Fungal community assembly in a long-term glacial chronosequence

Background/Question/Methods

We investigated the influence of dispersal limitation and habitat filtering on the composition and function of saprotrophic fungi along a 4000-year glacial chronosequence in the Great Lakes region. Distance between these sites, serving as a proxy for time since deglaciation, enabled us to investigate the influence of historical processes on present-day fungal communities. Further, monitoring of site characteristics enabled us to investigate the impact of environmental gradients on fungal communities. If dispersal limitation is at work at the regional scale, then a positive relationship should exist between fungal community dissimilarity and time since community assembly. Conversely, if habitat filtering is at work, then differences in fungal communities should be related to environmental changes across sites, and not to their age. Furthermore, if community composition and function are linked, then communities that are compositionally similar will share a high degree of functional similarity. We analyzed fungal community composition by constructing clone libraries from forest floor rDNA and profiled community functional potential using a functional gene microarray.

Results/Conclusions

A significant relationship between fungal b-diversity and geographic distance (P = 0.006), a proxy for time since deglaciation, provided evidence that dispersal limitation shapes saprotrophic fungal communities over unexpectedly long timeframes. Further, fungal metabolic potential and functional richness increased with site age. Despite attempts to minimize environmental variation, a direct relationship between phylogenetic b-diversity and biogeochemical differences across sites (P = 0.032) indicated habitat filtering further shapes fungal community composition. However, no single biogeochemical variable correlated to b-diversity, indicating that slight variation in multiple characteristics may collectively differentiate fungal communities. Despite differences in phylogenetic b-diversity across the chronosequence, significant differences in the functional gene composition did not occur. Contrary to our predictions, the composition of functional genes did not vary predictably with b-diversity. By minimizing variation in plant community composition, variance in functional gene composition may have also been constrained along with litter biochemistry. Our study provides evidence that dispersal limitation operates over centuries to millennia in structuring present-day fungal community composition and functional potential.