Phylogenetic diversity sheds light on the evolution of the unique fern flora of Mount Kinabalu, Borneo

Background and Aims Mount Kinabalu is an isolated, geologically young mountain (Pliocene and early Pleistocene) in northern Borneo harbouring a highly diverse flora with a high level of endemism. It is one of the global centres of fern diversity, but how this diversity has evolved has not been studied to date. We assess patterns of evolutionary legacies in the fern flora of Mount Kinabalu, with an emphasis on testing the tropical niche conservatism hypothesis.Methods We used several metrics of phylogenetic structure, including phylogenetic diversity (PD), mean nearest taxon distance (MNTD), mean pairwise distance (MPD), and their respective standardized effect sizes (PDses, MNTDses and MPDses), which represent different depths of evolutionary history, to investigate patterns of evolutionary legacies in the fern flora of Mount Kinabalu.Key Results We found that there was a decrease of PDses with elevation, reflecting a tropical origin of ferns and a gradual evolutionary adaptation to colder environments by fewer successive lineages. However, when separating the evolutionary old non-polypod lineages from the modern polypod radiation, we found a decrease of MNTD with elevation for the former, reflecting an ongoing upslope evolutionary trend, whereas for the modern polypod radiation, the inverse was true. Within the polypods, terrestrial and epiphytic species also showed different patterns, with the former showing a hump-shaped pattern for all phylogenetic diversity metrics, whereas the latter showed a more or less linear decline. Again, this suggests different evolutionary histories for ferns inhabiting the forest floor and canopy, due to the influence of the different habitat conditions. Finally, we found that mean annual temperature had a much stronger effect on phylogenetic diversity metrics than annual precipitation.Conclusions We found that the current fern assemblages on Mount Kinabalu show a strong evolutionary legacy that mirrors that of other mountains, suggesting a global consistency in phylogenetic diversity patterns of ferns along elevational transects, presumably due to a combination of dispersal between mountains and evolutionary convergence.