Are modern-day tropical high-mountain forests remnants of an extratropical paleoflora of Gondwanan ancestry?

Background/Question/Methods

Recent studies have presented diversity and clade–age relationships for South American forests that are inconsistent with the tropical conservatism hypothesis (TCH), contradicting the prediction that older woody families should be more frequent at lower latitudes and low elevations. We argue that a broader framework that considers the ancient history of the southern hemisphere (SH) flora and the recent history of the Andes can help explain these discordances. We evaluated the phylogenetic structure of the family Cunoniaceae -broadly distributed in Australasia, South America and southern Africa- to test whether lineages occurring at high latitudes and altitudes show evidence of evolutionary processes driven by the break-up of Gondwana and subsequent mountain uplift. We used predictions about phylogenetic clustering and overdispersion patterns of coexistence to compare different biomes or regions where species of this family are distributed. We propose that ancient biomes should exhibit a phylogenetic overdispersed pattern of coexistence in Cunionaceae species, while newer biomes at higher altitudes in the tropics, particularly high-Andean forests, should display a clustering pattern.

Results/Conclusions

Preliminary results indicate that high-Andean forests in the Neotropics are composed of phylogenetically clustered groups of Cunoniaceae species, represented mainly by the genus Weinmannia that occurs in the Andes from Argentina to Central America. Temperate regions at high latitudes in the SH, in contrast, show a phylogenetic overdispersed pattern of coexistence. We suggest that the Paleogene Andean uplift represented an opportunity for northward expansion of some temperate lineages. Thus, a few genus of Cunoniaceae occupied geologically recent high elevation environments, with lower mean temperatures, in modern-day tropical regions, radiating in this new environment. Moreover, considering the areal reduction of SH extratropical forests due to Antarctic freezing and the conservative evolution of traits within lineages, we propose that the Cunoniaceae family has experienced significant extinction processes, expressed in current overdispersion of modern lineages in SH temperate forests.