Emerging pantropical patterns
The observed compositional differences among geographic regions indicate a relatively high level of endemism even within different parts of the Neotropics. Because our sampling of the hyperdiverse mycota in these regions is incomplete, it is possible that more shared taxa will be identified through the accumulation of more field data in the future. Nonetheless, regional endemism is widespread in fungi, as many species in low- to mid-latitudes show dispersal limitation and pronounced phylogeographic patterns (e.g., Geml 2011; Peay et al . 2012; Branco et al . 2015), resulting in differences in the composition of regional species pools.
Despite the low number of shared species, environmental variables are still at least as important drivers of community composition of tropical fungi as geography (i.e. biogeographic history). Almost two-thirds of the putatively pantropical OTUs were indicators for an elevational zone. This suggests that even among fungi with widespread geographic distributions, many are habitat specialists for a certain elevational forest type. This observation is in agreement with the importance of abiotic factors in shaping the distributions of the most common fungi on a global scale (Větrovský et al. 2019).
Fungal communities in upper montane forests are particularly rich in taxa with high specificity and fidelity to their habitat, as shown by indicator species analyses. The upper montane forest likely represents a unique habitat with a higher proportion of species specialized to the characteristic environmental conditions there (i.e. lower temperature and pH, increased OM accumulation, higher relative humidity and cloud formation).
Likewise, the similarity of proportional richness of the functional groups among the three regions is remarkable (Fig. S5). We hypothesize that functional niche proportions may be driven by similar mechanisms of environmental filtering along elevation gradients throughout Neo- and Paleotropical forests, resulting in relatively predictable representations of the functional guilds in lowland forests: saprotrophs representing ca. 35-40%, plant pathogens 11-17%, wood decomposers 6-15%, animal pathogens 2-4%, and mycoparasites 1-2% of the OTUs (as calculated for the whole community including fungi with currently unknown function). In addition, ECM and non-ECM root-associated fungi represent 1-2% and 1-5% of OTUs in Neotropical and 4-8% and 1-2% in Paleotropical lowland rainforests, and reach 4-12% and 2-4% of OTUs in Neotropical upper montane forests, respectively.