxes of nutrients and for predicting responses of ecosystem functions to perturbations.